SATURATION CHARACTERISTICS AND SINGLE-FREQUENCY

Saturation characteristics and single-frequency operation of gas lasers 

In Chapter 11 we showed how, in some gas discharges under rather special conditions, a population inversion can be created which leads to amplification at visible wavelengths. 

By enclosing such an amplifier in one of the optical resonators whose properties were discussed in Chapter 12, oscillation at optical frequencies can be obtained. We now consider in more detail the intensity and frequency distribution of the radiation emitted by these laser oscillators. 

In lasers with inhomogeneously-broadened transitions it is found that oscillation usually occurs simultaneously on a ntunber of longitudinal cavity modes. The reasons for this behaviour are explained and we examine one method for stabilizing the intensities and inter-mode frequency differences in multi-mode operation. Next we consider several different techniques which have been used to obtain oscillation on a single longitudinal mode and this leads on naturally to a discussion of the output power versus oscillation frequency of single-frequency gas lasers. The experimental observation and theoretical interpretation of the Lamb dip is the main topic of section 13.8. 

The Lamb dip is an important manifestation of the saturation of the gain which occurs in inhomogeneously-broadened transitions when the oscillation frequency coincides with the centre of the laser line. A similar phenomenon occurs if the laser frequency is tuned close to the centre of an absorption line of a sample of atoms or molecules interacting with the standing wave field of the laser. This saturated absorption has become an important new technique in atomic and molecular spectroscopy since it removes the limit on the attainable resolution which was formerly imposed by the Doppler broadening of absorption lines. 

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