Radiation-balanced thin-disk laser system

Summary form only given. Traditional solid-state lasers are exothermic. The quantum defect between the pump and laser photons is a source of heat generated inside a laser medium. Heat generation results in increased temperature and stress in the laser medium, poor beam quality, and limits the average output power of the laser. Thin-disk and optical fiber lasers with their very high surface-to-volume ratio and guidance properties are excellent solutions for reducing heat generation. However, heat transport continues to remains a problem at very high powers, even in these lasers. The idea to cool solids with anti-Stokes fluorescence was first proposed by Pringsheim in 1929 [1]. In 1995, Epstein's research team observed for the first time the net radiation cooling by anti-Stokes fluorescence in solid state materials [2]. In 1999, Bowman [3] proposed a radiation-balanced (athermal) laser, in which cooling with anti-Stokes fluorescence completely offset the heat generated by the quantum defect. In 2002 the first operation of a bulk radiation-balanced solid state laser was experimentally demonstrated [4]. For athermal operation, the pump wavelength, λ<;sub>P<;/sub>, has to be chosen between the mean fluorescence wavelength, λ<;sub>F<;/sub>, and the wavelength of the laser emission, λ<;sub>L<;/sub>, that is λ<;sub>F<;/sub> <;λ<;sub>P<;/sub><; λ<;sub>L<;/sub>. In addition, the pump power has to be properly arranged at each point along the length of the laser rod. The precise control of the pump intensity at each point along the gain medium is essential for athermal operation of the laser, but this is extremely difficult to achieve with high accuracy [4].In this work, we propose and consider theoretically, a new scheme for an athermal laser, which consists of a series of radiation-balanced Yb3+:KGW thin disks placed inside a single resonator. For simplicity, we have shown a modular five disk arrangement in Fig. 1. Indeed, this allows for the number of disks to be increased incrementally to reach desired output laser intensity ILout. In our scheme each disk is mounted on a substrate with very high (theoretically 100%) reflectivity at the laser wavelength. The substrate replaces a heat sink in a traditional thin disk laser and performs a purely mechanical function. The thickness of each disk as for traditional thin disk lasers can be approximately 100-200 μm. The combined thin disks operate as a single rod placed in a resonator with the “end mirror” and the “end coupling” mirror on the either end of the cavity (Fig.1). Contrary to the rod geometry each of these thin disks can be pumped with the well developed schemes for thin disk laser pump systems; one can easily control the pump intensity at each disk with high accuracy. In addition, in our scheme the pump intensities at different disks do not influence each other [3]. Each disk operates independently and not only the pump intensity, but also the ion concentration in each disk can be changed dramatically. The thin disk geometry can reduce re-absorption of the anti-Stocks radiation significantly. In our simulations the thickness of the disks is 100 μm. The fluorescence lifetime of the 2F5/2 is τ ~ 334μs. The mean fluorescence wavelength is λF = 992 nm. The pump and laser wavelengths are λP = 1001nm and λL = 1040nm, respectively. The output laser intensity as a function of the “end coupling” mirror is illustrated in Fig.2. The high flexibility of our scheme, such as the modular design, precise control of the pump intensity in each disk, the changeable ion concentration from disk to disk, reduced re-absorption of anti-Stokes photons make this scheme promising for development of radiation-balanced lasers, leading to higher efficiencies at higher powers than has been possible so far.

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