Ambient index sensor using a Sagnac interferometer based on a D-shaped polarization maintaining fiber

A Sagnac loop interferometer based on a hybrid polarization maintaining fiber (PMF) is proposed and experimentally demonstrated for measurement of ambient index. The hybrid PMF consists of the PMF and the locally D-shaped PMF. The important key component is the locally D-shaped PMF. Since the core mode of the locally D-shaped PMF is directly interfaced to the external environment, the core mode is immediately changed by the variation of external index. The birefringence of the locally D-shaped PMF can generate the interference patterns. Consequently, the flexibility and simplicity in the sensing scheme can be provided by the ambient index change can be measured simply and flexibly since the peak wavelength shift is only monitored by changing the ambient index. The sensitivity of ambient index was measured to be -30 nm/RIU.