Properties of Pr3+∶YLF Red Laser with Weak Blue Absorption Pumping
Objective The development of the blue laser diode has made the visible solid-state laser generated by rare-earth-ion-doped material using laser diode(LD)pumping a research hotspot.Materials doped with trivalent praseodymium ions Pr3+are prospective gain material in the visible range,and yttrium lithium fluoride(Pr3+∶YLF)exhibits good output performance under blue LD pumping owing to its low phonon energy.Moreover,Pr3+∶YLF provides multiple emission wavelengths including green,orange,red,and deep red light wavelengths.To meet actual requirements,the output laser can be designed to achieve different wavelengths and polarization states.Thus,Pr3+∶YLF has become a common topic in the research on solid state lasers in the visible range.By contrast,ultraviolet and deep ultraviolet lasers are obtained by infrared light.Research and application progress in visible lasers has provided another scheme towards ultraviolet laser generation,which is directly performing frequency doubling of visible lasers to obtain ultraviolet lasers.Under these circumstances,we study the laser properties of Pr3+∶YLF crystal using a weak absorption pumping strategy.In this strategy,the band is pumped with weaker absorption in the crystal,reducing the amount of pump energy absorbed by the crystal and alleviating the thermal stress caused by uneven heat distribution.By quantitatively changing the pump spot size and beam distribution in Pr3+∶YLF,different output results are obtained.The influence of factors such as pump beam distribution on laser output is analyzed and discussed,and the conditions for high-quality operation of visible lasers are explored.Methods Despite good laser properties,the negative influence of the thermal effect of YLF crystals is poor,especially in terms of the crystal fracture caused by thermal stress.This is the primary concern restricting studies on YLF crystals.The thermal effects of a laser gain medium mainly include thermal photorefraction,thermal stress,and thermal lens.To reduce the thermal effect,different strategies,such as disk laser,side pumping,and wing pumping,have been adopted.Owing to the poor fracture property of YLF,the excessive thermal stress gradient caused by pumping absorption needs to be reduced.Therefore,we use a weak absorption pumping strategy.Results and Discussions The output characteristics of the red laser are shown in Fig.7.When the coupling ratio of the lens group is1∶2and the output coupler(OC)transmittance,Toc,is 1.27％,the maximum output power,PopMax,of the red laser is 2.46 W,with an oblique efficiency of 36.8％.The red laser performs best.Using a fiber spectrometer with a resolution of 0.076 nm,the center wavelength of the output laser spectrum is measured as 639.48 nm,with a linewidth of 0.23 nm and σ laser polarization state,with a polarization direction perpendicular to the crystal c-axis direction and a polarization ratio of 24.5 dB,as shown in Fig.7(d).When the coupling ratio of the lens group is 1∶2,considering the size of the pump spot,the quality of the pump beam,and the length of the crystal,the Rayleigh length of the blue pump light is closest to Pr3+∶YLF crystal length indicating that the laser cavity mode matching effect is better.A comparison of the experimental results shows that when the coupling ratio is 1∶2,the slope efficiency is the highest,the crystal output threshold power is the smallest,and the maximum output power is the largest.Conclusions We report a 640 nm Pr3+∶YLF red laser pumped by a semiconductor blue laser pump.In selecting the pump source,the weak absorption pumping strategy is adopted to reduce the absorption rate of the Pr3+∶YLF crystal to pump light to ensure that the absorption of the Pr3+∶YLF crystal to pump energy along the length direction is more even.The distribution of pump light in the Pr3+∶YLF crystal is improved by adjusting the coupling ratio of the lens group.The optimal coupling ratio of the lens group is 1∶2.Simultaneously,when the OC transmittance is 1.27％,the maximum output power of the Pr3+∶YLF red laser is 2.46 W,the slope efficiency is 36.8％,the output red light wavelength is 639.48 nm,and the line skew reaches 24.5 dB.The weak absorption pumping adopted in this study can not only reduce the overall absorption rate of the crystal but also effectively output the laser,providing a solution for slowing down the thermal effect of large-sized working substances in solid-state lasers.
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