Recording of absorption spectra by a three-beam integral technique with a tunable laser and external cavity

An integral method is considered for recording absorption using three laser beams transmitted through and reflected from an external cavity with the absorbing medium (R-ICOS). The method is the elaboration of a known single-beam ICOS method and allows suppression of the influence of radiation phase fluctuations in the resonator on recording weak absorption spectra. First of all, this reduces high-frequency instabilities and gives a possibility to record spectra during short time intervals. In this method, mirrors of the resonator may have moderate reflection coefficients. Capabilities of the method have been demonstrated by the examples of weak absorption spectra of atmospheric methane and natural gas in a spectral range around 1650 nm. With the mirrors having the reflection coefficients of 0.8–0.99, a spectrum can be recorded for 320 μs with the accuracy sufficient for detecting a background concentration of methane in atmosphere. For the acquisition time of 20 s, the absorption coefficients of ~2×10^-8 cm^-1 can be measured, which corresponds to a 40 times less molecule concentration than the background value.