High frequency Fabry-Perot phase modulator

We have constructed an efficient high frequency phase modulator using a KDP crystal modulator in a Fabry-Perot interferometer. We present the theoretical analysis for the performance of such a modulator and compare this with experimental results. We have obtained a modulation index of 0.6 at 2.7 GHz with 0.3 W of microwave power. This implies that the use of a Fabry-Perot cavity can reduce the amount of drive power needed to achieve a given modulation index by 3 orders of magnitude or more. Such a device is particularly useful in the ultraviolet region of the spectrum where one must use relatively inefficient modulator crystals.