Cylindrical vector beams (CVBs) featuring unique polarization distribution and donut-shaped beam profile have a significant impact in multiple fields, including stimulated emission depletion (STED) microscopy [1], optical trapping [2], surface plasmon excitation [3] and so on. The radially polarized beam, namely RP-CVB [4], is one of the typical CVBs. Several methods, such as spatial light modulator (SLM) [5,6], q-plate [7], S-waveplate [8], mode selective coupler (MSC) [9], long period fiber Bragg grating [10], mode superposition [11], photonic lantern [12], have been employed for RP-CVB generation. While spatial optical schemes have been widely used, the fiber structure presents a more flexible and compact alternative. Additionally, the ideal beam quality is inherent in both the generated and amplified RP-CVB within a fiber laser system.

To date, numerous works have focused on the amplification of RP-CVB within non-PM fiber laser systems. In 2009, T. Chubachi et al. reported the amplification of a RP-CVB by using an Yb-doped double-clad fiber with a 15 μm core diameter and 130 μm inner cladding diameter [13]. The corresponding slope efficiency and output power were measured to be 48% and 1.1 W, respectively. Although the intensity and polarization distributions were well-maintained during power amplification, noticeable degradation of more than 30% in mode purity was observed. In 2010, M. Fridman et al. reported the RP-CVB amplification in a large-mode-area (LMA) fiber, with a core diameter of 20 μm, achieving a 40-dB amplification [14]. However, the polarization purity was decreased by 10% relative to that of the seed laser. In 2014, S. Kanazawa et al. demonstrated the amplification of RP-CVB laser using an Yb-doped double-clad fiber with core and inner cladding diameters of 30 and 250 μm, in which the maximum output power is 21 W [15]. Despite the donut-shaped beam profile was maintained after amplification, a rotation in polarization distribution was observed due to the birefringence induced by fiber bending. A combination of quarter- and half-wave plates were proposed to restore the desired radial polarization [8,16]. In 2018, D. Lin et al. reported a picosecond MOPA system with a RP-CVB, in which the main amplifier was composed of an Yb-doped double-clad fiber that has a core diameter of 25 μm and an inner cladding diameter of 250 μm [17]. The radially polarized beam was scaled up to an average power of 106 W with a repetition rate of 5.468 MHz. However, the corresponding mode extinction ratio (MER) deteriorated from 98.4% to 84.2%.

The mentioned issues in above non-PM fiber laser systems, such as the influence of amplified spontaneous emission (ASE), LP01 mode (fundamental mode), birefringence, and mode crosstalk on RP-CVB characteristics, also exist in PM fiber laser systems. Furthermore, according to the aforementioned researches and our previous work [18], a RP-CVB can be obtained by precisely controlling a commercial, short, straight (or loosely coiled with a large bend diameter) non-PM fiber, but its mode purity and polarization purity both degenerate after power scaling. Although the circular symmetry of the fiber modes is broken by the strong birefringence of PM fiber, the PM fiber still exhibits potentiality for the RP-CVB amplification. On the one hand, a RP-CVB can be achieved by using a superposition of two appropriate linearly polarized eigenmodes (LP modes), and these LP modes with a high polarization ratio and an effective mode refractive index difference of 10−4 can be supported well in a PM fiber. Experimentally, the TM01 mode (radially polarized beam) has been obtained by using a superposition of LPx 11e and LPy 11o [19]. On the other hand, the mode crosstalk can be effectively suppressed due to the large birefringence of PM fiber, thus enabling the employment of a long and coiled fiber. As is mentioned above, the degradation of polarization purity and mode purity of RP-CVB during power amplification is hopefully resolved by using a PM fiber laser system. Hence, in order to obtain RP-CVB with high power and polarization purity, the amplification using a few-mode PM fiber amplifier should be thoroughly investigated theoretically and experimentally.

In this contribution, a 200 W-level few-mode PM fiber amplifier using a few home-made fiber devices and featuring a radially polarized beam is firstly demonstrated (to the best of our knowledge). Experimentally, a home-made fiberized mode convertor was employed to convert LP01 mode to TM01 mode (radially polarized beam), by which a TM01 mode seed laser with an output power of 2 W and a mode purity of 98.76% was prepared. The corresponding polarization purity was measured to be 98.5%. For avoiding the mode degeneracy and internal mode coupling, a home-made PM-combiner, featuring a smaller taper ratio than that of conventional combiner, was utilized in the experiment. In addition, a home-made PM-CPS without obvious loss for first-higher order mode was used. Finally, the radially polarized beam was scaled up to 192.1 W by using a few-mode PM fiber amplifier. The corresponding slope efficiency, mode purity, and polarization purity are 70%, 97%, and 98.2%, respectively. The polarization purity was only deceased by 0.3%, but it is 1.3% in our previous work within a hectowatt non-PM laser system. This indicates that this PM laser system effectively maintains a high polarization purity compared to a non-PM laser system. Moreover, the beam quality factor (M2) was measured to be about 2.2.