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A multipath XDP-type electro-optic crystal modulator

A design for a four-crystal compensated multipath transverse electro-optic crystal modulator of the XDP type, class 42m, is described. It is based on a novel crystal cut in which all the applied electric fields lie in the same plane (crystal thickness) thus making it possible to increase the crystal width to any required size. Placing the wide aperture crystals inside a cavity formed by two highly reflective plane mirrors, it is then possible for a laser beam, at a small angle of incidence, to propagate a number of times through the crystals, thus achieving an additive electro-optic effect. The modulator can be used with any wavelength for which the mirrors have an acceptably high reflection. Using four ADP crystals (15 mm length, 16.5 mm width, 2.5 mm thickness) in a cavity formed by two broadband dielectric mirrors with reflectance over 98% (0.4-0.7 mu m), we measured a half-wave voltage of 65 V at 633 nm for seven passes. The equivalent half-wave voltage for a 45 degrees x or y cut modulator would be 325 V. Reduction in the half-wave voltage is very important for construction of simpler, less power-consuming, inexpensive modulator drivers, particularly when driving the modulator to its frequency limits. The most serious limitation with the present design of the multipath modulator has been found to be the relatively poor extinction ratios obtained, typically 60:1 for seven passes with a 1 mW He-Ne laser, and 25:1 for five passes with a 5 mW Ar ion laser and a cavity length of 180 mm. Improvements are thought to be possible with an alternative cavity design.