(a) The OPA performance with respect to different cavity lengths at the pumping power of 10 W, where the propagation of solely lossy SPP (black curve), the signal in cavity with lengths of resonant length (red circle) and integer number times L_c (blue square), and lossy SPP after being pumped by a 3.32 μm cavity (green curve) are presented. (b) The output power of the signal wave varying with the initial phase difference at different pumping powers.