Temperature-Robust All-Fiber Demodulation of Optical Current Sensor Using Long-Period Grating on Birefringent Photonic Crystal Fiber

By incorporating a long-period fiber grating (LPFG) inscribed on birefringent photonic crystal fiber (BPCF) with a CO2 laser as a polarization analyzer, we demonstrate temperature-robust all-fiber demodulation of an optical fiber current sensor based on Faraday rotation. The LPFG written on BPCF, referred to as BPC-LPFG, exhibited polarization-dependent resonance dips with a polarization extinction ratio of >25 dB at 1555.94 nm at room temperature (i.e., similar to 25 degrees C). Due to the single-material property of pure silica BPCF, this BPC-LPFG can be employed as a temperature-robust fiber polarization analyzer for sensor demodulation. For ambient temperature changes from 25 degrees C to 50 degrees C, the resonance wavelength of the BPC-LPFG showed a slight redshift of similar to 0.2 nm resulting in a temperature sensitivity of similar to 8.0 pm/degrees C. At room temperature, all-fiber current demodulation up to 800 A (step: 100 A) was achieved at a fixed laser wavelength of 1555.94 nm, showing good linearity with an adjusted R-2 value of 0.99997. The same demodulation procedure was repeated for different ambient temperatures from 30 degrees C to 50 degrees C (step: 5 degrees C). Although the magnitude of demodulated signals slightly decreased with increasing temperature, sufficiently linear sensing results were obtained for every ambient temperature. Temperature-induced current measurement errors were analyzed for all applied currents, and at the maximum applied current (800 A) the temperature-induced error was evaluated to be typically less than 0.043%/degrees C, which corresponds to 0.344 A/degrees C. Consequently, temperature-robust current sensing up to 800 A was accomplished in a fiber-compatible way with good linearity using the BPC-LPFG.