Optical Parametric Oscillators
The interaction of a pump field with a second order nonlinear crystal produces two, entangled, lower-frequency photons via the process of spontaneous parametric down-conversion (SPDC). An optical parametric oscillator (OPO) can increase the number of entangled photon pairs generated by using stimulated parametric down-conversion .
By using a pump carrying orbital angular momentum (OAM) , this system is capable of producing an amplified source of photons entangled in their OAM. Photons entangled in their OAM are of interest for communications as each photon can be encoded to carry much larger amounts of information compare with using polarisation alone. Together with the added advantage of tuning the output frequency, this system may be capable of bringing major advances in high-bandwidth, secure communication closer to practical realisation.
 M. Förtsch et al., Nature Comm. 4, 1818 (2013)
 A. M. Yao, “Angular momentum decomposition of entangled photons with an arbitrary pump”, New J. Phys., 13, 053048 (2011).