Mid-infrared cylindrical vector beams have important applications in laser material processing, particle trapping, microscopy, and information storage. Common methods for the direct generation of these cylindrical vector beams in a solid-state laser are mainly based on the thermal birefringence effect of the gain medium, off-axis pumping regime, and polarization mode conversion element of the Q-plate. However, these typical generation methods have many intrinsic disadvantages, including poor stability, low lasing efficiency, and low output power. In contrast, a mid-infrared S-waveplate, which is a self-organized nanostructured grating that is inscribed in fused silica glass using a femtosecond laser, is an efficient mode-conversion element that can handle high power densities. We achieve high-power and stable mid-infrared cylindrical vector beams by incorporating this advantageous component into a mid-infrared Er∶YAP laser. In this study, highly efficient mid-infrared cylindrical vector beams are directly generated based on an S-waveplate acting as an intra-cavity polarization mode conversion device in an all-solid-state Er∶YAP laser. Radially and azimuthally polarized lasers can be easily fabricated and switched. In the continuous-wave operation mode, maximum average output powers of 136 mW and 133 mW are obtained for the radially and azimuthally polarized laser beams, respectively. Using an optical chopper, Q-switched pulsed cylindrical vector laser beams are also achieved, with the shortest pulse width being 124.73 ns and the single pulse energy being 13.84 μJ. These results provide insight into the structural design and experimental realization of mid-infrared structured lasers.

The experimental schematic of the mid-infrared all-solid-state cylindrical vector laser is shown in Fig. 1. The pump source was a fiber-coupled laser diode with a maximum pump power of 27 W. The center wavelength was locked at 976 nm with a spectral linewidth of less than 0.7 nm. The core diameter and its numerical aperture of the pig-tailed fiber were 105 μm and 0.22, respectively. A pair of coupling lenses was used to reimage the pump beam into the gain medium. A b-cut 5% (atomic fraction) Er∶YAP crystal with dimensions of 2 mm×2 mm×10 mm was selected as the gain medium. The Er∶YAP crystal was closely packed in a water-cooled heat sink at 16 °C to avoid thermal damage to the laser crystal. The linear resonator was composed of plane mirrors consisting of an input mirror (IM) and an output coupling mirror (OC). The IM had a high transmittance at 976 nm and high reflectivity at 2700?3000 nm, whereas the OC was coated with 5% reflectivity at 2700?3000 nm. An optical chopper was used as the active Q-switch. The total number of slits was 100, and the duty cycle was 50%. The rotation speed was continuously switched from 2 r/s to 100 r/s, corresponding to a modulating frequency range of 200 Hz to 10 kHz. A femtosecond-laser-inscribed self-organized nanostructured grating (S wave plate) was used as the intra-cavity polarization mode conversion device.

When the cylindrical vector laser is operated in the continuous wave regime, maximum output powers of 136 mW and 133 mW are obtained at an absorbed pump power of 14.07 W for the radially and azimuthally polarized laser emissions, respectively (Fig. 2). The doughnut-like intensity distributions of the radially and azimuthally polarized beams are shown in Fig. 3. After the optical chopper was inserted, the Q-switched pulse characteristics of the cylindrical vector laser output are studied. At a repetition rate of 5.20 kHz, maximum average output powers of 72 mW and 73 mW are achieved for the radially and azimuthally polarized beams, respectively, under a maximum absorbed pump power of 14.07 W (Fig. 4). The corresponding shortest pulse width and the peak power are 124.73 ns and 111 W, respectively, for the azimuthally polarized mode. Under a moderate pump power of 12 W, the pulse width increases from 176.47 ns to 244.16 ns when the repetition rate ranges from 5.2 kHz to 10 kHz, with the corresponding peak power varying from 37.05 kW to 17.61 kW (Fig. 5). The output wavelengths are centered at 2.73 μm in both continuous-wave and Q-switched operation regimes (Figs. 2 and 4). The output intensity profiles of the Q-switched cylindrical vector beams and their corresponding intensity distributions after passing through the polarizer are shown in Fig. 6, which indicates the generation of high-purity mid-infrared cylindrical vector beams.

We demonstrate the direct generation of mid-infrared cylindrical vector beams using an all-solid-state Er∶YAP laser. Radially and azimuthally polarized modes can be easily achieved and switched. The evolution of the intra-cavity polarization states is evaluated in terms of the Jones matrix, which satisfies the self-consistent transformation. In the continuous-wave mode, the radially and azimuthally polarized beams have output powers of 136 mW and 133 mW, respectively. Furthermore, by using an optical chopper as a Q-switch, we achieve pulsed cylindrical vector beams with the shortest pulse width being 124.73 ns at a repetition rate of 5.20 kHz. Moreover, the corresponding single pulse energy reaches 13.84 μJ. The output wavelength is centered at 2.73 μm in the continuous and Q-switched operation modes. The laser structure proposed in this study provides a simple and cost-effective scheme for the realization of nanosecond-level mid-infrared structured light fields in all-solid-state lasers.