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.