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Deep ultraviolet (DUV) femtosecond laser, which combines the advantages of high single-photon energy of DUV laser with high peak power of femtosecond laser, is widely used in scientific research, biomedicine, material processing and so on. However, in the process of generating DUV femtosecond laser based on nonlinear frequency conversion is encountered a problem that the group velocity mismatch caused by dispersion makes the temporal walk-off of the nonlinear frequency conversion larger than the pulse duration of the femtosecond laser, thus making the generation of the DUV femtosecond laser very difficult. In this work, based on a Yb-doped fiber femtosecond laser, the delay line is optimized to precisely compensate for the spatial and temporal walk-off, so DUV femtosecond laser possesses the following performances: the center wavelength is 206 nm, the repetition rate is 1 MHz, the maximum output power is 102 mW, the maximum conversion efficiency is 4.25% from near infrared to DUV, the root mean square (RMS) power stability is 0.88% within 3 h, and the peak-to-peak power stability is 3.75%. The evolution of laser spectra and beam quality in the process of second harmonic generation (SHG), fourth harmonic generation (FHG) and sum-frequency generation (SFG) are also systematically studied. The experimental results provide a basis for generating DUV femtosecond laser from femtosecond fiber lasers.
1. 引 言
深紫外激光具有单光子能量高、空间分辨率好、峰值功率高等优异性能, 被广泛应用于光谱检测、隐秘通讯、材料加工、生物医疗等领域中[1–5], 具有很大的市场前景和发展潜力. 如何研制出稳定、高效的深紫外激光光源是研究人员研究的热点, 常用的深紫外激光器有准分子气体激光器和固体激光器, 相比准分子气体激光器, 基于固体激光器非线性频率变换获得的深紫外光源具有高光束质量、高重频、高相干性等优点, 是目前科学研究的热点.
近年来, 科研人员对皮秒深紫外输出展开了广泛的研究[6–11]. 与皮秒深紫外光相比, 飞秒深紫外激光具有更窄的脉宽和更高的峰值功率, 其在材料精密加工方面有独特的优势[12,13]. 目前, 200 nm波段的飞秒深紫外激光主要依靠钛宝石激光器输出的基频光进行非线性四倍频得到[14–17]. 2008年, Wang等[15]利用钛宝石激光器实现了200 nm的飞秒深紫外激光输出. 2015年, 孟祥昊等[16]利用钛宝石激光器提供的可调谐光源, 实现了192.5—210 nm的可调谐飞秒深紫外激光输出. 受限于钛宝石晶体的激光特性, 目前基于钛宝石飞秒激光器产生的深紫外飞秒激光重复频率较低, 通常在kHz量级. 同时, 钛宝石飞秒激光器体积大、成本高, 应用推广相对比较难. 光纤飞秒激光器具有结构紧凑、光束质量好、成本低等优点, 采用飞秒光纤激光器作为基频光, 通过五倍频可以实现200 nm波段的深紫外激光输出, 2014年, Otsu等[18]通过掺镱光纤飞秒激光器输出了0.3 mW的206 nm深紫外激光. 对于飞秒量级的激光, 5次谐波产生过程中产生的时间走离通常大于脉冲持续时间, 因此高效的延迟方案是五倍频过程走离补偿的关键[19–21].
本文采用飞秒光纤激光器作为基频光, 基于延迟线系统补偿时空走离. 通过优化延迟线, 可以有效地补偿非线性频率变换过程中的时间走离和空间走离. 这是首次基于1030 nm掺镱光纤飞秒激光器实现百毫瓦量级的206 nm深紫外飞秒激光输出, 其平均输出功率达到102 mW, 重复频率为1 MHz, 功率稳定性为0.88% RMS (3 h), 峰峰值功率稳定性为3.75%, 红外至深紫外的转换效率达到4.25%.
2. 实验装置
图1为5次谐波产生的实验装置图. 掺镱飞秒光纤激光器输出的1030 nm基频光首先通过半波片和薄膜偏振片, 其目的是控制入射基频光的入射功率大小以及偏振态. 基频光经过LBO倍频晶体实现倍频后, 利用双色镜DM1将基频光和倍频光分开, 随后, 基频光进入延迟线系统, 通过调节延迟线补偿基频光和倍频光之间的时间走离和空间走离. 最后, 基频光和倍频光经过双色镜DM2后再次汇合, 其中倍频光经过BBO晶体再次倍频从而产生四倍频光输出, 之后基频光再和四倍频光经过BBO五倍频晶体进行Ι类相位匹配和频实现5次谐波产生. 采用CaF2佩林布洛卡棱镜将206 nm五倍频激光进行分离, 从而实现深紫外飞秒激光的独立输出.
图 1 实验装置示意图. λ/2, 半波片; TFP, 薄膜偏振片; M1—M5, 1030 nm反射镜; M6, M7, 515 nm反射镜; F1, 450 mm透镜; F2, 125 mm透镜; F3, 150 mm透镜; DM1, DM2, 双色镜; SHG, 3 mm LBO倍频晶体; FHG, 1 mm BBO四倍频晶体; FiHG, 1 mm BBO五倍频晶体; PP, 佩林布洛卡棱镜Fig. 1. Schematic of experimental setup. λ/2, half-wave plate; TFP, thin-film polarizer; M1—M5, plano mirror at 1030 nm; M6, M7, plano mirror at 515 nm; F1, 450 mm lenses; F2, 125 mm lenses; F3, 150 mm lenses; DM1, DM2, dichroic mirror; SHG, second harmonic generation, 3 mm LBO crystal; FHG, fourth harmonic generation, 1 mm BBO crystal; FiHG, fifth harmonic generation, 1 mm BBO crystal; PP, Pellin-Broca prism.3. 结果与讨论
实验采用1030 nm的掺镱飞秒光纤激光器, 使用YOKOGAWA公司生产的AQ6370B光谱分析仪测得基频光的中心波长为1029.3 nm, 其光谱宽度(1/e2处)约为13.5 nm, 图2(a)是基频光的光谱图, 脉冲宽度为433 fs, 图2(b)是基频光的脉宽图, 图2(b)中的插图为基频光光斑图, 椭圆度为0.9. 整个非线性频率变换过程分为倍频、四倍频以及和频3个部分.
图 2 (a) 基频光光谱图; (b) 基频光脉宽图, 插图为近场光斑图Fig. 2. (a) Spectrum of the fundamental frequency laser; (b) pulse width of the fundamental frequency laser, and the inset is the near-field beam profile of the fundamental frequency laser.如图1所示, 1030 nm基频光经过450 mm透镜聚焦, 测量得到基频光聚焦光斑大小约为258 μm, 经过3 mm LBO (θ = 90°, φ = 12.9°, S1, S2: AR-1030/515 nm)倍频晶体后, 当入射基频光为2.4 W时, 实现了1.1 W的绿光输出, 倍频转化效率为45.83%. 图3(a)是倍频光的平均功率和倍频转换效率随入射基频光功率变化关系图, 插图为最高输出功率下倍频光的近场光斑图, 椭圆度为0.88. 图3(b)为倍频光的光谱图, 使用Ocean insight公司生产的HR4000光谱分析仪测得倍频光的中心波长为515.6 nm, 其光谱宽度(1/e2处)约为3.6 nm, 估算倍频光的脉冲宽度在300 fs左右.
图 3 (a) 倍频光的平均功率和倍频转换效率随入射基频光功率变化关系图, 插图为最高平均功率输出时倍频光的近场光斑图; (b) 倍频光光谱图Fig. 3. (a) Average output power and conversion efficiency of the SH beam as functions of the fundamental power, inset, the near-field beam profile of the SH beam at maximum average power output; (b) spectrum of the SH.对于四倍频过程, 如图1所示, 基频光和倍频光经过150 mm透镜聚焦, 测量得到倍频光聚焦光斑大小约为284 μm, 经过1 mm BBO (θ = 50°, S1, S2: AR-515/258 nm)四倍频晶体后, 最高实现了0.23 W的257.5 nm紫外光输出, 四倍频转换效率为9.58%. 图4(a)是四倍频光的平均功率和四倍频转换效率随入射基频光功率变化关系图, 插图为最高功率输出时四倍频光的近场光斑图, 测量的光斑椭圆度为0.84. 图4(b)为四倍频光的光谱图, 使用长春新产业光电技术有限公司生产的分辨率为0.1 nm的Aurora 4000光谱仪测得四倍频光的中心波长为257.8 nm, 其光谱宽度(1/e2处)约为0.9 nm, 估算四倍频光的脉冲宽度在400 fs左右.
图 4 (a) 四倍频光的平均功率和四倍频转换效率随入射基频光功率变化关系图, 插图为最高功率输出时的四倍频光光斑图; (b) 四倍频光光谱图Fig. 4. (a) Average output power and conversion efficiency of the FH beam as functions of the fundamental power. Inset, the near-field beam profile of the FH beam at maximum average power output; (b) spectrum of the FH.在二倍频、四倍频过程中, 由于群速度走离, 导致在五倍频晶体前基频光与四倍频光在时域上分离. 通过理论模拟可以计算出, 基频光与四倍频光在五倍频之前总的时间走离量为1945.1 fs, 在五倍频晶体中的时间走离量为1327.1 fs, 具体见表1. 因此, 需要对两束光波进行时空走离补偿. 图1是基于延迟线系统补偿时间走离的实验装置图, 通过提高延迟线系统中位移台的精度, 可以实现对走离补偿进行更精确的调节, 从而获得更高的五倍频转换效率. 延迟线通常是放在四倍频之后, 但在实验过程中, 四倍频光对双色镜的损伤是很严重的. 因此, 为了避免四倍频光对双色镜的损伤, 本实验将延迟线放置在倍频之后.
表 1 基频光和四倍频光之间的时间走离Table 1. Delay time between fourth harmonic and fundamental frequency laser.LBO F2 F3 BBO (FHG) BBO (FiHG) Δt/fs 155.8 337.6 409.2 1042.5 1327.1 通过优化延迟线系统, 实现了最高输出功率达到102 mW的五倍频飞秒深紫外输出, 基频光功率为2.4 W, 转换效率是4.25%. 图5(a)是五倍频光的平均功率和转换效率随入射基频光功率变化关系图, 插图为最高平均输出功率下的五倍频光光束轮廓图, 测量的光斑椭圆度为0.77, 其中, 造成206 nm深紫外光束轮廓变椭的原因是双光子吸收导致的热畸变. 移动延迟线的位置可以改变时间走离补偿量, 从而影响五倍频转换效率. 图5(b)是五倍频输出功率随延迟线移动位置变化关系图, 其中, 原点对应的恰好是产生五倍频输出的位置. 图5(b)插图是当基频光功率在3 W时深紫外光对五倍频晶体的损伤, 因此, 五倍频的最高输出功率受到了限制. 图5(c)为实验测得的五倍频光谱图, 采用长春新产业光电技术有限公司生产的分辨率为0.1 nm的Aurora 4000光谱仪测得五倍频光的中心波长为208.1 nm, 其光谱宽度(1/e2处)约为0.6 nm, 傅里叶变换极限是229 fs. 图5(d)是206 nm飞秒深紫外输出的功率稳定性, 3 h功率稳定性是0.88% RMS, 峰峰值功率稳定性为3.75%, 由图5(d)可以看出, 206 nm五倍频光的平均输出功率总体呈下降趋势, 造成平均输出功率下降的原因是深紫外激光对五倍频晶体的损伤.
图 5 (a) 五倍频光的平均功率和五倍频转换效率随入射基频光功率变化关系图, 插图为最高功率输出时的五倍频光光斑图; (b) 五倍频光的平均功率随延迟线系统位置变化关系图, 插图为BBO晶体表面膜损伤; (c) 五倍频光谱图; (d) 功率稳定性测试Fig. 5. (a) Average output power and conversion efficiency of the FiH beam as functions of the fundamental power, inset, the near-field beam profile of the FiH beam at maximum average power output; (b) average output power of the FiH beam as functions of the location. Inset, damage to the surface film of the BBO crystal; (c) spectrum of the FiH; (d) power stability tests.4. 总 结
基于1030 nm的掺镱飞秒光纤激光器, 通过优化延迟线系统补偿时空走离, 获得了1 MHz, 206 nm的深紫外飞秒激光输出, 最高输出功率为102 mW, 近红外至深紫外的最高转换效率为4.25%, 功率稳定性是0.88% RMS (3 h), 峰峰值功率稳定性为3.75%. 与此同时, 系统研究了倍频、四倍频、和频过程中激光光谱、光束质量的演化过程, 这些实验现象为基于飞秒光纤激光器产生深紫外飞秒激光脉冲提供了实验依据. 这是首次基于1030 nm掺镱光纤飞秒激光器实现百毫瓦量级的206 nm深紫外飞秒激光输出, 有利于深紫外飞秒激光器向小型化、低成本的方向发展, 对科学研究与工业加工具有重要的价值.
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图 1 实验装置示意图. λ/2, 半波片; TFP, 薄膜偏振片; M1—M5, 1030 nm反射镜; M6, M7, 515 nm反射镜; F1, 450 mm透镜; F2, 125 mm透镜; F3, 150 mm透镜; DM1, DM2, 双色镜; SHG, 3 mm LBO倍频晶体; FHG, 1 mm BBO四倍频晶体; FiHG, 1 mm BBO五倍频晶体; PP, 佩林布洛卡棱镜
Fig. 1. Schematic of experimental setup. λ/2, half-wave plate; TFP, thin-film polarizer; M1—M5, plano mirror at 1030 nm; M6, M7, plano mirror at 515 nm; F1, 450 mm lenses; F2, 125 mm lenses; F3, 150 mm lenses; DM1, DM2, dichroic mirror; SHG, second harmonic generation, 3 mm LBO crystal; FHG, fourth harmonic generation, 1 mm BBO crystal; FiHG, fifth harmonic generation, 1 mm BBO crystal; PP, Pellin-Broca prism.
图 2 (a) 基频光光谱图; (b) 基频光脉宽图, 插图为近场光斑图
Fig. 2. (a) Spectrum of the fundamental frequency laser; (b) pulse width of the fundamental frequency laser, and the inset is the near-field beam profile of the fundamental frequency laser.
图 3 (a) 倍频光的平均功率和倍频转换效率随入射基频光功率变化关系图, 插图为最高平均功率输出时倍频光的近场光斑图; (b) 倍频光光谱图
Fig. 3. (a) Average output power and conversion efficiency of the SH beam as functions of the fundamental power, inset, the near-field beam profile of the SH beam at maximum average power output; (b) spectrum of the SH.
图 4 (a) 四倍频光的平均功率和四倍频转换效率随入射基频光功率变化关系图, 插图为最高功率输出时的四倍频光光斑图; (b) 四倍频光光谱图
Fig. 4. (a) Average output power and conversion efficiency of the FH beam as functions of the fundamental power. Inset, the near-field beam profile of the FH beam at maximum average power output; (b) spectrum of the FH.
图 5 (a) 五倍频光的平均功率和五倍频转换效率随入射基频光功率变化关系图, 插图为最高功率输出时的五倍频光光斑图; (b) 五倍频光的平均功率随延迟线系统位置变化关系图, 插图为BBO晶体表面膜损伤; (c) 五倍频光谱图; (d) 功率稳定性测试
Fig. 5. (a) Average output power and conversion efficiency of the FiH beam as functions of the fundamental power, inset, the near-field beam profile of the FiH beam at maximum average power output; (b) average output power of the FiH beam as functions of the location. Inset, damage to the surface film of the BBO crystal; (c) spectrum of the FiH; (d) power stability tests.
表 1 基频光和四倍频光之间的时间走离
Table 1. Delay time between fourth harmonic and fundamental frequency laser.
LBO F2 F3 BBO (FHG) BBO (FiHG) Δt/fs 155.8 337.6 409.2 1042.5 1327.1 
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[1] Tuschel D D, Mikhonin A V, Lemoff B E, Asher S A 2010 Appl. Spectrosc. 64 425
Google Scholar
[2] Kang Y F, Zhao J Y, Wu J X, Zhang L, Zhao J, Zhang Y Q, Zhao Y Q, Wang X F 2020 IEEE T. Electron Dev. 67 3391
Google Scholar
[3] Herman P R, Marjoribanks R S, Oettl A, Chen K, Konovalov I, Ness S 2000 Appl. Surf. Sci. 154 577
[4] Stern R S, Zierler S, Parrish J A 1980 Lancet 315 732
Google Scholar
[5] Vengris M, Gabryte E, Aleknavicius A, Barkauskas M, Ruksenas O, Vaiceliunaite A, Danielius R 2010 J. Cataract Refract. Surg. 36 1579
Google Scholar
[6] Kohler B, Andres T, Nebel A, Wallenstein R 2000 Conference on Lasers and Electro-Optics San Jose, The United States of America, May 9, 2000 p142
[7] Turcicova H, Novak O, Roskot L, Smrz M, Mocek T 2019 Opt. Express 27 24286
Google Scholar
[8] Willenberg B, Brunner F, Phillips C R, Keller U 2020 Optica 7 485
Google Scholar
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