Limitation Of Optical Gain In A High Density WDM System Using Raman Amplification

Abstract

summary we have investigated the properties of Ramanamplifications in an HDWDM system. We find that the achievablegain is limited due to the cross coupling among the channels and there exists an optimum pump power to yield a maximum amplifier gain. High gain is achievable for the use as a repeater amplifierand modest gain is obtainable when used as a post transmitter ampl- ifier. Based on the results, we conclude that Raman amplifiers are SPIE Vol 1171 Fiber Laser Sources and Amplifiers (1989) / 355 G = 10 log —————————— . (4) S^O) e-«Lwhere SjfL) is the power of Channel 1 at z=L with Raman amplificat­ ion .3. DISCUSSIONThe gain of Channel 1 versus the pump power is shown in Fig. 1. Inspection of Fig. 1 reveals that there are two regions can be characterized. One is the linear gain region in which G increases linearly with P(0), the other is the saturation region where G gradually reaches a maximum value and decreases as P(0) further increases. It is clear that the achievable gain for the use as post transmitter amplifiers (S1 (0)=0.1 mW) is much smaller than those used as repeater amplifiers (S1 (0)=0.1 j-tW) . The influence of channel number N on G is shown in Fig. 2 where gj and the total signal span (N-l)Au are keeping constant. The degradation of G as N increases is expected since more power of Channel 1 is depleted. When used as repeater amplifiers, G is insensitive to the channel number N in the linear region but degrades in the saturation regio- n. In the linear region, because Channel 1 obtains much more power from the pump than that depleted by the other channels, G is nearly independent of N. However, in the saturation region the depletion by the other channels becomes comparable to that obtained from the pump, thus degradation results. Since it is essentially operating in the saturation region when used as a post transmitter amplifier, G degrades as N increases. When pump power increases, the signal power of Channel 1 depleted by the other channels may exceeds that obtained from the pump so that the gain decreases. Therefore there exists an optimum pump power which results in maximum gain of Channel 1. As shown in the figures, for the use as a repeater amplifier, the amplifier gain up to 50 dB is achievable which corresponding to 250 km repeater spacing for 0.2 dB/km fiber loss. For the use as a post transmitter amplifier, more than 20 dB gain is still obtainable, corresponding to 100 km extended transmission length, and the constraint imposed by Raman crosstalk is relaxed because the depleted power of Channel 1 by the other channels is compensated by the pump.4. CONCLUSIONIn summary we have investigated the properties of Raman amplifications in an HDWDM system. We find that the achievable gain is limited due to the cross coupling among the channels and there exists an optimum pump power to yield a maximum amplifier gain. High gain is achievable for the use as a repeater amplifier and modest gain is obtainable when used as a post transmitter ampl­ ifier. Based on the results, we conclude that Raman amplifiers are