An optical parametric oscillator (OPO) consists of a crystal with second-order optical nonlinearity placed inside a cavity. This device performs down conversion of laser frequency to two lower frequencies, named 'signal' and 'idler'. In order for the OPO process to have significant efficiency it has to be phase-matched. A common method for obtaining quasi-phase-matching is the modulation of the sign of a ferroelectric crystal's nonlinearity by an external electric field (i.e. electric field poling).

Energy conservation requires that the photon energies of the signal and idler sum up to the pump photon energy. Therefore, pump-to-idler energy conversion efficiency is limited by quantum efficiency. For example, the quantum conversion efficiency of a 1 micron pump to a 4 micron idler is 25%.

In order to improve the pump-to-idler energy conversion efficiency we introduced an additional process into the crystal: the difference frequency generation (DFG) process between the signal and the idler. This process converts the signal frequency into the idler frequency and an additional intermediate frequency, and so raises the quantum limit for the pump-to-idler conversion efficiency by adding the pump-to-signal-to-idler conversion path. Simultaneous quasi-phase-matching of both the OPO and DFG processes throughout the entire crystal is achieved by using a quasiperiodic modulation of the crystal's nonlinearity, making it a nonlinear quasicrystal. Thus the two processes have long and overlapping interaction lengths, yielding high pump-to-idler conversion efficiency, as demonstrated experimentally.