Cavity-enhanced narrow bandwidth photon pairs generation at the telecom   wavelength with a triple resonance configuration
  In this paper, we present a narrow bandwidth source at the telecom wavelength based on type II PPKTP crystal inside an optical cavity. Simultaneous resonances of the pump and the two down-converted fields are achieved by special design of the cavity. This is a triple resonances optical parametric oscillator (OPO) far below threshold, which generates photon pairs at around 1560 nm with a bandwidth of 8 MHz. A coherence time of 27.7 ns is estimate by a time correlation measurement, and the estimated photon pairs generating rate is 134 s-1MHz-1mW-1. As entangled photon pairs in the telecom regime is suitable for long distance transmission and manipulation, this source is desirable for the demands of quantum communication. 
Zhi-Yuan Zhou, Dong-Sheng Ding, Yan Li, Fu-Yuan Wang, Bao-Seng Shi and   Guang-Can Guo
quant-ph physics.optics