光学相干层析成像大纤芯光纤探头的优化研究 下载: 820次
Developing a fiber probe with a high lateral resolution, long depth of focus, long working distance, and uniform axial light intensity is essential for endoscopic optical coherence tomography (OCT). Traditional beam shaping components, such as axicon lens and binary-phase mask, were minimized and adapted to miniature fiber probes for optimized output beams, but with a short working distance and reduced depth of focus gain compared to their bulk-optic counterparts. Alternatively, pure fiber techniques that simply splice fibers in series were proposed and demonstrated a significant enhancement in the imaging quality. The first demonstration of the concept suggested a phase mask consisting of a short section of overfilled graded-index fiber (Lorenser, 2012). However, the most recent progress indicated that using a step-index large core fiber as a coaxially focused multimode beam generator (Yin, 2017) or a high-efficient fiber-based filter (Ding, 2018) would be advantageous in terms of easy fabrication and increased depth of focus gain. However, full optimization of such probes is not straightforward, where the difficulty originates from the complexity of the light field by multimode interference and the arrangement flexibility of fiber components. Therefore, this study presents systematic research on the optimization of large core fiber probes. We discuss key design considerations for selecting fiber optics for mode excitation, number of modes, beam expansion method, and selecting the lens-free mode or spatial filter mode. We hope that our findings can be essential in designing the ultrathin fiber probe with improved performance for OCT imaging.
A unified equation quantifying the depth of focus gain was first deduced by comparing the Gaussian beam with the same minimal beam diameter. Then, the fiber mode theory was applied to demonstrate the light field manipulated by the large core fiber. The tunning length of the large core fiber was determined by its re-imaging property and modal dispersion. According to the relative position of the large core fiber to the pupil of the objective, the working modes of the probe were classified into two catalogs. Consequently, proximate equations of the output light field were deduced for the two working modes. Then, the optimization goals and restrictions were established in terms of the depth of focus gain, lateral resolution, uniformity of axial light intensity, working distance, and sidelobes. The fast simulation method based on the mode expansion was applied to search for the probe parameters according to the established goals and restrictions. We obtained the preferable structure and the maximum achievable performance of the large-core-fiber-based probe by comparing the optimized results under different working modes, beam expansion methods, and the number of modes. The fiber probe with optimized parameters was fabricated and interfaced to a swept-source OCT system. A conventional probe with the same minimal beam diameter was also fabricated for comparison. The same region of fresh lemon was scanned with a translational stage and imaged by the two probes to confirm whether the improved specifications led to corresponding enhancement in the imaging quality.
Similar to the binary phase filter that regulates the output beam by encoding the phases of annular zones on the aperture, the large core fiber can also adjust modal phases independently for the output beam manipulation. Additionally, the depth of focus, working distance, and lateral resolution are expected to increase with introduced higher modes. Although sidelobes become more significant with increased depth of focus, they can be mitigated by optimizing the modal power distribution. The large-core-fiber-based probe has less stringency on fabrication than the fiber phase mask. With a length tolerance of -28-+ 20 μm (Fig. 8), it is achievable for a commercial off-the-shelf fiber processing platform. The large-core-fiber-based probe features axially uniform light intensity compared with the coaxially focused multimode beam generator (Figs. 10 and 12). For the mode excitation device, we find that the graded-index fiber is superior to the tapered fiber in terms of a more robust splicing point. For the dual-mode interference, the amplitude ratio of the fiber mode can be tuned in the range of 0.2-0.3. For multimode interference, the graded-index fiber can be used with the no core fiber for efficient higher-order mode excitation (Fig. 12). For the working modes of the probe, we conclude that the spatial filter mode is advantageous in a larger depth of focus gain. Additionally, a longer working distance is attainable with beam relay optics (Table 4). The modal dispersion is the ultimate limit on the performance of the large-core-fiber-based probe. We confirmed that the maximum DOF gain of the probe was 3.8.
This study systematically investigates the optimization method of a large-core-fiber-based probe. By comparing the lateral resolution, working distance, and focal depth gain of various probe designs, we believe that the spatial filter design with beam relay optics and higher-order modes is beneficial for the probe performance. In addition to OCT imaging, the principle of the framework can be applied to optimize the output beam in laser scanning and photoacoustic imaging systems. Some presented elements of the work can also provide technical implications for non-imaging applications, such as fabrication of laser-fiber couplers and optical tweezers.
1 引言
光学相干层析成像(OCT)是一种通过低相干光源获取生物组织等散射介质二维或三维图像的高分辨率成像技术。与超声成像类似,OCT通过测量样本的反射或后向散射回波获得样本不同深度处的反射率,并结合一维或二维横向扫描获得样本的断层图像或三维图像。内窥OCT能通过细小的探头获得人体血管壁的10 μm分辨率图像,是心血管内窥领域用于评估冠状动脉结构形态的重要成像手段[1]。但是,很多与冠状动脉硬化发病机制相关的特征只能在细胞水平下观测,因此发展微米级分辨率的下一代OCT技术对于研究冠状动脉硬化的发病机制、提高硬化斑块的早期诊断率、评估再生血管化的术后恢复过程等具有重要意义[2-3]。此外,OCT探头在振动检测[4]等工业领域亦有所应用。
OCT的轴向分辨率主要取决于光源的光谱带宽。使用最先进的宽带光源能实现1~5 μm的轴向分辨率[5],但如果使用高数值孔径的物镜将横向分辨率提高到相同水平,则OCT的轴向视场将受限于光束极短的焦深,同时,其工作距也将迅速缩短。为了解决横向分辨率和焦深的矛盾,研究人员提出了多种方案,并实现了一个数量级的焦深拓展,这些方案包括准贝塞尔光照明[6-7]、数字聚焦[8-9]、动态聚焦[10-11]等。但是这些方法或需要相位稳定,或需要机械扫描,或需要使用两条光路分别实现照明和探测,难以应用于小型探头。
使用研磨抛光制作的微型轴锥镜[12]、基于软光刻工艺的微型二元相位板[9]、基于电子刻蚀和光刻工艺的超透镜[13]以及基于微纳3D打印的自由曲面微型镜片[14]已被用于探头焦深的扩展,但与台式系统相比,这些微型光学元件不仅制作成本高,而且探头的焦深拓展倍数有限。此外,一种仅需按一定顺序进行光纤熔接的纯光纤技术被提出来,并显示出了对图像质量的增强潜力,西澳大学的Lorenser等[15]在报道一种由过填充渐变折射率光纤组成的相位掩模板中首次阐述了此概念。最新的研究进展表明,将基于阶跃折射率的多模光纤作为同轴聚焦多模光束发生器[16-17]或作为高传输效率光纤型空间滤波器[18],在制造加工和拓展焦深方面更具优势。然而,这种基于大纤芯光纤(LCF)或多模光纤的探头的全面优化并不容易,其困难不仅来自多模干涉场的复杂性,还来自光纤之间排列组合的灵活性。本课题组亦围绕LCF探头进行了研究,研究内容包括设计输出光束可控的无透镜探头[19]、基于高传输效率光纤型空间滤波器的焦深拓展技术[18,20]、基于特征模展开的LCF探头快速仿真和优化方法[21],但没有对此类探头的设计和优化进行过系统报道。
本文针对LCF探头的优化设计进行了细致梳理,具体涉及探头结构的优化和出射光束的优化,并讨论了模式激发装置的设计、无透镜和空间滤波两种工作模式、模式干涉场的两种放大方式以及模式数量的选择。希望本研究能对超小OCT光纤探头的优化设计提供些许技术启示。
2 原理和方法
2.1 出射光束参数的表征
提高物镜的数值孔径能有效提高OCT系统的横向分辨率,但随着聚焦光斑的缩小,轴向视场和工作距将迅速减小。如
图 1. OCT系统的横向分辨率、轴向视场和工作距。(a)低数值孔径聚焦和高数值孔径聚焦下的高斯光束;(b)存在空间滤波器时光束的聚焦情况
Fig. 1. Lateral resolution, axial field of view, and working distance of optical coherence tomography (OCT) system. (a) Gaussian beams under low NA focusing and high NA focusing; (b) light focusing with spatial pupil filter
使用准贝塞尔光束是在更大轴向范围内保持高横向分辨率的一种实用方法。
2.2 LCF的光场调控原理
利用LCF的模式干涉是实现光场调制、DOF拓展和工作距拓展的有效途径[16,18-21]。如
图 2. 基于LCF的空间滤波器(下方三维图仅示意了从SMF某一环带出发的斜射光线;SMF:单模光纤;NCF:无芯光纤)
Fig. 2. Spatial filter based on large-core-fiber (LCF) ( only skew rays from a single annular area from SMF are depicted in the below panel demonstrating three-dimensional light rays; SMF: single mode fiber; NCF: no core fiber)
基于LCF的光场调制与基于相位板的光场调制[9, 15]在原理上有明显区别。相位板能直接调节空间域中各环带的相位,而LCF通过调节由正交Bessel函数构成的模式域中各模式的相位来间接调节各环带的幅值和相位。虽然相位板和LCF都通过调节长度来调控相位,但由于LCF具有远小于相位板的折射率对比度,因此LCF的相位变化对光纤长度相对不敏感。后者构成了LCF探头在制造方面的优势。
LCF的相位调节范围与其长度调节范围有关,而后者取决于LCF的节距Lp[23]。LCF的归一化有效折射率b0n不是均匀分布的,Lp一般较大,且LCF的长度在qLp~(q+1)Lp范围内(q为自然数)变化时能实现各模式相位差的独立调节。但为了防止模间色散[22]影响OCT系统的轴向分辨率,LCF的长度存在一个上限值。一般要求模间光程差Δl小于OCT系统的轴向分辨率ΔzOCT,因此LCF的长度调节范围为
LCF的模式幅值取决于激发条件。对于
2.3 LCF探头的出射光束
基于LCF的探头有两种工作模式。在无透镜模式下,LCF直接调控探头的出射光束,如
图 3. 基于LCF探头的两种工作模式以及光束的中继。(a)无透镜模式,LCF直接调控出射光束;(b)空间滤波模式,LCF作为空间滤波器,通过调控物镜(GIF)入瞳处的光场间接实现出射光束的调控;(c)(d)使用透镜 (GIF2)将LCF或GIF的出射光束中继到探头外侧
Fig. 3. Two working modes of LCF-based probes and relay of light beam. (a) Lens-free mode, where the output beam is regulated directly by LCF; (b) spatial filtering mode, where the output beam is regulated indirectly by LCF through controlling light field on the entrance pupil of the objective (GIF); (c)(d) output beams from LCF or GIF are relayed to the outside of probe by the lens (GIF2)
使用透镜将LCF或GIF的出射光束中继到探头外侧有望获得更大的工作距和DOF,如
2.4 LCF探头的参数优化
设计LCF探头的挑战之一是它的优化需要预先给定一组探头参数,然后基于此进行光传播仿真,才能确认出射光束的性能。因此,这种正向优化过程需要尝试所有可能的参数组合才能获得探头的最优参数。为了评估探头性能的优劣,在进行穷举优化前需要定义优化函数和束缚条件。
探头的优化目标是同时获得高横向分辨率、较长的DOF和工作距、均匀的轴向光强分布。为了同时保证较长的DOF和均匀的轴向光强分布,这里将最长连续焦深zc,DOF定义为光束直径小于或等于两倍MBD的连续区域的最大长度。若出射光束是轴向均匀的光束,则zc,DOF=zDOF;若出射光束不是轴向均匀的光束,则zc,DOF<zDOF。根据式(2),定义优化函数为
探头的优化变量是除了SMF外各段光纤的长度。其中:LCF的长度范围须满足式(4),以防止过大的模间色散;无芯光纤的长度上限根据光束在无芯光纤中衍射后刚好充满GIF2的入瞳确定,如
选择合适的仿真方法是实现探头参数优化的关键。一方面,该仿真方法需要具有较高的仿真精度。由于具有高横向分辨率的探头的聚焦光束一般会形成在靠近探头末端的区域,常用于设计空间滤波器的Fresnel近似[如式(7)所示],可能会造成较大的计算误差。另外,由于LCF的LP0n模仅在芯层区域符合Bessel光束,使用Bessel光束近似其出射光束[如式(6)所示]会引入较大的计算误差。另一方面,该仿真方法需要具有较快的速度。对于
基于特征模展开的光束仿真方法由于利用预先计算好的模场分布以及计算衍射光束在不同光纤之间模式的耦合效率,能实现对探头出射光束的快速仿真,非常适合基于LCF的探头设计。与光束传播法不同,当探头中的光纤长度发生改变时,基于特征模展开的光束仿真方法无需从头开始仿真,仅需重新计算各模式的幅值an和相位φn,因此有效提高了仿真速度。基于特征模展开的光束仿真方法的原理如
探头的出射光场实际上可视为GIF2各模式的衍射场的线性组合,即
2.5 LCF探头的成像效果评估
为了评估LCF探头的实际成像效果,将本团队制作的探头接入扫频OCT系统,对样本进行OCT成像。如
3 结果和分析
3.1 出射光束可调控的无透镜探头
基于LCF的探头的最简单结构是将LCF拉锥后直接与SMF熔接,其结构如
图 6. 无透镜模式下的可控输出光束[19]。(a)基于拉锥LCF的探头;(b)拉锥段长度对LCF模式能量的调控;(c)不同探头参数下出射光束的光强分布
Fig. 6. Controllable output beam under lens-free mode[19]. (a) Tapered-LCF-based probe; (b) mode power regulated by the length of tapering length; (c) intensity distributions of output beams under different probe parameters
为了演示参数LT和LLCF对探头出射光束的调控效果,这里考虑了LCF中主要含有LP01模而几乎没有模式干涉以及同时含有LP01和LP02模且有明显模式干涉的两种情况。当LT≈3.4 mm时,探头具有最高的传输效率且几乎只包含LP01模。当LT=1.2 mm时,从SMF基模到LCF的LP01模的耦合效率几乎是LP02模的2倍,此时a1,LCFΨ1,LCF(0)=a2,LCFΨ2,LCF(0),因而具有最明显的模式干涉。根据LCF末端LP02与LP01的模间相位差是2mπ还是(2m+1)π,LLCF也有两种不同的选取方式。LCF的长度选取范围由式(4)确定,由于所选的LCF模式数量少、模间色散较小,因此,为了获得较长的LLCF,这里取q=2,以尽可能地衰减LCF中的瞬态模式,同时也便于夹持。以上两种LT和LLCF的选取情况对应了4种探头参数组合LT,LLCF:探头Ⅰ(3.34 mm,3.03 mm),探头Ⅱ(3.34 mm,3.62 mm),探头Ⅲ(3.34 mm,3.62 mm),探头Ⅳ(1.2 mm,3.37 mm)。由于拉锥段会引入初始模间相位差,虽然探头Ⅰ和探头Ⅱ在LCF末端的模间相位差都等于2mπ,但它们的LLCF有所不同。探头Ⅱ与探头Ⅳ同理。探头Ⅰ~Ⅳ的出射光束的光强分布如
由于具有更短的硬端长度和相对更长的工作距,探头Ⅰ和Ⅲ被进一步研究,它们的出射光束参数见
表 1. 无透镜探头Ⅰ和Ⅲ的出射光束参数
Table 1. Parameters of output beams from lens-free probes Ⅰ and Ⅲ
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3.2 基于LCF的空间滤波器
上述出射光束可调控的无透镜探头有两个主要缺点:1)探头外径不均匀,尤其是SMF与LCF拉锥段细端的外径不同,容易断裂;2)有部分聚焦光束位于探头内部,因此探头的轴向视场相对有限。为了提高探头的机械稳定性,这里使用一段渐变折射率光纤GIF1(GIF50C)代替拉锥光纤,如
图 7. 基于光纤型空间滤波器的DOF拓展探头[18],其中黄色和白色框代表各段光纤的芯层。(a)探头原理图;(b)含空间滤波器探头的光强分布;(c)不含滤波器的传统探头的光强分布
Fig. 7. Probe with a fiber spatial filter and extended DOF[18], where the boxes with yellow and white colors represent fiber cores. (a) Schematic of probe; (b) light intensity distribution of probe with spatial filter; (c) light intensity distribution of traditional probe without filter
当模式幅值相等、相位相差π的奇数倍时,LCF末端的模式干涉场表现出一种中间暗、边缘亮的环形照明特征,如
表 2. 含有滤波器与不含滤波器探头的出射光束参数
Table 2. Parameters of output beams from probes with and without filter
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探头对制造的要求是一个重要的评估指标。由于探头采用全光纤结构,元件之间的光学对准在普通的光纤熔接机上可以自动实现;但控制各段光纤的长度在其公差范围内仍然是制造过程中的挑战。借助精密电动平移台,光纤的切割误差能达到-5~+5 μm以内;但如果探头中光纤的长度误差远大于这个范围,无论是制作优良率还是探头的性能都会有所提升。为了确定探头中光纤的长度误差,本文用探头的工作距(WD)、DOF增益(DOFG)和横向分辨率(用MBD表征横向分辨率)来评估光纤长度误差的影响。
图 8. 含有滤波器探头中各段光纤长度误差对探头性能的影响[18]。(a)GIF;(b)LCF;(c)NCF;(d)GIF2(图中纵坐标表示探头指标与零长度误差情况下的比值,其中DOFG、MBD和WD分别代表DOF增益、最小光束直径以及工作距的相对值)
Fig. 8. Influence of fiber length error of each fiber in the probe with filter on probe performance[18]. (a) GIF; (b) LCF; (c) NCF; (d) GIF2 (where Y-axis represents specifications ratio between the probe with fabrication errors and the probe without fabrication error, and DOFG, MBD, and WD represent relative values of DOF gain, minimal beam diameter, and working distance, respectively)
3.3 模式干涉场的两种放大方式
工作距是OCT内窥探头的关键指标之一。OCT在生物组织中的穿透深度约为1 mm,因此将探头的工作距提高到0.5 mm或以上最为理想。上述基于光纤型空间滤波器的探头在空气中的工作距为0.13 mm,但这一工作距仍存在一定的提升空间,而进一步提高其工作距有望拓宽其应用范围。
对于尺寸越小的光学系统来说,为保持其横向分辨率,其工作距一般也越短。因此,对于直径为125 μm的全光纤探头,为了实现优于4.4 μm的横向分辨率,要求其在空气中的工作距仅约为100 μm。在传统的全光纤探头中,无芯光纤通过衍射效应来扩大光束,以增加探头的工作距。但由于光纤透镜通光孔径的限制,单纯通过增加无芯光纤的长度来增加工作距的效果有限,而且过长的无芯光纤将导致光的传输效率降低。在3.2节基于光纤型空间滤波器的探头中,模式干涉场(MIF)被用于调控光纤透镜入瞳处的光场。通过在光纤透镜的入瞳处形成非高斯光束的光场分布,基于光纤型空间滤波器的探头有望缓解传统全光纤探头横向分辨率与工作距的矛盾。但对于
图 9. 基于模式干涉场成像放大的探头的原理图[20]
Fig. 9. Schematic of probe with a mode interference field expanded by imaging[20]
为了说明对MIF进行成像放大的必要性,这里提出6种典型的局部优化设计,其中探头Ⅰ、Ⅱ、Ⅲ对MIF进行成像放大(LGIF2=390 μm),而探头Ⅳ、Ⅴ、Ⅵ对MIF进行衍射放大(LGIF2=0)。为了使MIF被充分放大以充满GIF3的入瞳,LNCF被统一设置成300 μm。由于LNCF较长,探头Ⅳ、Ⅴ、Ⅵ的工作原理实际上更接近
图 10. 两种模式干涉场放大方式下探头出射光束的光强分布[20]
Fig. 10. Light intensity distributions of output beams from probes with mode interference fields expanded by two different ways[20]
表 3. 基于模式干涉场成像放大和衍射放大的探头的出射光束参数
Table 3. Output beams parameters from probes with mode interference fields expanded by imaging and by diffraction
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与3.2节基于光纤型空间滤波器的探头相比,基于衍射放大的探头Ⅵ将NCF从原来的150 μm增加到300 μm,使其工作距从原来的0.11 mm增加到0.14 mm。然而,单纯增加无芯光纤的长度导致其DOF拓展倍数下降至1.6。考虑到制作误差,探头Ⅵ的实际DOF拓展倍数可能更低,因此,本课题组认为通过牺牲DOF拓展倍数来换取工作距是难以接受的。通过优化GIF1和LCF的长度能在一定程度上增大DOF的拓展倍数。例如,对于同样采取衍射放大的探头Ⅴ来说,其优化后的工作距和DOF拓展倍数分别提升至0.15 mm和1.9。但如
图 11. 所制作的探头的显微图以及柠檬果肉的OCT成像效果图[20],其中黄色箭头表示探头端面,浅蓝色框线表示轴向视场。(a)基于成像放大的含滤波器的探头;(b)不含滤波器的传统探头
Fig. 11. Microscopy images of fabricated probes and OCT imaging of fresh lemon under the probes[20], where the yellow arrows indicate the end facets of the probes and the light blue dotted boxes represent the axial field of views. (a) Probe with a filter based on imaging expansion; (b) traditional probe without filter
3.4 双模干涉与多模干涉
通过引入更多的模式以及更高阶的模式有望获得更大的DOF增益;但是模式数量的增加,一方面意味着需要使用纤芯更粗和数值孔径更大的LCF,从而导致模间色散迅速增大,另一方面使MIF及其调控变得更加复杂,从而增大了探头优化的难度。本节主要解决在可接受的模间色散(小于OCT系统轴向分辨率的一半)前提下多模干涉探头的优化设计问题。
典型的双模干涉探头结构如
图 12. 具有不同模式数量和模式功率分布的探头设计。(a)双模干涉探头;(b)更低阶多模干涉探头;(c)更高阶多模干涉探头
Fig. 12. Probe designs with different mode numbers and mode power distributions. (a) Probe with dual-mode interference;(b) probe with lower-order-multimode interference; (c) probe with higher-order-multimode interference
与3.3节仅对双模干涉探头的LLCF进行全局优化不同,为了确保搜索到最优的探头参数组合,这里分别对更低阶多模干涉探头[如
表 4. 更低阶多模干涉和更高阶多模干涉探头的最优出射光束参数
Table 4. Optimized output beams parameters from the probes with lower-order-multimode interference and higher-order-multimode interference
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图 13. 更高阶多模干涉探头的出射光束与具有相同MBD的高斯光束的光强分布及参数对比[21]。(a)更高阶多模干涉探头出射光束的光强分布;(b)轴上光强曲线;(c)(d)轴向位置-横向分辨率曲线及虚线框区域的放大图;(e)工作距位置处的横向光强曲线
Fig. 13. Light intensity distribution and parameters comparisons between output beam from the probe with higher-order-multimode interference and Gaussian beam with a same MBD. (a) Light intensity distribution of output beam from the probe with higher-order-multimode interference; (b) axial light intensity curve; (c)(d) curve between axial positions and lateral resolution and zoom in view of the boxed area; (e) lateral intensity curves at the working distance
4 讨论与结论
本文系统研究了LCF探头的优化设计。与二元相位滤波器通过调节各环带相位实现光束调控[9]相似,LCF能通过独立调节各模式相位实现光束调控。此外,通过设计LCF的模式激发装置,还能实现模式幅值的调控。随着LCF中主要模式阶数的增加,高阶模式由于更接近Bessel光束而更容易获得较大的DOF增益,但同时出射光束的旁瓣强度也越大。而如何同时实现较大的DOF增益、较小的旁瓣强度和较高的光传输效率是设计模式激发装置的中心问题之一。作为一种光纤型滤波器,基于LCF的探头兼容常规光纤处理工艺,因此具有制作成本低、容易批量生产的特点。LCF与光纤相位板[15,29]相同,都通过调节长度来调控相位,但LCF的折射率对比度远小于光纤相位板的折射率对比度,因此基于LCF的探头对光纤长度相对不敏感。仿真结果显示LCF探头一般具有-28~+20 μm的制造误差,不仅进一步减小了制作难度,而且探头的制作优良率也会有所提升。与现有多模光纤探头技术[16]相比,本文所提出的LCF探头尺寸更小(外径仅为0.125 mm,是前者的1/4),而且其通过精细调控模间相位差,对由干涉相消引起的潜在轴向光强不均匀进行了改善。另外,由于LCF的芯层直径相对较小(25~50 μm),光束传播的衍射效应明显,因此基于几何光学以及菲涅耳衍射积分公式的仿真和设计方法不能准确计算和优化LCF探头的出射光场。设计LCF探头的另一挑战前文已提及,就是需要预先给定一组探头参数,然后基于此进行光传播仿真,才能确认出射光束的性能。因此,这种正向优化过程需要尝试所有可能的参数组合才能获得探头的最优参数。为此,本课题组提出了基于特征模展开的快速仿真方法,并提出了实用的束缚条件和优化函数,实现了探头参数的全局优化。只要LCF允许的模式数量足够多,理论上可以获得任意复杂的出射光场,并可以实现所需的工作距、横向分辨率和轴向视场。但随着LCF模式数量或者主要模式阶数的增加,其模间色散将随着LCF长度的增加而迅速增加,并轻易超出OCT的轴向分辨率,导致模式相位差的调节范围受限。因此,在可以接受的模间色散下,LCF探头存在一个DOF增益上限。
在LCF的模式激发装置方面,本文首先研究了基于拉锥光纤的光束可控探头。其中LCF通过拉锥工艺以及与SMF熔接得到,并且通过控制拉锥段的长度实现LCF内模式幅值的调控。在后来的光纤型空间滤波器设计中,改用具有与SMF外径相同的折射率渐变光纤实现SMF与LCF之间的光束耦合。由于后者具有均匀的探头外径,其机械强度显著提升。与拉锥光纤类似,渐变折射率光纤对双模干涉探头中LP02模与LP01模幅值比的调控范围为0.2~1.3。对于多模干涉探头,由于渐变折射率光纤只能减小光束的发散角,单纯依靠渐变折射率光纤无法激发LCF的更高阶模式。为了激发LCF的更高阶模式,在渐变折射率光纤之前增加了一段无芯光纤,并且先让SMF的出射光束在无芯光纤中充分放大后再由渐变折射率光纤聚焦成束腰直径更小、发散角更大的光束,最终在LCF中激发出包括LP02~LP05模的主要模式。
表 5. 工作模式、有无光束中继、光纤模式数量对LCF探头优化结果的影响
Table 5. Effects of work modes, being free of beam relay or not, and the number of fiber mode on optimized result of probes
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LCF末端的模式干涉场需要放大,以获得更长的工作距。通过研究两种模式干涉场的放大方式,包括传统的衍射放大和成像放大,本课题组发现基于成像放大的探头具有更长的工作距(增加至0.20 mm)和更大的DOF增益(增加至2.3)。至于衍射放大的探头,由于LCF末端的模式干涉场既不在探头物镜的前焦面附近,又不是关于前焦面的光学成像共轭面,其工作模式更接近上述无透镜探头基础上的光束中继。虽然其工作距从原来的0.11 mm增加到0.14 mm,但其DOF增益却从1.8下降到1.6。因此,与无透镜模式相比,空间滤波模式更有利于获得较大的DOF增益。
由于增加模式数量更有利于获得较大的DOF增益,在最后的探头优化中,本课题组不再局限于双模干涉。仿真结果表明,无论是更低阶多模干涉探头,还是更高阶多模干涉探头,它们都在DOF增益方面相较于双模干涉探头表现出了显著提升(前者的DOF增益为3.4,后者的DOF增益为3.8),但更高阶多模干涉探头具有更低的旁瓣强度,为主瓣强度的26%。因此,LCF探头的最优设计为更高阶多模干涉探头,其外径为0.125 mm,硬端长度为1.5 mm。在1.3 μm的中心波长下,该探头的横向分辨率为4.0 μm,工作距为0.19 mm,轴向视场为0.38 mm,旁瓣为26%,且具有均匀的轴向光强。
基于LCF的探头技术由于具有光束调控灵活和制造难度较小的优点,在优化微型OCT探头的出射光场和提高成像质量方面具有巨大潜力;但由于LCF与自聚焦光纤、无芯光纤之间存在复杂的排列组合,因此LCF的优化设计存在一定难度。本文系统阐述了LCF探头的优化方法。通过比较不同探头设计的横向分辨率、工作距和DOF增益这三个主要性能指标,本课题组认为空间滤波器的工作模式、对光束进行中继以及更多的模式更有利于提高探头的性能。除了OCT成像外,本文所涉及的优化方法还可以用于激光扫描成像系统、光声成像系统出射光束的优化。本课题组的一些工作对于激光器-光纤耦合器、光镊等非成像领域亦具有潜在的技术启示。
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Article Outline
邱建榕, 杨晨, 章程, 孟佳, 韩涛, 钱黄河, 陈沛哲, 杨璐, 刘智毅, 丁志华. 光学相干层析成像大纤芯光纤探头的优化研究[J]. 中国激光, 2022, 49(20): 2007201. Jianrong Qiu, Chen Yang, Cheng Zhang, Jia Meng, Tao Han, Huanghe Qian, Peizhe Chen, Lu Yang, Zhiyi Liu, Zhihua Ding. Optimization of Large-Core-Fiber-Based Fiber Probe for Optical Coherence Tomography[J]. Chinese Journal of Lasers, 2022, 49(20): 2007201.