Cascaded Sagnac loops embedded with two polarization maintaining photonic crystal fibers for highly-sensitive strain measurement

A strain sensor based on cascaded Sagnac loops with two PMPCFs has been experimentally demonstrated. Air holes distributed on the cross section of PMPCF go through the whole fiber which make the special fiber more sensitive to ambient force compared with conventional optical fiber. The lengths of the two PMPCFs are close, but not the same, to form envelopes on the total output spectrum and then obtain high resolution to the detected parameter by Vernier effect. The influences of the difference in length of the two PMPCFS on spectral envelopes are discussed. As the length of the PMPCF in reference loop increases, free spectrum range of the cascaded Sagnac loops (or envelope period) and magnification decrease. The sensitivities of this strain sensor are calculated by tracking upper- and lower-envelope. Results reveal that its average sensitivity is up to 45.15 pm/µε and the corresponding resolution is 0.44 µε as strain varies from 0 to 2000 µε. Compared with a single Sagnac loop, the sensitivity magnification is about 28.94. The sensing characteristics of a conventional PMF are studied to compare with the PMPCF. Moreover, reversibility has also been proved to possess low error rate. In short, the fiber strain sensor possesses high sensitivity and great reversibility.