Frequency stabilisation of a GaAlAs semiconductor diode laser to an absorption line of water vapour at 822 nm

An inexpensive GaAlAs single mode semiconductor diode laser operating in the 822 nm wavelength region is frequency stabilised at the line centre of a transition in the (2,1,1) vibration-rotation band of water vapour. A balanced photodetector is used for recording to improve the signal/noise (S/N) ratio. The laser frequency is locked at the line centre by using a simple current-feedback servocircuit. A long-term laser frequency stability of ∼4 MHz is attained, which is more than a hundred-fold improvement over the performance provided by the free running semiconductor diode laser. The corresponding Allan-variance measurement reaches σ(τ)=8.0 × 10^-12 for an integration time of 100 s.