We present a detailed study of the optical power spectrum and coherence properties of Fabry-Perot semiconductor lasers under gain-switched operation. We demonstrate that the distribution of longitudinal modes under gain-switched operation is described to high accuracy by a Gaussian envelope, in contrast to continuous wave (CW) lasers where the distribution is Lorentzian. We show that the minimum values of coherence under gain-switched operation are two orders of magnitude lower than under CW operation. We also demonstrate that intensity noise generated through interferometric conversion of mode partition noise is markedly different for gain-switched and CW lasers. The results are important for a host of potential applications that use low-coherence interferometry
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