Dynamic speckle spatial coherence measurements with single-mode optical fiber couplers

Abstract

Recent experiments have demonstrated the suitability of single-mode optical fiber couplers in heterodyne laser light scattering experiments.1 The optical fiber coupler obviates the wavefront matching requirement at the photodetector in bulk optic arrangements and provides a moveable point-like optical probe of the scattered electric field. Three single-mode fiber optic couplers are utilized to measure the spatial coherence of the amplitude, phase rate, and amplitude weighted phase rate fluctuations of light scattered by polystyrene spheres suspended in water and undergoing Brownian motion. Light scattered with a wavevector k s is divided by a beam splitter and collected by two single mode fibers initially aligned with their cores superpositioned. One fiber is mounted on a piezoelectric translator so that the core spacing may be varied. The light collected by each fiber is combined with a local oscillator field, mixed at individual photodetectors, and subsequently demodulated. Coherence measurements for the two demodulated outputs are determined by a spectrum analyzer as a function of the core spacing. Amplitude coher ence measurements are in agreement with the Van Cittert—Zernike theorem for the transverse coherence length. Phase rate and amplitude weighted phase rate coherence measurements are also presented.