Latest Posts:
MapYourTech
Tag

DWDM

Browsing
c+l
In Technical 4 Mins Read

Exploring the C+L Bands in DWDM Network

By

Exploring the C+L Bands in DWDM Network

DWDM networks have traditionally operated within the C-band spectrum due to its lower dispersion and the availability of efficient Erbium-Doped Fiber Amplifiers (EDFAs). Initially, the C-band supported a spectrum of 3.2 terahertz (THz), which has been expanded to 4.8 THz to accommodate increased data traffic. While the Japanese market favored the L-band early on, this preference is now expanding globally as the L-band’s ability to double the spectrum capacity becomes crucial. The integration of the L-band adds another 4.8 THz, resulting in a total of 9.6 THz when combined with the C-band.

What Does C+L Mean?

C+L band refers to two specific ranges of wavelengths used in optical fiber communications: the C-band and the L-band. The C-band ranges from approximately 1530 nm to 1565 nm, while the L-band covers from about 1565 nm to 1625 nm. These bands are crucial for transmitting signals over optical fiber, offering distinct characteristics in terms of attenuation, dispersion, and capacity.

c+l

C+L Architecture

The Advantages of C+L

The adoption of C+L bands in fiber optic networks comes with several advantages, crucial for meeting the growing demands for data transmission and communication services:

  1. Increased Capacity: One of the most significant advantages of utilizing both C and L bands is the dramatic increase in network capacity. By essentially doubling the available spectrum for data transmission, service providers can accommodate more data traffic, which is essential in an era where data consumption is soaring due to streaming services, IoT devices, and cloud computing.
  2. Improved Efficiency: The use of C+L bands makes optical networks more efficient. By leveraging wider bandwidths, operators can optimize their existing infrastructure, reducing the need for additional physical fibers. This efficiency not only cuts costs but also accelerates the deployment of new services.
  3. Enhanced Flexibility: With more spectrum comes greater flexibility in managing and allocating resources. Network operators can dynamically adjust bandwidth allocations to meet changing demand patterns, improving overall service quality and user experience.
  4. Reduced Attenuation and Dispersion: Each band has its own set of optical properties. By carefully managing signals across both C and L bands, it’s possible to mitigate issues like signal attenuation and chromatic dispersion, leading to longer transmission distances without the need for signal regeneration.

Challenges in C+L Band Implementation:

  1. Stimulated Raman Scattering (SRS): A significant challenge in C+L band usage is SRS, which causes a tilt in power distribution from the C-band to the L-band. This effect can create operational issues, such as longer recovery times from network failures, slow and complex provisioning due to the need to manage the power tilt between the bands, and restrictions on network topologies.
  2. Cost: The financial aspect is another hurdle. Doubling the components, such as amplifiers and wavelength-selective switches (WSS), can be costly. Network upgrades from C-band to C+L can often mean a complete overhaul of the existing line system, a deterrent for many operators if the L-band isn’t immediately needed.
  3. C+L Recovery Speed: Network recovery from failures can be sluggish, with times hovering around the 10-minute mark.
  4. C+L Provisioning Speed and Complexity: The provisioning process becomes more complicated, demanding careful management of the number of channels across bands.

The Future of C+L

The future of C+L in optical communications is bright, with several trends and developments on the horizon:

  • Integration with Emerging Technologies: As 5G and beyond continue to roll out, the integration of C+L band capabilities with these new technologies will be crucial. The increased bandwidth and efficiency will support the ultra-high-speed, low-latency requirements of future mobile networks and applications.
  • Innovations in Fiber Optic Technology: Ongoing research in fiber optics, including new types of fibers and advanced modulation techniques, promises to further unlock the potential of the C+L bands. These innovations could lead to even greater capacities and more efficient use of the optical spectrum.
  • Sustainability Impacts: With an emphasis on sustainability, the efficiency improvements associated with C+L band usage could contribute to reducing the energy consumption of data centers and network infrastructure, aligning with global efforts to minimize environmental impacts.
  • Expansion Beyond Telecommunications: While currently most relevant to telecommunications, the benefits of C+L band technology could extend to other areas, including remote sensing, medical imaging, and space communications, where the demand for high-capacity, reliable transmission is growing.

In conclusion, the adoption and development of C+L band technology represent a significant step forward in the evolution of optical communications. By offering increased capacity, efficiency, and flexibility, C+L bands are well-positioned to meet the current and future demands of our data-driven world. As we look to the future, the continued innovation and integration of C+L technology into broader telecommunications and technology ecosystems will be vital in shaping the next generation of global communication networks.

 

References:

by
Keep Reading
In Standards 4 Mins Read

Spectral Bands for Single Mode Optical Fiber Systems

By

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

by
Keep Reading
In Technical 2 Mins Read

Spectral Grids and WDM Techniques for Fiber Optic Capacity

By

Introduction

The telecommunications industry constantly strives to maximize the use of fiber optic capacity. Despite the broad spectral width of the conventional C-band, which offers over 40 THz, the limited use of optical channels at 10 or 40 Gbit/s results in substantial under utilization. The solution lies in Wavelength Division Multiplexing (WDM), a technique that can significantly increase the capacity of optical fibers.

Understanding Spectral Grids

WDM employs multiple optical carriers, each on a different wavelength, to transmit data simultaneously over a single fiber. This method vastly improves the efficiency of data transmission, as outlined in ITU-T Recommendations that define the spectral grids for WDM applications.

The Evolution of Channel Spacing

Historically, WDM systems have evolved to support an array of channel spacings. Initially, a 100 GHz grid was established, which was then subdivided by factors of two to create a variety of frequency grids, including:

  1. 12.5 GHz spacing
  2. 25 GHz spacing
  3. 50 GHz spacing
  4. 100 GHz spacing

All four frequency grids incorporate 193.1 THz and are not limited by frequency boundaries. Additionally, wider spacing grids can be achieved by using multiples of 100 GHz, such as 200 GHz, 300 GHz, and so on.

ITU-T Recommendations for DWDM

ITU-T Recommendations such as ITU-T G.692 and G.698 series outline applications utilizing these DWDM frequency grids. The recent addition of a flexible DWDM grid, as per Recommendation ITU-T G.694.1, allows for variable bit rates and modulation formats, optimizing the allocation of frequency slots to match specific bandwidth requirements.

Flexible DWDM Grid in Practice

#itu-t_grid

The flexible grid is particularly innovative, with nominal central frequencies at intervals of 6.25 GHz from 193.1 THz and slot widths based on 12.5 GHz increments. This flexibility ensures that the grid can adapt to a variety of transmission needs without overlap, as depicted in Figure above.

CWDM Wavelength Grid and Applications

Recommendation ITU-T G.694.2 defines the CWDM wavelength grid to support applications requiring simultaneous transmission of several wavelengths. The 20 nm channel spacing is a result of manufacturing tolerances, temperature variations, and the need for a guardband to use cost-effective filter technologies. These CWDM grids are further detailed in ITU-T G.695.

Conclusion

The strategic use of DWDM and CWDM grids, as defined by ITU-T Recommendations, is key to maximizing the capacity of fiber optic transmissions. With the introduction of flexible grids and ongoing advancements, we are witnessing a transformative period in fiber optic technology.

by
Keep Reading
submarinemap
In Technical 4 Mins Read

Exploring the Constraints on Fiber Pairs In Submarine Optical Fiber Networks

By

In the world of global communication, Submarine Optical Fiber Networks cable play a pivotal role in facilitating the exchange of data across continents. As technology continues to evolve, the capacity and capabilities of these cables have been expanding at an astonishing pace. In this article, we delve into the intricate details of how future cables are set to scale their cross-sectional capacity, the factors influencing their design, and the innovative solutions being developed to overcome the challenges posed by increasing demands.

Scaling Factors: WDM Channels, Modes, Cores, and Fibers

In the quest for higher data transfer rates, the architecture of future undersea cables is set to undergo a transformation. The scaling of cross-sectional capacity hinges on several key factors: the number of Wavelength Division Multiplexing (WDM) channels in a mode, the number of modes in a core, the number of cores in a fiber, and the number of fibers in the cable. By optimizing these parameters, cable operators are poised to unlock unprecedented data transmission capabilities.

Current Deployment and Challenges 

Presently, undersea cables commonly consist of four to eight fiber pairs. On land, terrestrial cables have ventured into new territory with remarkably high fiber counts, often based on loose tube structures. A remarkable example of this is the deployment of a 1728-fiber cable across Sydney Harbor, Australia. However, the capacity of undersea cables is not solely determined by fiber count; other factors come into play.

Power Constraints and Spatial Limitations

The maximum number of fibers that can be incorporated into an undersea cable is heavily influenced by two critical factors: electrical power availability and physical space constraints. The optical amplifiers, which are essential for boosting signal strength along the cable, require a certain amount of electrical power. This power requirement is dependent on various parameters, including the overall cable length, amplifier spacing, and the number of amplifiers within each repeater. As cable lengths increase, power considerations become increasingly significant.

Efficiency: Improving Amplifiers for Enhanced Utilisation

Optimising the efficiency of optical amplifiers emerges as a strategic solution to mitigate power constraints. By meticulously adjusting design parameters such as narrowing the optical bandwidth, the loss caused by gain flattening filters can be minimised. This reduction in loss subsequently decreases the necessary pump power for signal amplification. This approach not only addresses power limitations but also maximizes the effective utilisation of resources, potentially allowing for an increased number of fiber pairs within a cable.

Multi-Core Fiber: Opening New Horizons

The concept of multi-core fiber introduces a transformative potential for submarine optical networks. By integrating multiple light-guiding cores within a single physical fiber, the capacity for data transmission can be substantially amplified. While progress has been achieved in the fabrication of multi-core fibers, the development of multi-core optical amplifiers remains a challenge. Nevertheless, promising experiments showcasing successful transmissions over extended distances using multi-core fibers with multiple wavelengths hint at the technology’s promising future.

Technological Solutions: Overcoming Space Constraints

As fiber cores increase in number, so does the need for amplifiers within repeater units. This poses a challenge in terms of available physical space. To combat this, researchers are actively exploring two key technological solutions. The first involves optimising the packaging density of optical components, effectively cramming more functionality into the same space. The second avenue involves the use of photonic integrated circuits (PICs), which enable the integration of multiple functions onto a single chip. Despite their potential, PICs do face hurdles in terms of coupling loss and power handling capabilities.

Navigating the Future

The realm of undersea fiber optic cables is undergoing a remarkable evolution, driven by the insatiable demand for data transfer capacity. As we explore the scaling factors of WDM channels, modes, cores, and fibers, it becomes evident that power availability and physical space are crucial constraints. However, ingenious solutions, such as amplifier efficiency improvements and multi-core fiber integration, hold promise for expanding capacity. The development of advanced technologies like photonic integrated circuits underscores the relentless pursuit of higher data transmission capabilities. As we navigate the intricate landscape of undersea cable design, it’s clear that the future of global communication is poised to be faster, more efficient, and more interconnected than ever before.

 

Reference and Credits

https://www.sciencedirect.com/book/9780128042694/undersea-fiber-communication-systems

http://submarinecablemap.com/

https://www.telegeography.com

https://infoworldmaps.com/3d-submarine-cable-map/ 

https://gfycat.com/aptmediocreblackpanther 

by
Keep Reading
In Technical 2 Mins Read

Relationship between Q Factor and Data Rate or Modulation: Analysis and Examples

By

As the data rate and complexity of the modulation format increase, the system becomes more sensitive to noise, dispersion, and nonlinear effects, resulting in a higher required Q factor to maintain an acceptable BER.

The Q factor (also called Q-factor or Q-value) is a dimensionless parameter that represents the quality of a signal in a communication system, often used to estimate the Bit Error Rate (BER) and evaluate the system’s performance. The Q factor is influenced by factors such as noise, signal-to-noise ratio (SNR), and impairments in the optical link. While the Q factor itself does not directly depend on the data rate or modulation format, the required Q factor for a specific system performance does depend on these factors.

Let’s consider some examples to illustrate the impact of data rate and modulation format on the Q factor:

  1. Data Rate:

Example 1: Consider a DWDM system using Non-Return-to-Zero (NRZ) modulation format at 10 Gbps. If the system is properly designed and optimized, it may achieve a Q factor of 20.

Example 2: Now consider the same DWDM system using NRZ modulation format, but with a higher data rate of 100 Gbps. The higher data rate makes the system more sensitive to noise and impairments like chromatic dispersion and polarization mode dispersion. As a result, the required Q factor to achieve the same BER might increase (e.g., 25).

  1. Modulation Format:

Example 1: Consider a DWDM system using NRZ modulation format at 10 Gbps. If the system is properly designed and optimized, it may achieve a Q factor of 20.

Example 2: Now consider the same DWDM system using a more complex modulation format, such as 16-QAM (Quadrature Amplitude Modulation), at 10 Gbps. The increased complexity of the modulation format makes the system more sensitive to noise, dispersion, and nonlinear effects. As a result, the required Q factor to achieve the same BER might increase (e.g., 25).

These examples show that the required Q factor to maintain a specific system performance can be affected by the data rate and modulation format. To achieve a high Q factor at higher data rates and more complex modulation formats, it is crucial to optimize the system design, including factors such as dispersion management, nonlinear effects mitigation, and the implementation of Forward Error Correction (FEC) mechanisms.

by
Keep Reading
In Interviews 11 Mins Read

Q & A on Optical Fiber

By

Optical Fiber technology is a game-changer in the world of telecommunication. It has revolutionized the way we communicate and share information. Fiber optic cables are used in most high-speed internet connections, telephone networks, and cable television systems.

 

What is Fiber Optic Technology?

Fiber optic technology is the use of thin, transparent fibers of glass or plastic to transmit light signals over long distances. These fibers are used in telecommunications to transmit data, video, and voice signals at high speeds and over long distances.

What are Fiber Optic Cables Made Of?

Fiber optic cables are made of thin strands of glass or plastic called fibers. These fibers are surrounded by protective coatings, which make them resistant to moisture, heat, and other environmental factors.

How Does Fiber Optic Technology Work?

Fiber optic technology works by sending pulses of light through the fibers in a cable. These light signals travel through the cable at very high speeds, allowing data to be transmitted quickly and efficiently.

What is an Optical Network?

An optical network is a communication network that uses optical fibers as the primary transmission medium. Optical networks are used for high-speed internet connections, telephone networks, and cable television systems.

What are the Benefits of Fiber Optic Technology?

Fiber optic technology offers several benefits over traditional copper wire technology, including:

  • Faster data transfer speeds
  • Greater bandwidth capacity
  • Less signal loss
  • Resistance to interference from electromagnetic sources
  • Greater reliability
  • Longer lifespan

How Fast is Fiber Optic Internet?

Fiber optic internet can provide download speeds of up to 1 gigabit per second (Gbps) and upload speeds of up to 1 Gbps. This is much faster than traditional copper wire internet connections.

How is Fiber Optic Internet Installed?

Fiber optic internet is installed by running fiber optic cables from a central hub to the homes or businesses that need internet access. The installation process involves digging trenches to bury the cables or running the cables overhead on utility poles.

What are the Different Types of Fiber Optic Cables?

There are two main types of fiber optic cables:

Single-Mode Fiber

Single-mode fiber has a smaller core diameter than multi-mode fiber, which allows it to transmit light signals over longer distances with less attenuation.

Multi-Mode Fiber

Multi-mode fiber has a larger core diameter than single-mode fiber, which allows it to transmit light signals over shorter distances at a lower cost.

What is the Difference Between Single-Mode and Multi-Mode Fiber?

The main difference between single-mode and multi-mode fiber is the size of the core diameter. Single-mode fiber has a smaller core diameter, which allows it to transmit light signals over longer distances with less attenuation. Multi-mode fiber has a larger core diameter, which allows it to transmit light signals over shorter distances at a lower cost.

What is the Maximum Distance for Fiber Optic Cables?

The maximum distance for fiber optic cables depends on the type of cable and the transmission technology used. In general, single-mode fiber can transmit light signals over distances of up to 10 kilometers without the need for signal regeneration, while multi-mode fiber is limited to distances of up to 2 kilometers.

What is Fiber Optic Attenuation?

Fiber optic attenuation refers to the loss of light signal intensity as it travels through a fiber optic cable. Attenuation is caused by factors such as absorption, scattering, and bending of the light signal.

What is Fiber Optic Dispersion?

Fiber optic dispersion refers to the spreading of a light signal as it travels through a fiber optic cable. Dispersion is caused by factors such as the wavelength of the light signal and the length of the cable.

What is Fiber Optic Splicing?

Fiber optic splicing is the process of joining two fiber optic cables together. Splicing is necessary when extending the length of a fiber optic cable or when repairing a damaged cable.

What is the Difference Between Fusion Splicing and Mechanical Splicing?

Fusion splicing is a process in which the two fibers to be joined are fused together using heat. Mechanical splicing is a process in which the two fibers to be joined are aligned and held together using a mechanical splice.

What is Fiber Optic Termination?

Fiber optic termination is the process of connecting a fiber optic cable to a device or equipment. Termination involves attaching a connector to the end of the cable so that it can be plugged into a device or equipment.

What is an Optical Coupler?

An optical coupler is a device that splits or combines light signals in a fiber optic network. Couplers are used to distribute signals from a single source to multiple destinations or to combine signals from multiple sources into a single fiber.

What is an Optical Splitter?

optical splitter is a type of optical coupler that splits a single fiber into multiple fibers. Splitters are used to distribute signals from a single source to multiple destinations.

What is Wavelength-Division Multiplexing?

Wavelength-division multiplexing is a technology that allows multiple signals of different wavelengths to be transmitted over a single fiber. Each signal is assigned a different wavelength, and a multiplexer is used to combine the signals into a single fiber.

What is Dense Wavelength-Division Multiplexing?

Dense wavelength-division multiplexing is a technology that allows multiple signals to be transmitted over a single fiber using very closely spaced wavelengths. DWDM is used to increase the capacity of fiber optic networks.

What is Coarse Wavelength-Division Multiplexing?

Coarse wavelength-division multiplexing is a technology that allows multiple signals to be transmitted over a single fiber using wider-spaced wavelengths than DWDM. CWDM is used for shorter distance applications and lower bandwidth requirements.

What is Bidirectional Wavelength-Division Multiplexing?

Bidirectional wavelength-division multiplexing is a technology that allows signals to be transmitted in both directions over a single fiber. BIDWDM is used to increase the capacity of fiber optic networks.

What is Fiber Optic Testing?

Fiber optic testing is the process of testing the performance of fiber optic cables and components. Testing is done to ensure that the cables and components meet industry standards and to troubleshoot problems in the network.

What is Optical Time-Domain Reflectometer?

An optical time-domain reflectometer is a device used to test fiber optic cables by sending a light signal into the cable and measuring the reflections. OTDRs are used to locate breaks, bends, and other faults in fiber optic cables.

What is Optical Spectrum Analyzer?

An optical spectrum analyzer is a device used to measure the spectral characteristics of a light signal. OSAs are used to analyze the output of fiber optic transmitters and to measure the characteristics of fiber optic components.

What is Optical Power Meter?

An optical power meter is a device used to measure the power of a light signal in a fiber optic cable. Power meters are used to measure the output of fiber optic transmitters and to test the performance of fiber optic cables and components.

What is Fiber Optic Connector?

A fiber optic connector is a device used to attach a fiber optic cable to a device or equipment. Connectors are designed to be easily plugged and unplugged, allowing for easy installation and maintenance.

What is Fiber Optic Adapter?

A fiber optic adapter is a device used to connect two fiber optic connectors together. Adapters are used to extend the length of a fiber optic cable or to connect different types of fiber optic connectors.

What is Fiber Optic Patch Cord?

A fiber optic patch cord is a cable with connectors on both ends used to connect devices or equipment in a fiber optic network. Patch cords are available in different lengths and connector types to meet different network requirements.

What is Fiber Optic Pigtail?

A fiber optic pigtail is a short length of fiber optic cable with a connector on one end and a length of exposed fiber on the other. Pigtails are used to connect fiber optic cables to devices or equipment that require a different type of connector.

What is Fiber Optic Coupler?

A fiber optic coupler is a device used to split or combine light signals in a fiber optic network. Couplers are used to distribute signals from a single source to multiple destinations or to combine signals from multiple sources into a single fiber.

What is Fiber Optic Attenuator?

A fiber optic attenuator is a device used to reduce the power of a light signal in a fiber optic network. Attenuators are used to prevent

signal overload or to match the power levels of different components in the network.

What is Fiber Optic Isolator?

A fiber optic isolator is a device used to prevent light signals from reflecting back into the source. Isolators are used to protect sensitive components in the network from damage caused by reflected light.

What is Fiber Optic Circulator?

A fiber optic circulator is a device used to route light signals in a specific direction in a fiber optic network. Circulators are used to route signals between multiple devices in a network.

What is Fiber Optic Amplifier?

A fiber optic amplifier is a device used to boost the power of a light signal in a fiber optic network. Amplifiers are used to extend the distance that a signal can travel without the need for regeneration.

What is Fiber Optic Modulator?

A fiber optic modulator is a device used to modulate the amplitude or phase of a light signal in a fiber optic network. Modulators are used in applications such as fiber optic communication and sensing.

What is Fiber Optic Switch?

A fiber optic switch is a device used to switch light signals between different fibers in a fiber optic network. Switches are used to route signals between multiple devices in a network.

What is Fiber Optic Demultiplexer?

A fiber optic demultiplexer is a device used to separate multiple signals of different wavelengths that are combined in a single fiber. Demultiplexers are used in wavelength-division multiplexing applications.

What is Fiber Optic Multiplexer?

A fiber optic multiplexer is a device used to combine multiple signals of different wavelengths into a single fiber. Multiplexers are used in wavelength-division multiplexing applications.

What is Fiber Optic Transceiver?

A fiber optic transceiver is a device that combines a transmitter and a receiver into a single module. Transceivers are used to transmit and receive data over a fiber optic network.

What is Fiber Optic Media Converter?

A fiber optic media converter is a device used to convert a fiber optic signal to a different format, such as copper or wireless. Media converters are used to connect fiber optic networks to other types of networks.

What is Fiber Optic Splice Closure?

A fiber optic splice closure is a device used to protect fiber optic splices from environmental factors such as moisture and dust. Splice closures are used in outdoor fiber optic applications.

What is Fiber Optic Distribution Box?

A fiber optic distribution box is a device used to distribute fiber optic signals to multiple devices or equipment. Distribution boxes are used in fiber optic networks to route signals between multiple devices.

What is Fiber Optic Patch Panel?

A fiber optic patch panel is a device used to connect multiple fiber optic cables to a network. Patch panels are used to organize and manage fiber optic connections in a network.

What is Fiber Optic Cable Tray?

A fiber optic cable tray is a device used to support and protect fiber optic cables in a network. Cable trays are used to organize and route fiber optic cables in a network.

What is Fiber Optic Duct?

A fiber optic duct is a device used to protect fiber optic cables from environmental factors such as moisture and dust. Ducts are used in outdoor fiber optic applications.

What is Fiber Optic Raceway?

A fiber optic raceway is a device used to route and protect fiber optic cables in a network. Raceways are used to organize and manage fiber optic connections in a network.

What is Fiber Optic Conduit?

A fiber optic conduit is a protective tube used to house fiber optic cables in a network. Conduits are used in outdoor fiber optic applications to protect cables from environmental factors.

by
Keep Reading
In Interviews 5 Mins Read

DWDM related 25 Questions and Answers

By
  1. What is DWDM technology?

A: DWDM stands for Dense Wavelength Division Multiplexing, a technology used in optical networks to increase the capacity of data transmission by combining multiple optical signals with different wavelengths onto a single fiber.

  1. How does DWDM work?

A: DWDM works by assigning each incoming data channel a unique wavelength (or color) of light, combining these channels into a single optical fiber. This allows multiple data streams to travel simultaneously without interference.

  1. What is the difference between DWDM and CWDM?

A: DWDM stands for Dense Wavelength Division Multiplexing, while CWDM stands for Coarse Wavelength Division Multiplexing. The primary difference is in the channel spacing, with DWDM having much closer channel spacing, allowing for more channels on a single fiber.

  1. What are the key components of a DWDM system?

A: Key components of a DWDM system include optical transmitters, multiplexers, optical amplifiers, de-multiplexers, and optical receivers.

  1. What is an Optical Add-Drop Multiplexer (OADM)?

A: An OADM is a device that adds or drops specific wavelengths in a DWDM system while allowing other wavelengths to continue along the fiber.

  1. How does DWDM increase network capacity?

A: DWDM increases network capacity by combining multiple optical signals with different wavelengths onto a single fiber, allowing for simultaneous data transmission without interference.

  1. What is the typical channel spacing in DWDM systems?

A: The typical channel spacing in DWDM systems is 100 GHz or 0.8 nm, although more advanced systems can achieve 50 GHz or even 25 GHz spacing.

  1. What is the role of optical amplifiers in DWDM systems?

A: Optical amplifiers are used to boost the signal strength in DWDM systems, compensating for signal loss and enabling long-distance transmission.

  1. What is the maximum transmission distance for DWDM systems?

A: Maximum transmission distance for DWDM systems varies depending on factors such as channel count, fiber type, and amplification. However, some systems can achieve distances of up to 2,500 km or more.

  1. What are the primary benefits of DWDM?

A: Benefits of DWDM include increased network capacity, scalability, flexibility, and cost-effectiveness.

  1. What are some common applications of DWDM technology?

A: DWDM technology is commonly used in long-haul and metropolitan area networks (MANs), as well as in internet service provider (ISP) networks and data center interconnects.

  1. What is a wavelength blocker?

A: A wavelength blocker is a device that selectively blocks or filters specific wavelengths in a DWDM system.

  1. What are erbium-doped fiber amplifiers (EDFAs)?

A: EDFAs are a type of optical amplifier that uses erbium-doped fiber as the gain medium, providing amplification for DWDM systems.

  1. How does chromatic dispersion impact DWDM systems?

A: Chromatic dispersion is the spreading of an optical signal due to different wavelengths traveling at different speeds in the fiber. In DWDM systems, chromatic dispersion can cause signal degradation and reduce transmission distance.

  1. What is a dispersion compensating module (DCM)?

A: A DCM is a device used to compensate for chromatic dispersion in DWDM systems, improving signal quality and transmission distance.

  1. What is an optical signal-to-noise ratio (OSNR)?

A: OSNR is a measure of the quality of an optical signal in relation to noise in a DWDM system. A higher OSNR indicates better signal quality.

  1. How does polarization mode dispersion (PMD) affect DWDM systems?

A: PMD is a phenomenon where different polarization states of

ight travel at different speeds in the fiber, causing signal distortion and degradation in DWDM systems. PMD can limit the transmission distance and data rates.

  1. What is the role of a dispersion management strategy in DWDM systems?

A: A dispersion management strategy helps to minimize the impact of chromatic dispersion and PMD, ensuring better signal quality and longer transmission distances in DWDM systems.

  1. What is a tunable optical filter?

A: A tunable optical filter is a device that can be adjusted to selectively transmit or block specific wavelengths in a DWDM system, allowing for dynamic channel allocation and reconfiguration.

  1. What is a reconfigurable optical add-drop multiplexer (ROADM)?

A: A ROADM is a device that allows for the flexible addition, dropping, or rerouting of wavelength channels in a DWDM system, enabling dynamic network reconfiguration.

  1. How does DWDM support network redundancy and protection?

A: DWDM can be used to create diverse optical paths, providing redundancy and protection against network failures or service disruptions.

  1. What is the impact of nonlinear effects on DWDM systems?

A: Nonlinear effects such as self-phase modulation, cross-phase modulation, and four-wave mixing can cause signal degradation and limit transmission performance in DWDM systems.

  1. What is the role of forward error correction (FEC) in DWDM systems?

A: FEC is a technique used to detect and correct errors in DWDM systems, improving signal quality and transmission performance.

  1. How does DWDM enable optical network flexibility?

A: DWDM allows for the dynamic allocation and reconfiguration of wavelength channels, providing flexibility to adapt to changing network demands and optimize network resources.

  1. What is the future of DWDM technology?

A: The future of DWDM technology includes continued advancements in channel spacing, transmission distances, and data rates, as well as the integration of software-defined networking (SDN) and other emerging technologies to enable more intelligent and adaptive optical networks.

by
Keep Reading
In Standards 17 Mins Read

WDM Glossary

By

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)
by
Keep Reading
channel capacity
In Infographics 1 Min Read

Calculating Channel Capacity for DWDM links

By

The maximum data rate (maximum channel capacity) that can be transmitted error-free over a communications channel with a specified bandwidth and noise can be determined by the Shannon theorem. This is a theoretical maximum data transmission rate for all possible multilevel and multiphase encoding techniques.

As can be seen below that the maximum rate depends only on channel bandwidth and the ratio between signal power to noise power. There is no dependence on modulation method.

Rmax =Bolog2(OSNR+1)

where

    Rmax = maximum data rate for the channel (also known as channel capacity), Gbps

    Bo = optical channel passband, GHz

    OSNR = channel optical signal to noise ratio

Example:-

For a 62 GHz channel passband (for standard 200 GHz DWDM channel spacing) and an OSNR of 126 (21 dB) the maximum possible channel capacity is 433 Gbps.

As channel bandwidth decreases so does maximum transmission rate. For a 30 GHz channel passband (100 GHz DWDM channel spacing) and OSNR of 126 (21 dB) the maximum possible channel capacity is 216 Gbps.

by
Keep Reading