We propose a method that allows one to carry out a remote estimation of small concentrations of gases dissolved in water at large depths. The method is based on modulation laser spectroscopy combined with the application of a fiber-optical communication line. We demonstrate specific features and advantages of the method of modulation laser spectroscopy for the spectral analysis in aqueous media with large attenuation. We develop the structural scheme of a measuring system, whose main components are a one-mode fiber-optical communication line, a laser diode with tuning frequency, and a gauge in the form of a hollow ellipsoidal mirror reflector. The principle of self-focusing of radiation in the gauge is considered. The expedience of the use of modulation laser spectroscopy and a measuring system in the solution of problems of the analysis of aqueous media is evaluated.

Simultaneous multi-wave probing by using the differential absorption method could help to unveil fast chemical processes (for example -photo-stimulated) in atmosphere. The scheme of multi-wave finely tuned laser source, suitable for simultaneous probing a few atmosphere constituents and/or chemicals with the above mentioned method, is suggested and tested.

To achieve the maximum efficiency in single-frequency lasing mode of laser diode with external cavity, it is necessary to minimize losses in the optical system for the output beam and to provide the optimal frequency-selective feedback. In this paper, we have investigated the scheme of an external cavity diode laser (ECDL) based on phase volume holographic grating (VHG). Angular and spectral selectivities of the holographic grating allow to adjust the optical feedback with low losses in the cavity, and it can be used for the frequency narrowing of diode-array bars. Here, the optimal parameters of VHG and the temperature dependence of the diode laser bar spectrum have been studied, and the proposals on VHG design have been developed.