Practical Industrial Data Networks:Design, Installation and ...

92Practical Industrial Data Networks: Design, Installation and TroubleshootingAs a word of warning, the user should not look into fiber groups at the end of cables ifany fibers on the system at any location are connected to lasers. The infra-red light fromlasers cannot be seen by the human eye but can cause permanent eye damage.6.8.6.2 Insertion loss testingThe most common qualitative test that is carried out on a fiber optic system is to measurethe attenuation of the length of fiber. This figure will allow most elements of the systemdesign to be verified.Most insertion loss testing is carried out with a power source and a power meter.Firstly, the power meter is calibrated to the power source by connecting the twoinstruments together with a short piece of optic fiber approximately 2 m in length.Generally, the power source is set to transmit a level of –10 dBm and the power meterthen adjusted accordingly to read –10 dBm. Ensure that the level used to calibrate thepower meter is within the dynamic range of the power meter.There are four important points to check before commencing insertion loss testing.Firstly, ensure that the optic fiber type used for calibration purposes is the same opticfiber type that is to be tested for insertion loss. Secondly, the power meter and the powersource must operate at the same wavelength as the installed system equipment. Thirdly,the power meter and source must also use the same source and detector types (LED orlaser) that the transmitter and receiver in the installed system are to use. Fourthly, toavoid possible incorrect calibration, ensure that the same connectors are used forcalibration as are used in the installation.Once the power meter has been calibrated, then the power meter and source are takeninto the field and connected to the installed cable. The level that is read at the meter canbe used to calculate the insertion loss through the cable section under test. This willinclude the losses caused by the optic fiber, splices and the connectors. The test procedureis illustrated in Figure 6.11.If the power source and the power meter are calibrated in milliwatts, then the formulafor converting the loss figure to decibels is:Attenuation (dB) = − 10 Log ( Po / Pi)where:Po is power out of the fiberPi is power into the fiberTo calculate the insertion loss, subtract the dBm reading at the power meter from theinput power source value. For the example, shown in Figure 6.11 is the insertion loss in9.3 dB.It is recommended that the insertion loss measurement be performed in both directionsof an installed cable. The losses measured in each direction tend to vary becauseconnectors and splices sometimes connect unevenly and because the core diameter offibers tends to vary slightly. For example, if the core diameters of two fibers splicedtogether are 49.5 mm and 50.5 mm, light waves traveling from the thinner fiber into thethicker fiber will all enter the thicker fiber. For light traveling from the larger diameterfiber into the smaller diameter fiber, a small amount will be lost around the edges of theinterface between the two cores. A mismatch of this type could account for a differencein insertion loss in the two directions of 0.2 dB. If the fiber losses are different in eachdirection, then that fiber can be used in whichever direction gives it the best systemperformance.