Study on LD end-pumped multi-segment bonded Tm:YAG solid-state laser
Introduction
is in the atmospheric window and belongs to the safe band of human eyes, which contributes to many applications in laser medical, laser ranging, photoelectric countermeasures and laser radar [1], [2], [3]. The band laser generated by laser diode pumping Tm doped crystal is widely used in the field of mid-infrared laser due to its simple structure and high efficiency. Common single-doped Tm crystals include Tm:YAG, Tm:LuAG, Tm:YAP, Tm:YLF, etc [4], [5], [6]. However, only Tm:YAG and Tm:LuAG crystals can be directly pumped by laser diode to realize laser output wavelength of more than . While, compared with Tm:LuAG crystal, the Tm:YAG crystal has gradually become a research hotspot because of its good optical and thermal properties with high quantum efficiency. All the same, the Tm:YAG crystal is affected by the characteristics of the quasi-three-level system, and the emission cross section of Tm is small, which affects the thermal effect, resulting in a high threshold and limited output power [7], [8]. Therefore, in order to further increase the output power, we need to conduct a detailed study of Tm:YAG. In 1998, Bollig et al. obtained a 4.1 W output of 2. by a rational design of the cavity for the Tm:YAG crystal, in which it can be proved that the endless multi-segment bonded Tm:YAG is one of the effective methods to release the thermal effect to some extent [9]. In 2008, Y. L. Ju et al. introduced the LD end-pumped Tm:YAG double-ended bonded crystal at room temperature with the maximum output power of 6.37 W [10]. Since the bonded crystal can effectively reduce the temperature distribution in the crystal and alleviate the thermal deformation of the crystal end face, the means of using the bonding method to retard the thermal effect of the Tm:YAG crystal need to be specially focused.
In this paper, we introduce a double-ended continuous pump multi-segment bonded Tm:YAG laser. When the absorbed power is 26 W, the output power is 8.27 W, corresponding to light-to-light conversion efficiency is 31.8%, and the slope efficiency is 38%. It is also proved that the structure of multi-stage bonded Tm:YAG crystal can effectively alleviate the thermal effect of crystal.
Section snippets
Theoretical analysis
In this part, we will focus on the thermal problems of multi-segment bonded Tm:YAG crystal. Firstly, the multi-segment bonded crystal structure involved in this study should be introduced. As shown in Fig. 1, the multi-segment bonded Tm:YAG crystal model has a “12121” structure, in which, a portion of “1” is a YAG matrix and a portion of “2” is a doped Tm:YAG crystal.
The photon energy difference in the ion transition between the pumping band and the upper level is transferred thermally to the
Experimental device
The double-ended pumping multi-segment bonded Tm:YAG crystal experimental device diagram is shown in Fig. 8.
The LD pump in the experiment is a laser diode with a center wavelength of 785 nm and a maximum output power of 70 W. The fiber radius is and the numerical aperture is 0.22 N.A. The focus coupling ratio of the lens group is 7:15, and the focal lengths are =35 mm and =75 mm respectively. There is a plane-concave cavity in the experiment, and the full mirror was plated with a
Conclusion
In summary, we conduct an analysis of thermal effect about multi-segment bonded Tm:YAG crystal and its lasing characteristics under CW operation. The multi-stage bonded Tm:YAG temperature distribution and thermal focal length are analyzed. The maximum output power is 8.27 W with a center wavelength of 2014.52 nm. The optical–optical conversion efficiency is 31.8% and the slope efficiency is 38.5%. Compared with the two single-end bonded Tm:YAGs structure, the structure of multi-stage bonded
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work is supported by the Science and Technology Department Project of Jilin Province, China (No. 20190101004JH), the Natural Science Foundation of China (Grant NO. 11974060 and Grant NO. U19A2077).
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