High accuracy temperature and strain measurements are prerequisites for many modern industries to ensure safety, improve efficiency, and reduce greenhouse gas emissions. Traditional thermocouples or electronic devices often encounter challenges in temperature and strain measurement due to cross-sensitivity to surrounding perturbations, sensor’s drift at elevated temperature, or susceptibility to electromagnetic interference (EMI). To overcome these, fiber-optic sensors have gained popularity due to their unique advantages, including small size, multiplexing capacity, and immunity to EMI. In this work, we reported a novel approach to measure temperature using fiber optic Fabry-Pérot (FP) interferometer, which eliminates cross-strain sensitivity, shows linearity at high temperature, and provides high accuracy for a broad range. In addition, we developed another sensor for simultaneous measurement of temperature and strain using cascaded fiber Bragg grating (FBG)- silicon FP interferometer configuration.Our proposed temperature measurement method is based on an air-filled FP cavity, whose spectral notches shift due to a precise pressure variation in the cavity. For fabrication, a fused-silica tube is spliced with a single mode fiber at one end and a side-hole fiber at the other to form the FP cavity. The pressure in the cavity can be changed by passing air through the side-hole fiber causing the spectral shift, which is the measurand of temperature. We have developed two novel approaches based on this setup. The first approach employs two pressure values, their corresponding interferometric valley wavelengths, and the gas material’s constant (ɑ) to obtain temperature. A computer-controlled pressure calibration and sensor interrogation system has been developed with miniaturized instruments for this sensor operation. Experimental results show that the sensor has a high wavelength resolution (<0.2 pm) for minimal pressure fluctuation (2.5 × 10-3 psi) up to a broad temperature range (over 800 °C). We analyzed the effect of wavelength noise and pressure fluctuation on temperature resolution, which reveals that our developed system can obtain a high resolution (±0.32 °C) temperature measurement. The use of gas as the sensing material and the measurement mechanism also implies long-term stability and eliminates the cross-sensitivity to strain.In the second approach, we used a pair of FP cavities filled with gas of identical but variable pressure. One of the FPs (reference FP) is placed in the cold zone with a known temperature. The temperature of the measuring FP can be deduced by the spectral fringe shift vs. pressure of the two FPs. This method does not require measurement of the pressure or the knowledge of the optical properties of the gas. Hence it facilitates to make the instrumentation simpler and cost-effective and data acquisition faster. We have verified this method experimentally up to 800 °C. The sensor shows good linearity in the range. Long-term test conducted at 800 °C exhibited the stability of the sensor with fluctuations of ≤0.3% over a duration exceeding 100 hours.In addition to these air-filled FP interferometers, we have presented another novel sensor based on cascaded fiber Bragg grating (FBG)- silicon FP interferometer (FPI) for simultaneous measurement of temperature and strain. The sensor is composed of a 5 mm grating on a single mode fiber and a 100 μm silicon tip attached to the end of it by UV curable glue. The silicon tip is unbonded, and free from strain whereas the FBG is attached to the host structure. The sensor is tested from room temperature to 100 °C with varying strain up to ∼150 με. The silicon FPI provides high temperature sensitivity of 89 pm/°C unaffected by strain. On the contrary, the FBG is affected by both thermal and mechanical strain; the sensitivity of these are experimentally obtained as 32 pm/°C and 1.09 pm/με, respectively. With a high-speed spectrometer, the temperature and strain resolution of the FPI and FBG are found ±1.9 × 10-3 °C and ±0.042 με, respectively. Due to the small size, enhanced sensitivity and high resolution, this cascaded FBG-FPI sensor can be used in practical applications where accurate measurement of temperature and strain are required.
Read
