Exploring the thermodynamic limits of optical pumping
Most optically pumped solid state lasers represent essentially "brightness converters". They transform pump light with low beam quality into a laser beam of much better beam quality. Industrial high-power diode-pumped thin disk lasers are a good example.
Fig. 1: A diode-pumped thin-disk laser operating at 1030 nm is used for intra-cavity pumping of a second Yb:YAG disk. The second Yb:YAG disk lases at 1050 nm and has its own resonator. This new pumping scheme produces a very small quantum defect. Fig. 1: A diode-pumped thin-disk laser operating at 1030 nm is used for intra-cavity pumping of a second Yb:YAG disk. The second Yb:YAG disk lases at 1050 nm and has its own resonator. This new pumping scheme produces a very small quantum defect.
The goal of laser engineers is to achieve maximum energy efficiency when converting partially coherent light from the pump source into a fully coherent laser output. The second law of thermodynamics dictates that this process can not be achieved with 100% efficiency. A certain fraction of the pump energy always has to be converted into heat or another "useless" form of energy that carries away excess entropy of the pump light. The minimum energy difference is the "quantum defect" of that laser transition.
The interesting question arises what the maximum efficiency for the conversion process is and how closely one can approach it with a practical device. In typical solid state lasers, there are two effects that prevent us from achieving efficient laser action in a system with a low quantum defect. The absorption of the pump light is usually decreasing and thermal population of the lower laser level is increasing as the quantum defect becomes smaller. This means that very high pump power densities have to be achieved in a gain medium that is only weakly absorbing.
We want to address this challenge with the new laser concept of an intra-cavity pumped thin disk laser. A second Yb:YAG thin disk is placed into the resonator of an industrial, diode-pumped Yb:YAG thin disk laser. The diode-pumped Yb:YAG thin disk laser has no output coupling other than the absorption of the second disk. That second disk has its own laser resonator and produces a laser beam at a slightly longer wavelength than the first disk. For example, the first disk would produce 1030 nm transverse multimode output, the second 1050 nm TEM00. The second disk is experiencing very little heat due to the small quantum defect, enabling efficient TEM00-operation. Dispersive elements should allow pushing the two wavelengths very close together. This will allow us to explore the thermodynamic limits of optical pumping.
The project was funded by the DFG (Deutsche Forschungsgemeinschaft).