Abstract :
A novel and promising technique for the direct detection of wide-band microwave-frequency-modulated light has been analyzed and demonstrated. 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 generation of a transversely modulated electron beam at the photocathode, which modulation is then detected by the circuit. Results obtained with an experimental FM phototube confirm the analysis for small modulation indices, including a spurious effect due to aperturing of the swinging light beam. Improved versions of the FM phototube involving electron multiplication should provide shot noise limited operation at VHF or microwave frequencies. The ability of the FM phototube to utilize large-index modulation is unique and may lead to superior incoherent detection systems. In particular the signal to shot noise ratio is nearly that of an ideal heterodyne receiver, and the FM phototube system inherently discriminates against background light.
