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Realizing low loss air core photonic crystal fibers by exploiting an antiresonant core surround

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Abstract

The modal properties of an air core photonic crystal fiber which incorporates an anti-resonant feature within the region that marks the transition between the air core and the crystal cladding are numerically calculated. The field intensity at the glass/air interfaces is shown to be reduced by a factor of approximately three compared to a fiber with more conventional core surround geometry. The reduced interface field intensity comes at the expense of an increased number of unwanted core interface modes within the band gap. When the interface field intensity is associated with modal propagation loss, the findings are in accord with recent measurements on fabricated fibers which incorporate a similar antiresonant feature.

©2005 Optical Society of America

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Figures (8)

Fig. 1.
Fig. 1. Stacks for the fabrication of HC-PCF. The two cases (a) and (b) differ in the thickness of the central inserted core tube. In case (a), the central core tube is chosen so that the core surround of the fabricated fiber is similar to the strut width in the cladding. In case (b), the core tube is thicker so as to produce a core surround of antiresonant thickness.
Fig. 2.
Fig. 2. (a) The confinement loss of a silica annulus as a function of its thickness t, for core radii R=6 μm and R=10 μm. The wavelength is 1.55 μm. (b) The spatial distribution of the H-field intensity at the antiresonant thickness t=0.37 μm (marked on Fig. 2(a) by the downwards arrow) for the annulus of inner radius R=10 μm. An intensity minimum occurs at the inner interface at this thickness.
Fig. 3.
Fig. 3. (a) The normalized interface field intensity F, defined by Eq. (2), for a silica annulus plotted as a function of its thickness t. The traces are for inner radii R=6 μm and R=10 μm. The wavelength is 1.55 μm. (b) The light-in-glass power fraction η, defined by Eq. (3), as a function of t for the same inner radius values. Both F and η show minima close to the antiresonance thickness.
Fig. 4.
Fig. 4. Two designs of HC-PCF whose optical properties are to be compared. In case (a), the core surround has a thickness t=0.094Λ, where Λ is the cladding pitch, which gives antiresonance within the cladding band gap. In case (b), t=0.031Λ, which equals the strut-thickness within the cladding. Both the fibers (a) and (b) have identical cladding structures.
Fig 5.
Fig 5. The geometry of a cladding unit cell. The claddings of the fibers in Fig. 4 correspond to L 1/L 2=0.6 and w=0.031Λ, with Λ the lattice pitch.
Fig. 6.
Fig. 6. (a) The normalised interface field intensity F for HE11-like modes, plotted as a function of normalised wavenumber kΛ for the two fibers shown in Fig. 4. (b) The light-in-glass power fraction, η, as a function of normalized wavenumber for the HE11-like modes of these fibers.
Fig. 7.
Fig. 7. The normalised interface field intensity F plotted against the light-in-glass power fraction, η for the fiber with an antiresonant core surround. Each dot corresponds to a different wavenumber within the band gap.
Fig. 8.
Fig. 8. The mode field intensity distribution (log scale, 60 dB range shown), for the fiber shown in Fig 4(a), at the wavenumber kΛ=16.9 where the normalised interface field intensity F is minimized. Near nulls appear over much of the inner interface of the core surround.

Equations (3)

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t = ( 2 j + 1 ) λ ( 4 n gl 2 1 ) ,
F = ( ε 0 μ 0 ) 1 2 hole perimeters d s E 2 A c d S ( E H ) z ̂ ,
η = glass annulus d S ( E H ) . z ̂ A c d S ( E H ) . z ̂ .
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