Generation of cylindrically symmetric modes and orbital-angular-momentum modes with tilted optical gratings inscribed in high-numerical-aperture fibers

Abstract

Optical fiber with high numerical aperture (NA) can efficiently relieve the degeneracy of higher-order linearly polarized modes. The degeneracy relief is investigated in two types of high-NA fibers, i.e., low-index-cladding fiber and high-index-core fiber. A naked-core fiber, as with low-index cladding, can be used theoretically to generate the orbital-angular-momentum mode (OAMM) ${\mathrm{HE}}_{21}$ and the cylindrically symmetric modes (CSMs) ${\mathrm{TM}}_{01}$ and ${\mathrm{TE}}_{01}$. A high-index-core fiber incorporated with high-contrast-index structure can be used similarly to obtain OAMM ${\mathrm{HE}}_{31}$. The generation of both CSMs and OAMMs required tilted optical gratings to couple the fundamental core mode ${\mathrm{HE}}_{11}$ into these modes. The tilt angle and modulation period of the grating fringes can be calculated simply and visually with the method proposed in this article.

© 2014 Optical Society of America

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