Downconversion efficiency in backward optical parametric oscillators

Description

In optical parametric oscillators based on periodically poled (PP) non-linear quadratic materials, choosing a submicronic poling period allows generating the phase-matched signal and idler waves in opposite directions. In this case a distributed feedback mechanism is established, which enables optical parametric oscillation without the need of external mirrors or surface coatings. A backward mirrorless optical parametric oscillator (BMOPO), has been experimentally realized using a periodically-poled KTiOPO4 (PPKTP) crystal with sub-µm periodicity as a quasi-phase-matched (QPM) nonlinear medium. A remarkable property of the BMOPO is the strong asymmetry in spectral bandwidth between the signal and the idler pulses: the bandwidth of the co-propagating forward wave is comparable to the pump bandwidth, whereas that of the backward wave is typically one to two orders of magnitude narrower. It has been recently demonstrated both experimentally and numerically for a pump wave exhibiting deterministic phase modulation in the PPKTP BMOPO. This property is studied here in a guided wave configuration for broad incoherent pump pulses with bandwidth up to 4.7 THz. We demonstrate that in this case the higher the incoherence of the pump, the larger the amount of coherence transfer. The group velocities of the forward propagating waves can be perfectly matched by an appropriate waveguide profile in order to optimize the coherence transfer to the backward propagating wave. However, a nanometric QPM periodicity is required for perfect group velocity matching of the pump with the forward idler wave. Up to now such short periods have been only achieved by epitaxy in GaN. In order to have long enough waveguides we have studied this process in a PPGaN waveguide presenting stitching errors, which are very likely to occur during the fabrication process of the waveguide. We model a BMOPO in a fragmented GaN waveguide consisting of a sequence of submicronic PP elements separated by uniformly polarized connection sections representing the stitching errors. We have shown that in these fragmented waveguides the coherence transfer is nonintuitivelly almost as high as for a single piece PP waveguide because the generated coherent phase of the backscattered wave locks the phases of the forward propagating waves in such a way that the dynamics is almost insensitive to the short connection sections. We analyse on a large pump bandwidth range the downconversion efficiency versus the stochastic pump bandwidth for two different fragmented PPGaN BMOPOs and we compare it to the one piece periodically polarized PPGaN BMOPO.

Abstract

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