Lasers. Characterization of Laser Diodes by Measuring the Speckle Contrast at the End of a Multimode Fiber

Summary A novel and simple method is described to characterize semiconductor lasers which may produce excessive modal noise in a transmission system. The method is based on an analysis of the speckle contrast at the end of a suitable length of multimode fiber. An optimal length of measuring fiber is calculated and verified experimentally. The qualitative dependence of speckle contrast on fiber length is shown. The measured speckle contrast allows to define an “effective” number of laser spectral lines accounting for the modal noise.