In-fiber modal interferometer formed by offset-splicing in air-core photonic bandgap fiber

We present a compact in-fiber Mach–Zehnder interferometer (MZI) formed by an offset splicing technique in air-core photonic bandgap fiber (PBF). The MZI is implemented by using two cascaded offset-splicing joints which act as beam splitter and combiner. The influences of lateral offset amount and interferometer length on the performance of MZIs were investigated experimentally. Several in-fiber MZIs with different interferometer lengths were built and it is found that the interference dip wavelength spacing is inversely proportional to the interferometer length. The MZI relies on the interference occurring between the fundamental core mode (FCM) and a surface mode (SM) of the PBF. The potential applications of the proposed in-fiber MZI were further investigated as temperature and strain sensors. The temperature and longitudinal strain sensitivities of the MZI were measured to be 211.89 pm/(°C m) and −0.6 pm/με, respectively.