Limitations of the S-2 (spatially and spectrally resolved) imaging technique

Spatially and spectrally resolved imaging (S-2 imaging) is a technique that was developed as an alternative to beam quality (M-2) measurements to characterize the modal content of large mode area (LMA) fibers. While it is known that the success of S-2 imaging is highly dependent on the broadband source and the launch conditions into the fiber, the information resulting from this method is more limited than may appear at first glance. Experiments and numerical simulations are used to show that (a) the accuracy of the reconstructed LP01 mode profile varies greatly depending on launch conditions, and (b) there are always errors in the reconstructed LP11 mode profile. Not only do these findings reveal that the reconstructed mode areas cannot be accurately determined, but it also shows that the relative modal intensities can only be determined accurately under nearly perfect conditions of fundamental mode launch and propagation through the fiber. Despite these findings, the S-2 technique can still be used under nearly ideal launch and propagation conditions to identify the modes and their group delays, and accurately reconstruct only the fundamental mode. (C) 2021 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement