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Guidance properties of low-contrast photonic bandgap fibres

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Abstract

We investigate the guidance properties of low-contrast photonic band gap fibres. As predicted by the antiresonant reflecting optical waveguide (ARROW) picture, band gaps were observed between wavelengths where modes of the high-index rods in the cladding are cutoff. At these wavelengths, leakage from the core by coupling to higher-order modes of the rods was observed directly. The low index contrast allowed for bend loss to be investigated; unlike in index-guiding fibres, anomalous “centripetal” light leakage through the inside of the bend can occur.

©2005 Optical Society of America

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Figures (6)

Fig. 1.
Fig. 1. (a) A typical bandgap fibre cross-section, a schematic of an elliptical rod, and calculated intensity distributions of some modes of a rod. For elliptical rods, some modes split into odd/even versions about the minor axis. The modes are shown in the correct order for small deviations from circularity (b/a > 0.6). (b) The calculated confinement loss for a bandgap fibre with n r=1.465, n m=1.45 (1% index contrast), d/Λ=0.4 and circular rods, for different numbers of rings of rods around the 7-cell core. The cutoffs of modes of the rods are indicated. (c) and (d) The calculated confinement loss for the same fibre with 5 rings but for elliptical rods with b/a=0.8 and 0.6.
Fig. 2.
Fig. 2. Scanning electron micrographs of samples of fibres (left to right) B, C and D. Elliptical deformations are clearly seen in C and D. The black regions in C and D are (random) air bubbles that, apart from possibly contributing to the loss, did not appear to affect the guidance properties of the fibres.
Fig. 3.
Fig. 3. (a) Transmission in the first two bandgaps of fibre B (Λ=7.5 µm, length 0.15 m) for illumination of the core alone. Insets are filtered images of the fibre endface for wide illumination. (b) Transmission in the first 4 bandgaps of a sample of fibre C (Λ=10 µm, length 0.4 m). Insets are modes excited in the rods by focusing light into the core at the high-loss wavelengths for a sample of fibre C. (c) Transmission of fibre D (Λ=6 µm, length 0.4 m). Insets show the two modes supported by the core (LP01 and oLP11) at λ=1064 nm. The high-order mode cutoffs are indicated, corresponding to high-loss wavelengths. Spectra have been normalised to give 0 dB transmission in the first bandgap.
Fig. 4.
Fig. 4. (a) Cutback loss measurement on 15 m of a sample of fibre C with Λ=5 µm. Inset: typical near-field image of the fundamental mode in the first bandgap. (b) Normalised transmission of fibre C tapered from Λ=11.5 to 6.6 µm over 2.0 m to give a transmission window of 50 nm FWHM. For comparison, the transmission windows of uniform sections with Λ=6.6 and 11.5 µm were approximately 300 and 600 nm wide respectively.
Fig. 5.
Fig. 5. (a) Schematic index profile of straight and bent step-index fibres. The red line is the core mode’s n eff and is resonant with radiation modes in the cladding only on the outside of the bend. (b) A calculated plot of the n eff of the first two bands of a bandgap fibre, with the low-index line in red. (The core mode line will be only slightly below the low-index line, the exact trajectory being dependent on the size of the core.) (c) Schematic variation of the band n eff across a slightly bent bandgap fibre, for wavelengths at (left to right) the blue edge, middle and red edge of the bandgap.
Fig. 6.
Fig. 6. (a) Transmission spectra of a bandgap fibre for different bend radii. (b) Average loss as a function of bend radius for wavelength ranges at both edges and the middle of the bandgap, 600–700 nm corresponding to the blue edge, 770–980 nm to the middle and 1100–1270 nm to the red edge.

Equations (3)

Equations on this page are rendered with MathJax. Learn more.

V = 2 π N A r λ ,
N A = ( n r 2 n m 2 ) 1 2 .
n ( x ) = n o ( x ) [ 1 + ( 1 χ ) x R ] ,
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