The development of a crystal modulator using a multipath reflection technique

Bougas, Vassilis (1993) The development of a crystal modulator using a multipath reflection technique. Doctoral thesis, University of North London.


An AOP lumped crystal transverse modulator has been developed, based upon a novel crystal cut, which operates at much lower driving voltages than presently available commercial state of the art modulators. With the latter, the modulation bandwidth is limited by the requirement for driving voltages in the range of 115V to 250V, which are difficult and expensive to produce using existing semiconductor technology. The reduction in driving voltage leads to an increased modulation bandwidth; low voltage wideband drivers are readily available. The modulator's novelty lies in a composite crystal scheme which allows a light beam to be reflected back and forth many times. The voltage requirement is reduced by a factor dependent on the number of times the light beam passes through the crystals. This modulator does not require a half wave plate and can therefore operate at any optical wavelength as is also the case with some of the existing designs. But in addition, unlike the existing designs, this novel modulator allows for multicolour multibeam operation. Using four AOP crystals (15.0mm length, 16.5mm width and 2.5mm thickness) a half wave voltage of 65V has been achieved after seven passes. The extinction ratios obtained were in the range of 50:1 to 100:1. A theory developed and supported by experimental results shows that the extinction ratio depends upon the angle of propagation of the light bean through the crystals. Improvements in this area are suggested. Finally, a technique for extending the operational bandwidth of any modulator is presented. The bandwidth of the multipath modulator was 70MHz.

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