Speckle interferometry for spectral analysis of laser sources and multimode optical waveguides

Speckle phenomena in optical waveguides known as modal noise are useful for measuring the spectral width of laser sources down to 1 MHz or the bandwidth of multimode fibers up to 1 THz. This measurement is based on a theory relating the frequency correlation function of a speckle pattern to the light source spectrum and to the power transfer function of general waveguides. We demonstrate the feasibility of the method by investigating laser lines with a width of 300 MHz and 5 GHz, and by studying the transmission characteristics of short fiber pieces with bandwidths up to 37 GHz.