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How to choose OLTS, OTDR, OPM & test light source combination?

Fiber is playing as increasing role in most network installations, driven by the need for higher-bandwidth applications in data centers and backbone cabling system, 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 size is projected to reach USD 6.9 billion by 2024, up from USD 4.3 billion in 2019.

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

Optical loss tester (Optical Loss Test Set, OLTS). Inside the optical loss tester is a combination of optical power meter and optical fiber test optical flow. Its test process can be divided into two stages. The source power is tested first, and then the light passing through the device is tested. There is a difference in device loss between these two stages of testing.


OLTS: Required for Accurate Insertion Loss Testing

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 and is required by industry standards to ensure the link can meet the loss requirements for a given application. The test is performed with a light source which produces a continuous wave at specific wavelengths connected to one end of the fiber. A power meter with a photo detector is connected to the opposite end of the fiber link. The detector measures optical power at the same wavelengths produced by the light source. Working in concert, these devices determine the total amount of light lost.

Industry standards specify insertion loss limits for specific fiber applications, which is 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 insertion loss limits for a given application to determine if it passes. Note that a light source/power meter (LSPM) also accurately measures loss per industry standards but 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 a maximum length. Models such as the CertiFiber Pro™ measure both loss and length, providing a clear pass/fail result that will assure application support.

Optical Time Domain Reflectometer (OTDR). The working principle of the optical time domain reflectometer is similar to that of radar. Its essence is an optical radar. The optical time domain reflectometer test is performed by transmitting light pulses into the optical fiber, and then receiving the returned information at the optical time domain reflectometer port for analysis. In addition, there are a series of complicated calculations to analyze the loss of the optical fiber in the process of transmitting optical signals.

The combination of optical time domain reflectometer (OTDR), optical loss tester (OLTS), optical power meter (OPM) & test light source is very similar in function and application. They can be used for cable installation and outdoor wiring applications,test. So the question arises: Which one you like to choose? Today, let's talk about three commonly used test instruments: OTDR, OLTS, OPM & test light source combination.

OLTS and OTDR: A Winning Combination
When it comes to fiber testing, one may ask if an OTDR is used is an OLTS still necessary? The answer is yes. The use of an OLTS is required by industry standards to ensure application compliance because it accurately measures total fiber insertion loss. The use of an OTDR does not replace the OLTS because the total insertion loss measurement achieved with an OTDR is an inferred calculation that does not necessarily depict the total loss that will occur on a link once it is live. Especially in the case of multimode fibers, where standards specify precisely controlled launch conditions, OTDR tests are not as accurate or repeatable as an OLTS.

When testing or commissioning a significant number of links, the speed difference between the OLTS and OTDR becomes a significant issue. A high-performance OLTS, such as Fluke Networks’ CertiFiber Pro, can measure a duplex link at two wavelengths in under three seconds. Even a fast OTDR, such as Fluke Networks OptiFiber Pro, will take at least 12 seconds to characterize a fiber. However, to get an accurate measurement with an OTDR, the fiber must be tested in the reverse direction. This is made easy with the SmartLoop™ capability of the OptiFiber Pro, 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, one may ask if an OLTS is used and the fiber link passes, is an OTDR necessary? The answer to this question isn’t quite as simple. First, it’s important to understand that the specification for a given project must be followed. If the specification requires OTDR characterization (Tier 2 testing in TIA standards and extended testing in ISO/IEC standards), then an OTDR is indeed required along with OLTS insertion loss testing. 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. In fact, due to ever-tighter loss budgets and less room for error, many network owners and designers are setting not only overall loss budgets, but also loss budgets for individual splices and connectors, which can only be verified with an OTDR.

Additionally, it is recommended that OTDR characterization be done before OLTS insertion loss testing. The ability to measure the number, location and performance of each splice and connector with an OTDR allows problems to be corrected during the installation process and prior to final insertion loss testing with an OLTS rather than afterwards when the network is live. Further, the final OLTS insertion loss test results are required for final proof of compliance, so if testing fails and there is the need to troubleshoot with an OTDR, testing will have to be performed again with the OLTS. 

Optical Power Meter (OPM) & test light source combination. Using an optical power meter in combination with a stable test light source can measure connection loss, check continuity, and help evaluate fiber link transmission quality. Its functions are similar to OLTS. But because its single light source and two power meters can be used at the end of each link, the former is a bit more flexible compared to the other.

It will be introduced in terms of cost, ease of use, and application.

1. Cost: Compared with the cost of these three products, OTDR is generally more expensive. In addition, compared with the combination of OLTS and OPM & test light sources, OTDR requires professional personnel to operate, so its human cost will be higher. In addition, OTDR has high asset and management costs. If you use OTDR regularly, you can consider buying one. But if it is used only occasionally, then it is more reasonable to rent it from a cost perspective. As for the combination of OLTS and OPM & test light source, because the latter is composed of two devices (optical power meter and test light source), there is more inventory maintenance and management expenditure, so the use cost will be slightly higher than the optical loss tester some.


2. Ease of use: As mentioned in the previous paragraph, OTDR must be operated by a professional or experienced person, so from the aspect of ease of use, OTDR is slightly inferior. Because an inexperienced person will not operate the OTDR, it will take some time and energy to study and learn how to use it. As for the combination of OPM and test light source, since the wavelength of its light is not automatically synchronized during the test, it will be a bit more troublesome to operate than OLTS. Therefore, among these three, the optical loss tester is the easiest test equipment to use, and almost everyone can get started quickly.


3. Application: OTDR is mainly used for outdoor wiring test. Outdoor wiring involves the splicing of single-mode fiber to make it suitable for long-distance transmission. The role of OTDR is to test the quality of fiber splicing. However, after the link is completed, we also need to use OPM, test light source and reference cable to test the insertion loss of fiber splicing. OLTS is often used to test the end-to-end loss of an installed system. It is simple and reliable by using a two-way measurement method at multiple wavelengths. As mentioned above, the use of OPM and light source combinations is more flexible than OLTS. Generally, one OPM and light source combination can complete the measurement of two OLTS, and it is not necessary to measure the transmission power, so it is suitable for use in various places.


Through the above analysis

a. We can see that the cost of OTDR is the highest and suitable for professionals to use.

b. OLTS has the lowest cost and is easy to use.

c. OPM and light source combination are in between.


All in all, these fiber testing equipment is an indispensable tool in fiber optic networks, they can check out the problems in time before your fiber optic system is overloaded. As long as you are familiar with their characteristics, and according to the cost, ease of use, and specific application environment in the actual wiring test process, you will easily find the "it" that suits you best.


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