This part of IEC 60793 applies only to multimode, graded-index glass-core (category A1)
fibres. The test method is commonly used in production and research facilities, but is not
easily accomplished in the field.
This standard describes a method for characterizing the modal structure of a graded-index
multimode fibre. This information is useful for assessing the bandwidth performance of a fibre
especially when the fibre is intended to support a variety of launch conditions such as those
produced by standardized laser transmitters.
With this method, the output from a fibre that is single-mode at the test wavelength excites
the multimode fibre under test. The probe spot is scanned across the endface of the fibre
under test, and the optical pulse delay is determined at specified offset positions.
Two results can be produced from the same data. First, the difference in optical pulse delay
time between the fastest and slowest mode groups of the fibre under test can be determined.
The user specifies the upper and lower limits of radial offset positions over which the probe
fibre is scanned in order to specify desired limits of modal structure. The DMD data is then
compared to DMD specifications that have been determined by modeling and experimentation
to correspond to a minimum EMB for a range of transmitters. Second, the optical pulse
shapes can be combined using specific weights to determine a calculated effective modal
bandwidth (EMBc), and by calculating a sequence of EMBc values with different sets of
weights, a minimum EMBc can be calculated, corresponding to a range of transmitters.
The test quantifies the effects of interactions of the fibre modal structure and the source
modal characteristics excluding the source spectral interactions with fibre chromatic
dispersion. Adding the effects of chromatic dispersion and source spectral width will reduce
the overall transmission bandwidth, but this is a separate calculation in most transmission
models. In this test, the effects of non-zero spectral width are minimized but any residual
effects will tend to increase the DMD value and decrease the EMBc value.