Highly Localized Fiber Bragg Gratings With Strong Cladding Mode Inscribed by Femtosecond Laser

Highly localized fiber Bragg gratings (HLFBGs) with strong cladding mode have been successfully inscribed in hydrogen loaded single mode fiber by low intensity femtosecond laser and phase mask. The HLFBGs possess strong cladding mode resonances which are deep than 20 dB and wide than 100 nm in the transmission spectrum. Experimental results reveal that localized refractive index (RI) modulation in fiber core leads to the emergence of strong cladding mode and higher localization leads to stronger cladding mode resonances. The HLFBGs can withstand high temperature up to 800 °C, and the strength of cladding mode still remains about 8 dB after thermal annealing. With surrounding RI varying from 1.3330 to 1.4583, the cut-off wavelength of cladding mode shows a good linear shift with sensitivity of 527.43 nm/RIU. The HLFBGs are expected for using in chemical and biological detections.