Femtosecond Laser Refractive Index Modulated Multimode Fiber for High Sensitivity Vector Bending Measurement

A femtosecond (fs) laser fabricated refractive index (RI) modulated multimode fiber (MMF) for high sensitivity vector bending measurement is presented. A fs laser-induced local positive RI modulation in the MMF core results in a non-circular symmetric RI distribution of the MMF. Such a RI-modulated MMF used in single-mode-multimode-single-mode (SMS) fiber structure exhibits a bending-dependent spectral shift response. Due to the large core size of the MMF, the RI modulated area can be relatively far away from the fiber core axis, which in turn results in a greater effective RI change during bending. Experimental results show that the highest bending sensitivity of -45.8 nm/m-1 is achieved at 0° orientation with the resonant wavelength of 1554.4 nm. In addition to its high sensitivity and simple structure, this vector bending sensor exhibits ultra-low temperature crosstalk in bending measurements, as low as 0.0002 m-1/°C, which makes it an excellent candidate for accurate vector bending monitoring applications.