Femtosecond Laser Fabrication of Apodized FBG via Synergistic Control of Pulse Energy – Adjustable Slit

This study proposes a synergistic control method combining energy modulation and adjustable slit manipulation for refractive index modulation (RIM) area apodization during femtosecond laser point-by-point (PbP) fabrication of high quality apodized fiber Bragg gratings (AFBGs). We established an inverse coordination model between pulse energy and slit width to dynamically control the inscription area following Gaussian distribution, where higher pulse energy combined with narrower slit width creates longer inscription regions at the grating center, while lower energy with wider slit width produces shorter inscription regions at both grating ends. For experimental validation, we developed a femtosecond laser PbP collaborative inscription system. Results demonstrated a significant expansion of the single-pulse energy tuning range for PbP-FBG fabrication from 30 nJ to 150 nJ, effectively overcoming the key challenge of limited energy adjustability in PbP inscription. The fabricated 6-mm-long AFBG using polyimide-coated single-mode fiber exhibited remarkable performance characteristics: over 78% reflectivity and a side lobe suppression ratio (SLSR) exceeding 21.3 dB in the reflection spectrum. This approach provides enhanced flexibility for PbP based fabrication of high-quality AFBGs, thereby substantially improving FBG performance.