Abstract

The diode-pumped solid-state lasers that we are developing reach output powers of currently up to 620 W. Without adaptive techniques to compensate for the thermally induced lenses the generated beam quality and the pump power range DP for stable oscillation is limited by [1] where l is the oscillating laser wavelength, M² the beam propagation factor and D^* is the specific dioptric power of the thermal lens. Together with the radius R of the pumped cross section and the actual pump power P, D^* determines dioptric D power of the thermal lens according to In order to be able to operate the laser with good beam quality over a wide power range it is therefore necessary to adaptively compensate for the thermally induced lenses inside the resonator to reduce the overall intra-cavity value of D^*. We have demonstrated that this can be done very effectively with a self-adaptive negative thermal lens generated in a thin layer of a liquid or a gel [1] to compensate for the positive thermal lens in the laser rod. Figure 1 shows the total dioptric power measured inside a laser resonator. It shows that the dioptric power of the total thermal lens inside the cavity is reduced by more than an order of magnitude by the self adaptive compensating element. Figure 2 shows the effect of the compensating element on the stability of a given resonator. Without compensation the resonator gets unstable at an output power of about 35W due to the thermal lens in the Nd:YAG rod. With the compensating element the same resonator is stable up to the maximum available pump power. Current research is devoted to alternative implementations of the compensation technique, high-power experiments, and to the investigations of its limitations.

© 2001 EPS