Keywords: 
• Raman fiber laser /
• vortex beam /
• acoustically-induced fiber grating /
• orbital angular momentum

Abstract:   Objective  In recent years, vortex beams carrying orbital angular momentum (OAM) have attracted much attention due to their important research value and application prospects in optical communication, particle acceleration, particle manipulation, super-resolution imaging, and other fields. At present, vortex optical field can be mainly divided into two types, one is generated by spatial devices, such as the spatial light modulator, and the other is generated in fiber. Fiber lasers possess the advantages of compact structure, convenient thermal management, and high efficiency, which is conducive to realize the output of high-power and high-stability vortex beam. With the rapid development of big data, cloud computing, Internet of Things, and other technologies, it is urgent to expand the information capacity of communication systems. Vortex beams can greatly improve the capacity of communication systems because of their infinite orthogonality. At present, the output vortex beams of fiber lasers are mainly concentrated in the emission bands of rare earth ions such as ytterbium-doped, erbium-doped and thulium-doped, and the combination of space division, mode division and wavelength division multiplexing technology urgently needs to expand the wavelength range of the vortex beam. For this purpose, an all-fiberized vortex Raman fiber laser (RFL) is designed.   Methods  At present, many devices can realize vortex beam output in a fiber laser. Among them, the acoustically-induced fiber grating (AIFG) has the advantages of simple structure, wide wavelength tuning range, fast response speed and low insertion loss. Combined with the AIFG and RFL, when the output wavelength is converted by Raman frequency shift, there is no need to redesign and replace the mode conversion device. The RFL is built (Fig.1). The laser resonator is composed of a pair of fiber Bragg gratings and gain fiber. The AIFG is fused after the cavity. To control the polarization state of the output mode, a three-loop polarization controller (PC) is connected after AIFG. And the transmission spectrum of the LP₀₁ mode is tested (Fig.2(a)), which indicates that there is a linear relationship between the frequency and the wavelength. Once the suitable electrical signal is loaded on the AIFG, the output mode is converted to LP₁₁ mode, and the ring-shaped radially polarized light and vortex beam with topological charge l=±1 output can be realized by precise polarization control.   Results and Discussions   The variation curve of the output signal optical power with the pump optical power is shown (Fig.3(a)), where the black and red curves correspond to the output mode of LP₀₁ mode and LP₁₁ mode, respectively. The maximum output power of LP₁₁ mode is 69.6 W, with a slope efficiency of 90.6% and total optical efficiency of 83%. The output spectrum at the highest power is shown (Fig.3(b)). The central wavelength of the output laser is 1 134.72 nm, and the second-order Raman light with a central wavelength of 1 194.16 nm restricts further power improvement. At the highest power, the spectral purity of signal light reaches 99.72%. By adjusting the PC, the radially polarized light output can be obtained. The mode field distribution of the output beam is detected by rotating the linear polarizer (Fig.4(b1)-(b4)), which verifies the radially polarized TM₀₁ mode. Once there is a π/2 phase difference between the $ HE_{21}^{even} $ and $ HE_{{\text{2}}1}^{odd} $mode, the vortex beam can be realized through the superposition of the two modes, and the "Y-shaped" interference fringe can be detected through the self-interference (Fig.4(c)-(d)), which proves that the vortex beam with topological charge l=±1 is generated.   Conclusions  An all-fiberized Raman fiber laser with radially polarized light and vortex beam with topological charge l=±1 output is realized based on AIFG. It has the advantages of compact structure, wide wavelength tuning range, fast response speed and low insertion loss. The output central wavelength is 1 134.72 nm, with the spectral purity of 99.72%. The maximum output power is ~70 W, and the efficiency is 83%. As an all-fiberized mode conversion device, the AIFG is expected to be the key device to fill in the gaps in the spectrum of vortex beams due to its ultra-wide wavelength tuning ability and high power tolerable capacity, which could provide a reliable light source for the application and exploration of vortex beams in special waveband. By replacing the pump source and the gain fiber, the wavelength coverage can be extended further through cascaded Raman shift.