Since 1.7 μm lasers are located in the eye-safe wavelength band and also within the fingerprint absorption peaks of many important gas molecules, they have potentially important applications in biomedical, gas sensing, and other fields. Meanwhile, as a novel structured light field, vortex beams can have unique features like annular light intensity distribution, helical phase wavefront, and orbital angular momentum. Therefore, developing high-performance 1.7 μm vortex lasers and investigating involved technologies can further expand the application fields of the lasers, providing scientific significance and application prospects. It is generally difficult for traditional rare-earth-ion doped fibers and crystals to cover the 1.7 μm emission band, or they can only have very weak laser gain in this wavelength band. Additionally, the vortex beam generation usually relies on a free-space lasing structure. These factors ultimately result in a complex vortex lasing configuration operating in the 1.7 μm band with extremely poor integration and low output power. Thus, we employ a helical long-period fiber grating as a vortex mode converter, and propose a high-power all-fiber vortex laser based on a 1.7 μm random fiber laser (RFL) with half-opened cavity, producing a maximum output power of 2.09 W at 1690 nm. Benefiting from the all-fiber structure of the vortex RFL, the laser output shows excellent temporal stability with a short-term temporal fluctuation as low as 2.8%. The results can not only provide a feasible approach to achieve a compact 1.7 μm high-power vortex laser with excellent temporal stability, but also further expand its applications in laser medicine, gas detection, optical tweezers, biological imaging, and other fields.

First, a 1.7 μm high-power RFL is constructed based on the stimulated Raman scattering effect. Then, a helical long-period fiber grating is adopted as a vortex mode converter with the vortex mode conversion efficiency of about 97% corresponding to 16 dB, which can convert the 1.7 μm random lasing into a first-order vortex beam. In this sense, a 1.7 μm high-power vortex RFL with an all-fiber structure is achieved, with the maximum output power of 2.09 W and central wavelength of 1690 nm. Benefiting from the all-fiber structure of the vortex RFL, the whole lasing system has a compact configuration with sound integration and simple thermal management and thus can achieve high-power vortex beam output. Additionally, the vortex RFL shows excellent temporal stability (short-time temporal fluctuation as low as 2.8%), modeless resonant output, and low relative fluctuations. It is expected that the output power of the vortex RFL can be further enhanced by increasing the incident power of the 1.7 μm RFL and optimizing the performance of the helical long-period fiber grating.

The 1.7 μm high-power vortex random lasing is realized based on a 1.7 μm RFL and a helical long-period fiber grating. The maximum output power is 2.09 W and the central wavelength is 1690 nm (Fig. 3). Furthermore, Fig.3(b) shows the relationship between the output power and the slope efficiency of the 1.7 μm vortex RFL and the incident power. The output power of the vortex RFL increases almost linearly without obvious saturation signs for the whole power scaling range. By increasing the injection power of the 1.7 μm RFL and replacing the helical long-period fiber grating with better performance, the output power of vortex RFL can be further enhanced. Meanwhile, the topological charge of the vortex RFL is characterized based on a homemade Mach-Zender interferometer, where the vortex laser output is interfered with a reference beam (or the spherical wave). The topological charge is measured to be one, which means the first-order vortex beam (Fig. 4). Finally, the short-time lasing characteristics and the radio frequency (RF) spectrum of the 1.7 μm vortex RFL at the highest output power of 2.09 W are measured. Thanks to the inherent excellent temporal stability and modeless resonant output characteristics of random fiber lasing, the 1.7 μm vortex lasing output inherits the intrinsic advantages of RFL, exhibiting very low short-time temporal fluctuations of 2.8% without resonant cavity frequencies in the RF spectrum (Fig. 5).

We propose a 1.7 μm high-power vortex RFL with an all-fiber structure. The 1.7 μm high-power vortex RFL is realized based on a RFL with a half-open cavity and the helical long-period fiber grating. The maximum output power is 2.09 W and the central wavelength is 1690 nm. The vortex RFL shows excellent temporal stability, low relative intensity fluctuation, and modeless oscillation output. The short-time temporal fluctuations are as low as 2.8%. By increasing the injection power of the 1.7 μm RFL and replacing the helical long-period fiber grating with better performance, the output power of the vortex RFL can be further increased. The vortex RFL with higher topological charges can be realized by simply replacing the corresponding helical long-period fiber grating. This work provide a feasible scheme for the realization of high-performance 1.7 μm vortex lasers, which is expected to be applied to laser medicine, gas detection, optical tweezers, and bio-imaging fields.