Low-splicing-loss polarization-maintaining photonic-crystal-fiber-based Sagnac interferometer and its cladding-mode effect

A low-splicing-loss polarization-maintaining photonic-crystal-fiber (PMPCF)-based optical fiber Sagnac interferometer (OFSI) and its cladding-mode effect are demonstrated experimentally and analytically. An OFSI with minimum splicing loss of about 2.6 dB is achieved. The weak cladding modes induced by residual mode field diameter mismatch at the fusion splice cause a filtering effect on the transmission spectra of the OFSIs. This is especially noticeable for short length PMPCF-based OFSIs. Experimental results show that the relatively maximum ripple fluctuation induced by the cladding mode can reach 5.7%. The cladding-mode effect can be eliminated by slightly bending the PMPCF, but this introduces additional insertion loss.