面向分布式传感的散射增强光纤研究进展特邀综述
Fiber optic sensing technology has been widely applied in multiple fields and has received good feedback due to its advantages of strong anti-interference ability, small size, high sensitivity, long transmission distance, and intrinsic safety. Distributed sensing technology (OTDR, Φ-OTDR, and OFDR) based on Rayleigh scattering can achieve long-distance, large-scale, and multi-parameter monitoring, which has attracted more attention in applications. With the increasing demand for monitoring length and range in various application fields, the traditional methods of increasing light source power and detector detection limit have reached their peaks in increasing the system distance. The development of new scattering enhancement methods is urgent, so as to enhance the sensing distance of distributed sensing technology.
We review several ways to enhance the scattering light in fibers by enhancing their scattering coefficients and backscattering ability from the perspective of fibers, as well as the limitations and application scenarios of these methods. We also provide a detailed introduction to the latest scattering enhancement method, which enhances scattering by enhancing the backward collection coefficient and has potential development prospects in future distributed sensing.
The research progress of fiber optic scattering enhancement methods is as follows.
1) Enhancing fiber scattering by increasing the scattering coefficient. It is commonly used to increase the scattering coefficient of optical fibers through irradiation, microstructure, and nanoparticle doping to enhance the Rayleigh backscattering light of the fibers.
The irradiation method is to increase the refractive index disturbance in the fiber by ultraviolet or radiation irradiation. It is simple to operate and has continuous scattering enhancement. However, it will increase the loss of the optical fiber and reduce the sensing distance. At the same time, the preparation speed of the optical fiber is slow, requiring the removal of the coating layer and resulting in a decrease in mechanical strength. Therefore, the scattering enhanced fiber prepared in this way is difficult to apply to engineering environments.
The microstructure method refers to the formation of weak gratings, reflection points, Fabry Perot cavities, and other junction microstructures in optical fibers through ways such as ultraviolet, femtosecond, and arc discharge, resulting in significant refractive index changes. This method is flexible and has higher controllability, and it can be continuously prepared in large quantities without removing the coating layer and changing the mechanical strength of the optical fiber. However, it still increases the loss of the optical fiber and reduces the sensing distance, and the distribution of microstructures in the optical fiber is discrete, forming a minimum sensing area between two adjacent points, which reduces the spatial resolution of the distributed sensing system. This method is suitable for applications in sensing scenarios that do not require high spatial resolution.
The doping method of nanoparticles increases scattering in fibers by doping elements such as germanium, calcium, barium, gold, and magnesium. It has continuity, and the scattering enhancement is more obvious. In addition, it can be directly prepared through fiber drawing, which ensures the mechanical properties of the fiber. However, the high scattering enhancement also brings about a significant increase in losses. The losses of nano-doped fibers are generally two or three orders of magnitude higher than those of irradiation and microstructure and are generally applied in sensing scenarios with short distances and high signal-to-noise ratios.
2) Enhancing fiber scattering by increasing the backscattering collection coefficient. The method of increasing the backscatter collection coefficient to enhance fiber scattering theoretically does not increase the loss of the fiber, which mainly includes three types: plastic optical fiber, multimode optical fiber, and ultra long adiabatic tapered optical fiber.
Both polymer fiber and multimode fiber can increase the backward collection coefficient by increasing the numerical aperture, but the material absorption loss of polymer fiber itself is greater than that of quartz fiber. Therefore, it is generally applied in short-distance sensing scenarios. Multimode optical fibers have significant mode losses, and dispersion over long distances can degrade the spatial resolution of the system. It is commonly used in scenarios with lengths of kilometers.
Our team has proposed an ultra long tapered single-mode fiber that can increase the backward collection coefficient of the fiber to enhance scattering, without causing external losses. It can break through the distributed sensing long-distance limit of single-mode fiber and achieve sensing with an equal scattering signal-to-noise ratio at each point, and it can be applied to ultra long sensing scenarios with a length of above 150 km. Ultra long tapered single-mode fiber also has the advantage of enhancing the performance of fiber Bragg grating (FBG) arrays. Engraving FBG arrays on tapered optical fibers can effectively increase the remote reflection signal of FBG and expand the number of arrays, which has great development potential for future high-tech composite distributed sensors.
1 引言
自1970年世界上第一根低损耗光纤问世，光纤在通信和传感领域得到了飞速发展。凭借其抗干扰能力强、体型小、敏感度高、传送距离远、本征安全等优势，光纤传感技术已广泛应用于多个领域并得到良好的反馈。光纤传感技术根据传感方式可分为准分布式和分布式两类，准分布式传感方式的典型代表是光纤光栅和法布里-珀罗（法珀）腔，它们常应用于传感器数量为数十甚至上百的监测场景。随着石油化工、电力土木等领域对传感距离和空间分辨率需求的剧增，准分布式传感技术难以满足需求，分布式传感技术受到越来越多的关注。20世纪70年代末，随着光时域反射（OTDR）技术的出现，分布式光纤传感技术得到了迅速发展，出现了基于瑞利散射、拉曼散射及布里渊散射的分布式光纤传感系统，这些系统被广泛应用于温度、应力及其他传感领域。拉曼散射主要用于温度测量，布里渊散射在温度及应力传感方面都有应用，然而这两种散射光相对较弱，对于信号检测及后续处理要求较高，而瑞利散射光相对较强，在长距离传感应用上具有优势。
采用瑞利散射的光纤传感系统主要包括OTDR、相敏光时域反射（Φ-OTDR）和光频域反射（OFDR），它们可以通过感知外界参量引起的散射光强度、偏振及相位［1-2］的变化实现传感，偏振和相位的改变最终通过光强的变化来实现探测。光纤中的散射光信噪比（SNR；RSNR）是分布式传感系统的重要参数，直接决定了系统的极限传感长度，其由光源性能、散射强度、损耗和噪声共同决定，如
图 1. 瑞利散射传感分布式系统中的信噪比
Fig. 1. Signal-to-noise ratio in the distributed system of Rayleigh scattering sensing
光纤中散射源自内部存在的密度波动和折射率不均匀分布，当功率为P的探测光脉冲从被测光纤的一端通过耦合器或者环形器注入到被测光纤时，经过散射点发生散射，后向散射光经过原来的传播路径返回，被光电探测器接收，如
式中：z为光纤中发生瑞利散射的位置，亦为散射位置光纤初始端的距离；P0和τ分别为入射光脉冲功率和宽度；ν为光传播的群速度，为常数；αs和S分别为光纤瑞利散射衰减系数和后向散射收集系数；λ为入射光波长。
在光纤中，P0和τ是可以通过优化光源进行控制的，但其因受到光纤功率和非线性的限制存在上限，ν在光纤中为常数。因此，光纤中的散射光强度主要通过控制αs和S参数实现改变，这也是现阶段散射增强光纤的重要控制方式（
2 基于增加光纤散射系数的散射增强光纤
增强光纤中后向散射的最直接方式就是增加光纤中的不均匀性，产生更多的散射光，主要的方式有光纤辐照、写入微结构和掺杂纳米粒子。
2.1 光纤辐照
采用辐照光来控制光纤中的散射是最早运用的方法，其源于光纤的光敏性。从玻璃的微观结构来分析掺锗石英玻璃的光敏性主要源于玻璃结构中缺陷对紫外光的吸收。在锗硅酸盐玻璃中，Si、Ge与O形成四面体结构，每个四面体通过顶角氧连接形成空间网络结构［11-12］。当光纤中Ge以低价氧化物状态存在时，玻璃中会产生很多点缺陷，主要为中性氧空位缺陷和Ge2+缺陷。在紫外光的照射下，这些点缺陷吸收紫外光子能量，缺陷结构发生变化，变形的反应过程如
图 4. 光纤紫外光照射下缺陷变化模型
Fig. 4. Model of defect changes in optical fibers under ultraviolet light irradiation
早在1999年，Johlen就发现紫外曝光能够在光纤内部构成缺陷，从而引起瑞利散射光强度的提升［13］。载氢之后光纤的紫外光敏特性还可进一步增强［14］，再经过简单的紫外曝光可将瑞利后向散射光增强20 dB［15］，将光纤应变和温度传感分辨率提高一个数量级，这显示出散射增强光纤信噪比增强在分布式传感运用中的极大价值。2022年，美国 OFS公司报告了面向工程应用的1.5 km长度散射增强光纤，可实现散射增强7 dB左右，应用于DAS中声信噪比提高了39 dB，但光纤衰减提高至0.5 dB/km，这超过了普通单模光纤的衰减［9］。深圳大学王义平课题组［16］将散射增强光纤应用到OFDR传感系统中，实现散射增强37.3 dB，OFDR空间分辨率为2 mm，显示出散射增强光纤极大的应用潜力。
除了紫外光，射线辐照光纤也能引起光纤中缺陷数量的增加。2011，上海大学Wen等［17］研究发现，低水峰单模光纤在伽马射线辐照下瑞利散射系数增大了5倍，并证实了辐照瑞利散射改变主要是由电子在Ge和O原子周围电荷密度再分配引起的缺陷中心引起的，这项工作为光学损耗提供了一个新的解释，并揭示了辐照对瑞利散射影响的新机制，为增强光纤中的散射提供了新手段。在此基础上，北京航空航天大学Jin等［18］对100 m的Ge/P共掺杂光纤进行辐照并用于分布式传感，通过OTDR测试得到光纤的损耗为68.8 dB/km，这大大超过了单模光纤，散射光纤传感长度仅为100 m。
辐照增强光纤散射的方式可以很容易地用激光或者辐射源诱导，无需对准，操作简单。这种对整根光纤进行连续紫外曝光的方法，虽然能够大幅增强后向散射光信号强度，但光纤的损耗系数也随之增大，从而显著缩短了传感距离，且连续曝光方式的制备成本也较高，制造速度非常慢（大约为每秒数百微米），并且需要剥离纤维护套，降低了光纤的抗拉强度，因此应用到工程的长距离分布式传感中受到一定限制。
2.2 写入微结构
辐照光对光纤纤芯结构的折射率和密度控制具有随机性，借鉴于光纤中特殊结构［光纤光栅（FBG）和法珀腔］对输入光的反射作用，且光纤在线刻写技术［19］不断完善和成熟，在光纤中周期性写入微结构阵列可以进一步增强光纤中折射率和密度的不均匀性，从而增强散射，微结构阵列包括FBG阵列、法珀腔阵列和菲涅耳反射点，如
随着高功率激光技术的发展，高功率激光作用到光纤纤芯可以形成FBG、菲涅耳反射点结构，产生永久性的折射率改变和缺陷［25］，有效解决WFBG退化的问题。2017年，Yan等［26］使用飞秒激光诱导光纤纤芯纳米光栅的形成，其后向散射增强了40 dB，但传输损耗为15~41 dB/m。2019年，华中科技大学孙琪真团队［27］制备了具有大谱宽和离散分布位置［28］、强度可调［29］的微结构散射增强光纤来提升光纤的适用性，光纤散射增强可达30 dB。随后，王义平团队［30］尝试使用飞秒激光在标准单模光纤（SMF）中诱导永久的散射改变，光纤的瑞利后向散射强度提升了26 dB，损耗为3 dB/m。除了FBG阵列和菲涅耳反射点，其他微结构也可以实现相同的散射增强效果。超弱法珀干涉仪（IFPI）阵列可以提升信噪比20 dB［31］，纳米反射器可实现35 dB的散射增强（2.5 km长度上损耗为0.60 dB/km）［32］。微结构能有效增强光纤中的散射，微结构的热稳定性也被验证，在800 ℃的高温下飞秒激光刻写的散射增强光纤信号稳定［26］，张建中团队［33］采用飞秒激光在光纤中写入弱反射点，有效增强信噪比约30 dB，并成功应用于1000 ℃下的温度测量，这说明激光加工微结构的散射增强可以是永久的，相比FBG阵列，其环境适用性更强。激光写入微结构的方式简洁高效，但设备较为昂贵，能量利用率低，因此一种更简单、高效、成本低的替代方式——电弧放电法受到关注。光纤经过电极电弧放电，产生的高温热量加热熔融光纤，从而产生微小形变并诱发了折射率的改变，同时纤芯和包层的热膨胀系数差异导致残余应力释放，也会引起折射率的改变［34-36］。电弧放电在制备光纤锥、探头、微结构和长周期光纤光栅等方面应用广泛［37］，张建中团队［38］采用电弧放电方法在保偏光纤中产生强度可控的弱极化模耦合点，通过控制电弧放电强度和时间可实现近20 dB的散射增强，这为光纤结构上批量化制备弱散射点阵列提供了新的方案。
相对于辐照增强光纤中的散射，在光纤中写入微结构方式的可控性更强，灵活高效，无需剥除光纤涂覆层，光纤强度更高，同时得益于光纤加工技术的发展，光纤中微结构的刻写和控制技术日趋成熟，可以定制化各点的散射增强，实现分布上的散射控制。但微结构的分布在光纤中是离散的，两个相邻点之间构成最小传感区域，这降低了分布式传感系统的空间分辨率。
2.3 掺杂纳米粒子
光纤中的散射来自纤芯的密度起伏和不均匀，常用的辐照和微结构方式是借助激光对纤芯处理产生结构上的改变，但这种处理方式不稳定，控制难度较大。光纤中掺杂纳米粒子可以通过控制掺杂粒子密度和粒子尺寸直接控制折射率的不均匀分布，且可以直接制备出散射增强光纤，成为近年来广受关注的一种散射控制方式。早在1974年，美国Friebele等［39-40］就发现在光纤的制备过程中，可以通过掺杂一些金属离子来增强光纤的衰减特性［41］。这种方式控制光纤中的散射是连续的，可以很好地解决离散增强方式带来的空间分辨率降低的问题。Ge元素是最先掺入光纤的元素，物质的量分数为98%的GeO2掺杂纤维被验证具有29.2 dB散射信号强度［42］。除此之外，氧化镁（MgO）掺杂光纤由于制造方法相对简单，而且纤维可设计得与SMF纤维的尺寸相匹配，近年来获得广泛的关注。法国的Blanc等［43-45］对MgO掺杂光纤的制备流程、散射性能进行了一系列的研究。拉制过程中，温度较高，Si和碱土离子的分离产生富含MgO球形颗粒的化合物，该化合物在纤维中拉长，颗粒尺寸在几纳米到几百纳米水平［46］。其中纳米颗粒的尺寸直接决定了诱导的弹性散射和光学损耗，与SMF-28相比，20~160 nm的Mg纳米颗粒纤维增强了46.1 dB~47.5 dB的后向散射，但光损耗为292~298 dB/m［47-48］，而直径为100 nm的颗粒的后向散射增强强度为32.3 dB~45.2 dB，损失为27.8~33.1 dB/m［49-52］。最近，MgO基掺杂光纤的损耗被优化，衰减降低为14.3 dB/m，散射增量为48.9 dB，可以实现2.4~4.0倍的传感长度扩展［53］。除此之外，加拿大的Fuertes等［54-56］研究了Ga、Ba掺杂对光纤散射的影响，并在预制和光纤制造过程中改变纳米颗粒的特征，如颗粒分布大小、形态和密度，来进一步调控散射光强度［54］，光纤的损耗可以降低至0.1~0.2 dB/m，传感距离可延长至数百米，性能要优于Mg基掺杂光纤。荷兰的Wang等［57］开始开发具备传感性能的Au掺杂光纤器件，其性能与普通FBG接近，这促进未来Au掺杂光纤传感器件的发展。
纳米粒子掺杂光纤的散射增强具有连续性，散射增强更明显，可以通过光纤拉制直接制备，保证了光纤的力学性能。但高散射增强同时带来的是损耗剧烈增加，纳米掺杂光纤的损耗普遍高于辐照和微结构2~3个数量级，在提升分布式传感器系统信噪比的同时，缩短了传感距离，因此纳米掺杂散射增强光纤可适用于短距离的传感领域。由于辐照、微结构和纳米粒子掺杂方式均是通过增加光纤中的散射光实现散射增强，带来的必然结果是增加了光纤中的损耗，在增强散射的同时一定程度降低了传感长度。
综上，通过增加光纤中的缺陷数量和提升不均匀性实现散射增强的方式可以获得令人满意的效果，近年来这种散射增强光纤也开始应用到工程实践中，
表 1. 利用增加散射强度方式的散射增强光纤发展现状
Table 1. Current development status of scattering enhanced fiber by increasing scattering intensity
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图 6. 各种方式制备散射增强光纤的散射谱估计
Fig. 6. Scattering spectrum estimation of scattering enhanced fibers prepared by various methods
3 基于增强光纤后向收集系数的散射增强光纤
除了利用直接增大光纤散射系数的方式增强散射，还可以考虑控制光纤中散射后向收集能力的方式实现散射增强。这种方式可以避免较大损耗的产生，保持传感光纤的传感距离不受限制。后向收集系数S与光纤数值孔径（NA）和模场半径（w0）有关，直接受到光纤的相对折射率差（Δ）和半径（r）控制［58-60］，即
式中：Tf为假想温度；nco为纤芯折射率；V为归一化频率；kB为波尔兹曼常数；βT为绝热压缩比。结合
图 7. 光纤中的信噪比与半径和相对折射率差的关系
Fig. 7. Relationship among signal-to-noise ratio, radius, and relative refractive index difference in optical fibers
光纤半径的改变直接带来的是模场半径（R）的变化，折射率的改变直接带来的是数值孔径（NA）的变化。光纤中数值孔径和模场半径与散射收集系数的关系如
图 8. 光纤散射后向收集系数模型。（a）数值孔径；（b）模场半径
Fig. 8. Model of backward collection coefficient of fiber scattering. (a) Numerical aperture; (b) mode-field radius
采用增大数值孔径方式增强散射的光纤有聚合物光纤和多模光纤。聚合物光纤的直径能到1 mm，更大的散射截面［61］可收集更多的散射光，获得比普通石英光纤高31 dB的散射增强［62］，但聚合物光纤本身的吸收损耗和散射损耗大于石英光纤，损耗达到9.2~15 dB/km［63］，这说明聚合物光纤的材料增强了光纤的散射能力，因此聚合物光纤传感长度仅为数百米［64-65］，难以满足长距离传感需求。在石英光纤中多模光纤相对于单模光纤具有更大的数值孔径和较高的非线性阈值功率，具有更大的散射光俘获系数和支持更高的注入泵浦功率［66］，因此广泛应用于分布式传感中的分布式温度传感器（DTS）［67-68］、分布式声学传感器（DAS）［69］、分布式振动传感器（DVS）［70］以及复合传感器（DAS-DTS）［71］。对多模光纤结构的掺杂可以优化分布式传感性能，如
图 9. 多模光纤增强后向散射收集系数。（a）多模光纤结构；（b）散射谱
Fig. 9. Multimode fiber enhances the backscattering collection coefficient. (a) Structure of multimode fiber; (b) scattering spectrum
本课题组从光纤轴向折射率和半径分布入手，控制光纤长度方向上的俘获系数，提出一种长距离散射增强单模光纤——超长绝热锥形单模散射增强光纤（ULTF）［79］，通过增加光纤散射后向收集能力来增强散射，不带来额外的损耗。ULTF的一个案例如下：光纤长度为12.6 km，半径从4.50 μm线性单调递减至3.00 μm，光纤各个位置的归一化频率相同，均为2.166，ULTF的散射谱如
图 10. 超长绝热锥形光纤设计增强散射。（a）半径为4.5~3 μm、长度为12.5 km的ULTF的散射谱；（b）不同小端半径的散射增强结果和光纤长度
Fig. 10. Design of ultra long adiabatic tapered fiber to enhance scattering. (a) Scattering spectra of ULTF with a length of 12.5 km and radius of 4.5-3 μm; (b) scattering enhancement results and fiber length for different small end radius
在此基础上，设计并拉制ULTF用于验证散射增强光纤设计的正确性，光纤具体参数如
图 11. 超长绝热锥光纤的参数变化。（a）半径分布；（b）散射谱；（c）散射增强值
Fig. 11. Parameter changes of ULTF. (a) Radius distribution; (b) scattering spectrum; (c) scattering enhancement value
ULTF还具有调控FBG阵列反射光的能力，如
图 12. 锥形FBG阵列。（a）示意图；（b）光谱；（c）散射谱
Fig. 12. Tapered FBG array. (a) Schematic; (b) spectra; (c) scattering spectra
综上，控制光纤折射率和半径可以有效增强散射收集能力，提高接收的后向散射强度。对比各种光纤的散射增强性能，塑料光纤由于本身存在损耗，光纤散射增强了31 dB，最大传感距离仅有6.6 km［62］，多模光纤结构特点导致普遍存在的高阶模式损耗大于单模光纤的损耗，3 dB散射增强下传感长度能达到100 km［74］，但都小于单模光纤的传感长度150 km。本课题组提出的超长绝热锥形单模光纤不额外增加光纤中的损耗，预期能达到7 dB的散射增强，传感长度可达到154.2 km，可以突破单模光纤的传感距离极限，增强分布式传感系统的性能。
4 结论和展望
各种散射控制方式的对比如
图 13. 各种散射增强光纤的增强方式对比
Fig. 13. Comparison of various scattering enhanced fiber enhancement methods
散射增强光纤能有效增强分布式传感系统的信噪比，带来系统空间分辨率、传感精度的提升，并逐渐开始应用到工程实践中。散射增强光纤未来将朝着更高性能、更多功能和更多应用的方向发展：
1）光纤散射增强技术的开发。散射增强光纤的潜在优势可以极大地提升分布式传感系统的传感性能，特别是为各种分布式传感系统定制传感光纤，满足不同的传感需求，如传感长度、空间分辨率等。现阶段各种散射增强方式均存在各自的局限，基于现有的传感技术继续优化增强方案是持续的工作，同时开发新的散射增强技术，丰富散射控制方法，为散射控制应用提供更多选择也是一个重要的研究方向。
2）散射增强光纤的其他性能开发。散射增强光纤的散射增强能力是分布式光纤的需求，但分布式系统往往有许多限制，比如布里渊传感系统的阈值问题，激光器系统中的非线性问题，研究散射增强光纤的其他特性（偏振、色散、非线性等），丰富传感光纤的功能，进一步开发传感光纤在其他器件、系统的应用，将传感光纤发展为器件光纤，进一步扩大传感光纤的内核，可向集成化、功能化光纤发展。
3）散射增强光纤的应用开发。散射增强光纤的目的是增强光纤中的散射，优化分布式系统的性能，从而提升实际应用环境中的监测性能，将传感光纤技术向工程应用转化，形成研产循环，才能为传感光纤的开发提供源源不断的动力，推动散射增强光纤的长远发展。
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