Simple Method to Measure the Coherence Time of a Mode-Locked Laser

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Abstract
A simple method is described to measure the coherence time of a mode-locked pulse train. The temporal coherence function is obtained from an interference pattern produced by crossing the two laser pulses at a linear photodiode array (PDA) detector. Two wave-fronts generated from the same or from successive pulses are tilted with respect to each other and intersect the PDA at a small angle; the “visibility” of the resulting fringe pattern is then plotted as a function of the temporal offset between the interfering pulses. The coherence time of a mode-locked Ar+ laser (225 ps) and that of a synchronously pumped mode-locked dye laser (5.0 ps) have been measured.