Practical Attenuation Measurement

Attenuation measurement is carried out on the installed cable section by means of a stabilized light source and a power meter. Transmission is measured in both directions, at both 1310 and 1550nm. 
The connectors must be cleaned thoroughly before connecting the fibers and taking readings, since a dirty connector can affect the reading significantly

To prevent the light sources connector from becoming worn out, a dual connector patch cord should be connected to the output of the light source the other end of the patch cord should be provided with a bushing. The patch cord must remain connected to the light source for the entire period for measurement( removing it might change the coupling loss readings). Before measurement, the light source and the laser diode module should be turned on for at least 10 minutes (heating time) to ensure a stable light flow. Measurements should be carried out with un-modulated (CW) light.

 The light source is placed at the cables termination point, depending on the direction in which measurement is to be made. A reference value is then obtained from among five separate measurements i.e. after connection and disconnection of the bushing.

The connection of a stabilized light source and power meter for determining the reference value.

The second lowest reading is selected as the reference value. For example, if the following readings were obtained : -1.82,-2.10,-1.85,-1.92 and –1.88 dBm, -1.85 would be selected as the reference value.

The power meter is then shifted to the other end of the cable installation and each fiber is connected to the transmitter and receiver. Five readings are again taken

 When all readings have been taken the attenuation value for each fiber can be calculated. Only the second lowest reading is used in this calculation. For example, if the following readings , -10.56, -10.45, -10.49, -10.66 and –10.37 dBm, were obtained, the value for the calculation would be –10.45 dBm. The total attenuation in the fiber is obtained by substracting this value from the reference value.

Example:     (-1.85 – (-10.45) = 8.6 dB.

 Setup for measurement the cable attenuation of an operational installation.