Optical reflectometer

Optical reflectometer FOD 7302

Principle of operation

Reflectometers use special pulsed laser diodes to transmit a series of very short powerful light pulses over an optical cable. When transmitting pulses from the reflectometer through the cable, most of the light moves in the direction of the cable. Highly sensitive photodetectors measure the amount of light reflected or scattered during passage inside the cable. Using such measurements, the reflectometer detects the reasons for the decrease or reflection of the power of the source when light passes through the cable. For example, a small portion of the light is scattered in the other direction, which is due to the normal structure (and minor defects) of the glass of which the fiber is composed. The phenomenon of light scattering due to defects in fiber glass is called Rayleigh scattering. A certain percentage of scattering is expected for cables of a certain length based on the attenuation factor of the optical fiber. When a light pulse collides with connections, fractures, cracks, splices, steep bends or the end of a cable, it is reflected due to an unexpected change in refractive index.

This phenomenon is called Fresnel reflection. The ratio of the amount of reflected light except the backflash from the fiber itself to the source pulse is called reflectivity. This value is expressed in decibels and for passive optics is usually negative with values approaching zero, which indicates a greater reflectivity, and therefore a low quality of compounds with high losses. Reflectometry results are displayed as a graph, or reflectogram, reflected and scattered in the opposite direction of light compared to the distance along the fiber. One axis (Y) shows the power level, the other axis (X) shows the distance. In the left-to-right graph, the scattering value decreases because the path loss increases as light travels. Reflectograms have several common characteristics. Most reflectograms begin with an initial incoming pulse, which entails a Fresnel effect that occurs at the point of connection of the reflectometer. Like the pulse, the reflectometer response is a smooth curve tilted downward and interrupted by successive shifts. The gradual slope is due to Rayleigh scattering within each cable segment. The slope is interrupted by abrupt shifts that reflect the local deviations of the up and down graph. The loss is reflected as a downward shift in the graph. These shifts and point defects are usually due to the presence of joints, splices and breaks.

The end of the cable can be determined by a large emission, after which the reflectogram abruptly goes down. The output signal at the end of the reflectogram is due to the Fresnel reflection occurring at the end of the cable. The reflectogram allows you to confirm the compliance of all works and installation quality with the requirements of the standards, as well as the ability to support existing and future applications. For example, the total requirement for losses in splices should not exceed 0.3 dB, in connections - not more than 0.75 dB. The losses caused by such individual events are invisible in the application tests. If the splicing or connection does not meet the requirements, the installation technician can correct this directly on the work site.

Producers

Acterna

Anritsu

Fluke Networks

FOD

Tempo

Yokogawa

Svyazservice

SvyazPribor

See also

• Reflectogram
• Optical transistors
• Halide perovskites
• Optical computers