Fig. 1. Different cavity modes in OPO and Janus OPO designs. (a) A Gaussian-pumped OPO oscillating in a fundamental Gaussian mode. (b) An LG-pumped OPO with an LG cavity mode and an LG output mode. (c) A specially designed Janus OPO that is pumped by a Gaussian mode but outputs an LG mode. (d) A one-round-trip mode conversion without an imaging system. An LG mode passing through a VVW produces a hollow Gaussian beam, which evolves into a Gaussian-like mode after a certain propagation. However, the hollow Gaussian beam cannot recover itself without the equivalent lens as in panel (e) and neither can the LG mode. (e) A one-round-trip mode conversion inside a Janus OPO. The input coupler with a radius curvature of R1 can be seen as an equivalent lens with a focusing length of 2f=R1. Therefore, the light field after the VVW, which is set at the curvature center, will recover itself at the same position after being reflected by the input coupler.

Fig. 2. Experimental setup and Janus mode simulation. (a) The PPLN crystal, as the nonlinear medium, transforms one pump photon into a signal photon and an idle photon through the QPM parametric downconversion process. The input/output couplers are coated for high reflectivity at the signal wavelength. The FR, QWP, and VVW form a mode conversion setup inside the cavity. The QWP alters the vertical polarization of the signal beam to circular polarization so that the spin-OAM conversion can happen on the VVW to achieve the desired Gaussian-to-LG mode conversion. The output LG mode can be changed by rotating the QWP or replacing the VVW. FR is used to keep the signal wave to be vertically polarized inside the PPLN crystal. (b) Janus cavity modes for l=1, 2, and 4 and their cross sections at different distances of 0, 25, 50, 75, and 130 mm away from the input coupler.

Fig. 3. TC-controllable generation of high-purity LG(l,0) modes at the wavelength of 1550 nm. Modal analyses show high mode purities of 96.6%, 97.0%, 95.2%, and 93.7% for (a) LG(1, 0), (b) LG(–1, 0), (c) LG(2, 0), and (d) LG(4, 0) modes, respectively. The insets are the intensity patterns of the corresponding LG modes.

Fig. 4. Wavelength tunable high-purity LG modes. (a) Dependence of the conversion efficiencies of LG modes (l=1, 2, 4) on the wavelength ranging from 1500 to 1600 nm at a pump power of 4.2 W, showing high conversion efficiencies at the designed bandwidth. (b) Dependences of output powers of LG(1, 0) mode on the pump power at 1525, 1550, 1575, and 1600 nm, respectively, showing high-quality OPO output performances. (c) Modal analysis for the output LG(1, 0) mode at the wavelengths of 1525 and 1575 nm, showing the mode purity up to 97.1%.