I V Nikolaev

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.

Fast recording of weak absorption spectra in optical cavity using tunable laser

The possibilities of new spectral technique R-ICOS are demonstrated by an example of weak absorption spectra of methane. The technique uses the combination of the intensities of laser beams transmitted through and reflected by the gas-filled resonant optical cavity. Such an approach leads to damping signal fluctuations due to mismatching of laser and cavity modes. With cavity mirrors of moderate reflectivity of 0.8–0.99, the spectra of the quality enough to measure the trace methane concentrations in atmosphere can be recorded within 320 μs. For the acquisition time of 20 s, the sensitivity to absorption is α = 2 × 10−8 cm−1 (1σ), which is 40 times better than needed for the background methane monitoring in standard atmosphere.

Measurement of the concentration of water vapor in a glow discharge plasma

The absorption of water molecules in glow discharge plasma in moist inert gases with reduced pressure is studied by diode laser spectroscopy. A non-axial scheme with an external resonator is used, which allows the determination of kinetic temperature from Doppler broadening in order to obtain the partition function of water molecules, required for estimating the concentration of particles under non-equilibrium conditions.

Optimisation of external cavity parameters of a weak absorption laser spectrometer

We consider some peculiar features of the optimisation procedure of external optical cavity parameters of a laser spectrometer, caused by the use of a three-beam measurement scheme and the presence of losses in the mirrors. It is found that the maximum sensitivity to the absorption of an intracavity medium can be achieved only at a certain choice of the value and the ratio of the reflection coefficients of the mirrors. For example, registration of the spectra of the methane impurity in the atmosphere shows that in accordance with the calculation model, for the same resonator Q-factor the use of an input mirror with a smaller reflection coefficient allows the measurement sensitivity to be increased by approximately two times.