REFLECTANCE FLUCTUATIONS IN AN ABSORBING RANDOM WAVE-GUIDE

We study the statistics of the reflectance (the ratio of reflected and incident intensities) of an N-mode disordered waveguide with weak abs orption gamma per mean free path. Two distinct regimes are identified. The regime gamma N-2 much greater than 1 shows universal fluctuations , With increasing length L of the waveguide, the variance of the refle ctance changes from the value 2/15N(2), characteristic for universal c onductance fluctuations in disordered wires, to another value 1/8N(2), characteristic for chaotic cavities. The weak-localization correction to the average reflectance performs a similar crossover from the valu e 1/3N to 1/4N. In the regime gamma N-2 much less than 1, the large-L distribution of the reflectance R becomes very wide and asymmetric, P( R) proportional to (1-R)(-2) for R less than or equal to 1 -gamma N. ( C) 1996 American Institute of Physics.