Maksim V Spiridonov

Methods of Reference Signal and Phase Shifts in the Multipass Laser Schemes for the Detection of Trace Gas Impurities

The application of the methods using the phase shift of modulated radiation and ICOS for the measurement of relatively low concentrations of gas impurities is considered. The measurement sensitivities of 0.01 ppbv and 6 × 10−11 cm−1 with respect to concentration and absorption coefficient, respectively, are obtained for the NO2 molecules.

Recording weak absorption spectra by the phase-shift method with deep amplitude and frequency modulation using a diode laser and a high Q cavity

A method is suggested and developed for recording absorption spectra of particles in a high Q cavity from the phase delay of amplitude-modulated radiation generated by a tunable diode laser. The method features high spectral resolution and sensitivity. It is based on the experimentally established possibility of recording spectra in conditions of high-percentage modulation, under which the concomitant frequency deviation exceeds the bandwidth. This is achieved by processing in a special way the signals in the absorption recording channel of the spectrometer and concurrently measuring the radiation frequency variation parameters in real time. The method is experimentally implemented for the case of square-pulse modulation of the laser injection current. The absorption spectrum of water molecules is recorded in the region of 644 nm with a spectral resolution of 4:5 10 3 cm 1 and a sensitivity of 7 10 11 cm 1 in terms of the absorption coefficient. Comparative measurements are taken by another well-known high-sensitivity laser technique ICOS. The broadening coefficients for some weak absorption water lines by helium are determined. Possible ways are discussed of further development of the method suggested. (Some figures may appear in colour only in the online journal)

Measurements of the atmospheric concentrations of nitrogen dioxide using the radiation absorption in a spectral range of 415 nm

The trace atmospheric concentrations of NO2 are measured with the methods of the diode laser absorption spectroscopy and the phase shift of the modulated radiation. An electronic transition in a range of 415 nm is employed. For a measurement time of 20 ms, the sensitivity of the concentration measurements is about 10−11 relative to the concentration of air particles (0.02 ppbv). At a time interval of 5–10 min, the reproducibility of the absolute concentration measurements (0.1 ppbv) is limited by the low-frequency fluctuations of the optical parameters of the spectrometer and the calibration system and corresponds to the level determined by the Allan variance.

Measurement of the concentration ratio for {sup 13}N and {sup 12}N isotopes at atmospheric pressure by carbon dioxide absorption of diode laser radiation at {approx}2 {mu}m

The ratio of 12NO2 and 13CO2 concentrations in the human exhaled air is measured by the method of diode laser spectroscopy using a three-channel optical scheme and multipass cell. Unlike the previous measurements in the spectral range of ~4.3 μm with a resolved rotational structure at low pressure of selected samples, the present measurements are performed in the range of ~2 μm, in which weaker absorption bands of CO2 reside. In this case, it is possible to employ lasers and photodetectors operating at room temperature. The thorough simulation of the spectrum with collisional broadening of lines and employment of regression analysis allow one to take measurements at atmospheric pressure with the accuracy of ~0.04%, which satisfies the requirements to medical diagnostics of ulcers.