Distributed Analysis on the Spatial Mode Structure in a PANDA-Type Few-Mode Fiber By Brillouin Dynamic Gratings

We present the experimental analysis on the spatial mode structure in a PANDA-type few-mode fiber (PANDA-FMF) by the intermodal operation of Brillouin dynamic grating (BDG). Polarization-maintaining mode selective couplers are newly developed to selectively launch different optical modes for the pump and probe waves, and the characterization of the intra- and intermodal Brillouin scatterings in the PANDA-FMF is demonstrated. Distributions of effective refractive index (ERI) differences between six guided modes (LP
₀₁
^x
, LP
₀₁
^y
, LP
_11(o)
^x
, LP
_11(o)
^y
, LP
_11(e)
^x
, and LP
_11(e)
^y
) are measured by optical time-domain analysis of the intermodal BDG reflection spectra, and a unique ERI hierarchy among LP
₁₁
modes with different lobe orientations and polarization directions is reported for the first time in the PANDA-FMF. The analysis on the origin of measured ERI hierarchy is also presented by numerical simulations on the effect of core ellipticity.