[2012-JLT]Analysis of Generating Cylindrical Vector Beams Using a Few-mode Fiber Bragg Grating.pdf

3540 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 22, NOVEMBER 15, 2012 Analysis of Generating Cylindrical Vector Beams Using a Few-Mode Fiber Bragg Grating Zhongxi Lin, Anting Wang, Lixin Xu, Biao Sun, Chun Gu, and Hai Ming Abstract—An efficient numerical model is proposed to generate cylindrical vector (CV) beams by use of a few-mode fiber Bragg grating (FM-FBG) written directly in an ytterbium-doped fiber (YDF). Based on multilayer method, we analysis the transverse modes competition between and modes under var- ious conditions.We propose that a pure CV beam can be generated by completely depressing mode through a doughnut doping few-mode fiber or using a wavelength filter. Index Terms—Cylindrical vector beams, few-mode fiber Bragg grating, wavelength filter. I. INTRODUCTION C YLINDRICAL VECTOR (CV) beams, including radi-ally, azimuthally and hybrid polarized beams, have at- tracted greatly attention in recent years. Under a high numerical- aperture, the radially polarized beams can produce a strong lon- gitudinal component in focal point and the spot size is smaller than the diffraction limit of spatially homogenously polarized beams [1], which has many attractive applications, such as par- ticle acceleration [2], optical trapping [3], laser machining [4], [5], and materials processing [6]. Various methods have been used to generate CV beams, such as using an intracavity birefringent lens [7], [8], producing a space-variant polarization converters [9], [10] or sub-wave- length grating [11], [12]. All-fiber designs are also proposed [13]–[17], which enable the generation in an easy and flexible way. The degenerate mode includes radially , azimuthally and hybrid polarized beams. By exciting mode in a few-mode fiber, both and modes become switchable using polarization controllers (PCs) [16], [17]. However the presence of mode would have great effect on the polarization purity of the CV beams. In this paper, we present a numerical model to generate CV beams by usin