Fig. 1. Cross section of photonic crystal fiber and diagram of interferometer (SMF: single-mode fiber; PCF: photonic crystal fiber; CR: collapsed region)

Fig. 2. Transmission spectra at different lengths of collapsed regions. (a) Change in the length of collapsed region 2; (b) change in the length of collapsed region 1

Fig. 3. Experimental spectra of the interferometer with positive- and inverse-connection under different lengths of collapse regions

Fig. 4. Experimental schematic diagram of optical fiber sensing array

Fig. 5. Spectra under different strain conditions. The strain range is 0‒2000 με, and the spectra blue shift with the increase of strain

Fig. 6. Total output spectrum of cascaded interferometer

Fig. 7. Strain experimental results of interferometer after cascade. (a) Output spectra under different strain conditions and (b) relationship between corresponding wavelengths of three valleys and strain when strain experiments were carried out on an interferometer with a length of 10.5 mm of photonic crystal fiber; (c) output spectra under different strain conditions and (d) relationship between corresponding wavelengths of three valleys and strain when strain experiments were carried out on an interferometer with a length of 11.9 mm of photonic crystal fiber