Journal
JOURNAL OF LIGHTWAVE TECHNOLOGY
Volume 39, Issue 14, Pages 4784-4793Publisher
IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
DOI: 10.1109/JLT.2021.3078257
Keywords
Optical fibers; Optical fiber polarization; Optical fiber sensors; Fiber lasers; Couplings; Writing; Laser modes; Femtosecond laser direct writing; optical fiber sensors; refractive index sensing; titanium dioxide thin films; turn around point long period fiber gratings
Funding
- Ministry of Education and Science of Portuguese Government [SFRH/BD/110035/2015]
- Project On Chip Whispering Gallery Mode Optical Microcavities For Emerging Microcontaminant Determination In Waters - SAFE WATER - European Commission
- Directorate-General Communications Networks, Content and Technology (DG CONNECT) under the ERA-NET [7859/BI_B2-C/2019]
- Fundação para a Ciência e a Tecnologia [SFRH/BD/110035/2015] Funding Source: FCT
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A detailed study was conducted on turn around point long period fiber gratings with coupling to asymmetric cladding modes, fabricated using femtosecond laser direct writing. The fabrication process, coating with TiO2 film, and necessary corrections to achieve TAP condition with optimized sensitivity to the surrounding refractive index were discussed. LPFGs were coated with different thicknesses of TiO2 film and analyzed for wavelength sensitivity, showing maximum sensitivity when coated with a 30 nm thick film.
A detailed study of turn around point (TAP) long period fiber gratings (LPFGs) with coupling to the asymmetric cladding modes of a standard single-mode fiber (SMF-28e), fabricated by femtosecond (fs) laser direct writing was realized. The entire fabrication process, including the coating with different titanium dioxide (TiO2) film thicknesses of LPFGs and the corrections needed to achieve coated devices operating precisely in the TAP condition with coupling to the asymmetric cladding modes, was addressed. The significant fabrication details are also given, such as inscription periods, shape and localization of the refractive index modifications across the core. The fabrication process described allows the optimization of the LPFGs sensitivity in regards to the surrounding refractive index (SRI). Optimization of the writing parameters to obtain gratings working at the TAP for two different media surrounding the fiber (water and air) was achieved. It was demonstrated that for a grating period of 191.8 mu m, the LP1,12 mode exhibits a TAP at 1442.7 nm in air, and for a period of 192.5 mu m, the same mode exhibits a TAP at 1448.6 nm in water. The LPFGs operating at the TAP in air and water were coated with 10, 20, and 30 nm thin TiO2 film thicknesses and the spectral behavior characterized. The wavelength sensitivity to the surrounding refractive index (SRI) was assessed in the range between 1.3700 to 1.4120, and a maximum sensitivity of similar to 8051.4 nm/RIU was measured for the 192.5 mu m LPFG coated with a 30 nm thick TiO2 film.
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