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Optics Express

  • Editor: Michael Duncan
  • Vol. 11, Iss. 21 — Oct. 20, 2003
  • pp: 2762–2768
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Photonic crystal fiber design based on the V-parameter

Martin Dybendal Nielsen and Niels Asger Mortensen  »View Author Affiliations


Optics Express, Vol. 11, Issue 21, pp. 2762-2768 (2003)
http://dx.doi.org/10.1364/OE.11.002762


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Abstract

Based on a recent formulation of the V–parameter of a photonic crystal fiber we provide numerically based empirical expressions for this quantity only dependent on the two structural parameters—the air hole diameter and the hole-to-hole center spacing. Based on the unique relation between the V–parameter and the equivalent mode field radius we identify how the parameter space for these fibers is restricted in order for the fibers to remain single mode while still having a guided mode confined to the core region.

© 2003 Optical Society of America

1. Introduction

Theoretical descriptions of photonic crystal fibers (PCFs) have traditionally been restricted to numerical evaluation of Maxwell’s equations. In the most general case, a plane wave expansion method with periodic boundary conditions is employed [1

1. S. G. Johnson and J. D. Joannopoulos, “Block-iterative frequency-domain methods for Maxwell’s equations in a planewave basis,” Opt. Express 8, 173 (2001), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-8-3-173. [CrossRef] [PubMed]

] while other methods, such as the multipole method [2

2. T. P. White, B. T. Kuhlmey, R. C. McPhedran, D. Maystre, G. Renversez, C. M. de Sterke, and L. C. Botton, “Multipole method for microstructured optical fibers. I. Formulation,” J. Opt. Soc. Am. B 19, 2322 (2002). [CrossRef]

], take advantage of the localized nature of the guided modes and to some extend the circular shape of the air-holes. The reason for the application of these methods is the relatively complex dielectric cross section of a PCF for which rotational symmetry is absent.

The aim of this work is to provide a set of numerically based empirical expressions describing the basic properties such as cutoff and mode-field radius of a PCF based on the fundamental geometrical parameters only.

2. Fiber geometry and numerical method

We consider the fiber structure first studied by Knight et al. [3

3. J. C. Knight, T. A. Birks, P. S. J. Russell, and D. M. Atkin, “All-silica single-mode optical fiber with photonic crystal cladding,” Opt. Lett. 21, 1547 (1996). [CrossRef] [PubMed]

] and restrict our study to fibers that consist of pure silica with a refractive index of 1.444. The air holes of diameter d are arranged on a triangular grid with a pitch, Λ. In the center an air hole is omitted creating a central high index defect serving as the fiber core. A schematic drawing of such a structure is shown in the inset of the right panel in Fig. 1.

Depending on the dimensions, the structure comprises both single- and multi-mode fibers with large mode area as well as nonlinear fibers. The results presented here cover relative air hole sizes, d/Λ, from 0.2 to 0.9 and normalized wavelengths, λ/Λ, from around 0.05 to 2. The modeling is based on the plane-wave expansion method with periodic boundary conditions [1

1. S. G. Johnson and J. D. Joannopoulos, “Block-iterative frequency-domain methods for Maxwell’s equations in a planewave basis,” Opt. Express 8, 173 (2001), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-8-3-173. [CrossRef] [PubMed]

]. For the calculations of guided modes presented the size of the super cell was 8×8 resolved by 256×256 plane waves while for calculations on the cladding structure only, the super cell was reduced to a simple cell resolved by 32×32 planes waves.

3. The V-parameter and the relative mode-field radius

When attempting to establish a simple formalism for the PCF it is natural to strive for a result similar to the V–parameter known from standard fibers [4

4. A. W. Snyder and J. D. Love, Optical Waveguide Theory (Chapman & Hall, New York, 1983).

, 5

5. D. Marcuse, “Gaussian approximation of the fundamental modes of graded-index fibers,” J. Opt. Soc. Am. 68, 103 (1978). [CrossRef]

]. However, a simple translation is not straight forward since no wavelength-independent core- or cladding index can be defined. Recently, we instead proposed a formulation of the V–parameter for a PCF given by [6

6. N. A. Mortensen, J. R. Folkenberg, M. D. Nielsen, and K. P. Hansen, “Modal cut-off and the V-parameter in photonic crystal fibers,” Opt. Lett. 28, 1879 (2003). [CrossRef] [PubMed]

]

Fig. 1. left panel shows V PCF calculated from Eq. (1) for d/Λ ranging from 0.20 (lowest curve) to 0.70 in steps of 0.05. The dashed line indicates V PCF=π. The right panel shows the relative equivalent mode-field radius, w PCF/Λ plotted as function of V PCF for each of the 9 curves in the left panel. The inset shows a schematic drawing of the considered PCF structure.
Fig. 2. The left panel shows curves for constant values of V PCF in a normalized wavelength versus relative hole-size plot. The open circles indicate calculated data points with full lines to guide the eye. Similarly, the right panel shows curves for constant relative equivalent mode-field radius.
VPCF=2πΛλnFM2(λ)nFSM2(λ)
(1)

In the left panel of Fig. 1, calculated curves of V PCF as function of Λ/λ are shown for d/Λ ranging from 0.20 to 0.70 in steps of 0.05. In general, all curves are seen to approach constant levels dependent on d/Λ. The horizontal dashed line indicates the single-mode boundary V PCF=π. In the right panel, w PCF/Λ is plotted as function of V PCF for each of the 9 curves in the left panel and as seen all curves fold over a single curve. An empirical expression for w PCF/Λ can be found in Ref. [9

9. M. D. Nielsen, N. A. Mortensen, J. R. Folkenberg, and A. Bjarklev, “Mode-Field Radius of Photonic Crystal Fibers Expressed by the V-parameter,” Opt. Lett. 28, 2309 (2003). [CrossRef] [PubMed]

]. The mode is seen to expand rapidly for small values of V PCF and the mode-field radius saturates toward a constant value when V PCF becomes large. In fact, it turns out that w PCF/Λ≃1.00 for V PCF=1 and w PCF/Λ≃0.62 for V PCF=π. In the left panel of Fig. 2, curves corresponding to constant values of V PCF are shown in a λ/Λ versus d/Λ plot. In the right panel, curves of constant w PCF/Λ is shown, also in a λ/Λ versus d/Λ plot. Since there is a unique relation between w PCF/Λ and V PCF [9

9. M. D. Nielsen, N. A. Mortensen, J. R. Folkenberg, and A. Bjarklev, “Mode-Field Radius of Photonic Crystal Fibers Expressed by the V-parameter,” Opt. Lett. 28, 2309 (2003). [CrossRef] [PubMed]

] the curves naturally have the same shape.

When designing a PCF any combination of d and Λ is in principle possible. However, in some cases the guiding will be weak causing the mode to expand beyond the core and into the cladding region [10

10. T. P. White, R. C. McPhedran, C. M. de Sterke, L. C. Botton, and M. J. Steel, “Confinement losses in microstructured optical fibers,” Opt. Lett. 26, 1660 (2001). [CrossRef]

, 11

11. B. T. Kuhlmey, R. C. McPhedran, C. M. de Sterke, P. A. Robinson, G. Renversez, and D. Maystre, “Microstructured optical fibers: where’s the edge?,” Opt. Express 10, 1285 (2002), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-10-22-1285. [CrossRef] [PubMed]

] corresponding to a low value of V PCF. In the other extreme, the confinement will be too strong allowing for the guiding of higher-order modes [6

6. N. A. Mortensen, J. R. Folkenberg, M. D. Nielsen, and K. P. Hansen, “Modal cut-off and the V-parameter in photonic crystal fibers,” Opt. Lett. 28, 1879 (2003). [CrossRef] [PubMed]

, 7

7. B. T. Kuhlmey, R. C. McPhedran, and C. M. de Sterke, “Modal cutoff in microstructured optical fibers,” Opt. Lett. 27, 1684 (2002). [CrossRef]

]. Since both situations are governed by V PCF the design relevant region in a λ/Λ versus d/Λ plot can be defined. This is done in Fig. 3 where the low limit is chosen to be V PCF=1 where w PCF/Λ≃1. How large a mode that can be tolerated is of course not unambiguous. However, for w PCF~Λ leakage-loss typically becomes a potential problem in PCFs with a finite cladding structure. In non-linear PCFs it is for dispersion reasons often advantageous operating the PCF at V PCF≲1 and then a high number of air-hole rings is needed to achieve an acceptable level of leakage loss [12

12. W. H. Reeves, J. C. Knight, P. S. J. Russell, and P. J. Roberts, “Demonstration of ultra-flattened dispersion in photonic crystal fibers,” Opt. Express 10, 609 (2002), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-10-14-609. [CrossRef] [PubMed]

].

Fig. 3. Plot of the parameter space in terms of relative hole size and normalized wavelength divided into three regions by the boundaries defined byV PCF=1 andV PCF=π. In the upper red area the mode penetrates deeply into the cladding region and in lower blue region the structure supports a higher-order mode.

Finally, we note that the practical operational regime is also limited from the low wave-length side. In Ref. [13

13. N. A. Mortensen and J. R. Folkenberg, “Low-loss criterion and effective area considerations for photonic crystal fibers,” J. Opt. A: Pure Appl. Opt. 5, 163 (2003). [CrossRef]

] a low-loss criterion was formulated in terms of the coupling length zc =λ/[n FM(λ)-n FSM(λ)] between the FM and the FSM. In general scattering-loss due to longitudinal non-uniformities increases when z c increases and a PCF with a low z c will in general be more stable compared to one with a larger z c. Using n FM+n FSM≈2n FM≈2n silica we can rewrite Eq. (1) as

VPCF=Λλλzc(λ)
(2)

from which it is seen that a high value of the V–parameter is preferred over a smaller value. In Fig. (3) it is thus preferable to stay close to the single-mode boundary (V PCF~π) but in general there is a practical lower limit to the value of λ/Λ which can be realized because when λ/Λ≲0.1 one generally has that zcλ [13

13. N. A. Mortensen and J. R. Folkenberg, “Low-loss criterion and effective area considerations for photonic crystal fibers,” J. Opt. A: Pure Appl. Opt. 5, 163 (2003). [CrossRef]

].

4. V-parameter expression

Although the V–parameter offers a simple way to design a PCF, a limiting factor for using Eq. (1) is that a numerical method is still required for obtaining the effective indices. In analogy with expressions for standard fibers [5

5. D. Marcuse, “Gaussian approximation of the fundamental modes of graded-index fibers,” J. Opt. Soc. Am. 68, 103 (1978). [CrossRef]

] it would therefore be convenient to have an alternative expression only dependent on the wavelength, λ, and the structural parameters d and Λ. In Fig. 4, we show V PCF as function of λ/Λ (data are shown by open circles) for d/Λ ranging from 0.20 to 0.80 in steps of 0.05. Each data set in Fig. 4 is fitted to a function of the form

Fig. 4. Plot of V PCF as a function of relative wavelength λ/Λ for d/Λ ranging from 0.20 (lowest curve) to 0.80 in steps of 0.05.
VPCF(λΛ,dΛ)=A(dΛ)B(dΛ)×exp[C(dΛ)×λΛ]+1
(3a)

and the result is indicated by the full red lines. Eq. (3a) is not based on considerations of the physics of the V-parameter but merely obtained by trial and error in order to obtain the best representation of calculated data with the lowest possible number of free parameters. Prior to the fit, the data sets are truncated at V PCF=0.5 since w PCF≳2Λ in this region (see left panel in Fig. 1) and the data is thus not practically relevant. In Eq. (3a) the fitting parameters A, B, and C depend on d/Λ only. In order to extract this dependency, suitable functions (again obtained by trial and error) are fitted to the data sets for A, B, and C. We find that the data are well described by the following expressions

A(dΛ)=dΛ+0.457+3.405×dΛ0.904dΛ
(3b)
B(dΛ)=0.200×dΛ+0.100+0.027×(1.045dΛ)2.8
(3c)
C(dΛ)=0.630×exp(0.7550.171+dΛ)
(3d)

The above set of expressions, Eqs. (3), constitute our empirical expression for the V–parameter in a PCF with λ/Λ and d/Λ being the only input parameters. For λ/Λ<2 and V PCF>0.5 the expression gives values of V PCF which deviates less than 3% from the correct values obtained from Eq. (1).

Fig. 5. Plot of V PCF in the λ→0 limit as function of the relative air hole size (open circles). The full red line represents a fit to the data points and the horizontal dash line indicated the ESM limit V PCF=π. The insert shows a close-up of the intersection with the vertical line indicating the air hole size d/Λ≃0.43.

5. Endlessly single-mode criterion

The term endlessly single-mode (ESM) refers to PCFs which regardless of wavelength only support the two degenerate polarization states of the fundamental mode [14

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

]. In the framework of the V–parameter this corresponds to structures for which V PCF <π for any λ/Λ [6

6. N. A. Mortensen, J. R. Folkenberg, M. D. Nielsen, and K. P. Hansen, “Modal cut-off and the V-parameter in photonic crystal fibers,” Opt. Lett. 28, 1879 (2003). [CrossRef] [PubMed]

]. As seen in the left panel of Fig. 1 this corresponds to sufficiently small air holes. However, from the plot in Fig. 1 it is quite difficult to determine the exact d/Λ value for which V PCF=π for λ approaching 0. From Eq. (3) it is easily seen that the value may be obtained from

limλ0VPCF(λΛ,dΛ)=A(dΛ)B(dΛ)+1=π.
(4)

Fig. 5 illustrates this equation graphically where we have extrapolated the data in Fig. 4 to λ=0. From the intersection of the full line with the dashed line we find that d/Λ≃0.43 bounds the ESM regime. Solving Eq. (4) we get d/Λ≃0.44 and the deviation from the numerically obtained value is within the accuracy of the empirical expression.

6. Conclusion

Acknowledgments

We thank J. R. Folkenberg for stimulating discussion and M. D. Nielsen acknowledges financial support by the Danish Academy of Technical Sciences.

References and links

1.

S. G. Johnson and J. D. Joannopoulos, “Block-iterative frequency-domain methods for Maxwell’s equations in a planewave basis,” Opt. Express 8, 173 (2001), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-8-3-173. [CrossRef] [PubMed]

2.

T. P. White, B. T. Kuhlmey, R. C. McPhedran, D. Maystre, G. Renversez, C. M. de Sterke, and L. C. Botton, “Multipole method for microstructured optical fibers. I. Formulation,” J. Opt. Soc. Am. B 19, 2322 (2002). [CrossRef]

3.

J. C. Knight, T. A. Birks, P. S. J. Russell, and D. M. Atkin, “All-silica single-mode optical fiber with photonic crystal cladding,” Opt. Lett. 21, 1547 (1996). [CrossRef] [PubMed]

4.

A. W. Snyder and J. D. Love, Optical Waveguide Theory (Chapman & Hall, New York, 1983).

5.

D. Marcuse, “Gaussian approximation of the fundamental modes of graded-index fibers,” J. Opt. Soc. Am. 68, 103 (1978). [CrossRef]

6.

N. A. Mortensen, J. R. Folkenberg, M. D. Nielsen, and K. P. Hansen, “Modal cut-off and the V-parameter in photonic crystal fibers,” Opt. Lett. 28, 1879 (2003). [CrossRef] [PubMed]

7.

B. T. Kuhlmey, R. C. McPhedran, and C. M. de Sterke, “Modal cutoff in microstructured optical fibers,” Opt. Lett. 27, 1684 (2002). [CrossRef]

8.

J. R. Folkenberg, N. A. Mortensen, K. P. Hansen, T. P. Hansen, H. R. Simonsen, and C. Jakobsen, “Experimental investigation of cut-off phenomena in non-linear photonic crystal fibers,” Opt. Lett. 28, 1882 (2003). [CrossRef] [PubMed]

9.

M. D. Nielsen, N. A. Mortensen, J. R. Folkenberg, and A. Bjarklev, “Mode-Field Radius of Photonic Crystal Fibers Expressed by the V-parameter,” Opt. Lett. 28, 2309 (2003). [CrossRef] [PubMed]

10.

T. P. White, R. C. McPhedran, C. M. de Sterke, L. C. Botton, and M. J. Steel, “Confinement losses in microstructured optical fibers,” Opt. Lett. 26, 1660 (2001). [CrossRef]

11.

B. T. Kuhlmey, R. C. McPhedran, C. M. de Sterke, P. A. Robinson, G. Renversez, and D. Maystre, “Microstructured optical fibers: where’s the edge?,” Opt. Express 10, 1285 (2002), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-10-22-1285. [CrossRef] [PubMed]

12.

W. H. Reeves, J. C. Knight, P. S. J. Russell, and P. J. Roberts, “Demonstration of ultra-flattened dispersion in photonic crystal fibers,” Opt. Express 10, 609 (2002), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-10-14-609. [CrossRef] [PubMed]

13.

N. A. Mortensen and J. R. Folkenberg, “Low-loss criterion and effective area considerations for photonic crystal fibers,” J. Opt. A: Pure Appl. Opt. 5, 163 (2003). [CrossRef]

14.

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

OCIS Codes
(060.2280) Fiber optics and optical communications : Fiber design and fabrication
(060.2400) Fiber optics and optical communications : Fiber properties
(060.2430) Fiber optics and optical communications : Fibers, single-mode

ToC Category:
Research Papers

History
Original Manuscript: September 23, 2003
Revised Manuscript: October 14, 2003
Published: October 20, 2003

Citation
Martin Nielsen and Niels Mortensen, "Photonic crystal fiber design based on the V-parameter," Opt. Express 11, 2762-2768 (2003)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-11-21-2762


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References

  1. S. G. Johnson and J. D. Joannopoulos, �??Block-iterative frequency-domain methods for Maxwell�??s equations in a planewave basis,�?? Opt. Express 8, 173 (2001), <a href="http://www.opticsexpress.org/abstract.cfm?URI=OPEX-8-3-173">http://www.opticsexpress.org/abstract.cfm?URI=OPEX-8-3-173</a> [CrossRef] [PubMed]
  2. T. P. White, B. T. Kuhlmey, R. C. McPhedran, D. Maystre, G. Renversez, C. M. de Sterke, and L. C. Botton, �??Multipole method for microstructured optical fibers. I. Formulation,�?? J. Opt. Soc. Am. B 19, 2322 (2002). [CrossRef]
  3. J. C. Knight, T. A. Birks, P. S. J. Russell, and D. M. Atkin, �??All-silica single-mode optical fiber with photonic crystal cladding,�?? Opt. Lett. 21, 1547 (1996). [CrossRef] [PubMed]
  4. A. W. Snyder and J. D. Love, Optical Waveguide Theory (Chapman & Hall, New York, 1983).
  5. D. Marcuse, �??Gaussian approximation of the fundamental modes of graded-index fibers,�?? J. Opt. Soc. Am. 68, 103 (1978). [CrossRef]
  6. N. A. Mortensen, J. R. Folkenberg, M. D. Nielsen, and K. P. Hansen, �??Modal cut-off and the V�??parameter in photonic crystal fibers,�?? Opt. Lett. 28, 1879 (2003). [CrossRef] [PubMed]
  7. B. T. Kuhlmey, R. C. McPhedran, and C. M. de Sterke, �??Modal cutoff in microstructured optical fibers,�?? Opt. Lett. 27, 1684 (2002). [CrossRef]
  8. J. R. Folkenberg, N. A. Mortensen, K. P. Hansen, T. P. Hansen, H. R. Simonsen, and C. Jakobsen, �??Experimental investigation of cut-off phenomena in non-linear photonic crystal fibers,�?? Opt. Lett. 28, 1882 (2003). [CrossRef] [PubMed]
  9. M. D. Nielsen, N. A. Mortensen, J. R. Folkenberg, and A. Bjarklev, �??Mode-Field Radius of Photonic Crystal Fibers Expressed by the V�??parameter,�?? Opt. Lett. 28, 2309 (2003). [CrossRef] [PubMed]
  10. T. P. White, R. C. McPhedran, C. M. de Sterke, L. C. Botton, and M. J. Steel, �??Confinement losses in microstructured optical fibers,�?? Opt. Lett. 26, 1660 (2001). [CrossRef]
  11. B. T. Kuhlmey, R. C. McPhedran, C. M. de Sterke, P. A. Robinson, G. Renversez, and D. Maystre, �??Microstructured optical fibers: where�??s the edge?,�?? Opt. Express 10, 1285 (2002), <a href="http://www.opticsexpress.org/abstract.cfm?URI=OPEX-10-22-1285">http://www.opticsexpress.org/abstract.cfm?URI=OPEX-10-22-1285</a> [CrossRef] [PubMed]
  12. W. H. Reeves, J. C. Knight, P. S. J. Russell, and P. J. Roberts, �??Demonstration of ultra-flattened dispersion in photonic crystal fibers,�?? Opt. Express 10, 609 (2002), <a href="http://www.opticsexpress.org/abstract.cfm?URI=OPEX-10-14-609">http://www.opticsexpress.org/abstract.cfm?URI=OPEX-10-14-609</a> [CrossRef] [PubMed]
  13. N. A. Mortensen and J. R. Folkenberg, �??Low-loss criterion and effective area considerations for photonic crystal fibers,�?? J. Opt. A: Pure Appl. Opt. 5, 163 (2003). [CrossRef]
  14. T. A. Birks, J. C. Knight, and P. S. J. Russell, �??Endlessly single mode photonic crystal fibre,�?? Opt. Lett. 22, 961 (1997). [CrossRef] [PubMed]

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