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全固态激光器由于有发射截面大、导热率高的特点，具备体积小、寿命长、效率高、脉冲能量高、光束质量好等优点[1]，尤其是二极管泵浦的双波长固态激光器可以应用在环境监测[2]、测速激光雷达[3]、医疗仪器[4]、全息术[5]及太赫兹波产生[6]等领域。这些应用除了要求双波长外，还需要激光源具有其他特性，如：在太赫兹波的产生、测速雷达及全息术等应用中，两波长应具备间隔足够小[7]、两光束功率近乎相等[8]、两光束传输光路须重合等特性；在环境监测、医疗仪器设备等应用中，波长应可调谐，且输出脉冲的相对延迟应可控，以使其保持同步或连续输出[9]。
目前常见的几种获得双波长激光的方式为：1）利用同一晶体中的两个独立荧光光谱[10]；2）利用两种不同增益介质通过扩散键合的晶体[11]；3）利用两个同轴放置的OPO晶体进行非线性变频[12]。然而，这些产生两波长的方法存在波长不可调节、或两波长输出功率相差大、或两波长不共轴等不足，不能满足上述所有要求。光谱合束是实现不同波长激光束在空间重叠的一种有效方法[13]，每个激光器可独立工作，有利于输出脉冲的波长调谐、功率调节和延迟调整；组合光束的光束质量等于单个激光的光束质量，这样就可以满足上述所有要求。
目前光谱合束技术已经用于半导体激光器[14]、光纤激光器[15]和中红外量子级联激光器[16]。通过光谱合束的方法已经获得了功率千瓦级、光束质量束宽积为3.5 mm·mrad，并且可以输出十几个中心波长的二极管激光[17]。通过对光纤激光器进行光谱合束，获得了功率为几十千瓦、光束质量接近衍射极限的激光输出[18]。
Nd:YAG固体激光器由于其高增益和良好的热力学性能，是目前应用中重要的固体激光器[19]。Nd:YAG晶体的能级4F3/2-4F9/2、4F11/2和4F13/2中存在几十种跃迁，可以获得1.052~1.444 μm波段内的各种波长激光[20]。到目前为止，还未见对Nd:YAG固态激光器进行光谱合束的报道。
文中报道了用两个Nd:YAG脉冲固体激光器进行光谱合束的结构，两激光器可独立工作，通过光栅形成一个共同腔实现合束，光栅配合具有一定反射率的输出镜，同时控制两个Nd:YAG激光器以不同的波长工作，并使来自两个激光器的光束在空间重合，得到波长为1061 nm和1064 nm的双波长合束激光，脉冲能量为173 mJ，组束效率约为93%，光束质量因子M2为2.8×2.2，与单个Nd:YAG激光束的光束质量因子M2基本相同。满足波长间隔小、传输光路重合、波长可调谐等要求，可以应用于太赫兹波产生等领域。
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光谱合束技术来源于通信中所用波分复用原理。光谱合束的原理系统如图1所示，通过传输透镜将各个不同位置的光束以不同的入射角在光栅上重叠在一起，利用透射光栅的色散特性将各个不同入射角、不同波长的光束沿同一方向衍射，每个激光器的波长均与光栅色散和共同腔反馈匹配，通过输出镜的共同腔反馈确保将各个激光器锁定在不同波长，由此获得组束后的光束输出。组合光束的频谱宽度∆λ为：
$$ \Delta \lambda=\lambda_{3}-\lambda_{1}=\frac{d}{L_{1}} \times \omega \times \cos \beta $$ (1) 
图 1 光谱合束示意结构图
Figure 1. Schematic structure of the spectral beam combination of lasers
式中：d为光束间的中心距离；L1为变换透镜和光栅之间的长度（变换透镜的焦距）；ω为光栅的刻线宽度；β为光束在光栅上的入射角。
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图2为两个固体Nd:YAG激光器光谱合束的结构示意图。两激光器的增益介质相同，均以一个4 mm×4 mm×70 mm的1 at.%掺杂的Nd:YAG为芯层，四周键合3 at.%的Sm:YAG用于抑制寄生振荡及自发辐射放大（ASE），在两个Sm:YAG的外侧键合未掺杂的YAG，顶部和底部镀3 μm厚的SiO2膜层用来减少空气和晶体界面的反射损耗。增益介质用铟箔包裹并安装在金属热沉上。泵浦源采用两个808 nm二极管堆栈，泵浦源（LD1和LD2）放置于靠近晶体前端切角的位置，利用微透镜F1和F2对bar条快轴方向的光束进行准直，准直后整体光斑的尺寸约为宽10 mm、高3.8 mm。泵浦光直接入射至增益介质中并在两个侧面不断全反射，从而多次经过芯层以确保泵浦光几乎完全被吸收[21]，泵浦脉宽230 μs，重频5 Hz。当两激光器独立工作时，如图2所示，Laser1腔长约为720 mm，Laser2腔长约为740 mm；全反镜M2和M2' 镀1064 nm高反膜，输出镜M1和M1' 镀透过率为50%的1064 nm半反半透膜；腔内加入偏振片以实现两激光器偏振输出，满足组合光栅高效率的要求。实验中所用的光栅刻线密度为1600 lines/mm，中心波长为1060 nm，Littrow角为58°，尺寸为31.8 mm×12.3 mm，衍射效率约为94%，s光偏振，材质为石英，对s偏振光的衍射效率较高。
图 2 两个Nd:YAG激光器光谱合束结构图
Figure 2. Schematic structure of the spectral beam combination of two solid state Nd:YAG lasers
在合束过程中，为了使两激光器输出光束在透射光栅上重叠，Laser2输出的光束经过HR1和HR2两个全反镜后被反射到光栅上，并与Laser1输出光束相同的位置，此时两个激光器的光束由于波长相同，入射角不同，经过光栅后衍射角度不同，因此传输方向不同。在光栅后添加输出耦合镜OC，OC镀透过率为50%的1064 nm增透膜，移除Laser1的平面输出镜M1，调整OC，使Laser1从OC镜输出波长锁定在1064 nm，然后移除Laser2的M1'，调整HR1、HR2反射镜，将Laser2从OC镜输出波长调谐锁定到1061 nm。由于光栅的色散，波长不同、入射角不同的两个光束在外腔的作用下沿相同方向传播，获得双波长共轴输出。
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实验中选择1061 nm和1064 nm两波长，原因是在Nd:YAG中4F3/2-4F11/2的跃迁具有最低的阈值。HR2与光栅之间的长度L1为600 mm，Littrow角为58°，将频谱线宽宽度∆λ=3 nm代入公式（1），可以得到两光束的中心距离d为5.45 mm，然而在实验中可以布置的最短距离约为6 mm。根据实验参数计算可得激光的光谱宽度约为3.31 nm。图3为组束光束的测量光谱，1061 nm和1064 nm双波长激光输出中心波长分别为1061.5 nm和1064.6 nm，间隔3.1 nm，二者光谱线宽分别为0.5 nm和0.6 nm，与计算结果基本一致。由于腔内光束具有一定发散角，合束后每个激光器的线宽从0.1 nm增大到约0.5 nm，光束的光谱在脉冲持续时间内显示为定值。
图 3 合束后激光输出光束光谱图
Figure 3. Spectrum of the output beam of the spectrally beam combined solid-state Nd:YAG lasers
图4为两个激光器光束组束前后单脉冲能量与泵浦电流的关系。在泵浦功率430 mJ （电流200 A）时，Laser1和Laser2合束前的脉冲能量分别为94 mJ和92 mJ，光-光效率分别为22%和21.5%，合束后的脉冲能量为173 mJ，组束效率约为93%。损耗主要来源于光栅的衍射损耗，其在1064 nm处的衍射效率约为95%，经过输出耦合镜后的组合光束是线性s偏振光。由于透射光栅基质由熔融石英和高强度电介质材料制成，光栅损伤阈值较高，因此这些脉冲在较长时间内具有恒定的功率。
图 4 激光输出能量与泵浦电流关系
Figure 4. Measured output energy of Nd:YAG laser with respect to the current of the pump LD
使用焦距150 mm的透镜对组合输出光束聚焦，采用刀口法测量组合输出的激光光束质量，如图5所示。在水平方向（组束方向）上，合束前Laser1和Laser2的光束质量因子M2分别为2.7和2.1，合束后水平方向上的光束质量因子M2为2.8，合束后光束质量略微变差是由于HR1和HR2存在指向误差，导致两光束在透射光栅处没有完全重叠。在垂直方向（非合束方向）上，合束前Laser1和Laser2的光束质量因子M2分别为2.2和1.9，合束后在垂直方向上的M2为2.2。Laser2的光束质量比Laser1的好是因为其腔长更长。由此可以看出，合束后的激光光束质量在两个方向上均接近于单个固体Nd:YAG激光器的光束质量。
图 5 合束前后光束质量拟合曲线。（a）水平方向；（b）垂直方向
Figure 5. Beam quality measurement of the lasers before and after SBC using quadratic curve fitting. (a) Horizontal direction; (b) Vertical direction
两个激光器的泵浦电源串联在同一个电路中，使得两激光器同步驱动产生激光脉冲，在150 A电流下单个激光器的脉冲宽度约为90 μs，使用光电探头测量合束激光的脉冲轮廓，如图6所示，合束后的脉冲宽度略宽，约为118 μs。脉冲轮廓曲线上有一个陡峭的下降，说明两个脉冲光束在时域上并没有完全重叠。后续将改进时间控制系统，从而保证组合光束的脉冲宽度与单个Nd:YAG激光器的脉冲宽度尽可能相同。
图 6 合束后激光器瞬时脉冲形状
Figure 6. Temporal pulse shape of the combined laser
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文中提出了采用两个固态Nd:YAG激光器通过光谱合束的方式来获得双波长激光光源，所得组合光束的光束质量接近单个激光器的光束质量，合束后的脉冲能量为173 mJ，组束效率为93%。组合输出光束具有波长间隔小、两波长输出功率相近、两波长共轴的特点。光谱合束也可以应用在主动调Q Nd:YAG激光器上，从而获得纳秒级脉宽以及更高的峰值功率。此外，由于Nd3+:YAG附近存在诸多跃迁，采用该方法可以获得四种甚至更多不同中心波长的激光器（如>1052 nm、1061 nm、1064 nm和1072 nm）。
Dual wavelength output Nd: YAG solid-state laser based on spectral beam combining
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摘要: 报道了一种基于光谱合束的Nd:YAG固体激光器双波长光源。系统由两个固体Nd:YAG脉冲激光器通过光谱合束组合而成，两个固体Nd:YAG脉冲激光器可独立工作，有利于输出脉冲的波长调谐、功率调节和相对延迟调整。通过光栅的色散特性以及输出镜的共同外腔反馈将各个激光器锁定在不同波长， 从而实现合束，获得的激光源中心波长锁定在1061.5 nm和1064.6 nm，两谱线中心间距为3.1 nm，组合光束的输出能量为173 mJ，组合光束的光束质量因子M2为2.8 × 2.2；两个Nd:YAG激光器独立工作的输出能量分别为94 mJ和92 mJ，在合束方向上的光束质量因子M2分别为2.7和2.1，在非合束方向上的光束质量因子M2分别为2.2和1.9；组合光束的输出能量为两个Nd:YAG激光器能量总和的93%，组合光束的光束质量因子与单个Nd:YAG激光束的光束质量因子M2基本相同。该双波长激光源满足波长间隔小、输出功率大小相近、同光轴等要求，在太赫兹波产生、测速激光雷达以及医疗仪器等应用领域具有重要作用。Abstract:
Objective Solid-state lasers pumped by diode lasers have the advantages of small size, long lifetime, high efficiency, high energy and good beam quality because of the high emission cross section and the matching of the emission and the absorption spectral line. In particular, diode-pumped dual-wavelength solid-state lasers are attractive for environmental monitoring, laser radar for velocity measurement, medical instruments, holography, and terahertz generation. In addition to dual wavelengths, the following laser characteristics are required for these applications. The wavelength spacing between the two lasers must be small enough to generate terahertz waves; The two beams must have nearly equal power; The relative timing of the output pulses must be controllable, so that the output pulses can be in synchrony or in succession ; The two beams must be coincident and the wavelengths must be tunable. For this purpose, a dual wavelength excimer based on spectral beam combining (SBC) is designed in this paper. Methods SBC comes from the principle of wavelength division multiplexing semiconductor laser transmitters used in communications (Fig.1). Beam combining was achieved using a common external cavity containing a grating, which simultaneously forces each Nd:YAG laser to operate at a different controlled wavelength and forces the beams from the two lasers to coincide. The lasers are arranged in a line at the focal plane of a transform lens. The collimated beams overlap at the grating and the output coupler. The common external cavity forces the beams to copropagate, and each laser has a different wavelength. Results and Discussions The spectrum of the combined beam shows the wavelength spread is 3.1 nm (Fig.3). At a maximum current of 200 A, the pulse energy of Laser 1 is 94 mJ and Laser 2 is 92 mJ before beam combining. The pulse energy of the combined beam is as high as 173 mJ, which corresponds to a combining efficiency of 93% (Fig.4). In the horizontal direction (beam combining direction), the M2 is 2.7 and 2.1 for Laser 1 and Laser 2 before beam combining. The M2 of the combined beam in the horizontal direction is 2.8. In the vertical direction (no beam combining direction), the M2 is 2.2 and 1.9 for Laser 1 and Laser 2 before beam combining respectively. The M2 of the combined beam in the vertical direction is 2.2 (Fig.5). Conclusions In summary, we have reported a dual-wavelength laser source by SBC of two solid-state Nd:YAG lasers. The resultant output beam quality is similar to that from a single laser. For the combined beam, a pulse energy of 173 mJ, and a combining efficiency of 93% are obtained. We believe that SBC could also be applied to an actively Q-switched Nd:YAG laser to obtain ns pulse durations and higher peak powers. Furthermore, because of the existence of additional transitions of Nd3+: YAG, it is expected that four or more lasers with different wavelengths (e.g., 1 052 nm, 1 061 nm, 1 064 nm, and 1 072 nm) can be combined in the future. -
Key words:
- dual-wavelength /
- Nd:YAG lasers /
- spectral beam combining /
- output energy /
- beam quality
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图 1 光谱合束示意结构图
Figure 1. Schematic structure of the spectral beam combination of lasers
图 2 两个Nd:YAG激光器光谱合束结构图
Figure 2. Schematic structure of the spectral beam combination of two solid state Nd:YAG lasers
图 3 合束后激光输出光束光谱图
Figure 3. Spectrum of the output beam of the spectrally beam combined solid-state Nd:YAG lasers
图 4 激光输出能量与泵浦电流关系
Figure 4. Measured output energy of Nd:YAG laser with respect to the current of the pump LD
图 5 合束前后光束质量拟合曲线。（a）水平方向；（b）垂直方向
Figure 5. Beam quality measurement of the lasers before and after SBC using quadratic curve fitting. (a) Horizontal direction; (b) Vertical direction
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