Study On PDL Of Long-Period Fiber Gratings And Methods For Reducing PDL

Prof. Rao Yunjiang of our laboratory proposed a novel method to fabricate long-period fiber gratings (LPFG) by using high-frequency CO2 laser pulses, and this method stands out for its advantages, such as simple procedures, low cost, high writing efficiency, et al. This dissertation studies on the characteristics of Polarization Dependent Loss (PDL) of these LPFGs and explores the ways to reduce PDL.The novel LPFG is written on a single-mode fiber by exposuring to high-frequency CO2 laser pulses from one side of the fiber. For the side exposed to laser pulses, more laser power is transmitted and hence results in higher temperature, more residual stress release and larger refractive index change in the fiber. With the increase in the exposure time, the peak loss increases too, resulting in larger asymmetry in refractive index distribution on the cross-section of the LPFG. The birefringence induced becomes larger. Our study indicates that PDL in such a LPFG has a dual-peak structure that varies with the resonant wavelength. The maximum PDL tends to appear near the resonant peak and increase with its peak loss and is independent on the LPFG's period and resonant wavelength.The characteristics of PDL in Ultra Long-Period Fiber Gratings (ULPFG) by single-side exposure to high-frequency CO₂ laser pulses is similar to that of PDL in the LPFG by the same single-side exposure in terms of PDL features. This suggests that PDL in the novel ULPFG is independent on the fiber period and the orders of resonant peak, and only dependent on its resonant peak loss. Also, it is found that the relationship between Resonant Wavelength Separation (RWS) and the times of the orders of resonant peaks of the ULPFG is identical during testing RWS of the ULPFG.This dissertation presents four different methods to reduce PDL in the LPFG. These methods, including the dual-side exposure method, the fiber-twisting method, the LPFG-twisting method and the dual-side exposure method for the twisted fiber, according to the idea of improving the symmetry of the distribution of refractive index on the cross-section of the LPFG, which are all better in terms of PDL reduction over the single-side exposure method, proven by the experimental results.This study is of importance to realize practical fiber-optic devices with low PDL for applications in optical fiber communication and sensing.