Simultaneous measurement of strain and temperature based on a long-period fiber grating written on high birefringence photonic crystal fiber concatenated with a Faraday rotator mirror

Here we report simultaneous measurement of strain and temperature, carried out by incorporating a long-period fiber grating (LPFG) written on high birefringence photonic crystal fiber (HBPCF), referred to as an HBPC-LPFG, and a Faraday rotator mirror (FRM) concatenated in series with the LPFG. The HBPC-LPFG used as a sensor head was fabricated by irradiating CO2 laser pulses to unjacketed HBPCF with line-by-line technique. The HBPC-LPFG connected in series with the FRM exhibits a polarization-independent wavelength-dependent loss spectrum in its reflection spectrum, which has two resonance dips (RDs) with different cladding-mode orders, designated as RDs 1 and 2. As the two RDs in the fabricated HBPC-LPFG ended with the FRM have dissimilar cladding-mode orders, their strain sensitivities and their temperature sensitivities are different from each other. For the two RDs with resonance wavelengths of lambda(1) = similar to 1470.5 nm and lambda(2) = similar to 1495.1 nm, strain and temperature responses were investigated in an applied strain range from 0 to 1790 mu epsilon (step: 179 mu epsilon) and an ambient temperature range from 30 degrees C to 95 degrees C (step: 5 degrees C), respectively. The strain sensitivities of the RDs 1 and 2 were measured to be approximately -0.77 and -1.07 pm/mu epsilon at room temperature, and the temperature sensitivities of the RDs 1 and 2 were measured as similar to 10.44 and similar to 8.56 pm/degrees C without applied strain, respectively. Owing to their linear and independent responses to strain and temperature, strain and temperature changes applied to the HBPC-LPFG can be simultaneously estimated from the measured wavelength shifts of the RDs 1 and 2 using their predetermined strain and temperature sensitivities. (C) 2020 Society of Photo-Optical Instrumentation Engineers (SPIE)