中国激光, 2024, 51 (7): 0701020, 网络出版: 2024-03-29  

基于国产商用CLBO晶体的高转换效率、高功率深紫外266 nm激光

High‑Conversion‑Efficiency High‑Power Deep‑Ultraviolet 266 nm Laser Based on Domestic Commercially Available CLBO Crystal
俞航航 1,2张志韬 1,2玄洪文 1,3,*
作者单位
1 广东大湾区空天信息研究院,广东 广州 510700
2 广东省太赫兹量子电磁学重点实验室,广东 广州 510700
3 中国科学院大学,北京 100049
摘要
深紫外激光具有光子能量高、波长短等特点,在激光加工、半导体光刻等领域中具有重要的应用价值。固体激光非线性频率变换是实现高功率、高相干性深紫外激光输出的主要方式之一。采用全固态532 nm激光作为基频光、国产商用CsLiB6O10(CLBO)晶体作为频率变换晶体,在基频光功率为34.2 W时,实现了平均功率为14 W、重复频率为100 kHz、脉冲宽度为1.8 ns的266 nm深紫外激光输出,光-光转换效率达到41%。该深紫外光源具有效率高、结构紧凑的优点,验证了国产商用CLBO晶体的实用性,可进一步获得更稳定、更高功率的深紫外激光输出。
Abstract
Objective

Deep-ultraviolet (DUV) lasers have important applications in fields such as laser processing and semiconductor photolithography because of their high photon energy. A DUV laser source must have both a high output power and good beam quality for laser machining. Sum-frequency generation (SFG) in nonlinear optical crystals can be used in a DUV laser to produce shorter and even vacuum ultraviolet wavelengths such as 193 nm. Hence, high-power DUV lasers with a good beam quality have been a hot research topic in recent decades. On the other hand, it is now possible to use a domestic commercially produced CLBO crystal as a key nonlinear optical material for high-power DUV solid-state laser generation. However, the crystal quality for high-power DUV operation has not yet been verified because of the hydroscopic problem. Here, we report a high-power DUV laser output with a good beam quality and high conversion efficiency produced using domestic commercially available CLBO crystals, which demonstrates the potential to achieve a higher power DUV laser output.

Methods

In our study, we investigate DUV laser generation using CLBO crystals with different lengths. The research focuses on the characteristics of these high-power solid-state DUV light sources, including the output power, efficiency, power stability, beam quality, spectrum, and pulse width. The experimental setup is shown in Fig.1. The pump power is regulated by a combination of a half-wave plate (HWP) and polarizing beam splitter (PBS). The pump spot diameter is adjusted using a plano-convex lens (L1) with a focal length of f=+200 mm and plano-concave lens (L2) with a focal length of f=-100 mm. The dimensions of CLBO crystals are 5 mm×5 mm×10 mm and 5 mm×5 mm×20 mm, respectively. In both, cutting angle (θ) is 61.7°, and the two end-faces of the crystals are polished but have no coating. The CLBO crystals are heated to a temperature greater than 150 °C and exposed in a noble gas environment to avoid the hydroscopic problem. The 266 nm DUV laser output is spatially separated from the 532 nm pump light using an uncoated CaF2 prism.

Results and Discussions

A 20 mm long CLBO crystal pumped by a solid-state 532 nm laser generates a 266 nm DUV laser with an average power of 14 W, a repetition rate of 100 kHz, and a pulse width of 1.8 ns. The pump power is 34.2 W, and the optical conversion efficiency reaches 41%. The results are shown in Fig.2. Comparative experiments are conducted on a 10 mm long CLBO crystal using another light source system with a pump power density similar to that mentioned above. The output power of the 266 nm laser is 1.7 W with a pump power of 8 W, corresponding to an efficiency of 22%. This indicates that the crystal length is an important parameter to achieve a high conversion efficiency. The power stability of the 266 nm laser generated by the 20 mm long CLBO crystal reaches 1.52% within 10 min. There are several factors that influence the power stability, including the stability of the fundamental pump power, inhomogeneous temperature distribution of the crystal, and instability of the mechanics. The measured beam quality of the 266 nm laser at an output power of 7 W is shown in Fig.4. The transverse beam quality factor (Mx2) and longitudinal beam quality factor (My2) are 1.54 and 1.97, respectively. The inset shows the beam profile acquired at a distance of 1.5 m away from the crystal after beam expansion by a concave lens. The circular beam shape and homogenous distribution of the intensity also indicate the high beam quality of the generated 266 nm laser.

Conclusions

A nanosecond 532 nm fundamental laser and a 20 mm long domestic commercially available CLBO crystal are used to generate a high-power DUV solid-state laser at 266 nm, with an average power of 14 W and a conversion efficiency of 41%. The beam quality factors, Mx2 and My2, of the 266 nm laser have values of 1.54 and 1.97 at a power of 7 W, respectively. The root mean square value of the power stability at 10 W reaches 1.52% within 10 min. The temperature distribution and mechanical stress are the main factors influencing the DUV power stability. A 266 nm laser with higher power and better beam quality can be achieved by improving the temperature control system and mechanical design, as well as by increasing the pump power. This can be applied to laser machining, lithography, and vacuum ultra-violet laser generation in the future.

1 引言

深紫外激光(DUV)具有高光子能量、高空间分辨率和优良的聚焦性能等特点,被广泛应用于激光材料加工1-2、辐射测量3、半导体检测4、高能射线产生5及波导直写6等领域。相较于准分子激光器,固体深紫外激光器不仅具有体积小、高能效比的优势,而且具有高光束质量、高相干性的特点,可实现无掩模光刻。自20世纪80年代以来,以全固态激光为基频,通过偏硼酸钡(β-BaB2O4,BBO)、三硼酸锂(LiB3O5,LBO)、氟代硼铍酸钾(KBe2BO3F2,KBBF)、六硼酸铯锂(CsLiB6O10,CLBO)等人工晶体的非线性频率变换(倍频或和频),可实现高功率固态深紫外激光输出。2007年,Sudmeyer等7利用共振增强技术获得了平均功率为12.2 W的266 nm连续激光输出,1064 nm基频光至266 nm深紫外光的转换效率超过50%。该单频266 nm连续激光功率结果相较文献[8]的结果提高了10倍以上。2009年,Liu等9通过BBO晶体倍频获得了最高输出功率为14.8 W的266 nm深紫外激光,绿光至深紫外光的转换效率为18.3%。2016年,Nikitin等10通过LBO晶体和频产生了平均功率为3 W的连续266 nm激光。2022年,Cha等11同样通过LBO晶体和频产生了平均功率为12 W的257 nm深紫外纳秒脉冲激光。2014年,陈创天团队使用KBBF作为倍频晶体,获得了平均功率为7.86 W的266 nm激光输出12。在上述晶体中,BBO晶体的走离效应较大,影响高功率深紫外激光的光束质量;LBO晶体存在紫外诱导损伤及温度失调10-11;KBBF晶体因其层状习性,很难生长为大体块晶体,还需通过棱镜耦合装置(PCD)实现相位匹配13,难以支持更高功率的深紫外激光输出。

深紫外激光应用于激光加工时不仅需要高功率,还应具有高光束质量。另外,利用高平均功率、高光束质量的深紫外266 nm/258 nm激光,在非线性光学晶体中通过和频的方式,可进一步产生更短波长的深紫外乃至真空紫外波长激光。如193 nm激光可通过和频方式产生,可应用于干涉光刻、检测、量测设备,并可作为ArF准分子放大器的注入种子等14。CLBO晶体15具有走离效应小、可生长为大体块、深紫外区有效非线性系数高等优势,可用于产生高功率、高光束质量的固态深紫外激光。2003年和2006年,CLBO晶体输出的平均功率已达到40 W及28.4 W16-17,但相应的基频光的光束质量因子分别为M2=10及M2=6.5,因此深紫外激光的光束质量不会优于基频光。当提高基频光的光束质量时,通过CLBO晶体倍频获得的深紫外激光的光束质量也会同步提高。2017年,Xuan等18使用光纤和固体混合放大方式,获得了高功率1030 nm基频光,经过四倍频后获得了输出功率为10.5 W的258 nm激光,绿光至深紫外激光的转换效率超过40%;同时,在10 W的高输出功率下,深紫外激光的光束质量因子M2小于1.5。2020年,日本Spectronix公司使用皮秒基频光源,借助大口径CLBO晶体(尺寸为13 mm×13 mm×15 mm),获得了输出功率为14 W的266 nm激光,光-光转换效率达到54%,这也是目前绿光至深紫外的最高转换效率19。该结果中基频绿光的光束质量因子M2=1.2,深紫外激光的M2=1.5。2023年,该公司使用更高平均功率的泵浦基频光,将 266 nm激光的输出功率提高至53 W,其光束质量因子M2=1.9,光-光转换效率在32%左右20,打破了2003年以来大阪大学保持的深紫外激光最高输出平均功率的纪录。

作为高功率深紫外固体激光产生的关键非线性光学材料,CLBO晶体极易潮解,其质量需通过实际验证,以获得高光束质量、高转换效率、高平均功率的固态深紫外激光输出,满足我国科研与工业应用的需求21-22

本文以国产商用CLBO晶体作为非线性频率变换晶体,使用重复频率为100 kHz、平均功率为35 W的532 nm纳秒激光作为基频光,开展了高功率固态深紫外光源的研究,实现了最高平均功率为14 W的266 nm激光输出,光-光转换效率达到41%。当平均功率为7 W时,其横向光束质量因子(Mx2)和纵向光束质量因子My2分别为1.54和1.97。

2 实验装置

实验装置如图1所示,基频泵浦源为国产全固态532 nm激光器,脉冲宽度为2.7 ns,重复频率为100 kHz,最大输出功率约为35 W,光束质量因子M2为~1.5。利用半波片(HWP)和偏振分束器(PBS)组合对泵浦光功率进行调控,利用焦距为f=200 mm的平凸透镜(L1)和f=-100 mm的平凹透镜(L2)调节泵浦光斑直径。CLBO晶体的尺寸为5 mm×5 mm×20 mm,切角(θ)为61.7º,晶体两通光端面均已抛光但均未镀膜。CLBO晶体极易潮解,因此将其加热至150 ℃以上,并使其处于惰性气体环境中20。将CLBO晶体放置在旋转位移台上,便于在水平面内调节基频入射角度,以实现相位匹配。为避免分光器件损坏,使用未镀膜的氟化钙(CaF2)棱镜,在空间上分离产生的266 nm激光与532 nm泵浦光。

图 1. 基于CLBO晶体的266 nm激光实验装置图

Fig. 1. Experimental setup for 266 nm laser based on CLBO crystal

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3 实验结果与讨论

在实验中,首先通过白纸板在分光棱镜后观察到266 nm光信号(基频光的泵浦功率为0.7 W),然后继续增大泵浦光功率,通过不断调整泵浦光斑直径,并配合旋转晶体,调节入射光角度,获得最佳相位匹配,实现高功率泵浦下266 nm激光的高转换效率输出。为真实反映深紫外激光的输出功率,实验中首先对CaF2棱镜在266 nm波段的透过率进行测量标定。使用功率计分别测量插入CaF2棱镜前后266 nm激光的功率,当266 nm激光功率为0~7 W时,CaF2棱镜对其的实际透过率如图2(a)所示。考虑到测量过程中的误差,以算术平均值92.2%作为CaF2棱镜在高功率下对266 nm激光的透过率。随后测量了266 nm激光的输出功率及转换效率随泵浦功率的变化,如图2(b)所示。从出现266 nm信号开始,不断增加泵浦光功率到最大值34.2 W,CaF2棱镜后测得的266 nm激光功率为12.9 W。使用标定后的透过率92.2%计算得到对应的266 nm激光的功率为14 W,此时绿光至深紫外光的光-光转换效率达到41%。从图2(b)可见转换效率基本达到饱和状态,继续增大泵浦功率不会带来转换效率的进一步提升。266 nm激光的转换效率与晶体长度、温度和泵浦光斑大小等因素紧密相关,根据已有的报道,41%的转换效率接近于纳秒泵浦光源在20 mm长的CLBO晶体中所获得的最好深紫外转换效率。使用更长的晶体以及提高泵浦功率密度,理论上可以获得更大的转换效率。根据图2(b)的结果可以看出,使用更高功率的泵浦光可以获得更高功率的266 nm激光,但与此同时晶体内部严重的热效应会影响相位匹配,从而限制深紫外激光功率的继续提高。

图 2. 透过率及266 nm激光输出功率实验结果。(a) 266 nm激光在棱镜处的透过率;(b) 266 nm激光的输出功率和转换效率随注入泵浦功率的变化

Fig. 2. Experimental results of transmittance and output power of 266 nm laser. (a) Transmittance of 266 nm laser at prism; (b) output power and conversion efficiency of 266 nm laser versus injected pump power

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使用另一套光源系统及长度为10 mm的国产CLBO晶体开展了对比实验。泵浦源为国产532 nm激光器,脉宽约为10 ns,重复频率为5 kHz,输出功率约为8 W,光束质量因子M2约为1.3。实验装置与图1基本相同,CLBO晶体尺寸为5 mm×5 mm×10 mm。在泵浦功率密度接近 (约为150 MW/cm2) 的情况下,研究不同长度CLBO晶体下的转换效率和长时间的功率稳定性,如图3所示。当泵浦功率为8 W时,获得的266 nm激光的最高平均输出功率为1.7 W,转换效率约为22%。可见CLBO晶体长度是决定深紫外激光转换效率的重要因素之一,在适当的泵浦参数条件下,晶体长度成倍增大可以实现其光-光转换效率的倍增。但由于CLBO晶体对深紫外激光存在吸收、散射等,光-光转换效率会有上限存在。因此,对于CLBO晶体的深紫外倍频效率,可采用单位长度的光-光转换效率作为更合理的表征转换效率的参数。由此计算得出,在上述两个实验中,10 mm和20 mm长的CLBO晶体对应的单位长度光-光转换效率分别为2.20×10-2/mm和2.05×10-2/mm,两者的单位长度光-光转换效率基本一致。

表 1. 基于CLBO晶体的固态深紫外激光的研究进展

Table 1. Research progress of solid-state deep-ultraviolet laser based on CLBO crystal

YearWavelength /nmCrystalOutput power /WRepetition ratePulse durationM2 (DUV)Conversion efficiency (green to DUV)Ref.
200026615 mm long CLBO20.510 kHz46 ns>1019.4%23
200326615 mm long CLBO407 kHz80 ns>1020%16
200626640.15 mm long CLBO28.410 kHz80 ns>6.2423.7%17
200925830 mm long CLBO145 MHz1 ns~1.737.7%24
201525820 mm long CLBO36 kHz~5 ns<1.5~50%24
2016257.56 mm long CLBO6100 kHz4 ps>1.518.5%26
201725820 mm long CLBO10.510 kHz3 ns<1.542%18
202026615 mm long CLBO14200 kHz13 ps<1.254%19
202226615 mm long CLBO25.4600 kHz<8 ps~1.228.9%27
202326615 mm long CLBO531 MHz~14 ps~1.932%20
202326610 mm long CLBO1.75 kHz~7 ns~1.522%This work
202326620 mm long CLBO14100 kHz1.8 ns<2@7 W41%This work

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图 3. 266 nm激光输出功率和功率稳定性的测量结果。(a)使用10 mm长的CLBO晶体时266 nm激光的输出功率和转换效率随注入泵浦功率的变化;(b)使用10 mm长的CLBO晶体时266 nm激光的功率稳定性

Fig. 3. Measurement results of output power and power stability of 266 nm laser. (a) Output power and conversion efficiency of 266 nm laser versus injected pump power when 10 mm long CLBO crystal is used ; (b) power stability of 266 nm laser with 10 mm long CLBO crystal

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使用10 mm长的CLBO晶体时266 nm激光的功率稳定性如图3(b)所示,半小时内平均功率的均方根(RMS)约为4.39%。对于20 mm长的CLBO晶体,当平均输出功率为10 W左右时,测量其自由运转下10 min内的功率稳定性,结果如图4(a)所示,其功率抖动RMS为1.52%,高于泵浦光的RMS(~0.5%)。更长期的自由运转如1 h,功率会缓慢下降。实验上,精细调整放置CLBO晶体的旋转位移台,对于同一个晶体出光点,266 nm激光的输出功率会迅速回到10 W。如图3(b)所示,相较于使用20 mm长的晶体时的结果,使用10 mm长的CLBO晶体时266 nm激光的功率RMS较大。这是因为图3(b)中初始阶段的深紫外激光的功率衰减较快,对RMS数据的影响较大。虽然可以通过再次调整晶体角度恢复266 nm激光的功率,但目前所产生的深紫外激光的RMS抖动仍较大。泵浦光功率的稳定性、晶体内部温度场分布的均匀性19、机械稳定性及高功率深紫外激光对晶体的退化效应等是主要影响因素。上述问题可通过优化温度控制、改进机械设计并加入自动挪点等技术得到解决。

图 4. 266 nm激光功率稳定性和光束质量的测量结果。(a)使用20 mm长的CLBO晶体时266 nm激光功率稳定性的测量结果;(b)266 nm激光的光束质量(点为数据,线为拟合线),插图为使用凹透镜扩束后在1.5 m处测得的光斑

Fig. 4. Measurement results of power stability and beam quality of 266 nm laser. (a) Measured result of power stability of 266 nm laser with 20 mm long CLBO crystal; (b) beam quality of 266 nm laser (dot is data and curve is fitted curve) with beam spot measured at 1.5 m position after beam expansion using concave lens shown in inset

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使用热释电阵列相机对20 mm长的CLBO晶体对应的266 nm激光光斑大小进行了测量,并拟合得到其光束质量因子。由于使用了CaF2棱镜,在分光过程中266 nm光斑会产生附加畸变,故测量过程中使用二向色镜将泵浦光和266 nm激光分离。受限于二向色镜的损伤阈值,266 nm激光功率限制在7 W。相机分辨率为~85 μm,无法提供足够的像素点,因此使用凹透镜(f=-50 mm)和凸透镜(f=500 mm)组合对266 nm激光进行扩束,以增大束腰位置处光斑半径的大小。经扩束后光斑距离CLBO晶体约3.5 m,不同位置处的光斑测量值如图4(b)所示,拟合后得到266 nm激光的光束质量因子Mx2为1.54及My2为1.97,接近于532 nm泵浦光的M2(~1.5)。CLBO晶体沿y轴方向的走离效应以及532 nm基频泵浦光斑在y轴方向上相对较大的M2是造成y轴光束质量因子偏大的主要原因。在图4(b)中,x轴和y轴方向上的束腰位置有明显的偏离,主要是椭圆光斑形状的高功率泵浦光入射到晶体中在x轴和y轴方向上形成的热透镜焦距不同,因此光斑束腰大小和位置存在差异。对266 nm激光的远场光斑进行测量,结果如图4(b)中的插图所示。插图是使用凹透镜扩束后在距离其1.5 m处获得的光斑形状,光斑呈圆形且光强分布均匀,进一步验证了其光束质量相对较好。

表1总结了基于CLBO晶体的固态深紫外激光的研究进展。自2000年以来,基于CLBO晶体产生的深紫外激光的功率和效率随着晶体长度、泵浦功率和脉冲宽度不断变化。晶体长度集中在10~30 mm区间,皮秒泵浦光因扩大的泵浦光斑、大尺寸的CLBO晶体,更容易获得高转换效率。可以看到,在纳秒脉冲泵浦的结果中,本文使用国产商用10 mm和20 mm长的CLBO晶体,分别实现了22%和41%的转换效率,接近同类型实验的最好结果。

使用光纤光谱仪和双平面光电管分别测量高功率下266 nm深紫外激光的光谱和脉冲宽度,结果分别如图5(a)、(b)所示。测量得到的266 nm激光的脉冲宽度为1.8 ns,小于532 nm激光的脉冲宽度(2.7 ns)。窄化效应影响了泵浦光转换效率的上限。另一方面,理论上使用更窄线宽的泵浦源可以获得更好的深紫外激光转换效率。

图 5. 深紫外266 nm激光的光谱和脉宽。(a)光谱;(b)脉宽

Fig. 5. Spectrum and pulse width of deep-ultraviolet 266 nm laser. (a) Spectrum; (b) pulse width

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4 结论

利用纳秒532 nm激光器泵浦20 mm长的国产商用CLBO晶体,在34.2 W最大基频光入射功率下,实现了高功率、高效率的266 nm固态深紫外激光输出,其平均功率为14 W,光-光转换效率为41%;当平均功率为7 W时,其光束质量因子Mx2为1.54,My2为1.97。在国产10 mm长的CLBO晶体对比实验中,当532 nm基频光的功率为8 W时,也获得了1.7 W的输出功率。当输出功率为10 W时,266 nm激光在10 min内的功率稳定性RMS为1.52%。热分布、机械应力等是影响深紫外激光功率稳定性的主要原因。通过提升泵浦光功率及改进温度控制和机械设计等优化方案,可获得更高功率的深紫外激光并提高其光束质量,从而进一步推动高功率固态深紫外激光在工业科研领域中的应用和发展。

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俞航航, 张志韬, 玄洪文. 基于国产商用CLBO晶体的高转换效率、高功率深紫外266 nm激光[J]. 中国激光, 2024, 51(7): 0701020. Hanghang Yu, Zhitao Zhang, Hongwen Xuan. High‑Conversion‑Efficiency High‑Power Deep‑Ultraviolet 266 nm Laser Based on Domestic Commercially Available CLBO Crystal[J]. Chinese Journal of Lasers, 2024, 51(7): 0701020.

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