摘要
3μm激光在国防安全、生物医疗、光谱分析等领域具有广阔的应用前景。与非线性频率变换、半导体激光技术相比,利用掺Er的激光增益介质产生3μm激光是一种比较直接、高效的方法。随着3μm掺Er激光器在输出功率和效率方面的突破,半导体激光泵浦的3μm掺铒固体激光已成为热点研究方向。回顾了不同基质材料掺杂Er离子产生3μm激光的输出特性和研究进展;从铒离子能级结构出发,分析了铒离子间的能量传递上转换、激发态吸收等跃迁过程对3μm激光输出性能的影响,并对3μm稀土离子掺杂固体激光器的功率提升潜力和发展前景进行了展望。
Significance 3μm laser sources have a broad application prospect in the fields of national defense and security,biomedicine,spectral analysis,and so on.Compared with the nonlinear frequency conversion and semiconductor laser technologies,laser-diode pumping for Er-doped lasers to emit 3μm wavelength is a more direct and efficient method.The radiation from energy level^(4)I_(11/2)to^(4)I_(13/2)of Er^(3+) ions produces 3μm lasing.Since the lifetime of the upper level is shorter than that of the lower level,it is difficult to realize a continuous laser operation due to the“self-termination effect”from the classic laser theory.However,due to the inherent complex energy transfer process and Stark sub-level splitting of the Er^(3+)doped system,an efficient 3μm laser continuous wave operation has been realized in many Er-doped matrices such as Er:YAG,Er:YLF,and Er:Y_(2)O_(3).The main known problems linked with Er-doped 3μm lasers are self-termination and large quantum losses.If the Er^(3+) ions are doped into low phonon matrix,the probability of non-radiative transition is reduced and the fluorescence decay time at the upper laser level^(4)I_(11/2) is prolonged.This fact results in a low probability of selftermination.To reduce the self-termination effect in high phonon matrix,a high Er-doping level is required.With the increase of doping level,the spacing between Er^(3+) ions is shortened,which is beneficial to ion-ion energy transfer.However,using the high doped active medium affects thermal conductivity,which makes the thermal management of the laser system more difficult.The Er-doped laser gain medium is pumped by a semiconductor laser of 0.97μm to produce a 3μm laser with a large quantum loss.Theoretically,the laser efficiency is limited to~33%,which means that two-thirds of the pump power is wasted and deposited into the gain medium as parasitic heat,resulting in degradation of laser performance.Researchers have established a mathematical model to estimate the theoretical limit of the emission efficiency of 3μm Er:YAG laser,and they have found a simple analytical expression for the emission efficiency,which shows that the theoretical quantum efficiency can reach 59.8%due to the existence of an ETU process.So far,the maximum efficiency of 3μm lasers with Er^(3+)-doped gain media in experiments has reached to 50%in Er:LiYF with doping concentration (atomic fraction)of 15%.This result proves that with an optimized doping concentration,the 3μm laser efficiency can be effectively improved due to the ETU process.Using laser materials with a low Er^(3+) doping concentration and cascading two transitions (^(4)I_(11/2)→^(4)I_(13/2)→^(4)I_(15/2)),where the first^(4)I_(11/2)→^(4)I_(13/2)transition corresponds to the mid-IR~3μm laser and the second^(4)I_(13/2)→^(4)I_(15/2)corresponds to the eye-safe 1.6μm spectral region,respectively,provide a number of important benefits,including the increased overall efficiency of optical output and thermal management.In addition,the second eye-safe transition(^(4)I_(13/2)→^(4)I_(15/2))effectively depletes the lower laser level and sustains a positive inversion,as required for a CW operation.Progress The 3μm laser performance in various Er-doped host materials is summarized(Table 1),including continuous wave output,Q-switched pulse output,mode-locked pulse output,and 1.6μm and~3μm cascading outputs,etc.At present,the 3μm laser products are commercially available with Er:YAG and Er:YLF as gain media.Due to the strong heat generation inside Er:YAG with 50%atomic fraction,the side-pumping design is carried out,which results in a poor beam quality.Sesquioxides,which have a low phonon energy and high thermal conductivity,have immerged as a promising laser host material for~3μm laser operation in recent years.An efficient laser operation could be obtained with the sesquioxides at a low Er^(3+)concentration.With 2%(atomic fraction)Er:Y_(2)O_(3)ceramics as laser gain media,researchers have obtained a 14 Wlaser output at 2.8μm and cryogenically cooled temperature of 77K (Fig.4).In 2016,the output power is further increased to 24 W with a slope efficiency of 24%under the liquid nitrogen cooled condition.Research group from Jiangsu Normal University has obtained a 3.8Wlaser output at 2.7μm at room temperature with 7%(atomic fraction)Er:Y_(2)O_(3)ceramic,and the output power has been increased to more than 10 W.Conclusion and Prospects With its recent breakthrough in terms of output power and laser efficiency,the erbiumdoped 3μm laser has become an object of intense scientific research.With the improvement of high-quality low phonon energy laser gain media,especially the development of ceramic gain media,the 3μm laser performance can be further improved by optimizing the wavelength and spectral linewidth of pump sources,doping concentration of Er-doped laser gain media,and laser cavity parameters.Compared with semiconductor laser and nonlinear frequency conversion technologies,the laser-diode pumped Er-doped laser emitting 3μm wavelength is very promising,especially in the pulsed laser operation for producing high peak power and large pulse energy.
作者
王飞
丁曼曼
沈德元
王俊
唐定远
Wang Fei;Ding Manman;Shen Deyuan;Wang Jun;Tang Dingyuan(Jiangsu Collaborative Innovation Center of Advanced Laser Technology and Emerging Industry,Jiangsu Normal University,Xuzhou,Jiangsu 221116,China;Jiangsu Key Laboratory of Advanced Laser Materials and Devices,Jiangsu Normal University,Xuzhou,Jiangsu 221116,China;Jiangsu Institute of Mid Infrared Laser Applied Technology,Xuzhou,Jiangsu 221000,China)
出处
《中国激光》
EI
CAS
CSCD
北大核心
2022年第1期113-121,共9页
Chinese Journal of Lasers
基金
国家自然科学基金(62035007,61875078)。
关键词
激光器
掺铒固体激光
3μm激光
高功率
自终止
lasers
Er-doped solid state laser
3μm laser
high power
self-termination
