Design considerations and performance characteristics of microwave phototubes

Realization of the full communications potential of optical masers (lasers) requires the development of extremely broadband modulators and demodulators. Since general considerations for broadband light demodulators have been discussed previously, this paper presents specific design criteria and performance characteristics for traveling-wave microwave phototubes. Such a tube consists of a photoelectric cathode and a helical slow-wave circuit. Microwave-modulated light produces a current-modulated electron beam, which passes through the gun region and interacts with the helix. Hence, the tube acts like a traveling-wave tube amplifier in which the cathode is also the r-f input coupler. However, there are important differences between microwave phototubes and conventional TWT amplifiers. In particular, strong interaction can take place even at very low current levels since the beam can be almost purely current-modulated at the entrance to the helix. In addition, the output signal-to-noise ratio can be no better than its value at the cathode; hence, low-noise gun techniques cannot be used to suppress noise. The existence of several c-w lasers near one micron wavelength has stimulated the development of sensitive microwave phototubes for that spectral range. Particular processing techniques have been used to shift the spectral response of Ag-O-Cs photosurfaces toward longer wavelengths. Finally, performance data will be presented and compared with theoretical predictions.