Frequency doubling in the enhancement cavity with single focusing mirror

Laser power enhancement cavities are usually applied for frequency doubling of continuous radiation in nonlinear crystals, because of relatively small available CW power. Besides, external cavities allow to produce single-frequency radiation. Four- or three-mirror ring cavity design with two focusing mirrors is usually used in experimental and theoretical studies as well as in applications. In these schemes the first curved mirror focuses a relatively wide beam into nonlinear crystal and the second one reproduces the original beam from divergent one, which comes out of the crystal. We propose the three-mirror cavity design with single curved mirror, which is used both for focusing and reproducing of the beam. Two additional flat mirrors allow to direct the beams into the crystal from and back to the focusing mirror and thus to close the ring cavity of triangle form. Calculations show that stability of the single-focusing-mirror cavity has larger tolerance for angles of the beam incidence at the curved mirror in comparison with traditional schemes and broader stability region. It is analogous to a two-mirror concentric cavity, but has advantages of a ring cavity at the same time. The compact cavity is experimentally realized for frequency doubling of 488 nm argon laser single frequency radiation in BBO crystal. Power enhancement achieved in the cavity is about 30 times. Thus 244 nm second harmonic power of 33 mW at 660 mW pump power is produced having about 10 m coherence length.