Experimental Investigation Of High-power All-solid-state Single-frequency Laser

High-power all-solid-state single-frequency green lasers have been applied in many science and technology fields, such as quantum communication, holography, interferometry, generating deep ultraviolet radiation by frequency doubling and acting as the pump source of high-power Ti:sapphire laser and OPO, owning to its compact structure, perfect beam quality and low intensity noise. However, there are several obstacles involved in the power scaling and transformation to product of the high-power all-solid-state single-frequency green lasers as following:1) when we increase the pump power to further raise the output power of the laser, it decreases rather than increases with the increasing of the pump power and high-order modes are very likely to appear even under not very high pump power; 2) bistability-like phenomenon can be observered in the cases of increasing and decreasing the pump power accompanied by the sudden rise and fall of the output power of the laser, which will inevitably shorten the service life of the laser via the abrupt thermo-induced stress change in the intracavity elements. After longterm investigation, we find that an unconsidered factor in previous design of the laser——the thermal lens effect of the TGG crystal in the optical diode, plays an analogous role to the thermal lens effect of the gain medium in the high-power intracavity-frequency-doubled single-frequency laser and causes the above-mentioned problems. As a result, the present dissertation is focused on the influence of the thermal lens effect of the TGG crystal on laser performance and its relief and compensation measures. Furthermore, we investigated the influence of the curvature radius of the cavity mirrors on laser performance with the thermal lens effect of the TGG crystal taken into account.We firstly analysed the influence of thermal lens effect of the TGG crystal on laser performance by ABCD matrix then verified the theoretical expectation in a home-made solid-state laser, which indicated that the thermal lens effect of the TGG crystal didn't only exist but also had significant influence on laser performance——narrowing the stable range, reducing the pump power at the optimal operation point (OOP) and finally limiting the output power of the laser. The influence of the thermal lens effect of the TGG crystal could be passively mitigated by shortening the cavity length, however, shortening the cavity length couldn't eliminat the thermal lens effect of the TGG crystal and obvious bistability-like phenomenon was observed accompanied by the sudden rise and fall of the output power of the laser when the pump power was changed. We qualitively analysed the bistability-like phenomenon and further proved that the thermal lens effect of the TGG crystal was caused by the absorption of the intracavity fundamental radiation rather than the pump radiation, which was different from the mechanism of thermal lens effect of the gain medium.On the basis of above-mentioned work, we adopted DKDP crystal to adaptively and dynamicly compensate the thermal lens effect of the TGG crystal. First of all, we determined the cut direction of the DKDP crystal so that the angle between the optical axis of DKDP slice and the beam propagating direction was 30 degree, which made the thermal lens of the DKDP compensator as isotropic as possible to compensate the thermal lens of the TGG crystal effectively. Under the optimal cavity length, we firstly obtained the formula of the thermal focal length of the DKDP slice through the variation of the pump power at OOP in the compensation experiment of three pieces of DKDP slices with different thickness. On this basis, we found that a 1.65 mm-thick DKDP slice could completely compensate the thermal lens effect of the 8 mm-long TGG rod in our present experiment. As a result, we adopted the now available 1.6 mm-thick DKDP slice to compensate the thermal lens effect of the TGG crystal. After the compensation, the maximal output power of single-frequency 532 nm laser was raised from 14.7 W to 30.2 W and the bistability-like phenomenon was completely eliminated. In this way, we have obtained an all-solid-state continuous-wave intracavity-frequency-doubled single-frequency 532 nm laser with the highest output power in the world up to now.At last, we investigated the influence of the curvature radius of the cavity mirrors on laser performance contraposing the conclusion in the previous literatures that the high power solid-state laser should adope convex mirrors to compensate the thermal lens effect of the gain medium. Theoretical analysis showed that when the cavity length was kept in constant the threshold pump power and the pump power at the optimal operation point (OOP) decreased with the increase of the curvature radius of the cavity mirrors beside the gain medium. However, if the cavity length was shorten to a certain extent, the pump power at OOP could be recovered. The experimental results of three group of cavity mirrors with different curvature radius (R=1500 mm,3000 mm, and +?) were consistent with the theoretical expectation. Furthermore, we calculated the optimal cavity length under different curvature radius of the cavity mirrors, which could server as a guide line for the design of lasers with similar cavity structure. Finally, we obtained a 12.97 W 532 nm laser with the plane cavity mirrors which could stablely work in single longitudinal mode.Innovative work:1. We theoretically and experimentally investigated the influence of the thermal lens effect of the TGG crystal on laser performance and passively adapted to it by shortening the cavity length, by means of which the maximal output power of the single-frequency 532 nm laser was raised from 13.8 W to 18.7 W. In addition, we qualitively analysed the bistability-like phenomenon of the laser in the cases of increasing and decreasing the pump power and further proved that the thermal lens effect of the TGG crystal was caused by the absorption of the intracavity fundamental radiation rather than the pump radiation.2. We utilized DKDP slice to adaptively compensate the thermal lens effect of the TGG crystal in the high-power single-frequency laser, by means of which the bistability-like phenomenon of the laser was completely eliminated and the maximal output power of the single-frequency 532 nm laser was raised from 14.7 W to 30.2 W. To the best of our knowledge, this is the highest output power from the same type of single-frequency 532 nm laser in the world up to now.3. We investigated the influence of the curvature radius of the cavity mirrors on laser performance and proved that the laser adopting plane cavity mirrors could obtain similar performance to the laser adopting convex cavity mirrors only if we adjusted the cavity length to a certain extent.