Diffraction by random Ronchi gratings

In this work, we obtain analytical expressions for the near-and far-field diffraction of random Ronchi diffraction gratings where the slits of the grating are randomly displaced around their periodical positions. We theoretically show that the effect of randomness in the position of the slits of the grating produces a decrease of the contrast and even disappearance of the self-images for high randomness level at the near field. On the other hand, it cancels high-order harmonics in far field, resulting in only a few central diffraction orders. Numerical simulations by means of the Rayleigh–Sommerfeld diffraction formula are performed in order to corroborate the analytical results. These results are of interest for industrial and technological applications where manufacture errors need to be considered.

Fibre Bragg gratings fabrication in four core fibres

Due to the limitation of the lens effect of the optical fibre and the inhomogeneity of the laser fluence on different cores, it is still challenging to controllably inscribe different fibre Bragg gratings (FBGs) in multicore fibres. In this article, we reported the FBG inscription in four core fibres (FCFs), whose cores are arranged in the corners of a square lattice. By investigating the influence of different inscription conditions during inscription, different results, such as simultaneous inscription of all cores, selectively inscription of individual or two cores, and even double scanning in perpendicular core couples by diagonal, are achieved. The phase mask scanning method, consisting of a 244nm Argon-ion frequencydoubled laser, air-bearing linear transfer stage and cylindrical lens and mirror setup, is used to precisely control the grating inscription in FCFs. The influence of three factors is systematically investigated to overcome the limitations, and they are the defocusing length between the cylindrical lens and the bare fibre, the rotation geometry of the fibre to the irritation beam, and the relative position of the fibre in the vertical direction of the laser beam.