Intensity Fluctuations of Amplified Spontaneous Emission in Bidirectional Laser Amplifiers

Abstract

Intensity fluctuations of amplified spontaneous emission (ASE) in a bidirectional laser amplifier are analysed using a noise amplifier rate equation model. In contrast to the unidirectional case, in which saturation leads to laser-like intensity statistics, we have found that the saturation, as the mechanism for coupling between the counter-propagating beams, in general leads to a non-zero asymptotic value for the intensity fluctuations. For a symmetric amplifier the normalized variance of the output intensity fluctuations declines with increasing amplifier length from the initial value of unity to a limiting value of approximately 0·277. In contrast, when the noise sources of the two counter-propagating beams have unequal mean intensities, the normalized variance of the stronger beam declines more rapidly with length to less than 0·277 while for the weaker beam it reaches a limiting value greater than 0·277. We also find that there are features of the mode competition which are closely related to those observed in a two-mode ring laser.