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How to Choose OTDR?

The past few years have witnessed a boom in the use of fiber optics for network communications. To ensure the reliability and accessibility of fiber optic networks, more accurate and faster methods of assessing infrastructure integrity are essential. Therefore, choosing the right fiber test tool/equipment is critical: not only to meet the enhanced test requirements but also to help increase the reliability and value of the entire network. An OTDR is one of the most powerful test instruments for fiber optic cable testing.
Learning how to operate an OTDR properly is a very important skill for technicians in companies that manage and maintain fiber optic networks. Therefore, this article aims to provide some factors to be aware of when choosing an OTDR. Hope it helps you.

What Is OTDR?

OTDR (optical time-domain reflectometer) is used to test the performance of newly installed fiber links and detect problems that may exist in fiber links. Its purpose of it is to detect, locate, and measure elements at any location on a fiber optic link. An OTDR needs access to only one end of the link and acts like a one-dimensional radar system. By providing pictorial trace signatures of the fibers under test, it’s possible to get a graphical representation of the entire fiber optic link.
Just by injecting pulsed of light into one end of a fiber and analyzing the backscattered and reflected signals, an OTDR can thus measure:
Optical Distance
  • To elements: splices, connectors, splitters, multiplexers
  • To faults
  • To end of the fiber
Loss, Optical Return Loss (ORL)/Reflectance
  • Loss of splices and connectors
  • ORL of link or section
  • Reflectance of connectors
  • Total fiber attenuation

Why Do You Need an OTDR?

As we all know, fiber testing is an essential procedure to make sure that the network is optimized to deliver reliable and robust services without fault. So here are two reasons why an OTDR is needed.
First, service providers and network operators want to ensure that their investments in fiber networks are protected. Installers need to use OTDR to perform bi-directional tests and provide accurate cable documentation to certify their work. Of course, OTDRs can be used for troubleshooting problems such as break locations due to dig-ups.
Second, premises fiber networks have tight loss budgets and less room for error. Therefore installers have to test the overall loss budget with a light source and power meter, which is a big task. While OTDR can easily pinpoint the causes for excess loss and verify that splices and connections are within appropriate tolerances, which saves lots of time. Besides, it is also the only way to know the exact location of a fault or a break.

What and Where Will You Test?

Before choosing a suitable OTDR, ask yourself the following two questions.
Loss, reflectance, splicing alignment, and distance, which one are you going to test? Make sure the OTDR you choose can do what you want easily, quickly, and accurately. If you need to make a “live” test (like during a “hot cut”—splicing of fibers in a working cable), you need an OTDR that can do an active splice loss measurement in “real-time”.
Where are you going to do testing? A good understanding of the applications of an OTDR will help you make the right choice for specific needs. For example, what kind of networks will you test? LAN (local area network), metro, or long haul? What is the maximum distance you might have to test? 700 m, 25 km, 150 km?

For Outside Plants (OSP)

Service providers and network operators want to ensure that their investments in fiber networks are protected. In the outside fiber plant, every cable will be tested for end-to-end loss and with an OTDR to ensure the installation was properly made. Installers will be asked to use loss test sets (source and power meters) as well as OTDRs, performing bi-directional tests and providing accurate cable documentation to certify their work. Later, OTDRs can be used for troubleshooting problems such as break locations due to dig-ups.

For Premises, LAN/WAN, Data Centers, Enterprise

Premises fiber networks have tight loss budgets and less room for error. Installers should test the overall loss budget with a light source and power meter. OTDR testing is a best practice that can pinpoint the causes for excess loss and verify that splices and connections are within appropriate tolerances. It is also the only way to know the exact location of a fault or a break. Testing a fiber link with an OTDR also helps document the system for future verification.

Factors to Consider When Choosing the OTDR

For different test and measurement needs, there exist a great number of OTDR models, then how do select the right one? A comprehensive understanding of OTDR specifications and the application will help make the choice. Moreover, based on your specific need, you should answer the following questions before looking for an OTDR:
  • What kind of networks will you be testing?
  • What fiber type will you be testing? Multimode or single-mode?
  • What is the maximum distance you might have to test?
  • What kind of measurements will you perform? Construction, troubleshooting, or in-service?

And when choosing an OTDR, you should consider these factors:

  • Size and Weight—important if you have to climb up a cell tower or work inside a building
  • Display Size—5” should be the minimum requirement for a display size; OTDRs with smaller displays cost less but make OTDR trace analysis more difficult
  • Battery Life—an OTDR should be usable for a day in the field; 8 hours should be the minimum
  • Trace or Results Storage—128 MB should be the minimum internal memory with options for external storage such as external USB memory sticks
  • Bluetooth and/or WiFi Wireless Technology—wireless connectivity enables easily exporting test results to PCs/laptops/tablets
  • Modularity/Upgradability—a modular/upgradable platform will more easily match the evolution of your test needs; this may be costlier at the time of purchase but is less expensive in the long term
  • Post-Processing Software Availability—although it is possible to edit and document your fibers from the test instrument, it is much easier and more convenient to analyze and document test results using post-processing software

The Operation of the OTDR System

OTDR uses the effects of Rayleigh scattering and Fresnel reflection to measure the characteristics of an optical fiber. An OTDR uses a light backscattering technique to analyze fibers. In essence, by sending a pulse of light into the fiber and measuring the travel time (“time domain”) and strength of its reflections (“reflectometer”) from a point inside the fiber, it produces a characteristic trace, or profile, of the length vs. returned signal level on a display screen.
Then the trace will be analyzed on the spot, point out immediately for documentation of the system, or saved for later analysis. OTDR provides for automatic analysis of the raw trace data, thereby eliminating the need for extensive operator training.

Advantages of OTDR

People choose OTDR for its accuracy, measurement range, and ability to measure closely spaced events.
Accuracy
A figure of merit for the OTDR for a detector is accuracy. It means the correctness of the measurement, that is the difference between the measured value and the true value of the event being measured.
Measurement Range
Defined as the maximum attenuation that can be placed between the instrument and the event being measured, for which the instrument will still be able to measure the event within acceptable accuracy limits.
Instrument Resolution
Instrument resolution refers to a measure of how close two events can be spaced and still be recognized as two separate events. We all knew that the shorter the pulse duration and the shorter the data sampling interval, the better the instrument resolution, but the shorter the measurement range. To solve this situation, some OTDR manufacturers use a “masking” procedure to improve resolution.
Although operating an OTDR is not especially difficult, it requires familiarity with the model you are using. OTDRs are available with a variety of fiber types and wavelengths, including single-mode fiber, multimode fiber, 1310nm, 1550nm, 1625nm, etc.

Conclusion

As a good helper for optical cable testing, OTDR is widely used in many optical cable network tests today. Fiber testing plays an important role in ensuring that the network is optimized to provide reliable and robust service without failure. Choosing the right OTDR to test your network not only enhances its reliability but also improves the efficiency of your testing efforts and the quality of your documentation. Therefore, before choosing an OTDR, consider the application and specific needs of your test work to ensure it is suitable for your application.

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