Temperature-Independent Strain Measurement With High Sensitivity Using Chirped Fiber Bragg Gratings and Tunable Optoelectronic Oscillator

A temperature-insensitive strain measurement is designed and validated through experimental testing using chirped fiber Bragg gratings and a frequency-tunable optoelectronic oscillator (OEO). The system incorporates a microwave photonic filter that is formed by an incoherent broadband light source and a Michelson interferometer. Different from other works, the Michelson interferometer in this paper is implemented using two identical chirped fiber Bragg gratings (CFBGs). In this way, the broadband advantage of the light source remains, enabling the realization of a narrowband microwave photonic bandpass filter. What’s more, the dispersion of the CFBG allows to convert the strain-induced wavelength shifts into variations in the signal transmission delay. Consequently, the applied strain can finally be transformed into the oscillating frequency of the OEO, where the frequency shift is proportional to the dispersion coefficient of the CFBGs. This enables high-sensitivity and temperature-independent strain measurements by observing the oscillating frequency shift of the OEO. Experimental results demonstrate that the measurement sensitivity is 1.8 MHz/με with a CFBG dispersion coefficient of 33.3 ps/nm. In addition, the oscillating signal remains almost stable even when the temperature varies from 30 °C to 90 °C. The features of high sensitivity, temperature independence and easy demodulation make the proposed method highly promising for FBG sensing applications.