Surface Long-period Fiber Gratings Inscribed in Photonic Crystal Fibers

Abstract

We propose a new type of surface long-period fiber grating (LPFG) with enhanced sensitivities to ambient index and temperature, inscribed in photonic crystal fibers (PCFs). The PCF-based D-shaped fiber is fabricated by using a side-polishing technique and the surface LPFGs are formed by using a spin-coating method with a photoresist (PR) and a standard contact lithography technique. Two types of surface structures, such as LPFGs and a thin layer are induced in the PCF-based D-shaped fiber. For both cases, the resonant peaks, which result from the mode coupling between the D-shaped PCF and the surface PR structures, are created in the transmission spectrum of PCF-based surface LPFGs are created. The bandwidth of the PCF-based surface LPFGs is narrower than that of the PCF-based D-shaped fiber with a thin layer. However, the transmission peak depth of the PCF-based D-shaped fiber with a thin layer is stronger than that of the PCF-based surface LPFGs. The transmission characteristics of the PCF-based surface LPFGs and the PCF-based D-shaped fiber with a thin layer were measured for external temperature and index changes. As the ambient index was changed in a range from 1.30 to 1.45, the resonant wavelengths for both cases shifted to longer wavelength. The ambient index sensitivity of the PCF-based surface LPFG is higher than that of the PCF-based thin layer. The total resonant wavelength shifts for the PCF-based surface LPFGs and the PCF-based thin layer are measured to be 160.1 nm/RIU and 588.1 nm/RIU, respectively. As the applied temperature increases, the resonant wavelengths for both cases are shifted to shorter wavelength. The temperature sensitivity of the PCF-based surface LPFGs is also improved compared with that of the PCF-based thin layer. The temperature sensitivities for both cases are measured to be -0.3 and -0.4 nm/degrees C, respectively, in the temperature range from 30 to 90 degrees C. It is evident that the surface LPFGs induce resonant coupling and transmission peaks in the PCF-based D-shaped fiber and enhance the quality of the transmission characteristics and the sensitivities to the ambient index and the temperature.