Femtosecond Laser Inscribed Tilted Gratings for Leaky Mode Excitation in Optical Fibers

Abstract

We consider the excitation of leaky mode resonances (LMRs) in an optical fiber that originate from light coupling initiated by small angle (<21°) tilted fiber Bragg gratings (TFBGs), through sub-micrometric grating inscription using a femtosecond laser. The femtosecond laser inscription triggers the interaction between the fiber's guided mode and backward coupling to the LMRs, a process that requires tight control of the grating period, reflection angle, order and the strength of the modified refractive index. The TFBG itself exhibits very low spectral polarization dependence, whereas the LMRs display strong polarization dependence in specific wavelength ranges, allowing for easy mode selection and opening avenues for various applications related to sensing and optical filtering. The tailored-for-LMR-generation optical filters were characterized for torsion, temperature and strain, exhibiting a very large negative strain sensitivity of -2 pm/μϵ and a temperature sensitivity of 14.73 pm/°C that when compared with the grating response led to an exceptionally large strain response and remarkably well-behaved conversion matrix for the separation of strain and temperature in a single measurement. The LMR also proves to be insensitive to bend. The nature of LMRs is considered and confirmed by measuring their radiation field using an infrared camera, simulating their resonance wavelength using a finite element method and by experimentally observing their behavior, while changing the ambient refractive index.