IEC/TR 61292 6 Edition 1 0 2010 02 TECHNICAL REPORT Optical amplifiers – Part 6 Distributed Raman amplification IE C /T R 6 12 92 6 2 01 0( E ) ® L IC E N SE D T O M E C O N L IM IT E D R A N C H I/B[.]
General
This clause provides a brief introduction to the main concepts of Raman amplification Further information can be found IEC/TR 61292-3, ITU-T G.665, as well as in the bibliography.
Raman amplification process
Raman scattering, discovered by Sir Chandrasekhara Raman in 1928, is an inelastic scattering process where light interacts with matter molecules, resulting in a shift to a higher wavelength (lower energy) During this interaction, a light photon excites the molecules to a higher (virtual) energy state, which subsequently relaxes back to the ground state by emitting another photon along with vibration (acoustic) energy As a result, the emitted photon possesses less energy than the incident photon, leading to an increase in wavelength.
Stimulated Raman scattering describes a similar process whereby the presence of a higher wavelength photon stimulates the scattering process, i.e the absorption of the initial lower
Raman amplification in silica fibers involves the emission of a second higher wavelength photon from a wavelength photon, leading to signal amplification For instance, a ~1 550 nm signal is amplified by absorbing pump energy at ~1 450 nm Unlike doped optical fiber amplifiers (OFAs) such as EDFAs, which have a constant gain spectrum determined by dopants, Raman amplification's gain spectrum varies with the pump wavelength, achieving maximum gain at approximately 13 THz below the pump frequency.
Figure 1 – Stimulated Raman scattering process (left) and Raman gain spectrum for silica fibres (right)
A Raman amplifier fundamentally comprises a Raman pump laser, a fiber amplification medium, and a method for coupling the Raman pump with the input signal The key performance metric for a Raman amplifier is the on-off gain, which measures the output signal when the Raman pumps are active compared to when they are inactive This gain will be elaborated upon in section 6.2.1 In the small input signal regime, where pump power depletion is negligible, the on-off gain remains a critical parameter.
Raman amplifier can be approximated by eff
The on-off gain (G) is measured at 4.34 dB, where G is influenced by the Raman efficiency (C R) between the pump and signal, the coupled pump power (P), and the effective length of the fiber (L eff) concerning the Raman process.
≡ 1− − α where α p is the fibre attenuation coefficient at the pump WL
The Raman efficiency \( C_R \) is influenced by the separation between pump and signal wavelengths and their relative polarization When the polarizations are orthogonal, \( C_R \) equals zero, while maximum efficiency occurs with identical polarizations Often, a depolarized pump results in \( C_R \) being approximately half of its maximum value Additionally, as the pump and signal polarizations vary during propagation in the fiber amplification medium, \( C_R \) maintains an average value similar to that of a depolarized pump However, this variation may introduce a residual dependence on signal polarization, leading to polarization-dependent gain (PDG).
10 Raman coefficient in silica fibers
Pump - Signal wavelength difference (1/cm)
Ram an c oef fi ci ent X 1e-14 (m /W )
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Taking as an example conventional single mode fibre (SMF) and a depolarized pump with wavelength of 1 450 nm, then C R for a signal located at 1 550 nm is approximately
In the context of a long fiber with an effective length of approximately 17 km, a 500 mW pump yields about 15 dB of on-off gain, highlighting the low gain efficiency of the Raman process While utilizing highly non-linear fibers, such as Dispersion Compensating Fiber (DCF), can enhance gain efficiency, a significant fiber length of around 10 km remains necessary to achieve satisfactory gain levels.
Distributed vs lumped amplification
Optical Fiber Amplifiers (OFAs) are commonly used as discrete amplifiers, functioning within closed amplifier modules These modules are strategically positioned at discrete amplification sites at the end of each fiber span, allowing for the restoration of the transmission signal's power level that diminishes along the fiber This process is visually represented by the green curve in the accompanying diagram.
Raman amplifiers can function as discrete amplifiers, but they necessitate the use of specialized highly non-linear fiber, as illustrated in Figure 2 Their application is often restricted due to multi-path interference and other challenges, making lumped amplifiers like EDFAs more favorable in most situations.
Raman amplification can take place in any type of fibre, including the transmission fibre itself, unlike most Optical Fibre Amplifiers (OFAs) that require special doped fibres, such as Erbium doped fibre for EDFA This capability allows for distributed Raman amplification (DRA), where the transmission fibre is pumped to amplify the signal as it travels along the fibre The blue curve in Figure 2 illustrates the signal evolution for distributed amplification.
Raman amplification in a counter-propagating configuration enhances the optical signal-to-noise ratio by introducing the Raman pump power at the end of each span, allowing it to propagate against the signal This distributed Raman amplification (DRA) effectively prevents the signal from attenuating to low power levels, where noise becomes significant.
The net attenuation of the transmitted signal can be reduced, allowing for the launch of the signal into the transmission fiber with less power, which is crucial in applications sensitive to signal non-linear effects Additionally, Distributed Raman Amplification (DRA) can be implemented in a co-propagating ("forward") configuration, where the Raman pump power is introduced at the input of the span and travels alongside the signal Further details on the differences between these configurations will be explored in section 4.5.
Figure 2 – Distributed vs lumped amplification
Tailoring the Raman gain spectrum
The Raman gain spectrum's shape is influenced by the pump wavelength, with peak gain occurring around 100 nm above the pump wavelength This characteristic allows for amplification across various wavelength bands by selecting suitable pump wavelengths.
Pumps with varying wavelengths can be utilized to attain a flat broadband gain across a wide spectral range, as demonstrated in Figure 3.
Utilizing multiple pump wavelengths not only achieves flat broadband gain but also minimizes polarization dependent gain (PDG), a challenge often encountered with single pump systems This reduction in PDG can be enhanced by employing two pumps that operate at the same wavelength but with orthogonal polarization.
Figure 3 – The use of multiple pump wavelengths to achieve flat broadband gain
Forward and backward pumping configuration
DRA can be implemented in two configurations: forward (co-propagating), where the pump is introduced alongside the signal at the input, and backward (counter-propagating), where the pump is introduced at the end of the span, moving against the signal Both configurations yield the same on-off gain with a small input signal, differing only in the location along the span where amplification occurs.
NOTE Two pumps at different wavelength provide a total of 500 mW, resulting in 10 dB on-off gain across the
Figure 4 – Simulation results showing pump and signal propagation along an SMF span in forward (right plot) and backward (left plot) pumping configurations
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The primary benefit of the forward pumping configuration is that each decibel (dB) of Raman gain corresponds to an effective increase in the signal launch power by one dB, leading to a significant enhancement in performance.
OSNR system improvement However, there are a number of issues that reduce the overall effectiveness of the forward pumping configuration:
Raman gain in optical fibers occurs over a distance of several tens of kilometers, resulting in a maximum signal power that is lower than what would be achieved with a lumped amplifier at the span's start This phenomenon helps mitigate signal non-linear effects; however, as the effective launch power per channel rises, these non-linear effects can still pose challenges Consequently, there is a practical limit to the amount of forward Raman gain that can be effectively utilized.
Commercial semiconductor Raman pump lasers typically exhibit relative intensity noise (RIN) values around -115 dB/Hz In a forward pumping configuration, the extended walk-off length between the signal and pump leads to a substantial transfer of pump RIN to the signal This transfer can result in a cumulative system penalty over multiple spans, as elaborated in section 6.2.4.
As the input power of the composite signal increases, pump depletion occurs, leading to a reduction in Raman gain For instance, a pump power of 650 mW, designed to deliver a flat gain of 15 dB across the C-Band for single-mode fiber in the small signal regime, will only achieve approximately 8.5 dB of gain when the composite input signal reaches 20 dBm Additionally, pump depletion can cause significant transient effects when there are abrupt changes in the input signal.
In contrast to EDFAs, where transient effects can be mitigated through electronic feedback and feed-forward mechanisms, forward Distributed Raman Amplifiers (DRA) cannot completely eliminate these effects This limitation arises from the rapid response time of the Raman effect and the inherently distributed nature of the amplification process.
While the backward pumping configuration does not suffer from the above disadvantages, the
The improvement in Optical Signal-to-Noise Ratio (OSNR) is generally modest, as amplification occurs primarily in the final segments of the fiber span For instance, a 10 dB Raman gain in the backward configuration usually yields an OSNR enhancement of about 5 dB compared to a lumped amplifier delivering the same gain at the span's end Increasing the Raman gain beyond this point typically results in only an additional 1 dB to 2 dB OSNR improvement Further enhancements in OSNR usually require additional measures.
Complex multi-order Raman pumping schemes can achieve a gain of 2 dB by enhancing the Raman pump energy in the transmission fiber with additional pumps at shorter wavelengths This approach allows the Raman gain to occur deeper within the span, resulting in improved performance.
The backward pumping configuration typically enhances system performance with the same Raman pump power and is easier to implement Consequently, backward pumped Distributed Raman Amplifiers (DRA) are generally prioritized in deployment, while forward pumped DRA is utilized only in spans where backward pumping alone fails to provide adequate Optical Signal-to-Noise Ratio (OSNR) improvement.
Typical performance of DRA
DRA is primarily utilized in the C-Band to deliver moderate flat on-off gain ranging from 10 dB to 15 dB, predominantly in the backward configuration, with less frequent use in the forward configuration.
Figure 5 illustrates the gain for Single-Mode Fiber (SMF) in the C-Band, achieved through a triple pump backward Distributed Raman Amplifier (DRA) utilizing pump wavelengths of 1,424 nm (two pumps) and 1,452 nm (one pump) The system demonstrates a gain of approximately 10 dB.
A composite pump power of 450 mW is necessary, while 650 mW is needed for a 14 dB gain The equivalent noise figure (NF) of the backward Distributed Raman Amplifier (DRA) varies with different gains and is defined as the NF of a lumped amplifier that achieves the same gain and amount of amplified spontaneous emission (ASE) at the end of the span In a hybrid EDFA/Raman system, the backward DRA serves as a pre-amplifier for a conventional EDFA, supplying the additional gain needed to offset span loss.
The DRA exhibits a low effective noise figure (NF) and functions as a pre-amplifier, significantly influencing the overall NF of the combined EDFA-Raman amplifier With a typical EDFA NF of approximately 5 dB, the composite NF of the EDFA/Raman amplifier can be effectively determined.
Mecon Limited, located in Ranchi and Bangalore, is licensed for internal use of materials supplied by Book Supply Bureau In a scenario with 10 dB on-off Raman gain, the system achieves approximately 0 dB, leading to a 5 dB improvement in Optical Signal-to-Noise Ratio (OSNR) compared to a similar Erbium-Doped Fiber Amplifier (EDFA) setup.
NOTE The various curves correspond to different composite pump powers
Figure 5 – On-off gain and equivalent NF for SMF using a dual pump backward DRA with pumps at 1 424 nm and 1 452 nm
5 Applications of distributed Raman amplification
General
DRA offers two unique advantages compared to conventional amplifiers such as EDFAs:
The improved system OSNR and the capability to deliver flat gain across multiple transmission bands are significant advantages of DRA However, these benefits are counterbalanced by the high costs associated with DRA, primarily due to the substantial pump power required and various operational challenges Consequently, DRA is typically employed only in applications where it provides a notable advantage or when no other viable alternatives exist These specific applications will be explored in this section.
All-Raman systems
All-Raman systems are systems which utilize only Raman amplification, both DRA and lumped
Raman amplifiers By using only Raman amplification, such systems benefit from the inherent
OSNR improvement provided by DRA, and can be operated in wavelength ranges for which it is impossible or impractical to provide amplification with more common technologies such as
Raman systems, especially those operating in the L-Band, utilize EDFA technology, which is less efficient than in the C-Band However, L-Band systems enable longer transmission distances when paired with Non-zero dispersion shifted transmission fibre (NZDSF), making them ideal for ultra-long haul optical links exceeding 1500 km.
A typical configuration of an all-Raman amplification site is shown in Figure 6 The configuration comprises three Raman pump modules, one for backward DRA, one for forward
In a typical system, lumped Raman amplification is implemented within the DCF fiber, with an amplification site positioned every 80 km to deliver approximately 20 dB of net gain This is accomplished by utilizing both forward and backward DRA to achieve around 20 dB gain, while the DCF is pumped to ensure its net gain is zero, effectively compensating for the DCF's insertion loss, which is typically around 10 dB.
Raman gain and NF Vs wavelegth
MECON LIMITED is licensed for internal use in Ranchi and Bangalore, with materials supplied by the Book Supply Bureau The DCF exhibits relatively high Raman efficiency, which is attributed to its small effective area, allowing for a reduced amount of pump power needed for effective operation.
All-Raman systems are not only expensive but also challenging to upgrade for compatibility with reconfigurable optical add-drop multiplexers (ROADMs), which are essential for contemporary optical networks This difficulty arises from two main factors.
To compensate for the increased insertion loss of ROADM modules, additional lumped amplification is necessary One approach to achieve this is by increasing the pump power to the DCF, although this may result in higher multi-path interference (MPI) due to double Rayleigh backscattering Alternatively, employing a separate lumped amplification method can also be considered.
Raman amplifier, which further adds to the overall cost of the system
• Secondly, the transients resulting from system reconfiguration are difficult to suppress, especially in the case of forward DRA
For these reasons, the application of all-Raman systems is mainly limited to ultra-long haul point to point (i.e non-reconfigurable) optical links
Figure 6 – Typical configuration of an amplification site in an all-Raman system
Hybrid EDFA Raman systems
Long repeaterless links
Long repeaterless links, exceeding 150 km, are essential for various applications, including connecting islands and oil rigs, navigating through hostile or inaccessible terrains, and establishing connections in areas where repeater sites present security or logistical challenges.
Forward DRA Raman pump Mux Mux
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Utilizing backward Distributed Raman Amplification (DRA) can enhance the system's Optical Signal-to-Noise Ratio (OSNR) by 5 dB to 7 dB, influenced by the pump power For instance, a 700 mW Raman pump module, designed to deliver around 15 dB of on-off Raman gain across the C-Band, can achieve an OSNR improvement of about 6 dB, depending on the type of transmission fiber This enhancement enables an extension of the link reach by approximately 30 km.
To enhance longer links, both forward and backward Distributed Raman Amplification (DRA) can be utilized For instance, in a system equipped with a 20 dBm EDFA booster, incorporating a 700 mW forward DRA pump module can yield approximately 8.5 dB of Raman on-off gain, which translates to an OSNR improvement of around 7 dB, factoring in the insertion loss of the Raman pump module.
Utilizing forward and backward DRA with moderate pump power levels, such as 700 mW, can enhance the reach of repeaterless links by as much as 13 dB when compared to systems that rely solely on EDFA technology.
Long span masking in multi-span links
Multi-span links often utilize in-line EDFA repeaters every 80 km to 100 km; however, geographical constraints or practical considerations may necessitate longer spans In such scenarios, Distributed Raman Amplification (DRA) can enhance the Optical Signal-to-Noise Ratio (OSNR) margins needed for these extended spans Additionally, many systems are designed with in-line EDFAs that have a limited gain range while maintaining flat gain By improving OSNR, DRA enables the support of longer spans without compromising the standard EDFA used in the system, thereby enhancing overall system flexibility and utility.
Multi-span links, unlike static repeaterless links, are typically dynamic and necessitate ROADM functionality This inherent nature can lead to transient events that are challenging to manage when employing forward DRA Consequently, forward DRA is rarely utilized in these scenarios, making backward DRA the more prevalent choice.
High capacity long haul and ultra-long haul systems
In high-capacity systems, the Optical Signal-to-Noise Ratio (OSNR) becomes crucial as the number of spans increases Implementing backward Distributed Raman Amplification (DRA) in each span can significantly enhance OSNR, enabling the system to accommodate more spans and higher capacities For instance, incorporating 10 dB of backward DRA per span can lead to substantial improvements in performance.
(approximately 500 mW pump power), the system OSNR can be improved by about 5 dB compared to an equivalent EDFA only system, allowing a 3-fold increase in the reach of the system
6 Performance characteristics and test methods
General
This clause describes important performance parameters relevant to DRA, and considers tests methods for these parameters As discussed previously, a fundamental difference between
The performance of Distributed Raman Amplifiers (DRA) is influenced by the transmission fiber, necessitating a system-level characterization of amplifier performance rather than a device-level assessment Certain performance parameters specific to the Raman pump module can be independently specified and measured, while system-dependent parameters can be averaged across various types of transmission fiber to predict expected system performance This article will first explore the device-level characteristics before addressing the system-level performance.
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Performance of the Raman pump module
Pump wavelengths
The pump power spectrum from the Raman pump module's output is essential for determining the on-off Raman gain spectrum of signals in the connected fiber span This spectrum typically features multiple discrete wavelengths, which may arise from various pump sources By connecting the pump output to an Optical Spectrum Analyzer (OSA), often through an attenuator due to high power levels, one can obtain a list of wavelengths corresponding to the spectrum's peaks Additionally, the OSA can measure the width of each peak, commonly represented as Full Width at Half Maximum (FWHM) For most 14xx nm Fiber Bragg Grating (FBG) stabilized pump laser diodes available today, the FWHM ranges from 1 nm to 2 nm.
Pump output power
The pump power emitted from the Raman pump module at various wavelengths is a crucial factor influencing the on-off Raman gain spectrum Typically, in Raman pump modules, the power of each individual pump can be adjusted, allowing for modifications in the total output power and its distribution across different wavelengths The output power spectrum under specific operating conditions can be assessed by linking the pump output port to an Optical Spectrum Analyzer (OSA).
Raman pump module Fibre span
This article discusses the licensed use of a pump module by MECON LIMITED in Ranchi/Bangalore, specifically for internal purposes It highlights the impact of high pump power on the performance of an attenuator, leading to a compilation of pump powers linked to each specific pump wavelength Additionally, it mentions the capability to measure the total output power across all wavelengths by utilizing a high-power optical detector connected to the pump output port.
Pump degree-of-polarization (DOP)
The Degree of Polarization (DOP) of each pump wavelength influences the Polarization Dependent Gain (PDG) of the Distributed Raman Amplifier (DRA) The specific impact of a pump wavelength's DOP varies based on the DRA configuration (counter- or co-propagating), the characteristics and state of the transmission fiber, and the relative power and DOP of other pump wavelengths.
To determine the Degree of Polarization (DOP) for a specific pump wavelength, it is essential to activate only the relevant pumps within the pump module The Pump out port of the module must be linked to a rotating polarization analyzer Subsequently, the maximum and minimum power outputs (P max and P min) from the analyzer are measured, allowing for the calculation of the DOP.
DOP≡ − + Thus, a DOP of 100 % corresponds to a fully polarization pump wavelength, and a DOP of 0 % corresponds to a fully depolarized pump signal.
Pump relative intensity noise (RIN)
The Relative Intensity Noise (RIN) of a pump laser quantifies the intensity fluctuations of its output, measured in dB/Hz Within a specific bandwidth \( B \), the relative variance of the intensity fluctuation of the laser power is expressed as \( \sigma^2 / P^2 \equiv RIN \times B \) Additionally, since the Raman gain is directly proportional to the pump intensity, any fluctuations in intensity can affect the signal.
The system effect of the Raman pump relative intensity noise (RIN) is influenced by the RIN magnitude and the walk-off between the signal and pump, which affects the bandwidth of fluctuations transferred from the pump to the signal For standard 14xx nm pump laser diodes available today, the RIN value is around –115 dB/Hz The bandwidth of the transferred RIN fluctuations is determined by the relative group velocity (\(\Delta v\)) between the signal and pump, expressed as \(B \sim \frac{c^2}{L_{\text{eff}} \Delta v}\), where \(L_{\text{eff}}\) is the Raman effective length and \(c\) is the speed of light.
In counter-propagating Distributed Raman Amplifiers (DRA), the walk-off is rapid, approximately equal to the speed of light (\$Δv \sim c\$), resulting in a minimal impact from pump Relative Intensity Noise (RIN) Conversely, in co-propagating DRA, the walk-off between the signal and pump can vary based on fiber dispersion, making the influence of Raman pump RIN potentially significant.
The RIN of a pump laser in a Raman pump module can be measured by activating the laser and connecting the module's pump output port to a fast detector with a bandwidth greater than 100 MHz, followed by an electrical spectrum analyzer (ESA).
Insertion loss
The Raman pump module operates passively concerning the signal transmission, as the Distributed Raman Amplification (DRA) occurs within the transmission fiber A key performance metric of the module is the insertion loss, which affects the signal's overall noise performance In counter-propagating DRA, the insertion loss can be modeled and is illustrated in Figure 8 Conversely, in co-propagating DRA, the Raman pump module is typically positioned after a booster amplifier, meaning that the module's insertion loss directly impacts the signal launch power into the fiber span.
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The on-off gain (G on-off) and effective noise figure (F eff) of the Distributed Raman Amplifier (DRA) are crucial parameters, while G and F represent the gain and noise figure (NF) of a lumped amplifier that usually follows the Raman pump module.
Figure 8 – Model for signal insertion loss (IL) of a Raman pump module used for counter-propagating DRA
Out-of-band insertion loss refers to the insertion loss of wavelengths outside the designated transmission band, which is crucial for system performance For instance, an optical supervisory channel (OSC) typically operates at 1510 nm and is added and dropped at each repeater location The insertion loss encountered by the OSC within the Raman pump module significantly affects the OSC link budget.
In certain instances, the Optical Signal Conditioner (OSC) may be either omitted or included within the Raman pump module Therefore, it is essential to measure the OSC insertion loss, along with any other pertinent out-of-band insertion losses, between the appropriate ports of the pump module, rather than solely between the signal-in and signal-out ports.
The insertion loss of the Raman pump module may be measured in the same manner as for other types of OAs, as described in IEC 61290-7-1.
Other passive characteristics
The Raman pump module functions as a passive component regarding signal transmission between its input and output ports Consequently, it is essential to evaluate various performance characteristics typical of passive modules, including polarization mode dispersion (PMD) and reflectance.
IEC 61291-1, together with the relevant test methods.
System level performance
On-off signal gain
The primary performance metric of a Raman pump module is the on-off signal gain, which varies with different operating conditions, including pump power output and the type of transmission fiber used On-off signal gain is calculated by first measuring the signal level \( S_{\text{off}} \) at the output of the transmission fiber when the Raman pump module is off Next, the signal level \( S_{\text{on}} \) is measured at the same point when the module is activated under specified operating conditions The on-off gain is then expressed as \( G_{\text{on-off}} \) (dB) = \( S_{\text{on}} \) (dB) - \( S_{\text{off}} \) (dB).
Figure 9 illustrates the typical configurations for measuring on-off gain, with (a) representing co-propagating DRA and (b) depicting counter-propagating DRA The signal source can deliver either a single wavelength or multiple multiplexed wavelengths Utilizing an Optical Spectrum Analyzer (OSA) enables the measurement of signal powers (S on and S off) at each wavelength, facilitating simultaneous analysis.
Effective lumped amplifier for DRA
Pump module Lump amplifier following DRA
This article discusses the measurement of on-off gain at various wavelengths using a multi-wavelength signal source It highlights the capability of the Optical Spectrum Analyzer (OSA) to separate signal power from continuous signals, ensuring precise analysis for internal use at MECON Limited in Ranchi and Bangalore.
The ASE spectrum produced by the DRA allows for the measurement of the actual signal gain In cases where the signal is very weak, it may be necessary to subtract the ASE present within the signal wavelength This can be achieved by interpolating the ASE levels on either side of the signal.
Figure 9 – Typical configuration used to measure on of gain (a) for co-propagating DRA and (b) for counter-propagating DRA
A number of issues should be considered when measuring on-off gain:
The goal of on-off gain measurement is to determine the achievable gain for a specific type of transmission fiber To accurately assess this, it is important to measure the on-off gain when the fiber length significantly exceeds the Raman effective length For most commercially available transmission fibers, a length greater than 75 km is typically adequate to simulate the infinite fiber limit.
Excess loss between the pump module and the fibre span significantly affects the on-off gain by reducing the available pump power, thereby diminishing the overall gain To accurately measure the typical on-off gain for a specific fibre type, it is crucial to minimize this excess loss Additionally, when comparing typical results to actual measured on-off gain in the field, it is important to consider any excess loss accordingly.
In counter-propagating Distributed Raman Amplifiers (DRA), the signal power at the end of the span is typically weak, leading to the on-off gain being treated as small signal gain, which is not significantly influenced by the signal level Therefore, it is crucial that the input signal source is not overly strong to prevent pump depletion As a guideline, the input signal strength should ensure that the total output power with the Raman pumps activated is at least 20 dB lower than the Raman pump power at the output of the pump module.
In co-propagating Distributed Raman Amplifiers (DRA), high signal levels at the input can lead to pump depletion Consequently, the on-off gain for each wavelength is influenced by the total input power across all wavelengths, necessitating proper characterization.
Gain flatness
Gain flatness refers to the consistency of the on-off signal gain across a specific transmission band, such as the C-Band (1,529 nm to 1,564 nm) It is quantified by the difference between the maximum and minimum on-off gain at various signal wavelengths within that band, as previously outlined.
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The configuration of pump wavelengths and their respective power levels significantly affects gain flatness Many Raman pump modules offer preset power configurations to optimize gain flatness for various types of transmission fiber.
Polarization dependant gain (PDG)
PDG quantifies the fluctuation of on-off signal gain at a specific wavelength based on signal input polarization, defined as the difference between the maximum and minimum on-off gain across all polarization states It can be measured using a setup that incorporates a polarization controller in the signal source, allowing for the assessment of the difference between the maximum and minimum values of S on across various configurations of the polarization controller.
The Polarization Dependent Gain (PDG) is influenced by the arrangement of pump wavelengths, the Degree of Polarization (DOP) for each wavelength, and the polarization traits of the transmission fiber An increase in the number of wavelengths leads to a reduction in PDG, even if some wavelengths remain depolarized.
To accurately measure the Polarization Dependent Gain (PDG), it is essential to conduct the measurement over an extended duration, such as 24 hours This approach allows for the consideration of environmental changes that may impact the transmission fiber.
Equivalent noise figure
The equivalent noise figure is a crucial performance metric specifically for counter-propagating Distributed Raman Amplifiers (DRA), as it measures the noise performance of the DRA This figure primarily pertains to the signal-spontaneous noise factor, highlighting its significance in evaluating the amplifier's efficiency.
IEC 61290-3 pertains to a model lumped amplifier, illustrated in Figure 8, which exhibits identical on-off gain to the Distributed Raman Amplifier (DRA) and produces an equivalent amount of Amplified Spontaneous Emission (ASE) at the output of the fiber span, serving as the input to the Raman pump module.
The Raman ASE power density for a single polarization at the signal wavelength, denoted as ρASE, is defined at the output of the model lumped amplifier, which serves as the input to the Raman pump module According to IEC 61290-3, the equivalent noise figure is expressed as \( F_{eq} = \frac{2ρ_{ASE}}{(G_{on} - G_{off}) h ν} \), where \( h \) represents Planck's constant.
Planck’s constant and ν=c/λ is the signal frequency
To measure the amplified spontaneous emission (ASE) power density (\( \rho_{ASE} \)), an optical spectrum analyzer (OSA) can be employed as outlined in IEC 61290-3-1 Typically, the OSA is positioned after the Raman pump module, necessitating the consideration of the pump module's insertion loss in the measurement, as \( \rho_{ASE} \) is defined at the input to the pump module Alternatively, a portion of the power can be extracted at the input to the pump module and directed to the OSA, requiring an appropriate calibration factor for accurate measurement.
The typical equivalent noise figure of a counter-propagating DRA with an on-off gain of 10 dB is approximately -1 dB, significantly lower than the typical value of around 5 dB.
EDFA To translate this into OSNR system improvement, it is necessary to account for addition supplementary lumped amplification which may be required, as shown in Figure 8 (see also
Multi-path interference (MPI)
MPI in DRA is caused by double Rayleigh backscattering (DRB) which is amplified due to
Raman gain occurs when a portion of the signal is Rayleigh backscattered, traveling back to the source before being backscattered again This process generates a replica of the original signal that propagates in the same direction, leading to the phenomenon known as MPI.
In a typical transmission fibre without DRA, the level of DRB is very low (