Realization of double resonance for a bright frequency-tunable source of narrowband entangled photons

In this paper, we report an interesting phenomenon when precisely adjust the tuning crystal for double-resonance of a type-II configured parametric amplifier cavity, which is later verified as a cavity-enhanced effect in optics alignment. The theoretical result indicates that an angle accuracy error within 0.09^o is necessary to achieve a high contrast ratio of 100:1 for a cavity with a finesse of about 205, which is crucial but high-demanding to get a high-quality narrowband entanglement source. Meanwhile, we figure out a method to release such a high requirement and get high visibility in a moderate-accuracy alignment.