With 11 FiguresThe advent of dye lasers that are not only widely tunable but also highly monochromatic (H~nsch 1972) has had a profound impact on many areas of optical spectroscopy. However, the power and potential of tunable laser sources cannot be fully exploited unless the laser wavelength is accurately known as it is tuned. Unfortunately, conventional methods for measuring optical wavelengths tend to be either inadequate in their resolution and accuracy or else cumbersome and slow. Laser spectroscopists and other scientists soon recognized the need for new measuring instruments comparable in utility to the frequency counters commonly used in the microwave region. Research efforts directed at the development of such instruments, which became known generically as "laser wavemeters", began in the mid 1970s and rapidly expanded as dye lasers and, more recently, other tunable laser sources became available commercially. In this chapter we attempt to review the more successful laser wavemeters developed to date. It should be noted, however, that many problems related to laser wavelength metrology remain to be solved, and that wavemeter technology continues to develop and improve.An ideal wavemeter should be able to determine the exact laser wavelength quickly, conveniently, and accurately at any time. By "quickly" we mean that the measured wavelength should be available in a time short enough to permit the user to interact with the experiment. Optimally, the measurement and processing time for the wavemeter should be shorter than the human reaction time of a few hundredths of a second in order to facilitate operations such as tuning the laser to a particular wavelength. By "conveniently" we mean that the wavemeter should be reasonably compact and easy to acquire (by either building or purchasing) and to use. "Accurate" means different things depending on the laser and on the application, but in practice generally implies a measurement uncertainty below 10 -6 of the optical frequency (about 500 MHz for visible light).Most approaches to wavelength metrology are based either on comparison with a reference spectrum, on a spectrograph, on an interferometer, or on direct frequency measurement. We shall consider each of these approaches separately, but with emphasis on the interferometric techniques. These seem to provide the most likely candidates for practical wavemeters for the nearterm future, although the long-term prospects for wavemeters based on direct frequency measurements are promising.
