A system for detecting two or more analyte species present in a gaseous or liquid medium, the system comprising: a resonant optical cavity containing said medium and having at least three cavity mirrors, one of which is a cavity coupling mirror, the cavity having a plurality of optical resonance cavity modes; a first laser configured to emit continuous wave laser light, wherein the first laser is capable of being scanned whereby a mean optical frequency of the first laser is adjustable over a range of frequencies, and wherein the first laser is responsive to optical feedback light emerging from the cavity; a second laser configured to emit continuous wave laser light, wherein the second laser is capable of being scanned whereby a mean optical frequency of the second laser is adjustable over a range of frequencies, and wherein the second laser is responsive to optical feedback light emerging from the cavity; and, at least one detector configured to measure an intensity of the intra-cavity optical power of light circulating in the cavity and to generate a signal representing the intra-cavity optical power of light circulating in the cavity wherein the transmissivity of at least one of the cavity mirrors is selected such that the intensities of the optical feedback light impinging on the first and second lasers are below threshold intensity values so as to ensure that frequency hold interval ranges of the optical frequency of the first and second lasers are smaller than a free spectral range of the cavity, and wherein the system further comprises mode matching optics configured to couple the first laser light to the cavity via the cavity coupling mirror at an angle non perpendicular to the mirror reflective surface and couple the second laser light to the cavity via the cavity coupling mirror making the laser beam from the first laser impinging on the cavity coupling mirror and the laser beam from the second laser impinging on the cavity coupling mirror non-collinear. a resonant optical cavity containing said medium and having at least three cavity mirrors, one of which is a cavity coupling mirror, the cavity having a plurality of optical resonance cavity modes; a first laser configured to emit continuous wave laser light, wherein the first laser is capable of being scanned whereby a mean optical frequency of the first laser is adjustable over a range of frequencies, and wherein the first laser is responsive to optical feedback light emerging from the cavity; a second laser configured to emit continuous wave laser light, wherein the second laser is capable of being scanned whereby a mean optical frequency of the second laser is adjustable over a range of frequencies, and wherein the second laser is responsive to optical feedback light emerging from the cavity; and, at least one detector configured to measure an intensity of the intra-cavity optical power of light circulating in the cavity and to generate a signal representing the intra-cavity optical power of light circulating in the cavity wherein the transmissivity of at least one of the cavity mirrors is selected such that the intensities of the optical feedback light impinging on the first and second lasers are below threshold intensity values so as to ensure that frequency hold interval ranges of the optical frequency of the first and second lasers are smaller than a free spectral range of the cavity, and wherein the system further comprises mode matching optics configured to couple the first laser light to the cavity via the cavity coupling mirror at an angle non perpendicular to the mirror reflective surface and couple the second laser light to the cavity via the cavity coupling mirror making the laser beam from the first laser impinging on the cavity coupling mirror and the laser beam from the second laser impinging on the cavity coupling mirror non-collinear.