  We propose and demonstrate two methods for modal decomposition in multi-mode
fibres. Linearly polarized modes propagating in a slightly multi-mode fibre are
easily retrieved from intensity measurements at the fibre output surface. The
first method is an improvement of the so-called spectrally and spatially
imaging technique, which is limited to largemode-area optical fibers. The
second method is a new, simpler and faster solution for the characterization of
any kind of optical fibre, thus attractive in comparison to previously reported
methods, which are cumbersome, time-consuming and/or limited to large-more-area
fibres. Different kinds of multi-mode optical fibres are characterized. A
large-modearea photonic-bandgap fibre, a photonic-crystal small-core non-linear
fibre, and a standard index-stepped multi-mode fibre are characterized
successfully.
