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With the rapid advancements in fiber optic technology, OTDR testing has become an indispensable method to build, certify, maintain and troubleshoot fiber optic systems.
"OTDR" stands for Optical Time Domain Reflectometer which 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 connectors, 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, bad connectors and any excessive loss. An OTDR instrument can be a portable tool 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, and while single-ended (uni-directional) tests can be performed, dual-ended bi-directional OTDR testing will improve result accuracy. 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 OTDR tools can also be used to diagnose much shorter cables, such as internal aircraft and enterprise facility cabling such as structured cabling.
How Does an OTDR Work?
The OTDR injects pulsed light energy (pulsed optical power), generated by a laser diode, into one end of the optical fiber. A photodiode measures over time the returning light energy or optical power (reflected and scattered back) and converts it into an electrical value, amplified and sampled that is graphically displayed on a screen.
OTDRs measure and show the location and loss of passive optical network elements which are called OTDR trace “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 or optical power (backscattering effect).
Most of modern OTDR tools automatically select the optimal acquisition parameters for a particular fiber by sending out test pulses in a process known as auto-configuration, auto setup or auto test.
OTDR Testing Analogy
There are obvious parallels with the copper wire signal integrity testing that OTDR tools have 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.
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, also called trace, will show where each connector/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 a single 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.
OTDR specifications are important to understand so one can choose the right OTDR for a dedicated application.
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 reserved for 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 Fiber Testing, 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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