The thermodynamic limits of optical pumping - Investigation of a novel, intra-cavity pumped thin disk laser

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The fundamental thermodynamic limits of optical pumping of lasers shall be investigated. The performance that has been achieved over the past years by fiber lasers and thin disk lasers was made possible by replacing the incoherent pumping of classical rod lasers using gas discharge lamps by pumping the fibers and disks using spectrally and spatially partially coherent pump light from semiconductor lasers. The light from the semiconductor lasers only has a small quantum defect with respect to the laser wavelength. If the wavelength difference between pump light and laser light can be reduced even further, another increase of the brightness of fiber lasers and thin disk lasers can in principle be expected. However, in doing so, one is approaching a two-level laser system which is forbidden by the second law of thermodynamics. Technologically, this shows up in the form of two problems: On one hand, the absorption of the pump light is becoming increasingly weaker, while on the other hand the increasing thermal population of the lower laser level demands ever higher pump power densities in order to reach inversion. However, if the laser medium that is to be pumped is placed inside the resonator of the pump laser, both problems can be solved in an elegant way. This shall be realized for a high power laser for the first time. Looking at it from a thermodynamic viewpoint, we want to reduce the entropy in a two-step process: First, a conventional thin disk laser will be pumped by diode lasers. This laser will operate in a high transverse multi-mode and thus produces fairly coherent but not totally coherent light. This laser has no output coupler, it experiences losses only by the second laser disk which resides inside its resonator. This disk represents an impedance-matched coherent perfect absorber and shall produce spatially fully coherent laser light in the fundamental transverse mode at a wavelength that is just slightly longer.