A linear optical modulator with high FM sensitivity.
By using KDP inside a short laser cavity, a large linear frequency deviation has been achieved with a moderate modulation voltage, i.e., +/-44 Mc/s for +/-110 V. The modulation bandwidth is to 10 Mc/s, the modulation power is 2.5 W, and the optical output power is 0.4 mW (single mode, single frequency) at 6328 A. The piezoelectric resonances which occur at modulation frequencies in the 80 kc/s to 2 Mc/s range have been nearly eliminated by clamping the KDP in a rigid, but highly damped, mount. As a result, the change in FM sensitivity at the fundamental resonance, at 83 kc/s is only +/-5%. Noise loading tests, which simulate 2400 telephone channels, indicate that the FM distortion is negligible. Square wave tests verify that the distortion is indeed small by showing that the input pulses are reproduced faithfully at the output.
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Crawford Hill Laboratory, Bell Telephone Laboratories, Inc., Holmdel, New Jersey 07733, USA.