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How to use the OTDR launch cable properly

What is OTDR?

The OTDR(Optical Time Domain Reflectometer) is an device that tests the integrity of a fiber cable, it uses the inherent backscattering properties of the optical fiber to detect faults and categorize its condition. The OTDR sends high-power pulses of laser light down the fiber and captures the light that is reflected back. By measuring the timing and power levels of the return pulses, the device correlates the reflected information with physical locations along the fiber and display a “trace” that shows optical power versus distance. Attenuation of the fiber is displayed as the slope of the trace. Interruptions such as splices, connectors, bends, breaks or flaws in the fiber appear as transitions (event) that represent their nature and location.

What is otdr launch cable?

OTDR launch cables are developed to be connected with an OTDR and the test fiber to measure complete link loss of the fiber. They are also known as launch cords, launch leads, fiber rings, receive cables or pulse suppressor cables. Actually, it is nothing more than a fiber patch cord, but it is usually longer than the normal fiber patch cords.

OTDR Launch cable box FC SC LC

Why use a OTDR launch cable?

First we need to understand the OTDR dead zone.

OTDR dead zone is defined as the distance where the OTDR cannot detect or precisely localize any event or artifact on the fiber link. It is caused by a strong reflection (mainly caused by air gap at OTDR connection) and the subsequent recovery time of the OTDR detector. Once a strong reflection occurs, the power received by the photodiode can be more than 4,000 times higher than the backscattered power, which saturate the OTDR detector. Thus, it spends time to recover from its saturated condition. During the recovering time, it is unable to detect the backscattered signal accurately which results in corresponding dead zone on OTDR trace. This is similar to your eyes recovering from looking at the bright sun or the flash of a camera. In general, the higher the reflection, the longer the dead zone is. Furthermore, dead zone is also influenced by the pulse width. A bigger pulse width can increase the dynamic range while results in a longer dead zone.

The OTDR launch cable is compensation for the OTDR deadzone. With OTDR launch cable, we can view and measure all events especially those masked after the OTDR bulkhead.

The length of a launch cable?

Set the dynamic range & pulse width to be able to see entire fiber & sufficient backscatter.
Usually x6 pulse width is required for sufficient backscatter to measure the insertion loss. And if using too long of a launch cable will reduce the resolution of the measurements. Here x10 pulse width is suggested, please see the table below.

Pulse width(ns) Launch cable Length(m)
5 10
10 20
30 60
100 200
300 600
1000 2000

Important Tips

You must keep the Connectors are clean while testing.
Most of all failures are happened due to no cleaning or improper cleaning of the connectors. Contaminated or damaged connectors will cause high reflection which will saturate the OTDR detector.

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