Working Principle and Characteristics of OTDR

Updated on Apr 3, 2019 by

The Optical Time Domain Reflectometer (OTDR) is an essential tool used to test the integrity of fiber optic cables, which can be applied to evaluate the length of fiber cables, measure transmission and connection attenuation and to detect the fault location of fiber links as well. Based on these functions, it is commonly employed to fiber optical cable maintenance and construction. Moreover, OTDRs are most effective when testing long cables or cable plants with splices by illustrating where the cables are terminated and confirming the quality of the fibers, connections and splices.

How Does an OTDR Work?

Comparing with those sources and power meters which measure the loss of the fiber optic cable plant directly, the OTDR works indirectly. By duplicating the transmitter and receiver of the fiber optic transmission link, the source and meter make the measurement correlate well with actual system loss. However, the OTDR uses a unique optical phenomena "backscattered light" to make measurements along with reflected light from connectors or cleaved fiber ends, thus to measure loss indirectly.

During the process of OTDR testing, the instrument injects a higher power laser or fiber optic light source pulse into a fiber from one end of the fiber cable, with the OTDR port to receive the returning information. As the optical pulse is transmitted through the fiber, part of the scattered reflection will return to the OTDR. Only useful information returned could be measured by the OTDR detector which acts as the time or curve segments of fibers at different positions. By recording the time for signals from transmission to returning and the speed of transmission in fibers, the distance thus can be calculated. The following picture shows exactly how an OTDR works for fiber optic testing.


The Working Characteristics of OTDR

OTDR uses Rayleigh scattering and Fresnel reflections to measure fibers' characteristics. Rayleigh scattering refers to the irregular scattering generated as optical signals transmitting in the fiber. OTDR only measures the scattered light back on the OTDR port. The backscatter signal shows the attenuation degree (loss/distance) of the optical fiber, and will be tracked as a downward curve, illustrating the power of backscatter decreases. This is because both transmission signal and backscatter loss are attenuated.

Rayleigh scattering power is related to the wavelength of transmitted signal: the shorter the wavelength, the stronger the power, which means the backscatter lose generated by the trajectory of 1310nm signal will be higher than that of 1550nm signals.

In the higher wavelength region (more than 1500 nm), the Rayleigh scattering will continue to decrease, and another phenomenon called infrared attenuation (or absorption) will appear to increase thus to cause an increase of the overall attenuation values. Therefore, 1550nm wavelength has the lowest attenuation, which testifies why it is a long distance communication wavelength. Similarly, OTDR of 1550nm wavelength also has low attenuation, therefore it can be used for long distance testing as well.

Fresnel reflection falls into the category of discrete reflection that is caused by the individual point of the whole fibers. These points are the result of changes in reverse coefficient elements such as glass and air gap. At these points, a strong backscatter light will be reflected back. Therefore, OTDR uses the information of Fresnel reflection to locate the connection point, fiber optic terminal and breakpoints.

The Solution for OTDR Dead Zone

Then, let's take a look at an important OTDR specification which originated from Fresnel reflections, known as "dead zones". Basically, there are two types of dead zones: event and attenuation. Both of them are expressed in distance that vary in accordance with the power of those reflections. A dead zone refers to the length of time during which the detector is temporary blinded by a high amount of reflected light, until it recovers and can read light again. As the OTDR is working, time is converted into distance, therefore, more reflections lead to more time for the detector to recover, thus resulting a longer dead zone. Dead zone limits the operation of OTDR to a large extent, making it unable to locate and resolve faults. The following picture shows the dead zone of the OTDR.


The dead zone seems like a problem when using OTDR for testing, however, adapting visual fault locator (VFL) can serve as an effective solution for solving this issue. It works as a complement to the OTDR in cable troubleshooting since it successfully covers the range where OTDRs fail to monitor because of the dead zone. The visual fault locator is designed with a visible laser and universal adapter like FC, SC and ST etc, which contributes to locate faults on the fiber link easily. For instance, to locating the breakpoint, bending or cracking of the fiber optic cables and locate the fault of ODTR dead zone as well. The higher power of a visual fault locator can find breaks in fibers or high losses around connectors in simplex cables. If the light escapes at one break, it will be visible through the jacket of the fiber. Which is especially helpful in finding cable faults near the end of a cable, whereas the dead zone of the OTDR restrains its ability to resolve faults. Meanwhile, the VFL also can be applied to finding cracked fibers or bad splices where an OTDR unable to do so.


In conclusion, an OTDR tester is an optical radar in essence. By sending out a flash of bright light, it measures the intensity of echo or reflections. So, OTDRs are mainly used in the optical fiber installation and maintenance service of access networks (communications links between telephone exchanges) and user networks (communications links between user sites and telephone poles). And computation is used to display a trace and make a number of mathematical deductions. Fiberstore (FS.com) offers a wide range of OTDRs of reliable and excellent quality, for more information and details, kindly visit the website at www.fs.com or contact over sales@fs.com.

Related Article: Basics of OTDR (Optical Time-Domain Reflectometer)

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