Mode Recovery by S ^2 Imaging Without a Fourier Transform

In its standard implementation, spatially and spectrally resolved (S^2) imaging consists in recording near field intensity patterns as a function of the wavelength, followed by a Discrete Fourier Transform (DFT) to assess differential group delays, as well as optical power distribution between modes for the given injection as long as the higher-order mode powers are low. Here we present a variant of S^2-imaging which does not make use of DFT. The method is more robust to wavelength jitter of the laser source. Furthermore, in special cases, the method allows retrieving relative modal powers without constraints on HOM power.