ABSTRACT
The recognition of lasers as high frequency oscillators, whose spectral purity and relative frequency stability could be extremely precise, is as old as the laser itself. In spectroscopy, advances in frequency stabilisation techniques, coupled with increased tunability of laser sources at a number of wavelengths, has made possible dramatic advances in high resolution spectroscopy. Virtually all the main applications of frequency stable optical sources require, as a pre–requisite, that the spectral output of the laser consist of a single narrow band of frequencies, which is stabilised, either in absolute or relative terms with some level of precision. The intrinsic line width is dominated by frequency fluctuations caused by the random walk of the oscillation phase under the influence of spontaneous emission, (or quantum noise). The most convenient way to analyse the frequency stability of a laser output is to downshift its frequency to the radiofrequency region, where electronic frequency measurement is possible.
