A calorimeter for measuring the output energy of optical masers has been made. It consists essentially of several thermistor beads connected in a bridge circuit and placed in intimate contact with a carbon cone into which the maser beam is directed. The instrument is wide band and easily calibrated. A description of the calorimeter and some results of energy measurements of a ruby optical maser are presented in this paper.
Tingye Li and S. D. Sims, "A Calorimeter for Energy Measurements of Optical Masers," Appl. Opt. 1, 325-328 (1962)