A mirrorless optical parametric oscillator (MOPO) is a very special class of optical parametric oscillators, where the feedback for oscillation is realized by counter-propagating second-order nonlinear interaction without a need for external mirrors. This arrangement makes the MOPO a very simple, self-aligned and robust oscillator. The MOPO was first proposed theoretically in 1966  however its realization was only possible using the quasi-phase matching (QPM) approach due to requirement for compensation of large momentum mismatch in such interactions . Realization of first-order counter-propagating parametric oscillation requires structures with periodicity in the sub-μm range, which poses substantial fabrication challenges. In addition to its self-aligned nature, MOPO offers unique spectral properties , whereby the spectral contents of the pump are transferred to the forward wave, while the backward wave remains narrowband. So far, only nondegenerate MOPO oscillation has been demonstrated. However, it has been theoretically proposed that a degenerate MOPO could be used to realize a high-brightness biphoton source for quantum optics applications .
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