raman – MapYourTech

Stimulated Brillouin Scattering (SBS) in DWDM Networks

Technical 6 Mins Read

Stimulated Brillouin Scattering (SBS) is an inelastic scattering phenomenon that results in the backward scattering of light when it interacts with acoustic phonons (sound waves) in the optical fiber. SBS occurs when the intensity of the optical signal reaches a certain threshold, resulting in a nonlinear interaction between the optical field and acoustic waves within the fiber. This effect typically manifests at lower power levels compared to other nonlinear effects, making it a significant limiting factor in optical communication systems, particularly those involving long-haul transmission and high-power signals.

Mechanism of SBS

SBS is caused by the interaction of an incoming photon with acoustic phonons in the fiber material. When the intensity of the light increases beyond a certain threshold, the optical signal generates an acoustic wave in the fiber. This acoustic wave, in turn, causes a periodic variation in the refractive index of the fiber, which scatters the incoming light in the backward direction. This backscattered light is redshifted in frequency due to the Doppler effect, with the frequency shift typically around 10 GHz (depending on the fiber material and the wavelength of light).

The Brillouin gain spectrum is relatively narrow, with a typical bandwidth of around 20 to 30 MHz. The Brillouin threshold power $P_{th}$ can be calculated as:

$P_{th} = \frac{21 A_{eff}}{g_{B} L_{eff}}$

Where:

$A_{eff}$ is the effective area of the fiber core,
$g_{B}$ is the Brillouin gain coefficient,
$L_{eff}$ is the effective interaction length of the fiber.

When the power of the incoming light exceeds this threshold, SBS causes a significant amount of power to be reflected back towards the source, degrading the forward-propagating signal and introducing power fluctuations in the system.

Image credit: corning.com

Impact of SBS in Optical Systems

SBS becomes problematic in systems where high optical powers are used, particularly in long-distance transmission systems and those employing Wavelength Division Multiplexing (WDM). The main effects of SBS include:

Power Reflection:
- A portion of the optical power is scattered back towards the source, which reduces the forward-propagating signal power. This backscattered light interferes with the transmitter and receiver, potentially causing signal degradation.
Signal Degradation:
- SBS can cause signal distortion, as the backward-propagating light interferes with the incoming signal, leading to fluctuations in the transmitted power and an increase in the bit error rate (BER).
Noise Increase:
- The backscattered light adds noise to the system, particularly in coherent systems, where phase information is critical. The interaction between the forward and backward waves can distort the phase and amplitude of the transmitted signal, worsening the signal-to-noise ratio (SNR).

SBS in Submarine Systems

In submarine communication systems, SBS poses a significant challenge, as these systems typically involve long spans of fiber and require high power levels to maintain signal quality over thousands of kilometers. The cumulative effect of SBS over long distances can lead to substantial signal degradation. As a result, submarine systems must employ techniques to suppress SBS and manage the power levels appropriately.

Mitigation Techniques for SBS

Several methods are used to mitigate the effects of SBS in optical communication systems:

Reducing Signal Power:
- One of the simplest ways to reduce the onset of SBS is to lower the optical signal power below the Brillouin threshold. However, this must be balanced with maintaining sufficient power for the signal to reach its destination with an acceptable signal-to-noise ratio (SNR).
Laser Linewidth Broadening:
- SBS is more efficient when the signal has a narrow linewidth. By broadening the linewidth of the signal, the power is spread over a larger frequency range, reducing the power density at any specific frequency and lowering the likelihood of SBS. This can be achieved by modulating the laser source with a low-frequency signal.
Using Shorter Fiber Spans:
- Reducing the length of each fiber span in the transmission system can decrease the effective length over which SBS can occur. By using optical amplifiers to boost the signal power at regular intervals, it is possible to maintain signal strength without exceeding the SBS threshold.
Raman Amplification:
- SBS can be suppressed using distributed Raman amplification, where the signal is amplified along the length of the fiber rather than at discrete points. By keeping the power levels low in any given section of the fiber, Raman amplification reduces the risk of SBS.

Applications of SBS

While SBS is generally considered a detrimental effect in optical communication systems, it can be harnessed for certain useful applications:

Brillouin-Based Sensors:
- SBS is used in distributed fiber optic sensors, such as Brillouin Optical Time Domain Reflectometry (BOTDR) and Brillouin Optical Time Domain Analysis (BOTDA). These sensors measure the backscattered Brillouin light to monitor changes in strain or temperature along the length of the fiber. This is particularly useful in structural health monitoring and pipeline surveillance.
Slow Light Applications:
- SBS can also be exploited to create slow light systems, where the propagation speed of light is reduced in a controlled manner. This is achieved by using the narrow bandwidth of the Brillouin gain spectrum to induce a delay in the transmission of the optical signal. Slow light systems have potential applications in optical buffering and signal processing.

Summary

Stimulated Brillouin Scattering (SBS) is a nonlinear scattering effect that occurs at relatively low power levels, making it a significant limiting factor in high-power, long-distance optical communication systems. SBS leads to the backscattering of light, which degrades the forward-propagating signal and increases noise. While SBS is generally considered a negative effect, it can be mitigated using techniques such as power reduction, linewidth broadening, and Raman amplification. Additionally, SBS can be harnessed for beneficial applications, including optical sensing and slow light systems. Effective management of SBS is crucial for maintaining the performance and reliability of modern optical communication networks, particularly in submarine systems.

Stimulated Brillouin Scattering (SBS) is a nonlinear optical effect caused by the interaction between light and acoustic waves in the fiber.
It occurs when an intense light wave traveling through the fiber generates sound waves, which scatter the light in the reverse direction.
SBS leads to a backward-propagating signal, called the Stokes wave, that has a slightly lower frequency than the incoming light.
The effect typically occurs in single-mode fibers at relatively low power thresholds compared to other nonlinear effects like SRS.
SBS can result in power loss of the forward-propagating signal as some of the energy is reflected back as the Stokes wave.
The efficiency of SBS depends on several factors, including the fiber length, the optical power, and the linewidth of the laser source.
In WDM systems, SBS can degrade performance by introducing signal reflections and crosstalk, especially in long-haul optical links.
SBS tends to become more pronounced in narrow-linewidth lasers and fibers with low attenuation, making it a limiting factor for high-power transmission.
Mitigation techniques for SBS include using broader linewidth lasers, reducing the optical power below the SBS threshold, or employing SBS suppression techniques such as phase modulation.
Despite its negative impacts in communication systems, SBS can be exploited for applications like distributed fiber sensing and slow-light generation due to its sensitivity to acoustic waves.

Reference

https://link.springer.com/book/10.1007/978-3-030-66541-8
Image Credit : https://www.corning.com/au/en.html

Stimulated Raman Scattering (SRS) in DWDM Networks

Technical 6 Mins Read

Stimulated Raman Scattering (SRS) is a nonlinear optical phenomenon that results from the inelastic scattering of photons when intense light interacts with the vibrational modes of the fiber material. This scattering process transfers energy from shorter-wavelength (higher-frequency) channels to longer-wavelength (lower-frequency) channels. In fiber optic communication systems, particularly in Wavelength Division Multiplexing (WDM) systems, SRS can significantly degrade system performance by inducing crosstalk between channels.

Physics behind SRS

SRS is an inelastic process involving the interaction of light photons with the optical phonons (vibrational states) of the silica material in the fiber. When a high-power optical signal propagates through the fiber, a fraction of the power is scattered by the material, transferring energy from the higher frequency (shorter wavelength) channels to the lower frequency (longer wavelength) channels. The SRS gain is distributed over a wide spectral range, approximately 13 THz, with a peak shift of about 13.2 THz from the pump wavelength.

The basic process of SRS can be described as follows:

Stokes Shift: The scattered light is redshifted, meaning that the scattered photons have lower energy (longer wavelength) than the incident photons. This energy loss is transferred to the vibrational modes (phonons) of the fiber.
Amplification: The power of longer-wavelength channels is increased at the expense of shorter-wavelength channels. This power transfer can cause crosstalk between channels in WDM systems, reducing the overall signal quality.

Fig: Normalized gain spectrum generated by SRS on an SSMF fiber pumped at 1430 nm. The SRS gain spectrum has a peak at 13 THz with a bandwidth of 20–30 THz

The Raman gain coefficient $g_{R}$ describes the efficiency of the SRS process and is dependent on the frequency shift and the fiber material. The Raman gain spectrum is typically broad, extending over several terahertz, with a peak at a frequency shift of around 13.2 THz.

Mathematical Representation

The Raman gain coefficient $g_{R}$ varies with the wavelength and fiber properties. The SRS-induced power tilt between channels can be expressed using the following relation:

$SRS tilt (dB) = 2.17 \cdot \frac{L_{eff}}{A_{eff}} \cdot \frac{\partial g_{R}}{\partial λ} \cdot P_{out} \cdot Δ λ$ Where:

$L_{eff}$ is the effective length of the fiber,
$A_{eff}$ is the effective core area of the fiber,
$P_{out}$ is the output power,
$Δ λ$ is the wavelength bandwidth of the signal.

This equation shows that the magnitude of the SRS effect depends on the effective length, core area, and wavelength separation. Higher power, larger bandwidth, and longer fibers increase the severity of SRS.

Impact of SRS in WDM Systems

In WDM systems, where multiple wavelengths are transmitted simultaneously, SRS leads to a power transfer from shorter-wavelength channels to longer-wavelength channels. The main effects of SRS in WDM systems include:

Crosstalk:
- - - - SRS causes power from higher-frequency channels to be transferred to lower-frequency channels, leading to crosstalk between WDM channels. This degrades the signal quality, particularly for channels with lower frequencies, which gain excess power, while higher-frequency channels experience a power loss.
Channel Degradation:
- - - - The unequal power distribution caused by SRS degrades the signal-to-noise ratio (SNR) of individual channels, particularly in systems with closely spaced WDM channels. This results in increased bit error rates (BER) and degraded overall system performance.
Signal Power Tilt:
- - - - SRS induces a power tilt across the WDM spectrum, with lower-wavelength channels losing power and higher-wavelength channels gaining power. This tilt can be problematic in systems where precise power levels are critical for maintaining signal integrity.

SRS in Submarine Systems

SRS plays a significant role in submarine optical communication systems, where long transmission distances and high power levels make the system more susceptible to nonlinear effects. In ultra-long-haul submarine systems, SRS-induced crosstalk can accumulate over long distances, degrading the overall system performance. To mitigate this, submarine systems often employ Raman amplification techniques, where the SRS effect is used to amplify the signal rather than degrade it.

Mitigation Techniques for SRS

Several techniques can be employed to mitigate the effects of SRS in optical communication systems:

Channel Spacing:
- - - - Increasing the spacing between WDM channels reduces the interaction between the channels, thereby reducing the impact of SRS. However, this reduces spectral efficiency and limits the number of channels that can be transmitted.
Power Optimization:
- - - - Reducing the launch power of the optical signals can limit the onset of SRS. However, this must be balanced with maintaining adequate signal power for long-distance transmission.
Raman Amplification:
- - - - SRS can be exploited in distributed Raman amplification systems, where the scattered Raman signal is used to amplify longer-wavelength channels. By carefully controlling the pump power, SRS can be harnessed to improve system performance rather than degrade it.
Gain Flattening Filters:
- - - - Gain-flattening filters can be used to equalize the power levels of WDM channels after they have been affected by SRS. These filters counteract the power tilt induced by SRS and restore the balance between channels.

Applications of SRS

Despite its negative impact on WDM systems, SRS can be exploited for certain beneficial applications, particularly in long-haul and submarine systems:

Raman Amplification:
- - - - Raman amplifiers use the SRS effect to amplify optical signals in the transmission fiber. By injecting a high-power pump signal into the fiber, the SRS process can be used to amplify the lower-wavelength signal channels, extending the reach of the system.
Signal Regeneration:
- - - - SRS can be used in all-optical regenerators, where the Raman scattering effect is used to restore the signal power and quality in long-haul systems.

Summary

Stimulated Raman Scattering (SRS) is a critical nonlinear effect in optical fiber communication, particularly in WDM and submarine systems. It results in the transfer of power from higher-frequency to lower-frequency channels, leading to crosstalk and power imbalance. While SRS can degrade system performance, it can also be harnessed for beneficial applications such as Raman amplification. Proper management of SRS is essential for optimizing the capacity and reach of modern optical communication systems, especially in ultra-long-haul and submarine networks

Stimulated Raman Scattering (SRS) is a nonlinear effect that occurs when high-power light interacts with the fiber material, transferring energy from shorter-wavelength (higher-frequency) channels to longer-wavelength (lower-frequency) channels.
SRS occurs due to the inelastic scattering of photons, which interact with the vibrational states of the fiber material, leading to energy redistribution between wavelengths.
The SRS effect results in power being transferred from higher-frequency channels to lower-frequency channels, causing signal crosstalk and potential degradation.
The efficiency of SRS depends on the Raman gain coefficient, fiber length, power levels, and wavelength spacing.
SRS can induce signal degradation in WDM systems, leading to power imbalances and increased bit error rates (BER).
In submarine systems, SRS plays a significant role in long-haul transmissions, as it accumulates over long distances, further degrading signal quality.
Techniques like increasing channel spacing, optimizing signal power, and using Raman amplification can mitigate SRS.
Raman amplification, which is based on the SRS effect, can be used beneficially to boost signals over long distances.
Gain-flattening filters are used to balance the power across wavelengths affected by SRS, improving overall system performance.
SRS is particularly significant in long-haul optical systems but can also be harnessed for signal regeneration and amplification in modern optical communication systems.

Reference

https://link.springer.com/book/10.1007/978-3-030-66541-8
Image : https://link.springer.com/book/10.1007/978-3-030-66541-8 (SRS)

Dealing with RAMAN Fiber Links

Technical 6 Mins Read

RAMAN fiber links are widely used in the telecommunications industry to transmit information over long distances. They are known for their high capacity, low attenuation, and ability to transmit signals over hundreds of kilometers. However, like any other technology, RAMAN fiber links can experience issues that require troubleshooting. In this article, we will discuss the common problems encountered in RAMAN fiber links and how to troubleshoot them effectively.

Understanding RAMAN Fiber Links

Before we delve into troubleshooting, let’s first understand what RAMAN fiber links are. A RAMAN fiber link is a type of optical fiber that uses a phenomenon called Raman scattering to amplify light signals. When a light signal is transmitted through the fiber, some of the photons interact with the atoms in the fiber, causing them to vibrate. This vibration results in the creation of new photons, which have the same wavelength as the original signal but are out of phase with it. This process amplifies the original signal, allowing it to travel further without losing strength.

Common Issues with RAMAN Fiber Links

RAMAN fiber links can experience various issues that affect their performance. These issues include:

Loss of Signal

A loss of signal occurs when the light signal transmitted through the fiber is too weak to be detected by the receiver. This can be caused by attenuation or absorption of the signal along the fiber, or by poor coupling between the fiber and the optical components.

Signal Distortion

Signal distortion occurs when the signal is altered as it travels through the fiber. This can be caused by dispersion, which is the spreading of the signal over time, or by nonlinear effects, such as self-phase modulation and cross-phase modulation.

Signal Reflection

Signal reflection occurs when some of the signal is reflected back towards the source, causing interference with the original signal. This can be caused by poor connections or mismatches between components in the fiber link.

Troubleshooting RAMAN Fiber Links

Now that we have identified the common issues with RAMAN fiber links, let’s look at how to troubleshoot them effectively.

Loss of Signal

To troubleshoot a loss of signal, first, check the power levels at the transmitter and receiver ends of the fiber link. If the power levels are too low, increase them by adjusting the output power of the transmitter or by adding amplifiers to the fiber link. If the power levels are too high, reduce them by adjusting the output power of the transmitter or by attenuating the signal with a fiber attenuator.

If the power levels are within the acceptable range but the signal is still weak, check for attenuation or absorption along the fiber link. Use an optical time-domain reflectometer (OTDR) to measure the attenuation along the fiber link. If there is a high level of attenuation at a particular point, check for breaks or bends in the fiber or for splices that may be causing the attenuation.

Signal Distortion

To troubleshoot signal distortion, first, check for dispersion along the fiber link. Dispersion can be compensated for using dispersion compensation modules, which can be inserted into the fiber link at specific points.

If the signal distortion is caused by nonlinear effects, such as self-phase modulation or cross-phase modulation, use a spectrum analyzer to measure the spectral components of the signal. If the spectral components are broadened, this indicates the presence of nonlinear effects. To reduce nonlinear effects, reduce the power levels at the transmitter or use dispersion-shifted fiber, which is designed to minimize nonlinear effects.

Signal Reflection

To troubleshoot signal reflection, first, check for mismatches or poor connections between components in the fiber link. Ensure that connectors are properly aligned and that there are no gaps between the components. Use a visual fault locator (VFL) to identify any gaps or

scratches on the connector surface that may be causing reflection. Replace or adjust any components that are causing reflection to reduce interference with the signal.

Conclusion

Troubleshooting RAMAN fiber links can be challenging, but by understanding the common issues and following the appropriate steps, you can effectively identify and resolve any problems that arise. Remember to check power levels, attenuation, dispersion, nonlinear effects, and reflection when troubleshooting RAMAN fiber links.

FAQs

What is a RAMAN fiber link?
A RAMAN fiber link is a type of optical fiber that uses Raman scattering to amplify light signals.
What causes a loss of signal in RAMAN fiber links?
A loss of signal can be caused by attenuation or absorption along the fiber or by poor coupling between components in the fiber link.
How can I troubleshoot signal distortion in RAMAN fiber links?
Signal distortion can be caused by dispersion or nonlinear effects. Use dispersion compensation modules to compensate for dispersion, and reduce power levels or use dispersion-shifted fiber to minimize nonlinear effects.
How can I troubleshoot signal reflection in RAMAN fiber links?
Signal reflection can be caused by poor connections or mismatches between components in the fiber link. Use a VFL to identify any gaps or scratches on the connector surface that may be causing reflection, and replace or adjust any components that are causing interference with the signal.
What is an OTDR?
An OTDR is an optical time-domain reflectometer used to measure the attenuation along a fiber link.
Can RAMAN fiber links transmit signals over long distances?
Yes, RAMAN fiber links are known for their ability to transmit signals over hundreds of kilometers.
How do I know if my RAMAN fiber link is experiencing signal distortion?
Signal distortion can cause the signal to be altered as it travels through the fiber. This can be identified by using a spectrum analyzer to measure the spectral components of the signal. If the spectral components are broadened, this indicates the presence of nonlinear effects.
What is the best way to reduce signal reflection in a RAMAN fiber link?
The best way to reduce signal reflection is to ensure that connectors are properly aligned and that there are no gaps between components. Use a VFL to identify any gaps or scratches on the connector surface that may be causing reflection, and replace or adjust any components that are causing interference with the signal.
How can I improve the performance of my RAMAN fiber link?
You can improve the performance of your RAMAN fiber link by regularly checking power levels, attenuation, dispersion, nonlinear effects, and reflection. Use appropriate troubleshooting techniques to identify and resolve any issues that arise.
What are the advantages of using RAMAN fiber links?
RAMAN fiber links have several advantages, including high capacity, low attenuation, and the ability to transmit signals over long distances without losing strength. They are widely used in the telecommunications industry to transmit information over large distances.

Simplifying what and why of Raman Amplifier

Technical 6 Mins Read

Background Information

The Raman amplifier is typically much more costly and has less gain than an Erbium Doped Fiber Amplifier (EDFA) amplifier. Therefore it is used only for speciality applications.
The main advantage that this amplifier has over the EDFA is that it generates very less noise and hence does not degrade span Optical to Signal Noise Ratio (OSNR) as much as the EDFA.
Its typical application is in EDFA spans where additional gain is required but the OSNR limit has been reached.
Adding a Raman amplifier might not significantly affect OSNR, but can provide up to a 20dB signal gain.
Another key attribute is the potential to amplify any fiber band, not just the C band as is the case for the EDFA. This allows for Raman amplifiers to boost signals in O, E, and S bands (for Coarse Wavelength Division Multiplexing (CWDM) amplification application).
The amplifier works on the principle of Stimulated Raman Scattering (SRS), which is a nonlinear effect.
It consists of a high-power pump laser and fiber coupler (optical circulator).
The amplification medium is the span fiber in a Distributed Type Raman Amplifier (DRA).
Distributed Feedback (DFB) laser is a narrow spectral bandwidth which is used as a safety mechanism for Raman Card. DFB sends pulse to check any back reflection that exists in the length of fiber. If no High Back Reflection (HBR) is found, Raman starts to transmit.
Generally HBR is checked in initial few kilometers of fibers to first 20 Km. If HBR is detected, Raman will not work. Some fiber activity is needed after you find the problem area via OTDR.

Common Types of Raman Amplifiers

The lumped or discrete type Raman amplifier internally contains a sufficiently long spool of fiber where the signal amplification occurs.
The DRA pump laser is connected to the fiber span in either a counter pump (reverse pump) or a co-pump (forward pump) or configuration.
The counter pump configuration is typically preferred since it does not result in excessively high signal powers at the start of the fiber span, which can result in nonlinear distortions as shown in the image.

The advantage of the co-pump configurations is that it produces less noise.

Principle

As the pump laser photons propagate in the fiber, they collide and are absorbed by fiber molecules or atoms. This excites the molecules or atoms to higher energy levels. The higher energy levels are not stable states so they quickly decay to lower intermediate energy levels that release energy as photons in any direction at lower frequencies. This is known as spontaneous Raman scattering or Stokes scattering and contributes to noise in the fiber.

Since the molecules decay to an intermediate energy vibration level, the change in energy is less than the initial received energy at the time of molecule excitation. This change in energy from excited level to intermediate level determines the photon frequency since Δ f = Δ E / h. This is referred to as the Stokes frequency shift and determines the Raman gain versus frequency curve shape and location. The energy that remains from the intermediate level to ground level is dissipated as molecular vibrations (phonons) in the fiber. Since there exists a wide range of higher energy levels, the gain curve has a broad spectral width of approximately 30 THz.

At the time of the stimulated Raman scattering, signal photons co-propagate frequency gains curve spectrum, and acquires energy from the Stokes wave, that results in signal amplification.

Theory of Raman Gain

The Raman gain curve’s FWHM width is about 6THz (48 nm) with a peak at about 13.2THz under the pump frequency. This is the useful signal amplification spectrum. Therefore, in order to amplify a signal in the 1550 nm range the pump laser frequency is required to be 13.2THz below the signal frequency at about 1452 nm.

Multiple pump lasers with side-by-side gain curves are used to widen the total Raman gain curve.

Where fp = pump frequency, THz fs = signal frequency, THz Δ f v = Raman Stokes frequency shift, THz.

Raman gain is the net signal gain distributed over the fiber’s effective length. It is a function of pump laser power, fiber effective length, and fiber area.

For fibers with a small effective area, such as in dispersion compensation fiber, Raman gain is higher. Gain is also dependent on the signal separation from the laser pump wavelength, Raman signal gain is also specified and field measured as on/off gain. This is defined as the ratio of the output signal power with the pump laser on and off. In most cases the Raman ASE noise has little effect on the measured signal value with the pump laser on. However, if there is considerable noise, which can be experienced when the measurement spectral width is large, then the noise power measured with the signal off is subtracted from the pump on signal power in order to obtain an accurate on/off gain value. The Raman on/off gain is often referred to as the Raman gain.

Noise Sources

Noise created in a DRA span consists:

Amplified Spontaneous Emissions (ASE)
Double Rayleigh Scattering (DRS)
Pump Laser Noise

ASE noise is due to photon generation by spontaneous Raman scattering.

DRS noise occurs when twice reflected signal power due to Rayleigh scattering is amplified and interferes with the original signal as crosstalk noise.

The strongest reflections occur from connectors and bad splices.

Typically DRS noise is less than ASE noise, but for multiple Raman spans it can add up. In order to reduce this interference, Ultra Polish Connectors (UPC) or Angle Polish Connectors (APC) can be used. Optical isolators can be installed after the laser diodes in orer to reduce reflections into the laser. Also, span OTDR traces can help locate high-reflective events for repair.

Counter pump DRA configuration results in better OSNR performance for signal gains of 15 dB and greater. Pump laser noise is less of a concern because it usually is quite low with RIN of better than 160 dB/Hz.

Nonlinear Kerr effects can also contribute to noise due to the high laser pump power. For fibers with low DRS noise, the Raman noise figure due to ASE is much better than the EDFA noise figure. Typically, the Raman noise figure is –2 to 0 dB, which is about 6 dB better than the EDFA noise figure.

Raman amplifier noise factor is defined as the OSNR at the input of the amplifier to the OSNR at the output of the amplifier.

Noise figure is the dB version of noise factor.

The DRA noise and signal gain is distributed over the span fiber’s effective length.

Counter pump distributed Raman amplifiers are often combined with EDFA pre-amps to extend span distances. This hybrid configuration can provide 6dB improvement in the OSNR, which can significantly extend span lengths or increase span loss budget. Counter pump DRA can also help reduce nonlinear effects and allows for channel launch power reduction.

Functional Block Diagram for CoPropagating and Counter Propagating Raman Amplifier

Field Deployment architecture of EDFA and RAMAN Amplifiers:

Interesting to know:

Related Information

Planning Fiber Optic Networks by Bob Chomycz
https://www.cisco.com/c/en/us/products/collateral/optical-networking/ons-15454-series-multiservice-provisioning-platforms/data_sheet_c78-658538.html
Technical Support & Documentation – Cisco Systems