Comparison of methods for determining the coherence of semiconductor laser light based on interference fringe contrast and speckle pattern morphology

Abstract

We compare the classical technique for determining the coherence length of semiconductor laser light based on the slope of the interference-fringe visibility curve against a speckle technique based on the number of connected domains in the speckle-field interference pattern as determined using the morphological Euler number. The study was performed using a Michelson interferometer where the difference in optical path length between the legs can be adjusted over a range of several centimeters. We report the results from a comparison of these techniques for determining coherence of radiation from semiconductor lasers in the red (650 nm), green (532 nm), and blue (450 nm) regions. We show that the interference-fringe visibility curves and the curves for the morphological Euler numbers as a function of the phase delay induced by the adjustable arm of the interferometer are clearly correlated. The morphological radiation-coherence determination technique based on the speckle-field interference pattern is quite fast and is simpler than the traditional interference-fringe contrast-visibility technique.