Phase Shifted Long Period Fiber Grating Based On The Cycle Pressure

Long period fiber gratings (LPFGs) are new optical devices that can couple the core fundamental mode to various cladding modes. Because of their simple structure, no backward reflection and low insertion loss, LPFGs have been widely applied in the field of optical fiber communication and sensing.Since LPFGs have periods of hundreds of micrometers,they can be induced mechanically by applying pressure periodically on the fiber. The fabricated gratings, sharing most of the properties of the non-mechanically induced ones, are tunable and reversible and become a hot research topic. This article mainly focuses on the study of phase-shifted LPFGs based on periodically induced pressure.Firstly, based on coupled-mode theory,the transmission spectra of uniform LPFGs, especially phase-shifted LPFGs were numerical simulated. The effect of grating parameters on the transmission spectrum was analyzed in detail.Secondly, a kind of simple periodically grooved plate was designed. By pressing the fiber between periodically grooved plate and flat plate, the pressure points induced a periodic index modulation, and then LPFG was formed. The relationship between grating transmission spectrum and external pressure, grating period and Incident polarization was studied. By means of the fabricated gratings, EDFA spontaneous emission spectrum was flattened.Finally, a simple fabricated method of phase-shifted LPFGs based on periodically induced pressure in single mode fiber was proposed. It was fully illustrated that how the transmission spectrum can be tuned by changing the grating length, the phase shift and the applied pressure. After stripped the polymer coating, the changes of grating transmission spectra were discussed.The results show that LPFGs based on periodically induced pressure have the over coupling characteristics with the pressure increasing, Gratings resonant wavelength increases linearly as grating period increases, An EDFA gain spectrum flattened to within±1.3dBover the wavelength range from 1527nm to 1557nm was realized by using an LPFG. After polymer coating being removed, protecting tube and Refractive index matching liquid being added,the out-of-band loss of phase-shifted LPFGs transmission spectra significantly reduced,the resonant wavelengths shifted to short wavelength and the apophysis between two notches was became more obvious.