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Experimental and scalar beam propagation analysis of an air-silica microstructure fiber

C. E. Kerbage, B. J. Eggleton, P. S. Westbrook, and R. S. Windeler

Open Access Open Access

Abstract

We study the higher order guided modes in an air-silica microstructure fiber comprising a ring of six large air-holes surrounding a Germanium doped core. We characterize the modes experimentally using an intra-core Bragg grating. The experimentally observed modes are then accurately modeled by beam propagation simulations using an index profile similar to the observed fiber cross section. Theory and experiment confirm the presence of “inner cladding” modes with approximate cylindrical symmetry near the core, similar to conventional cladding modes, but which strongly exhibit the symmetry of the microstructure at large radius. Such modes are useful in fabricating robust tunable grating filters and we show that the Bragg grating is a useful diagnostic to measure their effective indices and intensity profiles.

©2000 Optical Society of America

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Figures (9)
Fig. 1.
Fig. 1. (a) Cross-section of the air-silica microstructure fiber. (b) Schematic diagram of simulated fiber

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Fig. 2.
Fig. 2. (a) Schematic diagram of Bragg grating in a fiber. Incident core mode is coupled to counter-propagating higher order cladding modes. (b) Transmission spectrum of a Bragg grating written in the core of a conventional fiber. The simulated profiles of the modes shown correspond to the first three order cladding modes (LP01, LP02, and LP03)

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Fig. 3.
Fig. 3. (a) Transmission Spectrum of Bragg grating in the ASMF. (b) First four order modes (A, B, C, D) confined to the inner cladding region.

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Fig. 6.
Fig. 6. Transmission spectrum of Bragg grating in ASMF with (blue) and without (red) index matching gel surrounding the fiber. The inset shows the cladding mode slightly affected by external index.

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Fig. 7.
Fig. 7. Setup experiment. Light incident from a tunable laser on the grating written in the core of the fiber is coupled back into different modes which are observed on the screen.

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Fig. 8.
Fig. 8. (a) Experimental and (b) simulated mode spectrum. (a) The measured spectrum, shown in black, is plotted as transmission loss (in arbitrary units) versus wavelength. (b) The simulated plot shows the relative power versus the wavelength calculated from the effective indices of each mode.

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Fig. 11.
Fig. 11. Mode (F) simulated (a) and observed (b). Some of the energy of the mode tunnels to the outer cladding.

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Fig. 12.
Fig. 12. (a) Experimental and (b) simulated mode spectrum. (a) The measured spectrum is plotted as transmission loss in arbitrary units versus wavelength. (b) The simulated mode spectrum, with an off-axis launch, is plotted as relative power versus wavelength. The simulated plot shows the excited odd modes which correspond to those observed on the transmission spectrum.

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Fig. 13.
Fig. 13. Simulated mode spectra of the ASMF under study (top) and of conventional fiber with a cladding diameter (D) of 40 mm (bottom), which is inverted in order to emphasize the location of the peaks.

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Tables (1)

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Table 1. The effective index differences between the core and the ith cladding mode, calculated using Eqs.(1) and (2), are compared to the simulated values. The wavelength values correspond to those at which the modes are observed

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Equations (2)

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λ core = 2 n core Λ
λ clad , i = ( n core + n clad , i ) Λ
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