Optical Time-Domain Reflectometers Pt. 1

An optical time-domain reflectometer (OTDR) is an opto-electronic instrument that is used to test optical fibers. OTDRs do this by injecting a series of light pulses into the fiber being tested. It then reads, from the same end of the fiber, scattered light (Rayleigh backscatter) and light that is reflected back from points along the length of fiber. (This is similar to the way that an electronic time-domain reflectometer measures reflections due to changes in the impedance of the conductive cable being tested.) The strength of the returned pulses and time are measured, and are plotted as a function of fiber length. Essentially, the longer it takes for a pulse to return to the sensor, the longer the distance to the event. OTDRs are often used to estimate the length and attenuation of a fiber, including splice loss and any loss related to connectors used in the installation of the fiber. Faults, such as breaks as well as optical return loss are also capable of being measured by OTDRs. In order to measure the attenuation of multiple fibers, it is a good practice to take measurements from each end of the fiber, and to average the results. This does create more work, but the result is a more accurate measurement of fiber attenuation. OTDRs are quite automated, with on board computers and graphical displays. Even though the incorporated technology does simplify the process considerably, proper operation and data interpretation is necessary for accurate trace measurement. Specially trained personnel and well calibrated tools are always  important resources in the arsenal of the fiber installation and inspection crew. Invaluable tools from manufacture through final installation, OTDRs are commonly used to characterize the length – as well as loss of fibers at any and all steps along the way. Due to the distances involved in final installations, as well as the potential for multiple splices and connections, they are inherently more challenging to measure. OTDR test results are usually saved and archived as a benchmark in case of later failure or for warranty claims. Fiber optic failures are very expensive in terms of repairs as well as lost service. This being the case, it is important to be able to save initial measurements – another value of the technology incorporated into most OTDRs today. The OTDR is available for a variety of fiber types and wavelengths, to suit specific applications. Generally speaking, an OTDR testing at longer wavelengths, such as 1550nm or 1625nm can be used to identify attenuation due to fiber problems as opposed to the more common splice or connector losses. The optical dynamic range is limited by the output power, pulse width, input sensitivity, and signal integration time. Higher pulse output power and input sensitivity combine directly to improve the range of measurement, and are usually fixed. Pulse width and signal integration time, however, are adjustable and require trade-offs that make them application-specific. Longer pulses improve the dynamic range and attenuation measurement resolution, but it comes at the cost of a lower distance resolution. A long pulse length allows you to measure attenuation over a longer length of fiber, but an event may appear to be much longer than it actually is. This would be useful for measuring the overall characterization of the link, but not very useful for locating a fault. A shorter pulse length improves the distance resolution, but reduces range and attenuation measurement resolution. The “apparent measurement length” of an event is commonly referred to as the “dead zone”. The table below illustrates the theoretical relationship between pulse length and dead zone.

Pulse length

Event dead zone

1 nsec 0.15 m (theoretically)
10 nsec 1.5 m (theoretically)
100 nsec 15 m
1 µsec 150 m
10 µsec 1.5 km
100 µsec 15 km

The “dead zone” is a topic of interest to many who use OTDRs. Dead zone is classified in one of two ways: Event Dead Zone, and Attenuation Dead Zone. The former is related to reflective discrete optical events. In an EDZ, the measured zone depends on a combination of pulse length (see table), and he reflection’s size. The latter is related to non-reflective events. In this case, the dead zone is dependent on pulse length and signal loss. In our next installment, we will look at reliability, quality and different types of OTDRs and OTDR-like test equipment. For more information and notification of updates, please sign up for our newsletter. If you would like to purchase an OTDR, or have one to sell, please visit efibertools.com.

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