Multifunctional Line-by-Line Tilted Waveguide Bragg Gratings Directly Written by Femtosecond Laser

We demonstrate the first line-by-line TWBGs in bulk glass by femtosecond laser direct writing, and apply a multi-layer parallel grating structure to enhance the grating strength in this paper. The tilted grating can radiate the light inside the waveguide outward from the sidewall by coupling the forward-propagating guided mode to the backward-propagating radiation mode, which appears as a wide loss band on the shorter wavelength side of the Bragg resonance peak in the transmission spectrum. We have elucidated the multifunctional optical filtering characteristics exhibited by TWBGs with different tilt angles. The TWBGs with tilt angles ranging from 3° to 40° exhibit an ultra-wide loss bandwidth in the range of 10 nm to 300 nm, thereby enabling broadband filtering. Furthermore, the 45° TWBG has strong polarization dependence with a polarization-dependent loss of 25 dB, rendering it suitable for on-chip polarization applications. In addition, the rising edge of the broadband loss band has a very good linearity, making it suitable for use as an edge filter. The proposed TWBG provides an efficient pathway to serve as on-chip filtering and polarization management devices.