Ultrafast-laser-induced negative-index fiber Bragg gratings with enhanced thermal stability

Negative-index fiber Bragg gratings (FBGs) were fabricated using 800 nm femtosecond laser overexposure and thermal regeneration. A positive-index type I-IR FBG was first inscribed in H-2-free fiber with a uniform phase mask, and then a highly polarization dependent phase-shifted FBG (PSFBG) was created from the type I-IR FBG by overexposure. Subsequently, the PSFBG was annealed at 800 degrees C for 12 hours. A negative-index FBG was obtained with a reflectivity of 99.22%, an insertion loss of 0.08 dB, a blue-shift of 0.83 nm, and an operating temperature of up to 1000 degrees C.