Break up of the azimuthal symmetry of higher order fiber modes

Abstract

Abstract:We investigate Bessel-like modes guided in a double cladding fiber where the outer cladding is an aircladding. For very high order LP 0X -modes, the azimuthal symmetry is broken and the mode is no longer linearly polarized. This is observed experimentally and confirmed numerically. The effect is investigated numerically using a full vectorial modesolver and is observed to be dependent on the fiber design. The effect on the diffraction free propagation distance of the modes is investigated using a fast Fourier transform propagation routine and compared to the properties of an ideal circularly symmetric mode. The free space properties of modes suffering from break up of azimuthal symmetry are also investigated experimentally by measuring the free space propagation of a LP 016 -mode excited in the double cladding fiber. (2012), p. CTu3M.6. 3. E. Li, X. Wang, and C. Zhang, "Fiber-optic temperature sensor based on interference of selective higher-order modes," Appl. Phys. Lett. 89(9), 091119 (2006).

The Bowtie Effect in Cylindrical Waveguides

“…for amplification [10] and nonlinear processes [11]- [14], the LP approximation is insufficient. More specifically, as reported for the first time in [15], the circular symmetry of Bessellike modes is broken when the mode order (i.e. the number of rings) increases sufficiently.…”

“…Qualitatively, (15) shows that higher values of n eff , i.e. those close to n c , imply lower magnitude of the oscillating term, compared to that of the constant term.…”

“…This occurs independently of the refractive index profile, as long as it is a function of the radius only. When |ν| = 1, this oscillation generates what has been denoted bowtie effect in Bessel-like modes, therefore named bowtie modes [15]. The generality of (9) ensures that the above conclusion applies to any cylindrical fiber.…”

Polymer-clad silica fibers for tailoring modal area and dispersion

“…This uniformly (e.g., linearly) polarized approximation gradually breaks down as the field strength at a large refractive index step increases, in any waveguide [14–16], because the radial and azimuthal components of the electric field must satisfy different boundary conditions. Consequently, the electric fields of the HE 1,m modes become quasi-radially polarized, while another set of vector modes, the EH 1,m modes (part of the LP 2,m mode in the scalar picture), become quasi-azimuthally polarized.…”

“…Since the polymer-clad fibers discussed herein have especially high index contrast (by design, to tailor dispersion), we expect the modes of these fibers to also show significant polarization non-uniformities Figure 3(a)–(d) shows the simulated intensity distribution overlaid with polarization projections, for a lower and higher order mode pairs comprising the almost degenerate HE 1,m and EH 1,m-1 modes for our fiber, simulated using a full-vectorial mode solver described in [17]. For lower mode orders, the LP approximation holds well, but for higher mode orders, the polarization, and even the intensity distribution along the azimuth, becomes non-uniform (these high order modes have been named “bow-tie” modes [15,16]). Stated differently, as mode order increases (and the mode field increasingly encounters the high-index step boundary), the HE 1,m modes’ polarization extinction ratio (PER), the power ratio between the maximum and minimum linear polarization projection of the mode, decreases dramatically [Fig.…”