ar X iv : c on d - m at / 9 60 71 19 v 1 1 7 Ju l 1 99 6 Reflectance Fluctuations in an Absorbing Random Waveguide

We study the statistics of the reflectance (the ratio of reflected and incident intensities) of an N -mode disordered waveguide with weak absorption γ per mean free path. Two distinct regimes are identified. The regime γN ≫ 1 shows universal fluctuations. With increasing length L of the waveguide, the variance of the reflectance changes from the value 2/15N, characteristic for universal conductance fluctuations in disordered wires, to another value 1/8N, characteristic for chaotic cavities. The weak-localization correction to the average reflectance performs a similar crossover from the value 1/3N to 1/4N . In the regime γN ≪ 1, the large-L distribution of the reflectance R becomes very wide and asymmetric, P (R) ∝ (1−R) for R ≪ 1− γN . PACS numbers: 78.20.Ci, 42.25.Bs, 05.40.+j Typeset using REVTEX