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The method of broad-band Intracavity Laser (ICL) Spectroscopy suggested in 1970 [1] consists of quenching the laser emission at the absorption-line frequencies of the species placed in a broad-band laser cavity, the generation band width of such a laser being much greater than the half-width of spectral line studied. In this case the laser emission spectrum has sharp dips at the frequencies of absorption lines, which can be recorded with an ordinary spectroscopic instrumentation. In the intracavity laser spectroscopy a laser itself is a nonlinear detector of a weak absorption and therefore the parameters of an intracavity spectrometer are first of all determined by the mechanism of the lasing process. The most important are the resonator mechanism caused by multipass travelling of radiation during the process ofgeneration and the threshold character oflasing process. The models describing the formation of a dip in the laser generation spectrum [2-8] can be divided into two groups. The first one asumes high sensitivity of the ICL-spectrscopy to be due to the threshold mechanism for which it is supposed the laser system is in a stationary stable state and all the parameters determining the sensitivity of the intracavity absorption are constant in time [7-8]. These models predict a strong influence of the spatial inhomogeneity of the inversion decay and a sharp increase of the ICL-spectrometer sensitivity near the generation threshold. The second group of the models is based on the assumption that even cw multimode lasers never reach a stable state of generation due to its breaks at a mode caused by different perturbation mechanisms, the life time of a mode being the basic quantity which determines the ICL-spectrometer sensitivity (so called resonator mechnism). Time sweep of the laser emission spectra confirm the fact that longitudinal modes of a cw laser have a fmite life time [9]. The analysis ofFourier spectra ofsuch a laser has revealed that at high spectral power density single modes oscillate at about 100 kHz frequency which corresponds to a 10 ts lifetime of a mode [10]. The decrease of the spectral power density results in the increase of a mode's lifetime, that in turn causes the increase of the ICLspectrometer sensitivity near the generation threshold. Experiments carried out under careftilly controlled conditions showed that the depth of a dip in the intracavity spectrum increases exponentially with the increase of mode's life time.9 This convicingly shows the validity of the resonator mechanism. The resonator mechnism allows one to explain the possibility of recording absorption coefficients as low as Kmin 1010...l01 'cm' during a 0.1 s time interval. As has been shown in Ref. 1 1 detection limit of ICLspectrometer based on broadband ranning wave dye laser is determined by quantum nature of radiation. However, it is only possible if the frequency variations of the laser gain coefficient in the vicinity of an absorption line is less than Kmin. The spectral stability of the gain coefficient is, in turn, determined by the broadening mecahnism in the active medium, i.e. by the ratio ofthe width ofhomogeneous amplification contour (or frequency filter width in resonator) to an absorption line halfwidth. The simplest way of excluding the frequency variation of gain coefficient is decreasing of the numbers of resonator optical elements. In that case the resolnator scheme becomes very simple and consists of 2 mirrors and an active element. Simultaneously, such simplicity decreases the spectral range of the spectrometer, so generation band covers only 20-30% ofgain profile width. The increase ofthe spectral tunable region using dispersive resonator leads to decreasing of the spectrometer sensitivity, so a scheme of the resonator of the ICL-spectrometer is determined by concrete task of investigation. Begining from 1970 the method of ICL-spectroscopy has been extensively developing and now there are about 500 papers in this field including several reviews13'7, devoted to the analysis of sensitivity of ICL-spectroscopy to the determination of quantitative information and to use of different schemes of the spectrometers. There was comprehensive analysis of the ICL-spectrometers for the aim of obtaining quantitative information performed at the Institute of Atmospheric Optics beginning 1972 when first ruby laser ICL-spectrometer had been designed here.12 It includes: 1) design of a complex of ICL-spectrometers based on Nd-glass, ruby, dye, and color 22 center lasers; 2) determination of absorption line parameters; 3) development of efective methods for elimination of spurious selection; 4) use of dispersive resonator in ICLspewometers. Obtained results were summarized in the monograph "Intracavity laser spectroscopy. Method and application" which was published in Russian in 1985. ' This monograph is the only monograph on Intracavity laser spectroscopy in the world but scientists abroad the Russia practically have no information on it. In this paper the main materials of the monograph are presented and the last results in ICLspectroscopy obtained at the Institute of Atmospheric Optics and at others scientific centers during last 10 years are su.mmurised.
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