How does one filter a multimode signal with single-mode quality by fiber Bragg gratings (FBGs) in a multicore fiber?

T.A. Goebel and his coauthors address this question in this Applied Optics paper. Multimode signals can be transformed into single-mode signals in a multicore fiber (MCF) by a photonic lantern. In astronomy applications, it is essential to suppress the hydroxyl emission lines with high quality. Here, the authors concentrate on the challenge to inscribe Bragg gratings with identical transfer characteristics into the individual cores of the MCF.

As the tuning of gratings by lifting the mean refractive index of each individual FBG does not improve the performance sufficiently, they focus on the homogeneity of the preform and thus the cores of the MCF. Two methods were investigated: preforms with selected core material filled into drilled holes in the cladding material, and stacked preforms. After making an FBG inscription, both methods lead to low deviation in Bragg-wavelengths of the different cores, high suppression at the center frequency and narrowband reflection spectra.
  
Experts in the field as well as researchers with a basic knowledge of fiber technology and fiber components can benefit from this paper.

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