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OLTS 和 OTDR：完整的测试战略

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概述

Fiber is playing an increasing role in most network installations, driven by the need for higher-bandwidth applications in data centers and backbone cabling systems, as well as emerging low-latency 5G and FTTX deployments in service provider networks. While copper continues to dominate horizontal cabling systems where few devices require more than 10 Gbps and many are powered via Power over Ethernet (PoE), the use of fiber cabling systems is on the rise wherever speeds are reaching 40 and 100 Gbps and beyond, or wherever there is a need for greater distance, noise immunity, and security. According to recent studies, the global fiber optics market is projected to reach USD 13.26 billion by 2033, up from USD 8.07 billion in 2023.

As fiber deployments become commonplace, network owners and technicians are paying more attention to the two crucial devices for testing fiber optical cables: the Optical Loss Test Set (OLTS) and the Optical Time Domain Reflectometer (OTDR).

• An OLTS provides the most accurate insertion loss measurement on a link by using a light source on one end and a power meter at the other to measure precisely how much light is coming out at the opposite end. 它要求按工业标准进行光纤测试。TIA 和 ISO 标注使用属于“1 层”描述 OLTS 测试。


• OTDR 通过将光脉冲传输到光纤中并测量每个脉冲反射的光量，表征各个焊接点和连接器的链路丢失情况。建议按工业标准进行光纤测试，这对于新兴的短距离单模应用至关重要，并且作为完整测试策略的一部分非常有价值。OTDR 和 OLTS 测试是指 TIA 标准的“2 层”测试，以及 ISO 标准中的“延伸”测试。

While the measurements taken by these two instruments seem similar, they perform distinct and essential roles. This article explains how these testers work, when to use them, and how they complement each other to ensure the performance of fiber optic links and maximizing customer satisfaction.

本页内容

• 什么是 OTDR？
• What Is an OLTS?
• Why You Should Test for Reflectance in Addition to Insertion Loss
• How to Read an OTDR Trace
• What Are the Benefits of Using an OTDR?
• How OLTS and OTDR Work Together
• The Advantages of Integrated OLTS and OTDR Test Results
• 附录：The Importance of Fiber Cleaning and Inspection
• 附录：What Is OTDR Event Mapping?

什么是 OTDR？

Unlike the OLTS, which measures the amount of light coming out of the far end, the OTDR measures the amount of light reflected back to the source. 通过计算近端和远端反射数量的差值，OTDR 可以推断出光纤中的损耗量。

OTDR 通过特制的脉冲激光二极管传输高功率光脉冲至光纤。Most light travels away from the source as the pulses travel down the fiber. High-gain light detectors in the OTDR measure any light that is reflected from each pulse. OTDR 通过上述测量方法，探测光纤中使源脉冲功率减弱或反射的事件。The normal structure of the fiber and minor defects in the glass also cause a small fraction of the pulse light to scatter in a different direction. 这种光被纤维中的杂质散射的现象被称为反向散射。

当光脉冲遇到连接、破损、破裂、接头、锐弯头或光纤末端时，会因折射率的变化而进行反射。这种反射称为 Fresnel 反射。Reflectance is the amount of light reflected, not including the backscatter, relative to the source pulse. It is expressed in dB units as a negative value for passive optics, with values closer to 0 representing larger reflectance, poorer connections, and greater losses. This measurement is the same as return loss, which is expressed as a positive value. Return loss indicates how much signal is lost by comparing input power to output power versus reflectance, which compares output power to the amount of light reflected. 对于反射率和回波损耗，此值与零的差值越大，则结果更好。(For more information on the difference between reflectance and return loss, read What is Return Loss?)

What Is an OLTS?

An OLTS is a mainstay for testing fiber optic cabling because it provides the most accurate method for determining the total loss of a link. It’s required by industry standards to ensure the link can meet the loss requirements for a given application.

An OLTS includes a light source and a power meter. The light source produces a continuous wave at specific wavelengths connected to one end of the fiber. The power meter uses a photodetector connected to the opposite end of the fiber link, which measures optical power at the same wavelengths the light source produces. These two devices work in concert to determine the total amount of light lost.

^{图 1：OLTS 测量时，在链路一端使用光源，在另一端使用功率计。Models such as the CertiFiber™ Pro maximize testing speed by testing two fibers simultaneously (duplex) using a light source and power meter at each end. Together, they determine the total amount of light lost on a link.}

Industry standards specify insertion loss limits for specific fiber applications as a combination of a loss budget and length. As required by both TIA 568-3.D and ISO/IEC 14763-3 standards for Tier 1 fiber optic testing, the loss measured with an OLTS is compared to the loss limits for a given application to determine if it passes. Note that a basic light source and power meter (LSPM) also accurately measures loss per industry standards, but it does not include some of the key OLTS features that facilitate testing, such as duplex testing, hands-free bidirectional testing, preloading of loss limits, length measurement, and other advanced features. Length is especially important, because application limits are a combination of a loss budget and maximum length. Models such as the Fluke Networks CertiFiber™ Pro measure both loss and length, providing a clear pass/fail result that assures application support.

^{图 2：OLTS 提供的结果显示光纤长度（如两类光纤）以及整体光耗（dB 为单位）。}

For multimode fiber optic testing that involves both lower-order modes (light that travels near the fiber core) and higher-order modes (light that travels closer to the cladding) that are inherently unstable, standards require the use of an encircled flux (EF) light source when using an OLTS. EF 合规光源控制进入线缆的光的模式，最终提供最精准、准确和可重复的测试结果。

Standards also recommend using the 1-jumper reference method when testing with an OLTS, since it includes the insertion loss values for the connections at both ends of the link, which more closely represents how the cabling plant will ultimately be used. The 1-jumper method references out the EF-compliant launch cord from the place where it connects to the light source to where it connects to the power meter.

In contrast, the 2-jumper method references out the connection between the two jumpers; that ultimately includes only one end connection in the loss measurement, providing only a partial depiction of the total loss. 三跳线方法引用两个连接器，排除了测试时两端连接的损耗。Some scenarios, such as testing links with connector types not supported by your testing equipment, will require a 2- or 3-jumper reference. (Read more about reference setting methods in our Demystifying Fiber Test Methods white paper.)

Why You Should Test for Reflectance in Addition to Insertion Loss

反射率对于新型短距离单模应用变得越来越重要，如 100GBASE-DR、200GBASE-DR4 和 400GBASE-DR4。While single-mode fiber applications historically have had larger loss budgets than multimode — 6.3 dB for 100 Gig over single-mode (100GBASE-LR4) versus just 1.9 dB for 100 Gig over multimode (100GBASE-SR4) — that is no longer the case with new short-reach single-mode applications. 这些新应用不仅需要更多了解低插入损耗要求，并且这些限值还取决于反射率。

Multimode transceivers are highly tolerant of reflection, but single-mode transceivers are not. 事实上，对于高功率单模激光器，太多的反射会损坏收发器。

在新的短距离单模应用中，IEEE 根据连接的数量和反射率，指定插入损耗限值。As shown in Figure 3 below, in a 100GBASE-DR4 application with four connectors that have a reflectance between -45 and -55 dB, the insertion loss is 3.0 dB (highlighted in red in the table). But adding four connectors with a reflectance between -35 and -45 dB, the insertion loss goes down to 2.7 dB (highlighted in yellow in the table). 注意：专用 OLTS 可测量反射率、最准确地测量回波损耗，其为正值。OTDR 可测量反射率，其为正数，此值由 IEEE 标准指定。

^{图 3：For emerging short-reach single-mode applications, IEEE standards specify insertion loss based on the number and reflectance of connections.}

How to Read an OTDR Trace

OTDR 可通过绘制反射和反向散射光，以及至光纤的路径以追踪结果，特别是在光纤链路中表征任何反射和非反射活动。

OTDR traces have several common characteristics:

• Most traces begin with an initial input pulse resulting from a Fresnel reflection occurring at the connection to the OTDR.
• Following this pulse, the OTDR trace is a line sloping downward and interrupted by gradual shifts. 随着光沿着光纤传播，插入损耗或反向散射衰减会导致结果逐渐下降。这种下降可能被代表迹线在向上或向下方向上的偏差的急剧移动中断。These shifts, or point defects, are typically caused by connectors, splices, or breaks.
• The end of the fiber is identified by a large spike (reflection), after which the trace drops dramatically down the Y axis.
• Finally, the output pulses at the end of the OTDR trace result from reflection occurring at the output of the fiber end face, referred to as “ghost” events that are technically non-existent events.

As shown in the trace example in Figure 4, the Y axis represents power level, and the X axis shows distance. 当您从左向右读取曲线图时，由于损耗随着距离的增加而增加，因此反向散射值会减小。Interpreting OTDR traces may seem intimidating to novice users, but they don’t have to be; some advanced OTDRs automatically interpret the trace and provide a detailed graphical map of events (see Appendix, What Is OTDR Event Mapping?).

^{图 4：Typical OTDR trace, showing length, a gradual decline in light strength, and events. (A) OTDR connector: Note the large reflectance makes it impossible to characterize the loss in the first connector. In this case, a launch fiber of about 300 ft. is being used. This allows the OTDR to characterize the first connector of the link under test (B). (C) shows two connectors that are too close together for the OTDR to properly characterize the loss in each. (D) is a loss event with no reflectance, likely a bad splice or APC connector. (E) shows a typical UPC connector with reflectance and loss. (F) depicts a connector with reflectance where the signal after the connector is stronger than before, often called a “gainer.” 这表明具有不同反向散射特性的连接光纤类型。(G) 是光纤的末端。Note that the strong reflection makes it impossible to determine if there is a connector there and what its performance is unless a “tail” fiber is attached.}

When using an OTDR, testing is done bidirectionally, since the loss of specific connectors and splices depends on the test direction. Even if two connected fibers are the same type (e.g., OM3, OM4, etc.), the fibers may have slight variations and different backscatter coefficients that can cause more light to be reflected after a connection than before. If OTDR testing is performed in only one direction, it can result in a measured loss value that is less than it actually is or even negative (referred to as a gainer — event F in Figure 4). Likewise, testing in the other direction, where less light is reflected after the connection, can result in a measured loss that is greater than actual loss. That’s why OTDR testing is performed bidirectionally, and the loss results are averaged to get a more accurate result.

当执行双向测试时，务必不断开待测试的发射和接收光纤，以在两个测试中保持相同的对齐方式并确保准确性。Thankfully, testers like the Fluke Networks OptiFiber™ Pro make it easy to test in both directions from one end by using a loop at the remote end of a duplex link and automatically averaging the two readings to provide a final loss measurement.

What Are the Benefits of Using an OTDR?

The OTDR has often been regarded as a troubleshooting tool, and indeed it’s valuable for locating events that are causing performance issues after the cabling plant is live. However, characterizing the entire link via an OTDR trace during initial testing offers several benefits for both the technician and the customer, and it can uncover potential problems that an OLTS may miss.

An OLTS calculates the total loss of the entire link in the most accurate and repeatable way, as required by industry standards, and a PASS or FAIL indicates whether the link falls within the maximum insertion loss for a given application. But specific event losses are completely invisible to an OLTS — which means that a good connection can hide a bad one. 这有何重要性？

A fiber link can contain several connectors and/or splices, and often different technicians perform terminations and splices, some of whom may be more skilled than others. Other disturbances, such as dirty fiber end faces, macrobends, and microbends can also occur within the link due to poor workmanship or other installation factors. Characterizing the fiber with an OTDR makes it possible to pinpoint the location of any fault and verify that the quality of the installation meets the design specifications for current and future applications. It also ensures that there are no unplanned loss events due to poor cable management or installation errors. It allows the technician to see the performance of specific connection points and their locations within the link, so it’s easy to identify any questionable connection points that may need to be addressed due to air gaps, poor fiber core alignment, lack of cleanliness, or other problems that can occur during installation. A link may even pass a loss test yet still fail to carry network traffic due to reflectance issues, and only the OTDR will find the problem. (Learn more in Short-Reach Single-mode Puts Reflectance on the Radar.)

For example, common requirements are that the loss associated with a splice should be no larger than 0.3 dB and that the loss associated with a connector should be no more than the manufacturer’s specification (typically 0.2 dB to 0.5 dB). With today’s stringent insertion loss requirements that have less room for error, identifying the location and loss of specific events in a fiber link becomes more critical than ever — especially considering that total loss can increase over time due to poor cable management, splice degradation, dirty fiber end faces, and even loss of power due to transmitter age.

Characterizing the fiber link with an OTDR also confirms precisely how many connections exist within a link, which is information that cannot be acquired with an OLTS. This is valuable for identifying when a link contains too many connection points due to a cross-connect or links being patched together, which can cause the end-to-end link to exceed loss limits for a given application.

How OLTS and OTDR Work Together

When it comes to fiber testing, if you use an OTDR, is an OLTS still necessary? 是。

Industry standards require the use of an OLTS to ensure application compliance because it accurately measures total fiber insertion loss. Using an OTDR does not replace the OLTS; the total insertion loss measurement achieved with an OTDR is an inferred calculation that doesn’t necessarily depict the total loss that will occur on a link once it is live. 特别是对于多节点光纤，当标准指定精确控制的发射条件时，OTDR 测试的准确性或可重复性不及 OLTS。

When testing or commissioning a sizable number of links, the speed difference between the OLTS and OTDR becomes a significant issue. A high-performance OLTS, such as the CertiFiber Pro, can measure a duplex link at two wavelengths in under three seconds. Even a fast OTDR, like the OptiFiber Pro, will take at least 12 seconds to characterize a fiber. To get an accurate measurement with an OTDR, the fiber must be tested in the reverse direction. The SmartLoop™ capability of the OptiFiber Pro makes this test easy, but it still requires an additional 12 seconds plus the time to swap the launch fibers, for a total testing time that is at least ten times longer than using an OLTS.

On the flip side, if you use an OLTS and the fiber link passes, is an OTDR necessary? 此问题的答案并非如此简单。

首先，务必了解指定项目应遵循的规格。如规格书要求 OTDR 表征（TIA 标准的 2 层测试，ISO/IEC 标准的延长测试），则确实需要 OTDR 以及 OLTS 插入损耗测试。If it is not specified, OTDR testing is technically not required — but it is highly recommended by both industry standards and experts due to the value of characterization and calculating reflectance in emerging short-reach single-mode applications. 事实上，光纤网络具有非常严格的损耗预算并严格控制误码超标，因此网络所有者和设计者不仅应制定总体损耗预算，还应制定每个接头和连接器的损耗、反射预算。

Additionally, OTDR characterization is recommended before OLTS insertion loss testing. The ability to measure the number, location, and performance of each splice and connector with an OTDR allows you to correct problems during the installation process and prior to final insertion loss testing with an OLTS, rather than after the network is live.

Further, the final OLTS insertion loss test results are required for definitive proof of compliance. If testing fails and you need to troubleshoot with an OTDR, you will also have to test again with the OLTS. Regardless of whether both testers are used as recommended, cleaning and inspection of fiber end faces is a must before testing (see Appendix, The Importance of Fiber Cleaning and Inspection).

The Advantages of Integrated OLTS and OTDR Test Results

Not only do an OLTS and OTDR complement each other for a complete testing strategy, but together they help protect the technician via comprehensive documentation. The combination of both an event trace and a total loss measurement that demonstrates compliance at the time of installation makes it very difficult for anyone to cast blame on the technician if performance issues arise down the road.

Having documented traces for each link also gives technicians and customers a frame of reference when troubleshooting to easily identify exactly what went wrong and where. For example, by comparing the original trace acquired during testing to the new trace, it can be easy to see if a new event happened due to a poor cable management, or if a connection point increased in loss over time due to contamination or other post-installation issues.

When it comes to selecting an OLTS and OTDR, select tools that are easy to use and capable of delivering test results and reports in an easy-to-understand format. It’s also a time and accuracy benefit when results from both can be integrated into a single test report for a given project via a test management and documentation service that allows technicians to upload results from both testers. Integration of the results from both the OLTS and OTDR provides complete, comprehensive documentation that satisfies customers, protects technicians, and facilitates troubleshooting once the cabling plant is live.

OLTS and OTDR testing are different yet critically complementary. It’s important to understand the differences between OLTS and OTDR testing and the benefits both provide, and it’s also important to recognize that together they perform a complementary role in the fiber testing process. And when an OLTS and OTDR are designed to work in tandem with each other, generating integrated documented results, their benefits are greatly enhanced.

附录：The Importance of Fiber Cleaning and Inspection

Regardless of whether you’re using an OLTS alone for Tier 1 testing or both an OLTS and OTDR for Tier 2 or extended testing, cleaning and inspection must be part of the process. Contaminated connections remain the number one cause of fiber-related problems and test failures; a single particle on the core of a fiber can cause loss and reflections. While an OTDR can expose dirty connections, cleaning and inspecting end faces before installation can reduce testing time and inaccuracies.

Any and all end faces, even brand-new connectors and factory-terminated plugs and pigtails, should be inspected for cleanliness before mating. This includes both ends of test reference cords, fiber jumpers, and pre-terminated trunk cables. Even interchangeable adapters used on test equipment should be inspected on a regular basis and cleaned when needed, since they too can accumulate debris. 部分制造商最近已成功改善了新工厂端接连接器的清洁度，但建议也应检查并清洁这些连接器（如有必要，即使新设备也需要清洁）。Remember that even the dust cover designed to protect the fiber end face can be a significant source of contamination.

Upon inspection, if cleaning is required, it’s important to use a specially designed fiber optic cleaning tool, such as the Fluke Networks Quick Clean™ cleaners. For more stubborn contamination, such as oils, a solvent specifically formulated for end face cleaning (such as a Fluke Networks Fiber Optic Solvent Pen) should be used. Isopropyl alcohol (IPA) was used for many years to clean fiber end faces, but today’s specialized solvents have a lower surface tension that makes them far more effective at enveloping debris for removal and dissolving contaminants. IPA can also leave behind a “halo” as it dries, which not only causes attenuation but also can be difficult to remove. 清洁后不应有溶剂留在端面上。

^{图 5：Specialized solvents (left) are far more effective at cleaning end faces than IPA that can leave behind a residue (right).}

附录：What Is OTDR Event Mapping?

When looking at an OTDR trace that graphically displays the characterization of a fiber link, experienced OTDR users can typically recognize reflective events for launch cords, connectors, mechanical splices, fusion splices, mismatched fibers, and the end of the link. They will also likely know that the little blips visible after the end of the link are ghosts and not real events to be concerned with. However, not everyone is a trace analysis expert, or perhaps a technician is simply out of practice.

Some advanced OTDRs come with highly developed logic that automatically interprets the trace and provides a detailed graphical map of events that indicates the location of connectors, splices, and anomalies. This event map is ideal for those who might not be proficient at reading a trace, and it can also be a valuable training tool to help technicians improve their trace interpretation skills. For example, if they are unsure what type of event they’re looking at on the trace, they can switch back and forth between the trace and the event map to help test their skills and verify exactly what they are seeing.

^{图 6：如右图所示，弯曲的特征是缺乏反射，在较长波长下损耗较高。高级 OTDR 可识别此类活动，并以易于解释的方式演示（左侧）。}