A standard oxide-cathode S-band traveling-wave tube has been used as an improvised microwave phototube to study the coherent light output from a ruby optical maser (laser). The laser's output consists of simultaneous, discrete optical components separated by the mode interval of 600 Mc/s between axial modes in the 12.5 cm laser rod. These components heterodyne in the TWT cathode to produce easily observed microwave outputs within the TWT bandwidth, corresponding to photobeats between third-through seventh-nearest-neighbor axial modes. This technique is a powerful tool for the study of optical masers, and also has important implications for communications via microwave-modulated light.
B. J. McMurtry and A. E. Siegman, "Photomixing Experiments with a Ruby Optical Maser and a Traveling-Wave Microwave Phototube," Appl. Opt. 1, 51-53 (1962)