Fiberoptic probe for measuring high frequency temperature fluctuations in combustion gases

A fiberoptic probe for measuring high frequency gas temperature fluctuations is described. The fiberoptic probe is an optical pyrometer in which the thermal radiation is transmitted through an optical fiber. The key element in the device is a high temperature optical fiber made of single crystal aluminia (sapphire), which remains transparent and nonemitting in the optical and near infrared at temperatures up to its melting point. Thermal radiation from the metallic thin film is collected by the high temperature fiber and transmitted through the conventional optical fiber to a detector assembly. The radiation is then transmitted through a band pass filter, and the intensity is measured using a photodetector. The frequency response of the fiberoptic probe is inherently superior to that of fine wire thermocouples. The output of a thermocouple depends on the internal temperature of the wire. The capability of measuring temperature fluctuations up to 14 KJz in combustor exhaust gases is demonstrated.