Applied Physics B, 2013, Volume 111, Issue 2, pp 305-311.
D. Liang, J. Almeida, E. Guillot.
CEFITEC, Departamento de Física, FCT, Universidade Nova de Lisboa, Campus de Caparica, 2829-516, Caparica, Portugal and
PROMES-CNRS, 7 rue du Four Solaire, 66120, Font Romeu-Odeillo-Via, France
To clarify the advantages of Cr:Nd:YAG ceramics rods in solar-pumped lasers, a fused silica light guide with rectangular cross-section is coupled to a compound V-shaped cavity within which a 7 mm diameter 0.1 at.% Cr:1.0 at.% Nd:YAG ceramic rod is uniformly pumped. The highly concentrated solar radiation at the focal spot of a 2 m diameter stationary parabolic mirror is transformed into a uniform pump radiation by the light guide. Efficient pump light absorption is achieved by pumping uniformly the ceramic rod within the V-shaped cavity. Optimum pumping parameters and solar laser output powers are found through ZEMAX© non-sequential ray-tracing and LASCAD© laser cavity analysis codes. 33.6 W continuous-wave laser power is measured, corresponding to 1.32 times enhancement over our previous results with a 4 mm diameter Nd:YAG single-crystal rod. High slope efficiency of 2.6 % is also registered. The solar laser output performances of both the ceramic and the single-crystal rods are finally compared, revealing the relative advantage of the Cr:Nd:YAG rod in conversion efficiency. Low scattering coefficient of 0.0018 cm−1 is deduced for the ceramic rod. Heat load is considered as a key factor affecting the ceramic laser output performance.
The direct conversion of sunlight into laser light is of ever-increasing importance because broadband, temporally constant sunlight is converted into laser light, which can be a source of narrowband, collimated, rapidly pulsed radiation with the possibility of obtaining extremely high brightness and intensity. Among the potential applications of solar lasers are earth, ocean, and atmospheric sensing; detecting, illuminating, and tracking hard targets in space; deep space communications. Renewable solarlasers are also very promising for terrestrial applications such as laser material processing. The direct excitation of large lasers by sunlight offers the prospect of a drastic reduction in the cost of coherent optical radiation for high average power applications. Up to now solar-pumped lasers have not been viewed from this perspective.
Fresnel lenses have attracted much more attentions of solar laser researchers in recent years. However, a solar laser head pumped by a Fresnel lens usually moves together with the whole tracking structure, an optical fiber thus becomes essential for the transportation of solar laser radiation from the laser head to a fixed material processing chamber. Beside a lot of practical inconveniences, fiber optic transmission loss occurs, which will inevitably deteriorate the final collection efficiency of the whole solar laser system.
The advantage of heliostat-parabolic mirror solar energy collection and concentration system is comparatively neglected by solar laser researchers in recent years. We have, however, insisted on adopting this system. The advantage of having a fixed laser head at the focus of a stationary parabolic mirror becomes much more pronounced when the processing chamber is to be installed nearby. It is therefore very meaningful to improve the performance of the solar laser pumped through a heliostat-parabolic mirror system. The ultra-high power heliostat-parabolic mirror system, such as the 1 MW solar furnace of PROMES-CNRS, in France, might well become a super solar laser power station in the future.
The radiation transmission and homogenization capacity of a fused silica light guide is combined with the light coupling properties of the compound V-shaped cavity to provide an efficient side-pumping to a 7 mm diameter 0.1 at% Cr: 1.0 at% Nd:YAG ceramic rod. 33.6 W continuous-wave solar laser power is measured, corresponding to 1.32 times enhancement over our previous result with a 4 mm diameter Nd:YAG single-crystal rod. The introduction of the rectangular cross-section light guide has also ensured a much more stable laser emission than other solar laser pumping schemes.