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

Abstract

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.