Cavity mirror reflectivity

Observation, in the laboratory, of the a-X system of O2 represents a greater challenge because it is much weaker than the b-X system. It has been observed using CRDS, a 1.5 m cavity and O2 at atmospheric pressure. Because the transition is in the near infrared, mirror reflectivity is very high thereby increasing the sensitivity. 

The relation between the overlap of the spontaneous emission spectrum and the cavity length is illustrated in Fig. 1.8, which shows the optical mode density of a short and a long cavity. Both cavities have the same mirror reflectivities and finesse. The natural emission spectrum of the active region is shown in Fig. 1.8(c). The best overlap between the resonant optical mode and the active region emission spectrum is obtained for the shortest cavity. Thus a cavity length of 2/2 provides the largest enhancement. 

Fig. 11. Chemical laser arrangement with flash initiation. 1 laser tube. 2 100% reflecting cavity mirror, 3 coupling mirror, 4 He-Ne laser for resonator alignment, 5 to gas manifold, 6 infrared detectors, 7 monochromator, 8 diode providing a triggering signal for the oscilloscope, 9 single spectral line, 10 total emission signal6)...

Fig. 21. Schematic representation of a subsonic C02 laser with purely chemical excitation (after Cool82)). A He and Fg injectors, H CO2 and NO inlet, C construction detail shown in B, L D2 mixing array, K part of the D2 inlet system which is shown in detail in J, D sodium chloride window, E totally reflecting cavity mirror with long focal length, M, F beam-folding (plane) mirrors, O partially reflecting cavity mirror for output coupling, N laser beam, G resonator housing flushed with nitrogen...

The small amount of radiation that leaks out of the cavity is measured with an appropriate detector. The decay lifetime is affected by the presence of an absorbing species within the cavity. The high finesse, resulting from extremely well fabricated high reflective coatings on the cavity mirrors, translates into an effective path length of tens of meters. If a molecular beam expansion is performed within the cavity, the direct absorption by molecular clusters can be observed. Considerable attention has been paid to the theory associated with the use of both pulsed and cw lasers. This method, while still in its infancy, promises to be another useftil tool in the characterization of hydrogen-bonded neutral molecular clusters. 

In order to attain a high sensitivity for the determination of a, the decay times should be as long as possible i.e., the reflectivity of the cavity mirrors should be as high as possible. The uncertainty Sr is mainly caused by the noise in the decay curves. A good signal-to-noise ratio therefore increases the accuracy of a. 

Resonator—Mirrors (or reflectors) making np the laser cavity inclnding the laser rod or tube. The mirrors reflect light back and forth to bnild np ampliflcation. 

Assume a short laser pulse with input power Pq is sent through an optical resonator with two highly reflecting mirrors (reflectivities R = R2 = R, and transmission T = 1 — R — A 1, where A includes all losses of the cavity from absorption, scattering, and diffraction, except those losses introduced by the absorbing sample). The pulse will be reflected back and forth between the mirrors (Fig. 6.12), while for each round-trip a small fraction will be transmitted through the end mirror and reach the detector. The transmitted power of the first output pulse is... 

As radiation source for the PA measurements an infrared CO2 waveguide laser was used in an intracavity configuration (100 W laser power) (see Fig. 3, above). A small resonant photoacoustic cell is placed inside the laser cavity between the output mirror and the waveguide tube. To reduce the laser beam radius so that no wall effects occur in the photoacoustic cell the laser beam is focused by a ZnSe lens (f = 254 mm) on a flat output mirror (reflectivity 92.1%). To generate the photoacoustic signal a mechanical chopper is placed inside the cavity. The photoacoustic cell possesses a resonator tube (length 100 mm, diameter 6 mm)... 

The nitrogen laser has such a high gain that a laser beam can be obtained even without cavity mirrors tlnough amphfied spontaneous emission (sometimes the term super-radiant laser is used). Normally, a totally reflecting mirror is used at one end of the gain tube while the window at the opposite... 

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