Systems and methods for measuring the isotope ratio of one or more trace gases and/or components of gas mixtures such as different gas species present in a gas mixture. The system includes a resonant optical cavity having two or more mirrors and containing a gas, the cavity having a free spectral range that equals the difference between frequencies of two measured absorption lines of different gas species in the gas, or of two different isotopes, divided onto an integer number. The system includes a continuous-wave tunable laser optically coupled with the resonant optical cavity and a detector system for measuring an absorption of laser light by the gas in the cavity. The detector system includes a photo-detector to measure an intensity of the intra-cavity light, or both a photo-acoustic sensor to measure photo-acoustic waves generated in the cavity and a photo-detector to measure an intensity of the intra-cavity light.