Fig. 1. Schematic illustration of the experimental set-up. L1 and L2: lenses; HWP: half-wave plate; PBS: polarization beam splitter; QWP: quarter-wave plate; M1 to M4: dielectric mirrors; and OC: 15% output coupler. The red and purple beams present the signal for channel 1 and channel 2, respectively.

Fig. 2. Working principle. (a) The evolution of signal beam profiles and polarization states through one round trip in the lower channel of OPO. Green arrows correspond to the polarization of light. (b) Jones matrices of basic optical elements and propagation beams.

Fig. 3. Spectral properties and beam profiles of the output. (a) Measured signal spectra (blue area and red area are for channel 1 and channel 2, respectively) by tuning cavity length of each channel. (b) Output intensity profiles for both channels. Corresponding lobe structure and interference pattern of the vortex beams are also shown here.

Fig. 4. Output power of the proposed OPO. (a), (b) Variations of the (a) signal output power and (b) pulse durations versus the central wavelength in different channels.

Fig. 5. (a) Far-field intensity distribution of the Gaussian signal beams and vortex signal beams of vortex order ls=±2 from channel 2. Tilted lens image and self-referencing of the vortex signal beams are also shown here. (b), (c) Variation of the signal output power and pump-signal conversion efficiency with the pump power for different vortex orders in channel 2.