The Laser Cavity

Active Q-switching occurs when laser light access to one of the mirrors in the cavity is controlled electro-optical cell, which works on the principle of affecting the passage of polarized light (see 

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To make an oscillator from an amplifier requires, in the language of electronics, positive feedback. In lasers this is provided by the active medium being between two mirrors, both of them highly reflecting but one rather less so in order to allow some of the stimulated radiation to leak out and form the laser beam. The region bounded by the mirrors is called the laser cavity. Various mirror systems are used but that shown in Figure 9.1, consisting of... 

One of the mirrors forming the laser cavity is as close to 100% reflecting as possible (99.5%) the other is coated to allow 1% of the radiation to emerge as the laser beam. 

Usually the laser cavity consists of one mirror that is almost 100% reflecting and one mirror that is partially reflecting and partially transmitting to allow emission of some of the light as the useful output of the laser. 

Spatial Profiles. The cross sections of laser beams have certain weU-defined spatial profiles called transverse modes. The word mode in this sense should not be confused with the same word as used to discuss the spectral Hnewidth of lasers. Transverse modes represent configurations of the electromagnetic field determined by the boundary conditions in the laser cavity. A fiiU description of the transverse modes requires the use of orthogonal polynomials. 

One variation in dye laser constmction is the ring dye laser. The laser cavity is a reentrant system, so that the laser light can circulate in a closed loop. The ring stmcture provides a high degree of stabiUty and a narrow spectral width. The spectral width of a conventional dye laser on the order of 40 GH2 is narrowed to a value as small as a few MH2. Such systems offer very high resolution in spectroscopic appHcations. 

A laser medium in a tube within the laser cavity, with a mirror at each end. A pump source must excite the atoms or molecules in the laser medium so that stimulated emission (Figure 1.3(b)) can occur. Lasers are classified by the nature of the medium, important ones being ... 

The small divergence of the laser beam, which is limited only by diffraction and by optical inhomogeneities of the laser medium or other optical components in the laser cavity, has several advantages for spectroscopists ... 

The linewidth Afl of such a single mode laser is determined by the bandwidth A of the laser cavity (which is inversely proportional to its g-factor), the laser frequency v and the output power P at this frequency. 

For the measurement of small absorption coefficients or refractive indices, it is often advantageous to place the probe inside the laser cavity ). The sensitivity is then increased by a factor which depends on the quality of the Q factor of the cavity and which can be very large (about 100 or more), since quite small changes in total absorption may cause large changes in laser intensity, especially if the laser is operated close above threshold. 

The absorption measurement via observation of the total fluorescence has advantages when the probe cannot be placed inside the laser cavity. It is not necessary to employ any monochromator or spectrograph. The spectral resolution limit, which is set by the finite Doppler width of the absorbing gas and which is already far lower than the resolution of most spectographs, may be drastically reduced by using an atomic or molecular beam perpendicular to the laser beam. 

One method which employs the saturable absorption of intracavity gaseous absorbers has turned out to be strikingly successful333) As explained in the last section, the absorption profile of a gas interacting with a monochromatic standing wave inside the laser cavity exhibits a sharp minimum at the center of the unsaturated ab-... 

The last two chapters discussed spectroscopic studies which used coincidences between laser lines and transitions in other atoms or molecules. These investigations have been performed either with lasers as external light sources, or inside the laser cavity. In the latter case coupling phenomena occur between the absorbing species and the laser emission, one example of which is the saturation effect employed in Lamb dip spectroscopy and laser frequency stabilization. This chapter will deal with spectroscopic investigations of the laser medium itself and some perceptions one may obtain from it. 

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