The frequency ratio of the fundamental 6940-Å radiation from a pulsed ruby optical maser and the second harmonic generated in piezoelectric crystals has been investigated by comparing the wavelengths of the first and second harmonics with known thorium spectral lines on photographs taken with a 15 000 lines-per-inch grating spectrometer. Two piezoelectric crystals, potassium dihydrogen phosphate and quartz, and one ferroelectric crystal, triglycine sulfate, were used to generate the second harmonic. In each case the generated second harmonic is twice the frequency of the primary radiation to within the limits imposed by the linewidth of the primary radiation and the resolution of the instrument. These restrictions limit the accuracy of the experiment to ±3 parts per million (ppm) for KDP, ±6 ppm for quartz, and ±4 ppm for TGS. Some spectral characteristics of high-energy pulses from the ruby optical maser as they might affect this experiment are given.
H. S. BOYNE and W. C. MARTIN, "Experimental Determination of the Frequency Ratio of Optical Harmonics," J. Opt. Soc. Am. 52, 880-884 (1962)