Measurement of In-Fiber Refractive Index Change Using a Mach–Zehnder Interferometer

We report, for the first time, the use of a fiber optic Mach-Zehnder interferometer to measure core refractive index changes written by an ultrafast laser irradiation. The core-offset interferometer was constructed by splicing a slightly misaligned stub of standard single-mode fiber between the lead-in and lead-out optical fibers. When the core refractive index of an in-fiber interferometer is alt...