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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 14, Iss. 13 — Jun. 26, 2006
  • pp: 6291–6296
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An improved photonic bandgap fiber based on an array of rings

J. M. Stone, G. J. Pearce, F. Luan, T. A. Birks, J. C. Knight, A. K. George, and D. M. Bird  »View Author Affiliations


Optics Express, Vol. 14, Issue 13, pp. 6291-6296 (2006)
http://dx.doi.org/10.1364/OE.14.006291


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Abstract

We describe the modeling, fabrication and characterization of a silica-core photonic bandgap fiber based on a 2-d array of raised-index cladding rings. The use of rings to form the cladding is shown to re-order the cladding modes in such a way as to broaden the photonic band gaps and reduce bend sensitivity. We compare the performance of the ring fiber with that of a similar fiber made using solid rods.

© 2006 Optical Society of America

1. Introduction

2. Computational analysis

Figures 1 and 2 show the DOS for an array of rings with pitch Λ, fixed outer diameter d=0.70Λ and various inner diameters D between D=0 (ie rods) and a maximum of D=0.91d. Figures 1 and 2 cover smaller and larger D (respectively) and are plotted on different scales, to follow features of interest in the plots. In both figures the photonic bandgaps, within which propagation in the cladding is forbidden, are highlighted in red; the regions of non-zero DOS are shown in grayscale with white representing regions of high DOS and black, regions of low DOS. The DOS for the array of rods shown at the top of Fig. 1 is typical of an all-solid PBG cladding; the structure of similar arrays of rods has been shown and discussed elsewhere [2

2. A. Argyros, T. A. Birks, S. G. Leon-Saval, C. M. B. Cordeiro, F. Luan, P. St, and J. Russell, “Photonic bandgap with an index step of one percent,” Opt. Express 13, 309–314 (2005). [CrossRef] [PubMed]

,3

3. G. Bouwmans, L. Bigot, Y. Quiquempois, F. Lopez, L. Provino, and M. Douay, “Fabrication and characterization of an all-solid 2D photonic bandgap fiber with a low-loss region (<20 dB/km) around 1550 nm,” Opt. Express 13, 8452–8459 (2005). [CrossRef] [PubMed]

,5

5. T. A. Birks, F. Luan, G. J. Pearce, A. Wang, J. C. Knight, and D. M. Bird, “Bend loss in all-solid bandgap fibres,” Opt. Express 14, 5688 (2006). [CrossRef] [PubMed]

,7

7. F. Luan, A. K. George, T. D. Hedley, G. J. Pearce, D. M. Bird, J. C. Knight, P. St., and J. Russell, “All-solid photonic band gap fiber,” Opt. Lett. 29, 2369–2371 (2004). [CrossRef] [PubMed]

]. The specific modes appearing in Fig. 1 have been identified using numerical computation, as the LPlm marked at the top of Fig. 1(a). The progressive replacement of high-index material with low-index material causes all modes to shift to higher frequencies, as would be expected from any process that reduces the effective V-value. However, the m>1 modes shift much faster than the m=1 modes, widening the frequency range between the m=1 and m>1 cutoffs of the low-l modes. Continuing the removal of high-index material in figure 2, the DOS in this frequency range is increasingly dominated by narrow m=1 bands with l>1, while the m>1 bands (including the second l=0 band) remain below the cutoff line. At the same time the bandgaps become shallower but broader.

Fig. 1. Photonic DOS for a triangular lattice of high-index inclusions (Δn=1.09%) in a low-index background (n=1.457, representing undoped silica). Specific guided modes occurring above the blue line are identified at the top of (a). The vertical axis is effective index neff=β/k; the horizontal axis is normalized frequency kΛ. The images to the right show the structure of the inclusions. Where shown, the ‘core line’ (the locus of the fundamental core-guided mode) is yellow.

3. Fabrication

We fabricated an all-solid PBGF with a high index 2-d ring matrix cladding using a multiple stacking process. Our starting material was a set of germanium-doped silica canes with a thin pure silica jacket (diameter 0.82mm, diameter ratio ~0.88). We formed the ring canes (the unit cell of our photonic crystal cladding) by inserting 49 individual canes to form a ring in the gap between a large silica central rod placed inside an outer silica tube. This structure was then drawn down to form ring canes of ~1.52mm diameter. These canes were stacked in a close-packed array, replacing the central cane with a plain silica cane to create the low-index core. Each ring in the final fiber is therefore actually a ring of closely-spaced rods, spaced by around 0.6µm. The ratio of the outer diameter of the ring to the pitch, d/Λ, of the final fiber was 0.72, and we have measured a nominal ring thickness of ~0.05d. We have estimated the effective index contrast of the ring to be 1.09% by using the average index in the starting rod. The slightly hexagonal nature of the rings which can be seen in Fig. 3(b) arose during the draw. Fibers of several diameters were drawn; the following characterization was carried out on a fiber of outer diameter 200µm, Λ~15µm and core diameter of ~20µm.

Fig. 2. Photonic DOS as shown in Fig. 1, but for a range of rings with D=0.6d to D=0.91d. The diameter of the thinnest rings (D=0.91d) was chosen so as to be similar to our experimental fiber. Note the change of both vertical and horizontal scales relative to Fig. 1; D=0.6d has been repeated here to assist in comparison.

4. Analysis

Fig. 3. The ring fiber. a) Optical micrograph of the fabricated fiber. The outer diameter is 200µm. High-index regions appear lighter in the image. b) Scanning electron micrograph of the rings close to the core. The individual germanium doped rods can just be identified within each ring. c) Near-field image at the fiber output face when the fiber input is illuminated by a broadband supercontinuum source. Fiber length is 90cm.
Fig. 4. Transmission spectra for a) the ring fiber and b) a rod fiber with a similar pitch and d/Λ. The overall shape of the transmitted spectra is due to the supercontinuum light source used in the experiments. The blue traces are the transmission through straight sections of fiber and the red traces are transmission through fibers with a bend diameter of 7.5cm. The numbers indicate the number of bright lobes measured in the high-l mode crossings at the indicated wavelengths. The peak at 1064 nm in b) and the intensity drop at ~470 nm in a) are due to the supercontinuum pump source and the short wavelength edge of the supercontinuum respectively.
Fig. 5. Near field images of unconfined modes excited by leakage from the core between bandgaps in the ring fiber (left, at a wavelength of 506 nm) and a rod fiber (right, at 736 nm). The same high-l mode (with 16 lobes; l=8) is observed at these wavelengths in the two structures.

In each case, in the straight fiber a series of low loss transmission windows can be seen, corresponding to the photonic band gaps. The changes to the cladding mode structure shown in Figs. 1 and 2 lead to significant changes to the locations and widths of the bands. The broad high-attenuation features in the rod fiber are eliminated from the ring fiber, so that the guiding regions in that structure are spectrally wider. The lowest-order bandgaps do not provide robust guidance in the ring fiber (as expected on the basis of the density-of-states calculations in Fig. 2). The crossings of the l=6, 7 and 8 modes in the rod fiber cause the narrow attenuation features marked in Fig. 4(b)). Our experimental observation of these modes is supported by detailed numerical computations (not shown). However, the shape of that transmission spectrum is dominated by the far broader features due to the crossings of lower-l cladding modes. In the ring fiber, these broad attenuation features have been swept away, leaving just the narrow loss regions due to crossings of the same high-l modes. The higher-order bandgaps in the ring structure suffer reduced bend loss when compared to those in the rod fiber. Although the actual wavelengths are a little different in the two cases, it should also be noted that the effective refractive index contrast is lower (by a factor of around 2) in the ring structure, as a result of the fabrication procedure. The improvement in bend loss performance is therefore marked, despite the bandgaps being shallower. We attribute the improvement to the very weak coupling between the core mode and the high-l modes of the cladding nodes.

5. Conclusions

We have modeled, fabricated and characterized an all-solid bandgap fiber with a 2-d ring matrix cladding, with the intention of suppressing higher-order radial modes of the high-index nodes in the cladding. This has the effect of increasing the widths of the photonic band gaps, and decreasing bend sensitivity. We have experimentally verified the improved performance by direct comparison of ring and rod fibers. Our fabrication technique enables fabrication of novel cladding structures in all-solid fibers, enabling us to construct advanced structures inside the unit cell of the photonic crystal. We anticipate that this procedure will lead to other designs of all-solid PBGF’s with enhanced characteristics.

Acknowledgments

This work was funded by the UK Engineering and Physical Sciences Research Council. J. C. Knight is a Research Fellow of the Leverhulme Trust. Ring - 506nm Rod - 736nm

References and links

1.

R. F. Cregan, B. J. Mangan, J. C. Knight, T. A. Birks, P. St., J. Russell, P. J. Roberts, and D. C. Allen, “Single-mode photonic band gap guidance of light in air,” Science 285, 1537–1539 (1999). [CrossRef] [PubMed]

2.

A. Argyros, T. A. Birks, S. G. Leon-Saval, C. M. B. Cordeiro, F. Luan, P. St, and J. Russell, “Photonic bandgap with an index step of one percent,” Opt. Express 13, 309–314 (2005). [CrossRef] [PubMed]

3.

G. Bouwmans, L. Bigot, Y. Quiquempois, F. Lopez, L. Provino, and M. Douay, “Fabrication and characterization of an all-solid 2D photonic bandgap fiber with a low-loss region (<20 dB/km) around 1550 nm,” Opt. Express 13, 8452–8459 (2005). [CrossRef] [PubMed]

4.

N. M. Litchinitser, S. C. Dunn, B. Usner, B. J. Eggleton, T. P. White, R. C. McPhedran, and C. M. de Sterke, “Resonances in microstructured optical waveguides,” Opt. Express 11, 1243–1251 (2003). [CrossRef] [PubMed]

5.

T. A. Birks, F. Luan, G. J. Pearce, A. Wang, J. C. Knight, and D. M. Bird, “Bend loss in all-solid bandgap fibres,” Opt. Express 14, 5688 (2006). [CrossRef] [PubMed]

6.

G. J. Pearce, T. D. Hedley, and D.M. Bird, “Adaptive curvilinear coordinates in a plane-wave solution of Maxwell’s equations in photonic crystals,” Phys. Rev. B 71, 195108 (2005). [CrossRef]

7.

F. Luan, A. K. George, T. D. Hedley, G. J. Pearce, D. M. Bird, J. C. Knight, P. St., and J. Russell, “All-solid photonic band gap fiber,” Opt. Lett. 29, 2369–2371 (2004). [CrossRef] [PubMed]

8.

T. A. Birks, J. C. Knight, and P. St. J. Russell, “Endlessly single-mode photonic crystal fiber,” Opt. Lett. 22961–963 (1997). [CrossRef] [PubMed]

9.

W. J. Wadsworth, N. Joly, J. C. Knight, T. A. Birks, F. Biancalana, and P. St.J. Russell, “Supercontinuum and four-wave mixing with Q-switched pulses in endlessly single-mode photonic crystal fibers,” Opt. Express 12, 299–309 (2004). [CrossRef] [PubMed]

OCIS Codes
(060.2280) Fiber optics and optical communications : Fiber design and fabrication
(060.2400) Fiber optics and optical communications : Fiber properties

ToC Category:
Photonic Crystal Fibers

History
Original Manuscript: May 26, 2006
Manuscript Accepted: June 18, 2006
Published: June 26, 2006

Citation
J. M. Stone, G. J. Pearce, F. Luan, T. A. Birks, J. C. Knight, A. K. George, and D. M. Bird, "An improved photonic bandgap fiber based on an array of rings," Opt. Express 14, 6291-6296 (2006)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-14-13-6291


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References

  1. R. F. Cregan, B. J. Mangan, J. C. Knight, T. A. Birks, P. St. J. Russell, P. J. Roberts, D. C. Allen, "Single-mode photonic band gap guidance of light in air," Science 285, 1537-1539 (1999). [CrossRef] [PubMed]
  2. A. Argyros, T. A. Birks, S. G. Leon-Saval, C. M. B. Cordeiro, F. Luan, P. St.J. Russell, "Photonic bandgap with an index step of one percent," Opt. Express 13, 309-314 (2005). [CrossRef] [PubMed]
  3. G. Bouwmans, L. Bigot, Y. Quiquempois, F. Lopez, L. Provino, M. Douay, "Fabrication and characterization of an all-solid 2D photonic bandgap fiber with a low-loss region (< 20 dB/km) around 1550 nm," Opt. Express 13, 8452-8459 (2005). [CrossRef] [PubMed]
  4. N. M. Litchinitser, S. C. Dunn, B. Usner, B. J. Eggleton, T. P. White, R. C. McPhedran, C. M. de Sterke, "Resonances in microstructured optical waveguides," Opt. Express 11, 1243-1251 (2003). [CrossRef] [PubMed]
  5. T. A. Birks, F. Luan, G. J. Pearce, A. Wang, J. C. Knight, D. M. Bird, "Bend loss in all-solid bandgap fibres," Opt. Express 14,5688 (2006). [CrossRef] [PubMed]
  6. sections I and II of G. J. Pearce, T. D. Hedley, D.M. Bird, "Adaptive curvilinear coordinates in a plane-wave solution of Maxwell’s equations in photonic crystals," Phys. Rev. B 71, 195108 (2005). [CrossRef]
  7. F. Luan, A. K. George, T. D. Hedley, G. J. Pearce, D. M. Bird, J. C. Knight, P. St.J. Russell, "All-solid photonic band gap fiber," Opt. Lett. 29, 2369-2371 (2004). [CrossRef] [PubMed]
  8. T. A. Birks, J. C. Knight and P. St. J. Russell, "Endlessly single-mode photonic crystal fiber," Opt. Lett. 22961-963 (1997). [CrossRef] [PubMed]
  9. W. J. Wadsworth, N. Joly, J. C. Knight, T. A. Birks, F. Biancalana, P. St.J. Russell, "Supercontinuum and four-wave mixing with Q-switched pulses in endlessly single-mode photonic crystal fibers," Opt. Express 12, 299-309 (2004). [CrossRef] [PubMed]

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