Broadband Electro-Optic Traveling-Wave Light Modulators

A detailed analysis has been carried out of a broadband traveling-wave electro-optic light modulator using single crystals of currently available materials. The structure analyzed is that first proposed by Rigrod and Kaminow in which a light beam is reflected back and forth across a microwave transmission line with the angle between optical and microwave phase velocity vectors chosen so that the component of the optical phase velocity vector in the direction of microwave propagation is equal to the microwave phase velocity. For generality both amplitude and phase modulation are considered in single crystals belonging to three different symmetry groups, Tdand D2d(Vd), (linear electro-optic effect) and Oh(quadratic effect), for two different orientations of the modulating field. The inclusion of such practical factors as microwave and optical loss results in an optimum design in which the modulator dimensions and operating temperature are uniquely determined by the optical and microwave dielectric properties of the modulating medium. The main conclusions of the analysis are: (i) Either cuprous chloride or suitably biased strontium titanate may be used to produce 50 per cent linear modulation over a bandwidth of 10 gc with less than 5 watts of modulating power, using structures a few centimeters long that have practical manufacturing and operating tolerances. (ii) The upper modulation frequency limit is set by a cutoff frequency which arises from the finite width of the optical beam. (iii) The presence of appreciable crystalline strain in the modulating medium requires that the modulating system consist of a simple phase modulator followed by some form of frequency-selective discriminator. Such a system requires monochromatic but not necessarily coherent light.