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Output characteristics of diode-pumped passively Q-switched Yb: CaYAlO4 pulsed laser based on a SESAM

Rui TANG Zi-ye GAO Zheng-mao WU Guang-qiong XIA

唐睿, 高子叶, 吴正茂, 夏光琼. 基于SESAM被动调Q的激光二极管泵浦Yb:CaYAlO4脉冲激光器[J]. 中国光学, 2019, 12(1): 167-178. doi: 10.3788/CO.20191201.0167
引用本文: 唐睿, 高子叶, 吴正茂, 夏光琼. 基于SESAM被动调Q的激光二极管泵浦Yb:CaYAlO4脉冲激光器[J]. 中国光学, 2019, 12(1): 167-178. doi: 10.3788/CO.20191201.0167
TANG Rui, GAO Zi-ye, WU Zheng-mao, XIA Guang-qiong. Output characteristics of diode-pumped passively Q-switched Yb: CaYAlO4 pulsed laser based on a SESAM[J]. Chinese Optics, 2019, 12(1): 167-178. doi: 10.3788/CO.20191201.0167
Citation: TANG Rui, GAO Zi-ye, WU Zheng-mao, XIA Guang-qiong. Output characteristics of diode-pumped passively Q-switched Yb: CaYAlO4 pulsed laser based on a SESAM[J]. Chinese Optics, 2019, 12(1): 167-178. doi: 10.3788/CO.20191201.0167

基于SESAM被动调Q的激光二极管泵浦Yb:CaYAlO4脉冲激光器

doi: 10.3788/CO.20191201.0167
基金项目: 

国家自然科学基金项目 61475127

国家自然科学基金项目 61575163

国家自然科学基金项目 61775184

详细信息
    作者简介:

    唐睿(1992-), 女, 黑龙江齐齐哈尔人, 硕士研究生, 主要从事全固态脉冲激光器方面的研究。E-mail:

    夏光琼(1970—),女,四川富顺人,教授,博士生导师,主要从事激光非线性动力学及相关应用方面的研究。E-mail:

  • 中图分类号: TN248.1

Output characteristics of diode-pumped passively Q-switched Yb: CaYAlO4 pulsed laser based on a SESAM

Funds: 

National Natural Science Foundation of China 61475127

National Natural Science Foundation of China 61575163

National Natural Science Foundation of China 61775184

More Information
    Author Bio:

    TANG Rui(1992—), female, from Qiqihar, Heilongjiang, is a master′s student who is mainly engaged in research on all-solid-state pulsed lasers. E-mail:tangrui199207@126.com

    XIA Guangqiong(1970—), female, from Fushun, Sichuan, is a professor and doctoral tutor who is mainly engaged in laser nonlinear dynamics and its related applications. E-mail:gqxia@swu.edu.cn

    Corresponding author: XIA Guang-qiong, E-mail:gqxia@swu.edu.cn
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出版历程
  • 收稿日期:  2018-03-14
  • 修回日期:  2018-04-09
  • 刊出日期:  2019-02-01

Output characteristics of diode-pumped passively Q-switched Yb: CaYAlO4 pulsed laser based on a SESAM

doi: 10.3788/CO.20191201.0167
    基金项目:

    国家自然科学基金项目 61475127

    国家自然科学基金项目 61575163

    国家自然科学基金项目 61775184

    作者简介:

    唐睿(1992-), 女, 黑龙江齐齐哈尔人, 硕士研究生, 主要从事全固态脉冲激光器方面的研究。E-mail:

    夏光琼(1970—),女,四川富顺人,教授,博士生导师,主要从事激光非线性动力学及相关应用方面的研究。E-mail:

    通讯作者: XIA Guang-qiong, E-mail:gqxia@swu.edu.cn
  • 中图分类号: TN248.1

摘要: 采用发射波长约为976 nm的半导体激光器作为泵浦源,Yb3+掺杂浓度为1.5at.%、通光长度为2 mm的Yb:CaYAlO4晶体作为增益介质,本文提出了一种基于半导体可饱和吸收镜(SESAM)被动调Q的激光二极管泵浦Yb:CaYAlO4以获取稳定脉冲输出的方案。通过合理设计谐振腔,实现了稳定的被动调Q激光脉冲输出,并分析了泵浦功率的大小对输出脉冲的重复频率、脉冲宽度、单脉冲能量以及脉冲峰值功率的影响。

English Abstract

唐睿, 高子叶, 吴正茂, 夏光琼. 基于SESAM被动调Q的激光二极管泵浦Yb:CaYAlO4脉冲激光器[J]. 中国光学, 2019, 12(1): 167-178. doi: 10.3788/CO.20191201.0167
引用本文: 唐睿, 高子叶, 吴正茂, 夏光琼. 基于SESAM被动调Q的激光二极管泵浦Yb:CaYAlO4脉冲激光器[J]. 中国光学, 2019, 12(1): 167-178. doi: 10.3788/CO.20191201.0167
TANG Rui, GAO Zi-ye, WU Zheng-mao, XIA Guang-qiong. Output characteristics of diode-pumped passively Q-switched Yb: CaYAlO4 pulsed laser based on a SESAM[J]. Chinese Optics, 2019, 12(1): 167-178. doi: 10.3788/CO.20191201.0167
Citation: TANG Rui, GAO Zi-ye, WU Zheng-mao, XIA Guang-qiong. Output characteristics of diode-pumped passively Q-switched Yb: CaYAlO4 pulsed laser based on a SESAM[J]. Chinese Optics, 2019, 12(1): 167-178. doi: 10.3788/CO.20191201.0167
    • Laser-diode-pumped all-solid-state pulse lasers have the advantages of compact structures, high efficiency, low cost, high peak pulsed laser output, narrow pulse width, high beam quality, etc.. They have been widely used in nonlinear optics, spectroscopy, biomedical diagnosis, materials processing and other fields[1-4]. Q-switching technology is one of the most important ways to create pulsed lasers and can be divided into active Q-switching[5] and passive Q-switching[6]. Active Q-switching requires an external device to control the loss of the resonant cavity, while passive Q-switching does this using the saturable absorption effect of nonlinear optical materials. Compared with the active Q-switched laser, the passive Q-switched laser has the advantages of simple structure, low cost, the ability to self-start and no need for an external power supply. The nonlinear saturable absorbers currently used for passive Q-switching are semiconductor saturable absorption mirrors(SESAM)[7-9], graphene[10], carbon nanotubes[11], topological insulators[12], transition metal dichalcogenides[13-14] and other, similar materials.

      Existing research shows that Yb3+ ion doped laser materials have the advantages of having a simple energy level structure, high quantum efficiency, wide emission spectrum, being unaffected by excited state absorption, up-conversion, concentration quenching, etc…. These are each very suitable for generating high power, near-infrared short-pulse laser beams[28-29]. Meanwhile, since a doped Yb3+ laser medium has an absorption wavelength matching the emission wavelength of its high-intensity InGaAs laser diode, a compact laser-diode-pumped all-solid-state laser can be constructed. Doped Yb3+ laser materials can be classified into glass, crystal, and ceramic, depending on the used process of preparation. They can also be classified by molecular structure, as shown in Tab. 1. In recent years, as a new type of erbium-doped gain material, Yb:CaYAlO4 has been attracting the attention of academics and industrial professionals, its output characteristics in continuous lasers and mode-locked pulsed lasers has been studied with more frequency. In 2010, Dongzhen Li et al. used the Czochralski method to grow Yb: CaYAlO4 crystals then determined their lifetime of fluorescence to be 426 μs and the full width at half maximum of their fluorescence spectrum to be at 61 nm[30]. In 2011, they further studied the continuous laser output of Yb:CaYAlO4 crystals. When the incident pump′s power is set to 8 W, a continuous laser with a maximum output power of 2.48 W is achieved with a slope efficiency of 55.4%[31]. Also in 2011, Tan et al. of Nanyang Technological University achieved a pulsed laser with a pulse width of 156 fs, repetition rate of 91 MHz and a maximum average output power of 740 mW[32] using a SESAM mode-locked Yb:CaYAlO4 laser. In 2015, Gao et al. achieved Kerr lens mode-locking with Yb:CaYAlO4 and produced an ultrashort pulse laser of 33 fs with a repetition rate of 115 MHz[33]. In 2016, Ma et al. of Nanyang Technological University used graphene as a saturable absorber to achieve mode-locking pulse output with a Yb:CaYAlO4 laser achieving a pulsed width of 30 fs, which is the shortest pulse generated by mode-locked Yb:CaYAlO4 lasers so far[34]. It is worth noting that, while current research on Yb:CaYAlO4 pulsed lasers is related to mode-locked lasers with no report on passive Q-switched lasers, understanding the performance of Q-switched pulses from Yb:CaYAlO4 crystal lasers helps further understand the optical properties of Yb:CaYAlO4 crystals. Therefore, it is necessary to research and explore the performance of passive Q-switched Yb:CaYAlO4 crystal lasers.

      表 1  按分子结构分类的Yb3+掺杂激光材料

      Table 1.  Yb3+ doped laser materials classified by molecular structure

      Molecular structure Laser Material
      Yttrium aluminum garnet Yb:Y3Al5O12[15]
      Sesquioxide Yb:Sc2O3[16]、Yb:Lu2O3[17]
      Aluminate Yb:CaGdAlO4[18]、Yb:CaYAlO4[19]
      Silicate Yb:Gd2SiO5[20]、Yb:Y2SiO5[21]
      Vanadate Yb:YVO4[22]、Yb:LuVO4[23]
      Borate Yb:Sr3Y(BO3)3[24]、Yb:Ca4GdO(BO3)3[25]
      Tungstate Yb:KY(WO4)2[26]、Yb:KG(WO4)2[27]

      This paper proposes a scheme that uses SESAM passive Q-switched laser diodes pumped with Yb:CaYAlO4 to output a pulse, then studies output characteristics of the emitted pulse. The experiment shows that a stable Q-switched laser pulse output can be obtained using this scheme. The slope efficiency of the output pulse was 7.25%, and the optical-optical efficiency was 5.97%. When the pump power was set to 5.27 W, the pulse width reached a minimum of 1.17 μs. At this time, the repetition frequency of the pulse was 73.67 kHz, the single pulse energy was 3.88 μJ, and the peak power was 3.32 W. When the pump power was set to 6.51 W, the output single pulse energy reached a maximum value of 3.97 μJ.

    • The experimental setup is schematically shown in Fig. 1, wherein the figures (a) and (b) correspond to the continuous laser output and the SESAM Q-switched pulsed laser output, respectively. The gain material used in the experiment was a Yb:CaYAlO4 crystal having a doping concentration of 1.5at.%, cut along the a direction. The crystal size was 3 mm×3 mm×2 mm and the light transmission length was 2 mm. In the experiment, in order to dissipate heat, the Yb:CaYAlO4 crystal was wrapped with indium foil, sandwiched with copper and also mounted on a water-cooled copper stage that was held at a temperature of 17 ℃. Since the absorption peak of Yb:CaYAlO4 crystals at room temperature are 979 nm, the absorption cross sections of σ and π polarization directions are 1.71×10-20 cm2 and 5.07×10-20 cm2, respectively, which is very suitable for pumping with laser diodes[30]. In this experiment, we used a pump source that is a multimode fiber-coupled output laser diode(BWT DS3-41212 0313) with a wavelength of (976±0.5) nm. The multimode fiber has a core diameter of 105 μm, a numerical aperture of 0.22, and a rated output power of 27 W. The pump light is focused at the center of the Yb:CaYAlO4 crystal through a 1:1 coupling focusing system so the pump light in the crystal has a diameter of approximately 105 μm. For the case of continuous laser output(as shown in Fig. 1(a)), the resonant cavity is composed of M1, M2 and OC. Among these, M1 is a planar dichroic mirror coated with an antireflective coating with high transmittance at (976±10) nm and a high reflective film with high reflectivity at a wavelength of 1 000-1 100 nm; M2 is a concave mirror with a curvature radius of 200 mm with a high-inversion dielectric film plated in the wavelength range of 940-1 100 nm; OC is a coupled output mirror with 0.5% transmittance in the 930-1 140 nm wavelength range. For the case of Q-switched pulse output, the resonant cavity is composed of M1, M2, M3 and SESAM. M3 is a concave mirror with a curvature radius of 300 mm. It is coated with a high-reverse film in the wavelength range of 900-1 100 nm. The SESAM has a saturated absorption rate of 0.7%, a saturated energy density of about 120 μJ/cm2, and a relaxation time of 1 ps. The wavelength range is 1 020-1 110 nm(center wavelength is 1 064 nm). In the experiment, a power meter(Thorlabs-PM100D, Thorlabs-CAL) was used to measure laser power, and a spectrum analyzer(Ando AQ6317C) was used to measure the laser spectrum. The optical signal was converted into an electrical signal by a photodetector and input into an oscilloscope(Agilent Technologies DSO9254A). The output laser pulse was then tested.

      图  1  Yb:CaYAlO4激光器实验装置图,其中(a)和(b)分别用于实现连续激光输出和被动调Q脉冲激光输出

      Figure 1.  Schematic setup of the Yb:CaYAlO4 laser experiment, where (a) and (b) are diagrams of the continuous-wave setup and passive Q-switched laser setup, respectively

    • First, using the optical path of Fig. 1(a), continuous laser output can be achieved by finely adjusting the position of each component in the cavity. The curve of the output power varying with the pump power curve and the laser spectrum when the pump power is 4.86 W are shown in Fig. 2. As can be seen from Fig. 2(a), the threshold pump power is 1.01 W, the slope efficiency is 10.23% and the optical-optical efficiency is 8.82%. In terms of slope efficiency, our results are quite different from the 55.4% reported in Ref.[31], which may be due to the low transmissivity of the output mirror(0.5%) used in our experiments. As can be seen from Fig. 2(b), the center wavelength of the output continuous laser is approximately 1 052 nm.

      图  2  连续激光工作下,激光器平均输出功率随泵浦功率变化关系(a)以及泵浦功率为4.86 W时的激光光谱(b)

      Figure 2.  Output power as a function of incident pump power(a) and the optical spectrum under a 4.86 W pump(b) for the laser operating at a continuous-wave

      By adding M3 and a SESAM, the experimental system for passive Q-switched pulse output can be constructed(as shown in Fig. 1(b)). In the experiment, when the pump power is 1.51W, there is a continuous laser output; when the pump power is increased to 1.59 W, the Q-switching pulse starts to appear but the output pulse is unstable at this time. When the pump power is increased to 2.40 W, there is a stable Q-switched laser. This relationship between stable Q-switched laser output power and the pump power is shown in Fig. 3. When the pump power is 7.76 W, the output power is 464 mW, the Q-switched output slope efficiency is 7.25%, and the optical-optical efficiency is 5.97%. By comparing Fig. 2(a) with Fig. 3, it can be seen that the Q-switched pulse′s output power is slightly smaller than the continuous laser′s output power because there is absorption loss in the SESAM and because the added cavity mirror increases the cavity loss. In addition, from the output power fitting curves of the continuous laser(Fig. 2(a)) and Q-switched laser(Fig. 3), it can be seen that the output power increases linearly with the increase of pump power and that there is no saturation effect. This is because no larger pump power is used in the experiment to prevent damage to the crystal.

      图  3  调Q脉冲平均输出功率随泵浦功率变化关系

      Figure 3.  Output power as a function of incident pump power for the laser operating at a Q-switched state

      The above experimental results show that the laser can achieve a stable Q-switched pulse output when the pump power is in the range of 2.40-7.76 W. In the following, we discuss the characteristics of the output pulse. Fig. 4(a) is a pulse train diagram at different time periods when the pump power is 4.04 W. At these times, the output pulse repetition frequency is 54.14 kHz and the pulse width is 1.31 μs. Fig. 4(b) is the same when the pump power is 6.10 W, its output pulse repetition frequency is 90.10 kHz and the pulse width is 1.33 μs. Finally, Fig. 4(c) corresponds to the case where the pump power is 7.76 W, the output pulse repetition frequency is 123.40 kHz and the pulse width is 1.83 μs. The first, second and the third columns respectively correspond to time windows of 1 000 μs, 100 μs, and 10 μs. It can be seen from the figure that the output pulse width is relatively uniform, the Q-switching effect is stable, and there is no pulse splitting.

      图  4  泵浦功率4.04 W(a), 6.10 W(b)以及7.76 W(c)时,不同时间窗口下的脉冲序列

      Figure 4.  Pulse trains at different time windows for pump powers of 4.04W (a), 6.10W(b), and 7.76W(c)

      The pulse output with the shortest pulse width in the experiment was obtained at a pump power of 5.27 W, whose pulse trains are shown in Fig. 5(a) and (b), respectively. It can be seen from these pulse trains that the output pulses are uniform and stable. Fig. 5(c) and (d) are the single pulse profile and the Q-switched laser spectrum, respectively. At this time, the Q-switched laser pulse width was 1.17 μs, the repetition frequency was 73.67 kHz, and the center wavelength was around 1 048.6 nm. In the experiment, the Q-switched laser pulse width is in the order of microseconds, which we speculate is due to the low transmittance of the output mirror[35].

      图  5  泵浦功率为5.27 W时,不同时间窗口下的调Q脉冲序列(a~c)以及激光光谱(d)

      Figure 5.  Q-switched pulse trains(a-c) and laser spectrums(d) at different time windows when the pump power is 5.27 W

      Finally, the output characteristics of the passive Q-switched Yb:CaYAlO4 laser are analyzed with regard to pulse width, repetition frequency, single pulse energy and peak power. Fig. 6 shows the repetition frequency and pulse width of the output pulsed laser as a function of pump power. As the pump power increases from 2.40 W to 7.76 W, the pulse repetition frequency continuously increases from 41.3 kHz to 123.4 kHz, while the pulse width decreases from 1.78 μs to 1.17 μs and then increases to 1.83 μs. In general, with a gradual increase of pump power, the pulse repetition frequency of the Q-switched laser gradually increases, while the pulse width tends to decrease rapidly and then gradually stabilize. It can be seen from the figure that the repetition frequency of the Yb:CaYAlO4 Q-switched pulse varies with the pump power in accordance with the characteristics of the Q-switched laser pulse described above. However, our experimental results show that when the pump power is relatively large, increasing the pump power will also increase the pulse width. The reason may be that when the pump power is large, the pulse repetition frequency is also large(greater than 70 kHz). Meanwhile, as the pump power increases, the pulse repetition frequency increases, the energy storage decreases in each period, the single-pass gain decreases, and the pulse needs to travel back and forth more within the cavity from its generation to formation, thereby causing the pulse to widen[36].

      图  6  重复频率和脉冲宽度随泵浦功率的变化关系

      Figure 6.  Relationship between repetition frequency and pulse width as a function of pump power

      Based on the experimentally measured average output power, pulse width, and repetition frequency, the single pulse energy and pulse peak power of the Q-switched laser pulse can be calculated. Fig. 7 shows the relationship between single pulse energy and pulse peak power as a function of pump power. It can be seen from the diagram that the single pulse energy increases first and then tends to stabilize with an increase in pump power. When the pump power is 6.51 W, the single pulse energy reaches 3.97 μJ; the pulse's peak power increases with the pump power and then decreases. When the pump power is 5.27 W, the highest value is reached with a peak power is 3.32 W.

      图  7  单脉冲能量和脉冲峰值功率随泵浦功率变化关系

      Figure 7.  Single pulse energy and pulse peak power as a function of pump power

    • A multimode fiber-coupled laser diode was used as a pump source, a Yb:CaYAlO4 crystal as a gain medium, and a SESAM as the passive Q-switched component to construct a passive Q-switched laser system. A stable Q-switched pulse output was achieved in the range of 2.40-7.76 W and the influence of pump power on the output of the Q-switched pulse was analyzed. The results show that output with the shortest pulse width achievable with this system(1.17 μs) can be obtained when the pump power is 5.27 W and the maximum single pulse energy(3.97 μJ) is obtained when the pump power is 6.51 W. It should be noted that the findings of this paper are only a preliminary attempt at creating a Yb:CaYAlO4 passive Q-switched laser. Therefore, the relevant parameters (such as output power, pulse width, efficiency, etc.) are awaiting refinement in future research. It is hoped that research in this paper will help promote the application of Yb:CaYAlO4 crystals in all-solid-state pulsed lasers.

      ——中文对照版——

    • 激光二极管泵浦的全固态脉冲激光器由于具有结构紧凑、效率高、成本低以及输出脉冲激光的峰值功率高、脉冲宽度窄、光束质量好等优点, 在非线性光学、光谱学、生物医学诊断、材料加工等领域得到广泛应用[1-4]。调Q技术是目前获取脉冲激光的重要方式之一, 按照调节方式可分为主动调Q[5]和被动调Q[6]。主动调Q需要利用外部器件来控制谐振腔的损耗, 而被动调Q则利用非线性材料的可饱和吸收效应控制腔内损耗。相比于主动调Q激光器, 被动调Q激光器具有结构简单、成本较低、可自启动、无需外加电源等优点。目前常被用于被动调Q的非线性可饱和吸收体有:半导体可饱和吸收镜(SESAM)[7-9]、石墨烯[10]、碳纳米管[11]、拓扑绝缘体[12]、过渡金属二硫族化合物[13-14]等等。

      已有的研究结果表明, Yb3+离子掺杂激光材料具有能级结构简单、量子效率高、发射谱宽以及不受激发态吸收、上转换、浓度淬灭影响等优点, 非常适合用于产生高功率、近红外的短脉冲激光[28-29]。同时, 由于掺杂Yb3+激光介质具有与高亮度InGaAs激光二极管的发射波长相匹配的吸收波长, 因此可构建结构紧凑的激光二极管泵浦全固态激光器。掺杂Yb3+激光材料按照制备工艺可以分为玻璃、晶体、陶瓷, 而按分子结构进行分类则如表 1所示。近年来, Yb:CaYAlO4作为一种新型的掺镱增益材料, 受到了相关领域学者和业界的广泛关注, 并对其连续激光和锁模脉冲激光的输出特性进行了相关研究。2010年, 李东振等人用Czochralski法生长出了Yb:CaYAlO4晶体, 并测定其荧光寿命为426 μs, 荧光光谱半高全宽为61 nm[30]; 2011年, 他们进一步研究了Yb:CaYAlO4晶体的连续激光输出性能, 在入射泵浦功率为8 W时, 获得了最大输出功率为2.48 W的连续激光, 斜率效率为55.4%[31]。2011年, 南洋理工大学Tan等人基于SESAM锁模Yb:CaYAlO4激光器获得了脉冲宽度为156 fs的脉冲激光, 重复频率为91 MHz, 最大平均输出功率为740 mW[32]。2015年, 高子叶等人首次在Yb:CaYAlO4中实现了克尔透镜锁模, 并且获取了33 fs的超短脉冲激光, 重复频率为115 MHz[33]。2016年, 南洋理工大学Ma等人使用石墨烯作为可饱和吸收体, 实现了Yb:CaYAlO4激光器锁模脉冲输出, 脉冲宽度为30 fs, 这是迄今为止锁模Yb:CaYAlO4激光器产生的最短脉冲[34]。值得注意的是, 一方面, 目前有关Yb:CaYAlO4脉冲激光器的研究均是关于锁模激光方面的, 未见被动调Q脉冲激光的相关报道; 另一方面, 了解Yb:CaYAlO4晶体的调Q脉冲激光性能有助于人们进一步了解Yb:CaYAlO4晶体的光学性能。因此, 有必要针对Yb:CaYAlO4晶体的被动调Q激光性能开展相关研究探索。

      本文提出了基于SESAM被动调Q的激光二极管泵浦Yb:CaYAlO4实现脉冲输出的方案, 并对该方案所获得的调Q脉冲的输出特性进行了研究。实验结果表明, 采用该方案可以获取稳定的调Q激光脉冲输出, 输出脉冲的斜率效率为7.25%, 光光效率为5.97%。当泵浦功率为5.27 W时, 脉冲宽度达到最小值1.17 μs。此时脉冲的重复频率为73.67 kHz, 单脉冲能量为3.88 μJ, 峰值功率为3.32 W; 而当泵浦功率为6.51 W时, 输出的单脉冲能量达到最大值3.97 μJ。

    • 实验装置示意如图 1所示, 其中图(a)和(b)分别对应连续激光和SESAM调Q获取脉冲激光输出的情形。实验中使用的增益材料是Yb:CaYAlO4晶体, 其掺杂浓度为1.5at.%, 沿着a向切割, 晶体尺寸为3 mm×3 mm×2 mm, 通光长度为2 mm。在实验中, 为了散热, 将Yb:CaYAlO4晶体用铟箔包裹、紫铜夹持后安装在温度保持为17 ℃的水冷铜台上。由于在室温下的Yb:CaYAlO4晶体吸收峰位于979 nm处, σ和π偏振方向的吸收截面分别为1.71×10-20 cm2和5.07×10-20 cm2, 从而非常适合采用激光二极管进行泵浦[30]。在该实验中, 采用的泵浦源为发射波长是(976±0.5) nm的多模光纤耦合输出激光二极管(BWT DS3-41212 0313), 其中多模光纤芯径为105 μm、数值孔径为0.22、额定输出功率为27 W。泵浦光经过1:1的耦合聚焦系统聚焦在Yb:CaYAlO4晶体中心, 因此晶体中泵浦光的直径约为105 μm。对于连续激光输出的情形(如图 1(a)所示), 谐振腔是由M1、M2、OC构成。其中, M1是平面双色镜, 镀有在(976±10) nm处具有高透射率的增透膜以及在1 000~1 100 nm波长处具有高反射率的高反膜; M2为凹面镜, 曲率半径为200 mm, 在940~1 100 nm波长范围内镀有高反介质膜; OC为耦合输出镜, 其在930~1 140 nm波长范围内具有0.5%透射率。而对于调Q脉冲输出的情形, 谐振腔则由M1、M2、M3以及SESAM构成。M3为凹面镜, 曲率半径为300 mm, 在900~1 100 nm波长范围内镀有高反介质膜; SESAM的饱和吸收率为0.7%, 饱和能流密度约为120 μJ/cm2, 弛豫时间为1 ps, 工作波长为1 020~1 110 nm(中心波长为1 064 nm)。实验中采用功率计(Thorlabs-PM100D、Thorlabs-CAL)来测量激光功率, 光谱分析仪(Ando AQ6317C)测量激光光谱, 光信号通过光电探测器转换成电信号后输入到示波器(Agilent Technologies DSO9254A)后对输出的激光脉冲序列进行测试。

    • 首先, 采用图 1(a)的光路, 通过精细调节腔内各元件位置可实现连续激光输出, 其输出功率随泵浦功率变化曲线、以及泵浦功率为4.86 W时的激光光谱如图 2所示。从图 2(a)可以看出, 阈值泵浦功率为1.01 W, 斜率效率为10.23%, 光光效率为8.82%。就斜率效率而言, 本文结果与文献[31]中报道的55.4%差距较大, 这可能是因为实验中采用的输出镜透射率比较低(0.5%)所致。从图 2(b)可知, 输出连续激光的中心波长位于1 052 nm附近。

      通过增加M3和SESAM, 可构成被动调Q获取脉冲输出的实验系统(如图 1(b)所示)。实验中, 当泵浦功率为1.51 W时, 有连续激光输出; 当泵浦功率增大到1.59 W时, 开始出现调Q脉冲, 但此时输出脉冲不稳定; 而当泵浦功率增加到2.40 W时, 激光器实现了稳定调Q运转。稳定调Q激光输出功率随泵浦功率变化关系如图 3所示, 在泵浦功率为7.76 W时输出功率为464 mW, 调Q脉冲输出斜率效率为7.25%, 光光效率为5.97%。通过比较图 2(a)图(3)可知:调Q脉冲输出功率比连续激光输出功率稍小, 这是因为SESAM中存在吸收损耗, 同时腔镜的增加也会增加腔的损耗。另外, 从连续激光(图 2(a))和调Q激光(图 3)输出功率的拟合曲线还可以看出:输出功率随泵浦功率增加近似线性增大, 还未出现饱和效应, 这是因为实验中为避免损坏晶体没有采用更大的泵浦功率。

      上面的实验结果表明, 当泵浦功率在2.40~7.76 W范围内, 激光器可实现稳定的调Q脉冲输出。接下来, 将讨论输出脉冲的特性。图 4(a)为泵浦功率为4.04 W时, 不同时间窗口下的脉冲序列图, 此时输出脉冲重复频率为54.14 kHz, 脉冲宽度为1.31 μs; 图 4(b)为泵浦功率为6.10 W时的情形, 此时输出脉冲重复频率为90.10 kHz, 脉冲宽度为1.33 μs; 图 4(c)对应泵浦功率为7.76 W时的情形, 此时输出脉冲重复频率为123.40 kHz, 脉冲宽度为1.83 μs。其中, 第一列、第二列以及第三列分别对应时间窗口为1 000 μs、100 μs以及10 μs。从图中可以看出输出脉冲宽度比较均匀, 调Q效果稳定, 没有出现脉冲分裂现象。

      实验中具有最短脉冲宽度的脉冲输出是在泵浦功率为5.27 W得到的, 其在时间窗口为1 000 μs和100 μs的脉冲序列分别如图 5(a)5(b)所示。从脉冲序列中可以看出, 输出脉冲序列均匀、稳定。图 5(c)5(d)分别是单脉冲轮廓图以及调Q激光的光谱。此时, 调Q激光脉冲宽度为1.17μs, 重复频率为73.67 kHz, 中心波长在1 048.6 nm附近。实验中获得的调Q激光脉冲宽度为微秒量级, 推测是由于输出镜透射率过低导致的[35]

      最后, 从脉冲宽度、重复频率、单脉冲能量以及峰值功率几个方面来分析被动调Q Yb:CaYAlO4激光器的输出特性。图 6给出了输出脉冲激光的重复频率和脉冲宽度随泵浦功率变化的曲线。随着泵浦功率从2.40 W增加到7.76 W, 脉冲重复频率从41.3 kHz持续增加到123.4 kHz, 而脉冲宽度从1.78μs先降低到1.17 μs然后再增加到1.83 μs。一般而言, 随着泵浦功率的逐渐增加, 调Q激光器脉冲重复频率逐渐提高, 而脉冲宽度呈现先快速降低然后逐渐趋于平稳的趋势。从该图可以看出, Yb:CaYAlO4调Q脉冲的重复频率随泵浦功率的变化趋势符合上述调Q激光脉冲的特征。但本文实验结果却显示, 当泵浦功率比较大的时候, 增加泵浦功率会导致脉冲宽度增加, 其原因可能是:在泵浦功率较大时, 导致脉冲重复频率较大(大于70 kHz), 此时随着泵浦功率的增加, 脉冲重复频率增加, 每周期内储能减少, 单程增益降低, 脉冲从产生到形成需要在腔内往返更多的次数, 从而导致脉冲变宽[36]

      根据实验测得的平均输出功率、脉冲宽度和重复频率, 可以计算出调Q激光脉冲的单脉冲能量和脉冲峰值功率。图 7给出了单脉冲能量和脉冲峰值功率随泵浦功率变化的关系曲线。从图中可以看出, 单脉冲能量随泵浦功率升高呈现先增加后趋于平稳的趋势, 当泵浦功率为6.51 W时, 单脉冲能量达到3.97 μJ; 脉冲峰值功率随泵浦功率升高呈现先增加后减小的趋势, 在泵浦功率为5.27 W时达到了最大值, 最大峰值功率为3.32 W。

    • 采用多模光纤耦合激光二极管作为泵浦源, Yb:CaYAlO4晶体为增益介质, SESAM作为被动调Q元件, 构建了一个被动调Q脉冲激光器系统。在泵浦功率为2.40~7.76 W时实现了稳定的调Q脉冲输出, 分析了泵浦功率对所产生的调Q脉冲输出特性的影响。结果表明:泵浦功率为5.27 W时, 可获得具有最短脉冲宽度(1.17 μs)的脉冲输出; 而泵浦功率为6.51 W时, 输出的脉冲具有最大单脉冲能量(3.97 μJ)。需要说明的是本文工作只是有关Yb:CaYAlO4被动调Q激光器的初步尝试, 因此相关参数指标(如输出功率、脉冲宽度、效率等)均有待在下一步工作中改进。本文的研究将有助于推动Yb:CaYAlO4晶体在全固态脉冲激光器中的应用。

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