摘要： 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 [1] 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 [2]. 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 [3], 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 [4]. In this work we report on the first realization of the degenerate MOPO generating counter-propagating signal and idler in the spectral band of optical communications. The MOPO was realized in periodically poled Rb-doped KTiOPO4 (PPRKTP) with a QPM period of 420 nm. The structure was fabricated by the coercive field engineering technique [5]. First, the crystals were patterned using a UV laser interference lithography. Then a coercive field grating was created by periodic ion-exchange. Finally, the crystals were poled using planar electrodes. The resulting domain pattern shows excellent uniformity over the whole crystal aperture, and a duty cycle close to 50% (see Fig 1(a)). The optical surfaces of the crystals were polished at an angle to prevent Fresnel-reflection feedback into the MOPO.