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Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 1 — Jan. 2, 2012
  • pp: 141–148
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Multicore composite single-mode polymer fibre

Sergio G. Leon-Saval, Richard Lwin, and Alexander Argyros  »View Author Affiliations


Optics Express, Vol. 20, Issue 1, pp. 141-148 (2012)
http://dx.doi.org/10.1364/OE.20.000141


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Abstract

We study, fabricate and characterise an all-solid polymer composite waveguide consisting of a multicore fibre for single-mode operation down to the visible. The individual cores of the multicore structure that forms the composite core are arranged such that they strongly interact. The behaviour and parameters of the multicore geometry are analysed to achieve true single-mode operation. The composite core fibre is fabricated with off-the-shelf poly-methyl-methacrylate (PMMA) and Zeonex 480R polymers.

© 2011 OSA

1. Introduction

Most fibre-based advanced photonics technologies use single-mode (SM) fibres. SM operation in fibre allows better control and understanding of the propagating light. One of the areas where SM fibres are essential is in sensor technology. The majority of all-fibre sensors are based on fibre Long Period- and Bragg-Gratings. These sensors exploit the resonance properties of the sole propagating fundamental mode along the fibre. The interest in SM polymer optical fibre (POF) gratings lies in strain sensing and monitoring in engineering and medical applications [1

1. M. C. J. Large, J. Moran, and L. Ye, “The role of viscoelastic properties in the strain testing using microstructured polymer optical fibres (mPOF),” Meas. Sci. Tech. 20 (2009).

]. This arises from existing interest in the use of polymer fibres for such applications [2

2. Z. Xiong, G. D. Peng, B. Wu, and P. L. Chu, “Highly tunable Bragg gratings in single-mode polymer optical fibres,” IEEE Photon. Technol. Lett. 11(3), 352–354 (1999). [CrossRef]

,3

3. M. Kiesel, K. Peters, T. Hassan, and M. Kowalsky, “Behaviour of intrinsic polymer optical fibre sensor for large-strain applications,” Meas. Sci. Technol. 8(10), 3144–3154 (2007). [CrossRef]

], with PMMA fibres used for strains of up to 15% [4

4. S. Kiesel, K. Peters, T. Hassan, and M. Kowalsky, “Large deformation in-fibre polymer optical fibre sensor,” IEEE Photon. Technol. Lett. 20(6), 416–418 (2008). [CrossRef]

,5

5. M. C. J. Large, D. Blacket, and C.-A. Bunge, “Microstructured polymer optical fibres compared to conventional POF: novel properties and applications,” IEEE Sens. J. 10(7), 1213–1217 (2010). [CrossRef]

]. The high elastic limit of polymers and the ability to have low-loss single-mode fibres in the visible makes SM POF uniquely useful in such strain sensing applications.

Whilst SM fibre technology is abundant in silica, POF technology has focused almost entirely on large-core multimode, step- and graded-index fibres. The best state of the art low loss POF fibres are generally fabricated by co-extrusion, such as perfluorinated fibres [6

6. O. Ziemann, J. Krauser, P. E. Zamzow, and W. Daum, POF Handbook (Springer, Berlin 2008).

]. The extrusion method, even though very good for large-core fibres, is not suitable for the fabrication of the very small cores required for SM operation. In general, achieving true SM operation in polymer fibres down to the visible regime is not straight forward due to the non trivial process of doping drawable polymers to the required refractive index and maintaining good optical quality. The fabrication of small doped polymer cores is intrinsically difficult due to much higher dopant diffusion seen in polymers [7

7. L. L. Blyer, T. Salmon, W. White, M. Dueser, W. A. Reed, Ch. S. Coappen, Ch. Ronaghan, P. Wiltzius, and X. Quan, “Performance and reliability of graded-index polymer optical fibres,” in Proceedings of the 47th (International Wire and Cable Symposium, Inc., 1998), pp. 241–245.

] compared to silica doped cores. In spite of these problems, all-solid SM POF have been fabricated [8

8. D. Bosc and C. Toinen, “Fully polymer single-mode optical fibre,” IEEE Photon. Technol. Lett. 4(7), 749–750 (1992). [CrossRef]

,9

9. D. W. Garvey, K. Zimmerman, P. Young, J. Tostenrude, J. S. Townsend, Z. Zhou, M. Lobel, M. Dayton, R. Wittorf, M. G. Kuzyk, J. Sounick, and C. W. Dirk, “Single-mode nonlinear-optical polymer fibres,” J. Opt. Soc. Am. B 13(9), 2017 (1996). [CrossRef]

], but they are typically SM only in the infrared, suffer from high loss (in excess of 10’s dB/m), and are generally not commercially available [6

6. O. Ziemann, J. Krauser, P. E. Zamzow, and W. Daum, POF Handbook (Springer, Berlin 2008).

]. The most successful approach for SM operation in the visible in polymer fibres is the use of microstructured polymer fibres (mPOF) which can be designed to be SM at any wavelength [10

10. M. C. J. Large, L. Poladian, G. W. Barton, and M. A. van Eijkelenborg, Microstructured Polymer Optical Fibres (Springer, 2007).

,11

11. A. Argyros, “Microstructured polymer optical fibres,” J. Lightwave Technol. 27(11), 1571–1579 (2009). [CrossRef]

]. Another recent approach is a hybrid fabrication technique between commercial perfluorinated POF and mPOF technology. A standard graded-index perfluorinated large-core multimode fibre was inserted into a PMMA cladding with holes (the air holes were needed due to the much lower refractive index of the perfluorinated polymer, n = 1.34, compared to PMMA, n = 1.49) and drawn to fibre together until the large multimode fibre was reduced to form the SM core of the final fibre [12

12. G. Zhou, C. F. J. Pun, H. Tam, A. C. L. Wong, C. Lu, and P. K. A. Wai, “Single-mode perfluorinated polymer optical fibres with refractive index of 1.34 for biomedical applications,” IEEE Photon. Technol. Lett. 22(2), 106–108 (2010). [CrossRef]

]. However, this fibre was also designed for operation in the IR.

In general polymer rods/tubes with tailored refractive indices are not commercially available or indeed easy to fabricate. On the other hand, different off-the-shelf drawable polymer materials can be purchased in an affordable and accessible manner from many different manufacturers. Some of the most common polymers are, PMMA (n = 1.49), Polystyrene (n = 1.59), Polycarbonate (n = 1.58) and Zeonex 480R (n = 1.53). A SM step-index fibre can be made by using these raw polymers in a straight-forward manner, however due to the high refractive index differences between these, the core sizes needed for SM operation would have to be extremely small, making the fibres impractical compared to their silica counterparts.

An interesting alternative for the fabrication of SM fibres is to have a composite core formed by an array of evanescent-field coupled cores. If the individual cores are are sufficiently small and appropriately spaced, the resulting multicore fibre (MCF) can be tailored to behave as a true SM waveguides with a single-mode, composite core. Such MCF waveguides were initially reported in silica to behave as SM waveguides in the IR, with the aim of increasing the effective mode area for fibre amplifiers and lasers [13

13. M. M. Vogel, M. Abdou-Ahmed, A. Voss, and T. Graf, “Very-large-mode-area, single-mode multicore fiber,” Opt. Lett. 34(18), 2876–2878 (2009). [CrossRef] [PubMed]

]. However, a later report demonstrated that those waveguides were not operating in a SM regime but in a few-moded one [14

14. J. M. Fini, “Large-mode-area multicore fibers in the single-moded regime,” Opt. Express 19(5), 4042–4046 (2011). [CrossRef] [PubMed]

]. Large-core MCFs having quasi-SM behaviour have been exploited in fibre laser applications. In those waveguides the SM propagation was achieved by phase locking [15

15. Y. Huo, P. Cheo, and G. King, “Fundamental mode operation of a 19-core phase-locked Yb-doped fiber amplifier,” Opt. Express 12(25), 6230–6239 (2004). [CrossRef] [PubMed]

,16

16. L. Michaille, C. R. Bennett, D. M. Taylor, T. J. Shepherd, J. Broeng, H. R. Simonsen, and A. Petersson, “Phase locking and supermode selection in multicore photonic crystal fiber lasers with a large doped area,” Opt. Lett. 30(13), 1668–1670 (2005). [CrossRef] [PubMed]

] and self-organizing [17

17. E. J. Bochove, P. K. Cheo, and G. G. King, “Self-organization in a multicore fiber laser array,” Opt. Lett. 28(14), 1200–1202 (2003). [CrossRef] [PubMed]

] of the fundamental supermode, but not by a true cut-off mechanism within the fibre core (the fibre was again few-moded).

This paper presents an analysis of the behaviour of these composite core waveguides, investigating the core-spacing and size required in order to achieve true SM operation down to 650 nm by using off-the-shelf polymer materials. We use PMMA and Zeonex 480R polymer for the analysis as well as for the fabrication of the final all-solid SM polymer fibre. A characterisation of the final fibre showing the onset of SM operation is also presented.

2. Multicore composite core for SM operation

For the first part of the analysis we began by considering the case of a standard SM step-index fibre made of Zeonex/PMMA. The refractive indices of PMMA and Zeonex are 1.4894 and 1.5226 at 650nm respectively. The size of the core needs to be D = 1.522 µm to be SM. To study the behaviour of a composite core we divided this solid core into 19 adjacent (i.e. with a d/Λ = 1) Zeonex submicron cores with the same total area. The size of the composite core can be considered to be 4.5 times the spacing (4.5 × Λ). Hence the submicron cores were calculated by dividing the diameter of the standard core by 4.5 (d = D/4.5), resulting in a composite core with d/Λ = 1, d = 345 nm and Λ = 345 nm (Fig. 1
Fig. 1 (left) Dispersion plots for the fundamental mode (LP01) of the composite cores with different d/Λ ratios while fixing multicores size constant. (right) Schematic of the studied multicore composite cores and calculated LP01 modes at 810 nm for the different d/Λ ratios, at the same scale.
(top right)). The independent Zeonex submicron cores are not effective waveguides on their own with a V-parameter or normalized frequency (V = πd/λ (nco2-ncl2)1/2) of about 0.5 [18

18. R. J. Black, J. Lapierre, and J. Bures, “Field evolution in doubly clad lightguides,” IEE Proc. Pt. J. 134(2), 105 (1987). [CrossRef]

].

Effectively, this multicore composite core approach can be used to tailor the neff of the modes (i.e. the fibre core refractive index) in the same way that doping technology is used in the fabrication of standard SM silica fibres, and effectively results in a step-index fibre with a core of a tailored refractive index. Like all step-index fibres, the fundamental mode has no defined cut-off, but at long wavelengths stops to be effectively guided. This is also seen for these composite core fibres in Fig. 3, for example at 1050 nm the neff of the fundamental mode approached that of the cladding at d/Λ = 0.25, making the fibre an ineffective waveguide. This analysis has identified the range of parameters required to achieve SM operation in such multicore composite-core waveguides.

3. PMMA/Zeonex multicore composite core fibre fabrication

The entire structure was drawn to a 6 mm preform while vacuum was applied to eliminate the gaps between the Zeonex rods and the PMMA cladding, as shown in Fig. 4(b). The final d/Λ obtained in the preform was 0.35. A reduction in d/Λ was expected due to the collapse of the holes during the preform draw and also due to the smaller diameter of the Zeonex rods (480 µm) compared to the holes in the PMMA (500 µm). Once the microstructured core was achieved, the 6 mm preform was inserted into a 12.7/6.35 mm (OD/ID) PMMA jacket and stretched to 6 mm again. Due to the high reduction in diameter needed to achieve the final core size of about 4-5 µm, this sleeving process was repeated 4 times before the final fibre was drawn. Two different fibre diameters were fabricated, 330 (Fig. 4(c)) and 390µm. The core sizes for the 330 and 390 µm fibres were measured using a microscope to be approximately 4.5 and 5.3 µm respectively (measured corner-to-corner of the multicore composite core). Considering the corner-to-corner distance to be equal to 4Λ + d (d = size of submicron cores and Λ = spacing) and a d/Λ = 0.35 in the composite cores, parameters for the cores can be estimated to be d = 362 nm, Λ = 1.034 µm for the 330 µm fibre (Fig. 5(a)
Fig. 5 Optical photographs of (a) 4.5µm composite core of the 330µm fibre; (b) 330µm fibre near- and far-field images of the guided LP11 mode at 550nm in a 1.2m fibre length; (c) calculated LP01 mode profile for the 330µm fibre at 650nm; (d) 330µm fibre near- and far-field images of the guided fundamental mode LP01 at 650nm in a 1.2m fibre length; (e) 5.3µm composite core of the 390µm fibre; and (f) 390µm fibre near- and far-field images of the guided LP11 mode at 650nm in a 1.2m fibre length.
); and d = 428 nm, Λ = 1.222 µm for the 390 µm one (Fig. 5(e)). Considering the final submicron individual Zeonex core size of approximately 360 nm in the final composite core, a total remarkable reduction of almost 200,000 times was achieved from the 60 mm diameter original Zeonex 480R rod.

The 330 µm fibre was found to be clearly operating in the SM regime at 650 nm as can be seen by its near- and far-field outputs (Fig. 5(d)). The fibre was bent in order to observe any spatial intensity variations on the mode field profile which would have indicated multimode operation. The 330 µm fibre showed a strong unperturbed SM operation at 650 nm while bending. However, while using the 10 nm band-pass filter centred at 600 nm a slight variation in the spatial intensity mode profile was observed while bending the fibre. This indicated that the second mode LP11 was present, and that the wavelength cut-off for this fibre was somewhere in the range of 605 to 645 nm. We characterized the 390 µm fibre following the same procedure and the wavelength cut-off of the fibre was found to be somewhere in the range of 705 to 745 nm. We further characterised the 330 µm fibre by using a HeNe laser and found that the fibre was definitely operating in the single-mode regime narrowing the range of cut-off to 633 to 645 nm. The fibre losses were found to be 8 dB/m at the HeNe wavelength. We measured the material losses of our Zeonex and PMMA materials at HeNe wavelength to be 6.5 and 0.35 dB/m respectively. Considering our composite core to be ~10% Zeonex and ~90% PMMA at the d/Λ = 0.35, the theoretical composite bulk material loss would be around 1 dB/m. However, as it is clear from the calculated and measured mode profiles, light intensity will tent to localize in the higher refractive index areas, i.e Zeonex. This effect will indeed represent a larger effective bulk material loss than the theoretical 1 dB/m.

4. Conclusion

A new concept for the realisation of SM polymer fibres using standard available PMMA and Zeonex polymers has been presented. We have shown and analysed multicore composite core geometries for achieving true SM operation down to 650 nm wavelengths. The range of parameters used were not exhaustive, however, this analysis clearly results in a detailed understanding of the parameters and trade-offs. We have shown that this multicore composite core approach can be used to tailor the fibre core properties in the same way that doping technology is used in the fabrication of standard SM silica fibres. Furthermore, this approach could lead to more interesting and complex core index geometries that cannot be achieved with standard doping technologies. Two different core sizes were fabricated and characterized, their cut-off wavelength properties were measured and found to agree with the theory. The fabricated 330 µm SM composite multicore polymer fibre had a very similar core size, MFD and dispersion behaviour to its silica counterpart (i.e. Thorlabs SM600).

Acknowledgements

Sergio G. Leon-Saval is supported by an Australian Research Council Australian Postdoctoral Fellowship. Alexander Argyros is supported by an Australian Research Council Australian Research Fellowship. This work was performed in part at the OptoFab node of the Australian National Fabrication Facility, a company established under the National Collaborative Research Infrastructure Strategy to provide nanofabrication and microfabrication facilities for Australian researchers. Sergio G. Leon-Saval would like to dedicate this paper to his newly born niece, Alejandra.

References and links

1.

M. C. J. Large, J. Moran, and L. Ye, “The role of viscoelastic properties in the strain testing using microstructured polymer optical fibres (mPOF),” Meas. Sci. Tech. 20 (2009).

2.

Z. Xiong, G. D. Peng, B. Wu, and P. L. Chu, “Highly tunable Bragg gratings in single-mode polymer optical fibres,” IEEE Photon. Technol. Lett. 11(3), 352–354 (1999). [CrossRef]

3.

M. Kiesel, K. Peters, T. Hassan, and M. Kowalsky, “Behaviour of intrinsic polymer optical fibre sensor for large-strain applications,” Meas. Sci. Technol. 8(10), 3144–3154 (2007). [CrossRef]

4.

S. Kiesel, K. Peters, T. Hassan, and M. Kowalsky, “Large deformation in-fibre polymer optical fibre sensor,” IEEE Photon. Technol. Lett. 20(6), 416–418 (2008). [CrossRef]

5.

M. C. J. Large, D. Blacket, and C.-A. Bunge, “Microstructured polymer optical fibres compared to conventional POF: novel properties and applications,” IEEE Sens. J. 10(7), 1213–1217 (2010). [CrossRef]

6.

O. Ziemann, J. Krauser, P. E. Zamzow, and W. Daum, POF Handbook (Springer, Berlin 2008).

7.

L. L. Blyer, T. Salmon, W. White, M. Dueser, W. A. Reed, Ch. S. Coappen, Ch. Ronaghan, P. Wiltzius, and X. Quan, “Performance and reliability of graded-index polymer optical fibres,” in Proceedings of the 47th (International Wire and Cable Symposium, Inc., 1998), pp. 241–245.

8.

D. Bosc and C. Toinen, “Fully polymer single-mode optical fibre,” IEEE Photon. Technol. Lett. 4(7), 749–750 (1992). [CrossRef]

9.

D. W. Garvey, K. Zimmerman, P. Young, J. Tostenrude, J. S. Townsend, Z. Zhou, M. Lobel, M. Dayton, R. Wittorf, M. G. Kuzyk, J. Sounick, and C. W. Dirk, “Single-mode nonlinear-optical polymer fibres,” J. Opt. Soc. Am. B 13(9), 2017 (1996). [CrossRef]

10.

M. C. J. Large, L. Poladian, G. W. Barton, and M. A. van Eijkelenborg, Microstructured Polymer Optical Fibres (Springer, 2007).

11.

A. Argyros, “Microstructured polymer optical fibres,” J. Lightwave Technol. 27(11), 1571–1579 (2009). [CrossRef]

12.

G. Zhou, C. F. J. Pun, H. Tam, A. C. L. Wong, C. Lu, and P. K. A. Wai, “Single-mode perfluorinated polymer optical fibres with refractive index of 1.34 for biomedical applications,” IEEE Photon. Technol. Lett. 22(2), 106–108 (2010). [CrossRef]

13.

M. M. Vogel, M. Abdou-Ahmed, A. Voss, and T. Graf, “Very-large-mode-area, single-mode multicore fiber,” Opt. Lett. 34(18), 2876–2878 (2009). [CrossRef] [PubMed]

14.

J. M. Fini, “Large-mode-area multicore fibers in the single-moded regime,” Opt. Express 19(5), 4042–4046 (2011). [CrossRef] [PubMed]

15.

Y. Huo, P. Cheo, and G. King, “Fundamental mode operation of a 19-core phase-locked Yb-doped fiber amplifier,” Opt. Express 12(25), 6230–6239 (2004). [CrossRef] [PubMed]

16.

L. Michaille, C. R. Bennett, D. M. Taylor, T. J. Shepherd, J. Broeng, H. R. Simonsen, and A. Petersson, “Phase locking and supermode selection in multicore photonic crystal fiber lasers with a large doped area,” Opt. Lett. 30(13), 1668–1670 (2005). [CrossRef] [PubMed]

17.

E. J. Bochove, P. K. Cheo, and G. G. King, “Self-organization in a multicore fiber laser array,” Opt. Lett. 28(14), 1200–1202 (2003). [CrossRef] [PubMed]

18.

R. J. Black, J. Lapierre, and J. Bures, “Field evolution in doubly clad lightguides,” IEE Proc. Pt. J. 134(2), 105 (1987). [CrossRef]

OCIS Codes
(060.2280) Fiber optics and optical communications : Fiber design and fabrication
(060.4005) Fiber optics and optical communications : Microstructured fibers

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: October 3, 2011
Revised Manuscript: December 11, 2011
Manuscript Accepted: December 13, 2011
Published: December 19, 2011

Citation
Sergio G. Leon-Saval, Richard Lwin, and Alexander Argyros, "Multicore composite single-mode polymer fibre," Opt. Express 20, 141-148 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-1-141


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References

  1. M. C. J. Large, J. Moran, and L. Ye, “The role of viscoelastic properties in the strain testing using microstructured polymer optical fibres (mPOF),” Meas. Sci. Tech. 20 (2009).
  2. Z. Xiong, G. D. Peng, B. Wu, and P. L. Chu, “Highly tunable Bragg gratings in single-mode polymer optical fibres,” IEEE Photon. Technol. Lett. 11(3), 352–354 (1999). [CrossRef]
  3. M. Kiesel, K. Peters, T. Hassan, and M. Kowalsky, “Behaviour of intrinsic polymer optical fibre sensor for large-strain applications,” Meas. Sci. Technol. 8(10), 3144–3154 (2007). [CrossRef]
  4. S. Kiesel, K. Peters, T. Hassan, and M. Kowalsky, “Large deformation in-fibre polymer optical fibre sensor,” IEEE Photon. Technol. Lett. 20(6), 416–418 (2008). [CrossRef]
  5. M. C. J. Large, D. Blacket, and C.-A. Bunge, “Microstructured polymer optical fibres compared to conventional POF: novel properties and applications,” IEEE Sens. J. 10(7), 1213–1217 (2010). [CrossRef]
  6. O. Ziemann, J. Krauser, P. E. Zamzow, and W. Daum, POF Handbook (Springer, Berlin 2008).
  7. L. L. Blyer, T. Salmon, W. White, M. Dueser, W. A. Reed, Ch. S. Coappen, Ch. Ronaghan, P. Wiltzius, and X. Quan, “Performance and reliability of graded-index polymer optical fibres,” in Proceedings of the 47th (International Wire and Cable Symposium, Inc., 1998), pp. 241–245.
  8. D. Bosc and C. Toinen, “Fully polymer single-mode optical fibre,” IEEE Photon. Technol. Lett. 4(7), 749–750 (1992). [CrossRef]
  9. D. W. Garvey, K. Zimmerman, P. Young, J. Tostenrude, J. S. Townsend, Z. Zhou, M. Lobel, M. Dayton, R. Wittorf, M. G. Kuzyk, J. Sounick, and C. W. Dirk, “Single-mode nonlinear-optical polymer fibres,” J. Opt. Soc. Am. B 13(9), 2017 (1996). [CrossRef]
  10. M. C. J. Large, L. Poladian, G. W. Barton, and M. A. van Eijkelenborg, Microstructured Polymer Optical Fibres (Springer, 2007).
  11. A. Argyros, “Microstructured polymer optical fibres,” J. Lightwave Technol. 27(11), 1571–1579 (2009). [CrossRef]
  12. G. Zhou, C. F. J. Pun, H. Tam, A. C. L. Wong, C. Lu, and P. K. A. Wai, “Single-mode perfluorinated polymer optical fibres with refractive index of 1.34 for biomedical applications,” IEEE Photon. Technol. Lett. 22(2), 106–108 (2010). [CrossRef]
  13. M. M. Vogel, M. Abdou-Ahmed, A. Voss, and T. Graf, “Very-large-mode-area, single-mode multicore fiber,” Opt. Lett. 34(18), 2876–2878 (2009). [CrossRef] [PubMed]
  14. J. M. Fini, “Large-mode-area multicore fibers in the single-moded regime,” Opt. Express 19(5), 4042–4046 (2011). [CrossRef] [PubMed]
  15. Y. Huo, P. Cheo, and G. King, “Fundamental mode operation of a 19-core phase-locked Yb-doped fiber amplifier,” Opt. Express 12(25), 6230–6239 (2004). [CrossRef] [PubMed]
  16. L. Michaille, C. R. Bennett, D. M. Taylor, T. J. Shepherd, J. Broeng, H. R. Simonsen, and A. Petersson, “Phase locking and supermode selection in multicore photonic crystal fiber lasers with a large doped area,” Opt. Lett. 30(13), 1668–1670 (2005). [CrossRef] [PubMed]
  17. E. J. Bochove, P. K. Cheo, and G. G. King, “Self-organization in a multicore fiber laser array,” Opt. Lett. 28(14), 1200–1202 (2003). [CrossRef] [PubMed]
  18. R. J. Black, J. Lapierre, and J. Bures, “Field evolution in doubly clad lightguides,” IEE Proc. Pt. J. 134(2), 105 (1987). [CrossRef]

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