A transverse wave phototube for detection of microwave-frequency-modulated light

This paper describes the theoretical and experimental investigation of a novel means of detecting microwave-frequency-modulated light signals. The system consists of an optical dispersing element followed by a photocathode and transverse-field interaction circuit. The disperser causes the light beam to swing back and forth in accordance with the instantaneous optical frequency, resulting in the generation of a transversely modulated electron beam at the photocathode, which is then detected by the circuit. Results obtained with an experimental FM phototube system confirm the analyses of the optical and electron beam mechanisms for a small modulation index. Improved versions of the FM phototube are suggested, and it is shown that the approach may represent a superior means of directly detecting optical communications or reconnaissance signals as modulation techniques improve. In particular, the FM phototube can in principle utilize high-index modulation to increase the signal-to-shot noise ratio to a value only three dB below that of an ideal heterodyne detector, while inherently discriminating against background light.