Abstract: Contaminants are detected optically at concentrations below 1 part-per-million (ppm) and extending to a level approaching 1 part-per-trillion (ppt) by using intracavity laser spectroscopy (ILS) techniques. A diode laser-pumped solid-state laser (the ILS laser) is employed as a detector. The ILS laser comprises an ion-doped crystal medium contained in a laser cavity optically pumped by a diode laser pump laser. A gas sample containing gaseous contaminant species is placed inside the laser cavity and on one side of the ion-doped crystal. The output signal from the ILS laser is detected and analyzed to identify the gaseous species (via its spectral signature). The concentration of the gaseous species can be determined from the spectral signature as well. Advantageously, the diode laser pump laser is relatively small and compact in comparison to other sources of optical pumping.