Improvement of pulsed optical parametric oscillator by spatio-temporal shaping of the pump beam

Summary form only. During the past years high-energy, nanosecond OPOs have been the subject of numerous investigations because of their potential applications as powerful tunable sources. However, they require short cavity length to achieve high conversion efficiency and large pump cross-section to avoid optical damages. Typical cavities correspond to high Fresnel number and produce beams of bad spatial quality. An increase in OPO output energy frequently leads to a reduction in OPO brightness. We propose a new method based on spatio-temporal shaping of the pump wave to improve the transverse properties of OPO emission at high energy level. Extracavity optical parametric generation is usually performed in two steps (at least). The pump pulse is split in two parts of unequal intensity. The weakest pulse, once confined in a tightly focused beam serves for the actual OPG, while the pulse of high energy is used for amplification during a second pass through the nonlinear crystal. This permits most of the times to preserve a good beam profile. The same idea has been applied to OPO. At the beginning of the pumping the pump beam size is chosen to be small enough to ensure a transverse single mode oscillation in the cavity and energetic enough to start the oscillation. Then the following part of the pump beam, bringing the main part of the energy, is spread on a larger cross section to amplify the oscillating wave.