Cavity modes longitudinal mode

The cavity of a laser may resonate in various ways during the process of generation of radiation. The cavity, which we can regard as a rectangular box with a square cross-section, has modes of oscillation, referred to as cavity modes, which are of two types, transverse and axial (or longitudinal). These are, respectively, normal to and along the direction of propagation of the laser radiation. 

Schematic plot of laser intensity as a function of frequency and of five equally spaced longitudinal cavity modes.

These quantities characterize a Fabry-Perot cavity, or a laser cavity. I is the cavity spacing, and 21 is the round-trip path length. The free spectral range is the wavenumber interval between successive longitudinal cavity modes. 

Fig. 4. Scan of cavity modes in the presence of a sample and sample holder. The most intense resonances are the longitudinal resonances, which are the resonances of the fundamental Gaussian beam. Radial and azimuthal modes are also present and appear as shoulders on the longitudinal resonances. Note that the higher order radial and azimuthal modes are slightly dispersive.

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. 

A pair of low-reflectivity etalons (finesse of 1.8) reduces the BRF oscillating bandwidth to that of a single longitudinal cavity mode, the thin etalon of 7.5 cm FSR or -mm physical thickness, and the thick etalon of 0.33-cm FSR and 10-mm physical thickness. Two etalons of low finesse give less loss for the selected mode than a single high-finesse etalon, due to a more complete overlap of the interfering beams (less walk-off loss). In Fig. I la the thin-etalon transmission functions for the three central orders are shown as solid lines, and the products of the BRF and thin etalon functions (the composite filter) for several more orders are shown as dashed. The... 

Mode locked laser Optical oscillator in which the longitudinal cavity modes are locked in phase, generating a periodic (i.e., pulsed) optical output. 

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