A sensitive fluid sensor for detecting fluids and particularly trace fluids. The sensor may be adjustable for detecting fluids of various absorption lines. To effect such adjustment, a tunable laser may be used. The laser may be an edge emitting diode, a VCSEL or other tunable source. The detection apparatus of the sensor may incorporate a sample cell through which a laser light may go through. The sample cell may comprise a tunable ring-down cavity. The ring-down cavity may be a ring laser cavity like that of a ring laser gyroscope. There may be a photo detector proximate to the ring down cavity connected to a processor.