Unambiguous Peak Identification of a Silicon Fabry-Perot Temperature Sensor Assisted with an In-Line Fiber Bragg Grating

Wavelength interrogation has been widely used for fiber-optic sensors based on Fabry-Perot interferometers (FPIs) because of its high measurement resolution. However, phase ambiguity often limits the measurement range to be less than one free-spectral range (FSR) of the FPI. Here, we investigate a simple but effective method that can eliminate the phase ambiguity of a silicon FPI temperature sensor using a fiber Bragg grating (FBG) fabricated on the lead-in fiber close to the silicon FPI. The reflection spectrum of the sensor shows sinusoidal fringes from the FPI and a single peak from the FBG. Assuming the FBG and the silicon FPI have similar temperatures, the FBG peak can be used merely as a marker to identify an FPI fringe peak without ambiguity even when it shifts by more than an FSR. Although the temperature resolution of the FBG is lower than that of the silicon FPI, the measurement range of the silicon FPI can be greatly improved without sacrificing the measurement resolution. Using a temperature sensor based on a 75-μm-thick silicon FPI, we demonstrated temperature measurement over a range of-60 to 140 °C (corresponding to a spectral shift of about five FSRs) with a resolution of 0.0033 °C. The measurement range with unambiguous peak recognition is expected to be larger than demonstrated in the experiment and only limited by the spectral bandwidth of the sensor system and the capability of materials used for the sensor construction.