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Efficient third harmonic generation of 355 nm picosecond laser pulse

Cheng Meng-Yao Wang Zhao-Hua He Hui-Jun Wang Xian-Zhi Zhu Jiang-Feng Wei Zhi-Yi

Efficient third harmonic generation of 355 nm picosecond laser pulse

Cheng Meng-Yao, Wang Zhao-Hua, He Hui-Jun, Wang Xian-Zhi, Zhu Jiang-Feng, Wei Zhi-Yi
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  • Picosecond laser with high-repetition-rate and high pulse energy is widely favorite in many scientific and industrial applications. Some nonlinear crystals can be used to efficiently convert a near-infrared laser into a green laser or an ultraviolet laser which has a higher photon energy and a smaller focal area. Especially for high-quality and high-speed transparent hard material fabrication, green or ultraviolet picosecond laser has been found to possess unique advantages. In this paper, the experiments on high-efficiency second-harmonic-generation (SHG) and third-harmonic-generation (THG) by using a home-made all-solid-state picosecond laser amplifier and an LBO crystal are reported. The picosecond laser amplifier consists of a seed source, a regenerative amplifier and a two-stage single-pass amplifier. The seed source is a commercial all-solid-state picosecond oscillator with a pulse duration of 8.3 ps and a repetition rate of 68 MHz. The repetition rate is reduced from 68 MHz to 500 kHz by an electro-optic Pockels cell (PC), and the period doubling bifurcation is minimized by reducing the duration of high voltage in PC. Both the regenerative amplifier and the two-stage single-pass amplifier are pumped by three 30-W continuous-wave fiber-coupled laser diodes. After the regenerative amplifier, the seed laser is amplified to 4.86 W with a repetition rate of 500 kHz at 1064 nm. Then the laser power is increased to 23.2 W by a two-stage single-pass amplifier, and the M2 value of the amplified laser in the X direction and in the Y direction are 1.330 and 1.235, respectively. The final pulse duration is 13.4 ps, which is slightly stretched in the amplification chain compared with the seed pulse duration (8.3 ps). For high-efficiency SHG and THG from near-infrared to green and ultraviolet, we carefully study the optical characteristics of some nonlinear crystals, such as LBO, BBO, BIBO, CLBO, etc., and we find that the LBO crystal, which has a high damage threshold, small walk-off and high nonlinear coefficient, is the best choice for both SHG and THG. Then the parameters of the two crystals for SHG and THG are specially designed according to the phase matching condition, the walk-off and the laser parameter. As a result, a 4-mm-long type-I phase matching LBO with cutting angle of θ = 90° and φ = 11.6° is used for SHG, and a 3-mm-long type-II phase matching LBO with cutting angle of θ = 42.2° and φ = 90° is used for THG. Finally, we realize high-efficiency frequency conversion with SHG power of 12.7 W at 532 nm and THG power of 9.25 W at 355 nm. The corresponding optical-optical conversion efficiencies reach 54.7% and 39.6%, respectively.
      Corresponding author: Wang Zhao-Hua, zhwang@iphy.ac.cn ; Wei Zhi-Yi, zywei@iphy.ac.cn
    [1]

    Erny C, Heese C, Haag M, Gallmann L, Keller U 2009 Opt. Express 17 1340

    [2]

    王鹏, 赵环, 王兆华, 李德华, 魏志义 2006 物理学报 55 4161

    Wang P, Zhao H, Wang Z H, Li D H, Wei Z Y 2006 Acta Phys. Sin. 55 4161

    [3]

    何会军, 蒋建旺, 程梦尧, 宋贾俊, 王兆华, 方少波, 魏志义 2018 光子学报 47 0914002

    He H J, Jiang J W, Cheng M Y, Song J J, Wang Z H, Fang S B, Wei Z Y 2018 Acta Photon. Sin. 47 0914002

    [4]

    Muhammad N, Whitehead D, Boor A, Oppenlander W, Liu Z, Li L 2012 Appl. Phys. A 106 607

    [5]

    Weck A, Crawford T H R, Wilkinson D S, Haugen H K, Preston J S 2008 Appl. Phys. A 90 537

    [6]

    张菲, 段军, 曾晓雁, 李祥友 2009 中国激光 36 3143

    Zhang F, Duan J, Zeng X Y, Li X Y 2009 Chin. J. Las. 36 3143

    [7]

    Rauch T, Delmdahl R, Pfeufer V, Mondry M 2009 Laser Tech. J. 6 20

    [8]

    Norreys P A, Zepf M, Moustaizis S, Fews A P, Zhang J, Lee P, Bakarezos M, Danson C N, Dyson A, Gibbon P, Loukakos P, Neely D, Walsh F N, Wark J S, Dangor A E 1996 Phys. Rev. Lett. 76 1832

    [9]

    Liu J X, Wang W, Wang Z H, Lü Z G, Zhang Z Y, Wei Z Y 2015 Appl. Sci. 5 1590

    [10]

    Offerhaus H L, Godfried H P, Witteman W J 1996 Opt. Commun. 128 61

    [11]

    Dörring J, Killi A, Morgner U, Lang A, Lederer M, Kopf D 2004 Opt. Express 12 1759

    [12]

    毛小洁, 秘国江, 庞庆生, 邹跃 2013 中国激光 38 1002004

    Mao X J, Bi G J, Pang Q S, Zou Y 2013 Chin. J. Las. 38 1002004

    [13]

    Zhu P, Li D J, Liu Q Y, Chen J, Fu S J, Shi P, Du K M, Loosen P 2013 Opt. Lett. 38 4716

    [14]

    Borsutzky A, Briinger R, Huang C H, Wallenstein R 1991 Appl. Phys. B 52 55

    [15]

    Ghotbi M, Sun Z, Majchrowski A, Michalski E, Kityk I V, Ebrahim Z M 2006 Appl. Phys. Lett. 89 173124

    [16]

    Ueda K, Orii Y,Takahashi Y, Okada G, Mori Y, Yoshimura M 2016 Opt. Express 24 30465

    [17]

    Guo L, Wang G L, Zhang H B, Cui D F, Wu Y C, Lu L, Zhang J Y, Huang J Y, Xu Z Y 2007 Appl. Phys. B 88 197

    [18]

    Yoshida H, Fujita H, Nakatsuka M, Yoshimura M, Sasaki T, Kamimura T, Yoshida K 2006 Jpn. J. Appl. Phys. 45 766

    [19]

    王正平, 滕冰, 杜晨林, 许心光, 傅琨, 许贵宝, 王继扬, 邵宗书 2003 物理学报 52 2176

    Wang Z P, Teng B, Du C L, Xu X G, Fu K, Xu G B, Wang J Y, Shao Z S 2003 Acta Phys. Sin. 52 2176

    [20]

    Wu Y C, Sasaki T, Nakai S, Yokotani A, Tang H G, Chen C T 1993 Appl. Phys. Lett. 62 2614

    [21]

    Wu B C, Chen N, Chen C T, Deng D Q, Xu Z Y 1989 Opt. Lett. 14 1080

    [22]

    Chen L Y, Bai Z X, Pan Y L, Chen M, Li G 2013 Opt. Eng. 52 086107

  • 图 1  全固态皮秒激光放大器及SHG与THG光路图(HR, 高反镜; DM, 双色镜; LD, 二极管激光器; HT 1064 HR 532, 在1064 nm处高透, 在532 nm处高反)

    Figure 1.  Structure of all-solid-state picosecond laser amplifier, SHG and THG (HR, high reflectivity mirror; DM, dichroic mirror; LD, laser diode; HT 1064 HR 532: high transmittance @ 1064 nm and high reflectivity @ 532 nm).

    图 2  种子光和基频光自相关曲线(sech2拟合) (a)种子光自相关曲线; (b)基频光自相关曲线

    Figure 2.  Self-reference curve of seed laser and fundamental laser (sech2 fitting): (a) Self-referencecurve of seed laser; (b) self-reference curve of fundamental laser.

    图 3  基频光光谱图和光束质量(M2)图 (a)基频光光谱图; (b)基频光光束质量(M2)图

    Figure 3.  Wavelength and beam quality (M2) of fundamental laser: (a) Wavelength of fundamental laser; (b) beam quality (M2) of fundamental laser.

    图 4  SHG与THG过程中光束空间走离及补偿的原理示意图

    Figure 4.  Schematic diagram of space walk-off and compensation for SHG and THG.

    图 5  SHG激光功率稳定性

    Figure 5.  Power stability of SHG power.

    图 6  THG激光功率曲线图

    Figure 6.  THG output power versus input power.

    图 7  (a) SHG与(b) THG光谱图

    Figure 7.  Wavelength of (a) SHG and (b) THG.

    表 1  非线性晶体参数

    Table 1.  Parameters of nonlinear crystal.

    晶体透射范围/nm损伤阈值/GW·cm–2deff 11/pm·V–1deff 22/pm·V–1走离角1/mrad走离角2/mrad
    LBO160—260036.3[18]0.83 (xy)0.53 (yz)7.06 (xy)9.30 (yz)
    BBO185—260018.27[18]2.012.0255.8672.33
    KDP177—170020[18]0.260.3228.0630.12
    BIBO286—25003.4[19]2.96 (yz)3.9 (yz)25.74 (yz)67.86 (yz)
    CLBO180—257027.3[18]0.380.5231.4137.16
    CBO170—300026[20]1.01 (xz)1.19 (xy)31.17 (xz)16.25 (xy)
    注: 1, 1064 nm倍频输出532 nm的条件下; 2, 1064 nm与532 nm和频输出355 nm的条件下
    DownLoad: CSV
  • [1]

    Erny C, Heese C, Haag M, Gallmann L, Keller U 2009 Opt. Express 17 1340

    [2]

    王鹏, 赵环, 王兆华, 李德华, 魏志义 2006 物理学报 55 4161

    Wang P, Zhao H, Wang Z H, Li D H, Wei Z Y 2006 Acta Phys. Sin. 55 4161

    [3]

    何会军, 蒋建旺, 程梦尧, 宋贾俊, 王兆华, 方少波, 魏志义 2018 光子学报 47 0914002

    He H J, Jiang J W, Cheng M Y, Song J J, Wang Z H, Fang S B, Wei Z Y 2018 Acta Photon. Sin. 47 0914002

    [4]

    Muhammad N, Whitehead D, Boor A, Oppenlander W, Liu Z, Li L 2012 Appl. Phys. A 106 607

    [5]

    Weck A, Crawford T H R, Wilkinson D S, Haugen H K, Preston J S 2008 Appl. Phys. A 90 537

    [6]

    张菲, 段军, 曾晓雁, 李祥友 2009 中国激光 36 3143

    Zhang F, Duan J, Zeng X Y, Li X Y 2009 Chin. J. Las. 36 3143

    [7]

    Rauch T, Delmdahl R, Pfeufer V, Mondry M 2009 Laser Tech. J. 6 20

    [8]

    Norreys P A, Zepf M, Moustaizis S, Fews A P, Zhang J, Lee P, Bakarezos M, Danson C N, Dyson A, Gibbon P, Loukakos P, Neely D, Walsh F N, Wark J S, Dangor A E 1996 Phys. Rev. Lett. 76 1832

    [9]

    Liu J X, Wang W, Wang Z H, Lü Z G, Zhang Z Y, Wei Z Y 2015 Appl. Sci. 5 1590

    [10]

    Offerhaus H L, Godfried H P, Witteman W J 1996 Opt. Commun. 128 61

    [11]

    Dörring J, Killi A, Morgner U, Lang A, Lederer M, Kopf D 2004 Opt. Express 12 1759

    [12]

    毛小洁, 秘国江, 庞庆生, 邹跃 2013 中国激光 38 1002004

    Mao X J, Bi G J, Pang Q S, Zou Y 2013 Chin. J. Las. 38 1002004

    [13]

    Zhu P, Li D J, Liu Q Y, Chen J, Fu S J, Shi P, Du K M, Loosen P 2013 Opt. Lett. 38 4716

    [14]

    Borsutzky A, Briinger R, Huang C H, Wallenstein R 1991 Appl. Phys. B 52 55

    [15]

    Ghotbi M, Sun Z, Majchrowski A, Michalski E, Kityk I V, Ebrahim Z M 2006 Appl. Phys. Lett. 89 173124

    [16]

    Ueda K, Orii Y,Takahashi Y, Okada G, Mori Y, Yoshimura M 2016 Opt. Express 24 30465

    [17]

    Guo L, Wang G L, Zhang H B, Cui D F, Wu Y C, Lu L, Zhang J Y, Huang J Y, Xu Z Y 2007 Appl. Phys. B 88 197

    [18]

    Yoshida H, Fujita H, Nakatsuka M, Yoshimura M, Sasaki T, Kamimura T, Yoshida K 2006 Jpn. J. Appl. Phys. 45 766

    [19]

    王正平, 滕冰, 杜晨林, 许心光, 傅琨, 许贵宝, 王继扬, 邵宗书 2003 物理学报 52 2176

    Wang Z P, Teng B, Du C L, Xu X G, Fu K, Xu G B, Wang J Y, Shao Z S 2003 Acta Phys. Sin. 52 2176

    [20]

    Wu Y C, Sasaki T, Nakai S, Yokotani A, Tang H G, Chen C T 1993 Appl. Phys. Lett. 62 2614

    [21]

    Wu B C, Chen N, Chen C T, Deng D Q, Xu Z Y 1989 Opt. Lett. 14 1080

    [22]

    Chen L Y, Bai Z X, Pan Y L, Chen M, Li G 2013 Opt. Eng. 52 086107

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  • Received Date:  08 April 2019
  • Accepted Date:  26 April 2019
  • Available Online:  16 August 2019
  • Published Online:  01 June 2019

Efficient third harmonic generation of 355 nm picosecond laser pulse

    Corresponding author: Wang Zhao-Hua, zhwang@iphy.ac.cn
    Corresponding author: Wei Zhi-Yi, zywei@iphy.ac.cn
  • 1. School of Physics and Optoelectronic Engineering, Xidian University, Xi’an 710071, China
  • 2. Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

Abstract: Picosecond laser with high-repetition-rate and high pulse energy is widely favorite in many scientific and industrial applications. Some nonlinear crystals can be used to efficiently convert a near-infrared laser into a green laser or an ultraviolet laser which has a higher photon energy and a smaller focal area. Especially for high-quality and high-speed transparent hard material fabrication, green or ultraviolet picosecond laser has been found to possess unique advantages. In this paper, the experiments on high-efficiency second-harmonic-generation (SHG) and third-harmonic-generation (THG) by using a home-made all-solid-state picosecond laser amplifier and an LBO crystal are reported. The picosecond laser amplifier consists of a seed source, a regenerative amplifier and a two-stage single-pass amplifier. The seed source is a commercial all-solid-state picosecond oscillator with a pulse duration of 8.3 ps and a repetition rate of 68 MHz. The repetition rate is reduced from 68 MHz to 500 kHz by an electro-optic Pockels cell (PC), and the period doubling bifurcation is minimized by reducing the duration of high voltage in PC. Both the regenerative amplifier and the two-stage single-pass amplifier are pumped by three 30-W continuous-wave fiber-coupled laser diodes. After the regenerative amplifier, the seed laser is amplified to 4.86 W with a repetition rate of 500 kHz at 1064 nm. Then the laser power is increased to 23.2 W by a two-stage single-pass amplifier, and the M2 value of the amplified laser in the X direction and in the Y direction are 1.330 and 1.235, respectively. The final pulse duration is 13.4 ps, which is slightly stretched in the amplification chain compared with the seed pulse duration (8.3 ps). For high-efficiency SHG and THG from near-infrared to green and ultraviolet, we carefully study the optical characteristics of some nonlinear crystals, such as LBO, BBO, BIBO, CLBO, etc., and we find that the LBO crystal, which has a high damage threshold, small walk-off and high nonlinear coefficient, is the best choice for both SHG and THG. Then the parameters of the two crystals for SHG and THG are specially designed according to the phase matching condition, the walk-off and the laser parameter. As a result, a 4-mm-long type-I phase matching LBO with cutting angle of θ = 90° and φ = 11.6° is used for SHG, and a 3-mm-long type-II phase matching LBO with cutting angle of θ = 42.2° and φ = 90° is used for THG. Finally, we realize high-efficiency frequency conversion with SHG power of 12.7 W at 532 nm and THG power of 9.25 W at 355 nm. The corresponding optical-optical conversion efficiencies reach 54.7% and 39.6%, respectively.

    • 高重频皮秒激光放大器具有平均功率高、脉冲能量大、光束质量好等优点, 在光参量放大[1]、同步抽运[2,3]、激光微加工[4,5]等科学研究与工业加工领域中具有广泛应用. 同时, 利用非线性晶体将近红外激光转换通过二倍频(second harmonic generation, SHG)及三倍频(third harmonic generation, THG)转化为绿光或紫外光后, 可使激光具有更高的光子能量与更小的聚焦面积, 不但可以有效提高加工精度, 也可对一些特殊表面进行加工[6,7], 或用于表面物理实验[8]等研究中. 特别是在如蓝宝石等透明硬质材料的切割中, 绿光或紫外皮秒激光器具有独特的优势.

      通常由激光振荡器直接产生的超短脉冲激光的能量非常低, 为了产生高平均功率、高能量的皮秒激光, 需要对振荡器输出的种子激光进行放大; 这可以采用再生放大技术[9]或者主振荡功率放大(master oscillator power amplifier, MOPA)技术[10]. 高重频再生放大中, 由于脉冲间隔小于增益介质的上能级寿命, 需要消除倍周期分叉效应[11]以实现稳定的脉冲输出; 而采用MOPA结构放大时不会产生此效应, 但由于缺少对应重复频率的皮秒种子源[12], 需要对种子源脉冲选单后再放大, 这样会降低放大效率. 结合两种技术的特点, 我们采用再生放大加两级单通放大的方案来实现高功率皮秒激光放大. 其对再生放大器输出功率要求低, 可以较为容易地消除倍周期分叉效应, 从而实现稳定的高功率高能量基频光输出.

      在高效率THG中, 一般先使用非线性晶体对基频光进行倍频, 而后再将倍频光和基频光进行和频, 产生紫外激光, 常用的非线性晶体有LBO[13], BBO[14], KDP[14], BIBO[15], CLBO[16], CBO[17]等非线性晶体, 这些晶体的物理性质和光学性质如表1[18-20]所列.

      晶体透射范围/nm损伤阈值/GW·cm–2deff 11/pm·V–1deff 22/pm·V–1走离角1/mrad走离角2/mrad
      LBO160—260036.3[18]0.83 (xy)0.53 (yz)7.06 (xy)9.30 (yz)
      BBO185—260018.27[18]2.012.0255.8672.33
      KDP177—170020[18]0.260.3228.0630.12
      BIBO286—25003.4[19]2.96 (yz)3.9 (yz)25.74 (yz)67.86 (yz)
      CLBO180—257027.3[18]0.380.5231.4137.16
      CBO170—300026[20]1.01 (xz)1.19 (xy)31.17 (xz)16.25 (xy)
      注: 1, 1064 nm倍频输出532 nm的条件下; 2, 1064 nm与532 nm和频输出355 nm的条件下

      Table 1.  Parameters of nonlinear crystal.

      表1可以看出, 相比于其他晶体, LBO晶体具有损伤阈值高、走离角度小等优点, 并且LBO晶体具有稳定的物理化学性质[21], 因此选用LBO晶体开展倍频与和频实验.

      2013年, 北京工业大学的Chen等[22]报道了利用LBO晶体和再生放大器实现的皮秒紫外激光, 他们利用可以输出6.6 mJ基频光的Nd:YAG再生放大器实现了2 mJ的355 nm皮秒紫外激光输出, 激光的重复频率为1 kHz, THG效率为33.3%. 这种大能量的皮秒紫外激光在科学研究中有着较为广泛的应用, 但是由于激光器的重复频率较低, 因此无法在工业加工中使用. 同年, 德国亚琛工业大学的Zhu等[13]也利用LBO晶体在1 MHz的重复频率下实现了39.1 W的355 nm皮秒紫外激光输出, THG效率为46%; 该实验中基频光来自于一套多级多通放大系统, 选单后种子光经过多级多通放大后在1 MHz重复频率下被放大到84 W; 这套系统可以实现较高的平均功率, 但是存在结构复杂、成本高等缺点. 本文利用一套自建的全固态皮秒激光放大器实现了高效率的SHG与THG. 全固态皮秒激光放大器由种子源、再生放大器和两级单通放大器组成, 输出23.2 W平均功率、13.4 ps脉冲宽度、500 kHz重复频率的1064 nm基频光, 基频光在xy方向上的M2分别为1.330和1.235. 随后利用两块LBO晶体作为SHG和THG晶体, 分别得到了中心波长位于532 nm处12.7 W的SHG激光和中心波长位于355 nm处9.25 W的THG激光, 相应的光光转换效率分别为54.7%和39.8%. 并对倍频光进行了24 h的稳定性测试, 其均方根误差(root mean square, RMS)小于0.2%. 这套激光器系统具有转换效率高、平均功率高、结构简单、低成本等优点, 可用于光参量放大、激光加工、材料表面处理等科学研究或工业生产领域.

    2.   全固态皮秒激光放大器
    • 实验中设计研制的全固态皮秒激光放大器及SHG与THG的光路图如图1所示.

      Figure 1.  Structure of all-solid-state picosecond laser amplifier, SHG and THG (HR, high reflectivity mirror; DM, dichroic mirror; LD, laser diode; HT 1064 HR 532: high transmittance @ 1064 nm and high reflectivity @ 532 nm).

      全固态皮秒激光放大器由种子源、再生放大器和两级单通放大器组成. 其中振荡器采用了一台最大输出功率为2 W、重复频率为68 MHz、输出脉冲宽度8.3 ps的皮秒激光种子源(HighQ公司). 我们在振荡器与再生放大器之间插入了一个隔离器以避免回光入射到振荡器中影响锁模稳定性. 再生放大器中采用一块20 mm长、a-cut、0.15 at.%掺杂浓度的Nd:YVO4晶体作为增益介质, 而两级单通放大器中采用两块10 mm长、a-cut、0.3 at.%掺杂浓度的Nd:YVO4晶体作为增益介质. 为了实现更高的效率, 所有晶体表面均镀有对808 nm和1064 nm高透的介质膜; 同时所有晶体均用铟箔包裹后通过铜制通水热沉进行散热, 冷却水温度均为20 ℃. 放大激光系统采用3个最大输出功率为30 W的光纤耦合半导体激光器(laser diode, LD)作为抽运源, 分别用于再生放大器和两级单通放大器中. 再生放大器可以将振荡器种子能量由nJ量级放大到μJ量级, 重复频率经过普克尔盒(Pockels cell, PC)选单为500 kHz. 由于高重频皮秒再生放大器中的脉冲间隔(2 μs)远远小于增益介质的上能级寿命(120 μs), 因此在再生放大器中容易产生倍周期分叉效应; 需要进行优化, 并以降低输出功率为代价, 减小再生腔中PC的加压时间来实现无分叉的稳定脉冲输出. 最终, 再生放大器将种子光的脉冲能量放大到9.72 μJ, 重复频率为500 kHz, 平均功率为4.86 W. 此时, 再生腔的抽运功率为11.9 W, 对应的光光效率为40.8%; 再生腔的加压时间为205 ns.

      再生放大激光的输出平均功率较低, 需要通过单通放大提高输出平均功率. 第一级单通放大中, 在25 W的抽运功率与4.6 W的入射功率下, 输出功率为14.5 W. 第二级单通放大中, 在25 W的抽运功率下, 输出功率为23.2 W. 两级单通放大后输出的基频光为平行激光, 光斑半径约为1 mm. 我们利用一台最大扫描宽度为55 ps的自相关仪(PulseCheck, APE)分别测量了种子光与基频光的脉冲宽度, 测得基频光的脉冲宽度为13.4 ps, 与种子光的脉冲宽度(8.3 ps)相比有一定的展宽, 种子光与基频光的自相关曲线如图2所示.

      Figure 2.  Self-reference curve of seed laser and fundamental laser (sech2 fitting): (a) Self-referencecurve of seed laser; (b) self-reference curve of fundamental laser.

      也利用光谱仪(HR400CG-UV-NIR, Ocean Optics)和光束质量分析仪(M2-200S, SPIRICON)分别测量了基频光的光谱和光束质量(M2), 如图3所示. 图3(a)是基频光的光谱, 可以看出基频光光谱的中心波长位于1064 nm处; 图3(b)是基频光的光束质量(M2), 其在xy方向上的M2分别为1.330和1.235, 表明基频光具有较好的光束质量.

      Figure 3.  Wavelength and beam quality (M2) of fundamental laser: (a) Wavelength of fundamental laser; (b) beam quality (M2) of fundamental laser.

    3.   频率变换实验
    • 频率变换实验分为倍频实验与和频实验, 其光路图如图1所示. 在基频光入射前, 一个λ/2波片和薄膜偏振片(thin film laser polarizer, TFP)用于调节入射光的功率; 另一个λ/2波片用于调节入射激光的偏振方向, 以调节倍频效率. 由于聚焦入射时可以提高激光的功率密度, 从而在较短的晶体长度下实现高效率的频率变换, 并减小晶体内的走离效应, 因此在晶体前使用一个焦距为100 mm的平凸透镜聚焦入射激光, 晶体与平凸透镜的距离约为90 mm.

      为了实现高效率的相位匹配, 在倍频时, 选用一类相位匹配, LBO晶体的切割角为θ = 90°, ϕ = 11.6°; 在和频时, 选用二类相位匹配, LBO晶体的切割角为θ = 42.2°, ϕ = 90°. 倍频与和频晶体表面均镀有对1064, 532和355 nm激光高透的介质膜, 同时晶体用铟箔包裹后通过铜制通水热沉进行冷却, 冷却水温度为20 ℃.

      为了实现较高的和频效率, 利用商业软件SNLO对非线性晶体的长度与和频效率的关系进行了计算. 首先假设入射到倍频晶体上的基频光为高斯光束, 单脉冲能量约为46.4 μJ, 当和频效率最高时, 倍频效率应为40%, 此时假设倍频时没有能量损耗, 可得到倍频后剩余基频光的单脉冲能量为27.8 μJ, 倍频光的单脉冲能量为18.6 μJ. 当激光聚焦入射到和频晶体上时, 和频晶体上的剩余基频光峰值功率密度约为7 GW/cm2; 根据这个峰值功率密度, 计算得到当和频晶体长度为3 mm时, 和频效率最高, 因此选用3 mm长的LBO晶体作为和频晶体.

      在确定了和频晶体长度后, 需要通过计算基频光与倍频光在晶体中走离效应来确定倍频晶体长度, 如图4所示. 在和频晶体, 选用二类匹配, 因此倍频光与基频光在经过和频晶体时, 会在yz平面内产生走离, 走离角为9.3 mrad, 经过3 mm长晶体后, 走离距离为28 μm; 而倍频晶体, 选用一类相位匹配, 倍频光与基频光会在xy平面内走离, 走离角为7 mrad, 因此选用4 mm长的倍频晶体时, 其在空间上走离距离也为28 μm, 正好可以补偿基频光与倍频光的走离, 提高和频效率.

      Figure 4.  Schematic diagram of space walk-off and compensation for SHG and THG.

      最终, 在SHG时, 使用5 mm × 5 mm截面的4 mm长LBO晶体, 切割角度为θ = 90°, ϕ = 11.6°, 满足一类相位匹配; THG时, 和频部分使用一块5 mm × 5 mm截面的3 mm长LBO晶体, 切割角为θ = 42.2°, ϕ = 90°, 满足二类相位匹配.

      在SHG实验中, 通过仔细调节激光入射角度、偏振方向和SHG晶体与聚焦透镜的距离, 最终在23.2 W的基频光下得到了12.7 W的SHG激光, 中心波长为532 nm, SHG效率为54.7%. 也测量了SHG激光24 h的功率稳定性, 如图5所示, 其24 h内的功率稳定性(RMS)小于0.2%, 表明SHG激光有很好的功率稳定性.

      Figure 5.  Power stability of SHG power.

      在进行THG实验时, 为了使和频输出功率达到最大, 需要在SHG实验的基础上, 减小倍频晶体与聚焦透镜的距离, 使倍频效率降低到约40%左右; 此时在SHG晶体后约1 mm处插入和频晶体, 就可在和频晶体后得到高效率的THG激光输出. 通过仔细调节基频光的各个参数, 最终, 在23.2 W基频光下, 得到了9.25 W的THG激光, 中心波长为355 nm, THG效率为39.8%, 其功率曲线如图6所示.

      Figure 6.  THG output power versus input power.

      还测量了SHG和THG的光谱, 如图7所示. 从图7可以看出SHG和THG的中心波长分别为532 nm和355 nm.

      Figure 7.  Wavelength of (a) SHG and (b) THG.

    4.   总 结
    • 利用一套自建的全固态皮秒激光放大器进行了高效率的SHG与THG实验. 自建的全固态皮秒激光放大器由种子源、再生放大器、两级单通放大器三部分组成, 最终可以输出23.2 W平均功率、13.4 ps脉冲宽度、1064 nm中心波长的基频光, 基频光的重复频率为500 kHz, 其在xy方向上的M2分别为1.330和1.235. 随后进行的非线性频率变换中, 利用LBO晶体进行了SHG与THG实验, 分别得到了中心波长位于532 nm处12.7 W的SHG激光和中心波长位于355 nm处9.25 W的THG激光, 相应的光光转换效率分别为54.7%和39.8%. 这种激光器具有转换效率高、平均功率高、结构简单等特点, 可以广泛地用于科学研究与工业加工中, 特别是在如蓝宝石等透明硬质材料的切割中具有重要的价值.

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