Volume 52 Issue 3
Mar.  2023
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Jin Zhuang, Li Jing, Jiang Menghua, Liu Youqiang, Qin Wenbin, Cao Yinhua, Wang Zhiyong. Influence of output mirror free external cavity spectral beam combining structure on feedback efficiency[J]. Infrared and Laser Engineering, 2023, 52(3): 20220446. doi: 10.3788/IRLA20220446
Citation: Jin Zhuang, Li Jing, Jiang Menghua, Liu Youqiang, Qin Wenbin, Cao Yinhua, Wang Zhiyong. Influence of output mirror free external cavity spectral beam combining structure on feedback efficiency[J]. Infrared and Laser Engineering, 2023, 52(3): 20220446. doi: 10.3788/IRLA20220446

Influence of output mirror free external cavity spectral beam combining structure on feedback efficiency

doi: 10.3788/IRLA20220446
Funds:  Beijing Municipal Natural Science Foundation (4174085)
  • Received Date: 2022-06-28
  • Rev Recd Date: 2022-09-07
  • Publish Date: 2023-03-25
  •   Objective   The external cavity spectral beam combining technology has two structures of open loop (without output coupler) and closed loop (with output coupler). The main difference between them is that the wavelength locked feedback beam of each light-emitting unit is different. Among them, the output coupler free external cavity spectral beam combining structure uses the beam returning from re-diffraction along the 0th-order and 1st-order diffraction direction as the feedback light, which avoids the waste of the beam and overcomes many problems of the feedback locking of the −1st order diffraction beam, and realizes the high-efficiency wavelength beam combination. For spectral beam combining technology, the feedback efficiency of the external cavity determines the stability of wavelength locking and even the success or failure of beam combination. Compared with the general closed-loop structure, the output coupler free external cavity spectral beam combining structure can only obtain enough feedback beams to achieve wavelength locking by controlling the parameters of the external cavity, and this external cavity structure is relatively complex, and the feedback efficiency variation caused by some factors in the external cavity are more obvious than the closed-loop structure. Therefore, for this structure, a simulation system is established to study the influencing factors of the external cavity feedback efficiency.  Methods   The efficiency model of output coupler free external cavity spectral beam combining structure is constructed, and the expression of spectral beam combining efficiency is deduced. According to the expression, the length of the external cavity, the telescope filtering system and the "Smile" effect have great influence on the feedback efficiency of this structure. The output coupler free external cavity spectral beam combining structure in Zemax is established, and the level of the feedback quantity of the two diffraction cavities in this system and the change in feedback quantity caused by changing the cavity length, the influence of the 1st-order diffraction cavity added to the telescope system on the feedback efficiency and quality of the combined beam, and the influence of the "Smile" effect on the feedback beam intensity are studied respectively.   Results and Discussions   According to the output coupler free external cavity spectral beam combining simulation system, the 1st-order diffraction light feedback power accounts for 2.73% of the output power, while the 0th-order diffraction light feedback power only accounts for 0.15% of the output power (Fig.3). As the distance of the external cavity increases from 27 mm to 558 mm, the feedback power decreases from 1.76 W to about 1.45 W (Fig.4), and the feedback beam crosstalk will occur (Fig.5); When different telescope filtering systems are inserted into the 1st-order diffraction cavity, the feedback power of the external cavity remains basically unchanged and the size of the slow-axis beam spot remains about 4 mm (Fig.8); The feedback power steadily diminishes and the combined spot size gradually increases as the degree of "Smile" effect rises from 0 μm to 1 μm. The feedback power of the 500-mm long focal length cylindrical lens inserted in the fast axis direction under the impact of the "Smile" effect at 1 μm is essentially the same as that without the "Smile" effect, which lessens the influence of the "Smile" effect on the feedback power.  Conclusions   The factors impacting on the external cavity's feedback efficiency are analyzed using the output coupler free external cavity spectral beam combining efficiency model. The effects of the external cavity length, the filter structure of the telescope, and the "Smile" effect on the feedback efficiency of the 0th-order and 1st-order diffracted beams are studied, respectively, using the output coupler free external cavity spectral beam combining simulation system built in Zemax. The results show that: (1) The feedback beam is dominated by the 1st-order diffracted beam, and wavelength locking of the external cavity is essential. As the length of the external cavity rises, the feedback power falls and the beam crosstalk increases; (2) The telescope filter structure can effectively filter the stray beam with large deflection angle and accurately feed back the beam to the original light-emitting unit; (3) The degree of "Smile" effect has a particularly negative influence on feedback efficiency and output beam quality. Although the "Smile" effect can be lessened by inserting a long focal length cylindrical lens in the fast axis direction, the beam quality after beam combination won't be noticeably enhanced. The research on the feedback efficiency of the external cavity can be used as a guide when designing the parameters for the output coupler external cavity spectral beam combining structure.
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Influence of output mirror free external cavity spectral beam combining structure on feedback efficiency

doi: 10.3788/IRLA20220446
  • 1. Beijing Engineering Research Center of Laser Technology, Faculty of Materials and Manufacturing, Beijing University of Technology, Beijing 100124, China
  • 2. Key Laboratory of Trans-scale Laser Manufacturing Technology,Ministry of Education, Beijing University of Technology, Beijing 100124, China
  • 3. Institute of Laser Engineering, Faculty of Materials and Manufacturing, Beijing University of Technology, Beijing 100124, China
  • 4. Beijing Aeronautical Technology Research Center, Beijing 100076, China
Fund Project:  Beijing Municipal Natural Science Foundation (4174085)

Abstract:   Objective   The external cavity spectral beam combining technology has two structures of open loop (without output coupler) and closed loop (with output coupler). The main difference between them is that the wavelength locked feedback beam of each light-emitting unit is different. Among them, the output coupler free external cavity spectral beam combining structure uses the beam returning from re-diffraction along the 0th-order and 1st-order diffraction direction as the feedback light, which avoids the waste of the beam and overcomes many problems of the feedback locking of the −1st order diffraction beam, and realizes the high-efficiency wavelength beam combination. For spectral beam combining technology, the feedback efficiency of the external cavity determines the stability of wavelength locking and even the success or failure of beam combination. Compared with the general closed-loop structure, the output coupler free external cavity spectral beam combining structure can only obtain enough feedback beams to achieve wavelength locking by controlling the parameters of the external cavity, and this external cavity structure is relatively complex, and the feedback efficiency variation caused by some factors in the external cavity are more obvious than the closed-loop structure. Therefore, for this structure, a simulation system is established to study the influencing factors of the external cavity feedback efficiency.  Methods   The efficiency model of output coupler free external cavity spectral beam combining structure is constructed, and the expression of spectral beam combining efficiency is deduced. According to the expression, the length of the external cavity, the telescope filtering system and the "Smile" effect have great influence on the feedback efficiency of this structure. The output coupler free external cavity spectral beam combining structure in Zemax is established, and the level of the feedback quantity of the two diffraction cavities in this system and the change in feedback quantity caused by changing the cavity length, the influence of the 1st-order diffraction cavity added to the telescope system on the feedback efficiency and quality of the combined beam, and the influence of the "Smile" effect on the feedback beam intensity are studied respectively.   Results and Discussions   According to the output coupler free external cavity spectral beam combining simulation system, the 1st-order diffraction light feedback power accounts for 2.73% of the output power, while the 0th-order diffraction light feedback power only accounts for 0.15% of the output power (Fig.3). As the distance of the external cavity increases from 27 mm to 558 mm, the feedback power decreases from 1.76 W to about 1.45 W (Fig.4), and the feedback beam crosstalk will occur (Fig.5); When different telescope filtering systems are inserted into the 1st-order diffraction cavity, the feedback power of the external cavity remains basically unchanged and the size of the slow-axis beam spot remains about 4 mm (Fig.8); The feedback power steadily diminishes and the combined spot size gradually increases as the degree of "Smile" effect rises from 0 μm to 1 μm. The feedback power of the 500-mm long focal length cylindrical lens inserted in the fast axis direction under the impact of the "Smile" effect at 1 μm is essentially the same as that without the "Smile" effect, which lessens the influence of the "Smile" effect on the feedback power.  Conclusions   The factors impacting on the external cavity's feedback efficiency are analyzed using the output coupler free external cavity spectral beam combining efficiency model. The effects of the external cavity length, the filter structure of the telescope, and the "Smile" effect on the feedback efficiency of the 0th-order and 1st-order diffracted beams are studied, respectively, using the output coupler free external cavity spectral beam combining simulation system built in Zemax. The results show that: (1) The feedback beam is dominated by the 1st-order diffracted beam, and wavelength locking of the external cavity is essential. As the length of the external cavity rises, the feedback power falls and the beam crosstalk increases; (2) The telescope filter structure can effectively filter the stray beam with large deflection angle and accurately feed back the beam to the original light-emitting unit; (3) The degree of "Smile" effect has a particularly negative influence on feedback efficiency and output beam quality. Although the "Smile" effect can be lessened by inserting a long focal length cylindrical lens in the fast axis direction, the beam quality after beam combination won't be noticeably enhanced. The research on the feedback efficiency of the external cavity can be used as a guide when designing the parameters for the output coupler external cavity spectral beam combining structure.

    • 半导体激光器具有效率高、体积小、质量轻、寿命长、波长丰富和可以直接电驱动等诸多优点,这些特性使得半导体激光器在科研和工程领域得到了非常重要的应用[1]。但由于受到光束质量的限制以及单个半导体激光器的输出功率有限,通常只能适用于其他激光器的泵浦源或者对光束质量要求不高的应用领域。如何提升半导体激光器的光束质量和输出功率一直以来都是国内外研究热点[2]。光谱合束是提高半导体激光阵列光束质量的一种有效方法,其采用衍射光栅作为色散元件,利用外腔使每个发光单元具有不同波长,通过衍射光栅的色散作用使激光在近场和远场均空间重叠,从而获得输出功率为所有发光单元之和、且光束质量接近单个发光单元的激光[3]

      2000年,由美国麻省理工学院林肯实验室V. Daneu等提出光谱合束概念,并将其应用于半导体激光阵列的合束[4-5]。此后,国内外许多机构针对半导体激光阵列外腔光谱合成开展了一系列理论和实验研究。北京工业大学[6]、中国科学院长春光学精密机械与物理研究所[7-8]、中国科学院半导体研究所[9]、中国工程物理研究院等相继取得了一系列实验进展[10]。四川大学对光谱合束系统的输出光束特性、合成效率以及串扰行为等进行了一系列的理论研究[11-13]

      2016年,Teradiode公司设计了一种无输出耦合镜的光谱合成结构,将0级和1级衍射光用作反馈光束[14]。2018~2019年,中国工程物理研究院基于单个半导体短阵列以及叠阵,分别进行了无输出耦合镜外腔光谱合束实验研究[15-16]。无输出耦合镜外腔光谱合束利用0级和1级衍射光束作为反馈光,避免了光束的浪费且克服了−1级衍射光束反馈锁定的多种问题,实现了高效率波长合束。与闭环结构相比,该外腔结构的反馈光束由0级和沿1级衍射方向光束再次衍射返回的光束提供,只能通过控制外腔结构参数来获得足够的反馈光束实现波长锁定,反馈效率决定了波长锁定的稳定性甚至光谱合束的成败,并且此种外腔结构相对复杂,外腔中的一些因素造成的反馈效率变化相比闭环结构更加明显,更需要对外腔结构进行优化设计。文中研究了无输出耦合镜外腔光谱合束结构中两臂距离、外腔等效反射率、望远镜滤波结构及 "Smile"效应对反馈效率的影响,可以为外腔结构参数设计提供依据。

    • 图1为无输出耦合镜外腔光谱合束结构示意图,其中半导体激光阵列水平放置,前腔面镀增透膜。空间位置不同的发光单元出射的光束经传输透镜后沿不同角度入射到衍射光栅上,部分0级衍射光和沿1级衍射方向光束再次衍射后经全反镜反馈回原发光单元,将每个发光单元锁定在不同的起振波长,起振波长满足光栅方程,合束激光沿−1级衍射方向输出。

      Figure 1.  Structure of diagram external cavity spectral beam combining without output coupler

      半导体激光阵列发光面位于传输柱透镜Lens1的焦平面上,全反镜HR1位于传输透镜Lens4的焦平面上,两传输透镜Lens1和Lens4组成远焦望远镜系统,光栅到两个传输透镜的距离均为透镜焦距,在水平方向类似4f像传递系统。从半导体激光阵列上各单元发出的光束经过快轴准直镜FAC和慢轴准直镜SAC进行快慢轴准直,然后通过传输透镜Lens1叠加到衍射光栅上,−1级衍射光束作为合束光束输出,透过光栅的0级衍射光束经过传输透镜Lens4在全反镜HR1处成光源的实像,然后被全反镜HR1反射沿原光路返回,传输透镜Lens4作为返回光的变换透镜并且将光束叠加到衍射光栅上。该光束的一部分光透过光栅返回到发光单元,另一部分光沿光栅1级衍射方向传输,经过全反镜HR2反射再次返回到衍射光栅,并再次经历衍射过程,一部分透射到光栅−1级合束方向,剩余光衍射后沿光栅0级衍射方向传输。因而实现各个发光单元波长锁定的反馈光束由两部分组成:0级衍射光束经HR1反射回光栅的透射返回光和沿1级衍射方向的衍射光束再次衍射后经全反镜HR1原路返回发光单元的返回光。为滤除大偏角的1级衍射光束造成的反馈光串扰,在1级衍射光束光路中引入带有狭缝滤波器的望远镜结构(由Lens2和Lens3组成)。图中,R1R2分别为半导体激光阵列后端面和前端面的反射率,T1T3分别为传输透镜Lens1和传输透镜Lens4的透过率,ηg为衍射光栅的−1级衍射效率,为了计算方便,假设望远镜滤波结构的整体透过率为T2

      根据Lang-Kobayashi理论模型[17],将半导体激光阵列各子发光单元与外腔构成的谐振腔等效为如图2所示的结构,其中,R1R2分别为半导体激光阵列的后端面和前端面的反射率,Rex1Rex2为外腔等效反射率,ηg 为光栅衍射效率(通常为−1 级衍射效率),Ld表示半导体激光阵列内腔长度,Lex表示外腔长度。

      Figure 2.  Equivalent resonator of the external cavity spectral beam combining structure without output coupler

      根据Lang-Kobayashi理论模型,在稳态工作条件下,载流子密度Nc为:

      阈值载流子密度$N_{{\rm{T}}} $为:

      在有光束反馈和光束串扰的情况下,从半导体激光器前端面输出光强I为:

      进一步得到该发光单元经过光谱合束后的输出光强Ij为:

      式中:N0为无反馈时的载流子密度;GN为微分增益因子;ω 为发光单元的角频率;τ为外腔往返时间;τc为载流子寿命;τd为内腔往返时间;τp为光子寿命;τ′为返回到原发光单元的串扰光束外腔往返时间;k为反馈强度参数;k′为串扰光束反馈强度参数;R为单位体积载流子注入率;R2为半导体激光阵列的前端面反射率;E为光场初始振幅;ηex为外腔耦合效率;η0为光栅的0级衍射效率;η1为光栅的1级衍射效率。

      在无光反馈时,即k=k′=0则该发光单元前端面输出光强I0为:

      于是,该发光单元的合成效率ηj[13]为:

      式中:$ k = \left( {1 - {R_2}} \right){\left( {{R_3}/{R_2}} \right)^{1/2}} $

      式中:R3为外腔各光学元件等效反射率。

      每个发光单元经光栅-外腔合成系统之后的效率主要与内腔特性(内腔折射率n、内腔长度Ld、半导体激光器前端面反射率R2、光子寿命τp)和外腔特性(外腔长度Lex、外腔等效反射率R3、变换透镜透射率T1、光栅0级和1级衍射效率分别为η0η1,外腔耦合效率ηex、外腔往返时间τ′)有关。此外,受到串扰光束影响,合成效率还与返回至自身串扰光的反馈强度和其他单元串扰光的反馈光有关。

      故经光谱合成系统后的合成效率ηsbc可以表示为:

      由公式(7)可知,对于无输出耦合镜外腔光谱合束结构,外腔长度Lex、外腔等效反射率R3、变换透镜透射率T1T3、望远镜滤波系统整体透射率T2、光栅衍射效率η0η1、外腔耦合效率ηex、外腔往返时间τ′均会影响反馈光束强度以及波长锁定的稳定性,进而影响合束效率及光束质量。

      下文分别研究无输出耦合镜外腔光谱合束结构的两臂距离(即外谐振腔腔长)、外腔等效反射率、望远镜滤波系统及 "Smile"对反馈效率的影响。

    • 根据建立的无输出耦合镜外腔光谱合束结构,采用具有19个发光单元的半导体激光阵列作为光源进行模拟。发光单元条宽为100 μm,相邻发光单元间距为500 μm,中心波长为976 nm,准直前快轴和慢轴发散全角分别为8°和47°,使用有效焦距fFAC =0.09 mm和fSAC=2.2 mm的快慢轴准直镜对快慢轴方向光束分别进行准直,两个传输透镜焦距均为fT=400 mm。选用高衍射效率的透射式光栅,光栅线密度为1851.85 lines/mm,波长在(976±20) nm范围内衍射效率$\geqslant $93%。

    • 无输出耦合镜外腔光谱合束结构由光栅的0级和1级衍射光束提供反馈,用于外腔波长锁定。因此两路反馈量的高低直接决定了外部谐振腔波长锁定的稳定性。基于Zemax对19个发光单元的半导体阵列无输出耦合镜外腔光谱合束结构的反馈光场强度进行仿真,如图3所示,其中图(c)为合束过程示意图,图中采用相同颜色表示同次衍射光。在距离发光单元−10 μm处放置一个探测器,分别观测两路的反馈量。首先探测到0级衍射光经光栅直接透射回发光单元的部分光束功率占输出功率的比例为0.15%,可以看出此部分反馈量较弱,对于波长锁定影响较小。而另一部分0级衍射光束经光栅衍射到1级衍射光方向的光束功率占输出功率的比例为2.73%,此部分光束会经过全反镜HR2反射再次返回到衍射光栅,小部分透射到光栅−1级合束方向,剩余光束衍射后沿光栅0级衍射方向传输并再次反射回光栅经历衍射。经对比发现1级衍射光决定了参与反馈光束的绝大部分,对外腔的波长锁定至关重要。

      Figure 3.  (a) Feedback spot of the 0th-order diffracted beam; (b) Feedback spot of the 1st-order diffracted beam; (c) Schematic diagram of the feedback of the 0th-order and 1st-order diffracted beams

    • 改变全反镜HR2与光栅的距离,研究反馈光束的变化,结果如图4所示。随着距离的增加,反馈到发光单元的光束辐射强度在逐渐下降,同时反馈功率也在逐渐降低,整体减缓趋势较弱。当全反镜HR2与光栅的距离大于传输透镜一倍焦距时可以发现反馈功率迅速下降。其原因在于,1级衍射光束随距离的增加呈发散趋势,故经全反镜HR2反馈回发光单元时光束宽度愈发增大,以至于超出快慢轴准直镜的尺寸,导致部分反馈光束无法经过聚焦正确返回原发光单元,从而反馈功率大幅下降。同时部分1级衍射光束会经光栅透射到−1级衍射光方向(即合束方向),由于光束呈发散状态,故随着全反镜HR2距离的增加,透射的杂散光束会造成合束光斑慢轴尺寸逐渐增大。虽然此部分光束功率不高,但会造成合束后的光束质量下降,需要采取一些措施对其进行抑制,如在外腔中加入柱面望远镜系统、滤波光阑等,这些措施同时也会对合束效率有所影响,造成合束效率略微下降。

      Figure 4.  (a) Influence of the feedback power and feedback radiation intensity on the distance of the HR2; (b) Influence of the size of the slow axis of the combined beam spot on the distance of the HR2

      图5为HR2与光栅距离不同时反馈光束的分布图,结果表明,随着距离增加,其反馈到原发光单元的光束逐渐出现串扰现象。由于1级衍射光束随距离的增加呈发散趋势,若直接经全反镜反馈回发光单元,则随着距离增加,反馈光束重叠程度逐渐增大,会造成光束不能正确反馈回原发光单元,而反馈至相邻甚至其它较远发光单元造成光束串扰现象。

      Figure 5.  Variation of the feedback beam intensity distribution with the distance of the HR2

      图6为反馈光束分布图,图中不同波长的光束分别对应一种色彩,反馈光束应和初始出射光束颜色相同才会代表光束正确反馈。由于入射光束经传输透镜叠加到呈Littrow角的衍射光栅上,会有部分光束直接反射到1级衍射方向上,若此部分光束再经衍射光栅反射回发光单元,会造成和初始发光单元的出射光束相比上下颠倒,即第一个发光单元出射的光束会反馈至第19个发光单元,这样会造成激光模式锁定混乱,甚至锁定失败。所以需要在1级衍射方向上加入准直镜进行光束校正及滤除,使光束正确反馈。

      Figure 6.  Diffracted beam feedback. (a) Correct beam feedback; (b) Incorrect beam feedback

    • 在1级衍射光束方向加入柱面望远镜系统,利用其聚焦特性结合光阑进行光束滤波,将大偏角杂散光束过滤掉,并将衍射光束再一次进行准直,可以更好的抑制相邻发光单元之间的光束串扰。此外柱面望远镜系统还能校正部分反馈光束错位,使光束正确反馈回原发光单元进行锁定,同时望远镜系统放大倍数会影响1级衍射光束方向的外腔长度以及滤波效果。

      在1级衍射臂加入望远镜系统,如图7中Lens2和Lens3组成的望远镜滤波系统。单从滤波效果来看,望远镜系统放大倍数越高则聚焦光斑越小,杂散光偏转角度越大,则光阑滤除杂散光束效果越好,但不同放大倍数的望远镜系统会导致全反镜距离不同。而且部分直接反射到1级衍射方向的光束呈发散状态,故望远镜系统中第一个柱面透镜摆放要在大于一倍焦距以上才会逐渐呈现较好的聚焦效果,而且第一个透镜位置会决定其聚焦后焦点的距离,太近会导致聚焦长度更长,则光束在经过第二个透镜时近似平行光束,不会准直而会聚焦。

      Figure 7.  Telescope diaphragm filter system in Zemax

      不同放大倍数的望远镜系统模拟结果如图8所示,结果表明,在1级衍射光路中插入望远镜系统,其倍数导致的距离不同对反馈功率影响不大。同时望远镜系统对发散的1级衍射光束进行准直,抑制相邻合束单元之间的串扰,减少大偏角的杂散光束对于合束光斑尺寸的影响。

      Figure 8.  Variation of feedback power and combined slow-axis beam size with different multiple telescope systems

    • "Smile"效应的存在,会导致非光谱合束方向上发光单元的光束错位、离轴以及指向性偏差等问题,这些偏差会使部分光束不能返回到原发光单元中形成波长锁定,导致该发光单元发出的光束与其他发光单元发出的光束在合束后无法空间重合从而降低激光在非光谱合束方向(一般指快轴方向)的光束质量。若"Smile"效应程度严重,甚至会造成发光单元完全无反馈,从而该发光单元波长锁定失败,不能实现光谱合束。另外若光束反馈到半导体激光阵列的非发光区域,则容易对半导体激光阵列本身造成损坏,因此应尽量避免该种情况出现。光谱合束结构中在"Smile"效应影响下的快轴方向光束反馈示意如图9,通常5 μm的"Smile"引起2倍光束质量的降低[18]

      Figure 9.  Feedback reduction and misalignment caused by the "Smile" effect (fast axis direction)

      尤其对于无输出耦合镜外腔光谱合束结构,是利用光栅能量较低的0级和1级衍射光束来实现发光单元的反馈锁定,则 "Smile"效应对此种结构影响更为突出。为研究"Smile"效应对反馈光束和串扰的影响,基于100 mm和10 mm的柱面透镜组成的10倍望远镜系统,对无输出耦合镜外腔光谱合束结构研究"Smile"效应对反馈效率的影响以及抑制方法。其反馈光分布及合束光斑模拟结果如图10图11所示,随着"Smile"效应的增强,反馈功率逐渐下降,从而导致波长锁定的稳定性变差,并且在光谱合束方向会导致合束光斑变化。

      慢轴方向光斑尺寸随着"Smile"效应增大而增大,但趋势较弱。而在快轴方向光斑尺寸增大趋势较为明显,随着"Smile"效应程度的严重会造成快轴方向光束质量的快速下降,因此需要在光路中采取措施来抑制"Smile"效应。

      快轴方向采用插入长焦距柱透镜的方法可以减少"Smile"效应的影响,图12(a)为快轴方向加入长焦距柱透镜后光束反馈示意图。"Smile"效应会导致半导体激光阵列的垂直位移,而光束通过快轴准直镜后,垂直位移就会转化为角度偏移,从而造成反馈量损失。在快轴方向插入柱面透镜Lens5,其与快轴准直镜FAC组成望远镜系统,使所有激光单元的主光线能垂直入射全反镜HR到上,并沿原光路返回发光单元,形成有效反馈,极大地补偿了"Smile"效应造成的反馈损失,提高了反馈效率[19],但是此种"Smile"效应补偿方法并不会较大程度改善系统的光束质量。在1 μm 的"Smile"效应影响程度下,快轴方向上加入焦距f 1=500 mm的柱面透镜Lens5,经模拟得到的反馈光束分布及合束光斑如图12(b)所示,此时反馈功率占输出功率的比例2.63%,可以看出每个发光单元的反馈量和无"Smile"效应时基本一致,保证了足够的反馈量,但光谱合束后快轴方向上的光束半径并未减小过多,故此结构不会较大程度改善"Smile"效应造成的光束质量下降。在快轴方向插入柱面透镜组成望远镜系统,也可以适用一般商用半导体激光阵列的闭环光谱合束结构,但当"Smile"效应更为明显的时,合束后的光束快轴方向光束质量较差。

      Figure 10.  Feedback beam intensity distribution and combined beam spot as a function of severity of "Smile" effect

      Figure 11.  Influence of "Smile" effect degree on feedback power and combined beam spot radius

      Figure 12.  (a) Corrected feedback beam of fast-axis telescope system; (b) Corrected feedback beam and combined beam spot

    • 相比于闭环光谱合束结构,无输出耦合镜外腔光谱合束结构利用光栅0级和1级衍射光束反馈来实现发光单元的波长锁定,−1级衍射光束用于合束输出。此种结构虽然提高了合束效率,但是反馈量的高低由0级和1级衍射光束的利用率直接决定,也即决定了外腔波长锁定的稳定性。文中对无输出耦合镜外腔光谱合束结构影响反馈效率的因素进行了理论研究,研究结果表明:(1)1级衍射光决定了参与反馈光束的绝大部分,对外腔的反馈锁定至关重要,且1级衍射光束方向的外腔长度增长会导致反馈功率的降低和光束串扰加剧;(2)在1级衍射光束方向加入望远镜滤波系统可以有效滤除大偏角杂散光束改善光束质量,同时校正由于传输透镜导致的部分光束偏移,使光束正确反馈回原发光单元;(3)对于无输出耦合镜外腔光谱合束结构,"Smile"效应的影响程度更加明显,更容易导致反馈光束的错位,离轴以及完全不反馈,因此需要在外腔中采用光学手段进行改善。经模拟可知在快轴方向加入柱面透镜与快轴准直镜形成望远镜系统能极大程度减缓"Smile"效应导致的反馈损失,虽然该方法不会较大程度改善合束的光束质量,但为一般商用高"Smile"效应半导体激光阵列用于光谱合束带来可能性。文中研究可以为无输出耦合镜外腔光谱合束结构参数设计提供参考。

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