Optical fibres – Part 1-49: Measurement methods and test procedures – Differential mode delay
|Publication Date:||1 August 2018|
|ICS Code (Fibres and cables):||33.180.10|
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 document 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 range of launch conditions, for example, those produced by standardized laser transmitters.
With this method, the output from a probe fibre that is single-moded at the test wavelength excites the multimode fibre under test. The probe spot is scanned across the end-face of the fibre under test at specified radial positions, and a set of response pulses are acquired at these positions.
Three specifiable parameters can be derived from the collected set of data.
• The first parameter, differential modal delay (DMD), is the difference in optical pulse delay time between the fastest and slowest mode groups of the fibre under test. DMD specifications place limits on modal delay over a specified range of probe fibre radial offset positions. DMD specifications are determined by modeling and experimentation to correspond to a minimum effective modal bandwidth (EMB) for the expected range of transmitters used in a link at a given performance level.
• The second specifiable parameter is derived by combining the pulses using sets of specific radial weights to determine an approximation of a set of pulses from typical transmitters. Using Fourier transforms, the calculated effective modal bandwidth (EMBc) is determined for each weight set. The minimum of these EMBc values (minEMBc) is the specifiable parameter.
• The third specifiable parameter, the computed overfilled launch bandwidth, OMBc, is determined in a manner similar to EMBc, but by applying just one weight set to the set of pulses; this weight set corresponds to the overfilling condition, where all mode groups are equally excited.
The test's intent is to quantify the effects of interactions of the fibre modal structure and the source modal characteristics excluding the source's spectral interaction with fibre chromatic dispersion. Adding the effects of fibre chromatic dispersion and the source spectral characteristics will reduce the overall transmission bandwidth, but this is a separate calculation in most transmission models. In this test, the contribution of chromatic dispersion is controlled by limiting the spectral width of usable test sources. Practical test sources will have non-zero spectral width and will thus slightly distort the DMD, minEMBc and OMBc values. These chromatic dispersion effects are considered in Annex A.
NOTE Comparison between IEC 60793-1-49 and ITU recommendations: ITU-T Recommendation G.650.1  contains no information on how to measure the DMD of a graded-index multimode fibre.