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Because a light wave, together with frequency and amplitude, is also characterized by phase and polarization state, in intracavity laser spectroscopy (ICLS) it is appropriate to attempt to realize not only the effect of the accumulation of the values of the absorption of the light wave, as this was done in Ref. 1, but also the changes of phase and state of polarization of laser radiation upon placing a phase or optically active object, respectively, in the laser cavity. In addition, one assumes that numerous passages of the generated radiation through a spectral apparatus (for example, an interference device) placed in the laser cavity might improve its spectral characteristics radically. However, along the way of appreciably broadening the possibilities of ICLS it is clear one might meet serious difficulties. Among them are: 1) The absence in existing lasers of polarization coherence of the generated radiation. 2) The possibility of generating nonlinear processes in the object placed in one of the arms of an intracavity interferometer. 3) With the anticipated increase in the accuracy of spectral interferometric measurements in ICLS, there might appear those effects which existed previously (in classical spectral interferometry) but were hidden under the broad contour of interference fringes (IF). For example, the appearance of the distinction of the real shape of the absorption line from Lorenlzian in the refraction curve, which might be unsymmetric and not be described by the generally customary Selmeier equation. 4) It was not evident earlier that, when solving applied problems (because of the increased accuracy of measurements of the positions of interference fringes in the spectrum), one is able to transfer this level of accuracy to the whole complex of accompanying spectroscopic and interferometric measurements (the methods of treating of the experimental data, errors of determining ofthe associated quantities which enter the computational equations). 5)Itis not clear how one can define the linear state of polarization if radiation in the cavity at the beginning of the generation pulse, which subsequently might undergo optical rotation inside the laser in correspondence with the number ofpassages ofthe generated radiation through the optically active object. This enumeration might be extended. In addition to the indicated difficulties, others, which can not always be predicted beforehand, might be expected. As will be clear from the following, this was confirmed completely during the performance of subsequent investigations. It is necessary to note that solutions to the problem of increasing the sensitivity of optical measurements in ICLS were proposed earlier. These are works2 which used certain types of laser processing a resonant response to the modulation of the quality factor of their cavities at the frequency of the relaxation oscillations for a significant increase in the sensitivity of polarization and interference measurements. This idea might be used, for example, for investigating the optical activity induced in an object and of phase objects whose thickness might be modulated in time. In Ref.3 they increased (to a factor of 100) the sensitivity of polarization measurements as the result of placing of the object being investigated inside an anisotropic cavity of an optical quantum generator of monochromatic radiation. Physical problems arising from the realization of ideas that might lead to the establishment of phase, polarization, and interference ICLS with linear (in correspondence with the number of passages of the laser radiation through the cavity) increase in the sensitivity and precision of optical measurements go far beyond the realm of problems which arose earlier during the establishment of absorption ICLS. However, a lot of work is obviously needed to realize possibilities that might reveal themselves to researchers after the development of these new directions in ICLS. The review of work performed under the direction of the authors Optics Department of St.Petersburg University for the last ten years on the development of the physical basis of phase, polarization, and interference ICLS is contained in this exposition. For a discussion of the material, we start from the scheme of the investigations on intracavity laser spectroscopy shown in Fig. 1.
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