Abstract

Optical feedback can reduce the linewidth of a semiconductor laser by several orders of magnitude, but it can also cause line broadening (the so-called coherence collapse). While these effects are well understood, the influence of feedback on spatial coherence has received less attention. Here we propose a simple technique based on speckle analysis, to discriminate feedback effects on spatial and temporal coherence. Speckle is a granular, noisy spatial structure produced by the interference of coherent waves. The contrast of the speckles decreases when the laser is under optical feedback or current modulation [1], which cause a chaotic output. To discriminate the effects of the excitation of temporal and spatial modes, we perform experiments with an edge-emitting laser: we compare the contrast of speckle images recorded using either a multimode (MM) or a single-mode (SM) fiber and an optical diffuser (OD), and we also compare the optical spectra after light propagates in the MM or SM fiber. In the regime of coherence collapse we find that: 1) the spectra measured using the MM or SM fiber are the same and 2) the speckle contrast (SC) measured using the MM fiber is significantly lower than that measured using the SM fiber. This difference is interpreted as due to the destabilization of spatial modes which lower the spatial coherence of the laser light and reduce the SC when the MM fiber is used, but which are filtered out by the SM fiber and therefore, the SC measured with the SM fiber is not reduced.

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