Cavity longitudinal mode

Th efore, the sensitivity can be increased by intracavity operation of the photoacoustic cell. Such a tracegas analyzer with a resonant cell operated in the cavity of a tunable COj laser is described by Shtrikman and Slatkine The cell could be operated without any windows in the lowest-order longitudinal mode, allowing the determination of the ethylene concentration in the laboratory air as a function of time. Windowless operation leads to additional pickup of ambient acoustic noise. The authors estimate that with a closed cell and a 30 W laser, concentrations lowra-than 10" (3 10" cm ) could be measured... 

The tuning characteristics of a diode laser are not very satisfying for spectroscopic applications. The gain curve comprises mostly one to three longitudinal modes of the short cavity and it is tuned across them by the current, which modifies both temperature and the refractive index. One of these modes can be selected externally by a grating spectrograph of high transmission. 

The frequency spacing of the longitudinal modes in a resonator with plane mirrors is 3 x 10 Hz, which shows that only one mode of the cavity is excited. In this case the resonator length L has to be synchroneously tuned when the laser wavelength is tuned (see Vol. 1, Sect. 5.5). 

FIGURE 3 Two distinct longitudinal modes occupying the same spatial region of the laser optical cavity. 

Ihnable laser oscillator A laser resonator, or cavity, integrating intracavity frequency selective elements such as gratings, etalons, or prisms, or a combination of these. Usually this definition applies to laser devices capable of narrow-hnewidth, or single-longitudinal mode, emission. 

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