Since the invention of the laser, the determination of the wavelength or frequency of such a highly monochromatic light source has been a major issue. Interferometric wavelength measurement techniques run out of steam at the 10−10 level due to unavoidable geometric wavefront distortions, and absolute frequency counting schemes for optical frequencies on the order of several hundred terahertz have to be used. This is because time can be measured with by far the highest precision as compared to any other physical quantity, and counting the number of cycles in a second is as accurate as the clock that is used to determine the duration of the second. Since the value of the speed of light was defined in 1983 to be precisely c = 299,792,458 m/s in vacuum, the conversion between wavelength and frequency measurements can be done without loss in accuracy.
While it has been extremely difficult in the past to measure absolute optical frequencies, a small frequency difference or gap between two laser frequencies can be measured rather simply by superimposing the two beams on a photodetector and monitoring a beat signal. The first experiments of this kind date back to the advent of continuous-wave (CW) HeNe lasers in the early 1960s. Modern commercial fast photodiodes and microwave frequency counters make it possible to directly count frequency differences up to the order of 100 GHz.
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