OTDR Testing

A Look at the Latest OTDR Testing Procedures and Equipment

Certify, maintain, and troubleshoot your fiber optic systems better with industry-leading OTDR test equipment and procedures.

OTDR Testing

With the rapid advancements in fiber optic technology, OTDR testing has become an indispensable method to certify, maintain and troubleshoot fiber optic systems.

An Optical Time Domain Reflectometer (OTDR) is an instrument used to create a virtual “picture” of a fiber optic cable run. The analyzed data can provide insight into the integrity of the fibers, as well as the connections and splices along the length of the cable run. Once this information has been captured and stored, it can be recalled as needed to evaluate the degradation of the same cable run over time.

Although originally intended for long haul fiber optic applications, newer generation OTDRs can also be used to diagnose much shorter runs, such as internal aircraft and industrial facility cabling such as structured cabling.

Contact sales to learn more about VIAVI OTDR Testing Equipment today!

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The Origins of OTDR Testing

The use of fiber optics in digital communication applications is so prevalent today that it is often taken for granted. This hasn’t always been the case, however.  In fact, the advent of fiber optics is truly a case of innovation colliding with application. 

For centuries, artisans and craftsmen worked to perfect the creation of long, thin glass fibers or wires. By the late 19th century, experiments had demonstrated the ability of light to travel through a curved glass substrate and retain a significant portion of its original intensity.  The breakthrough practical application for this technology came in the late 1960’s and early 1970’s as newly developed laser optics, ultra-transparent thin glass fibers, and digital signaling were combined to form the foundation of the fiber optic communication networks we know today. By 1991, all-optic systems could carry up to 100 times more information than traditional cable with electronic amplifiers.

As the use of fiber optic has increased exponentially, analytic and diagnostic technology such as OTDR testing have emerged and continue to improve their functionality, accuracy and feature sets.

OTDR Testing Analogy

There are obvious parallels with the copper wire signal integrity testing that OTDR has gradually supplanted as the technology has shifted to fiber optics. However, to visualize the premise behind OTDR testing, a more useful analogy might be found with Ultrasound technology. 

In medical imaging applications, high frequency (≥20KHz) inaudible sound waves are produced by the vibrating elements of an ultrasound transducer. Much like light waves, these sound waves are either absorbed, reflected back to the source, or scattered in multiple directions, depending on the distance from the transducer and the nature of the material being analyzed. The frequency, direction and intensity of the sound waves returning back to the transducer provide enough data to create detailed and accurate images of internal anatomical features.

In much the same way, OTDR testing utilizes a high intensity laser light emitted through a connecting cable at one end of the fiber optic cable run, at a pre-defined pulse interval (or pulse width). The OTDR test instrument analyzes the intensity and time stamp of the light either directly reflected or scattered back to the source location, painting a detailed picture of the entire optical fiber run condition, including information on splices, connections, bends and terminations.

OTDR Test Terminology

Understanding the science behind OTDR begins with a few basic concepts that are integral to the OTDR testing process.

  • Attenuation

    The reduction in power of the light signal as it is transmitted.  Attenuation is expressed in decibels per kilometer (dB/km). The degradation in signal may be due to splices, connections or the inherent loss within the optical fiber itself. Understanding the attenuation of the system is crucial when evaluating the overall performance.

  • Backscatter

    A term used to describe the diffuse reflection of light waves back in the direction from which they originated. The amount of backscatter is one indicator of total attenuation, since light traveling back to the source represents a loss in downstream signal intensity. In OTDR testing, the amount of backscattered light is only about one-millionth of the test pulse.

  • Reflectance

    A measure of the proportion of light striking a surface which is reflected off of it. Unlike backscattered light, reflected light is returned more directly to the light source rather than being diffused in many directions. Connections and splices will reflect back to the source, allowing proper OTDR testing to determine the position, changes in condition, and signal loss from these elements.

  • Refraction

    Refraction is the bending of light waves as they pass from one material type to another. The amount of light reflected is determined by the differences in the index of refraction of two fibers joined through splicing, impurities in the glass fiber, material changes in a connector, or any other material change contained within the cable run.

OTDR Testing Procedure

The OTDR test process is dependent on the equipment type and the fiber optic cable run being tested, as well as the objective(s) of the test. However, there are some common OTDR testing procedures that are fundamental to any application.

  • Reference Cables

    The first important step in setting up an OTDR test is the proper connection of launch and receive cables at either end of the cable run.

    The launch cable is the link between the OTDR and the cable run, it is used to stabilize the test pulse and to enable the OTDR to recover from transmitting the test pulse in order to ‘see’ or characterize the first connector of the cable run.  The mating connector selected must be compatible, to minimize the reflectance from this junction. Imagine a hose bib with a loose or crooked connection to the hose itself, causing water to leak and project backwards from the junction. With OTDR testing, a similar result is too much laser light reflected by the poor connection and/or the air gap between connector and cable end. Poor launch connections/conditions like this cause the OTDR’s receiver to become overloaded and greatly reduces the laser pulse power delivered into the fiber run being tested, meaning you will only ‘see’ or characterize a shorter initial section of the fiber. A receive cable at the far end of the run provides a monument that can help to accurately measure overall length as well as loss at the final connector of the run. Learn more about fiber characterization.

  • OTDR Testing Parameters

    The real expertise in utilizing state-of-the-art OTDR comes from understanding OTDR testing parameters available on the instruments and optimizing them for resolution and accuracy. OTDR testing parameter settings typically include the following.

    • Range: Sets appropriate range (distance) based on the overall cable run length
    • Pulse Width: Sets the duration of each laser pulse emitted
    • Acquisition Time: Sets the time duration for averaging the measurements of reflected light
    • Refractive Index: Matches the index of the cable material being tested

    In general, the length of the cable run will govern the level of resolution that can be achieved through equipment settings. Testing a longer run may require compromising the sensitivity. Longer averaging times can also contribute to better resolution by increasing the signal-to-noise ratio, thereby “smoothing” the data presented in the test curve.

    During the OTDR test setup, loss thresholds for the overall system, as well as each connection and splice individually, can be pre-programmed. These might be based on industry or project specific OTDR testing standards. System markers may be used to indicate virtual start and stop points for each element under test.

  • OTDR Auto Test

    Although many OTDR testing models include an “Auto Test” feature that allows the device to automatically determine the optimum settings for your system, it is important to understand what these underlying settings are and how they may impact your results. Although this feature can save a significant amount of setup time, it might equate to the “auto-focus” mode of a camera that can be improved upon in the hands of a professional photographer.  

Interpreting the OTDR Test Results

Once the OTDR test is completed, the system will display the OTDR test results in both numeric and graphical formats.  The x-axis of the display will indicate distance while the y-axis will display signal loss in dB. The graph will show where each connection, splice or break is located, with the signal loss and reflection characteristic of each element clearly visible.

The length of the fiber is calculated based on the index of refraction of the glass in the fiber. Therefore, it is important for this value to be set correctly to generate accurate OTDR test results.

The precise amount of time required for the test pulse to be sent and reflected (or scattered) back to the receiver is analyzed to pinpoint connector, splice and other loss event locations.   

If loss thresholds were initially set, Pass or Fail will be indicated for each element of the cable run. It is entirely possible to have a passing cable run with one or more failing elements or vice versa. This is when the aforementioned data storage from previous OTDR testing can be handy for troubleshooting.

Types of OTDR Test Equipment

Although feature sets and cost vary significantly, there are two predominant types of OTDR test equipment available on the market today. 

  • Full Feature OTDR

    This is the term traditionally used to describe large display, feature-rich, non-portable test equipment with a direct AC (outlet) power source. This type of OTDR test equipment may also include expansion capability in the form of plug-in ports connected to the main frame. Although the cost is typically higher than portable OTDR’s, this type of equipment may be called for when a high level of accuracy, sensitivity or long-range measurement (with its inherent higher pulse intensity) are required to generate the most accurate OTDR test results.

  • Hand Held OTDR

    As the name implies, hand-held OTDRs are typically battery-powered, lightweight and portable instruments optimized for use in the field. Since the compact form factor may compromise computing power, most include USB connectivity for retrieval of test results for more detailed PC based analysis and reporting. These devices are useful for diagnosing high loss points and end-to-end loss. Hand-held devices in the “micro” category, including limited functionality devices such as fiber break locators, may weigh less than 1 kg.

Calibrating OTDR Test Equipment

For all measurement equipment, periodic calibration is essential to monitor and correct equipment bias and reset relevant functions based on reference standards. While some favor a gold standard cable such as the “Golden Fibre” created by NPL, others have proposed an electronic/optical simulation approach to calibration that requires no physical reference standard.

In industries where the accuracy of OTDR test results is essential, the IEC 61746 standard for calibration, as well as the TIA/EIA-455-226 (adopted from the IEC standard) are recognized.

The IEC standard includes specific practices for calibrating point to point accuracy, linearity, attenuation, power output and delay, among other attributes.  Given the complexity, OTDR calibration is best left to OTDR equipment manufacturers or certified calibration labs.

The Future of OTDR Testing

Providing more functionality, accuracy and resolution at a lower price point is an ongoing challenge.  Improvement in OTDR auto test algorithms may lower the barrier to entry for technicians and increase acceptance.  Similarly, improvements related to overcoming reflectance overload issues inherent to short cable runs may help to expand the application of OTDR technology into new arenas.

With fiber optic technology, the byproduct of centuries of drawn glass craftsmanship have combined with modern innovation and optimization to create a revolutionary new way to meet the communication needs of our global society. As the data-load demands on our fiber optic networks continue to increase, OTDR test capabilities must continue to improve in order to meet these challenges.

Without technology such as OTDR testing, advanced application of fiber optics would not be feasible.  The ability to “see” inside thousands of miles of optical fiber no thicker than a human hair has become both an incredible accomplishment and a practical necessity. 

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