Gigacycle bandwidth coherent light traveling-wave phase modulator

A continuous duty coherent light phase modulator has been constructed which exhibits a bandwidth in the kilomega-cycles. A modulation index of unity has been obtained with a modulation peak power of about 12 watts. This power level is independent of the bandwidth of the modulation. This is contrasted with known microwave amplitude light modulators which require peak modulation power in the kilowatts and exhibit bandwidths of the order of 30 Mc. The extremely wide bandwidth operation with the low modulation power is obtained by applying the modulation voltage to the electro-optic material by means of a traveling-wave structure of the transmission line type. The direction of propagation of the light and of the modulation are colinear and the transmission line is designed so that the light progresses through the crystal in synchronism with the modulation on the line. The magnitude of the angular modulation was determined by illuminating the modulator with a ruby laser and examining the individual spectral components of the output with an S-band traveling-wave microwave phototube receiver. Similar to the behavior of conventional FM systems, a minimum in the carrier at a modulation index of 3.75 was observed as well as the appearance of the appropriate sidebands. These experiments were performed at 30 Mc and 1 Gc. It appears that the bandwidth of the modulator should be several octaves greater than 1 Gc, but this was not determined experimentally for equipment reasons. The attenuation of the 1 Gc modulation through the modulator and the balun at either end was 2 db and the attenuation of the light through the electro-optic crystals was about 6 db. A good portion of this later loss undoubtedly occurred at the interfaces between crystals.