Offset-enhanced slow light in femtosecond laser-fabricated Bragg gratings

We report a strength-enhanced waveguide second-order line-Bragg grating (WLBG) directly written with femtosecond laser in bulk glass by using "offset" to exploit the slow-light effect. This design eschews the use of multiple waveguides and/or waveguide bundles for light guiding. Instead, it only employs a single-laser-pass waveguide (SLPWG) with a refractive index change of 1.1 × 10^-3, to achieve effective light propagation. The SLPWG is first written as a core-shell ellipsoid unit by a single-laser pass. Subsequently, a line-grating is written on top, with an offset to accommodate for the already modified refractive index from the waveguide along the vertical direction of different offset values 0 µm, 5 µm, 10 µm, and 15 µm. The enhanced slow-light effect for WLBG is studied theoretically and experimentally. Optimal performance occurs at a 10 µm offset, exhibiting a maximum group delay of 35 ps and a derived slow-down factor (SDF) of up to 1.54, with a 12.5 dB transmission dip and a propagation loss of 1.16 dB/cm, in vertical polarization. The experimental SDF results demonstrate the potential of our design for future applications in creating slow-wave structures via grating dispersion for compact photonic integrated devices, applying it to microfluid devices that can increase the light-liquid interaction path for the detection of refractive index change caused by variations in fluid concentration and composition, directly incorporating it into the hardened glass of cellphone screens for embedded sensors, as well as integrating it into optical antennas within smart glass windows that can enhance light-matter interactions for enabling real-time monitoring of environmental changes and improving wireless communications.