Learn how to select an OTDR that is appropriate for your fiber optic testing needs.
Certify, maintain, and troubleshoot your fiber optic systems better with industry-leading OTDR test equipment and procedures.
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.
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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.
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.
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.
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.