Theoretical model and spectrum characteristics of superimposed photonic crystal fiber grating

Abstract

The superimposed gratings have attracted considerable interest because they can extend the potential applications of gratings. Superimposed gratings are fabricated by inscribing multiple gratings at the same section of the fiber, and they can demonstrate various features simultaneously. A number of optical devices based on superimposed gratings have been reported, such as multi-wavelength filters, beam shapers, ultrahigh repetition rate optical pulse generators, etc. Photonic crystal fiber (PCF) can bring new optical characteristics by changing the sizes, spacings and arrangements of the air holes in the fiber. In this paper, we present the spectra of the superimposed gratings inscribed in a photonic crystal fiber. A numerical mode is proposed based on the V-I transmission matrices. The traditional cosinoidal variation of refractive index is replaced with a square-type refractive index variation, and the scattering occurs at a localized discrete location. According to the simulations, the reflection spectra and time delays of a superimposed Bragg grating and superimposed chirped Bragg grating are analyzed. A superimposed Bragg grating and a superimposed chirped Bragg grating are fabricated in the single mode photosensitive PCFs under the irradiation of a 193 nm ultraviolet laser. The superimposed Bragg grating is composed of four subgratings with resonance wavelengths at set spacings. And under a phase mask displacement of 1.03 mm, the superimposed chirped Bragg grating has a periodic resonance with a period of 0.82 nm. The results show that the spectrum of superimposed Bragg grating can be flexibly customized by the parameters of each subgrating. Superimposed chirped Bragg gratings have good linear group delays and flat periodic resonance amplitudes, and the resonance period can be adjusted by displacing the phase mask. The grating spectra obtained from experiments are in good agreement with the theoretical analyses. The research results in this paper provide an important basis for designing, fabricating, and applying the superimposed PCF gratings.