Generation of mid-infrared wavelengths larger than 4.0 μm in a mirrorless counterpropagating configuration

We report that mid-infrared wavelengths larger than 4.0 μm are efficiently generated in a periodically poled lithium niobate as a counterpropagating backward optical parametric oscillator. In comparison with the forward optical parametric oscillator, the backward configuration does not require a cavity mirror to establish laser oscillations. Also, the changing curves of the threshold intensity with and without the idler absorption for beyond 4.0 μm generation are quite different. Whether the idler absorption is considered or not, the conversion efficiency of both the signal and the idler will always increase with the pump intensity above the threshold. However, as the idler absorption cannot be neglected, the signal becomes larger than that without the idler absorption, while the idler gets smaller, and the threshold intensity is lower than that in forward QPM optical parametric generation without a cavity mirror only when the idler wavelength is less than 5.6 μm. The design that is presented offers practical mid-infrared generation with a low pump threshold.