Investigation of the thermal lens effect of the TGG crystal in high-power frequency-doubled laser with single frequency operation.

The thermal lens effect of the TGG crystal is investigated theoretically and experimentally. The theoretical analysis is demonstrated by the experimental measurements on a home-made frequency-doubled Nd:YVO(4) laser with single-frequency operation. In the presence of the thermal lens effect of the TGG crystal, the output power can be optimized by shortening the distance between the cavity mirrors of M(3) and M(4) (two plane-concave mirrors placed at two sides of the second-harmonic generator). Consequently, a single-frequency laser with output power of 18.7 W at 532 nm is obtained. The power stability and the beam quality M(2) are better than ±0.4% for 5 hours and 1.08, respectively. Meanwhile, we observe and discuss a bistability-like phenomenon of the laser in the cases of increasing and decreasing the incident pump power.