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掺Er3+晶体是如图2所示的一种准三能级结构。最常见的是采用波长1 µm (或1.45 µm)的泵浦光将掺Er3+晶体中的基态光子由4I15/2跃迁至4I11/2(或4I13/2),随后大部分的激发态光子从4I13/2跃迁至4I15/2。由于掺Er3+晶体的材质有所不同,激发态光子由高能级向低能级跃迁的光子波长在1.53~1.64 µm之间。为获得1.6 µm附近波段激光,科研人员对其开展了大量的研究工作。
表1 是近年来以掺Er3+晶体为增益介质获得1.6 µm附近波段激光的相关案例。如北京理工大学的宋睿等人[17]采用种子注入技术搭配对称泵浦双Er: YAG的结构,获得重复频率200 Hz、单脉冲能量22.75 mJ、脉宽223.1 ns、线宽2.46 MHz、中心波长1.645 µm波段的单频激光脉冲输出,其光束质量因子M2为1.15,能量稳定度约为0.5%。实验中获得的激光脉冲不仅线宽窄、光束质量好,而且能量稳定度高。然而实验中并没有获得单脉冲能量和重复频率均高的激光脉冲,通过表1中的案例也不难发现,这是由于掺Er3+晶体的吸收效率低、晶体内的寄生激光多且晶体的导热率低,这些使得晶体的热负荷很大[18]。不仅如此,Er3+的光子跃迁截面小,在固体和光纤的掺Er3+增益介质中要获得高效率的4I15/2→4I11/2泵浦吸收跃迁很难,并且当使用掺Er3+晶体的掺杂浓度较大时又会出现淬灭效应。这些原因导致该类激光器几乎无法获得高重频、大能量的激光脉冲[19-22]。
表 1 掺Er3+晶体为增益介质获得1.6 µm附近波段激光的相关研究进展
Table 1. Example of obtaining a laser in the band near 1.6 µm using Er3+doped crystal as the gain medium
Center wavelength/µm Single-pulse energy/mJ Pulse width/ns Frequency/Hz M2x,M2y Energy stability Linewidth/MHz Year 1.64 120 100 30 2, 2.5 - - 2014[23] 1.645 2.9 160 100 - - - 2015[24] 1.645 10.1 205 200 1.4, 1.34 1.5% 2.44 2018[25] 1.645 20.3 110 200 1.27, 1.3 0.61% 4.59 2019[26] 1.645 28.6 159 200 1.37, 1.09 2.1% 3.4 2020[27] 1.645 22.75 223.1 200 1.16, 1.15 0.5% 2.46 2021[17] 1.54 1.3 10 100 - 0.28% - 2023[28] -
SRS是一种基于三阶非线性过程扩展现有激光光谱范围的有效方式,通过选用不同波段的泵浦光源和拉曼晶体,可将现有激光的波长从紫外扩展到近红外波段。具体过程是当一个入射的泵浦光光子与一个热振子碰撞时会产生一个斯托克斯光子和一个受激声子,当泵浦光的光子与新的声子碰撞后,又会再产生一个新的斯托克斯光子和一个受激声子,由此而产生一个受激声子的“雪崩过程”,进而产生斯托克斯光[29]。在获得1.6 µm附近波段的方法中,SRS是一个非常有效的方案,它不仅具有较高的光-光转化效率,而且还可以通过自动相位匹配消除激光的热失相问题,进而输出高光束质量的激光。
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基于BaWO4[30-31]、SrWO4[32]、Ba(NO3)2[33-34]、BaTeMo2O9[35]、YVO4和KGd(WO4)2等传统增益介质拉曼激光器的相关研究进展和上述晶体的拉曼频移、拉曼线宽、导热系数如表2和表3所示。由此可知,这些非线性晶体的拉曼频移小,一般用波长大于1.3 μm波段激光来泵浦这些非线性晶体获得1.6 µm附近波段激光;并且传统拉曼晶体的热导率不高,这会导致拉曼频移过程中晶体的热效应严重,因此使用上述拉曼晶体获得1.6 μm波段激光的最大输出功率一般不超过瓦级。不仅如此,在泵浦光和拉曼介质线宽的作用下,输出拉曼光的线宽一般在nm量级。
表 2 传统增益介质的拉曼频移、拉曼线宽、导热系数
Table 2. Raman frequency shift, Raman linewidth, heat conductivity of conventional gain media
Crystal Raman material Raman shift
/cm−1Raman linewidth
/cm−1Heat conductivity/W·m−1·K−1 Spectral transmission
/μmBa(NO3)2 1047 0.4 1.17 0.35-1.8 KGd(WO4)2 901 5.4 2.6 0.34-5.5 BaWO4 926 1.6 3.0 0.26-3.7 BaTeMo2O9 921 5.6 1.26 0.38-5.53 SrWO4 924.23 3.0 3.133 0.263-3.2 YVO4 890 3.0 5.2 0.4-5 表 3 传统增益介质拉曼激光器的相关研究进展
Table 3. Relevant research progress of conventional gain dielectric Raman lasers
Crystal Raman
materialPump light
wavelength/µmRaman light
wavelength/µmOutput power/W Output laser
frequencyLight-light conversion
efficiencyStokes order Linewidth/nm Year Ba(NO3)2 1.32 1.56 0.25 1 Hz 48% 1 - 1995[36] KGd(WO4)2 1.35 1.537 1.2×10−5 1 kHz 10% 1 20 2005[37] BaWO4 1.3 1.536 0.7 15 kHz 44% 1 - 2012[38] BaTeMo2O9 1.342 1.531 0.83 25 kHz 7.7% 1 0.06 2013[39] SrWO4 1.444 1.664 1.16 10 kHz 4.2% 1 - 2016[40] Ba(NO3)2 1.319 1.53 5 50 Hz - 1 - 2016[41] Nd:YVO4 1.342 1.524 0.685 - 4.8% 1 0.3 2021[42] 为将1 µm波段脉冲激光频移至1.6 µm附近波段,白俄罗斯B. I. Stepanov Institute of Physics的V. A. Lisinetskii等人[43]采用外腔拉曼的设计,利用重复频率20 Hz、脉宽10 ns、单脉冲能量300 mJ、光束质量因子M2约为3、中心波长1.064 µm的Nd: YAG激光器做泵浦源,2块Ba(NO3)2晶体作为拉曼介质,经过三阶斯托克斯频移后获得93 mJ的1.599 µm激光输出,其脉冲持续时间为9 ns,功率为1.8 W,能量转换效率约为47%。而此时会有大量泵浦光的能量以热量的形式留在非线性晶体内。由于传统拉曼晶体的热导率低且热膨胀系数大,导致其无法满足输出高重频、大能量激光的需求。
综上,传统拉曼晶体的单次拉曼频移量小,无法获得长波长频移。因此,一般选用1.3 μm波段激光来泵浦拉曼增益介质,通过一阶斯托克斯频移获得1.6 μm附近波段激光脉冲输出。当使用技术成熟的波长为1 μm的Nd: YAG激光器做泵源时,需要经历三阶以上斯托克斯频移后才能得到,这个过程不仅技术要求高,而且随着斯托克斯频移阶数的增加,拉曼介质内部热量也会急剧增加。因此在设计高重频的拉曼激光器时,应选用高热导率的拉曼晶体,同时采用有效的散热方式将晶体内的热量带走,以避免出现严重的热效应,引起光斑畸变而损伤晶体。不仅如此,当基频光经过三阶以上斯托克斯频移时,晶体内的四波混频还会展宽激光的光谱,使得输出激光的线宽更宽,从而导致其应用受限。上述原因均限制了拉曼频移技术在获得高重频、大能量1.6 µm附近波段激光领域的应用。
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相比之下,金刚石的透射光谱范围覆盖了紫外、可见光、红外至无线电波范围如图3所示[44]。且金刚石晶体的拉曼频移为1332.3 cm−1,其室温条件下的拉曼增益线宽1.5 cm−1。不仅如此,金刚石的拉曼增益系数为12 cm/GW@1.064 μm,且它的二阶斯托克斯输出激光与泵浦光能量之间存在直接的正比关系[45]。因此金刚石晶体凭借优异的拉曼特性和宽波长范围的透明度[46-50]可以将波长1 μm的基频光通过二阶斯托克斯频移获得1.49 μm波段激光输出。金刚石还具有优异的热物性,其热导率高达$ {\text{2\;2}}00{\text{ W}}/\left( {{\text{m}} \cdot {\text{K}}} \right) $[51-52],是传统拉曼晶体的140多倍,且热膨胀系数低至$ 1.1 \times {10^{ - 6}}\;{{\text{K}}^{ - 1}} $[49]。如此优异的热稳定性,再给其配备合适的散热装置,使得金刚石即使在高温、高强度的严苛条件下依然可以呈现优异的非线性性能,而且还可以输出高光束质量的二阶斯托克斯光。此外,金刚石的大禁带避免了金刚石晶体在高温下产生电荷载流子,因此,即使在很高的温度下,金刚石依然可以保持高透明度。以上这些优点使得金刚石成为SRS技术中获得高重频、大能量长波段激光中最有前途的非线性介质。
为了获得高光束质量的拉曼激光,澳大利亚Macquarie大学的McKay A等人[53]以金刚石为拉曼晶体,使用波长1.064 µm的Nd: YVO4激光器做为泵浦源,在重复频率36 kHz、脉宽20 ns、光束质量因子M2为3.0泵浦光泵浦下,经二阶斯托克斯频移输出16.2 W的1.485 µm激光脉冲,光-光效率高达40%。其输出1.485 µm光的光束质量因子${{{M}}^{\text{2}}}$为$ {\text{1}}.{\text{17}} \pm 0.08 $,相比泵浦光提升了2.7倍,这是由于在SRS的过程中,通过合理的谐振腔设计,可以使拉曼光的相位畸变有效的消散在声子场中,从而大大提升输出斯托克斯光的光束质量。当拉曼光在谐振腔中多次往返后光束质量会无限趋近于TEM00的理想高斯光束。
金刚石拉曼激光器不仅可以提升输出光的光束质量,而且可以获得高功率激光输出。Macquarie 大学的Williams等人[45]通过功率为259 W、光束质量因子M2小于1.2、重频40 Hz、脉宽250 µs的1.06 µm的光泵浦金刚石,经过二阶斯托克斯频移,将泵浦光的波长从1.06 µm频移至1.49 µm,输出光的功率为114 W,光-光转换效率为44%,其实验装置如图4所示。随后Williams和Bai等人[54]在1.064 µm泵浦光输入功率823 W条件下获得了功率302 W、波长1.49 µm的激光输出。
此外,金刚石拉曼激光器通过合理的谐振腔设计也可以获得一定波长范围调谐的激光脉冲输出。英国University of Strathclyde的Casula等人验证了这一点。实验中使用波长为1.18 µm的半导体激光器泵浦金刚石晶体,实验装置如图5所示[55],通过旋转位于圆盘半导体激光器谐振腔中的双折射滤波器,使输出的激光波长在1.375~1.415 µm范围内调谐,线宽为0.1 nm。
虽然,基于金刚石的拉曼激光器可以获得高重频、大能量、波长可调谐且光束质量好的激光脉冲输出,但是由于金刚石的拉曼频移范围也十分有限,无法仅通过二阶斯托克斯频移将现有的且技术成熟的高功率1 μm波段激光频移到1.6 μm波段,且高阶斯托克斯频移的技术难度高、效率低。因此想要获得1.6 μm波段激光,基频光中心波长需大于1.3 μm,如广东晶体与激光技术工程研究中心的Ma等就使1.342 μm的基频光经一阶斯托克斯后获得1.634 μm波段激光[56]。这也是很少有使用波长1 μm的激光泵浦金刚石晶体获得1.6 μm波段激光的案例被报道的主要原因[41]。
Research progress of high-frequency and high-energy solid state lasers at 1.6 µm (invited)
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摘要: 1.6 µm附近波段激光不仅属于人眼安全波段,而且处于大气传输窗口,不仅如此,高重频、大能量的1.6 µm附近波段激光还可携带高分辨率、大数据量的信息远距离传输。近年来随着晶体制备和镜片镀膜工艺的提高,通过直接泵浦增益介质和频率转换技术获得1.6 µm附近波段的激光在重复频率、能量和光束质量等方面都得到了很大进展。首先,介绍了直接泵浦掺Er3+晶体、受激拉曼频移和光参量振荡产生1.6 µm附近波段激光的原理和研究进展;其次,总结了三种方案在获得1.6 µm附近波段激光的优点和缺点;最后,分析了它们在获得高重频、大能量1.6 µm附近波段激光的应用前景。针对光参量振荡输出激光光束质量较差的问题,文中进行分析并给出相应解决方法,最后对通过光参量振荡获得较好光束质量、高重频、大能量1.6 µm附近波段激光的发展前景进行了展望。
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关键词:
- 高重频 /
- 大能量 /
- 光参量振荡 /
- 1.6 μm附近波段激光 /
- 受激拉曼散射 /
- 直接泵浦掺Er3+晶体
Abstract:Significance The laser near 1.6 µm is not only the safe band of human eyes, but also the transmission window in the atmosphere. The high-frequency and high-energy laser close to 1.6 µm can also carry information with high resolution and large amount of data at a longer distances. In recent years, with the improvement of crystal preparation and lens coating technology, the 1.6 µm band laser obtained by directly pumping gain media and frequency conversion technology has greatly improved the parameters such as repetition frequency, energy and beam quality. In this paper, the principles and research progress of 1.6 μm laser generated by erbium-doped crystal direct pumping, optical parametric oscillation and stimulated Raman frequency shift are introduced, the advantages and disadvantages of the above three schemes in 1.6 μm laser are analyzed, and their application prospect in 1.6 μm high-repetition rate and high-energy laser is prospected. The problem of poor output beam quality when high-frequency and high-energy lasers is obtained near 1.6 µm is also analyzed, and several enhancement examples are given. The development prospect of obtaining better beam quality and high-frequency and high-energy lasers by optical parametric oscillation near 1.6 µm is discussed. Progress Firstly, the energy level conversion process of the laser near 1.6 µm directly generated by pumping Er3+ doped crystals is given. However, the low absorption efficiency of pump light, the small photon transition cross section, the high number of parasitic lasers in the crystal and the low thermal conductivity of the crystal make the thermal load on the crystal very high. All these reasons limit its application in obtaining high-repetition rate and high-energy lasers at about 1.6 µm band. Then the process of obtaining stokes light by stimulated Raman frequency shift is described. Raman lasers based on conventional Raman gain materials such as BaWO4, SrWO4, Ba(NO3)2, BaTeMo2O9, GdVO4, YVO4 and KGd(WO4)2 are analysed, as their low Raman gain coefficients and the low thermal conductivity and thermal expansion coefficients of the crystals lead to the inability of these non-linear crystals to obtain high re-frequency, large-energy wavelength band lasers near 1.6 µm. In contrast, the high and low thermal expansion coefficients of diamond and its transparency over a wide wavelength range make up for some shortcomings of traditional Raman crystals, but the Raman frequency shift is only 1 332.3 cm−1, so it is still impossible to convert the existing and technically mature high-power 1 µm band lasers to the 1.6 µm band with second-order Stokes frequency shift. These reasons limit the application of stimulated Raman shifts to obtain high-frequency and high-energy lasers near 1.6 µm. Finally, the OPO technique based on KTA and KTP crystals is presented for application in obtaining a human-safe laser output in the wavelength band near 1.6 µm with wide wavelength tuning, higher beam quality, high heavy frequencies and large energy. Although the spot quality of laser output of OPO technology is poor in the wavelength band near 1.6 µm, it is possible to obtain laser output with high repetition rate, high energy and good beam quality in the wavelength band near 1.6 µm with reasonable resonator design, phase matching method of nonlinear crystal, selection of pump wave shape and pulse width, and use of a Gaussian mirror and a quasi-monolithic 90° image rotation, which is certainly what researchers in OPO technology are working hard to achieve. Conclusions and Prospects The high-frequency, high-energy laser near 1.6 µm is of great significance because it meets the needs of long-distance and high-data transmission without causing unintentional harm to people nearby. The main methods for obtaining lasers in the 1.6 µm band are pump light direct pumping of Er3+ doped crystals, SRS and OPO techniques. However, the low absorption efficiency of Er3+ crystals, the low thermal conductivity of the gain medium and the short lifetime of the energy level of the crystals make them unable to meet the requirements of high-repetition rate and high energies. The SRS technique is only capable of shifting the 1 µm band to near 1.49 µm due to the low thermal conductivity of the existing Raman medium and the limited Raman frequency shift, while the OPO technique is capable of achieving high-frequency and high-energy output near 1.6 µm by adjusting the parameters of the pump light and resonant cavity with a good nonlinear crystal. Although the beam quality of the output light is not good, laser pulses with good beam quality can be obtained through proper optimization, and there is much room for improvement in the current methods to solve this problem. -
表 1 掺Er3+晶体为增益介质获得1.6 µm附近波段激光的相关研究进展
Table 1. Example of obtaining a laser in the band near 1.6 µm using Er3+doped crystal as the gain medium
Center wavelength/µm Single-pulse energy/mJ Pulse width/ns Frequency/Hz M2x,M2y Energy stability Linewidth/MHz Year 1.64 120 100 30 2, 2.5 - - 2014[23] 1.645 2.9 160 100 - - - 2015[24] 1.645 10.1 205 200 1.4, 1.34 1.5% 2.44 2018[25] 1.645 20.3 110 200 1.27, 1.3 0.61% 4.59 2019[26] 1.645 28.6 159 200 1.37, 1.09 2.1% 3.4 2020[27] 1.645 22.75 223.1 200 1.16, 1.15 0.5% 2.46 2021[17] 1.54 1.3 10 100 - 0.28% - 2023[28] 表 2 传统增益介质的拉曼频移、拉曼线宽、导热系数
Table 2. Raman frequency shift, Raman linewidth, heat conductivity of conventional gain media
Crystal Raman material Raman shift
/cm−1Raman linewidth
/cm−1Heat conductivity/W·m−1·K−1 Spectral transmission
/μmBa(NO3)2 1047 0.4 1.17 0.35-1.8 KGd(WO4)2 901 5.4 2.6 0.34-5.5 BaWO4 926 1.6 3.0 0.26-3.7 BaTeMo2O9 921 5.6 1.26 0.38-5.53 SrWO4 924.23 3.0 3.133 0.263-3.2 YVO4 890 3.0 5.2 0.4-5 表 3 传统增益介质拉曼激光器的相关研究进展
Table 3. Relevant research progress of conventional gain dielectric Raman lasers
Crystal Raman
materialPump light
wavelength/µmRaman light
wavelength/µmOutput power/W Output laser
frequencyLight-light conversion
efficiencyStokes order Linewidth/nm Year Ba(NO3)2 1.32 1.56 0.25 1 Hz 48% 1 - 1995[36] KGd(WO4)2 1.35 1.537 1.2×10−5 1 kHz 10% 1 20 2005[37] BaWO4 1.3 1.536 0.7 15 kHz 44% 1 - 2012[38] BaTeMo2O9 1.342 1.531 0.83 25 kHz 7.7% 1 0.06 2013[39] SrWO4 1.444 1.664 1.16 10 kHz 4.2% 1 - 2016[40] Ba(NO3)2 1.319 1.53 5 50 Hz - 1 - 2016[41] Nd:YVO4 1.342 1.524 0.685 - 4.8% 1 0.3 2021[42] -
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