Temporal coherence behavior of a semiconductor laser under strong optical feedback

We investigate the temporal coherence properties of a semiconductor laser (SL) under strong continuous-wave optical feedback (OF). The temporal coherence length z(c) of the 830-nm laser is computed from the inverse Fourier transform of the time-averaged emission spectrum at different feedback strengths. The case of a confocal OF configuration is considered in the experiments. For a given feedback intensity R (0 less than or equal to R less than or equal to 0.76), z(c) exhibits a hump-like dependence with increasing bias current I. The peak location of the hump shifts to lower I values with increasing R, but the functional dependence of z(c) with I is generally maintained. For a fixed I, z(c) decreases monotonically with R, indicating that feedback may be utilized to control the coherence of the laser light source. For R > 0.76, no fixed z(c) values are obtained because the spectra are very unstable. The power output of the laser is also analyzed using the Lang-Kobayashi (LK) model and the predictions are compared with experimental data. The results are utilized to establish the optimal conditions for the use of the laser as an optical signal detector. (C) 1999 Published by Elsevier Science B.V. All rights reserved.