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 build, 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 route. The analyzed data can provide insight into the integrity of the fibers, as well as any passive optical component such as the connections, splices, splitters and multiplexers along the cable path.

Once this information has been captured, analyzed and stored, it can be recalled as needed to evaluate the degradation of the same cable over time.

The OTDR is also the only tool capable of troubleshooting any fiber optic cable failures by locating the distance to the fault and identifying the type of fault-like breaks, bends and any excessive loss. An OTDR instrument can be portable or it can be rack-mounted and placed for permanent monitoring in the network such that an alarm can be triggered if the fiber is compromised. 

Common issues that OTDRs find are signal loss due to connector problems, fiber bends, crushes and breaks. Rayleigh OTDR measurements are used for this technique. Raman and Brillouin OTDR measurements can be used to predict breaks and monitor fiber health by making temperature and strain measurements. The three techniques form a powerful tool set to manage your fiber or to utilize your fiber for Distributed Fiber Optic Sensing. Many issues that gradually damage the fiber can be remediated before a service outage impacts a customer. 

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

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

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How Does an OTDR Work?

The OTDR injects pulsed light energy, generated by a laser diode, into one extremity of the optical fiber. A photodiode measures over time the returning light energy (reflected and scattered back) and converts it into an electrical value, amplified and sampled that is graphically displayed on a screen.

OTDRs measure the location and loss of passive optical network elements also called “events”. The location or distance to each event is calculated from the round-trip time of the light pulse traveling along the fiber. The loss is calculated from the amplitude value of the returned signal (backscattering effect).

Most of modern OTDRs automatically select the optimal acquisition parameters for a particular fiber by sending out test pulses in a process known as auto-configuration or test auto.

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.

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 primary step before connecting any device to reference cables and to the fiber to be tested is the inspection of each connector that will be mated for the measurement (OTDR port, reference cables, patch panels, etc.)Learn more about the VIAVI Inspect Before You Connect methodology on our Fiber Inspection page.

    The next step in setting up an OTDR test is the proper connection of launch and receive cables at either end of the fiber link. The launch cable is the link between the OTDR and the fiber link, 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 fiber under test. 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 fiber 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. Newer auto tests utilize multiple pulse widths to choose the optimum ones to accurately characterize the close events at the beginning of the link, the splices or splitters in the middle and the far end sections of a fiber link. 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, also called trace, will show where each connection, splice or break is located, with the signal loss and reflection characteristic of each element clearly visible. Good OTDR test equipment will translate this trace into an iconic linear view where each element and event is represented as an easy-to-read icon, with pass/fail information visible immediately, and the name of each component/event clearly shown.

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. 

  • Benchtop

    This term typically describes OTDR test equipment used in the laboratory and production plants. Benchtop devices can be placed on a laboratory workbench or in a production test bay, and usually have a larger display and 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, such as optical switching devices for MPO test. Although the cost is typically higher for this type of OTDR test equipment it may be called for when a high level of accuracy, sensitivity or long-range measurement (with its inherent higher pulse intensity) is required to generate the most accurate OTDR test results.

  • Hand-held OTDR

    As the name implies, hand-held OTDR test equipment is lightweight (less than 1kg), portable, typically battery-powered and optimized for use in the field. The price / performance ratio is optimized for contractors and fiber installers to build, certify and troubleshoot optical cables for multiple applications. The user interface is usually simple and straightforward, so any technicians can operate it and understand OTDR test results with minimal training. The various connectivity features available, such as the Wi-Fi or bBluetooth, facilitate the workflow to get the job orders and transfer the test results.

  • Embedded or Rack-Mounted OTDR

    Embedded OTDRs are the size of an electronic board. They are designed as small as possible to fit in network equipment or monitoring systems. They are usually produced in large volumes, like electronic components, to optimize the cost. The increasing need for continuous and proactive monitoring of the fiber network infrastructure make them a capital component for the future. Rack-mounted OTDRs are combined with an optical switch to automatically rotate across many fibers with a routine. The routine can prioritize very critical fibers or important customers. These fiber monitoring applications can be used for in-service monitoring or dark fiber for out-of-service monitoring.

OTDR Specifications

OTDR specifications are important to understand so one can choose the right OTDR for a dedicated application.

  • Dynamic range

    The dynamic range is one of the most important characteristics of an OTDR since it determines the maximum observable length of a fiber. The higher the dynamic range, the higher the signal-to-noise ratio and the better the trace and event detection. The dynamic range is relatively difficult to determine since there is no standard computation method used by all the manufacturers. The dynamic range is defined as the difference between the extrapolated point of the backscatter trace at the near end of the fiber and the upper level of the noise floor after the fiber end. Dynamic range is expressed in decibels (dB). The measurement is performed over a three minute period, and the results are averaged.

  • Event dead zone

    The event dead zone (EDZ) is the minimum distance that distinguishes two consecutive unsaturated reflective events (typically two connections). In the case where the reflective events are more closely spaced than the EDZ, the OTDR will show them as one event. EDZ is dependent on the pulse width. The smaller the pulse width the smaller the EDZ.

  • Attenuation dead zone

    The attenuation dead zone (ADZ) defined in the IEC 61745 standard is the minimum distance after a reflective (ex: connector) or attenuation (ex: splice) event, where a non-reflective event (ex: splice) can be measured. In the case where the events are more closely spaced than the ADZ, they will show as one event on the trace. ADZ is dependent on the pulse width. The smaller the pulse width the smaller the ADZ.

  • Wavelengths

    OTDRs measure according to wavelength. The typical wavelengths used are 850 nm and 1300 nm for multimode fiber and 1310 nm, 1550 nm and 1625 nm for singlemode fiber. Filtered 1625 nm or 1650 nm can be used for maintenance to avoid interference with the traffic wavelength. C-/D-WDM wavelengths are used to commission, upgrade and troubleshoot singlemode fiber links carrying C- or D-WDM channels.

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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