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Virtual Journal for Biomedical Optics

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  • Editor: Gregory W. Faris
  • Vol. 3, Iss. 11 — Oct. 22, 2008
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Broadband single-mode operation of standard optical fibers by using a sub-wavelength optical wire filter

Yongmin Jung, Gilberto Brambilla, and David J. Richardson  »View Author Affiliations


Optics Express, Vol. 16, Issue 19, pp. 14661-14667 (2008)
http://dx.doi.org/10.1364/OE.16.014661


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Abstract

We report the use of a sub-wavelength optical wire (SOW) with a specifically designed transition region as an efficient tool to filter higher-order modes in multimode waveguides. Higher-order modes are effectively suppressed by controlling the transition taper profile and the diameter of the sub-wavelength optical wire. As a practical example, single-mode operation of a standard telecom optical fiber over a broad spectral window (400~1700 nm) was demonstrated with a 1µm SOW. The ability to obtain robust and stable single-mode operation over a very broad range of wavelengths offers new possibilities for mode control within fiber devices and is relevant to a range of application sectors including high performance fiber lasers, sensors, photolithography, and optical coherence tomography systems.

© 2008 Optical Society of America

1. Introduction

2. Idealized sub-wavelength optical wire for higher-order mode filtering

2.1. Working principle, fabrication and optical transmission properties

Figure 1 represents an idealized SOW for higher-order mode filtering, which is composed of two conical transition tapers and a central uniform waist region. If the conical transition tapers are adiabatic, guided modes in the core of the multimode fiber (LP 01, LP 11 in Fig. 1(a)) are continuously mode converted to guided cladding modes in the SOW on a one-to-one basis by the down-taper and are then coupled back into guided modes in the multimode fiber by the up-taper. The evolution of the spatial profile of the first two guided modes along the transition tapers is also shown. However, higher-order modes can be effectively suppressed by controlling the SOW diameter in the uniform waist region [15

15. L. Tong, J. Lou, and E. Mazur, “Single-mode guiding properties of subwavelength-diameter silica and silicon wire waveguides,” Opt. Express 12, 1025–1035 (2004). [CrossRef] [PubMed]

], which serves to restrict the propagation of the higher-order modes along the entire length of the SOW and thereby constrains the number of guided modes at the taper output. In general, when the diameter of the optical fiber is considerably smaller than the wavelength of the guided light, the optical fiber can operate as a single-mode sub-wavelength-diameter waveguide with an air cladding. The single mode operation range is determined by the mode cut-off conditions commonly used for larger waveguides [1

1. K. Okamoto, Fundamentals of Optical Waveguides (Elsevier Academic, London, 2006).

,15

15. L. Tong, J. Lou, and E. Mazur, “Single-mode guiding properties of subwavelength-diameter silica and silicon wire waveguides,” Opt. Express 12, 1025–1035 (2004). [CrossRef] [PubMed]

,16

16. M. Sumetsky, Y. Dulashko, P. Domachuk, and B. J. Eggleton, “Thinnest optical waveguide: experimental test,” Opt. Lett. 32, 754–756 (2007). [CrossRef] [PubMed]

] and single-mode operation is limited to a spectral range of just a few hundred nanometers; at short wavelengths the SOW still supports a finite number of high-order modes. By designing an appropriate non-adiabatic transition taper as shown in Fig. 1(b), it is possible to convert high-order core modes to higher-order cladding modes or radiation modes which are not supported by the SOW. The different mode evolution (adiabatic for fundamental mode and non-adiabatic for higher-order modes) allows only the transverse single mode to propagate along the waveguide, which permits single-mode operation for a conventional fiber over an extremely wide range of wavelengths. Low-loss SOWs were manufactured with the aid of the well-established single-stage conventional “flame-brushing” technique [17

17. F. Bilodeau, K. O. Hill, S. Faucher, and D. C. Johnson, “Low-loss highly overcoupled fused couplers: Fabrication and sensitivity to external pressure,” J. Lightwave Technol. 6, 1476–1482 (1988). [CrossRef]

]. A standard telecom optical fiber (Corning SMF-28) was selected as a simple example of a fiber providing multimode operation at short wavelengths. The profile of the conical transition tapers in the experiment was approximated by a decreasing exponential function (transition region length ~25mm, uniform waist length ~4mm, relative taper elongation rate=0 [18

18. T. A. Birks and Y. W. Li, “The shape of fiber tapers,” J. Lightwave Technol. 10, 432–438 (1992). [CrossRef]

]) and was achieved by ensuring appropriate control of the translation stage movement during the tapering process [8

8. G. Brambilla, V. Finazzi, and D. J. Richardson, “Ultra-low-loss optical fiber nanotapers,” Opt. Express 12, 2258–2263 (2004), http://www.opticsinfobase.org/abstract.cfm?URI=oe-12-10-2258. [CrossRef] [PubMed]

,18

18. T. A. Birks and Y. W. Li, “The shape of fiber tapers,” J. Lightwave Technol. 10, 432–438 (1992). [CrossRef]

].

Fig. 1. An idealized sub-wavelength optical wire (SOW) for higher-order mode filtering in a two mode optical fiber. (a) Adiabatic transition tapers provide continuous mode evolution from core modes to cladding modes, and vice versa. All modes launched into the fiber are collected at the fiber output. (b) The use of non-adiabatic tapers for higher-order core modes transfers power to higher-order cladding modes or radiation modes which are not guided by the SOW. Higher-order taper modes are effectively filtered out by controlling the SOW diameter.

Fig. 2. Spectral response of the tapered fibers: (a) Transmission spectra of a 5m telecom fiber with a SOW for different SOW outer diameters. Interference between different modes occurs for tapers with 70µm diameter, while single mode operation is observed for the whole range of scanned wavelengths for tapers with 1µm diameter. (b) Comparison between the transmission spectra of a standard fiber (Corning SMF-28) without (in black) and with (in red) a 1µm SOW. λ c_ LP 11 represents the cut-off wavelength for the bi-lobed LP 11 mode which exists only for wavelengths shorter than λ c_ LP 11. Similarly, λ c_ LP 21 and λ c_ LP 02 represent the cut-offs for the higher order modes LP 21 and LP 02, respectively.

2.2. Stability of single-mode operation

Fig. 3. Far-field imaging of telecom fibers without (a,c,e) and with SOW (b,d,f) at a wavelength of 632.8nm (He-Ne laser) and 488 nm (Ar laser), respectively.

3. Analytic modeling of mode evolution

To allow a more detailed understanding of the optical mode evolution along the transition taper, the adiabaticity of the transition taper has been examined by calculating the beat length and taper angle necessary to ensure adiabatic behavior [19

19. J. D. Love, W. M. Henry, W. J. Stewart, R. J. Black, S. Lacroix, and F. Gonthier, “Tapered single-mode fibres and devices - Part 1: Adiabaticity criteria,” IEE Proceedings-J 138, 343–354 (1991).

,20

20. R. J. Black, S. Lacroix, F. Gonthier, and J. D. Love, “Tapered single-mode fibres and devices - Part 2: Experimental and theoretical quantification,” IEE Proceedings-J 138, 355–364 (1991).

]. A profile is called adiabatic for a mode when it does not induce any power transfer to any other mode. Figure 4(a) and (b) plot the calculated effective indices of the guided modes as a function of the core guidance parameter (V-value) for an input wavelength λ=1µm. In an ideal adiabatic transition taper, the taper angle is small enough so that the LP 01, LP 11 core modes can be considered unperturbed as they evolve from being core-guided to cladding-guided. In particular, the beat length z between two modes having propagation constants β 1 and β 2 in a fiber with radius ρ has been assumed to be the defining factor determining the transition between lossless and lossy tapers. For distances larger than the beating length the two modes do not exchange power and the taper is adiabatic. This results in the critical angle Ω being defined as:

Ω=ρz=ρ(β1β2)2π
(1)

Fig. 4. Effective index versus core guidance parameter V (=2π ρ NA/λ, where ρ is the SOW radius, NA the numerical aperture and λ the wavelength) for the first three (a) LP 0m and (b) LP 1m guided modes. n clad and n eff represent the cladding and the mode effective indices, respectively. The difference of effective indices between LP n0 and LP n1 modes allows the determination of the taper adiabatic angle Ω through equation 1. (c) Adiabatic profiles for LP 01 (black) and LP 11 (red) modes. The real taper profile has greater angles than the LP 11 adiabatic curve between inverse taper ratios ρ(z)/ρ 0=0.65 and 0.8, meaning that the LP 11 mode will be converted in LP 1m (m>1) modes for ρ(z)/ρ 0<0.8.

4. Conclusions

In conclusion, a compact simple scheme for stable single-mode operation of multimode waveguides has been developed and validated. Different mode evolutions (adiabatic for fundamental mode and non-adiabatic for higher-order modes) along sub-wavelength-sized waveguides produce significantly different losses. Higher-order modes are effectively suppressed by controlling the transition taper profile and the diameter of the sub-wavelength optical wire. While the transition taper transfers power from the second and third order modes to unguided and higher order modes, the small diameter SOW brings further suppression by constraining the number of guided cladding modes. As an example, single-mode operation of a standard telecom optical fiber over a broad spectral window (400~1700nm) was demonstrated with a 1µm SOW. The stable single-mode operation could be very beneficial for various applications in fiber lasers, sensors, photolithography, and optical coherence tomography.

Acknowledgments

The authors thank the Engineering and Physical Sciences Research Council UK for financial support; GB gratefully acknowledges the Royal Society (UK) for his Research Fellowship.

References and links

1.

K. Okamoto, Fundamentals of Optical Waveguides (Elsevier Academic, London, 2006).

2.

J. Nilson, W. A. Clarkson, R. Selvas, J. K. Sahu, P. W. Turner, S. -U. Alam, and A. B. Grudinin, “High-power wavelength-tunable cladding-pumped rare-earth-doped silica fiber lasers,” Opt. Fiber Technol. 10, 5–30 (2004). [CrossRef]

3.

O. S. Wolfbeis, “Fiber-optic chemical sensors and biosensors,” Anal. Chem. 74, 2663–2678 (2002). [CrossRef] [PubMed]

4.

B. A. Flusberg, E. D. Cocker, W. Piyawattanametha, J. C. Jung, E. L. M. Cheung, and M. J. Schnitzer, “Fiber-optic fluorescence imaging,” Nature Methods 2, 941–950 (2005). [CrossRef] [PubMed]

5.

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

6.

P. St. J. Russell, “Photonic crystal fibers,” Science 299, 358–362 (2003). [CrossRef] [PubMed]

7.

L. Tong, R. R. Gattass, J. B. Ashcom, S. He, J. Lou, M. Shen, I. Maxwell, and E. Mazur, “Subwavelength-diameter silica wires for low-loss optical wave guiding,” Nature 426, 816–819 (2003). [CrossRef] [PubMed]

8.

G. Brambilla, V. Finazzi, and D. J. Richardson, “Ultra-low-loss optical fiber nanotapers,” Opt. Express 12, 2258–2263 (2004), http://www.opticsinfobase.org/abstract.cfm?URI=oe-12-10-2258. [CrossRef] [PubMed]

9.

S. Moon and D. Y. Kim, “Effective single-mode transmission at wavelengths shorter than the cutoff wavelength of an optical fiber,” IEEE Photon. Technol. Lett. 17, 2604–2606 (2005). [CrossRef]

10.

Denis Donlagic, “In-line higher order mode filters based on long highly uniform fiber tapers,” J. Lightwave Technol. 24, 3532–3539 (2006). [CrossRef]

11.

M. Sumetsky, Y. Dulashko, and A. Hale, “Fabrication and study of bent and coiled free silica nanowires: Self-coupling microloop optical interferometer,” Opt. Express 12, 3521–3531 (2004), http://www.opticsinfobase.org/abstract.cfm?URI=oe-12-15-3521. [CrossRef] [PubMed]

12.

D. -I. Yeom, E. C. Mägi, M. R. E. Lamont, M. A. F. Roelens, L. Fu, and B. J. Eggleton, “Low-threshold supercontinuum generation in highly nonlinear chalcogenide nanowires,” Opt. Lett. 33, 660–662 (2008), http://www.opticsinfobase.org/abstract.cfm?URI=ol-33-7-660 [CrossRef] [PubMed]

13.

F. Xu, P. Horak, and G. Brambilla, “Optical microfiber coil resonator refractometric sensor,” Opt. Express . 15, 7888–7893 (2007), http://www.opticsinfobase.org/abstract.cfm?URI=oe-15-12-7888. [CrossRef] [PubMed]

14.

V. I. Balykin, K. Hakuta, F. Le Kien, J. Q. Liang, and M. Morinaga, “Atom trapping and guiding with a subwavelength-diameter optical fiber,” Phys. Rev. A70, 011401 (2004).

15.

L. Tong, J. Lou, and E. Mazur, “Single-mode guiding properties of subwavelength-diameter silica and silicon wire waveguides,” Opt. Express 12, 1025–1035 (2004). [CrossRef] [PubMed]

16.

M. Sumetsky, Y. Dulashko, P. Domachuk, and B. J. Eggleton, “Thinnest optical waveguide: experimental test,” Opt. Lett. 32, 754–756 (2007). [CrossRef] [PubMed]

17.

F. Bilodeau, K. O. Hill, S. Faucher, and D. C. Johnson, “Low-loss highly overcoupled fused couplers: Fabrication and sensitivity to external pressure,” J. Lightwave Technol. 6, 1476–1482 (1988). [CrossRef]

18.

T. A. Birks and Y. W. Li, “The shape of fiber tapers,” J. Lightwave Technol. 10, 432–438 (1992). [CrossRef]

19.

J. D. Love, W. M. Henry, W. J. Stewart, R. J. Black, S. Lacroix, and F. Gonthier, “Tapered single-mode fibres and devices - Part 1: Adiabaticity criteria,” IEE Proceedings-J 138, 343–354 (1991).

20.

R. J. Black, S. Lacroix, F. Gonthier, and J. D. Love, “Tapered single-mode fibres and devices - Part 2: Experimental and theoretical quantification,” IEE Proceedings-J 138, 355–364 (1991).

OCIS Codes
(060.2340) Fiber optics and optical communications : Fiber optics components
(220.4000) Optical design and fabrication : Microstructure fabrication
(230.1150) Optical devices : All-optical devices
(220.4241) Optical design and fabrication : Nanostructure fabrication

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: June 30, 2008
Revised Manuscript: August 14, 2008
Manuscript Accepted: August 28, 2008
Published: September 3, 2008

Virtual Issues
Vol. 3, Iss. 11 Virtual Journal for Biomedical Optics

Citation
Yongmin Jung, Gilberto Brambilla, and David J. Richardson, "Broadband single-mode operation of standard optical fibers by using a sub-wavelength optical wire filter," Opt. Express 16, 14661-14667 (2008)
http://www.opticsinfobase.org/vjbo/abstract.cfm?URI=oe-16-19-14661


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References

  1. K. Okamoto, Fundamentals of Optical Waveguides (Elsevier Academic, London, 2006).
  2. J. Nilson, W. A. Clarkson, R. Selvas, J. K. Sahu, P. W. Turner, S. -U. Alam, and A. B. Grudinin, "High-power wavelength-tunable cladding-pumped rare-earth-doped silica fiber lasers," Opt. Fiber Technol. 10, 5-30 (2004). [CrossRef]
  3. O. S. Wolfbeis, "Fiber-optic chemical sensors and biosensors," Anal. Chem. 74, 2663-2678 (2002). [CrossRef] [PubMed]
  4. B. A. Flusberg, E. D. Cocker, W. Piyawattanametha, J. C. Jung, E. L. M. Cheung, and M. J. Schnitzer, "Fiber-optic fluorescence imaging," Nat. Methods 2, 941-950 (2005). [CrossRef] [PubMed]
  5. T. A. Birks, J. C. Knight, and P. St. J. Russell, "Endlessly single-mode photonic crystal fiber," Opt. Lett. 22, 961-963 (1997). [CrossRef] [PubMed]
  6. P. St. J. Russell, "Photonic crystal fibers," Science 299, 358-362 (2003). [CrossRef] [PubMed]
  7. L. Tong, R. R. Gattass, J. B. Ashcom, S. He, J. Lou, M. Shen, I. Maxwell, and E. Mazur, "Subwavelength-diameter silica wires for low-loss optical wave guiding," Nature 426, 816-819 (2003). [CrossRef] [PubMed]
  8. G. Brambilla, V. Finazzi, and D. J. Richardson, "Ultra-low-loss optical fiber nanotapers," Opt. Express 12, 2258-2263 (2004), http://www.opticsinfobase.org/abstract.cfm?URI=oe-12-10-2258. [CrossRef] [PubMed]
  9. S. Moon and D. Y. Kim, "Effective single-mode transmission at wavelengths shorter than the cutoff wavelength of an optical fiber," IEEE Photon. Technol. Lett. 17, 2604-2606 (2005). [CrossRef]
  10. D. Donlagic, "In-line higher order mode filters based on long highly uniform fiber tapers," J. Lightwave Technol. 24, 3532-3539 (2006). [CrossRef]
  11. M. Sumetsky, Y. Dulashko, and A. Hale, "Fabrication and study of bent and coiled free silica nanowires: Self-coupling microloop optical interferometer," Opt. Express 12, 3521-3531 (2004), http://www.opticsinfobase.org/abstract.cfm?URI=oe-12-15-3521. [CrossRef] [PubMed]
  12. D. -I. Yeom, E. C. Mägi, M. R. E. Lamont, M. A. F. Roelens, L. Fu, and B. J. Eggleton, "Low-threshold supercontinuum generation in highly nonlinear chalcogenide nanowires," Opt. Lett. 33, 660-662 (2008), http://www.opticsinfobase.org/abstract.cfm?URI=ol-33-7-660. [CrossRef] [PubMed]
  13. F. Xu, P. Horak, and G. Brambilla, "Optical microfiber coil resonator refractometric sensor," Opt. Express. 15, 7888-7893 (2007), http://www.opticsinfobase.org/abstract.cfm?URI=oe-15-12-7888. [CrossRef] [PubMed]
  14. V. I. Balykin, K. Hakuta, F. Le Kien, J. Q. Liang, and M. Morinaga, "Atom trapping and guiding with a subwavelength-diameter optical fiber," Phys. Rev. A 70, 011401 (2004).
  15. L. Tong, J. Lou, and E. Mazur, "Single-mode guiding properties of subwavelength-diameter silica and silicon wire waveguides," Opt. Express 12, 1025-1035 (2004). [CrossRef] [PubMed]
  16. M. Sumetsky, Y. Dulashko, P. Domachuk, and B. J. Eggleton, "Thinnest optical waveguide: experimental test," Opt. Lett. 32, 754-756 (2007). [CrossRef] [PubMed]
  17. F. Bilodeau, K. O. Hill, S. Faucher, and D. C. Johnson, "Low-loss highly overcoupled fused couplers: Fabrication and sensitivity to external pressure," J. Lightwave Technol. 6, 1476- 1482 (1988). [CrossRef]
  18. T. A. Birks and Y. W. Li, "The shape of fiber tapers," J. Lightwave Technol. 10, 432-438 (1992). [CrossRef]
  19. J. D. Love, W. M. Henry, W. J. Stewart, R. J. Black, S. Lacroix, and F. Gonthier, "Tapered single-mode fibres and devices - Part 1: Adiabaticity criteria," IEE Proceedings-J 138, 343-354 (1991).
  20. R. J. Black, S. Lacroix, F. Gonthier, and J. D. Love, "Tapered single-mode fibres and devices - Part 2: Experimental and theoretical quantification," IEE Proceedings-J 138, 355-364 (1991).

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