A device of detection of gas in trace amounts by a semiconductor laser coupled to a resonant optical cavity containing a chemical species to be analyzed. The device comprises a resonant optical cavity containing a chemical species to be analyzed; a semiconductor laser, coupled by optical feedback to the optical cavity and capable of being frequency-scanned; a means for adjusting the laser-cavity coupling rate; a means for finely adjusting the optical feedback phase; and a means for measuring the light transmitted by the cavity.