TEMPERATURE FIBER-OPTIC POINT SENSORS: COMMERCIAL TECHNOLOGIES AND INDUSTRIAL APPLICATIONS

Temperature fiber-optic point-sensors have been commercialized for about two decades. Among the various available optical sensing technologies, only few ones have lead to commercial successes. For instance, temperature could be measured by fluorescence decay time of a phosphorus compound excited with UV light; the higher the temperature, the faster the decay. Semiconductor band-gap thermal properties could also be used for temperature sensing. As an example for GaAs, the wavelengths transmission cut-off is increasing al linearly with temperature (~0.3 nm/°C). By analyzing with a spectrometer the reflected spectrum of a white-light source, temperature could be deduced. Interferometry-based technology such as Fabry-Perot is also a field-proven method for accurate measurement of temperature in various applications. In this case, the optical path of a Fabry-Perot cavity is changing with temperature (~20 nm/°C). Finally with fiber Bragg grating technology, a Bragg grating is written inside the fiber and reflects a given wavelength which is slightly shifting with temperature (~10 pm/°C). Several industrial applications involving temperature measurement contributed to the development of commercial optical fiber sensing technologies. Those include for instance the real-time temperature monitoring of hot spots in high-power transformers, of semiconductor plasma etchers or of microwave chemical reactors. Applications in the food industry or in the medical field are now also appearing.