Measurement of the absorption with a diode laser characterised by a detection threshold governed by the shot noise of its radiation

The example of detection of Cl atoms in a glow discharge was used in an investigation of the ultimate capabilities of the method of low-absorption measurement with the aid of tunable diode lasers. The excess laser noise and low-frequency fluctuations of the transmission of the optical channel were compensated by a double-beam optical system with logarithmic subtraction of the measuring and reference beam signals. Simultaneous modulation of the laser radiation wavelength and of the glow discharge current at frequencies ~1 — 1  kHz, and detection of the difference-frequency signal made it possible to reach a detection threshold limited by the laser-radiation shot noise. The limit reached in this way corresponded, for a lock-in detection time constant 1 s, to a minimum detectable absorption of 2 × 10^–7, which for a discharge 15 cm long was equivalent to a Cl concentration of 6000 atoms cm^–3 in a metastable excited state.