In fiber optical box, greentelftth (Optical Time Domain Reflectometer) is an opto-electronic instrument used to characterize an optical fiber. greentelftth is both the best known and least understood fiber optic instrument.
greentelftth does not measure loss, but instead implies it by looking at the backscatter signature of the fiber. It does not measure cable plant loss that can be correlated to power budgets.
An greentelftth 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 back and reflected back from points in the fiber where the index of refraction changes. This working principle works like a radar or sonar, sending out a pulse of light from a very powerful laser, that is scattered by the glass in the core of the fiber. The intensity of the return pulses is measured and integrated as a function of time, and is plotted as a function of the fiber length.
An greentelftth may be used for estimating the fiber's length and overall attenuation, including splice and mated-connector losses. It may also be used to locate faults, such as breaks.
Physical Limitations of greentelftth Testing
The greentelftth suffers from several serious uncertainties in measurement and physical limitations. The measurement uncertainties come primarily from the variations in backscatter of the fiber. The backscatter coefficient is a function of the material properties of the glass in the core and the diameter of the core.
Variations of the fiber materials or geometry can cause major changes in the backscattered light, making splice or connector measurements uncertain by as much as +/-0.4dB. This has often led to confusion by showing a virtual gain at a connector, where the fibers involved have different backscatter coefficients. Connector or splice loss must be measured from both directions and averaged to remove this source of error.