Femtosecond Laser Direct Writing of On-Chip Tilted Waveguide Bragg Gratings With Controllable Cladding Modes for Refractive Index Sensing

We present a novel variant of the on-chip tilted waveguide Bragg grating (TWBG), wherein the tilted grating is constructed through the tilted stacking of 20 sets of line-by-line sub-gratings. This structural design not only attains the characteristic of a tilted grating but also significantly enhances Bragg resonance. By utilizing femtosecond laser direct writing technology, the TWBGs are inscribed near the surface of the bulk glass, enabling refractive index (RI) sensing through monitoring of the wavelength shifts of the resonance peaks induced by a few cladding modes. The responses of the TWBGs with different writing depths and tilt angles to surrounding RI change have been investigated, and the maximum sensitivity of 13° tilted grating is 62.47 nm/RIU at the writing depth of 6 μm. Furthermore, the peaks of the cladding mode resonance and Bragg resonance show similar temperature responses, with temperature sensitivities of 16.67 pm/°C and 18.78 pm/°C, respectively. This feature enables the elimination of temperature crosstalk. The proposed structure can control the number of cladding modes and their effective refractive index by changing the writing depth and tilt angle, providing a new approach for the realization of on-chip biochemical sensing in different RI ranges.