What is an OTDR?

OTDR, short for optical time-domain reflectometer, is an optoelectronic instrument used to characterize an optical fiber. It can be considered as the optical equivalent of an electronic time-domain reflectometer.

OTDR injects a series of optical pulses into the fiber under test. It also extracts, from the same end of the fiber,light that is scattered or reflected back from points along the fiber. The strength of the return pulses is measured and integrated as a function of time, and is plotted as a function of fiber length.

It may be used for estimating the fiber length and overall attenuation, including splice and mated-connector losses. It may also be used to locate faults, such as breaks, and to measure optical return loss. To measure the attenuation of multiple fibers, it is advisable to test from each end and then average the results, however this considerable extra work is contrary to the common claim that testing can be performed from only one end of the fiber.

In addition to required specialized optics and electronics, OTDRs have significant computing ability and a graphical display, so they may provide significant test automation. However, proper instrument operation and interpretation of an OTDR trace still requires special technical training and experience.

(Reference: FIBERSTORE OTDR Tutorial)

How Does an OTDR Work?

OTDR fiber tester works indirectly by using a unique phenomena of fiber to imply loss, unlike fiber optic light sources and power meters which measure the loss of the fiber optic cable plant directly by duplicating the transmitter and receiver of the fiber optic transmission links. It works like a radar. It first to send a signal for optical fiber, and then observe what return from one point to the information. This process will be repeated, then the results were averaged and to be displayed in the form of track, the track is described within the whole period of optical fiber (or the state) of the fiber on the strength of the signal.

As light travels along the fiber, a small proportion of it is lost by Rayleigh scattering. Rayleigh scattering is caused by the irregular scattering signal along the fiber produced. Given fiber optic transceiver parameters, the Rayleigh scattering power can be marked out. If the wavelength is known, it is proportional to the signal of pulse width, the longer backscattering, the stronger power. Rayleigh scattering power is related to the wavelength of emission signal, the shorter wavelength, the stronger power. That is to say, 1310nm signal path of the Rayleigh backscattering is higher than 1550 nm Rayleigh backscattering.

OTDR uses Rayleigh scattering to represent the characteristics of fiber optic. OTDR measurements back to part of scattering light to the OTDR port. As the light is scattered in all directions, some of it just happens to return back along the fiber towards the light source. This returned light is called backscatter as shown below.

The backscatter power is a fixed proportion of the incoming power and as the losses take their toll on the incoming power, the returned power also diminishes as shown in the figure.

OTDR uses the backscattered light to make its measurements. It sends out a very high power pulse and measures the light coming back. It can continuously measure the returned power level and hence deduce the losses encountered on the fiber.

Any additional losses such as connectors and fusion splices have the effect of suddenly reducing the transmitted power on the fiber and hence causing a corresponding change in backscatter power. The position and degree of the losses can be ascertained. At any point in time, the light the OTDR sees is the light scattered from the pulse passing through a region of the fiber.

Think of the OTDR pulse as being a virtual source that is testing all the fiber between itself and the OTDR as it moves down the fiber.Since it is possible to calibrate the speed of the pulse as it passes down the fiber, the OTDR can correlate what it sees in backscattered light with an actual location in the fiber. Thus it can create a display of the amount of backscattered light at any point in the fiber.

There are some calculations involved. Remember the light has to go out and come back, so you have to factor that into the time calculations, cutting the time in half and the loss calculations, since the light sees loss both ways. The power loss is a logarithmic function, so the power is measured in dB.

The amount of light scattered back to the OTDR is proportional to the backscatter of the fiber, peak power of the OTDR test pulse and the length of the pulse sent out. If you need more backscattered light to get good measurements, you can increase the pulse peak power or pulse width as shown in the picture.

Some events like connectors show a big pulse above the backscatter trace. That is a reflection from a connector, splice or the end of the fiber. They can be used to mark distances or even calculate the back reflection of connectors or splices, another parameter we want to test in single mode systems.

OTDRs are generally used for testing with a launch cable and may use a receive cable. The launch cable allows the OTDR to settle down after the test pulse is sent into the fiber and provides a reference connector for the first connector on the cable under test to determine its loss. A receive cable may be used on the far end to allow measurements of the connector on the end of the cable under test also.

FiberStore OTDRs are available with a variety of fiber types and wavelengths, including single mode fiber, multimode fiber, 1310nm, 1550nm, 1625nm, etc.. We also supply OTDRs of famous brands, such as JDSU MTS series, EXFO FTB series, YOKOGAWA AQ series and so on. OEM portable and handheld OTDRs (manufactured by FiberStore) are also available.Click for the OTDR price.

Since the 1990s, many small form factor (SFF) fiber optic connectors have been developed to fill the interest in devices that may fit into tight spaces and permit denser packing of connections such as fiber patch cables. Some are miniaturized versions of older connectors, built around a 1.25mm ferrule rather than the 2.5mm ferrule used in ST, SC and FC connectors. Others are based on smaller versions of MT-type ferrule for multi fiber connections, or other brand new designs.

Most of these SFF connectors have a push-and-latch design that adapts easily to duplex connectors. LC, MU, E2000 and MT-RJ would be the most typical small form factor fiber optic connectors.

1. LC Connector
LC is short for Lucent Connector, licensed by Lucent. LC connector may also be called "Little Connector". It resembles a typical RJ45 telephone jack externally, while a miniature version of the SC connector internally. LC connector uses a 1.25mm ceramic (zirconia) ferrule instead of the 2.5mm ferrule and add a push-and-latch design providing pull-proof stability in system rack mounts. LC connectors are highly favored for single mode applications such as high-density connections, SFP transceivers, XFP transceivers, etc.. Generally, LC connectors can be found in simplex and duplex, single mode and multimode versions.

2. MU Connector
MU is brief for Miniature Unit, produced by NTT. MU connector is known as "mini SC" and is popular in Japan. It has push-pull mechanism, utilizing a 1.25mm ferrule the same as LC connector. MU connectors' applications include high-speed data communications, voice networks, telecommunications, and dense wavelength division multiplexing (DWDM). They're also utilized in multiple optical connections so that as a self-retentive mechanism in backplane applications. MU connectors can be found in simplex and duplex versions.

3. E2000 Connector
E2000, as known as laser shock hardening (LSH), is a technology generally used in Telecom, DWDM systems. E2000 connector is also called LX.5 connector. It looks like a miniature SC connector externally, like the MU connector utilizing a 1.25mm ferrule. You can easily install, with a snap-in and push-pull latching mechanism which clicks when fully inserted. E2000 connector includes a spring-loaded shutter which fully protects the ferrule from dust and scratches. The shutter closes automatically once the connector is disengaged, locking out impurities which could later result in network failure, and locking in possibly damaging lasers. When it's connected to the adapter the shutter opens automatically. E2000 connectors are available in single mode and multimode versions. FiberStore provides both E2000 to ST fiber patch cable and fibre optic patch cables E2000 LC rich in quality and best price.

4. MT-RJ Connector
MT-RJ is short for Mechanical Transfer Registered Jack. MT-RJ connector's overall dimensions are comparable like a RJ45 connector. MT-RJ connector dose not make use of a 1.25mm ferrule, and it is design is derived from MT ferrule. It features a miniature two-fiber ferrule with two guide pins parallel to the fibers on the outside. The guides pins align ferrules precisely when mating two MT-RJ connectors. MT-RJ connectors are designed with male-female polarity which means male MT-RJ connector has two guide pins and feminine MT-RJ connector has two holes instead. MT-RJ connectors are utilized in intra building communication systems. Since they are designed as plugs and jacks, like RJ-45 telephone connectors, adapters can be used with a few designs, but are not required for all. MT-RJ connectors are available in duplex version only and multimode version only given that they use the two-fiber ferrules.