Compact vector fiber-optic displacement sensor using an asymmetric Mach-Zehnder interferometer

We demonstrate a fiber-optic Mach-Zehnder interferometer for vector displacement measurement. The sensor is a compact structure in which a short length of thin-core fiber (TCF) is sandwiched between two single mode fibers (SMFs) by core-aligned fusion splicing. The TCF stub contains a refractive index modification (RIM) region which serves as a mode-coupling window over the interface between the TCF's core and cladding. We employed femtosecond laser-based direct inscription to achieve precise position of the RIM over a chosen length close to the core-cladding interface of the TCF and ensure large and stable refractive index increase in the inscribed region. Strong coupling between core and cladding modes is generated at the RIM, resulting in a well-defined interference spectrum in transmission. The transmission spectrum exhibits strong bending dependence and direction discrimination due to the azimuthally asymmetrical distribution of the RIM over the fiber's cross section. We achieved vector displacement measurement by wavelength interrogation of the interference spectrum. In the frequency spectrum, the intensity of a cladding mode resonance presents linear response to displacement as well as temperature independence.