A novel all-fiber broadband circular polarizer

A chiral fiber grating (CFG) formed by twisting a high-birefringence (Hi-Bi) fiber with a linearly varying twist rate is proposed to realize an all-fiber broadband circular polarizer. For the first time, by coating CFG with a higher index material and using the leakage property of leaky modes, the coupling of the core mode to a leaky mode is introduced to achieve a desired extinction ratio. Further, by using a unified coupled-mode analysis and then by an analytical analogy to damped oscillations, the power evolution of the core mode resulted from the coupling is investigated. The condition that the power attenuates the most quickly is thus identified, and an explicit optimization scheme is provided for a compact design. Moreover, for a circular polarizer with the given specifications such as the extinction ratio and the operation bandwidth, the design parameters are rather easy to be determined. Finally, a circular polarizer with an extinction ratio better than 25dB within a bandwidth over 60nm is demonstrated by simulations. The length of the polarizer is only 9cm, which is much compacter than those obtained by the previous schemes based on CFGs, to the best of our knowledge.