LASER-DIODE COHERENCE LENGTH VARIATION FOR BALANCING FIBER OPTIC INTERFEROMETERS

A visible light (675-nm) laser diode is used as a variable coherence length source to allow the rapid adjustment to equal optical pathlengths of a fiber interferometer. The diode's spectral width, measured for decreasing drive current, changes from less than 0.2 to 16 nm. The corresponding change in coherence length is from 2.3 mm to 28 mum, bridging a significant portion of the gap between a gas laser and a broadband source, The fiber interferometer's air paths are first adjusted with the laser at high drive current, then fine tuned as the current is reduced below threshold. An experimentally observed interferogram confirms the theoretical prediction for fringe visibility at a current near threshold.