In recent years, vortex beams carrying orbital angular momentum (OAM) have caught much attention due to their research significance and application prospects. With the applications of vortex beams in sensing, measurement, and high-capacity optical communication, the output bandwidth and wavelength tunability of vortex beams have become a research focus. Breaking through the emission wavelength limitation of rare-earth doped fiber, and the device of broadband mode conversion is the basis for realizing the output of special band/broadband vortex light. Currently, many devices can realize vortex beam output in a fiber laser. However, most devices are designed and manufactured according to the target wavelength. The acoustically-induced fiber grating (AIFG) achieves mode conversion by acousto-optic coupling in passive fibers. When the operating wavelength changes, it only needs to change the frequency of the loaded electric signal, without re-designing and replacing the parameters of the mode conversion device. Theoretically, it has an extremely wide operating bandwidth. Considering the above requirements, the structure of random Raman fiber laser (RRFL) based on distributed Rayleigh backscattering is adopted to realize broadband vortex beams by combining the AIFG.
By combining the AIFG and RRFL, when the output wavelength is converted by Raman frequency shift, there is no need to redesign and replace the mode conversion device. The transmission spectrum of the LP01 mode is tested in Fig. 1(b), which indicates that there is a high efficiency of mode conversion from 1000 to 1700 nm. The RRFL is built as shown in Fig. 2. An amplified spontaneous emission (ASE) source including two amplification stages is utilized as the pump source which is then coupled into the half-open cavity of RRFL by wavelength division multiplexing (WDM). The half-open cavity is formed by a high-reflective (HR) optical fiber mirror which is attached to the WDM, a piece of gain fiber, and a homemade fiber endcap. The reflectance of the HR mirror is more than 99.5% at 1-2 μm, and anti-reflection coating is conducted on the endcap to evade unwanted end feedback. The gain fiber is the commercial CS980 fiber with a length of 500 m. 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.
When the pump power reaches the Raman threshold, the pump energy begins to transfer to the Raman Stokes. By integrating the output spectrum, the variation curves of the Raman optical power of each order are calculated, as shown in Fig. 3(a). When the pump power reaches 76 W, the output wavelength reaches 1513.7 nm by the six-stage Raman shift, with a power of 23.6 W and a total efficiency of 31.1%. With the cascaded wavelength conversion, the purity and efficiency of high-order Raman light decrease, which is shown in Figs. 3(c) and 3(d). With the increasing output wavelength, the loss of gain fiber rises with the incomplete conversion of each stage, which results in a gradual efficiency decrease. Once there is a π/2 phase difference between the eigenmodes by controlling the polarization controller (PC), the vortex beam can be realized via the superposition of the two modes, and the “Y-shaped” interference fringe can be detected by the self-interference experiment (Fig. 5), which proves that the vortex beam with topological charge l=±1 is generated.
We propose an all-fiber high-power RRFL with vortex beam output in the 1.1-1.5 μm band. Based on the cascaded Raman shift and broadband AIFG, the output of vortex beams with topological charges l=±1 at 1133.9, 1197.6, 1260.5, 1331.8, 1414.5, and 1513.7 nm wavelengths is realized, and the topological charge is verified by self-interference experiments. After the six-stage conversion, the power at 1513.7 nm wavelength is 23.6 W, with an efficiency of 31.1%. The ultra-wide wavelength tuning capability of the AIFG is expected to make it a key device to fill the spectral gap of vortex beams and can provide a reliable light source for the application of special wavelength vortex beams. By replacing the pump source, gain fiber, and related devices, the wavelength coverage of the vortex beam can be further expanded in other wavebands, and the application of vortex light in multi-dimensional optical communication and interaction between light field and matter can be further expanded.
