Study on the spectrum and far-field pattern of broad-area semiconductor lasers toward speckle suppression by optical feedback

Laser diodes working at 940 nm are potential in video surveillance systems, but the speckle problem needs to be solved. Both the spectrum and far-field pattern of the broad-area semiconductor lasers (BALs) are important for speckle suppression of multimode fibers, but the relation between them has not been studied. In this paper, the time-averaged spectrum and far-field pattern of the BALs under optical feedback are studied in experiments and simulations. Effective spectral bandwidth and spot fill factor are introduced to evaluate the effective temporal coherence of the BALs and the relative numerical aperture of the coupled light. In-phase change between the effective spectral bandwidth and spot fill factor is observed under different feedback intensities, due to the spatial separation of the longitudinal modes. Low speckle contrast is obtained under 10% to ∼20% optical feedback, while stronger feedback may reduce the speckle suppression efficiency. A spectral bandwidth of 15.3 nm is observed under 10% feedback, but the spectral hole-burning is serious. The spectral hole-burning is partly suppressed by tuning the length of the external cavity. What we believe to be a novel low frequency fluctuation is observed in BALs at a high pump current, which may have some relation with the spatial bistability of the lateral modes. The spatiotemporal dynamic of BALs is simulated by a multimode model of BALs, and the spatial separation of the longitudinal modes is demonstrated. This work provides insight into the spectral and spatial characteristics of the BALs under optical feedback.