Abstract: A sensing system for high sensitivity spectroscopic measurements is described. An optically pumped passively Q-switched laser emits laser light of a first frequency at a sensing site. Frequency-converting crystals coupled to the laser convert the laser output to light of a second frequency which illuminates the sensing site and produces a return radiation that is characteristic of the material at the site. A pump source coupled to the laser by an optical fiber pumps the laser with light of a third frequency that is efficiently propagated over the fiber. A sensor senses the radiation returning from the sensing site. The sensor is either an optical detector located at the site or an optical fiber having one end located at the site and the other end coupled to a remote optical detector.