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Optical Amplifiers (OAs) are key parts of today’s communication world. They help send data under the sea, land and even in space .In fact it is used in all electronic and telecommunications industry which has allowed human being develop and use gadgets and machines in daily routine.Due to OAs only; we are able to transmit data over a distance of few 100s too 1000s of kilometers.

Classification of OA Devices

Optical Amplifiers, integral in managing signal strength in fiber optics, are categorized based on their technology and application. These categories, as defined in ITU-T G.661, include Power Amplifiers (PAs), Pre-amplifiers, Line Amplifiers, OA Transmitter Subsystems (OATs), OA Receiver Subsystems (OARs), and Distributed Amplifiers.

amplifier

Scheme of insertion of an OA device

  1. Power Amplifiers (PAs): Positioned after the optical transmitter, PAs boost the signal power level. They are known for their high saturation power, making them ideal for strengthening outgoing signals.
  2. Pre-amplifiers: These are used before an optical receiver to enhance its sensitivity. Characterized by very low noise, they are crucial in improving signal reception.
  3. Line Amplifiers: Placed between passive fiber sections, Line Amplifiers are low noise OAs that extend the distance covered before signal regeneration is needed. They are particularly useful in point-multipoint connections in optical access networks.
  4. OA Transmitter Subsystems (OATs): An OAT integrates a power amplifier with an optical transmitter, resulting in a higher power transmitter.
  5. OA Receiver Subsystems (OARs): In OARs, a pre-amplifier is combined with an optical receiver, enhancing the receiver’s sensitivity.
  6. Distributed Amplifiers: These amplifiers, such as those using Raman pumping, provide amplification over an extended length of the optical fiber, distributing amplification across the transmission span.
Scheme of insertion of an OAT

Scheme of insertion of an OAT
Scheme of insertion of an OAR
Scheme of insertion of an OAR

Applications and Configurations

The application of these OA devices can vary. For instance, a Power Amplifier (PA) might include an optical filter to minimize noise or separate signals in multiwavelength applications. The configurations can range from simple setups like Tx + PA + Rx to more complex arrangements like Tx + BA + LA + PA + Rx, as illustrated in the various schematics provided in the IEC standards.

Building upon the foundational knowledge of Optical Amplifiers (OAs), it’s essential to understand the practical configurations of these devices in optical networks. According to the definitions of Booster Amplifiers (BAs), Pre-amplifiers (PAs), and Line Amplifiers (LAs), and referencing Figure 1 from the IEC standards, we can explore various OA device applications and their configurations. These setups illustrate how OAs are integrated into optical communication systems, each serving a unique purpose in enhancing signal integrity and network performance.

  1. Tx + BA + Rx Configuration: This setup involves a transmitter (Tx), followed by a Booster Amplifier (BA), and then a receiver (Rx). The BA is used right after the transmitter to increase the signal power before it enters the long stretch of the fiber. This configuration is particularly useful in long-haul communication systems where maintaining a strong signal over vast distances is crucial.
  2. Tx + PA + Rx Configuration: Here, the system comprises a transmitter, followed by a Pre-amplifier (PA), and then a receiver. The PA is positioned close to the receiver to improve its sensitivity and to amplify the weakened incoming signal. This setup is ideal for scenarios where the incoming signal strength is low, and enhanced detection is required.
  3. Tx + LA + Rx Configuration: In this configuration, a Line Amplifier (LA) is placed between the transmitter and receiver. The LA’s role is to amplify the signal partway through the transmission path, effectively extending the reach of the communication link. This setup is common in both long-haul and regional networks.
  4. Tx + BA + PA + Rx Configuration: This more complex setup involves both a BA and a PA, with the BA placed after the transmitter and the PA before the receiver. This combination allows for both an initial boost in signal strength and a final amplification to enhance receiver sensitivity, making it suitable for extremely long-distance transmissions or when signals pass through multiple network segments.
  5. Tx + BA + LA + Rx Configuration: Combining a BA and an LA provides a powerful solution for extended reach. The BA boosts the signal post-transmission, and the LA offers additional amplification along the transmission path. This configuration is particularly effective in long-haul networks with significant attenuation.
  6. Tx + LA + PA + Rx Configuration: Here, the LA is used for mid-path amplification, while the PA is employed near the receiver. This setup ensures that the signal is sufficiently amplified both during transmission and before reception, which is vital in networks with long spans and higher signal loss.
  7. Tx + BA + LA + PA + Rx Configuration: This comprehensive setup includes a BA, an LA, and a PA, offering a robust solution for maintaining signal integrity across very long distances and complex network architectures. The BA boosts the initial signal strength, the LA provides necessary mid-path amplification, and the PA ensures that the receiver can effectively detect the signal.

Characteristics of Optical Amplifiers

Each type of OA has specific characteristics that define its performance in different applications, whether single-channel or multichannel. These characteristics include input and output power ranges, wavelength bands, noise figures, reflectance, and maximum tolerable reflectance at input and output, among others.

For instance, in single-channel applications, a Power Amplifier’s characteristics would include an input power range, output power range, power wavelength band, and signal-spontaneous noise figure. In contrast, for multichannel applications, additional parameters like channel allocation, channel input and output power ranges, and channel signal-spontaneous noise figure become relevant.

Optically Amplified Transmitters and Receivers

In the realm of OA subsystems like OATs and OARs, the focus shifts to parameters like bit rate, application code, operating signal wavelength range, and output power range for transmitters, and sensitivity, overload, and bit error ratio for receivers. These parameters are critical in defining the performance and suitability of these subsystems for specific applications.

Understanding Through Practical Examples

To illustrate, consider a scenario in a long-distance fiber optic communication system. Here, a Line Amplifier might be employed to extend the transmission distance. This amplifier would need to have a low noise figure to minimize signal degradation and a high saturation output power to ensure the signal remains strong over long distances. The specific values for these parameters would depend on the system’s requirements, such as the total transmission distance and the number of channels being used.

Advanced Applications of Optical Amplifiers

  1. Long-Haul Communication: In long-haul fiber optic networks, Line Amplifiers (LAs) play a critical role. They are strategically placed at intervals to compensate for signal loss. For example, an LA with a high saturation output power of around +17 dBm and a low noise figure, typically less than 5 dB, can significantly extend the reach of the communication link without the need for electronic regeneration.
  2. Submarine Cables: Submarine communication cables, spanning thousands of kilometers, heavily rely on Distributed Amplifiers, like Raman amplifiers. These amplifiers uniquely boost the signal directly within the fiber, offering a more distributed amplification approach, which is crucial for such extensive undersea networks.
  3. Metropolitan Area Networks: In shorter, more congested networks like those in metropolitan areas, a combination of Booster Amplifiers (BAs) and Pre-amplifiers can be used. A BA, with an output power range of up to +23 dBm, can effectively launch a strong signal into the network, while a Pre-amplifier at the receiving end, with a very low noise figure (as low as 4 dB), enhances the receiver’s sensitivity to weak signals.
  4. Optical Add-Drop Multiplexers (OADMs): In systems using OADMs for channel multiplexing and demultiplexing, Line Amplifiers help in maintaining signal strength across the channels. The ability to handle multiple channels, each potentially with different power levels, is crucial. Here, the channel addition/removal (steady-state) gain response and transient gain response become significant parameters.

Technological Innovations and Challenges

The development of OA technologies is not without challenges. One of the primary concerns is managing the noise, especially in systems with multiple amplifiers. Each amplification stage adds some noise, quantified by the signal-spontaneous noise figure, which can accumulate and degrade the overall signal quality.

Another challenge is the management of Polarization Mode Dispersion (PMD) in Line Amplifiers. PMD can cause different light polarizations to travel at slightly different speeds, leading to signal distortion. Modern LAs are designed to minimize PMD, a critical parameter in high-speed networks.

Future of Optical Amplifiers in Industry

The future of OAs is closely tied to the advancements in fiber optic technology. As data demands continue to skyrocket, the need for more efficient, higher-capacity networks grows. Optical Amplifiers will continue to evolve, with research focusing on higher power outputs, broader wavelength ranges, and more sophisticated noise management techniques.

Innovations like hybrid amplification techniques, combining the benefits of Raman and Erbium-Doped Fiber Amplifiers (EDFAs), are on the horizon. These hybrid systems aim to provide higher performance, especially in terms of power efficiency and noise reduction.

References

ITU-T :https://www.itu.int/en/ITU-T/Pages/default.aspx

Image :https://www.chinacablesbuy.com/guide-to-optical-amplifier.html

As the 5G era dawns, the need for robust transport network architectures has never been more critical. The advent of 5G brings with it a promise of unprecedented data speeds and connectivity, necessitating a backbone capable of supporting a vast array of services and applications. In this realm, the Optical Transport Network (OTN) emerges as a key player, engineered to meet the demanding specifications of 5G’s advanced network infrastructure.

Understanding OTN’s Role

The 5G transport network is a multifaceted structure, composed of fronthaul, midhaul, and backhaul components, each serving a unique function within the overarching network ecosystem. Adaptability is the name of the game, with various operators customizing their network deployment to align with individual use cases as outlined by the 3rd Generation Partnership Project (3GPP).

C-RAN: Centralized Radio Access Network

In the C-RAN scenario, the Active Antenna Unit (AAU) is distinct from the Distribution Unit (DU), with the DU and Central Unit (CU) potentially sharing a location. This configuration leads to the presence of fronthaul and backhaul networks, and possibly midhaul networks. The fronthaul segment, in particular, is characterized by higher bandwidth demands, catering to the advanced capabilities of technologies like enhanced Common Public Radio Interface (eCPRI).

CRAN
5G transport network architecture: C-RAN

C-RAN Deployment Specifics:

  • Large C-RAN: DUs are centrally deployed at the central office (CO), which typically is the intersection point of metro-edge fibre rings. The number of DUs within in each CO is between 20 and 60 (assume each DU is connected to 3 AAUs).
  • Small C-RAN: DUs are centrally deployed at the metro-edge site, which typically is located at the metro-edge fibre ring handover point. The number of DUs within each metro-edge site is around 5~10

D-RAN: Distributed Radio Access Network

The D-RAN setup co-locates the AAU with the DU, eliminating the need for a dedicated fronthaul network. This streamlined approach focuses on backhaul (and potentially midhaul) networks, bypassing the fronthaul segment altogether.

5G transport network architecture: D-RAN
5G transport network architecture: D-RAN

NGC: Next Generation Core Interconnection

The NGC interconnection serves as the network’s spine, supporting data transmission capacities ranging from 0.8 to 2 Tbit/s, with latency requirements as low as 1 ms, and reaching distances between 100 to 200 km.

Transport Network Requirement Summary for NGC:

ParameterRequirementComments
Capacity0.8-2 Tbit/sEach NGC node has 500 base stations. The average bandwidth of each base station is about 3Gbit/s, the convergence ratio is 1/4, and the typical bandwidth of NGC nodes is about 400Gbit/s. 2~5 directions are considered, so the NGC node capacity is 0.8~2Tbit/s.
Latency1 msRound trip time (RTT) latency between NGCs required for DC hot backup intra-city.
Reach100-200 kmTypical distance between NGCs.

Note: These requirements will vary among network operators.

The Future of 5G Transport Networks

The blueprint for 5G networks is complex, yet it must ensure seamless service delivery. The diversity of OTN architectures, from C-RAN to D-RAN and the strategic NGC interconnections, underscores the flexibility and scalability essential for the future of mobile connectivity. As 5G unfolds, the ability of OTN architectures to adapt and scale will be pivotal in meeting the ever-evolving landscape of digital communication.

References

https://www.itu.int/rec/T-REC-G.Sup67/en

The advent of 5G technology is set to revolutionise the way we connect, and at its core lies a sophisticated transport network architecture. This architecture is designed to support the varied requirements of 5G’s advanced services and applications.

As we migrate from the legacy 4G to the versatile 5G, the transport network must evolve to accommodate new deployment strategies influenced by the functional split options specified by 3GPP and the drift of the Next Generation Core (NGC) network towards cloud-edge deployment.

5G
Deployment location of core network in 5G network

The Four Pillars of 5G Transport Network

1. Fronthaul: This segment of the network deals with the connection between the high PHY and low PHY layers. It requires a high bandwidth, about 25 Gbit/s for a single UNI interface, escalating to 75 or 150 Gbit/s for an NNI interface in pure 5G networks. In hybrid 4G and 5G networks, this bandwidth further increases. The fronthaul’s stringent latency requirements (<100 microseconds) necessitate point-to-point (P2P) deployment to ensure rapid and efficient data transfer.

2. Midhaul: Positioned between the Packet Data Convergence Protocol (PDCP) and Radio Link Control (RLC), the midhaul section plays a pivotal role in data aggregation. Its bandwidth demands are slightly less than that of the fronthaul, with UNI interfaces handling 10 or 25 Gbit/s and NNI interfaces scaling according to the DU’s aggregation capabilities. The midhaul network typically adopts tree or ring modes to efficiently connect multiple Distributed Units (DUs) to a centralized Control Unit (CU).

3. Backhaul: Above the Radio Resource Control (RRC), the backhaul shares similar bandwidth needs with the midhaul. It handles both horizontal traffic, coordinating services between base stations, and vertical traffic, funneling various services like Vehicle to Everything (V2X), enhanced Mobile BroadBand (eMBB), and Internet of Things (IoT) from base stations to the 5G core.

4. NGC Interconnection: This crucial juncture interconnects nodes post-deployment in the cloud edge, demanding bandwidths equal to or in excess of 100 Gbit/s. The architecture aims to minimize bandwidth wastage, which is often a consequence of multi-hop connections, by promoting single hop connections.

The Impact of Deployment Locations

The transport network’s deployment locations—fronthaul, midhaul, backhaul, and NGC interconnection—each serve unique functions tailored to the specific demands of 5G services. From ensuring ultra-low latency in fronthaul to managing service diversity in backhaul, and finally facilitating high-capacity connectivity in NGC interconnections, the transport network is the backbone that supports the high-speed, high-reliability promise of 5G.

As we move forward into the 5G era, understanding and optimizing these transport network segments will be crucial for service providers to deliver on the potential of this transformative technology.

Reference

https://www.itu.int/rec/T-REC-G.Sup67-201907-I/en


In today’s world, where digital information rules, keeping networks secure is not just important—it’s essential for the smooth operation of all our communication systems. Optical Transport Networking (OTN), which follows rules set by standards like ITU-T G.709 and ITU-T G.709.1, is leading the charge in making sure data gets where it’s going safely. This guide takes you through the essentials of OTN secure transport, highlighting how encryption and authentication are key to protecting sensitive data as it moves across networks.

The Introduction of OTN Security

Layer 1 encryption, or OTN security (OTNsec), is not just a feature—it’s a fundamental aspect that ensures the safety of data as it traverses the complex web of modern networks. Recognized as a market imperative, OTNsec provides encryption at the physical layer, thwarting various threats such as control management breaches, denial of service attacks, and unauthorized access.

OTNsec

Conceptualizing Secure Transport

OTN secure transport can be visualized through two conceptual approaches. The first, and the primary focus of this guide, involves the service requestor deploying endpoints within its domain to interface with an untrusted domain. The second approach sees the service provider offering security endpoints and control over security parameters, including key management and agreement, to the service requestor.

OTN Security Applications

As network operators and service providers grapple with the need for data confidentiality and authenticity, OTN emerges as a robust solution. From client end-to-end security to service provider path end-to-end security, OTN’s applications are diverse.

Client End-to-End Security

This suite of applications ensures that the operator’s OTN network remains oblivious to the client layer security, which is managed entirely within the customer’s domain. Technologies such as MACsec [IEEE 802.1AE] for Ethernet clients provide encryption and authentication at the client level.Following are some of the scenerios.

Client end-to-end security (with CPE)

Client end-to-end security (without CPE)
DC, content or mobile service provider client end-to-end security

Service Provider CPE End-to-End Security

Service providers can offer security within the OTN service of the operator’s network. This scenario sees the service provider managing key agreements, with the UNI access link being the only unprotected element, albeit within the trusted customer premises.

OTNsec

Service provider CPE end-to-end security

OTN Link/Span Security

Operators can fortify their network infrastructure using encryption and authentication on a per-span basis. This is particularly critical when the links interconnect various OTN network elements within the same administrative domain.

OTN link/span security
OTN link/span security

OTN link/span leased fibre security
OTN link/span leased fibre security

Second Operator and Access Link Security

When services traverse the networks of multiple operators, securing each link becomes paramount. Whether through client access link security or OTN service provider access link security, OTN facilitates a protected handoff between customer premises and the operator.

OTN leased service security
OTN leased service security

Multi-Layered Security in OTN

OTN’s versatility allows for multi-layered security, combining protocols that offer different characteristics and serve complementary functions. From end-to-end encryption at the client layer to additional encryption at the ODU layer, OTN accommodates various security needs without compromising on performance.

OTN end-to-end security (with CPE)
OTN end-to-end security (with CPE)

Final Observations

OTN security applications must ensure transparency across network elements not participating as security endpoints. Support for multiple levels of ODUj to ODUk schemes, interoperable cipher suite types for PHY level security, and the ability to handle subnetworks and TCMs are all integral to OTN’s security paradigm.

Layered security example
Layered security example

This blog provides a detailed exploration of OTN secure transport, encapsulating the strategic implementation of security measures in optical networks. It underscores the importance of encryption and authentication in maintaining data integrity and confidentiality, positioning OTN as a critical component in the infrastructure of secure communication networks.

By adhering to these security best practices, network operators can not only safeguard their data but also enhance the overall trust in their communication systems, paving the way for a secure and reliable digital future.

References

More Detail article can be read on ITU-T at

https://www.itu.int/rec/T-REC-G.Sup76/en

Fiber optics has revolutionized the way we transmit data, offering faster speeds and higher capacity than ever before. However, as with any powerful technology, there are significant safety considerations that must be taken into account to protect both personnel and equipment. This comprehensive guide provides an in-depth look at best practices for optical power safety in fiber optic communications.

Directly viewing fiber ends or connector faces can be hazardous. It’s crucial to use only approved filtered or attenuating viewing aids to inspect these components. This protects the eyes from potentially harmful laser emissions that can cause irreversible damage.

Unterminated fiber ends, if left uncovered, can emit laser light that is not only a safety hazard but can also compromise the integrity of the optical system. When fibers are not being actively used, they should be covered with material suitable for the specific wavelength and power, such as a splice protector or tape. This precaution ensures that sharp ends are not exposed, and the fiber ends are not readily visible, minimizing the risk of accidental exposure.

Optical connectors must be kept clean, especially in high-power systems. Contaminants can lead to the fiber-fuse phenomenon, where high temperatures and bright white light propagate down the fiber, creating a safety hazard. Before any power is applied, ensure that all fiber ends are free from contaminants.

Even a small amount of loss at connectors or splices can lead to a significant increase in temperature, particularly in high-power systems. Choosing the right connectors and managing splices carefully can prevent local heating that might otherwise escalate to system damage.

Ribbon fibers, when cleaved as a unit, can present a higher hazard level than single fibers. They should not be cleaved or spliced as an unseparated ribbon unless explicitly authorized. When using optical test cords, always connect the optical power source last and disconnect it first to avoid any inadvertent exposure to active laser light.

Fiber optics are delicate and can be damaged by excessive bending, which not only risks mechanical failure but also creates potential hotspots in high-power transmission. Careful routing and handling of fibers to avoid low-radius bends are essential best practices.

Board extenders should never be used with optical transmitter or amplifier cards. Only perform maintenance tasks in accordance with the procedures approved by the operating organization to avoid unintended system alterations that could lead to safety issues.

Employ test equipment that is appropriate for the task at hand. Using equipment with a power rating higher than necessary can introduce unnecessary risk. Ensure that the class of the test equipment matches the hazard level of the location where it’s being used.

Unauthorized modifications to optical fiber communication systems or related equipment are strictly prohibited, as they can introduce unforeseen hazards. Additionally, key control for equipment should be managed by a responsible individual to ensure the safe and proper use of all devices.

Optical safety labels are a critical aspect of safety. Any damaged or missing labels should be reported immediately. Warning signs should be posted in areas exceeding hazard level 1M, and even in lower classification locations, signs can provide an additional layer of safety.

Pay close attention to system alarms, particularly those indicating issues with automatic power reduction (APR) or other safety mechanisms. Prompt response to alarms can prevent minor issues from escalating into major safety concerns.

Raman Amplified Systems: A Special Note

Optical_safety

Raman amplified systems operate at sufficiently high powers that can cause damage to fibre or other components. This is somewhat described in clauses 14.2 and 14.5, but some additional guidance follows:

Before activating the Raman power

–           Calculate the distance to where the power is reduced to less than 150 mW.

–           If possible, inspect any splicing enclosures within that distance. If tight bends, e.g., less than 20mm diameter, are seen, try to remove or relieve the bend, or choose other fibres.

–           If inspection is not possible, a high resolution OTDR might be used to identify sources of bend or connector loss that could lead to damage under high power.

–           If connectors are used, it should be verified that the ends are very clean. Metallic contaminants are particularly prone to causing damage. Fusion splices are considered to be the least subject to damage.

While activating Raman power

In some cases, it may be possible to monitor the reflected light at the source as the Raman pump power is increased. If the plot of reflected power versus injected power shows a non‑linear characteristic, there could be a reflective site that is subject to damage. Other sites subject to damage, such as tight bends in which the coating absorbs the optical power, may be present without showing a clear signal in the reflected power versus injected power curve.

Operating considerations

If there is a reduction in the amplification level over time, it could be due to a reduced pump power or due to a loss increase induced by some slow damage mechanism such as at a connector interface. Simply increasing the pump power to restore the signal could lead to even more damage or catastrophic failure.

The mechanism for fibre failure in bending is that light escapes from the cladding and some is absorbed by the coating, which results in local heating and thermal reactions. These reactions tend to increase the absorption and thus increase the heating. When a carbon layer is formed, there is a runaway thermal reaction that produces enough heat to melt the fibre, which then goes into a kinked state that blocks all optical power. Thus, there will be very little change in the transmission characteristics induced by a damaging process until the actual failure occurs. If the fibre is unbuffered, there is a flash at the moment of failure which is self-extinguishing because the coating is gone very quickly. A buffered fibre could produce more flames, depending on the material. For unbuffered fibre, sub-critical damage is evidenced by a colouring of the coating at the apex of the bend.

Conclusion

By following these best practices for optical power safety, professionals working with fiber optic systems can ensure a safe working environment while maintaining the integrity and performance of the communication systems they manage.

For those tasked with the maintenance and operation of fiber optic systems, this guide serves as a critical resource, outlining the necessary precautions to ensure safety in the workplace. As the technology evolves, so too must our commitment to maintaining stringent safety standards in the dynamic field of fiber optic communications.

References

https://www.itu.int/rec/T-REC-G/e

In the pursuit of ever-greater data transmission capabilities, forward error correction (FEC) has emerged as a pivotal technology, not just in wireless communication but increasingly in large-capacity, long-haul optical systems. This blog post delves into the intricacies of FEC and its profound impact on the efficiency and cost-effectiveness of modern optical networks.

The Introduction of FEC in Optical Communications

FEC’s principle is simple yet powerful: by encoding the original digital signal with additional redundant bits, it can correct errors that occur during transmission. This technique enables optical transmission systems to tolerate much higher bit error ratios (BERs) than the traditional threshold of 10−1210−12 before decoding. Such resilience is revolutionizing system design, allowing the relaxation of optical parameters and fostering the development of vast, robust networks.

Defining FEC: A Glossary of Terms

inband_outband_fec

Understanding FEC starts with grasping its key terminology. Here’s a brief rundown:

  • Information bit (byte): The original digital signal that will be encoded using FEC before transmission.
  • FEC parity bit (byte): Redundant data added to the original signal for error correction purposes.
  • Code word: A combination of information and FEC parity bits.
  • Code rate (R): The ratio of the original bit rate to the bit rate with FEC—indicative of the amount of redundancy added.
  • Coding gain: The improvement in signal quality as a result of FEC, quantified by a reduction in Q values for a specified BER.
  • Net coding gain (NCG): Coding gain adjusted for noise increase due to the additional bandwidth needed for FEC bits.

The Role of FEC in Optical Networks

The application of FEC allows for systems to operate with a BER that would have been unacceptable in the past, particularly in high-capacity, long-haul systems where the cumulative noise can significantly degrade signal quality. With FEC, these systems can achieve reliable performance even with the presence of amplified spontaneous emission (ASE) noise and other signal impairments.

In-Band vs. Out-of-Band FEC

There are two primary FEC schemes used in optical transmission: in-band and out-of-band FEC. In-band FEC, used in Synchronous Digital Hierarchy (SDH) systems, embeds FEC parity bits within the unused section overhead of SDH signals, thus not increasing the bit rate. In contrast, out-of-band FEC, as utilized in Optical Transport Networks (OTNs) and originally recommended for submarine systems, increases the line rate to accommodate FEC bits. ITU-T G.709 also introduces non-standard out-of-band FEC options optimized for higher efficiency.

Achieving Robustness Through FEC

The FEC schemes allow the correction of multiple bit errors, enhancing the robustness of the system. For example, a triple error-correcting binary BCH code can correct up to three bit errors in a 4359 bit code word, while an RS(255,239) code can correct up to eight byte errors per code word.

fec_performance

Performance of standard FECs

The Practical Impact of FEC

Implementing FEC leads to more forgiving system designs, where the requirement for pristine optical parameters is lessened. This, in turn, translates to reduced costs and complexity in constructing large-scale optical networks. The coding gains provided by FEC, especially when considered in terms of net coding gain, enable systems to better estimate and manage the OSNR, crucial for maintaining high-quality signal transmission.

Future Directions

While FEC has proven effective in OSNR-limited and dispersion-limited systems, its efficacy against phenomena like polarization mode dispersion (PMD) remains a topic for further research. Additionally, the interplay of FEC with non-linear effects in optical fibers, such as self-phase modulation and cross-phase modulation, presents a rich area for ongoing study.

Conclusion

FEC stands as a testament to the innovative spirit driving optical communications forward. By enabling systems to operate with higher BERs pre-decoding, FEC opens the door to more cost-effective, expansive, and resilient optical networks. As we look to the future, the continued evolution of FEC promises to underpin the next generation of optical transmission systems, making the dream of a hyper-connected world a reality.

References

https://www.itu.int/rec/T-REC-G/e

Optical networks are the backbone of the internet, carrying vast amounts of data over great distances at the speed of light. However, maintaining signal quality over long fiber runs is a challenge due to a phenomenon known as noise concatenation. Let’s delve into how amplified spontaneous emission (ASE) noise affects Optical Signal-to-Noise Ratio (OSNR) and the performance of optical amplifier chains.

The Challenge of ASE Noise

ASE noise is an inherent byproduct of optical amplification, generated by the spontaneous emission of photons within an optical amplifier. As an optical signal traverses through a chain of amplifiers, ASE noise accumulates, degrading the OSNR with each subsequent amplifier in the chain. This degradation is a crucial consideration in designing long-haul optical transmission systems.

Understanding OSNR

OSNR measures the ratio of signal power to ASE noise power and is a critical parameter for assessing the performance of optical amplifiers. A high OSNR indicates a clean signal with low noise levels, which is vital for ensuring data integrity.

Reference System for OSNR Estimation

As depicted in Figure below), a typical multichannel N span system includes a booster amplifier, N−1 line amplifiers, and a preamplifier. To simplify the estimation of OSNR at the receiver’s input, we make a few assumptions:

Representation of optical line system interfaces (a multichannel N-span system)
  • All optical amplifiers, including the booster and preamplifier, have the same noise figure.
  • The losses of all spans are equal, and thus, the gain of the line amplifiers compensates exactly for the loss.
  • The output powers of the booster and line amplifiers are identical.

Estimating OSNR in a Cascaded System

E1: Master Equation For OSNR

E1: Master Equation For OSNR

Pout is the output power (per channel) of the booster and line amplifiers in dBm, L is the span loss in dB (which is assumed to be equal to the gain of the line amplifiers), GBA is the gain of the optical booster amplifier in dB, NFis the signal-spontaneous noise figure of the optical amplifier in dB, h is Planck’s constant (in mJ·s to be consistent with Pout in dBm), ν is the optical frequency in Hz, νr is the reference bandwidth in Hz (corresponding to c/Br ), N–1 is the total number of line amplifiers.

The OSNR at the receivers can be approximated by considering the output power of the amplifiers, the span loss, the gain of the optical booster amplifier, and the noise figure of the amplifiers. Using constants such as Planck’s constant and the optical frequency, we can derive an equation that sums the ASE noise contributions from all N+1 amplifiers in the chain.

Simplifying the Equation

Under certain conditions, the OSNR equation can be simplified. If the booster amplifier’s gain is similar to that of the line amplifiers, or if the span loss greatly exceeds the booster gain, the equation can be modified to reflect these scenarios. These simplifications help network designers estimate OSNR without complex calculations.

1)          If the gain of the booster amplifier is approximately the same as that of the line amplifiers, i.e., GBA » L, above Equation E1 can be simplified to:

osnr_2

E1-1

2)          The ASE noise from the booster amplifier can be ignored only if the span loss L (resp. the gain of the line amplifier) is much greater than the booster gain GBA. In this case Equation E1-1 can be simplified to:

E1-2

3)          Equation E1-1 is also valid in the case of a single span with only a booster amplifier, e.g., short‑haul multichannel IrDI in Figure 5-5 of [ITU-T G.959.1], in which case it can be modified to:

E1-3

4)          In case of a single span with only a preamplifier, Equation E1 can be modified to:

Practical Implications for Network Design

Understanding the accumulation of ASE noise and its impact on OSNR is crucial for designing reliable optical networks. It informs decisions on amplifier placement, the necessity of signal regeneration, and the overall system architecture. For instance, in a system where the span loss is significantly high, the impact of the booster amplifier on ASE noise may be negligible, allowing for a different design approach.

Conclusion

Noise concatenation is a critical factor in the design and operation of optical networks. By accurately estimating and managing OSNR, network operators can ensure signal quality, minimize error rates, and extend the reach of their optical networks.

In a landscape where data demands are ever-increasing, mastering the intricacies of noise concatenation and OSNR is essential for anyone involved in the design and deployment of optical communication systems.

References

https://www.itu.int/rec/T-REC-G/e

Forward Error Correction (FEC) has become an indispensable tool in modern optical communication, enhancing signal integrity and extending transmission distances. ITU-T recommendations, such as G.693, G.959.1, and G.698.1, define application codes for optical interfaces that incorporate FEC as specified in ITU-T G.709. In this blog, we discuss the significance of Bit Error Ratio (BER) in FEC-enabled applications and how it influences optical transmitter and receiver performance.

The Basics of FEC in Optical Communications

FEC is a method of error control for data transmission, where the sender adds redundant data to its messages. This allows the receiver to detect and correct errors without the need for retransmission. In the context of optical networks, FEC is particularly valuable because it can significantly lower the BER after decoding, thus ensuring the accuracy and reliability of data across vast distances.

BER Requirements in FEC-Enabled Applications

For certain optical transport unit rates (OTUk), the system BER is mandated to meet specific standards only after FEC correction has been applied. The optical parameters, in these scenarios, are designed to achieve a BER no worse than 10−12 at the FEC decoder’s output. This benchmark ensures that the data, once processed by the FEC decoder, maintains an extremely high level of accuracy, which is crucial for high-performance networks.

Practical Implications for Network Hardware

When it comes to testing and verifying the performance of optical hardware components intended for FEC-enabled applications, achieving a BER of 10−12 at the decoder’s output is often sufficient. Attempting to test components at 10−12 at the receiver output, prior to FEC decoding, can lead to unnecessarily stringent criteria that may not reflect the operational requirements of the application.

Adopting Appropriate BER Values for Testing

The selection of an appropriate BER for testing components depends on the specific application. Theoretical calculations suggest a BER of 1.8×10−4at the receiver output (Point A) to achieve a BER of 10−12 at the FEC decoder output (Point B). However, due to variations in error statistics, the average BER at Point A may need to be lower than the theoretical value to ensure the desired BER at Point B. In practice, a BER range of 10−5 to 10−6 is considered suitable for most applications.

Conservative Estimation for Receiver Sensitivity

By using a BER of 10−6 for component verification, the measurements of receiver sensitivity and optical path penalty at Point A will be conservative estimates of the values after FEC correction. This approach provides a practical and cost-effective method for ensuring component performance aligns with the rigorous demands of FEC-enabled systems.

Conclusion

FEC is a powerful mechanism that significantly improves the error tolerance of optical communication systems. By understanding and implementing appropriate BER testing methodologies, network operators can ensure their components are up to the task, ultimately leading to more reliable and efficient networks.

As the demands for data grow, the reliance on sophisticated FEC techniques will only increase, cementing BER as a fundamental metric in the design and evaluation of optical communication systems.

References

https://www.itu.int/rec/T-REC-G/e

Signal integrity is the cornerstone of effective fiber optic communication. In this sphere, two metrics stand paramount: Bit Error Ratio (BER) and Q factor. These indicators help engineers assess the performance of optical networks and ensure the fidelity of data transmission. But what do these terms mean, and how are they calculated?

What is BER?

BER represents the fraction of bits that have errors relative to the total number of bits sent in a transmission. It’s a direct indicator of the health of a communication link. The lower the BER, the more accurate and reliable the system.

ITU-T Standards Define BER Objectives

The ITU-T has set forth recommendations such as G.691, G.692, and G.959.1, which outline design objectives for optical systems, aiming for a BER no worse than 10−12 at the end of a system’s life. This is a rigorous standard that guarantees high reliability, crucial for SDH and OTN applications.

Measuring BER

Measuring BER, especially as low as 10−12, can be daunting due to the sheer volume of bits required to be tested. For instance, to confirm with 95% confidence that a system meets a BER of 10−12, one would need to test 3×1012 bits without encountering an error — a process that could take a prohibitively long time at lower transmission rates.

The Q Factor

The Q factor measures the signal-to-noise ratio at the decision point in a receiver’s circuitry. A higher Q factor translates to better signal quality. For a BER of 10−12, a Q factor of approximately 7.03 is needed. The relationship between Q factor and BER, when the threshold is optimally set, is given by the following equations:

The general formula relating Q to BER is:

bertoq

A common approximation for high Q values is:

ber_t_q_2

For a more accurate calculation across the entire range of Q, the formula is:

ber_t_q_3

Practical Example: Calculating BER from Q Factor

Let’s consider a practical example. If a system’s Q factor is measured at 7, what would be the approximate BER?

Using the approximation formula, we plug in the Q factor:

This would give us an approximate BER that’s indicative of a highly reliable system. For exact calculations, one would integrate the Gaussian error function as described in the more detailed equations.

Graphical Representation

ber_t_q_4

The graph typically illustrates these relationships, providing a visual representation of how the BER changes as the Q factor increases. This allows engineers to quickly assess the signal quality without long, drawn-out error measurements.

Concluding Thoughts

Understanding and applying BER and Q factor calculations is crucial for designing and maintaining robust optical communication systems. These concepts are not just academic; they directly impact the efficiency and reliability of the networks that underpin our modern digital world.

References

https://www.itu.int/rec/T-REC-G/e

While single-mode fibers have been the mainstay for long-haul telecommunications, multimode fibers hold their own, especially in applications where short distance and high bandwidth are critical. Unlike their single-mode counterparts, multimode fibers are not restricted by cut-off wavelength considerations, offering unique advantages.

The Nature of Multimode Fibers

Multimode fibers, characterized by a larger core diameter compared to single-mode fibers, allow multiple light modes to propagate simultaneously. This results in modal dispersion, which can limit the distance over which the fiber can operate without significant signal degradation. However, multimode fibers exhibit greater tolerance to bending effects and typically showcase higher attenuation coefficients.

Wavelength Windows for Multimode Applications

Multimode fibers shine in certain “windows,” or wavelength ranges, which are optimized for specific applications and classifications. These windows are where the fiber performs best in terms of attenuation and bandwidth.

#multimodeband

IEEE Serial Bus (around 850 nm): Typically used in consumer electronics, the 830-860 nm window is optimal for IEEE 1394 (FireWire) connections, offering high-speed data transfer over relatively short distances.

Fiber Channel (around 770-860 nm): For high-speed data transfer networks, such as those used in storage area networks (SANs), the 770-860 nm window is often used, although it’s worth noting that some applications may use single-mode fibers.

Ethernet Variants:

  • 10BASE (800-910 nm): These standards define Ethernet implementations for local area networks, with 10BASE-F, -FB, -FL, and -FP operating within the 800-910 nm range.
  • 100BASE-FX (1270-1380 nm) and FDDI (Fiber Distributed Data Interface): Designed for local area networks, they utilize a wavelength window around 1300 nm, where multimode fibers offer reliable performance for data transmission.
  • 1000BASE-SX (770-860 nm) for Gigabit Ethernet (GbE): Optimized for high-speed Ethernet over multimode fiber, this application takes advantage of the lower window around 850 nm.
  • 1000BASE-LX (1270-1355 nm) for GbE: This standard extends the use of multimode fibers into the 1300 nm window for Gigabit Ethernet applications.

HIPPI (High-Performance Parallel Interface): This high-speed computer bus architecture utilizes both the 850 nm and the 1300 nm windows, spanning from 830-860 nm and 1260-1360 nm, respectively, to support fast data transfers over multimode fibers.

Future Classifications and Studies

The classification of multimode fibers is a subject of ongoing research. Proposals suggest the use of the region from 770 nm to 910 nm, which could open up new avenues for multimode fiber applications. As technology progresses, these classifications will continue to evolve, reflecting the dynamic nature of fiber optic communications.

Wrapping Up: The Place of Multimode Fibers in Networking

Multimode fibers are a vital part of the networking world, particularly in scenarios that require high data rates over shorter distances. Their resilience to bending and capacity for high bandwidth make them an attractive choice for a variety of applications, from high-speed data transfer in industrial settings to backbone cabling in data centers.

As we continue to study and refine the classifications of multimode fibers, their role in the future of networking is guaranteed to expand, bringing new possibilities to the realm of optical communications.

References

https://www.itu.int/rec/T-REC-G/e

When we talk about the internet and data, what often comes to mind are the speeds and how quickly we can download or upload content. But behind the scenes, it’s a game of efficiently packing data signals onto light waves traveling through optical fibers.If you’re an aspiring telecommunications professional or a student diving into the world of fiber optics, understanding the allocation of spectral bands is crucial. It’s like knowing the different climates in a world map of data transmission. Let’s explore the significance of these bands as defined by ITU-T recommendations and what they mean for fiber systems.

#opticalband

The Role of Spectral Bands in Single-Mode Fiber Systems

Original O-Band (1260 – 1360 nm): The journey of fiber optics began with the O-band, chosen for ITU T G.652 fibers due to its favorable dispersion characteristics and alignment with the cut-off wavelength of the cable. This band laid the groundwork for optical transmission without the need for amplifiers, making it a cornerstone in the early days of passive optical networks.

Extended E-Band (1360 – 1460 nm): With advancements, the E-band emerged to accommodate the wavelength drift of uncooled lasers. This extended range allowed for greater flexibility in transmissions, akin to broadening the canvas on which network artists could paint their data streams.

Short Wavelength S-Band (1460 – 1530 nm): The S-band, filling the gap between the E and C bands, has historically been underused for data transmission. However, it plays a crucial role in supporting the network infrastructure by housing pump lasers and supervisory channels, making it the unsung hero of the optical spectrum.

Conventional C-Band (1530 – 1565 nm): The beloved C-band owes its popularity to the era of erbium-doped fiber amplifiers (EDFAs), which provided the necessary gain for dense wavelength division multiplexing (DWDM) systems. It’s the bread and butter of the industry, enabling vast data capacity and robust long-haul transmissions.

Long Wavelength L-Band (1565 – 1625 nm): As we seek to expand our data highways, the L-band has become increasingly important. With fiber performance improving over a range of temperatures, this band offers a wider wavelength range for signal transmission, potentially doubling the capacity when combined with the C-band.

Ultra-Long Wavelength U-Band (1625 – 1675 nm): The U-band is designated mainly for maintenance purposes and is not currently intended for transmitting traffic-bearing signals. This band ensures the network’s longevity and integrity, providing a dedicated spectrum for testing and monitoring without disturbing active data channels.

Historical Context and Technological Progress

It’s fascinating to explore why we have bands at all. The ITU G-series documents paint a rich history of fiber deployment, tracing the evolution from the first multimode fibers to the sophisticated single-mode fibers we use today.

In the late 1970s, multimode fibers were limited by both high attenuation at the 850 nm wavelength and modal dispersion. A leap to 1300 nm in the early 1980s marked a significant drop in attenuation and the advent of single-mode fibers. By the late 1980s, single-mode fibers were achieving commercial transmission rates of up to 1.7 Gb/s, a stark contrast to the multimode fibers of the past.

The designation of bands was a natural progression as single-mode fibers were designed with specific cutoff wavelengths to avoid modal dispersion and to capitalize on the low attenuation properties of the fiber.

The Future Beckons

With the ITU T G.65x series recommendations setting the stage, we anticipate future applications utilizing the full spectrum from 1260 nm to 1625 nm. This evolution, coupled with the development of new amplification technologies like thulium-doped amplifiers or Raman amplification, suggests that the S-band could soon be as important as the C and L bands.

Imagine a future where the combination of S+C+L bands could triple the capacity of our fiber infrastructure. This isn’t just a dream; it’s a realistic projection of where the industry is headed.

Conclusion

The spectral bands in fiber optics are not just arbitrary divisions; they’re the result of decades of research, development, and innovation. As we look to the horizon, the possibilities are as wide as the spectrum itself, promising to keep pace with our ever-growing data needs.

Reference

https://www.itu.int/rec/T-REC-G/e

The world of optical communication is intricate, with different cable types designed for specific environments and applications. Today, we’re diving into the structure of two common types of optical fiber cables, as depicted in Figure below, and summarising the findings from an appendix that examined their performance.

cableA_B
#cable

Figure

Cable A: The Stranded Loose Tube Outdoor Cable

Cable A represents a quintessential outdoor cable, built to withstand the elements and the rigors of outdoor installation. The cross-section of this cable reveals a complex structure designed for durability and performance:

  • Central Strength Member: At its core, the cable has a central strength member that provides mechanical stability and ensures the cable can endure the tensions of installation.
  • Tube Filling Gel: Surrounding the central strength member are buffer tubes secured with a tube filling gel, which protects the fibers from moisture and physical stress.
  • Loose Tubes: These tubes hold the optical fibers loosely, allowing for expansion and contraction due to temperature changes without stressing the fibers themselves.
  • Fibers: Each tube houses six fibers, comprising various types specified by the ITU-T, including G.652.D, G.654.E, G.655.D, G.657.A1, G.657.A2, and G.657.B3. This array of fibers ensures compatibility with different transmission standards and conditions.
  • Aluminium Tape and PE Sheath: The aluminum tape provides a barrier against electromagnetic interference, while the polyethylene (PE) sheath offers physical protection and resistance to environmental factors.

The stranded loose tube design is particularly suited for long-distance outdoor applications, providing a robust solution for optical networks that span vast geographical areas.

Cable B: The Tight Buffered Indoor Cable

Switching our focus to indoor applications, Cable B is engineered for the unique demands of indoor environments:

  • Tight Buffered Fibers: Unlike Cable A, this indoor cable features four tight buffered fibers, which are more protected from physical damage and easier to handle during installation.
  • Aramid Yarn: Known for its strength and resistance to heat, aramid yarn is used to reinforce the cable, providing additional protection and tensile strength.
  • PE Sheath: Similar to Cable A, a PE sheath encloses the structure, offering a layer of defense against indoor environmental factors.

Cable B contains two ITU-T G.652.D fibers and two ITU-T G.657.B3 fibers, allowing for a blend of standard single-mode performance with the high bend-resistance characteristic of G.657.B3 fibers, making it ideal for complex indoor routing.

Conclusion

The intricate designs of optical fiber cables are tailored to their application environments. Cable A is optimized for outdoor use with a structure that guards against environmental challenges and mechanical stresses, while Cable B is designed for indoor use, where flexibility and ease of handling are paramount. By understanding the components and capabilities of these cables, network designers and installers can make informed decisions to ensure reliable and efficient optical communication systems.

Reference

https://www.itu.int/rec/T-REC-G.Sup40-201810-I/en

Introduction

An unamplified link is a connection between two devices or systems that does not use an amplifier to boost the signal. This type of link is common in many applications, including audio, video, and data transmissions. However, designing a reliable unamplified link can be challenging, as several factors need to be considered to ensure a stable connection.

In this guide, we’ll walk you through the steps to design a reliable and efficient unamplified link. We’ll cover everything from understanding unamplified links to factors to consider before designing a link, step-by-step instructions for designing a link, testing and troubleshooting, and more.

Understanding Unamplified Links

Before we dive into designing a unamplified link, it’s essential to understand what they are and how they work.

An unamplified link is a connection between two devices or systems that does not use an amplifier to boost the signal. The signal travels through the cable without any amplification, making it susceptible to attenuation, or signal loss.

Attenuation occurs when the signal strength decreases as it travels through the cable. The longer the cable, the more attenuation the signal experiences, which can result in a weak or unstable connection. To prevent this, several factors need to be considered when designing an unamplified link.

Factors to Consider Before Designing a Unamplified Link

Designing a reliable unamplified link requires considering several factors to ensure a stable connection. Here are some of the essential factors to consider:

Cable Type and Quality

Choosing the right cable is crucial for designing a reliable unamplified link. The cable type and quality determine how well the signal travels through the cable and the amount of attenuation it experiences.

For example, coaxial cables are commonly used for video and audio applications, while twisted pair cables are commonly used for data transmissions. The quality of the cable also plays a significant role in the signal’s integrity, with higher quality cables typically having better insulation and shielding.

Distance

The distance between the two devices or systems is a critical factor to consider when designing a unamplified link. The longer the distance, the more attenuation the signal experiences, which can result in a weak or unstable connection.

Signal Loss

Signal loss, also known as attenuation, is a significant concern when designing a unamplified link. The signal loss is affected by several factors, including cable type, cable length, and cable quality.

Connectors

Choosing the right connectors is essential for designing a reliable unamplified link. The connectors must match the cable type and have the correct impedance to prevent signal reflections and interference.

Designing a Unamplified Link: Step by Step

Designing a unamplified link can be challenging, but following these step-by-step instructions will ensure a reliable and efficient connection:

Step 1: Choose the Right Cable

Choosing the right cable is crucial for designing a reliable unamplified link. You need to consider the cable type, length, and quality.

For video and audio applications, coaxial cables are commonly used, while twisted pair cables are commonly used for data transmissions. The cable length should be as short as possible to minimize signal loss, and the cable quality should be high to ensure the signal’s integrity.

Step 2: Determine the Distance

The distance between the two devices or systems is a critical factor to consider when designing a unamplified link. The longer the distance, the more attenuation the signal experiences.

You need to determine the distance between the devices and choose the cable length accordingly. If the distance is too long, you may need to consider using a different cable type or adding an amplifier.

Step 3: Calculate the Signal Loss

Signal loss, also known as attenuation, is a significant concern when designing a unamplified link. You need to calculate the signal loss based on the cable type, length, and quality.

There are several online calculators that can help you determine the signal loss based on the cable specifications. You need to make sure the signal loss is within the acceptable range for your application.

Step 4: Choose the Right Connectors

Choosing the right connectors is essential for designing a reliable unamplified link. The connectors must match the cable type and have the correct impedance to prevent signal reflections and interference.

You need to choose connectors that are compatible with your devices and have the correct gender (male or female). It’s also essential to choose connectors that are easy to install and remove.

Step 5: Assemble the Cable

Once you have chosen the right cable and connectors, you need to assemble the cable. You need to follow the manufacturer’s instructions carefully and make sure the connectors are securely attached to the cable.

It’s also essential to check the cable for any damage or defects before using it. A damaged or defective cable can result in a weak or unstable connection.

Testing and Troubleshooting the Unamplified Link

After designing the unamplified link, you need to test it to ensure it’s working correctly. You can use a signal tester or a multimeter to test the signal strength and quality.

If you experience any issues with the connection, you may need to troubleshoot the link. You can check the cable for any damage or defects, make sure the connectors are securely attached, and verify the devices’ compatibility.

Conclusion

Designing a reliable unamplified link requires considering several factors, including cable type and quality, distance, signal loss, and connectors. By following the step-by-step instructions outlined in this guide, you can design a reliable and efficient unamplified link for your application.

FAQs

  1. What is an unamplified link, and when is it used?
    • An unamplified link is a connection between two devices or systems that does not use an amplifier to boost the signal. It is used in many applications, including audio, video, and data transmissions, where a stable and reliable connection is required.
  2. What factors should I consider when designing a unamplified link?
    • Some of the essential factors to consider when designing a unamplified link include cable type and quality, distance between the devices, signal loss, and connectors.
  3. Can I use any cable for a unamplified link?
    • No, you cannot use any cable for a unamplified link. You need to choose the right cable type, length, and quality based on your application’s requirements.
  4. What connectors should I use for a unamplified link?
    • You need to choose connectors that are compatible with your devices and have the correct gender (male or female). The connectors must also match the cable type and have the correct impedance to prevent signal reflections and interference.
  5. How do I troubleshoot a faulty unamplified link?
    • If you experience any issues with the connection, you can troubleshoot the link by checking the cable for any damage or defects, making sure the connectors are securely attached, and verifying the devices’ compatibility. You can also use a signal tester or a multimeter to test the signal strength and quality.

Designing a reliable unamplified link requires careful consideration of several factors. By choosing the right cable, calculating the signal loss, choosing the right connectors, and assembling the cable correctly, you can ensure a stable and efficient connection. Testing and troubleshooting the link can help you identify any issues and ensure the link is working correctly.

Discover the best Q-factor improvement techniques for optical networks with this comprehensive guide. Learn how to optimize your network’s performance and achieve faster, more reliable connections.

Introduction:

In today’s world, we rely heavily on the internet for everything from work to leisure. Whether it’s streaming videos or conducting business transactions, we need fast and reliable connections. However, with so much data being transmitted over optical networks, maintaining high signal quality can be a challenge. This is where the Q-factor comes into play.

The Q-factor is a metric used to measure the quality of a signal transmitted over an optical network. It takes into account various factors, such as noise, distortion, and attenuation, that can degrade signal quality. A higher Q-factor indicates better signal quality, which translates to faster and more reliable connections.

In this article, we will explore effective Q-factor improvement techniques for optical networks. We will cover everything from signal amplification to dispersion management, and provide tips for optimizing your network’s performance.

TOC:

  1. Amplification Techniques
  2. Dispersion Management
  3. Polarization Mode Dispersion (PMD) Compensation
  4. Nonlinear Effects Mitigation
  5. Fiber Cleaning and Maintenance

Amplification Techniques:

Optical amplifiers are devices that amplify optical signals without converting them to electrical signals. There are several types of optical amplifiers, including erbium-doped fiber amplifiers (EDFAs), semiconductor optical amplifiers (SOAs), and Raman amplifiers.

EDFAs are the most commonly used optical amplifiers. They work by using an erbium-doped fiber to amplify the signal. EDFAs have a high gain and low noise figure, making them ideal for long-haul optical networks.

SOAs are semiconductor devices that use a gain medium to amplify the signal. They have a much smaller footprint than EDFAs and can be integrated into other optical components, such as modulators and receivers.

Raman amplifiers use a process called stimulated Raman scattering to amplify the signal. They are typically used in conjunction with EDFAs to boost the signal even further.

Dispersion Management:

Dispersion is a phenomenon that occurs when different wavelengths of light travel at different speeds in an optical fiber. This can cause distortion and degradation of the signal, resulting in a lower Q-factor.

There are several techniques for managing dispersion, including:

  • Dispersion compensation fibers: These are fibers designed to compensate for dispersion by introducing an opposite dispersion effect.
  • Dispersion compensation modules: These are devices that use a combination of fibers and other components to manage dispersion.
  • Dispersion-shifted fibers: These fibers are designed to minimize dispersion by shifting the zero-dispersion wavelength to a higher frequency.

Polarization Mode Dispersion (PMD) Compensation:

Polarization mode dispersion is a phenomenon that occurs when different polarization states of light travel at different speeds in an optical fiber. This can cause distortion and degradation of the signal, resulting in a lower Q-factor.

PMD compensation techniques include:

  • PMD compensators: These are devices that use a combination of wave plates and fibers to compensate for PMD.
  • Polarization scramblers: These are devices that randomly change the polarization state of the signal to reduce the impact of PMD.

Nonlinear Effects Mitigation:

Nonlinear effects can occur when the optical signal is too strong, causing distortion and degradation of the signal. These effects can be mitigated using several techniques, including:

  • Dispersion management techniques: As mentioned earlier, dispersion management can help reduce the impact of nonlinear effects.
  • Nonlinear compensation: This involves using specialized components, such as nonlinear optical loops, to compensate for nonlinear effects.
  • Modulation formats: Different modulation formats,such as quadrature amplitude modulation (QAM) and coherent detection, can also help mitigate nonlinear effects.

    Fiber Cleaning and Maintenance:

    Dirty or damaged fibers can also affect signal quality and lower the Q-factor. Regular cleaning and maintenance of the fibers can help prevent these issues. Here are some tips for fiber cleaning and maintenance:

    • Use proper cleaning tools and materials, such as lint-free wipes and isopropyl alcohol.
    • Inspect the fibers regularly for signs of damage, such as bends or breaks.
    • Use protective sleeves or connectors to prevent damage to the fiber ends.
    • Follow the manufacturer’s recommended maintenance schedule for your network components.

    FAQs:

    1. What is the Q-factor in optical networks?

    The Q-factor is a metric used to measure the quality of a signal transmitted over an optical network. It takes into account various factors, such as noise, distortion, and attenuation, that can degrade signal quality. A higher Q-factor indicates better signal quality, which translates to faster and more reliable connections.

    1. What are some effective Q-factor improvement techniques for optical networks?

    Some effective Q-factor improvement techniques for optical networks include signal amplification, dispersion management, PMD compensation, nonlinear effects mitigation, and fiber cleaning and maintenance.

    1. What is dispersion in optical fibers?

    Dispersion is a phenomenon that occurs when different wavelengths of light travel at different speeds in an optical fiber. This can cause distortion and degradation of the signal, resulting in a lower Q-factor.

    Conclusion:

    Achieving a high Q-factor is essential for maintaining fast and reliable connections over optical networks. By implementing effective Q-factor improvement techniques, such as signal amplification, dispersion management, PMD compensation, nonlinear effects mitigation, and fiber cleaning and maintenance, you can optimize your network’s performance and ensure that it meets the demands of today’s data-driven world.

  • With these techniques in mind, you can improve your network’s Q-factor and provide your users with faster, more reliable connections. Remember to regularly inspect and maintain your network components to ensure optimal performance. By doing so, you can keep up with the ever-increasing demands for high-speed data transmission and stay ahead of the competition.In conclusion, Q-factor improvement techniques for optical networks are crucial for maintaining high signal quality and achieving faster, more reliable connections. By implementing these techniques, you can optimize your network’s performance and meet the demands of today’s data-driven world. Keep in mind that regular maintenance and inspection of your network components are key to ensuring optimal performance. With the right tools and techniques, you can boost your network’s Q-factor and provide your users with the best possible experience.

WDM Glossary

Following are some of the frequent used DWDM terminologies.

TERMS

DEFINITION

Arrayed Waveguide Grating (AWG)

An arrayed waveguide grating (AWG) is a passive optical device that is constructed of an array of waveguides, each of slightly different length. With a AWG, you can take a multi-wavelength input and separate the component wavelengths on to different output ports. The reverse operation can also be performed, combining several input ports on to a single output port of multiple wavelengths. An advantage of AWGs is their ability to operate bidirectionally.

AWGs are used to perform wavelength multiplexing and demultiplexing, as well as wavelength add/drop operations.

Bit Error Rate/Q-Factor (BER)

Bit error rate (BER) is the measure of the transmission quality of a digital signal. It is an expression of errored bits vs. total transmitted bits, presented in a ratio. Whereas a BER performance of 10-9 (one bit in one billion is an error) is acceptable in DS1 or DS3 transmission, the expected performance for high speed optical signals is on the order of 10-15.

Bit error rate is a measurement integrated over a period of time, with the time interval required being longer for lower BERs. One way of making a prediction of the BER of a signal is with a Q-factor measurement.

C Band

The C-band is the “center” DWDM transmission band, occupying the 1530 to 1562nm wavelength range. All DWDM systems deployed prior to 2000 operated in the C-band. The ITU has defined channel plans for 50GHz, 100GHz, and 200GHz channel spacing. Advertised channel counts for the C-band vary from 16 channels to 96 channels. The C-Band advantages are:

  • Lowest loss characteristics on SSMF fiber.
  • Low susceptibility to attenuation from fiber micro-bending. EDFA amplifiers operate in the C-band window.

Chromatic Dispersion (CD)

The distortion of a signal pulse during transport due to the spreading out of the wavelengths making up the spectrum of the pulse.

The refractive index of the fiber material varies with the wavelength, causing wavelengths to travel at different velocities. Since signal pulses consist of a range of wavelengths, they will spread out during transport.

Circulator

A passive multiport device, typically 3 or 4 ports, where the signal entering at one port travels around the circulator and exits at the next port. In asymmetrical configurations, there is no routing of traffic between the port 3 and port 1.

Due to their low loss characteristics, circulators are useful in wavelength demux and add/drop applications.

Coupler

A coupler is a passive device that combines and/or splits optical signals. The power loss in the output signals depends on the number of ports. In a two port device with equal outputs, each output signal has a 3 dB loss (50% power of the input signal). Most couplers used in single mode optics operate on the principle of resonant coupling. Common technologies used in passive couplers are fused-fiber and planar waveguides.

WAVELENGTH SELECTIVE COUPLERS

Couplers can be “tuned” to operate only on specific wavelengths (or wavelength ranges). These wavelength selective couplers are useful in coupling amplifier pump lasers with the DWDM signal.

Cross-Phase Modulation (XPM)

The refractive index of the fiber varies with respect to the optical signal intensity. This is known as the “Kerr Effect”. When multiple channels are transmitted on the same fiber, refractive index variations induced by one channel can produce time variable phase shifts in co-propagating channels. Time varying phase shifts are the same as frequency shifts, thus the “color” changes in the pulses of the affected channels.

DCU

A dispersion compensation unit removes the effects of dispersion accumulated during transmission, thus repairing a signal pulse distorted by chromatic dispersion. If a signal suffers from the effects of positive dispersion during transmission, then the DCU will repair the signal using negative dispersion.

TRANSMISSION FIBER

  • Positive dispersion (shorter “blue” ls travel faster than longer “red” ls) for SSMF
  • Dispersion value at 1550nm on SSMF = 17 ps/km*nm

DISPERSION COMPENSATION UNIT (DCU)

  • Commonly utilizes Dispersion Compensating Fiber
  • Negative dispersion (shorter “blue” ls travel slower than longer “red” ls) counteracts the positive dispersion of the transmission fiber… allows “catch up” of the spectral components with one another
  • Large negative dispersion value … length of the DCF is much less than the transmission fiber length

Dispersion Shifted Fiber (DSF)

In an attempt to optimize long haul transport on optical fiber, DSF was developed. DSF has its zero dispersion wavelength shifted from the 1310nm wavelength to a minimal attenuation region near the 1550nm wavelength. This fiber, designated ITU-T G.653, was recognized for its ability to transport a single optical signal a great distance before regeneration. However, in DWDM transmission, signal impairments from four-wave mixing are greatest around the fiber’s zero-dispersion point. Therefore, with DSF’s zero-dispersion point falling within the C-Band, DSF fiber is not suitable for C-band DWDM transmission.

DSF makes up a small percentage of the US deployed fiber plant, and is no longer being deployed. DSF has been deployed in significant amounts in Japan, Mexico, and Italy.

Erbium Doped Fiber Amplifier (EDFA)

PUMP LASER

The power source for amplifying the signal, typically a 980nm or 1480nm laser.

ERBIUM DOPED FIBER

Single mode fiber, doped with erbium ions, acts as the gain fiber, transferring the power from the pump laser to the target wavelengths.

WAVELENGTH SELECTIVE COUPLER

Couples the pump laser wavelength to the gain fiber while filtering out any extraneous wavelengths from the laser output.

ISOLATOR

Prevents any back-reflected light from entering the amplifier.

EDFA Advantages are:

  • Efficient pumping
  • Minimal polarization sensitivity
  • High output power
  • Low noise
  • Low distortion and minimal crosstalk

EDFA Disadvantages are:

  • Limited to C and L bands

Fiber Bragg Grating (FBG)

A fiber Bragg grating (FBG) is a piece of optical fiber that has its internal refractive index varied in such a way that it acts as a grating.  In its basic operation, a FBG is constructed to reflect a single wavelength, and pass the remaining wavelengths.  The reflected wavelength is determined by the period of the fiber grating.

If the pattern of the grating is periodic, a FBG can be used in wavelength mux / demux applications, as well as wavelength add / drop applications.  If the grating is chirped (non-periodic), then a FBG can be used as a chromatic dispersion compensator.

Four Wave Mixing (FWM)

The interaction of adjacent channels in WDM systems produces sidebands (like harmonics), thus creating coherent crosstalk in neighboring channels. Channels mix to produce sidebands at intervals dependent on the frequencies of the interacting channels.  The effect becomes greater as channel spacing is decreased.  Also, as signal power increases, the effects of FWM increase. The presence of chromatic dispersion in a signal reduces the effects of FWM.  Thus the effects of FWM are greatest near the zero dispersion point of the fiber.

Gain Flattening

The gain from an amplifier is not distributed evenly among all of the amplified channels.  A gain flattening filter is used to achieve constant gain levels on all channels in the amplified region.  The idea is to have the loss curve of the filter be a “mirror” of the gain curve of the amplifier.  Therefore, the product of the amplifier gain and the gain flattening filter loss equals an amplified region with flat gain.

The effects of uneven gain are compounded for each amplified span.  For example, if one wavelength has a gain imbalance of +4 dB over another channel, this imbalance will become +20 dB after five amplified spans.  This compounding effect means that the weaker signals may become indistinguishable from the noise floor.  Also, over-amplified channels are vulnerable to increase non-linear effects.

Isolator

An isolator is a passive device that allows light to pass through unimpeded in one direction, while blocking light in the opposite direction.  An isolator is constructed with two polarizers (45o difference in orientation), separated by a Faraday rotator (rotates light polarization by 45o).

One important use for isolators is to prevent back-reflected light from reaching lasers.  Another important use for isolators is to prevent light from counter propagating pump lasers from exiting the amplifier system on to the transmission fiber.

L Band

The L-band is the “long” DWDM transmission band, occupying the 1570 to 1610nm wavelength range. The L-band has comparable bandwidth to the C-band, thus comparable total capacity. The L-Band advantages are:

  • EDFA technology can operate in the L-band window.

Lasers

A LASER (Light Amplification by the Stimulated Emission of Radiation) produces high power, single wavelength, coherent light via stimulated emission of light.

Semiconductor Laser (General View)

Semiconductor laser diodes are constructed of p and n semiconductor layers, with the junction of these layers being the active layer where the light is produced.  Also, the lasing effect is induced by placing partially reflective surfaces on the active layer. The most common laser type used in DWDM transmission is the distributed feedback (DFB) laser.  A DFB laser has a grating layer next to the active layer.  This grating layer enables DFB lasers to emit precision wavelengths across a narrow band.

Mach-Zehnder Interferometer (MZI)

A Mach-Zehnder interferometer is a device that splits an optical signal into two components, directs each component through its own waveguide, then recombines the two components.  Based on any phase delay between the two waveguides, the two re-combined signal components will interfere with each other, creating a signal with an intensity determined by the interference.  The interference of the two signal components can be either constructive or destructive, based on the delay between the waveguides as related to the wavelength of the signal.  The delay can be induced either by a difference in waveguide length, or by manipulating the refractive index of one or both waveguides (usually by applying a bias voltage). A common use for Mach-Zehnder interferometer in DWDM systems is in external modulation of optical signals.

Multiplexer (MUX)

DWDM Mux

  • Combines multiple optical signals onto a single optical fiber
  • Typically supports channel spacing of 100GHz and 50GHz

DWDM Demux

  • Separates individual channels from the aggregate DWDM signal

Mux/Demux Technology

  • Thin film filters
  • Fiber Bragg gratings
  • Diffraction gratings
  • Arrayed waveguide gratings
  • Fused biconic tapered devices
  • Inter-leaver devices

Non-Zero Dispersion Shifted Fiber (NZ-DSF)

After DSF, it became evident that some chromatic dispersion was needed to minimize non-linear effects, such as four wave mixing.  Through new designs, λ0 was now shifted to outside the C-Band region with a decreased dispersion slope.  This served to provide for dispersion values within the C-Band that were non-zero in value yet still far below those of standard single mode fiber.  The NZ-DSF designation includes a group of fibers that all meet the ITU-T G.655 standard, but can vary greatly with regard to their dispersion characteristics.

First available around 1996, NZ-DSF now makes up about 60% of the US long-haul fiber plant.  It is growing in popularity, and now accounts for approximately 80% of new fiber deployments in the long-haul market. (Source: derived from KMI data)

Optical Add Drop Multiplexing (OADM)

An optical add/drop multiplexer (OADM) adds or drops individual wavelengths to/from the DWDM aggregate at an in-line site, performing the add/drop function at the optical level.  Before OADMs, back to back DWDM terminals were required to access individual wavelengths at an in-line site.  Initial OADMs added and dropped fixed wavelengths (via filters), whereas emerging OADMs will allow selective wavelength add/drop (via software).

Optical Amplifier (OA)

POSTAMPLIFIER

Placed immediately after a transmitter to increase the strength on the signal.

IN-LINE AMPLIFIER (ILA)

Placed in-line, approximately every 80 to 100km, to amplify an attenuated signal sufficiently to reach the next ILA or terminal site.  An ILA functions solely in the optical domain, performing the 1R function.

PREAMPLIFIER

Placed immediately before a receiver to increase the strength of a signal.  The preamplifier boosts the signal to a power level within the receiver’s sensitivity range.

Optical Bandwidth

Optical bandwidth is the total data carrying capacity of an optical fiber.  It is equal to the sum of the bit rates of each of the channels.  Optical bandwidth can be increased by improving DWDM systems in three areas: channel spacing, channel bit rate, and fiber bandwidth. The current benchmark for channel spacing is 50GHz. A 2X bandwidth improvement can be achieved with 25GHz spacing.

CHANNEL SPACING

Current benchmark is 50GHz spacing. A 2X bandwidth improvement can be achieved with 25GHz spacing.

Challenges:

  • Laser stabilization
  • Mux/Demux tolerances
  • Non-linear effects
  • Filter technology

CHANNEL BIT RATE

Current benchmark is 10Gb/s. A 4X bandwidth improvement can be achieved with 40Gb/s channels. However, 40Gb/s will initially require 100GHz spacing, thus reducing the benefit to 2X.

Challenges:

  • PMD mitigation
  • Dispersion compensation
  • High Speed SONET mux/demux

FIBER BANDWIDTH

Current benchmark is C-Band Transmission. A 3X bandwidth improvement can be achieved by utilizing the “S” & “L” bands.

Challenges:

  • Optical amplifier
  • Band splitters & combiners
  • Gain tilt from stimulated Raman scattering

Optical Fiber

Optical fiber used in DWDM transmission is single mode fiber composed of a silica glass core, cladding, and a plastic coating or jacket.  In single mode fiber, the core is small enough to limit the transmission of the light to a single propagation mode.  The core has a slightly higher refractive index than the cladding, thus the core/cladding boundary acts as a mirror.  The core of single mode fiber is typically 8 or 9 microns, and the cladding  extends the diameter to 125 microns.  The effective core of the fiber, or mode field diameter (MFD), is actually larger than the core itself since transmission extends into the cladding.  The MFD can be 10 to 15% larger than the actual fiber core.  The fiber is coated with a protective layer of plastic that extends the diameter of standard fiber to 250 microns.

Optical Signal to Noise Ratio (OSNR)

Optical signal to noise ratio (OSNR) is a measurement relating the peak power of an optical signal to the noise floor.  In DWDM transmission, each amplifier in a link adds noise to the signal via amplified spontaneous emission (ASE), thus degrading the OSNR.  A minimum OSNR is required to maintain good transmission performance.  Therefore, a high OSNR at the beginning of an optical link is critical to achieving good transmission performance over multiple spans.

OSNR is measured with an optical signal analyzer (OSA).  OSNR is a good indicator of overall transmission quality and system health.  Therefore OSNR is an important measurement during installation, routine maintenance, and troubleshooting activities.

Optical Supervisory Channel

The optical supervisory channel (OSC) is a dedicated communications channel used for the remote management of optical network elements.  Similar in principal to the DCC channel in SONET networks, the OSC inhabits its own dedicated wavelength.  The industry typically uses the 1510nm or 1625nm wavelengths for the OSC.

Polarization Mode Dispersion (PMD)

Single mode fiber is actually bimodal, with the two modes having orthogonal polarization.  The principal states of polarization (PSPs, referred to as the fast and slow axis) are determined by the symmetry of the fiber section.  Dispersion caused by this property of fiber is referred to as polarization mode dispersion (PMD).

Raman

Raman fiber amplifiers use the Raman effect to transfer power from the pump lasers to the amplified wavelengths. Raman Advantages are:

  • Wide bandwidth, enabling operation in C, L, and S bands.
  • Raman amplification can occur in ordinary silica fibers

Raman Disadvantages are:

  • Lower efficiency than EDFAs

Regenerator (Regen)

An optical amplifier performs a 1R function (re-amplification), where the signal noise is amplified along with the signal.  For each amplified span, signal noise accumulates, thus impacting the signal’s optical signal to noise ratio (OSNR) and overall signal quality.  After traversing a number of amplified spans (this number is dependent on the engineering of the specific link), a regenerator is required to rebaseline the signal. A regenerator performs the 3R function on a signal.  The three R’s are: re-shaping, re-timing, and re-amplification.  The 3R function, with current technology, is an optical to electrical to optical operation (O-E-O).    In the future, this may be done all optically.

S Band

The S-band is the “short” DWDM transmission band, occupying the 1485 to 1520nm wavelength range.  With the “S+” region, the window is extended below 1485nm. The S-band has comparable bandwidth to the C-band, thus comparable total capacity. The S-Band advantages are:

  • Low susceptibility to attenuation from fiber micro-bending.
  • Lowest dispersion characteristics on SSMF fiber.

Self Phase Modulation (SPM)

The refractive index of the fiber varies with respect to the optical signal intensity.  This is known as the “Kerr Effect”.  Due to this effect, the instantaneous intensity of the signal itself can modulate its own phase.  This effect can cause optical frequency shifts at the rising edge and trailing edge of the signal pulse.

SemiConductor Optical Amplifier (SOA)

What is it?

Similar to a laser, a SOA uses current injection through the junction layer in a semiconductor to stimulate photon emission.  In a SOA (as opposed to a laser), anti-reflective coating is used to prevent lasing. SOA Advantages are:

  • Solid state design lends itself to integration with other devices, as well as mass production.
  • Amplification over a wide bandwidth

SOA Disadvantages are:

  • High noise compared to EDFAs and Raman amplifiers
  • Low power
  • Crosstalk between channels
  • Sensitivity to the polarization of the input light
  • High insertion loss
  • Coupling difficulties between the SOA and the transmission fiber

Span Engineering

Engineering a DWDM link to achieve the performance and distance requirements of the application. The factors of Span Engineering are:

Amplifier Power – Higher power allows greater in-line amplifier (ILA) spacing, but at the risk of increased non-linear effects, thus fewer spans before generation.

Amplifier Spacing – Closer spacing of ILAs reduces the required amplifier power, thus lowering the susceptibility to non-linear effects.

Fiber Type – Newer generation fiber has less attenuation than older generation fiber, thus longer spans can be achieved on the newer fiber without additional amplifier power.

Channel Count – Since power per channel must be balanced, a higher channel count increases the total required amplifier power.

Channel Bit Rate – DWDM impairments such as PMD have greater impacts at higher channel bit rates.

SSMF

Standard single-mode fiber, or ITU-T G.652, has its zero dispersion point at approximately the 1310nm wavelength, thus creating a significant dispersion value in the DWDM window.  To effectively transport today’s wavelength counts (40 – 80 channels and beyond) and bit rates (2.5Gbps and beyond) within the DWDM window, management of the chromatic dispersion effects has to be undertaken through extensive use of dispersion compensating units, or DCUs.

SSMF makes up about one-third of the deployed US terrestrial long-haul fiber plant.  Approximately 20% of the new fiber deployment in the US long-haul market is SSMF. (Source: derived from KMI data)

Stimulated Raman Scattering (SRS)

The transfer of power from a signal at a lower wavelength to a signal at a higher wavelength.

SRS is the interaction of lightwaves with vibrating molecules within the silica fiber has the effect of scattering light, thus transferring power between the two wavelengths.  The effects of SRS become greater as the signals are moved further apart, and as power increases.  The maximum SRS effect is experienced at two signals separated by 13.2 THz.

Thin Film Filter

A thin film filter is a passive device that reflects some wavelengths while transmitting others.  This device is composed of alternating layers of different substances, each with a different refractive index.  These different layers create interference patterns that perform the filtering function.  Which wavelengths are reflected and which wavelengths are transmitted is a function of the following parameters:

  • Refractive index of each of the layers
  • Thickness of the layers
  • Angle of the light hitting the filter

Thin film filters are used for performing wavelength mux and demux.  Thin film filters are best suited for low to moderate channel count muxing / demuxing (less than 40 channels).

WLA

Optical networking often requires that wavelengths from one network element (NE) be adapted in order to interface a second NE.  This function is typically performed in one of three ways:

  • Wavelength Adapter (or transponder)
  • Wavelength Converter
  • Precision Wavelength Transmitters (ITU l)

The major advantage of using the coherent detection techniques is that both the amplitude and the phase of the received optical signal can be detected, extracted  and measured accordingly. This method helps in  sending information by modulating either the amplitude, or the phase, or the frequency of an optical carrier. In the case of digital communication systems, the three possibilities give rise to three modulation formats known as amplitude-shift keying (ASK), phase-shift keying (PSK), and frequency-shift keying (FSK) 

Use of coherent detection may allow a more efficient use of fiber bandwidth by increasing the spectral efficiency of WDM system. Sometimes it has been seen that the receiver sensitivity can be improved by up to 20 dB compared with that of IM/DD systems BER, and hence the receiver sensitivity.

There are two types of transponders

  • non coherent transponders 
  • coherent transponders

non coherent transponders:

These transponders involve IM/DD (Intensity Modulation/Direct Detection) technique also known as OOK method for transmission of signal. In IM/DD the intensity, or power, of the light beam from a laser or a light-emitting diode (LED) is modulated by the information bits and no phase information is needed. Due to this nature, no local oscillator is required for IM/DD communication, which greatly eases the cost of the hardware.

coherent transponders:

The basic idea behind coherent detection consists of combining the optical signal coherently with a continuous-wave (CW) optical field before it falls on the photodetector. The CW field is generated locally at the receiver using a narrow line width laser, called the local oscillator (LO). With the mixing of the received optical signal with the LO output can improve the receiver performance.

 

Optical Standards

https://www.itu.int/en/ITU-T/techwatch/Pages/optical-standards.aspx

https://en.wikipedia.org/wiki/ITU-T

ITU-T Handbook

ITU-T Study Group 15 – Networks, Technologies and Infrastructures for Transport, Access and Home

ITU-T Video Tutorial on Optical Fibre Cables and Systems

 

Recommendations for which ITU-T test specifications are available
ITU-T Recommendations specifying test procedures are available for the following Recommendations:

 

Optical fibre cables:

  • G.652 (2009-11) Characteristics of a single-mode optical fibre and cable
  • G.653 (2010-07) Characteristics of a dispertion-shifted, single-mode optical fibre and cable
  • G.654 (2010-07) Characteristics of a cut-off shifted, single-mode optical fibre and cable
  • G.655 (2009-11) Characteristics of a non-zero dispersion-shifted single-mode optical fibre and cable
  • G.656 (2010-07) Characteristics of a fibre and cable with non-zero dispersion for wideband optical transport
  • G.657 (2009-11) Characteristics of a bending-loss insensitive single-mode optical fibre and cable for the access network

Characteristics of optical components and subsystems:

  • G.662 (2005-07) Generic characteristics of optical amplifier devices and subsystems
  • G.663 (2011-04) Application related aspects of optical amplifier devices and subsystems
  • G.664 (2006-03) Optical safety procedures and requirements for optical transport systems
  • G.665 (2005-01) Generic characteristics of Raman amplifiers and Raman amplified systems
  • G.666 (2011-02) Characteristics of PMD compensators and PMD compensating receivers
  • G.667 (2006-12) Characteristics of adaptive chromatic dispersion compensators

Optical fibre submarine cable systems:

  • G.973 (2010-07) Characteristics of repeaterless optical fibre submarine cable systems
  • G.974 (2007-07) Characteristics of regenerative optical fibre submarine cable systems
  • G.975.1 (2004-02) Forward error correction for high bit-rate DWDM submarine systems
  • G.977 (2011-04) Characteristics of optically amplified optical fibre submarine cable systems
  • G.978 (2010-07) Characteristics of optical fibre submarine cables

 

When the bit error occurs to the system, generally the OSNR at the transmit end is well and the fault is well hidden.
Decrease the optical power at the transmit end at that time. If the number of bit errors decreases at the transmit end, the problem is non-linear problem.
If the number of bit errors increases at the transmit end, the problem is the OSNR degrade problem. 

 

General Causes of Bit Errors

  •  Performance degrade of key boards
  • Abnormal optical power
  • Signal-to-noise ratio decrease
  • Non-linear factor
  • Dispersion (chromatic dispersion/PMD) factor
  • Optical reflection
  • External factors (fiber, fiber jumper, power supply, environment and others)

As defined in G.709 an ODUk container consist of an OPUk (Optical Payload Unit) plus a specific ODUk Overhead (OH).    OPUk OH information is added to the OPUk information payload to create anOPUk.  It includes information to support the adaptation of client signals.Within the OPUk overhead there is the payload structure identifier (PSI) that includes the     payload type (PT).  The payload type (PT) is used to indicate the composition of the OPUk signal.

When an ODUj signal is multiplexed into an ODUk, the ODUj signal is first extended with frame alignment overhead and   then mapped into an Optical channel Data Tributary Unit (ODTU). Two different types of ODTU are   defined in G.709:

– ODTUjk ((j,k) = {(0,1), (1,2), (1,3), (2,3)}; ODTU01,ODTU12,ODTU13 and ODTU23) in which an ODUj signal is mapped via the asynchronous mapping procedure (AMP), defined in clause 19.5 of G.709.

– ODTUk.ts ((k,ts) = (2,1..8), (3,1..32), (4,1..80)) in which a lower order ODU (ODU0, ODU1, ODU2, ODU2e, ODU3, ODUflex) signal is mapped via the generic mapping procedure (GMP), defined in clause 19.6 of  G.709.

When PT is assuming value 20 or 21,together with OPUk type (K=1,2,3,4), it is used to discriminate two different  ODU multiplex structure ODTUGx :

 Value 20: supporting ODTUjk only,

– Value 21: supporting ODTUk.ts or ODTUk.ts and ODTUjk.

The discrimination is needed for OPUk with K =2 or 3, since OPU2 and OPU3 are able to support both the different ODU multiplex structures.For OPU4 and OPU1, only one type  of ODTUG is supported: ODTUG4 with PT=21 and ODTUG1 with PT=20.The relationship between PT and TS granularity, is in the fact that the twodifferent ODTUGk discriminated by PT and OPUk  are characterized by two different TS granularities of the relatedOPUk, the former at 2.5   Gbps, the latter at 1.25Gbps.

Auto-Negotiation for fiber optic media segments turned out to be sufficiently difficult to achieve that most Ethernet fiber optic segments do not support Auto-Negotiation. During the development of the Auto-Negotiation standard, attempts were made to de‐ velop a system of Auto-Negotiation signaling that would work on the 10BASE-FL and 100BASE-FX fiber optic media systems.

However, these two media systems use different wavelengths of light and different signal timing, and it was not possible to come up with an Auto-Negotiation signaling standard that would work on both. That’s why there is no IEEE standard Auto-Negotiation sup‐ port for these fiber optic link segments. The same issues apply to 10 Gigabit Ethernet segments, so there is no Auto-Negotiation system for fiber optic 10 Gigabit Ethernet media segments either.

The 1000BASE-X Gigabit Ethernet standard, on the other hand, uses identical signal encoding on the three media systems defined in 1000BASE-X. This made it possible to develop an Auto-Negotiation system for the 1000BASE-X media types, as defined in Clause 37 of the IEEE 802.3 standard.

This lack of Auto-Negotiation on most fiber optic segments is not a major problem, given that Auto-Negotiation is not as useful on fiber optic segments as it is on twisted- pair desktop connections. For one thing, fiber optic segments are most often used as network backbone links, where the longer segment lengths supported by fiber optic media are most effective. Compared to the number of desktop connections, there are far fewer backbone links in most networks. Further, an installer working on the back‐ bone of the network can be expected to know which fiber optic media type is being connected and how it should be configured.

When Ethernet was developed it was recognized that the use of repeaters to connect segments to form a larger network would result in pulse regeneration delays that could adversely affect the probability of collisions. Thus, a limit was required on the number of repeaters that could be used to connect segments together. This limit in turn limited the number of segments that could be interconnected. A further limitation involved the number of populated segments that could be joined together, because stations on populated segments generate traffic that can cause collisions, whe   reas non-populated segments are more suitable for extending the length of a network of interconnected segments. A result of the preceding was the ‘‘5-4-3 rule.’’ That rule specifies that a maximum of five Ethernet segments can be joined through the use of a maximum of four repeaters. In actuality, this part of the Ethernet rule really means that no two communicating Ethernet nodes can be more than two repeaters away from one another. Finally, the ‘‘three’’ in the rule denotes the maximum number of Ethernet segments that can be populated. Figure illustrates an example of the 5-4-3 rule for the original bus-based Ethernet.

The Optical Time Domain Reflectometer (OTDR) is useful for testing the integrity of fiber optic cables. An optical time-domain reflectometer (OTDR) is an optoelectronic instrument used to characterize an optical fiber. An OTDR is the optical equivalent of an electronic time domain reflectometer. It injects a series of optical pulses into the fiber under test. It also extracts, from the same end of the fiber, light that is scattered (Rayleigh backscatter) or reflected back from points along the fiber. The strength of the return pulses is measured and integrated as a function of time, and plotted as a function of fiber length.

Using an OTDR, we can:

1. Measure the distance to a fusion splice, mechanical splice, connector, or significant bend in the fiber.

2. Measure the loss across a fusion splice, mechanical splice, connector, or significant bend in the fiber.

3. Measure the intrinsic loss due to mode-field diameter variations between two pieces of single-mode optical fiber connected by a splice or connector.

4. Determine the relative amount of offset and bending loss at a splice or connector joining two single-mode fibers.

5. Determine the physical offset at a splice or connector joining two pieces of single-mode fiber, when bending loss is insignificant.

6. Measure the optical return loss of discrete components, such as mechanical splices and connectors.

7. Measure the integrated return loss of a complete fiber-optic system.

8. Measure a fiber’s linearity, monitoring for such things as local mode-field pinch-off.

9. Measure the fiber slope, or fiber attenuation (typically expressed in dB/km).

10. Measure the link loss, or end-to-end loss of the fiber network.

11. Measure the relative numerical apertures of two fibers.

12. Make rudimentary measurements of a fiber’s chromatic dispersion.

13. Measure polarization mode dispersion.

14. Estimate the impact of reflections on transmitters and receivers in a fiber-optic system.

15. Provide active monitoring on live fiber-optic systems.

16. Compare previously installed waveforms to current traces.

The maintenance signals defined in [ITU-T G.709] provide network connection status information in the form of payload missing indication (PMI), backward error and defect indication (BEI, BDI), open connection indication (OCI), and link and tandem connection status information in the form of locked indication (LCK) and alarm indication signal (FDI, AIS).

 

 

 

 

Interaction diagrams are collected from ITU G.798 and OTN application note from IpLight

“In analog world the standard test message is the sine wave, followed by the two-­tone signal  for more rigorous tests.  The property being optimized is generally  signal-to-noise ratio (SNR). Speech  is  interesting, but does not lend itself easily to mathematical analysis, or measurement. 

ln digital world a binary sequence, with a known pattern of ‘ 1’ and ‘0’ ,  i s common .   It i s more common  to measure Bit error  rates (BER) than  SNR, and this is simplified by the fact that  known binary sequences are easy to generate and reproduce. A common sequence is the pseudo random  binary sequence.”

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“A PRBS (Pseudo Random Binary Sequence) is a binary PN (Pseudo-Noise) signal. The sequence of binary 1’s and 0’s exhibits certain randomness and auto-correlation properties.Bit-sequences like PRBS are used for testing transmission lines and transmission equipment because of their randomness properties.Simple bit-sequences are used to test the DC compatibility of transmission lines and transmission equipment.”

**********************************************************************************************************************************************************

 Pseudo-Random-Bit-Sequence (PRBS) is used to simulate random data for transmission across the link.The different types of PRBS and the suggested data-rates for the different PRBS types are described in the ITU-T standards O.150, O.151, O.152 and O.153.In order to properly simulate real traffic, a pseudo-random bit sequence (PRBS) is also used. The rate of the PRBS can range between 2^-9 and 2^-31. Typically, for higher-bit-rate devices, a high-rate PBRS pattern is preferable so that the device under test is effectively stressed

**********************************************************************************************************************************************************

 Bit-error measurements are an important means of assessing the performance of digital transmission. It is necessary to specify reproducible test sequences that simulate real traffic as closely as possible. Reproducible test sequences are also a prerequisite to perform end-to-end measurement.  Pseudo-random bit sequences (PRBS) with lengths of 2n – 1 bits are the most common solution to this problem.

PRBS bit-pattern are generated in a linear feed-back shift-register. This is a shift-register with a xored– feedback of the output-values of specific flip-flops to the input of the first flip-flop.2*X (X = PRBS shift register length). 

Example : PRBS-Generation of the sequence 2^9  -1 :

 

PRBS_TYPE   

 

ERROR TYPE   

 

Note:(PRBS) of order 31 (PRBS31), which is the inverted bit stream.

G(x) = 1 + x28 + x31 (1)

The advantage of using a PRBS pattern for BER testing is that it is a deterministic signal with properties similar to those of a random signal for the link , i. e. of white noise.

Bit error counting

Whereas a mask of the bit errors in the stream can be created by ANDing the received bytes after coalescing them with the locally generated PRBS31 pattern, counting the number of bits set in this mask in order to calculate the BER is a bit tricky. So we need to follow this

 

Typical links are designed for BERs better than 10-12

The Bit Error Ratio (BER) is often specified as a performance parameter of a transmission system, which needs to be verified during investigation. Designing an experiment to demonstrate adequate BER performance is not, however, as straightforward as it appears since the number of errors detected over a practical measurement time is generally small. It is, therefore, not sufficient to quote the BER as simply the ratio of the number of errors divided by the number of bits transmitted during the measurement period, instead some knowledge of the statistical nature of the error distribution must first be assumed.

The bit error rate (BER) is the most significant performance parameter of any digital communications system. It is a measure of the probability that any given bit will have been received in error. For example a standard maximum bit error rate specified for many systems is 10-9. This means that the receiver is allowed to generate a maximum of 1 error in every 109 bits of information transmitted or, putting it another way, the probability that any received bit is in error is 10-9.

 The BER depends primarily on the signal to noise ratio (SNR) of the received signal which in turn is determined by the transmitted signal power, the attenuation of the link, the link dispersion and the receiver noise. The S/N ratio is generally quoted for analog links while the bit-error-rate (BER) is used for digital links. BER is practically an inverse function of S/N. There must be a minimum power at the receiver to provide an acceptable S/N or BER. As the power increases, the BER or S/N improves until the signal becomes so high it overloads the receiver and receiver performance degrades rapidly.

 The formula used to calculate residual BER assumes a gaussian error distribution:

C = 1 – e–nb

C = Degree of confidence required

(0.95 = 95% confidence)

n = No. of bits examined with no error found.

b = Upper bound on BER with a confidence C

(b = 10–15)

To determine the length of time, that is, the number of bits needed to test for (at a given bit rate), requires the above equation to be transposed:

n = loge(1 – C)/b

 

So, to test for a residual BER of 10–13 with a 95% confidence limit requires a test pattern equal to 3 x 1013 bits. This equates to only 0.72 hours using an OC-192c/STM-64c payload rather than 55.6 hours using an STS-3c/VC-4 bulk filled payload (149.76 Mb/s).The graph in Figure plots test time versus residual BER and shows the difference in test time for OC-192c/STM-64c payloads versus an OC-48c/STM-16c payload.The graphs are plotted for different confidence limits and they clearly indicate that the payload capacity is the dominant factor in improving the test time and not the confidence limit. Table 1 shows the exact test times for each BER threshold and confidence limit.

 

collected from::Product Note-OmniBER

FEC codes in optical communications are based on a class of codes know as Reed-Solomon.

Reed-Solomon code is specified as  RS (nk), which means that the encoder takes k data bytes and adds parity bytes to make an n bytes codeword. A Reed-Solomon decoder can correct up to t bytes in the codeword, where 2t=n – k.

 

ITU recommendation G.975 proposes a Reed-Solomon (255, 239). In this case 16 extra bytes are appended to 239 information-bearing bytes. The bit rate increase is about 7% [(255-239)/239 = 0.066], the code can correct up to 8 byte errors [255-239/2 =8] and the coding gain can be demonstrated to be about 6dB.

The same Reed-Solomon coding (RS (255,239)) is recommended in ITU-T G.709. The coding overhead is again about 7% for a 6dB coding gain. Both G.975 and G.709 improve the efficiency of the Reed-Solomon by interleaving data from different codewords. The interleaving technique carries an advantage for burst errors, because the errors can be shared across many different codewords. In the interleaving approach lies the main difference between G.709 and G.975: G.709 interleave approach is fully standardized,while G.975 is not.

The actual G.975 data overhead includes also one bit for framing overhead, therefore the bit rate exp ansion is [(255-238)/238 = 0.071]. In G.709 the frame overhead is higher than in G.975, hence an even higher bit rate expansion. One byte error occurs when 1 bit in a byte is wrong or when all the bits in a byte are wrong. Example: RS (255,239) can correct 8 byte errors. In the worst case, 8 bit errors may occur, each in a separate byte so that the decoder corrects 8 bit errors. In the best case, 8 complete byte errors occur so that the decoder corrects 8 x 8 bit errors.

There are other, more powerful and complex RS variants (like for example concatenating two RS codes) capable of Coding Gain 2 or 3 dB higher than the ITU-T FEC codes, but at the expense of an increased bit rate (sometimes as much as 25%).

FOR OTN FRAME: Calculation of RS( n,k) is as follows:-

*OPU1 payload rate= 2.488 Gbps (OC48/STM16)

 

*Add OPU1 and ODU1 16 bytes overhead:

 

3808/16 = 238, (3808+16)/16 = 239

ODU1 rate: 2.488 x 239/238** ~ 2.499Gbps

*Add FEC

OTU1 rate: ODU1 x 255/239 = 2.488 x 239/238 x 255/239

=2.488 x 255/238 ~2.667Gbps

 

NOTE:4080/16=(255)

**Multiplicative factor is just a simple math :eg. for ODU1/OPU1=3824/3808={(239*16)/(238*16)}

Here value of multiplication factor will give the number of times  for rise in the frame size after adding header/overhead.

As we are using Reed Soloman(255,239) i.e we are dividing 4080bytes in sixteen frames (The forward error correction for the OTU-k uses 16-byte interleaved codecs using a Reed- Solomon S(255,239) code. The RS(255,239) code operates on byte symbols.).

Hence 4080/16=255…I have understood it you need to do simpler maths to understand..)