Basics of OTDR (Optical Time-Domain Reflectometer)

OTDR, short for optical time-domain reflectometer, is an optoelectronic instrument used to characterize an optical fiber. It injects a series of optical pulses into the fiber under test and extracts, from the same end of the fiber, light that is scattered (Rayleigh backscatter) or reflected back from points along the fiber.

OTDR, short for optical time-domain reflectometer, is an optoelectronic instrument used to characterize an optical fiber. It injects a series of optical pulses into the fiber under test and extracts, from the same end of the fiber, light that is scattered (Rayleigh backscatter) or reflected back from points along the fiber.  It can offer you an overview of the whole system you test and can be used for estimating the fiber length and overall attenuation, including splice and mated-connector losses. It can also be used to locate faults, such as breaks, and to measure optical return loss.

How does an OTDR work?

OTDR is used to test the performance of newly installed fiber links and detect problems that may exist in them. Its purpose is to detect, locate, and measure elements at any location on a fiber optic link. OTDR works like radar—it sends pulse down the fiber and looks for a return signal, creating a display called a “trace”or “signature” from the measurement of the fiber.

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.Unlike sources and power meters, which measure the loss of the fibre optic cable plant directly, the OTDR works indirectly. The source and meter duplicate the transmitter and receiver of the fibre optic transmission link, so the measurement correlates well with actual system loss.

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.

When do you need an OTDR? 

You can use an OTDR to locate a break or similar problem in a cable run, or to take a snapshot of fibers before turning an installation over to a customer. This snapshot, which is a paper copy of the ODTR trace, gives you a permanent record of the state of that fiber at any point in time. This can help installers when fibers have been damaged or altered after installation, proving where responsibility for the damage lies. In fact, some customers will demand OTDR testing as a condition for system acceptance.

Although OTDRs are not especially accurate for loss testing, they can be used to conduct loss testing on long, outdoor runs of singlemode fiber where access to both ends of the cable isn’t practical. It can also be helpful for preventive maintenance procedures, such as routine checkups on a facility’s fibers.

Characteristics of OTDR

Rayleigh scattering refers to the irregular scattering generated when the optical signals transmitting in the fiber. OTDR only measure the scattered light back on the OTDR port. The backscatter signal show the attenuation degree (loss/distance) of the optical fiber, and will be tracked as a downward curve, illustrating the power of backscatter is decreasing, this is because that both transmission signal and backscatter loss are attenuated.given the optical parameters, Rayleigh scattering power can be marked, if the wavelength is know, it is proportional with the pulse width of the signal: the longer the pulse width, the stronger backscatter power. Rayleigh scattering power is also related to the wavelength of transmitted signal: the shorter the wavelength, the power is stronger. That is to say, the backscatter loose generated by the trajectory of 1310nm will higher than that of 1550nm signals.

In the higher wavelength region (more than 1500nm), the Rayleigh scattering will continue to decrease, and the other one phenomenon which called infrared attenuation (or absorption) will appear to increase and cause an increase the overall attenuation values. Therefore, 1550nm wavelength is the lowest attenuation, this also explains why it is a long distance communication wavelength. Naturally, these phenomena will return to affect the OTDR. OTDR of 1550nm wavelength is also have low attenuation, so it can be used for long distance testing. While as the high attenuation wavelength 1310nm or 1625nm, OTDR testing distance is bound to be limited, because the test equipment need to test a sharp front in the OTDR trace, and the end of the spikes will quickly fall into the noise area.

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.


OTDRs are invaluable test instruments that can illuminate problems in your optical fiber before they bring your system to its knees. Once you’re familiar with its limitations and how to overcome them, you’ll be prepared to detect and eliminate your optical fiber events. Fiber-MART can offer OTDRs are available with a variety of fiber types and wavelengths, including single mode fiber, multimode fiber, 1310nm, 1550 nm, 1625 nm, etc.. And we also supply OTDRs of famous brands, such as AFL Noyes OFL & FLX series, JDSU MTS series, EXFO FTB series, YOKOGAWA AQ series and so on. OEM portable and handheld OTDRs (manufactured by Fiber-Mart) are also available.Pls not hesitate to contact us for any question, for more information, welcome to visit or E-mail:

Author: Fiber-MART.COM

eShop of Fiber Optic Network, Fiber Cables & Tools

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