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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 5 — Mar. 5, 2007
  • pp: 2468–2475
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A fractal-based fibre for ultra-high throughput optical probes

S. T. Huntington, B. C. Gibson, J. Canning, K. Digweed-Lyytikäinen, J. D. Love, and V. Steblina  »View Author Affiliations


Optics Express, Vol. 15, Issue 5, pp. 2468-2475 (2007)
http://dx.doi.org/10.1364/OE.15.002468


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Abstract

A core component of all scanning near-field optical microscopy (SNOM) systems is the optical probe, which has evolved greatly but still represents the limiting component for the system. Here, we introduce a new type of optical probe, based on a Fractal Fibre which is a special class of photonic crystal fibre (PCF), to directly address the issue of increasing the optical throughput in SNOM probes. Optical measurements through the Fractal Fibre probes have shown superior power levels to that of conventional SNOM probes. The results presented in this paper suggest that a novel fibre design is critical in order to maximize the potential of the SNOM.

© 2007 Optical Society of America

1. Introduction

During the last quarter of a century, the SNOM has undergone numerous improvements to the point where several commercial companies supply these instruments in various forms worldwide [1

1. E. H. Synge, “A suggested method for extending the microscopic resolution into the ultramicroscopic region,” Phil. Mag. 6, 356 (1928).

]-[4

4. E. Betzig, M. Isaacson, and A. Lewis, “Collection mode near-field scanning optical microscopy,” Appl. Phys. Lett. 51, 2088–2090 (1987). [CrossRef]

]. A core component of all SNOM systems is the optical probe, which has evolved greatly but still represents the limiting component for the system. In order to capture non-diffraction limited optical images, the SNOM utilizes an optical probe, with a sub-wavelength aperture, to deliver an optical signal to or receive optical information from a sample of interest. If the probe is very close to the sample, the resolution is commensurate with the size of this aperture. The optical probe can be manufactured using a number of different techniques [5

5. E. Betzig, J. K. Trautman, T. D. Harris, J. S. Weiner, and R. L. Kostelak, “Breaking the diffraction barrier - optical microscopy on a nanometric scale,” Science , 251, 1468–1470 (1991). [CrossRef] [PubMed]

]-[10

10. M. Chaigneau, G. Ollivier, T. Minea, and G. Louarn, “Nanoprobes for near-field optical microscopy manufactured by substitute-sheath etching and hollow cathode sputtering,” Rev. Sci. Instrum. 77, 103702 (2006). [CrossRef]

], but typically these probes are fabricated using optical fibres that are tapered down to nanometer-scale tips and the outside is coated with a thin layer of metal [5

5. E. Betzig, J. K. Trautman, T. D. Harris, J. S. Weiner, and R. L. Kostelak, “Breaking the diffraction barrier - optical microscopy on a nanometric scale,” Science , 251, 1468–1470 (1991). [CrossRef] [PubMed]

]. The key problem with this type of probe is the excessive loss that occurs, which effectively limits the applications of the microscope [11]. Low power throughput means slow scanning speeds. The high loss arises firstly from attenuation through the sub-wavelength aperture, which is unavoidable, and secondly from the interaction of light with the metal coating in the tapered region of the tip. We intend to address the latter of these mechanisms by reducing the interaction between the transmitted light and the probes’ metal coating. Using a combination of PCF technology and a new type of fibre called a Fractal Fibre (special class of PCF), we will present a new type of ultra high throughput probe that will maximize the potential of the SNOM.

2. Fractal fibre concept

When standard optical fibre (eg. SMF-28e) is used to produce tapered metal-coated SNOM probes, dopant diffusion occurs during the tapering process due to the high temperatures involved and there is a substantial change in fibre geometry [12

12. S. T. Huntington, S. J. Ashby, M. C. Elias, and J. D. Love, “Direct measurement of core profile diffusion and ellipticity in fused-taper fibre couplers using atomic force microscopy,” Electron. Lett. , 36, 121–123 (2000). [CrossRef]

]. This can be observed schematically in Fig. 1(a).

Fig. 1. (a). Evolution of the refractive index profile in a standard tapered fibre which shows the dopant diffusion, (b) mode evolution along a standard metal coated taper which shows a strong interaction between the field and the coating at the tip and (c) metal coated tapered Fractal fibre which depicts minimal interaction between the modal field and the metal coating.

As a result, the modal field will spread across the whole fibre and there is an interaction between the field and the metal coating, leading to strong attenuation, as shown in Fig. 1(b). However, the interaction of the modal field and the metal coating can be reduced, and the throughput optical power can be increased, if a single material PCF is used instead of a doped fibre, as depicted in Fig. 1(c). In this case the holes scale down with the taper, and the dopant diffusion problem is eliminated due to the absence of any fibre dopants. The tapering of PCFs post manufacture is well understood [13

13. S. T. Huntington, J. Katsifolis, B. C. Gibson, J. Canning, K. Lyytikainen, J. Zagari, L. W. Cahill, and J. D. Love, “Retaining and characterising nano-structure within tapered air-silica structured optical fibers,” Opt. Express , 11, 98–104 (2003), http://www.opticsinfobase.org/abstract.cfm?URI=oe-11-2-98. [CrossRef] [PubMed]

]-[16

16. Y. K. Lizé, E. C. Magi, V. G. Ta’eed, J. A. Bolger, P. Steinvurzel, and B. J. Eggleton, “Microstruc-tured optical fiber photonic wires with subwavelength core diameter,” Opt. Express , 12, 3209–3217 (2004), http://www.opticsinfobase.org/abstract.cfm?URI=oe-12-14-3209. [CrossRef] [PubMed]

], with non-SNOM applications ranging from microfluidics [17

17. C. Kerbage and B. J. Eggleton, “Tunable microfluidic optical fiber gratings,” Appl. Phys. Lett. , 82, 1338–1340 (2003). [CrossRef]

], to supercontinuum generation [18

18. S. G. Leon-Saval, T. A. Birks, W. J. Wadsworth, P. St. J. Russell, and M. W. Mason, “Su-percontinuum generation in submicron fibre waveguides,” Opt. Express , 12, 2864–2869 (2004), http://www.opticsinfobase.org/abstract.cfm?URI=oe-12-13-2864. [CrossRef] [PubMed]

], couplers [19

19. B. H. Lee, J. B. Eom, J. Kim, D. S. Moon, U. C. Paek, and G. H. Yang, “Photonic crystal fiber coupler,” Opt. Lett. , 27, 812–814 (2002). [CrossRef]

] and low-loss transition to conventional optical fibers [20

20. T. A. Birks, G. Kakarantzas, P. St. J. Russell, and D. F. Murphy, “Photonic crystal fiber devices,” in Fiber-based Component Fabrication, Testing, and Connectorization, Proc. SPIE , 4943, 142–151 (2002). [CrossRef]

].

In 1975, Benoít Mandelbrot coined the term Fractal, and published his ideas two years later, which describe structures that include shapes that are recursively constructed or self-similar, that is, a shape that appears similar at all scales of magnification and is therefore often referred to as ‘infinitely complex’ [21

21. B. MandelbrotFractals: Form, Chance and Dimension (W. H. Freeman and Co., San Francisco, 1977).

]. The novel fibre, presented here for the first time, is based on the Fractal pattern and was designed to maximize the optical throughput of near-field probes. Standard PCFs are typically manufactured using a technique of stacking silica capillary tubes of equal size in a hexagonal lattice pattern [22

22. T. A. Birks, J. C. Knight, and P. S. J. Russell, “Endlessly single-mode photonic crystal fibre,” Opt. Lett. 22, 961–963 (1997). [CrossRef] [PubMed]

]. However, the Fractal fibre consists of a series of rings of holes whose cross-sectional area increases with increasing distance from the center core region in such a manner that the average effective index of the fibre decreases with increasing distance from the core region. The fibre requires a π/n rotation of the ring of holes, where n is the number of holes in each ring, with respect to each consecutive ring of holes so that the holes are not aligned. A schematic representation of the cross-section of the Fractal fibre is shown in Fig. 2(a).

Fig. 2. (a). Schematic representation of the Fractal fibre cross-section which clearly shows the π/n rotation of each consecutive ring of holes from the core along with the doubling of the radius of the holes in each consecutive ring, and (b) depicts when the Fractal fibre is tapered to form a near-field probe, the each inner ring of holes collapses into the core region and the modal field is then confined by successive rings of holes with minimal variation to the modal properties.

3. Fractal fibre fabrication

The fabrication of the Fractal fibre posed a host of new challenges that had not previously been dealt with by other fibre fabrication facilities. As previously mentioned, standard PCFs are typically produced from stacking silica capillary tubes of similar size in a lattice structure. The production of the Fractal fibre had the complex challenge of trying to incorporate capillary tubes with significantly varied internal holes sizes. Although the hole collapse rates were different from the center to the outer edges of the preform, the drawing rates were maintained with scaling for the preform and cane, similar to normal PCF fabrication. However, in fibre form, the internal hole collapse rates were dissimilar to standard PCF fabrication. An intricate capillary stacking procedure was implemented in order to produce the Fractal preform. A picture of the stacked capillary tubes can be observed in Fig. 3(a) and the subsequently fused preform and fibre cane are shown in Figs. 3(b) and (c), respectively.

Fig. 3. (a). Stacked capillary tubes (outer diameter ∼ 25mm). Gentle collapsing of the outer layer reveals an octagonal profile determined by the use of eight capillaries. Interstitial gap fillers are also used to reduce the size of interstitial (unwanted) holes and minimize the fine adjustment in pressure control as much as possible, (b) Fractal preform after fusing and (c) the resulting fibre cane.

4. Fabrication and characterization of fibre probes

Carbon-dioxide laser-based pulling methods were used to taper and break optical fibres to tip diameters in the order of 50 nm [23

23. G. A. Valaskovic, M. Holton, and G. H. Morrison, “Parameter control, characterization, and optimization in the fabrication of optical fibre near-field probes,” Appl. Opt. 34, 1215–28, (1995). [CrossRef] [PubMed]

]-[25

25. B. Gibson, S. Huntington, S. Rubanov, P. Olivero, K. Digweed-Lyytikäinen, J. Canning, and J. Love, “Exposure and characterization of nano-structured hole arrays in tapered photonic crystal fibres using a combined FIB/SEM technique,” Opt. Express 13, 9023–9028 (2005), http://www.opticsinfobase.org/abstract.cfm?URI=oe-13-22-9023. [CrossRef] [PubMed]

]. This technique is the industry standard which is used for the production of optical fibre-based near-field probes. In addition to the Fractal fibre, a single-mode fibre and PCF were also tapered in order to accurately compare the optical throughput properties of the each probe. The cross-section of the single-mode fibre, PCF and Fractal fibre can be observed in Figs. 4(a), 4(b) and 4(c), respectively.

The single-mode, PCF and Fractal fibres were each tapered using a custom-built carbon dioxide laser-based pulling system. For each sample, a 5 cm length of fibre was stripped of its acrylate coating using methylene chloride and then cleaned with ethanol. The partially stripped fibre was then held under tension between two pulling arms and a carbon dioxide laser was used to heat the fibre. The motorized arms pulled the fibre in opposite directions until the fibre thinned and finally snapped to form two individual tapered optical fibres with overall taper lengths of approximately 1.25 mm and tip diameters in the order of 50 nm. All of the fibre tapers described in this paper were fabricated at room temperature and no additional gas was used to pressurize the holes during the tapering.

Fig. 4. CCD images of the cross-sections of a piece of (a) single-mode fibre, (b) PCF and (c) Fractal fibre. The diameter of each fibre type is 125 microns.

The three different probe samples can be observed in Fig. 5, where (a) corresponds to a single-mode (conventional) probe, (b) shows a PCF probe and (c) depicts the Fractal fibre probe.

Fig. 5. CCD images of 488nm light transmission through (a) single-mode fibre probe, (b) photonic crystal fibre probe and (c) Fractal fibre probe. In each case, the tapered probes were immersed in a fluorescent solution which has an index of refraction greater than that of the silica fibre cladding.

Figure 5(a) shows a substantial amount of optical leakage from the single-mode probe. The surrounding solution is fluorescing at the point where the light is not confined within the optical probe. This exit point for the light occurs at approximately 40 micrometers from the probe tip and it is clear that the pumped blue light does not propagate through to the end of the probe.

By observing this image, it is clear that a metal coating is required in order to enable propagation within the remaining length of the probe tip. Figure 5(b) shows that a greater fraction of the launched blue light propagates through to the PCF probe tip. This can be confirmed by the bright blue point at the tip of the probe. The transmission properties of the PCF probe are greatly enhanced compared to the conventional single-mode taper. However, there is some optical leakage of the propagating light within a few microns from the tip. This is most likely due to some diffraction effects as the hole-to-hole pitch within the tapered hexagonal air-silica lattice approaches the 1st order Bragg condition.

5. Conclusion

When tapered to form an optical probe, it has been shown that the Fractal fibre structure enables light to be confined almost all the way to the tip of the probe. The Fractal fibre represents a completely innovative step in the development of unique fibre structures which are designed to minimize loss. This fibre has applications far beyond those indicated here, including a reduction in the strict criteria for drawing that are normally imposed for standard fibres, leading to cheaper and faster manufacturing in the photonics industry. The Fractal Fibre has many special features still to be investigated, for example, extremely low bend loss compared to traditional fibres.

The fabrication of ultra high throughput optical probes directly allows scanning probe microscopy to further access the fields of Nanotechnology and Biotechnology. Low power probes that limit imaging speeds restrict the applicability of these SNOM techniques. Ultra high optical throughput Fractal probes have the real potential of making a significant contribution to the field. Fractal probes have the capability to enable the use of near field microscopy for the examination of biological processes in real time in addition to high speed nano-engineering procedures.

Acknowledgments

This project is proudly supported by the International Science Linkages programme established under the Australian Government’s innovation statement Backing Australia’s Ability. The authors would also like to acknowledge J. Digweed, J. Zagari and B. Ashton for their assistance with fibre preparation and useful discussions.

References and links

1.

E. H. Synge, “A suggested method for extending the microscopic resolution into the ultramicroscopic region,” Phil. Mag. 6, 356 (1928).

2.

D. W. Pohl, W. Denk, and M. Lanz, “Optical stethoscopy: Image recording with resolution lambda/20,” Appl. Phys. Lett. 44, 651–653 (1984). [CrossRef]

3.

U. Ch. Fischer, U. T. Drig, and D. W. Pohl, “Near-field optical scanning microscopy in reflection,” Appl. Phys. Lett. 52, 249–251 (1988) [CrossRef]

4.

E. Betzig, M. Isaacson, and A. Lewis, “Collection mode near-field scanning optical microscopy,” Appl. Phys. Lett. 51, 2088–2090 (1987). [CrossRef]

5.

E. Betzig, J. K. Trautman, T. D. Harris, J. S. Weiner, and R. L. Kostelak, “Breaking the diffraction barrier - optical microscopy on a nanometric scale,” Science , 251, 1468–1470 (1991). [CrossRef] [PubMed]

6.

R. C. Davis, C. C. Williams, and P. Neuzil, “Micromachined submicrometer photodiode for scanning probe microscopy,” Appl. Phys. Lett. , 66, 2309–2311 (1995). [CrossRef]

7.

T. Niwa, Y. Mitsuoka, K. Kato, S. Ichihara, N. Chiba, M. Shin-Ogi, K. Nakajima, H. Muramatsu, and T. Sakuhara, “Optical microcantilever consisting of channel waveguide for scanning near-field optical microscopy controlled by atomic force,” J. of Micros. , 194388–392 (1999). [CrossRef]

8.

P. Hoffmann, B. Dutoit, and R. Salathe, “Comparison of mechanically drawn and protection layer chemically etched optical fibre tips,” Ultramicroscopy , 61, 165–170 (1995). [CrossRef]

9.

S. Mononobe and M. Ohtsu, “Fabrication of a pencil-shaped fibre probe for near-field optics by selective chemical etching,” J. Lightwave Technol. , 14, 2231–2235, (1996). [CrossRef]

10.

M. Chaigneau, G. Ollivier, T. Minea, and G. Louarn, “Nanoprobes for near-field optical microscopy manufactured by substitute-sheath etching and hollow cathode sputtering,” Rev. Sci. Instrum. 77, 103702 (2006). [CrossRef]

11.

http://www.nanonics.co.il/.

12.

S. T. Huntington, S. J. Ashby, M. C. Elias, and J. D. Love, “Direct measurement of core profile diffusion and ellipticity in fused-taper fibre couplers using atomic force microscopy,” Electron. Lett. , 36, 121–123 (2000). [CrossRef]

13.

S. T. Huntington, J. Katsifolis, B. C. Gibson, J. Canning, K. Lyytikainen, J. Zagari, L. W. Cahill, and J. D. Love, “Retaining and characterising nano-structure within tapered air-silica structured optical fibers,” Opt. Express , 11, 98–104 (2003), http://www.opticsinfobase.org/abstract.cfm?URI=oe-11-2-98. [CrossRef] [PubMed]

14.

E. C. Magi, P. Steinvurzel, and B. J. Eggleton, “Tapered photonic crystal fibers,” Opt. Express , 12, 776–784 (2004), http://www.opticsinfobase.org/abstract.cfm?URI=oe-12-5-776. [CrossRef] [PubMed]

15.

Y. Youk, D. Y. Kim, and K. W. Park, “Guiding properties of a tapered photonic crystal fiber compared with those of a tapered single-mode fiber,” Fiber Integrated Opt. , 23, 439–446 (2004). [CrossRef]

16.

Y. K. Lizé, E. C. Magi, V. G. Ta’eed, J. A. Bolger, P. Steinvurzel, and B. J. Eggleton, “Microstruc-tured optical fiber photonic wires with subwavelength core diameter,” Opt. Express , 12, 3209–3217 (2004), http://www.opticsinfobase.org/abstract.cfm?URI=oe-12-14-3209. [CrossRef] [PubMed]

17.

C. Kerbage and B. J. Eggleton, “Tunable microfluidic optical fiber gratings,” Appl. Phys. Lett. , 82, 1338–1340 (2003). [CrossRef]

18.

S. G. Leon-Saval, T. A. Birks, W. J. Wadsworth, P. St. J. Russell, and M. W. Mason, “Su-percontinuum generation in submicron fibre waveguides,” Opt. Express , 12, 2864–2869 (2004), http://www.opticsinfobase.org/abstract.cfm?URI=oe-12-13-2864. [CrossRef] [PubMed]

19.

B. H. Lee, J. B. Eom, J. Kim, D. S. Moon, U. C. Paek, and G. H. Yang, “Photonic crystal fiber coupler,” Opt. Lett. , 27, 812–814 (2002). [CrossRef]

20.

T. A. Birks, G. Kakarantzas, P. St. J. Russell, and D. F. Murphy, “Photonic crystal fiber devices,” in Fiber-based Component Fabrication, Testing, and Connectorization, Proc. SPIE , 4943, 142–151 (2002). [CrossRef]

21.

B. MandelbrotFractals: Form, Chance and Dimension (W. H. Freeman and Co., San Francisco, 1977).

22.

T. A. Birks, J. C. Knight, and P. S. J. Russell, “Endlessly single-mode photonic crystal fibre,” Opt. Lett. 22, 961–963 (1997). [CrossRef] [PubMed]

23.

G. A. Valaskovic, M. Holton, and G. H. Morrison, “Parameter control, characterization, and optimization in the fabrication of optical fibre near-field probes,” Appl. Opt. 34, 1215–28, (1995). [CrossRef] [PubMed]

24.

R. L. Williamson and M. J. Miles, “Melt-drawn scanning near-field optical microscopy probe profiles,” J. Appl. Phys. 80, 4804–4812, (1996). [CrossRef]

25.

B. Gibson, S. Huntington, S. Rubanov, P. Olivero, K. Digweed-Lyytikäinen, J. Canning, and J. Love, “Exposure and characterization of nano-structured hole arrays in tapered photonic crystal fibres using a combined FIB/SEM technique,” Opt. Express 13, 9023–9028 (2005), http://www.opticsinfobase.org/abstract.cfm?URI=oe-13-22-9023. [CrossRef] [PubMed]

26.

J. Love, W. Henry, W. Stewart, R. Black, S. Lacroix, and F. Gonthier, “Tapered singlemode fibres and devices Part1: Adiabaticity criteria,” IEE Proc. J. Optoelectron. , 138, 343–354, (1991). [CrossRef]

OCIS Codes
(060.2270) Fiber optics and optical communications : Fiber characterization
(060.2310) Fiber optics and optical communications : Fiber optics
(060.2350) Fiber optics and optical communications : Fiber optics imaging
(120.4610) Instrumentation, measurement, and metrology : Optical fabrication
(180.5810) Microscopy : Scanning microscopy

ToC Category:
Microscopy

History
Original Manuscript: December 19, 2006
Manuscript Accepted: February 18, 2007
Published: March 5, 2007

Virtual Issues
Vol. 2, Iss. 4 Virtual Journal for Biomedical Optics

Citation
S. T. Huntington, B. C. Gibson, J. Canning, K. Digweed-Lyytikäinen, J. D. Love, and V. Steblina, "A fractal-based fibre for ultra-high throughput optical probes," Opt. Express 15, 2468-2475 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-5-2468


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References

  1. E. H. Synge, "A suggested method for extending the microscopic resolution into the ultramicroscopic region," Phil. Mag. 6, 356 (1928).
  2. D. W. Pohl, W. Denk, and M. Lanz, "Optical stethoscopy: Image recording with resolution lambda/20," Appl. Phys. Lett. 44, 651-653 (1984). [CrossRef]
  3. U. Ch. Fischer, U. T. Drig, and D. W. Pohl, "Near-field optical scanning microscopy in reflection," Appl. Phys. Lett. 52, 249-251 (1988) [CrossRef]
  4. E. Betzig, M. Isaacson, and A. Lewis, "Collection mode near-field scanning optical microscopy," Appl. Phys. Lett. 51, 2088-2090 (1987). [CrossRef]
  5. E. Betzig, J. K. Trautman, T. D. Harris, J. S. Weiner and R. L. Kostelak, "Breaking the diffraction barrier - optical microscopy on a nanometric scale," Science,  251, 1468-1470 (1991). [CrossRef] [PubMed]
  6. R. C. Davis, C. C. Williams and P. Neuzil, "Micromachined submicrometer photodiode for scanning probe microscopy," Appl. Phys. Lett.,  66, 2309-2311 (1995). [CrossRef]
  7. T. Niwa, Y. Mitsuoka, K. Kato, S. Ichihara, N. Chiba,M. Shin-Ogi, K. Nakajima, H. Muramatsu and T. Sakuhara, "Optical microcantilever consisting of channel waveguide for scanning near-field optical microscopy controlled by atomic force," J. of Micros.,  194388-392 (1999). [CrossRef]
  8. P. Hoffmann, B. Dutoit and R. Salathe, "Comparison of and protection layer chemically etched optical fibre tips," Ultramicroscopy,  61, 165-170 (1995). [CrossRef]
  9. S. Mononobe andM. Ohtsu, "Fabrication of a pencil-shaped fibre probe for near-field optics by selective chemical etching," J. Lightwave Technol.,  14, 2231-2235, (1996). [CrossRef]
  10. M. Chaigneau, G. Ollivier, T. Minea and G. Louarn, "Nanoprobes for near-field optical microscopy manufactured by substitute-sheath etching and hollow cathode sputtering," Rev. Sci. Instrum. 77, 103702 (2006). [CrossRef]
  11. http://www.nanonics.co.il/.
  12. S. T. Huntington, S. J. Ashby, M. C. Elias, and J. D. Love, "Direct measurement of core profile diffusion and ellipticity in fused-taper fibre couplers using atomic force microscopy," Electron. Lett.,  36, 121-123 (2000). [CrossRef]
  13. S. T. Huntington, J. Katsifolis, B. C. Gibson, J. Canning, K. Lyytikainen, J. Zagari, L. W. Cahill, and J. D. Love, "Retaining and characterising nano-structure within tapered air-silica structured optical fibers," Opt. Express,  11, 98-104 (2003), http://www.opticsinfobase.org/abstract.cfm?URI=oe-11-2-98. [CrossRef] [PubMed]
  14. E. C. Magi, P. Steinvurzel, and B. J. Eggleton, "Tapered photonic crystal fibers," Opt. Express,  12, 776-784 (2004), http://www.opticsinfobase.org/abstract.cfm?URI=oe-12-5-776. [CrossRef] [PubMed]
  15. Y. Youk, D. Y. Kim, and K.W. Park, "Guiding properties of a tapered photonic crystal fiber compared with those of a tapered single-mode fiber," Fiber Integrated Opt.,  23, 439-446 (2004). [CrossRef]
  16. Y. K. Lizé, E. C. Magi, V. G. Ta’eed, J. A. Bolger, P. Steinvurzel, and B. J. Eggleton, "Microstructured optical fiber photonic wires with subwavelength core diameter," Opt. Express,  12, 3209-3217 (2004), http://www.opticsinfobase.org/abstract.cfm?URI=oe-12-14-3209 [CrossRef] [PubMed]
  17. C. Kerbage and B. J. Eggleton, "Tunable microfluidic optical fiber gratings," Appl. Phys. Lett.,  82, 1338-1340 (2003). [CrossRef]
  18. S. G. Leon-Saval, T. A. Birks, W. J. Wadsworth, P. St. J. Russell, and M. W. Mason, "Supercontinuum generation in submicron fibre waveguides," Opt. Express,  12, 2864-2869 (2004), http://www.opticsinfobase.org/abstract.cfm?URI=oe-12-13-2864. [CrossRef] [PubMed]
  19. B. H. Lee, J. B. Eom, J. Kim, D. S. Moon, U. C. Paek, and G. H. Yang, "Photonic crystal fiber coupler," Opt. Lett.,  27, 812-814 (2002). [CrossRef]
  20. T. A. Birks, G. Kakarantzas, P. St. J. Russell, and D. F. Murphy, "Photonic crystal fiber devices," in Fiber-based Component Fabrication, Testing, and Connectorization,Proc. SPIE,  4943, 142-151 (2002). [CrossRef]
  21. B. Mandelbrot, Fractals: Form, Chance and Dimension (W. H. Freeman and Co., San Francisco, 1977).
  22. T. A. Birks, J. C. Knight, and P. St. J. Russell, "Endlessly single-mode photonic crystal fibre," Opt. Lett. 22, 961-963 (1997). [CrossRef] [PubMed]
  23. G. A. Valaskovic, M. Holton and G. H. Morrison, "Parameter control, characterization, and optimization in the fabrication of optical fibre near-field probes," Appl. Opt. 34, 1215-28, (1995). [CrossRef] [PubMed]
  24. R. L. Williamson and M. J. Miles, "Melt-drawn scanning near-field optical microscopy probe profiles," J. Appl. Phys. 80, 4804-4812, (1996). [CrossRef]
  25. B. Gibson, S. Huntington, S. Rubanov, P. Olivero, K. Digweed-Lyytik ainen, J. Canning, and J. Love, "Exposure and characterization of nano-structured hole arrays in tapered photonic crystal fibres using a combined FIB/SEM technique," Opt. Express 13, 9023-9028 (2005), http://www.opticsinfobase.org/abstract.cfm?URI=oe-13-22-9023. [CrossRef] [PubMed]
  26. J. Love, W. Henry, W. Stewart, R. Black, S. Lacroix, and F. Gonthier, "Tapered singlemode fibres and devices Part1: Adiabaticity criteria," IEE Proc. J. Optoelectron.,  138, 343-354, (1991). [CrossRef]

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