Homodyne detection of multimode optical signals

The feasibility of coherent hoinodyne detection for multimode optical sources is experimentally investigated. By employing the self-homodyne detection, it is observed that a considerable amount of mixing can take place between the signal and local oscillator fields despite the relatively small coherence length of multimode lasers. The experiment was carried out using an a.ll-a.tmosplieric set-up, uecessitating a thorough calculation of power losses due to beam misahgnment. Power losses in the interference term for radial and angular misalignments were calculated for first order Gaussian beams. It is seen that radial misalignment is intolerable lor beam radii ratios smaller than 0.35 whereas angular misalignment becomes intolerable for angular deviations greater than a few tens of milli degrees.