An apparatus for measuring environmental parameters comprising an optical fiber-based sensor having thermally-induced diffraction gratings therein which are stable at very high temperatures for many hours. The diffraction gratings are, preferably, formed in an optical fiber by exposure to light from an infrared laser and do not degrade at high temperatures. A system for measuring an environmental parameter includes an optical fiber-based sensor, a light source, and a detector. According to a method of measuring an environmental parameter, the optical fiber-based sensor is positioned within a high-temperature environment having a parameter desired for measurement. The light source directs light into the optical fiber-based sensor. The detector measures the differential diffraction of the light output from the optical fiber-based sensor and determines a value of the environmental parameter based, at least in part, upon a known correlation between the differential diffraction and the environmental parameter.