Fig. 1. Optical design of ring-cavity configuration, in which red, blue, and green lines indicating potential routes of Stokes wave through rotating the angle of M4

Fig. 2. Structural diagram of experimental setup. (a) Ring-cavity THz parametric oscillator in which blue and red lines indicating pump wave and Stokes wave; (b) crystal profile

Fig. 3. THz wave output frequency and Stokes wave length versus tuning angle at pump energy of 150.30 mJ

Fig. 4. THz wave output energy versus frequency at pump energy of 150.30 mJ

Fig. 5. Comparison of cavity type between conventional linear-cavity THz parametric oscillator (solid line) and ring cavity THz parametric oscillator (dashed line)

Fig. 6. THz wave output energy versus pump energy for linear-cavity and ring-cavity parametric oscillators at THz frequency of 1.50 THz