A fluid sensor comprises a solid state laser comprising a solid state laser gain material in a laser cavity, wherein light emitted from the laser gain material is resonant in the laser cavity. The fluid sensor further comprises means for modulating light in the laser cavity to induce an acoustic or pressure wave caused by absorption of the resonant wave, and means located in the laser cavity for detecting the acoustic or pressure wave. The fluid sensor may be used for detecting a fluid such as a gas. The fluid sensor may be operable to detect very low levels of gas leaking from the surface of the earth or fugitive from a man-made object such as a processing plant. As an example, the gas may be ethane.