Mirrorless optical parametric oscillators with stitching faults: backward downconversion efficiency and coherence gain versus stochastic pump bandwidth

Abstract

Parametric downconversion in submicronic periodically poled (PP) quadratic materials allows for the generation of signal and idler waves in opposite directions. The distributed feedback mechanism enables a backward mirrorless optical parametric oscillator (BMOPO), which has been experimentally realized using a periodically poled KTiOPO (PPKTP) crystal with 800 nm periodicity. A remarkable spectral property of the BMOPO is that the bandwidth of the forward co-propagating wave is comparable to the pump bandwidth, whereas that of the backward wave is typically several orders of magnitude narrower. In the backward idler configuration realized in PPKTP the backward idler exhibits a coherence gain of two orders of magnitude. In this paper we will consider the backward signal configuration, where the effect of coherence enhancement of the backward signal wave is especially pronounced for exact group velocity matching of the co-propagating pump and idler waves. This quasi-phase-matched (QPM) scheme requires a 335 nm scale poling periodicity, which may be achieved in GaN waveguides, combining e-beam lithography and epitaxy. However, during the lithography step necessary to obtain the PP material, stitching faults may occur. The BMOPO device is then formed by a sequence of 200 micron long PP elements stuck together. We assume that the stitching faults do not exceed half of the poling period in order to prevent the parametric reverse process. The original three-wave QPM model with the explicit periodicity of the nonlinear parametric coupling is necessary for taking into account the periodicity faults in the junctions. Thanks to the high performance computing facility available at Nice University, we analyze the parametric downconversion efficiency and the coherence gain of the backward signal versus the stochastic pump bandwidth in a large pump-bandwidth range. We show that if the stitching faults do not exceed a quarter of the poling period, i.e., less than 80 nm, which is technically feasible, the efficiency is comparable to the BMOPO in the perfect PP material. In any case, for an incoherent pump of broad spectral bandwidth we obtain a coherence gain of almost three orders of magnitude.