All-fiber second-order optical vortex generation based on strong modulated long-period grating in a four-mode fiber

Abstract

We propose an effective all-fiber method to generate a high-order optical vortex (OV) via twisting a strong modulated long-period fiber grating (LPFG) written in a four-mode fiber (4MF). With a special design and optimization of the procedures of CO-laser irradiation, an LPFG with strong period deformation is achieved in the 4MF. Based on this LPFG, we can directly convert the linear polarization (LP) fiber fundamental mode (LP) to the high-order LP core mode (LP) with efficiency of 99.7% and then transform the LP mode into a high-order OV mode (±2 order). This is the first time, to the best of our knowledge, that ±2-order OV modes have been experimentally generated with just one fiber grating in an all-fiber-system.

Generation of Orbital Angular Momentum Modes Using Fiber Systems

“…The mode couple module is used to couple the fundamental mode to a specific lth-order CV mode. It is usually composed of the fiber gratings [31][32][33][34][35][36][37][38][39][40][41][42][43][44] and the fiber couplers [45][46][47][48]. As mentioned above, the lth-order CV modes are composed of four degenerated modes.…”

“…For the other type, the OAM modes can be generated by combining two vector modes (HE l+1,m or EH l−1,m ) with a π/2 phase shift, where the generated OAM modes have the SAM at the same time. There have been many reports about the generation of these two types OAM modes using LPFGs [32,34,35,37,43].…”

“…Thus, the LP-OAM can be obtained by combining the even and odd LP l,1 mode having the same polarized direction with a ±π/2 phase shift. The first order LP-OAM modes, composed of LP a 11 and LP b 11 mode with a π/2 phase shift, using a two-mode fiber (TMF), a mechanical LPFG, a mechanical rotator and metal slabs, as shown in Figure 8a, and the second order LP-OAM modes, composed of LP a 21 and LP b 21 mode with a π/2 phase shift, utilizing an LPFG written in a four-mode fiber induced by CO 2 laser irradiation, have been experimentally demonstrated in [35] and [37], respectively.…”

“…The OAM modes obtained by [34,35,37,43] are all generated by a combination of two or four degenerated modes in the same mode group and the associated eigenmodes must maintain a stable phase and polarization. In addition, in ideal cylindrical waveguides such as fiber, the propagation constants of the TE 01 and TM 01 mode are different.…”

“…Compared with spatial generating methods, fiber generating methods have shown their certain advantages. A converter applied in fiber methods can be fiber gratings [31][32][33][34][35][36][37][38][39][40][41][42][43][44], mode selective couplers [45][46][47][48], and microstructure optical fibers [49][50][51] and photonic lanterns [52,53]. As a cylindrical waveguide, fiber is a natural beam shaper for OAM beams.…”