High-Quality Waveguide Bragg Gratings in Planar Lightwave Circuits Fabricated by Femtosecond Laser Direct Writing

This research pioneers the application of a femtosecond laser Burst-Mode technique for the fabrication of waveguide Bragg gratings (WBGs) within planar lightwave circuits (PLCs). Distinguished by its advantages in miniaturization, integration, and low loss, PLC technology emerges as a promising frontier in optical advancements. Our method significantly mitigates permanent damage to waveguides by employing lower pulse energy for multiple exposures at the same location, thereby enhancing the spectral properties of PLC-WBGs. Through meticulous exploration of the grating order's impact on WBG subpeaks, we refined our parameter optimization process. This led to the successful creation of a first-order WBG utilizing the femtosecond laser Burst-Mode technique, which yielded an outstanding return loss of approximately 32 dB. Further, we innovated upon this technique by integrating it with geometric apodization. Maintaining consistent grating length and laser energy parameters allowed for the fabrication of a double-apodized PLC-WBG, which introduced greater refractive index modulation (RIM) and achieved a Bragg resonance attenuation of roughly 12 dB and a side-mode suppression ratio (SMSR) of about 18 dB. Moreover, we developed stress waveguides adjacent to the core using the femtosecond laser, incorporating stressors with WBG. This innovation produced a WBG with significantly low birefringence of 5.66 × 10⁻⁶ and a minimal polarization-dependent loss (PDL) of 0.8 dB. The high-quality WBGs fabricated through these advanced techniques offer vast potential for applications across diverse fields, such as multi-parameter sensing, optical communication systems, and fiber lasers.