Optical vortex lasers, with good beam quality in the mid-infrared spectral region, has many interesting applications such as super-resolution molecular absorption microscopy and molecular spectroscopy. The optical parametric oscillator (OPO) has been established as the most direct method to change the wavelength and transition the orbital angular momentum (OAM) of an optical vortex pump beam. A single-idler resonant cavity can produce a high-quality mid-infrared vortex output. However, one of the main challenges has been to manage the transfer of OAM from the pump beam to the mid-infrared idler output, especially given the significant wavelength difference—over three times—between the 1 μm pump and 3.5 μm idler beam. This discrepancy complicates achieving high spatial overlap efficiency between the pump and idler vortex modes in the optical vortex pumped idler-resonant parametric oscillator. By choosing cavity mirrors with the correct radius of curvature, a half-symmetric OPO system can facilitate the transfer of the pump beam's OAM to the idler output, ultimately producing a high-quality mid-infrared vortex beam.

In the paper, the idler single resonant optical vortex parametric oscillator based on KTA was examined. A conventional flash-lamped Q-switched Nd∶YAG laser (with a Gaussian spatial form, pulse duration of 25 ns, wavelength of 1.064 μm, and pulse repetition frequency of 50 Hz) was employed as the pump source. The laser output was converted into a first-order optical vortex beam using a spiral phase plate. This beam was then focused into a non-critically phase-matched KTA crystal with dimension of 5 mm×5 mm×30 mm. A plane-parallel cavity was formed using M1, which had high transmission for the pump and high reflection for the idler output beam, and an OC that had high transmission for the pump and signal beams, and a partial reflectivity (80%) for the 3.5 μm (idler) beam. A plane-concave cavity was created using a plane-concave M2 (with a curvature radius of 500 mm) that was anti-reflection coated for the pump field and high-reflection coated for the idler beam. An OC, which was partially reflective (R=80%) for the idler field and high-transmitting for the pump and signal fields, was used. The pump beam was observed using a conventional CCD camera. The spatial forms and wavefronts of the signal and idler outputs were measured with a pyroelectric camera (Spiricon Pyrocam III; with a spatial resolution of 75 μm). A lateral shear interferometer with a Mach-Zehnder geometry was used, allowing the optical vortex output to interfere with its own copy, given a proper lateral displacement.

By using an input mirror with an appropriate radius of curvature and a flat output mirror, plane-parallel and plane-concave cavities are established, respectively. This setup enables the selective transfer of the pump beam's orbital angular momentum to either the signal or idler outputs. The plane-concave cavity produces a high-quality mid-infrared vortex beam with M² factors of 2.1 and 2.2 in the two orthogonal directions, as shown in Fig. 4. We achieve 0.82 mJ of 3.468 μm mid-infrared vortex output and 3.04 mJ of 1.535 μm near-infrared vortex output, with a maximum pump energy of 20.6 mJ. This corresponds to slope efficiencies of 28.21% and 7.62%, as depicted in Fig. 5. The transfer principle of OAM is theoretically elucidated by considering the spatial overlap efficiency between pump and idler fields in the two cavities. The spectral bandwidths (FWHM) of the signal and idler outputs are measured as Δλ_s=0.85 nm and Δλ_i=1.08 nm (Fig. 3), respectively.

To produce high beam quality and high energy vortex laser in the near/mid-infrared band, an idler-resonant mid-infrared optical vortex parametric oscillator, formed by a 1 μm optical vortex pumped KTA, is constructed. We obtain 0.82 mJ of 3.468 μm mid-infrared vortex output and 3.04 mJ of 1.535 μm near-infrared vortex output at the maximum pump energy of 20.6 mJ, corresponding to a slope efficiency of 28.21% and 7.62%, respectively. With appropriate radius curvature of the cavity mirrors, the plane-concave OPO system enables the OAM of the pump beam transfer to the idler output, and it delivers high beam quality mid-infrared vortex beam. Combined with the advantages of the idler single resonant optical vortex parametric oscillator, the beam quality factors of mid-infrared idler beam in the horizontal and vertical directions are Mx2≈2.1 and My2≈2.2, respectively, and the spectral bandwidths of near/mid-infrared vortex are Δλ_s=0.85 nm andΔλ_i=1.08 nm, respectively.