Amplified Spontaneous Emission Noise in Pulse-Pumped Raman Amplifiers

In this paper we discuss amplified spontaneous emission (ASE) generated in Raman amplifiers that are counter-pumped with trains of pulses. Our experimental and theoretical results show that if the peak power of the pump pulses is too high, the ASE output from the amplifier can be significantly higher than from a CW-pumped amplifier providing the same gain. 

Therefore, this places a lower limit on the duty cycle of pump pulses one can use for a given level of Raman gain. Furthermore, we report an additional ASE enhancement if there is insufficient walk-off between the pump pulses and co-propagating ASE to average the effects of higher frequency pump intensity noise. 

As a result, pulse-pumped Raman amplifiers can tolerate less pump intensity noise when made from .ber with a zero-dispersion wavelength located midway between the pump and signal wavelengths.