OPTICAL FREQUENCY MEASUREMENTS

The history of the measurement of coherent optical frequencies is reviewed. The author summarizes some of the important techniques of laser frequency measurement, describes some illustrative frequency-synthesis chains, lists accurate measurements of the speed of light, and discusses laser frequency standards. Recent proposals for more efficient harmonic generation and for laser-based frequency standards are also discussed. The development of frequency measurement from the cesium frequency standard to the visible is traced. No replacement of the cesium frequency standard is imminent, but a methane-stabilized laser 'clock' has been demonstrated; it provides a less accurate, but interesting alternative to a cesium clock at a much higher frequency. Two related aspects of optical frequency measurements, the speed of light and the redefinition of the meter, are also discussed.