High pulse energy and symmetrical far field from an optical parametric oscillator in the red spectral range

Remote sensing applications often require high energy laser-like beams with narrow bandwidth and low divergence at wavelengths that cannot be reached directly with ordinary laser materials. The common way of generating such wavelengths is through optical parametric oscillators (OPOs), sumor difference frequency generators, or a combination of these. For the new beam to be generated with high conversion efficiency and good beam quality, careful design of the nonlinear optical conversion stage is necessary, in particular with respect to beam size, nonlinear crystal length, and in the case of an OPO, resonator length and output coupling. For an OPO with narrow beams it is usually possible to obtain a good beam quality with a simple resonator because the small gain region provides enough spatial filtering to suppress higher order modes. The interplay between pump beam size and crystal length also is important both for the conversion efficiency and the beam quality. The pump beam size affects the pump intensity, and if this is reduced, the nonlinear crystal must be longer to have sufficient conversion. On the other hand, if the crystal is too long or the pump intensity is too high, then back-conversion (the reverse of the desired nonlinear process) will reduce both the conversion efficiency and the beam quality. For a plane-wave (or top-hat) pump beam it is fairly straight-forward to find the optimal combination of beam size and crystal length, but as most pump beams have a strong transversal and temporal variation of their intensity, finding the optimal OPO parameters becomes increasingly difficult.