基于太赫兹时域光谱系统共光路光纤耦合系统的设计及误差分析
The term “terahertz radiation” typically refers to the frequency range of 0.1 THz to 10 THz in electromagnetic waves, positioning terahertz waves between microwaves and infrared. Due to the unique frequency band of THz waves, they exhibit several distinctive characteristics. (1) Transience: The signal amplitude of THz pulses is very low, yet they possess a noticeable peak value, making them valuable in time resolution research applications. (2) Spectral resolution function: Experimental THz radiation sources typically consist of only a few pulses, each covering a spectral range containing the vibrational and rotational energy levels of numerous macromolecules, facilitating substance identification. (3) Safety: The photon energy at 1 THz frequency is approximately 4 meV, and terahertz radiation does not easily disrupt the molecular structure of the detected substance when applied in medical imaging. (4) Penetration: With a wavelength falling between microwaves and millimeter waves, terahertz waves can pass through small particles in the air. Given these unique properties of THz rays, THz technology holds significant application prospects in safety inspection, communication technology, terahertz radar, astronomy, biomedical imaging, chemical identification, materials science, and other fields. Consequently, the generation, detection, and application studies of terahertz waves constitute a prominent research area.
Utilizing a double-lens transmission matrix, an initial double-lens structure was designed, encompassing both aperture and thickness considerations. Subsequently, leveraging Gaussian beam transmission characteristics, the size and position of the beam waist were meticulously determined, optimizing the entire optical system. This process enables the creation of a fiber coupling system with a small aperture and high efficiency. The optical simulation software ZEMAX was employed to scrutinize the initial fiber coupling system’s design, aligning with terahertz time-domain spectroscopy (THz-TDS) and fiber coupling technology features. The optical simulation software was further utilized to trace the system’s light, facilitating the preliminary establishment of the placement angle and position between optical components, such as the delay line and light source, ensuring successful light recovery. Concurrently, the delay line and the coupled optical system were configured to avoid mutual interference, allowing for the optimization of the fiber coupling system’s structure. This optimization aimed to achieve higher coupling efficiency and improved beam quality. Considering the practicalities of the experimental installation process, the mechanical structure of the entire module was designed based on the optical specifications. This approach ensures that all system components can be installed and adjusted cohesively. The coupling lens’s mechanical structure was devised as a five-dimensional adjustment structure, characterized by its simplicity, convenient machining and assembly, compactness, high stability without a transmission gap, and five degrees of freedom for three-dimensional translation and two-dimensional angle rotation.
The collimated coupled optical system’s single-mode fiber coupling efficiency is illustrated in Fig. 2. When the system was positioned in front of the rotating delay line, the light followed a path reflected back through the delay line, coupling to the original fiber and rendering the system lens entirely symmetrical, resulting in a high coupling efficiency with a single-mode fiber. Figures. 4 and 5 depict the optimized collimated coupling optical system with its single-mode fiber coupling efficiency and the actual optical path diagram. The observed coupling efficiency with a single-mode fiber was 76.27%, approaching the ideal coupling efficiency of 81.45%. Accounting for Fresnel reflection loss at the incident end face of the fiber, the maximum coupling efficiency was reduced to 78%, closely aligning with the system’s actual coupling efficiency. Simultaneously, the single-mode fiber coupling efficiency reached 97.25% for physical optical propagation. Consequently, following system optimization, the coupling efficiency was markedly high, meeting the specified coupling requirements.
The fiber THz-TDS transceiver-integrated coupling system differs from traditional fiber THz-TDS by incorporating a delay line with the fiber in the coupling aspect, simplifying the structure of the fiber coupling system. An optical system with high coupling efficiency was designed based on the principles of a Gaussian beam relay and the characteristics of a double lens. The single-mode fiber coupling lens model was developed using ZEMAX software, and the system underwent optimization to enhance the coupling efficiency of the single-mode fiber. The results demonstrate that the coupling efficiency reached 97.25%, meeting the high-efficiency coupling requirements for single-mode fibers in terahertz time-domain spectroscopy systems. This not only provides a guiding direction for the design of coupling lenses but also contributes to the advancement of miniaturized terahertz time-domain spectroscopy instruments.
1 引言
自太赫兹波的特性被发现以来,太赫兹先进技术便引起了国内外研究人员的极大兴趣,尤其是太赫兹时域光谱仪器,更是吸引了国内外厂商的普遍关注,而且其已成为我国的高科技产业之一[1]。国外高科技公司很早就已开始在太赫兹时域光谱技术[2]领域投入大量研究。更细、更轻、尺寸小、抗干扰能力强的光纤式太赫兹时域光谱系统(THz-TDS)与在自由空间传输的系统相比,具有使用寿命更长、体积更小、成本更低等特性。目前人们更多研究的是如何提高飞秒激光光纤的耦合效率[3]。光纤耦合一开始被应用于空间探测,之后被应用于激光雷达技术[4],后来才逐渐被应用于不同领域。实现光纤耦合的技术众多,主要有直接耦合法和间接耦合法[5],两种方法各有优缺点。
2018年,华中科技大学的黎小姝[6]将红外飞秒激光器作为光源,结合光纤耦合技术设计了一款应用于THz-TDS的光纤传输系统,通过模拟实验获得了最高可达73.8%的耦合效率;2019年,况耀武等[7]结合单模光纤温度适应性成功设计了耦合光学系统;2023年,朱尚典等[8]基于光束准直、空间光束合成、偏振光合成、光纤耦合等技术设计了输出功率为272.4 W、光光转换效率为85.1%的光纤耦合系统。目前,关于太赫兹时域光谱系统内光纤耦合技术的研究较少,多数是空间激光通信中空间激光-单模光纤的相关研究。高耦合效率的光纤耦合系统能够产生高质量的泵浦光,提高光电导天线的载流子浓度和载流子运动速度的变化率,进而影响最后所产生的太赫兹信号的脉冲宽度和脉冲功率。
笔者采用双透镜和旋转延迟线设计收发一体共光路光学系统,通过模拟实验优化出耦合效率较高、光束质量较好、稳定性较高的系统,要求该光学系统能够收发自如、减少传输损耗,同时将返回的飞秒激光顺利耦合进单模光纤。延迟线经过了设计论证[9]。光纤发出的光在自由空间内以一定角度进行发散传输,经过耦合系统透镜后变为准直光,确定系统与旋转延迟线的摆放位置,使准直光打在该延迟线工作区域角度范围的平面反射镜上并经反射镜反射回入射光路。两相邻反射楔形结构具有一定范围的空闲角度,该空闲角度应当为工作区域角度的整数倍,以保证延迟时间的连续性。基于激光与光纤的耦合原理,笔者使用光学仿真软件ZEMAX对耦合系统的关键透镜参数进行优化设计,得到更高的单模光纤耦合效率,随后对整个系统进行倾斜、偏心等仿真模拟,分析各种误差对光纤耦合的影响。
2 耦合效率理论分析
通常,光纤激光器发射的自由空间激光近似符合
式中:
式中:
式中:
令
光束波长
图 2. 理想单模光纤耦合效率随 变化的曲线
Fig. 2. Coupling efficiency of ideal single-mode fiber varies with β
3 准直透镜的耦合机理分析
3.1 双透镜特性分析
现实中,光线通过光学系统后,会因为视场、系统口径等产生各种像差,导致成像出现弯曲、会聚不集中、畸变等问题。光纤式光学耦合系统是小像差系统,所以球差在单模光纤耦合系统中的影响最大,可以通过主动使透镜离焦来校正部分球差,也可以通过双透镜光学系统将正负透镜组合,从而使色差和彗差相互校正[14]。基于双透镜设计的准直耦合光学系统具有调节方便、能量损失少、会聚光斑小的优点,能够实现高效耦合。
光线通过单透镜系统的几何成像公式[15]为
式中:
当单透镜的主距分别为
将
综合
其中,
式中:
3.2 高斯光束在光学系统中的传播特性
确定准直耦合光学系统后,为提高准直光学系统与单模光纤的耦合效率,需要保证高斯光束经过光学系统后,其光束束腰半径与接收光纤的模式匹配。因此,需要先从理论与原理上对高斯光束进行分析计算[16]。在柱面坐标系中,假设
假设传输的高斯光束参数为
式中:
在传输过程中,高斯光束遵守高斯光学的ABCD定理,结合
高斯光束在物方空间束腰到像方空间束腰处的传输矩阵为
式中:
联立式(
前文通过
4 准直透镜的设计与优化分析
4.1 准直透镜设计
ZEMAX软件能够对耦合透镜组中的关键光学元件进行参数优化[17],并可在软件中进行仿真模拟,得到透镜与单模光纤的耦合效率。基于双透镜设计的单模光纤耦合透镜系统具有光线准直、聚集光束、方便调节等特点,而且该光学系统的结构简单紧凑,不容易出现装调错误。
根据
实验所用光纤为保偏光纤,为保证后续实验顺利进行,在仿真计算时已进行了保偏处理。初始准直耦合光学系统中单模光纤的耦合效率如
4.2 透镜的优化分析
基于ZEMAX软件对光学系统透镜进行设计和优化,在序列模式下对耦合透镜进行优化,用均方根和波前函数对光学系统的透镜参数进行完善,得到了最佳的光学耦合系统。该系统的单模光纤耦合效率较高,优化后的准直耦合光学系统的单模光纤耦合效率如
优化后,系统的口径减小,可以配合实验室光源等器件。同时,优化后的点列图光斑均方根半径与艾里斑半径相比初始光学系统更小,如
4.3 单模光纤耦合原理及误差分析
接下来分析飞秒激光在单模保偏光纤中的传输原理,同时分析导致单模光纤耦合效率降低的系统参数和外部因素以及将聚集光束完全耦合传输进光纤的条件。首先,若要保证激光能够完全耦合进光纤,则光束与单模光纤耦合需要满足一定的匹配条件,即
激光束到达光纤端面的聚焦光斑直径
对于利用一个或多个薄透镜将激光束间接耦合的光纤传输系统来说,有三种失配误差会降低单模光纤的耦合效率。第一种误差如
图 7. 耦合对准误差分析原理图。(a)光束相对光轴倾斜;(b)光束相对光轴偏移;(c)光束束腰半径与光纤模场半径不匹配
Fig. 7. Schematic diagrams of coupling alignment error analysis. (a) Beam tilt relative to optical axis; (b) beam offset relative to optical axis; (c) beam waist radius mismatch with fiber mode field radius
高斯光束分析与物理光学传播分析都可以用于耦合效率的分析,二者的主要区别在于高斯光束分析一般用于完美基模TEM00光束,不考虑系统内光学像差及孔径的影响,而物理光学传播分析则考虑了系统内光学像差及孔径的影响。
图 8. 高斯传播单模光纤耦合效率与径向失配的关系
Fig. 8. Relationship between coupling efficiency of Gaussian propagation single mode fiber and radial mismatch
图 9. 高斯传播单模光纤耦合效率与角向失配的关系
Fig. 9. Relationship between coupling efficiency of Gaussian propagation single mode fiber and angular mismatch
图 10. 物理传播单模光纤耦合效率与径向失配的关系
Fig. 10. Relationship between coupling efficiency of physical propagation single mode fiber and radial mismatch
图 11. 物理传播单模光纤耦合效率与角向失配的关系
Fig. 11. Relationship between coupling efficiency of physical propagation single mode fiber and angular mismatch
通过ZEMAX可以模拟光纤偏心或者光源倾斜等误差。首先分析径向失配,即光学系统的光学元件沿X或Y方向偏心。当光学系统单独从初始位置沿X方向偏心1 mm时,单模耦合效率从76%降到74%左右;当光学系统单独从初始位置沿Y方向偏心1 mm时,单模耦合效率基本保持76%不变。之后,随着偏心增大,二者的降低趋势基本相同。当光学系统同时沿X、Y方向偏心1 mm时,耦合效率变为72%,光线损耗相比沿X、Y方向单独偏心时更大,之后,耦合效率与偏心角度的关系如
当物理光学传播从初始位置沿X方向偏心1 mm时,光纤耦合效率从97%降为95%;当物理光学传播从初始位置沿Y方向偏心1 mm时,光纤耦合效率降为96.6%。之后,随着偏心增大,二者的降低趋势基本相同。当物理光学传播从初始位置同时沿X、Y方向偏心1 mm时,耦合效率变为94%,如
接下来分析光学系统的光学元件沿X或Y方向倾斜,也就是所谓的角向失配。当光束沿X方向倾斜0.01°时,高斯单模光纤的耦合效率从76%降为74%;当光束沿Y方向倾斜0.01°时,高斯单模光纤的耦合效率基本保持为76%,光纤损耗从-1.1765 dB变为-1.3723 dB。当光束同时沿X、Y方向倾斜时,单模光纤耦合效率的下降趋势更快,光纤损耗较单独方向倾斜时更高,如
由
4.4 实验评估
实验室使用的光源为光纤激光器,其输出波段覆盖1500~1600 nm,主波长为1550 nm,且功率可调。发射和接收光纤均采用单模保偏光纤(PM1550),通过仿真实验判断该光学准直耦合系统达到了单模光纤耦合使用条件。
探测光与泵浦光之间的时间延迟是通过光学延迟线旋转来实现的,因此可以实现太赫兹脉冲的采样扫描。当延迟距离为ΔL时,对应的时间延迟为Δt=ΔL/c,其中c为光速。如果ΔL变大,扫描的数据点数就会变少,系统的扫描速度就会变快。在时间延迟逐步增加过程中,脉冲扫描点的位置是不断改变的。
光源从光纤中出射,通过透镜组后被准直,准直光线在传输过程中被旋转延迟线反射回透镜组,进入透镜后被耦合进发射光纤。
在开始进行单模光纤耦合实验时,应该先校准光学系统透镜与单模光纤的相对位置。为了保证实验在较为稳定的环境下进行,需要减小外界振动对实验平台的影响,因此将实验器材固定在光学平台上,并将光学系统中的准直耦合透镜安装在多维调整结构台上,同时使用单模光纤将双透镜与实验光纤激光器、光功率计连接。为了保证准直耦合透镜中心在一条光轴上,利用可见光对光学系统的双透镜进行调节。最后进行不同维度的微调,使输出功率达到最大,即达到最高耦合效率。光学实验实物图如
图 13. 光纤输出功率与耦合误差的关系。(a)输出功率与径向失配的关系;(b)输出功率与角向失配的关系
Fig. 13. Relationship between fiber output power and coupling error. (a) Output power versus radial mismatch; (b) output power versus angular mismatch
5 结论
基于光纤式THz-TDS设计了收发一体式光纤耦合系统。与传统的光纤式THz-TDS不同的是,所设计的光纤耦合系统将延迟线与光纤结合,简化了光纤耦合系统的结构。基于高斯光束传播原理与双透镜特性,初步设计了高耦合效率的光学系统。分析了光学准直透镜与光纤耦合时的3种耦合误差所导致的耦合效率降低的现象,借助ZEMAX软件设计了单模光纤耦合透镜模型,并对该系统进行了优化,提高了单模光纤的耦合效率。结果显示,物理传播单模光纤的耦合效率达到了97.25%,高斯传播单模光纤的耦合效率达到了76.27%。为实现单模光纤的高效耦合,光束需要从光纤端面正入射,并且高斯光束的束腰位置应位于光纤端面上,同时束腰直径应等于光纤模场直径。即,光束与光纤模式越匹配,单模光纤耦合效率就会越高。得到的结论与太赫兹时域光谱系统的单模光纤高效耦合要求相符,为耦合透镜的设计提供了一种途径,同时为实现小型化太赫兹时域光谱仪提供了参考。
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Article Outline
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