  Mode-dependent loss (MDL) is known to be a major issue in space-division
multiplexed (SMD) systems. Its effect on performance is complex as it affects
both the data carrying signal and the accumulated amplification noise. In this
paper we propose a procedure for characterizing the MDL of SDM systems by means
of standard measurements that are routinely performed on SDM setups. The figure
of merit that we present for quantifying MDL incorporates the effect on the
transmitted signal and the noise and is directly related to the spectral
efficiency reduction.
