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 altered, it changes the phase of the core light. Since the phase of light propagating in the cladding (reference arm) remains unchanged, the transmission fringe pattern of the interferometer observes a spectral shift. The spectral shift was used to quantify the effective core refractive index change in a standard single-mode fiber. Measurements of effective refractive index changes as high as 0.01356 and as low as 0.000475 are reported in this letter.