The refractive index of a material such as silicon changes rapidly with temperature at wavelengths near the band edge. This phenomenon can be used to sense temperature if the silicon or other material is used as a Fabry-Perot etalon. As temperature changes and the index varies, the etalon's spectral reflection characteristics will be affected. Using this effect and a spectral distribution readout technique it is possible to construct an optical temperature sensor with many desirable properties. These include a wide temperature range, typically -100 to + 400°C, long-term stability, insensitivity to connector and fiber losses, tolerance of fiber bending effects and rapid response. These sensors are fabricated using techniques similar to those used to manufacture semiconductor devices. This results in low production costs and the ability to produce sensors with reproducible performance.