Frequency stabilization of single

Frequency stabilization using the Lamb dip. The power output versus frequency characteristic of a single-mode laser provides one way of detecting changes in the oscillation frequency and Fig.13.15 shows how this may be used to 

Block diagram of laser frequency stabilization scheme using the Lamb dip. (After Bloom (1968).) 

The combination of moderately high operating pressures and large pressure shifts effectively rules out any self-stabilized laser oscillator as a primary standard. On the other hand, such oscillators may be used as sources to generate saturated absorption resonances in molecular systems, as discussed in section 13.9.2. Such resonances have several advantages for frequency stabilization schemes  

A large number of these accidental coincidences of molecular absorption lines with gas laser transitions are now known and some of the most important examples are listed by Hall (1973) in a review of saturated absorption spectroscopy. 

The spectral distribution and frequency stability of the output of these stabilized lasers cannot be studied by the methods of conventional high-resolution spectroscopy. Instead it is usually necessary to construct two stabilized lasers, one of which may be regarded as a local oscillator, and to investigate the frequency stability using optical heterodyne techniques. As shown in Fig.13.17 the outputs of both lasers are superimposed coherently on the surface of a square-law detector, such as a photodiode or photomultiplier. The detector output signal then contains a component corresponding to the beat note or frequency difference between the two lasers. The power spectrum of this optical heterodyne signal may be displayed directly on an r.f. spectrum analyser and its mean frequency determined pre- 

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The increased frequency stability of single-mode lasers offers new possibilities in high-resolution spectroscopy, and new techniques for measuring frequencies up to 10 Hz have enabled absolute frequency determinations of infrared laser lines. This opens the way for much more accurate measurements of the speed of light, the basic parameter involved in most spectroscopic quantities. 

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