scope:

Insertion loss is the most frequent measurement performed on a fibre optic link. The avionic system designer will want to know or predict the insertion loss of a link to determine its performance. Aircraft manufacturers will want to measure the insertion loss of harness components during assembly and before it is delivered to the customer to highlight faults and to provide a record of the performance of the harness at the beginning of its lifetime (footprinting). The insertion loss will be measured at intervals during the lifetime of the aircraft to discover or identify faults and any gradual degradation in performance of the harness. There is, however, one problem. It is difficult to collect reliable and consistent measurements of the insertion loss on any multi-mode fibre optic harness where the distance between components is relatively small (less than 100 metres). The reason is that the insertion loss of a component or a harness depends on the power distribution of the light injected into it. This leads to very large differences in the measured value of the insertion loss [1] depending on the power distribution of the source used to make the measurement. This Part of EN 4533 will explain the measurement problem and the techniques used to overcome them in greater detail.