Research On Solar Pumped Cr/Nd:YAG Ceramic Laser

Solar-pumped lasers have potential applications in space solar power system（SSPS）, atmospheric and ocean sensing, deep space communications and other fields.Currently, the key issues of solar-pumped solid-state lasers are generally researched asfollowing:（a）, the threshold pumped power is higher.（b）, the volume is large.（c）, it isharder to maintain. In this paper, based on Cr/Nd:YAG ceramic laser medium, severalcrucial questions are researched including the basic characteristic of laser medium, thedesign of Fresnel lens, and design and experimental scheme of the lower thresholdpumped power.The spectrum characteristic of Cr/Nd:YAG ceramic laser medium and energytransition mechanism are firstly researched. With the same dilution Nd³⁺concentration,both absorption spectrum of Nd:YAG crystal and Nd/Cr:YAG ceramic were comparied,The effective absorption cross section of Cr³⁺is larger than Nd³⁺at their absorbancepeaks. The stimulated emission spectrum was measured, when Nd³⁺was excited at808nm, and the calculation result of effective stimulated cross-section was3×10^-19cm².The stimulated cross-section was largly enhanced because of resonance energy transferbetween Cr³⁺and Nd³⁺, when Cr³⁺and Nd³⁺were exicted by solar spectrum at the sametime. The stimulated cross-section was three times larger than Nd:YAG crystal. Thetheory and experiment two kinds of methods were used to analyze the effective lifetimeof Nd upper level of Cr/Nd:YAG ceramic excieted by tunable pulsed dye laser between579nm and601nm.The results show that the lifetime of Nd upper level for Cr/Nd:YAGceramic was dependent of various exicting spectum. In this paper, based on solarspectrum focused by Fresnel lens, the calculation result of the effective lifetime of Ndlaser upper level was0.55ms. Therefore, Cr/Nd:YAG ceramic was used for solarpumped laser which was helpful for realizing the laser output on the low irradiancedays.On the design of Fresnel lens, the design method of Fresnel lens was proposed onmatching spectrum focused by Frsenel lens and the absorption spectrum of Cr/Nd:YAGceramic. Firstly, On the basis of geometrical principle and predictor-correctingalgorithm, the parameters of Fresnel lens with diameter600mm and focused length600mm was calculated, comparing with three design schemes which influencedmaximum power density and maximum absorption densiy of5mm×3mmCr/Nd:YAGceramic. Maximum foucsed power density design was18.8%higher than the designbased on the purpose of image projection, the absorption power density of Cr/Nd:YAG ceramic is16.3%higher than the design based on the purpose of image projection in themeantime. The design based on the prupose of obtaining maximum absorb powerdensity is about20%higher than image projection design. The design method of Fresnellens for the prupose of obtaining maximum absorb power denstiy is useful for obtainingthe laser output on low irradiance days in conclusion.In the theory, the physical model of the solar-pumped with Fresnel lenses andCr/Nd:YAG ceramic had been set up, on the basis of which the laser outputcharacteristics of foreign reported paperwere analyzed, and the experimental scheme oflow threshold pumping power was proposed. Firstly, the pumping efficiency wascalculated based on the trace ray algorithm, the conception of threshold pumping powerdensity and the physical model which aveage pumping beam waist was along withincident sunlight power were proposed. The model overlapping integral and efficiencyof laser oscillation were calculated. Secondly, the calculation result of laser outputcharacteristics were consistent with reported experimental result by qualitativelyanalyzing. Based on above physical model, for1.3m²Fresnel lens, choosingФ5mm×50⁶0mm Cr/Nd:YAG ceramic laser medium was effectively decreasing lowpumping threshold power and helpful for obtaining laser oscillation on low irradiancedays.On the source sunlight760W/m²and800W/m², laser output power ofsolar-pumped laser system were stuied with Fresnel lens of1.3m²surface area, conicalcavity with25mm in inlet diameter and9mm in outlet diameter and Ф5mm×60mmCr/Nd:YAG ceramic. On the two kinds of conditions, laser output power had achieved0.62W and1.4W repectively when the reflectivity of the output coupling mirror is97%.0.58W and0.94W have achieved when the reflectivity of the output coupling mirror is99%. On the basis of thisexperimental scheme, the calculation result of effectivethreshold pumping power is about190W, which was lower than the reported272Wfrom foreign literatures. Finally, distribution function of pumping power was establishedaccoring to power density model, and the reason of the experimental results performedbelow expectations was explained. Under ideal experimental conditions, the laser outputcharacteristics were calculated and analyzed.