Asymmetric GaN planar waveguides grown on (0001) sapphire by molecular beam epitaxy are promissing submicro-metric periodically poled (PP) structures required for quasi-phase-matching the counterpropagatingthree-wave parametric interactions [1, 2]. However, the realization of cm long waveguides presenting a submicronic periodicity is not possible without moving the sample under the writing beam. This displacement introduces stitching errors. The device must then be formed by a chain of multiple PPGaN elements, each of about 100 mm long, jointed by uniformly polarized domains of mm length representing stitching errors. A mirrorless optical parametric oscillator (MOPO) was experimentally realized in a bold KTiOPO4 crystal by using the periodically-poled PPKTP with sub-mm periodicity [3]. Now we investigate a MOPO for a fragmented PPGaN waveguide. One of the remarkable properties of the PPKTP bulk MOPO with a chirped pump was the strong asymmetry in the spectral bandwidth: the bandwidth of the forward signal was comparable to that of the pump, whereas the bandwidth of the backward idler was typically one to two orders of magnitude narrower [4, 5]. Quite surprisingly here, the structure presenting stitching errors is almost as efficient as a perfect one because the generated coherent phase of the backscattered wave locks the phases of the forward propagating waves in such a way that they are almost unsensitive to the junctions where quasi-phase-matching is not preserved. Regardless if the phase modulation in the pump is deterministic or stochastic, the backward parametric wave always has a bandwidth that is narrow compared to that of the forward wave.
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