The Research Of All-fiber Circular Polarizer Based On Chiral Fiber Grating

Though circular polarization was widely utilized in the optical communication, optical sensor, and optical detection area, still there are not very well technology to polarize.. At present circular polarized light was generated through the linearly polarized light. So as for the traditional circular polarizer, it was assembled and installed with discrete optical device, such as 1/4 wave plate and line polarizer. Such apparatus will bring about many inconvenience when worked. In order to broaden the use of circular polarization, this paper put forward a new suggestion of making all-fiber circular polarizer. This circular polarizer was based on the chiral fiber grating, which was made by spinning the high birefringence fiber. Such circular polarizer was based on line polarization maintaining fiber, which can be easily got in the society. Additional, such circular polarizer can be easily linked with the optical fiber, so such device will have a very bright future.This paper was based on the chiral fiber gratings.Coupled mode theory is widely-used theory for the analysis and modeling of optical device. By means of coupled mode theory, this paper firstly analysis the mode coupling in the chiral fiber grating. This analysis reveals the scheme of polarization-selectivity between coupled modes and the necessary conditions for achieving polarizer.Similar with traditional gratings, chiral fiber gratings can also be classified as long-period chiral fiber gratings and short-period chiral fiber grating according to the period length. This paper aimed at the long-period fiber grating, detailed analyzed such gratings'property. Take the right-handed rotating chiral fiber grating for example, when twist pitch was uniform, at some wavelength, right-handed core circular polarization mode's power can totally coupled to the cladding circular polarization mode. When leak the cladding circular mode's power, left-handed circular polarization can be generated. This paper also gave a detail presentation to explain how optical fiber's parameters effect circular polarizer's characteristics.Since such long-period chiral fiber gratings with uniform period can couple the core circular mode's energy to cladding circular mode at certain wavelength. The bandwidth of such a circular polarizer is about several nanometers, which will bring about inconvenience for most applications. In order to improve the bandwidth characteristic, this paper put forward a new method to achieve a broader bandwidth polarizer. To achieve a broadband circular polarizer, we utilize the adiabatic coupling principle to analysis the coupling between tapered modes. This paper also give detail suggestions to achieve all-fiber, broad bandwidth, good extinction ratio circular polarizer with small size.This paper also analyzed the short-period chiral fiber grating with theoretically analysis and numerical simulation. The simulation discovered that when forward transmitted core circular-polarized mode coupled with reverse transmitted cladding circular-polarized mode. The 3dB bandwidth of the core mode's reflectivity spectrum is quite narrow, so such chiral fiber grating is suitable as circular polarization filter. This paper also discussed the coupling between forward transmitted core circular-polarized mode and reverse transmitted cladding circular-polarized mode. Take the right-handed short-period chiral fiber grating for example, left-handed polarized light was generated at the forward and reverse direction. In addition, this paper also discussed the characteristics of short chirped-period chiral fiber gratings .