Demodulation based on a long-period grating in photonic crystal fiber with differential processing for highly birefringent fiber loop mirror temperature sensor

Demodulation based on a long-period grating (LPG) in a photonic crystal fiber (PCF) with differential processing was proposed to eliminate the noise of fiber sensors and raise the accuracy of the highly-birefringent fiber loop mirror (HiBi-FLM) temperature sensor. Utilizing the property temperature insensitive and the filtering function of the LPG in PCF, the intensities of two signals located respectively within the positive and negative linear region of the LPG's transmission spectrum can be monitored simultaneously and accurately. The noise of fiber sensors is eliminated effectively by the algorithm of two signals, since the two signals (also including the noise of fiber sensors) transmit through the same way to the monitor. Experimental results show that the proposed demodulation eliminates not only all of power fluctuations in the system, but also raises the accuracy of the sensor. The value of the algorithm remains constant when the power of light source varies ±10% and the relative error between the measured temperature change and the true change is only 0.34%. Compared with the sensing without the algorithm, the accuracy increases from~90.5% to ~99.7%.