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Feb 23

OTDRs Never Go Out of Style

By: Larry Johnson

The optical time domain reflectometer (OTDR) is now thirty-six years old and aging gracefully.

While fiber optic cables had been installed in North America since 1977, one major concern was and still is: how do we accurately locate a fault? This is the major reason that OTDRs exist. Fortunately, as the communications industry has matured, so has the OTDR.

Why Use It?

Today’s OTDRs address quality control (QC), quality assurance (QA) for optical fibers and cables as well as acceptance testing and troubleshooting installed links in the field. Fiber and cable manufacturers make use of the OTDR’s QC features to perform attenuation and length measurements at a variety of wavelengths based on the type of fiber being tested. These tests are more common in factory settings, often in conjunction with optical switches to allow quick and efficient testing of large numbers of optical fibers.

As fiber counts in cables have increased, the level of automation has paralleled this growth, providing opportunities to increase the OTDR’s value to service providers by incorporating optical switches to monitor live and dark fibers.

The OTDR’s dominant role for service providers and contractors is in QA roles — which is far more extensive and complex than QC testing. Modern OTDRs must address multiple tests and measurement tasks focused mostly around attenuation, but as the critical nature of reflections and their impact on system performance increase, the OTDR is essential for these measurements. It must also be able to perform length measurements for approximate physical locations of events such as splices, fiber stress points (macrobends and microbends), passive devices such as splitters and fiber breaks.

The OTDR is also the easiest instrument to use to measure component reflectance and span optical return loss (ORL) values. The importance of reflection testing cannot be overstated. Reflectance and ORL values are critical for achieving desired bit error rates, as Fresnel reflections from connectors can disrupt the efficient operation of the laser diodes in fiber optic transmitters.

Transmitter manufacturers define the quality of signal based on the level of attenuation and the ORL values for fiber spans. A single contaminated connector can affect the component reflectance, which in turn affects the span’s ORL value. Component reflectance and ORL testing should be requirements for all end-to-end OTDR tests on single-mode fibers.

New Challenges

As system data rates increase, the need for fiber characterization (FC) continues to challenge the industry. In some cases where optical amplifiers are installed, identifying, locating and re-terminating high-loss connectors and splices is required due to higher reflection and attenuation issues with legacy terminations. Older terminations were limited by two issues: fiber tolerances and the type of polish on the connectors. Single-mode fiber tolerances for core, cladding dimensions, ovality and concentricity continue to improve. Older fibers simply have more opportunity for higher loss connections.

Many legacy connectors had either flat polishes or original physical contact (PC) polishes with reflectance levels of 30-40 dB. Even the improved super PC (SPC) polishes of the late 1990s with most ST and SC connectors have much greater reflectance levels (45 dB) than the 50-65 dB reflectance levels for today’s ultra physical contact (UPC) and angled physical contact (APC) polishes.

Identifying high loss splices and connectors as well as high reflectance connectors can easily be performed by the OTDR. However if the readings are too high to meet today’s standards or system performance levels, these older terminations may have to be replaced with UPC or APC connectors.

The Chicken and the Egg Question

Is the technology driving the development of improved OTDRs? Or are the users driving their needs? Each group will have their own perspectives. The manufacturers have done a great job at developing smaller, lighter, less costly instruments. They have done so while improving technical requirements: greater dynamic range options, shorter and longer laser pulse widths, increased waveform and data storage, longer battery life, and interfaces for exporting data via Bluetooth or Wi-Fi. At the same time maintaining an instrument that is easy to operate.

OTDRs have also grown from AC powered mainframe OTDRs to handheld mini OTDRs.

The development of modular OTDR platforms allows for greater flexibility to add various optical tools such as power meters, visual fault locators, inspection scopes or other modules for advanced fiber testing, wifi or copper testing.

OTDR function selection screen

For installers and contractors, quick and accurate documentation is essential for acceptance of the installation along with attenuation, reflectance and distance measurements. Unless the operator understands the automatic functions of the instrument, there is greater likelihood of mistakes and/or changes to be required.

The industry continues to upgrade the OTDR to operate with “auto” functions that allow for quick multi-wavelength testing, bi-directionally averaged waveforms and trace analysis. The days of the traditional OTDR trace has expanded to feature event tables and trace maps. Event tables identify each span and event with their distances, attenuation, and reflectance along with icons to match the waveform for quick identification. The advent of icon based reports in place of the OTDR traces has been a welcome addition for field personnel. One word of caution – pass/fail or red/green icons can sometimes not tell the entire story. Learn to properly set up and use your OTDR and learn how to examine and interpret the OTDR trace. This is bound to be useful when troubleshooting a problem at some point.

An OTDR event table

Setting up the OTDR with desired preset test conditions for maximum attenuation and reflection values will easily identify only those fibers that do not meet either component or span values and show pass/fail notations. These settings can be set automatically or customized for the span under test. For older installations, older settings can be used when needed.

What is Old is New Again

The role of the OTDR as a maintenance tool should not be overlooked. Already the ITU has developed several recommendations, such as L.41, for wavelength assignment for maintenance roles. These recommendations focus mostly on the 1625 nm or 1650 nm for “out of band” testing. As these wavelengths are extremely sensitive to macro and microbends, service providers will also be able to identify possible problems unseen at the 1550 nm wavelength.

Another application of the OTDR is the integration with optical switches. These allow an OTDR to function 24/7 while scanning and comparing stored waveforms either on live or dark fibers. This is best at hub sites where multiple cables can be monitored as desired.

The OTDR is an indispensable test and maintenance tool. It has grown with the fiber optic communication industry to address new technologies while maintaining its key role as the best instrument for troubleshooting optical fiber spans. For users, it is still key that the operators understand the abilities of the OTDR and know how it can be used to address requirements of today’s fiber optic communication systems.

Article used by permission