IEC TR 61292-6:2010 pdf – Optical amplifiers – Part 6: Distributed Raman amplification.
• Signal non-linear effects: Since the Reman gain occurs a few tens of km within the fibre. the maximum signal power within the span is less than what would occur if a lumped amplifiar with equivalent gain were to be placed at the beginning of the span. While this reduces signal non-linear effects, these can still become an issue when the effective launch power per channel increases, thus placing a practical limit on the amount of forward Reman gain that can be used.
• Pump relative intensity noise (RIN): Typical commercial semi-conductor Raman pump lasers have RIN values of the order of -115 dB/Hz. In forward pumping configuration there Is a long walk-off length between signal and pump, which results In significant transference of the pump RIN to the signal, thus resulting in a system penalty which can accumulate along many spans. This is discussed In more detail in 6.2.4.
• Pump depletion: As the composite signal Input power increases pump depletion occurs. resultin9 in reduction of the Raman gain. For example. 650 mW of pump power configured to provide 15 dB flat gain across the C-Band for SMF fibre in the small signal regime, will only provide about 8.5 dB of gain when the composite input signal is 20 dBm. Pump depletion can also lead to large transient effects when the input signal changes abruptly (e.g. due to channel addidrop). Unlike EDFA’s where transient effect can be suppressed using electronic feed-back and feed-forward mechanisms, such effects cannot be fully suppressed in forward DRA due to the fast response time of the Raman effect and the distributed nature of the amplification.
While the backward pumping configuration does not suffer from the above disadvantages, the OSNR improvement is typically more modest since the amplification occurs in the last few tens of km of the fibre span. For example. 10 dB of Reman gain in the backward configuration will typically result in about 5 dB OSNR improvement (relative to a lumped amplifier providing the same gain at the end of the span), while Increasing the Raman gain further will only result in an additional I dB to 2 dB OSNR improvement. Further OSNR improvement (typically another 1 to 2 dB) can be achieved using complex multi-order Raman pumping schemes, which involve boosting the Reman pump energy In the transmission fibre with additional pumps at even shorter wavelengths. Thus the Reman gain occurs deeper within the span, leading to improved
Overall, the backward pumping configuration usually provides better system performance fo the same amount of Raman pump power, and is simpler to implement. Thus, in most systems backward pumped DRA is usually deployed first. and then forward pumped DRA only for those spans where backward pump DRA alone cannot supply sufficient OSNR improvement.
4.6 TypIcal performance of DRA
As we shall see In Clause 5, DRA Is most often used to provide moderate (10 dB to 15 dB) flat on-off gain in the C-Band, most often in the backward configuration, and less often in the forward configuration.
Figure 5 shows the gain for SMF In the C-Band provided by a triple pump backward DRA with pump wavelengths of 1 424 nm (two pumps) and 1 452 nm (one pump). For 10 dB gain about 450 mW of composite pump power is required, whereas for 14 dB gain 650 mW pump power Is required. The figure also shows the equivalent NF figure of the backward DRA for different gains, which is defined as the NF of an equivalent lumped amplifier (generating the same gain and same amount of ASE) placed at the end of the span (see 6.3.4 for further detail). In a hytxid EDFA1Raman system (see 53) backward DRA is used as a pre-amplifier for a conventional EDFA which provides the remaining gain required to compensate the span loss. Since the DRA has a very low effective NF. and since it acts as a pre-amplifier, it mainly determines the NF of the combined EDFA Raman amplifier.