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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 16, Iss. 13 — Jun. 23, 2008
  • pp: 9261–9275
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Rigorous sufficient conditions for index-guided modes in microstructured dielectric waveguides

Karen K. Y. Lee, Yehuda Avniel, and Steven G. Johnson  »View Author Affiliations


Optics Express, Vol. 16, Issue 13, pp. 9261-9275 (2008)
http://dx.doi.org/10.1364/OE.16.009261


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Abstract

We derive a sufficient condition for the existence of indexguided modes in a very general class of dielectric waveguides, including photonic-crystal fibers (arbitrary periodic claddings, such as “holey fibers”), anisotropic materials, and waveguides with periodicity along the propagation direction. This condition provides a rigorous guarantee of cutoff-free index-guided modes in any such structure where the core is formed by increasing the index of refraction (e.g. removing a hole). It also provides a weaker guarantee of guidance in cases where the refractive index is increased “on average” (precisely defined). The proof is based on a simple variational method, inspired by analogous proofs of localization for two-dimensional attractive potentials in quantum mechanics.

© 2008 Optical Society of America

1. Introduction

The most common guiding mechanism in dielectric waveguides is index guiding (or “total internal reflection”), in which a higher-index core is surrounded by a lower-index cladding εc (ε is the relative permittivity, the square of the refractive index in isotropic non-magnetic materials). A schematic of several such dielectric waveguides is shown in Fig. 1. In particular, we suppose that the waveguide is described by a dielectric function ε(x,y,z)=εc(x,y,z)+Δε(x,y,z) such that: ε, εc, and Δε are periodic in z (the propagation direction) with period a (a → 0 for the common case of a waveguide with a constant cross-section); that the cladding dielectric function εc is periodic in xy (e.g. in a photonic-crystal fiber), with a homogeneous cladding (e.g. in a conventional fiber) as a special case; and the core is formed by a change Δε in some region of the xy plane, sufficiently localized that ∫|1/ε-1/εc|<∞ (integrated over the xy plane and the unit cell in z). This includes a very wide variety of dielectric waveguides, from conventional fibers [Fig. 1(a)] to photonic-crystal “holey” fibers [Fig. 1(b)] to waveguides with a periodic “grating” along the propagation direction [Fig. 1(c)] such as fiber-Bragg gratings and other periodic waveguides. We exclude metallic structures (i.e, we require ε>0) and make the approximation of lossless materials (real ε). We allow anisotropic materials. The case of substrates (e.g. for strip waveguides in integrated optics [12–14

12. R. G. Hunsperger, Integrated Optics: Theory and Technology (Springer-Verlag, 1982).

]) is considered in Sec. 5. We also consider only non-magnetic materials (relative permeability μ=1), although a future extension to magnetic materials should be straightforward. Intuitively, if the refractive index is increased in the core, i.e. if Δε is non-negative, then we might expect to obtain exponentially localized index-guided modes, and this expectation is borne out by innumerable numerical calculations, even in complicated geometries like holey fibers [2–5

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

].

Fig. 1. Schematics of various types of dielectric waveguides in which our theorem is applicable. Light propagates in the z direction (along which the structure is either uniform or periodic) and is confined in the xy direction by a higher-index core compared to the surrounding (homogeneous or periodic) cladding.

However, an intuitive expectation of a guided mode is far from a rigorous guarantee, and upon closer inspection there arise a number of questions whose answers seem harder to guess with certainty. First, even if Δε is strictly non-negative, is there a guided mode at every wavelength, or is there the possibility of e.g. a long-wavelength cutoff (as was initially suggested in holey fibers [15

15. 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, 1285–1290 (2002). [PubMed]

], but was later contradicted by more careful numerical calculations [16

16. S. Wilcox, L. Botten, C. M. de Sterke, B. Kuhlmey, R. McPhedran, D. Fussell, and S. Tomljenovic-Hanic, “Long wavelength behavior of the fundamental mode in microstructured optical fibers,” Opt. Express 13 (2005). [CrossRef] [PubMed]

])? Second, what if Δε is not strictly non-negative, i.e. the core consists of partly increased and partly decreased index; it is known in such cases, e.g. in “W-profile fibers” [17

17. S. Kawakami and S. Nishida, “Characteristics of a doubly clad optical fiber with a low-index inner cladding,” IEEE J. Quantum Electron. 10, 879–887 (1974). [CrossRef]

] that there is the possibility of a long-wavelength cutoff for guidance, but precisely how much decreased-index regions does one need to have such a cutoff? Third, under some circumstances it is possible to obtain a “single-polarization” fiber, in which the waveguide is truly single-mode (as opposed to two degenerate polarization modes as in a cylindrical fiber) [18–23

18. T. Okoshi and K. Oyamoda, “Single-polarization single-mode optical fibre with refractive-index pits on both sides of core,” Electron. Lett. 16, 712–713 (80). [CrossRef]

]—our theorem can be extended, similar to Ref. 9

9. A. Bamberger and A. S. Bonnet, “Mathematical analysis of the guided modes of an optical fiber,” SIAM J. Math. Anal. 21, 1487–1510 (1990). [CrossRef]

, to a condition for two guided modes, and we will explore the consequences for single-polarization fibers in a subsequent paper. It turns out that all of these questions can be rigorously answered (in the sense of sufficient conditions for guidance) for the very general geometries considered in Fig. 1, without resorting to approximations or numerical computations.

Fig. 2. Example dispersion relation of a simple 2d dielectric waveguide in air (inset) for the TM polarization (electric field out of the plane), showing the light cone, the light line, the fundamental (cutoff-free) guided mode, and a higher-order guided mode with a cutoff.

We will proceed as follows. First, in Sec. 2, we review the mechanism of index guiding, state our result (a sufficient condition for the existence of index-guided modes), and discuss some important special cases. In Sec. 3, we first prove this theorem for the simplified special case of a homogeneous cladding εc, where the proof is much easier to follow. Then, in Sec. 4, we generalize the proof to arbitrary periodic claddings, such as for holey photonic-crystal fibers (with some algebraic details left to the appendix). In Sec. 5, we discuss a few contexts that go beyond the initial assumptions of our theorem: substrates, material dispersion, and finite-size effects. Finally, we offer some concluding remarks in Sec. 6 discussing future directions.

2. Statement of the theorem

First, let us review the basic description of the eigenmodes of a dielectric waveguide [1

1. A. W. Snyder and J. D. Love, Optical Waveguide Theory (Chapman and Hall, London, 1983).

, 5

5. J. D. Joannopoulos, S. G. Johnson, J. N. Winn, and R. D. Meade, Photonic Crystals: Molding the Flow of Light, 2nd ed. (Princeton Univ. Press, 2008).

]. In a waveguide as defined above, the solutions of Maxwell’s equations (both guided and nonguided) can be written in the form of eigenmodes H(x,y,z)e iβz-iωt (via Bloch’s theorem thanks to the periodicity in z) [5

5. J. D. Joannopoulos, S. G. Johnson, J. N. Winn, and R. D. Meade, Photonic Crystals: Molding the Flow of Light, 2nd ed. (Princeton Univ. Press, 2008).

], where ω is the frequency, β is the propagation constant, and the magnetic-field envelope H(x,y,z) is periodic in z with period a (or is independent of z in the common case of a constant cross section, a → 0). A plot of ω versus β for all eigenmodes is the “dispersion relation” of the waveguide, one example of which is shown in Fig. 2. In the absence of the core (i.e. if Δε=0), the (non-localized) modes propagating in the infinite cladding form the “light cone” of the structure [2–5

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

]; and at each real β there is a fundamental (minimum-ω) space-filling mode at a frequency ωc(β) with a corresponding field envelope H c [2–5

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

]. Such a light cone is shown as a shaded triangular region in Fig. 2. Below the “light line” ωc(β), the only solutions in the cladding are evanescent modes that decay exponentially in the transverse directions [2–5

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

, 24

24. P. Kuchment, “The Mathematics of Photonic Crystals,” in Mathematical Modeling in Optical Science, G. Bao, L. Cowsar, and W. Masters, eds., Frontiers in Applied Mathematics, pp. 207–272 (SIAM, Philadelphia, 2001).

]. Therefore, once the core is introduced (Δε≠0), any new solutions with ω<ωc must be guided modes, since they are exponentially decaying in the cladding far from the core: these are the index-guided modes (if any). Such guided modes are shown as lines below the light cone in Fig. 2: in this case, both a lowest-lying (“fundamental”) guided mode with no low-frequency cutoff (although it approaches the light line asymptotically as ω → 0) and a higher-order guided mode with a low-frequency cutoff are visible. Since a mode is guided if ω<ωc, we will prove the existence of a guided mode by showing that ω has an upper bound <ωc, using the variational (min–max) theorem for Hermitian eigenproblems [5

5. J. D. Joannopoulos, S. G. Johnson, J. N. Winn, and R. D. Meade, Photonic Crystals: Molding the Flow of Light, 2nd ed. (Princeton Univ. Press, 2008).

].

[Modes that lie beneath the light cone are not the only type of guided modes in microstructured dielectric waveguides. While all the guided modes in a traditional, homogeneous-cladding fiber lie below the light line and are confined by the mechanism of index-guiding, there can also be bandgap-guided modes in photonic-crystal fibers [2–5

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

]. These bandgap-guided modes lie above the cladding light line and are therefore not index-guided. Bandgap-guided modes always have a low-frequency cutoff (since in the long-wavelength limit the structure can be approximated by a “homogenized” effective medium that has no gap [25

25. D. R. Smith, D. C. Vier, T. Koschny, and C. M. Soukoulis, “Electromagnetic parameter retrieval from inhomogeneous materials,” Phys. Rev. E 71, 036,617 (2005). [CrossRef]

]). We do not consider bandgap-guided modes in this work; sufficient conditions for such modes to exist were considered by Ref. 26

26. P. Kuchment and B. Ong, “On guided waves in photonic crystal waveguides,” in Waves in Periodic and Random Media, vol. 339 of Contemporary Mathematics, pp. 105–115 (AMS, Providence, RI, 2003).

.]

We will derive the following sufficient condition for the existence of an index-guided mode in a dielectric waveguide at a given β: a guided mode must exist whenever

Dc*·(ε1εc1)Dc<0,
(1)

where the integral is over xy and one period in z and D c is the displacement field of the cladding’s fundamental mode. From this, we can immediately obtain a number of useful special cases:

  • There must be a cutoff-free guided mode if Δε≥0 everywhere (i.e., if we only increase the index to make the core).
  • For a homogeneous cladding (and isotropic media), there must be a cutoff-free guided mode if ∫(1/ε-1/εc)<0 (similar to the earlier theorem of Ref. 9

    9. A. Bamberger and A. S. Bonnet, “Mathematical analysis of the guided modes of an optical fiber,” SIAM J. Math. Anal. 21, 1487–1510 (1990). [CrossRef]

    , but generalized to include waveguides periodic in z and/or cores Δε that do not have compact support).
  • More generally, a guided mode has no long-wavelength cutoff if eq. (1) is satisfied for the quasi-static (ω → 0, β → 0) limit of D c.

Equation (1) can also be extended to a sufficient condition for having two guided modes (or, equivalently, a necessary condition for single-polarization guidance), when the cladding fundamental mode is doubly degenerate. We explore this generalization, analogous to a result in Ref. 9

9. A. Bamberger and A. S. Bonnet, “Mathematical analysis of the guided modes of an optical fiber,” SIAM J. Math. Anal. 21, 1487–1510 (1990). [CrossRef]

for homogeneous claddings, in another manuscript currently being prepared.

3. Waveguides with a homogeneous cladding

That is, we take the dielectric function ε(x,y) to be of the form:

ε(x,y)=εc+Δε(x,y),
(2)

where Δε is an an arbitrary change in ε that forms the core of the waveguide. For convenience, we define a new function Δ by:

Δ(x,y)ε1εc1.
(3)

The only constraints we place on Δε are that ε be real and positive and that ∫|Δ|dxdy be finite, as discussed above. Now, we wish to show that there must always be a (cutoff-free) guided mode as long as Δε is “mostly positive,” in the sense that:

Δ(x,y)dxdy<0,
(4)

Since Equation (4) is independent of ω or β, the existence of guided modes will hold at all frequencies (cutoff-free).

The foundation for the proof is the existence of a variational (min–max) theorem that gives an upper bound for the lowest eigenfrequency ω min. In particular, at each β, the eigenmodes H(x,y)e iβz-iωt satisfy a Hermitian eigenproblem [5

5. J. D. Joannopoulos, S. G. Johnson, J. N. Winn, and R. D. Meade, Photonic Crystals: Molding the Flow of Light, 2nd ed. (Princeton Univ. Press, 2008).

]:

β×1εβ×H=Θ̂βH=ω2c2H,
(5)

where

β+ẑ,
(6)

with Equation (5) defining the linear operator Θ^ β. In addition to the eigenproblem, there is also the “transversality” constraint [5

5. J. D. Joannopoulos, S. G. Johnson, J. N. Winn, and R. D. Meade, Photonic Crystals: Molding the Flow of Light, 2nd ed. (Princeton Univ. Press, 2008).

, 24

24. P. Kuchment, “The Mathematics of Photonic Crystals,” in Mathematical Modeling in Optical Science, G. Bao, L. Cowsar, and W. Masters, eds., Frontiers in Applied Mathematics, pp. 207–272 (SIAM, Philadelphia, 2001).

]:

β·H=0
(7)

(the absence of static magnetic charges). From the Hermitian property of Θ^ β, the variational theorem immediately follows [5

5. J. D. Joannopoulos, S. G. Johnson, J. N. Winn, and R. D. Meade, Photonic Crystals: Molding the Flow of Light, 2nd ed. (Princeton Univ. Press, 2008).

]:

ωmin2(β)c2=infβ·H=0H*·Θ̂βHdxdyH*·Hdxdy
(8)

That is, an upper bound for the smallest eigenvalue is obtained by plugging any “trial function” H(x,y), not necessarily an eigenfunction, into the right-hand-side (the “Rayleigh quotient”), as long as H is “transverse” [satisfies Equation (7)]. [Technically, we must also restrict ourselves to trial functions where the integrals in Equation (8) are defined, i.e.the trial functions must be in the appropriate Sobolev space H(∇β×).] Conversely, if Equation (7) is not satisfied, it is easy to make the numerator of the right-hand-side (which involved ∇β×H) zero, e.g. by setting H=∇φ+iβφ for any φ(x,y), so transversality of the trial function is critically important to obtaining a true upper bound.

Now, we merely need to find a transverse trial function such that the variational upper bound is below the light line of the cladding, which will guarantee a guided fundamental mode. For a homogeneous, isotropic cladding εc, the light line is simply ω 2 c/c 2=β 2/εc, and so the condition for guided modes becomes:

εcH*·Θ̂βHdxdyβ2H*·Hdxdy
=εc1εβ×H2dxdyβ2H2dxdy<0,
(9)

where in the second line we have integrated by parts. Although the basic idea of using the variational principle to estimate eigenvalues is well known, the challenge is to find an estimate that establishes localization even for arbitrarily small Δε>0 and/or for arbitrarily low frequencies.

The problem of bound states in quantum mechanics is conceptually very similar. There, given a potential function V(x,y) in two dimensions with ∫|V|<∞, one wishes to show that ∫V<0 (attractive) implies the existence of a bound state: an eigenfunction of the Schrödinger operator -∇2+V with eigenvalue (energy)<0. Again, this is a Hermitian eigenproblem and there is a variational theorem [31

31. C. Cohen-Tannoudji, B. Din, and F. Laloë, Quantum Mechanics (Hermann, Paris, 1977).

], so one merely needs to find some trial wavefunction ψ for which the Rayleigh quotient is negative in order to obtain a bound state. In one dimension, finding such a trial function is simple—for example, an exponentially decaying function e -α|x| (or a Gaussian eαx2 ) will work for sufficiently small α—and the proof is sometimes assigned as an undergraduate homework problem [32

32. D. ter Haar, Selected Problems in Quantum Mechanics (Academic Press, New York, 1964).

]. In two dimensions, however, finding a trial function is more difficult—in fact, no function of the form f(αr) (where r is the radius x2+y2 ) will work (without more knowledge of the explicit solution for V) [11

11. K. Yang and M. de Llano, “Simple variational proof that any two-dimensional potential well supports at least one bound state,” Am. J. Phys. 57, 85–86 (1989). [CrossRef]

]—and the earliest proofs of the existence of bound modes used more complicated, non-variational methods [27

27. B. Simon, “The bound state of weakly coupled Schrödinger operators in one and two dimensions,” Ann. Phys. 97, 279–288 (1976). [CrossRef]

, 30

30. E. N. Economou, Green’s functions in quantum physics (Springer, 2006).

]. However, an appropriate trial function for a variational proof was eventually discovered [9

9. A. Bamberger and A. S. Bonnet, “Mathematical analysis of the guided modes of an optical fiber,” SIAM J. Math. Anal. 21, 1487–1510 (1990). [CrossRef]

, 29

29. H. Picq, “Détermination et calcul numérique de la première valeur propre d’opérateurs de Schrödinger dans le plan,” Ph.D. thesis, Université de Nice, Nice, France (1982).

], and a simpler trial function e(r+1)α was later proposed independently by Yang and de Llano [11

11. K. Yang and M. de Llano, “Simple variational proof that any two-dimensional potential well supports at least one bound state,” Am. J. Phys. 57, 85–86 (1989). [CrossRef]

].

In the present electromagnetic case, we found that the following trial function, inspired by the quantum case above [11

11. K. Yang and M. de Llano, “Simple variational proof that any two-dimensional potential well supports at least one bound state,” Am. J. Phys. 57, 85–86 (1989). [CrossRef]

], works. That is, we can prove the existence of waveguided modes for a homogeneous cladding using the trial function, in cylindrical (r,ϕ) coordinates:

H=r̂γcosϕϕ̂(rγ)sinϕ,
(10)

where

γ=γ(r)=e1(r2+1)α
(11)

for some α>0, and ()′ is the derivative with respect to r. Clearly, H in eq. (10) reduces to an -polarized plane wave propagating in the direction as α → 0 (and hence γ → 1). This is a key property of the trial function: in the limit of no localization (α=0, Δε=0) it should recover a fundamental (lowest-ω) solution of the infinite cladding. Also, by construction, it satisfies the transversality condition (7) (which is why we chose this particular form). We chose γ slightly differently from Ref. 11

11. K. Yang and M. de Llano, “Simple variational proof that any two-dimensional potential well supports at least one bound state,” Am. J. Phys. 57, 85–86 (1989). [CrossRef]

for convenience only (to make sure it is differentiable at the origin and goes to 1 for α → 0). For future reference, the first two r derivatives of γ are:

γ=2αr(r2+1)α1γ,
(12)
γ=2α(r2+1)α1γ[1+2αr2(r2+1)α1+2(1α)r2(r2+1)1],
(13)

and are plotted along with γ in Fig. 3.

What remains is, in principle, merely a matter of algebra to verify that this trial function, for sufficiently small α, satisfies the variational condition (9). In practice, some care is required in appropriately bounding each of the integrals and in taking the limits in the proper order, and we review this process below.

Fig. 3. Plot of γ [eq. (11)], γ′/α [eq. (12)], and γ″/α [eq. (13)] versus r for α=0.1. All three functions go to zero for r → ∞, with no extrema other than those shown.

We substitute the trial function (10) for H into the left-hand side of eq. (9):

εc1ε(r)(+iβẑ)×H(r,ϕ)2d2rβ2H2d2r
=εc(1εc+Δ(r))ẑ1r[r(rHϕ)Hrϕ]+iβẑ×H2d2rβ2H2d2r
=εc(1εc+Δ(r))(sin2ϕr2{[r(rγ)]γ}2+β2H2)d2rβ2H2d2r
=εc(1εc+Δ(r))sin2ϕr2(3rγ+r2γ)2d2rεcβ2Δ(r)H2d2r
(14)

We proceed to show that the last line of the above expression is negative in the limit α → 0, thus satisfying the condition for the existence of bound modes. We first examine the second term of eq. (14):

limα0β2Δ(r)H2d2r=β2Δ(r)d2r<0.
(15)

The key fact here is that we are able to interchange the α → 0 limit and the integral in this case, thanks to Lebesgue’s Dominated Convergence Theorem (LDCT) [33

33. E. Hewitt and K. Stromberg, Real and Abstract Analysis (Springer, 1965).

]: whenever the absolute value of the integrand is bounded above (for sufficiently small α) by an α-independent function with a finite integral, LDCT guarantees that the α → 0 limit can be interchanged with the integral. In particular, the absolute value of this integrand is bounded above by |Δ| multiplied by some constant (since |H| is bounded by a constant: |γ|≤1 and |′| is also easily seen to be bounded above for sufficiently small α), and |Δ| has a finite integral by assumption. Since limα → 0|H|2=1, we obtain eq. (4), which is negative by assumption.

Now we must show that the remaining first term of eq. (14) goes to zero as α → 0, completing our proof. This term is proportional to (ε -1 c+Δ), but the Δ terms trivially go to zero by the same arguments as above: Δ allows the limit to be interchanged with the integration by LDCT, and as α → 0 the γ′ and γ″ terms go to zero. The remaining ε -1 c terms can be bounded above by a sequence of inequalities as follows:

limα0002πsin2ϕr2(3rγ+r2γ)2rdrdϕ
=16πlimα00α2r3(r2+1)2α2γ2[2+αr2(r2+1)α1+(1α)r2(r2+1)1]2dr
16πlimα00α2r(r2+1)2α1γ2[2+α(r2+1)α+(1α)]2dr
=16πlimα01α2t4α1e22t2α[(3α)+αt2α]2dt
8πlimα00αue22u[(3α)+αu]2du
=8πe2limα0α[38α2+12α(3α)+14(3α)2]=0.
(16)

From the first to second line, we substituted eqs. (12) and (13) and simplified. From the second to third line, we bounded the integrand above by flipping negative terms into positive ones and replacing r 2 with r 2+1. From the third to the fourth line, we made a change of variables t 2=r 2+1. Then, from the fourth to fifth line, we made another change of variable u=t , and bounded the integral above by changing the lower limit from u=1 to u=0. The final integral can be performed exactly and goes to zero, completing the proof.

4. General periodic claddings

  • transverse periodicity in the cladding material (photonic-crystal fibers),
  • a core and cladding that are periodic in z with period a (a → 0 for the z-invariant case),
  • anisotropic εc and Δε materials (ε is a 3×3 positive-definite Hermitian matrix).

In particular, we consider dielectric functions of the form:

ε(x,y,z)=εc(x,y,z)+Δε(x,y,z),
(17)

where the cladding dielectric tensor εc(x,y,z)=εc(x,y,z+a) is z-periodic and also periodic in the xy plane (with an arbitrary unit cell and lattice), and the core dielectric tensor change Δε(x,y,z)=Δε(x,y,z+a) is z-periodic with the same period a. Both εc and the total ε must be positive-definite Hermitian tensors. As defined in eq. (3), we denote by Δ the change in the inverse dielectric tensor. Similar to the isotropic case, we require that ∫|Δij| be finite for integration over the xy plane and one period of z, for every tensor component Δij. We also require that the components of ε -1 c be bounded above.

The proof is similar in spirit to that of the homogeneous-cladding case. At each β, the eigenmodes H(x,y,z)e iβz-iωt satisfy the same Hermitian eigenproblem (5) and transversality constraint (7) as before. We have a similar variational theorem to eq. (8) [5

5. J. D. Joannopoulos, S. G. Johnson, J. N. Winn, and R. D. Meade, Photonic Crystals: Molding the Flow of Light, 2nd ed. (Princeton Univ. Press, 2008).

], except that, in the case of z-periodicity, we now integrate over one period in z as well as over x and y.

ωmin2(β)c2=infβ·H=0H*·Θ̂βHH*·H.
(18)

As before, to prove the existence of a guided mode we will find a trial function H such that this upper bound, called the “Rayleigh quotient” for H, is below the light line ωc(β)2/c 2. The corresponding condition on H can be written [similar to eq. (9)]:

H*·Θ̂βHωc2(β)c2H*·H<0.
(19)

We considered a variety of trial functions, inspired by the Yang and de Llano quantum case [11

11. K. Yang and M. de Llano, “Simple variational proof that any two-dimensional potential well supports at least one bound state,” Am. J. Phys. 57, 85–86 (1989). [CrossRef]

], before finding the following choice that allows us to prove the condition (19). Similar to eq. (10), we want a slowly decaying function proportional to γ(r)=e1(r2+1)α , from eq. (11), that in the α → 0 (weak guidance) limit approaches the cladding fundamental mode H c. As before, the trial function must be transverse (∇β·H=0), which motivated us to write the trial function in terms of the corresponding vector potential. We denote by A c the vector potential corresponding to the cladding fundamental mode H c=∇β×A c. In terms of A c and γ, our trial function is then:

H=β×(γAc)=γHc+γ×Ac.
(20)

Substituting eq. (20) into the left hand side of our new guidance condition (19), we obtain five categories of terms to analyze:

  1. terms that contain Δ=ε -1-ε -1 c,
  2. terms that cancel due to the eigenequation (5),
  3. terms that have one first derivative of γ,
  4. terms that have (γ′)2,
  5. terms that have γγ″ or (γ″)2.

Category (i) will give us our condition for guided modes, eq. (1), while category (ii) will be cancelled exactly in eq. (19). We show in the appendix that all of the terms in category (iii) exactly cancel one another. The terms in categories (iv) and (v) all vanish in the α → 0 limit; we distinguish them because category (v) turns out to be easier to analyze. There are no terms with γ″ alone, as these can be integrated by parts to obtain category (iii) and (iv) terms. In the appendix, we provide an exhaustive listing of all the terms and how they combine as described above. In this section, we only outline the basic structure of this algebraic process, and explain why the category (iv) and (v) terms vanish as α → 0.

Category (i) consists only of one term:

limα0H*·(β×Δβ×H)
=Hc*·(β×Δβ×Hc)
=(β×Hc)*·Δ(β×Hc)
=ωc2c2Dc*·ΔDc
(21)

From the first to the second line, we interchanged the limit with the integration, thanks to the LDCT condition as in Sec. 3, since the magnitudes of all of the terms in the integrand are bounded above by the tensor components |Δij| multiplied by some α-independent constants, and |Δij| has a finite integral by assumption. (In particular, the A c fundamental mode and its curls are bounded functions, being Bloch-periodic, and γ and its first two derivatives are bounded for sufficiently small α.) The result is precisely the left-hand side of eq. (1), which is negative by assumption.

Next, we would like to cancel ωc2c2H*·H by the eigen-equation (5). Thus, we examine the term ∫H*·(∇β×ε-1 cγ∇β×H c) (which comes from the term where the right-most curl falls on H c rather than γ) below:

H*·(β×εc1γβ×Hc)
=H*·(γβ×εc1β×Hc+(γ)×εc1β×Hc)
=H*·γωc2c2Hc+H*·(γ×εc1β×Hc)
=H*·ωc2c2HH*·ωc2c2γ×Ac+H*·(γ×εc1β×Hc)
(22)

From the second to the third lines, we used the eigenequation (5), and from the third to the fourth lines we used the definition (20) of H in terms of H c. The first term of the last line above cancels ωc2c2H*·H in eq. (19). The second and third terms contain two category (iii) terms: ωc2c2γHc·(γ×Ac) and cγγ·E c×H*c, both of which will be exactly cancelled as described in the appendix, as well as some category (iv) and (v) terms.

The category (iv) integrands are all of the form (γ′)2 multiplied by some bounded function (a product of the various Bloch-periodic fields as well as the bounded ε -1 c). This integrand can then be bounded above by replacing the bounded function with the supremum B of its magnitude, at which point the integral is bounded above by 2πB 0(γ′)2 rdr. However, such integrands were among the terms we already analyzed in the homogeneous-cladding case, in eq. (16), and we explicitly showed that such integrals go to zero as α → 0.

In summary, we have shown that, if eq. (1) is satisfied, then the variational upper bound for our trial function [eq. (20)] is below the light line, and therefore an index-guided mode is guaranteed to exist. The special cases of this theorem, as discussed in the introduction, immediately follow.

5. Substrates, dispersive materials, and finite-size effects

In this section, we briefly discuss several situations that lie outside of the underlying assumptions of our theorem: waveguides sitting on substrates, dispersive (ω-dependent) materials, and finite-size claddings.

An optical fiber is completely surrounded by a single cladding material, but the situation is quite different in integrated optical waveguides. There, it is common to have an asymmetrical cladding, with air above the waveguide and a low-index material (e.g. oxide) below the waveguide, such as in strip or ridge waveguides [12–14

12. R. G. Hunsperger, Integrated Optics: Theory and Technology (Springer-Verlag, 1982).

]. In such cases, it is well known that the fundamental guided mode has a low-frequency cutoff even when the waveguide consists of strictly nonnegative Δε [12

12. R. G. Hunsperger, Integrated Optics: Theory and Technology (Springer-Verlag, 1982).

, 14

14. C.-L. Chen, Foundations for Guided-Wave Optics (Wiley, 2006). [CrossRef]

]. This does not contradict our theorem because we required the cladding to be periodic in both transverse directions, whereas a substrate is not periodic in the vertical direction.

We have also assumed non-dispersive materials in our proof. What happens when we have more realistic, dispersive materials? Suppose that ε depends on ω but has negligible absorption (so that guided modes are still well-defined). For a given ω, we can construct a frequency-independent ε structure matching the actual ε at that ω, and apply our theorem to determine whether there are guided modes at ω. The simplest case is when Δε≥0 for all ω, in which case we must still obtain cutoff-free guided modes. The theorem becomes more subtle to apply when Δε<0 in some regions, because not only must one perform the integral of eq. (1) to determine the existence of guided modes, but the condition (1) is for a fixed β while the integrand is for a given frequency, and the frequency of the guided mode is unknown a priori.

6. Concluding remarks

Also using a variational approach, Ref. 26

26. P. Kuchment and B. Ong, “On guided waves in photonic crystal waveguides,” in Waves in Periodic and Random Media, vol. 339 of Contemporary Mathematics, pp. 105–115 (AMS, Providence, RI, 2003).

proved a sufficient condition for the existence of bandgap-guided modes in two dimensions. However, the condition established in that work was fairly strong, requiring a minimum defect size to guarantee the existence of a guided mode, whereas numerical studies in one and two dimensions have suggested that no minimum defect size may be required [5

5. J. D. Joannopoulos, S. G. Johnson, J. N. Winn, and R. D. Meade, Photonic Crystals: Molding the Flow of Light, 2nd ed. (Princeton Univ. Press, 2008).

], similar to the index-guided case proved here. If there is, in fact, no minimum defect size for gap-guided modes in linear defects, it is possible that a trial function similar to the one here could be applied to an approach like that of Ref. 26

26. P. Kuchment and B. Ong, “On guided waves in photonic crystal waveguides,” in Waves in Periodic and Random Media, vol. 339 of Contemporary Mathematics, pp. 105–115 (AMS, Providence, RI, 2003).

.

Appendix: All Rayleigh-quotient terms

In this appendix, we provide an exhaustive listing of all the terms that appear when the trial function [eq. (20)] is substituted into eq. (19) (the condition to be satisfied, a rearrangement of the Rayleigh quotient bound). Since the terms that contain Δ [category (i)] were already fully analyzed in Sec. 4 (since for these terms the limits could be trivially interchanged), we consider only the remaining terms involving ε c(r). More specifically, the only non-trivial term to analyze is the Δ-free part of the left-most integral in eq. (19):

H*·(β×εc1β×H)
=H*·(β×εc1γβ×Hc)+H*·(β×εc1γ×Hc)
+H*·(β×εc1β×(γ×Ac)).
(23)

We have already seen, in eq. (22), that the first term breaks down into a term that cancels ωc2c2H*·H in eq. (19), via the eigen-equation, and two other terms. Removing the terms cancelled by the eigenequation, and substituting iωccEc for ε -1 cβ×H c (Ampère’s law), we have:

ωc2c2H*·(γ×Ac)+H*·[γ×(iωccEc)]
+H*·β×εc1[γ×Hc+β×(γ×Ac)]
=ωc2c2[γHc+γ×Ac]*·(γ×Ac)iωccγγ·(Ec×Hc*)
iωcc(γ×Ac)*·(γ×Ec)+(γβ×Hc)*·εc1[γ×Hc+β×(γ×Ac)]
+(γ×Hc)*·εc1[γ×Hc+β×(γ×Ac)]
+(β×γ×Ac)*·εc1[γ×Hc+β×(γ×Ac)]
=ωc2c2γHc*·(γ×Ac)ωc2c2γ×Ac2iωccγγ·(Ec×Hc*)
iωcc(γ×Ac)*·(γ×Ec)+iωccγEc*·(γ×Hc)
+iωccγ(β×Ec)*·(γ×Ac)+iωcc(γ×Ec)*·(γ×Ac)
+(γ×Hc)*·εc1(γ×Hc)
+((γ×Hc)*·εc1[β×(γ×Ac)]+c.c.)
+(β×(γ×Ac))*·εc1(β×(γ×Ac)).
(24)

Above, the first “=” step is obtained by substituting the trial function for H, integrating some of the ∇β× operators by parts, and distributing the derivatives of γ H c by the product rule. The second step is obtained by using Ampère’s law again, combined with integrations by parts and the product rule; “c.c.” stands for the complex conjugate of the preceding expression. Continuing, we obtain:

=ωc2c2γHc*·(γ×Ac)ωc2c2γ×Ac22iωccγγ·{Ec×Hc*}
+(iωcc(γ×Ac)·(γ×Ec)*+c.c.)+ωc2c2γHc*·(γ×Ac)
+(γ×Hc)*·εc1(γ×Hc)+((γ×Hc)*·εc1[β×(γ×Ac)]+c.c.)
+(β×(γ×Ac))*·εc1(β×(γ×Ac))
(25)

In obtaining this expression, we have grouped terms into complex-conjugate pairs and used Faraday’s law to replace ∇β×E c with iωccHc . At this point, we have two ωc2c2γHc*·(γ×Ac) terms that exactly cancel. All of the remaining terms, except for iωccγγ·{Ec×Hc*} , are multiples of two first or higher derivatives of γ, corresponding to category (iv) and (v) terms, which we proved to vanish in Sec. 4.

2iωccγγ·{Ec×Hc*}
=2iωc2cγ2·{Ec×Hc*}
=iωcc·(γ2{Ec×Hc*})+iωccγ2·({Ec×Hc*})
(26)

The first term of the last line is zero by the divergence theorem (transforming it into a surface integral at infinity), since γ → 0 at infinity. For the second term, the integrand is the divergence of the time-average Poynting vector ℜ{E c×H*c}, which equals the time-average rate of change of the energy density [34

34. J. D. Jackson, Classical Electrodynamics, 3rd ed. (Wiley, New York, 1998).

], which is identically zero for any lossless eigenmode (such as the cladding fundamental mode).

Acknowledgments

We are grateful to M. Ghebrebrhan and G. Staffilani at MIT for helpful discussions.

References and links

1.

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

2.

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

3.

A. Bjarklev, J. Broeng, and A. S. Bjarklev, Photonic Crystal Fibres (Springer, New York, 2003). [CrossRef]

4.

F. Zolla, G. Renversez, A. Nicolet, B. Kuhlmey, S. Guenneau, and D. Felbacq, Foundations of Photonic Crystal Fibres (Imperial College Press, London, 2005). [CrossRef]

5.

J. D. Joannopoulos, S. G. Johnson, J. N. Winn, and R. D. Meade, Photonic Crystals: Molding the Flow of Light, 2nd ed. (Princeton Univ. Press, 2008).

6.

R. Ramaswami and K. N. Sivarajan, Optical Networks: A Practical Perspective (Academic Press, London, 1998).

7.

C. Elachi, “Waves in active and passive periodic structures: A review,” Proc. IEEE 64, 1666–1698 (1976). [CrossRef]

8.

S. Fan, J. N. Winn, A. Devenyi, J. C. Chen, R. D. Meade, and J. D. Joannopoulos, “Guided and defect modes in periodic dielectric waveguides,” J. Opt. Soc. Am. B 12, 1267–1272 (1995). [CrossRef]

9.

A. Bamberger and A. S. Bonnet, “Mathematical analysis of the guided modes of an optical fiber,” SIAM J. Math. Anal. 21, 1487–1510 (1990). [CrossRef]

10.

H. P. Urbach, “Analysis of the domain integral operator for anisotropic dielectric waveguides,” Journal on Mathematical Analysis 27 (1996).

11.

K. Yang and M. de Llano, “Simple variational proof that any two-dimensional potential well supports at least one bound state,” Am. J. Phys. 57, 85–86 (1989). [CrossRef]

12.

R. G. Hunsperger, Integrated Optics: Theory and Technology (Springer-Verlag, 1982).

13.

B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics (Wiley, 1991). [CrossRef]

14.

C.-L. Chen, Foundations for Guided-Wave Optics (Wiley, 2006). [CrossRef]

15.

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, 1285–1290 (2002). [PubMed]

16.

S. Wilcox, L. Botten, C. M. de Sterke, B. Kuhlmey, R. McPhedran, D. Fussell, and S. Tomljenovic-Hanic, “Long wavelength behavior of the fundamental mode in microstructured optical fibers,” Opt. Express 13 (2005). [CrossRef] [PubMed]

17.

S. Kawakami and S. Nishida, “Characteristics of a doubly clad optical fiber with a low-index inner cladding,” IEEE J. Quantum Electron. 10, 879–887 (1974). [CrossRef]

18.

T. Okoshi and K. Oyamoda, “Single-polarization single-mode optical fibre with refractive-index pits on both sides of core,” Electron. Lett. 16, 712–713 (80). [CrossRef]

19.

W. Eickhoff, “Stress-induced single-polarization single-mode fiber,” Opt. Lett. 7 (1982). [CrossRef] [PubMed]

20.

J. R. Simpson, R. H. Stolen, F. M. Sears, W. Pleibel, J. B. Macchesney, and R. E. Howard, “A single-polarization fiber,” IEEE J. Lightwave Technol. 1, 370–374 (1983). [CrossRef]

21.

M. J. Messerly, J. R. Onstott, and R. C. Mikkelson, “A broad-band single polarization optical fiber,” IEEE J. Lightwave Technol. 9, 817–820 (1991). [CrossRef]

22.

H. Kubota, S. Kawanishi, S. Koyanagi, M. Tanaka, and S. Yamaguchi, “Absolutely single polarization photonic crystal fiber,” IEEE Photon. Tech. Lett. 16, 182–184 (2004). [CrossRef]

23.

M.-J. Li, X. Chen, D. A. Nolan, G. E. Berkey, J. Wang, W. A. Wood, and L. A. Zenteno, “High bandwidth single polarization fiber with elliptical central air hole,” IEEE J. Lightwave Technol. 23, 3454–3460 (2005). [CrossRef]

24.

P. Kuchment, “The Mathematics of Photonic Crystals,” in Mathematical Modeling in Optical Science, G. Bao, L. Cowsar, and W. Masters, eds., Frontiers in Applied Mathematics, pp. 207–272 (SIAM, Philadelphia, 2001).

25.

D. R. Smith, D. C. Vier, T. Koschny, and C. M. Soukoulis, “Electromagnetic parameter retrieval from inhomogeneous materials,” Phys. Rev. E 71, 036,617 (2005). [CrossRef]

26.

P. Kuchment and B. Ong, “On guided waves in photonic crystal waveguides,” in Waves in Periodic and Random Media, vol. 339 of Contemporary Mathematics, pp. 105–115 (AMS, Providence, RI, 2003).

27.

B. Simon, “The bound state of weakly coupled Schrödinger operators in one and two dimensions,” Ann. Phys. 97, 279–288 (1976). [CrossRef]

28.

L. D. Landau and E. M. Lifshitz, Quantum Mechanics (Addison-Wesley, 1977).

29.

H. Picq, “Détermination et calcul numérique de la première valeur propre d’opérateurs de Schrödinger dans le plan,” Ph.D. thesis, Université de Nice, Nice, France (1982).

30.

E. N. Economou, Green’s functions in quantum physics (Springer, 2006).

31.

C. Cohen-Tannoudji, B. Din, and F. Laloë, Quantum Mechanics (Hermann, Paris, 1977).

32.

D. ter Haar, Selected Problems in Quantum Mechanics (Academic Press, New York, 1964).

33.

E. Hewitt and K. Stromberg, Real and Abstract Analysis (Springer, 1965).

34.

J. D. Jackson, Classical Electrodynamics, 3rd ed. (Wiley, New York, 1998).

35.

J. A. Kong, Electromagnetic Wave Theory (Wiley, New York, 1975).

36.

S. G. Johnson, M. L. Povinelli, M. Soljačić, A. Karalis, S. Jacobs, and J. D. Joannopoulos, “Roughness losses and volume-current methods in photonic-crystal waveguides,” Appl. Phys. B 81, 283–293 (2005). [CrossRef]

37.

P. Yeh, A. Yariv, and E. Marom, “Theory of Bragg fiber,” J. Opt. Soc. Am. 68, 1196–1201 (1978). [CrossRef]

OCIS Codes
(060.2310) Fiber optics and optical communications : Fiber optics
(130.2790) Integrated optics : Guided waves
(060.4005) Fiber optics and optical communications : Microstructured fibers
(060.5295) Fiber optics and optical communications : Photonic crystal fibers

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: March 20, 2008
Revised Manuscript: April 17, 2008
Manuscript Accepted: April 18, 2008
Published: June 9, 2008

Citation
Karen K. Lee, Yehuda Avniel, and Steven G. Johnson, "Rigorous sufficient conditions for index-guided modes in microstructured dielectric waveguides," Opt. Express 16, 9261-9275 (2008)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-13-9261


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References

  1. A. W. Snyder and J. D. Love, Optical Waveguide Theory (Chapman and Hall, London, 1983).
  2. P. Russell, "Photonic crystal fibers," Science 299, 358-362 (2003). [CrossRef] [PubMed]
  3. A. Bjarklev, J. Broeng, and A. S. Bjarklev, Photonic Crystal Fibres (Springer, New York, 2003). [CrossRef]
  4. F. Zolla, G. Renversez, A. Nicolet, B. Kuhlmey, S. Guenneau, and D. Felbacq, Foundations of Photonic Crystal Fibres (Imperial College Press, London, 2005). [CrossRef]
  5. J. D. Joannopoulos, S. G. Johnson, J. N. Winn, and R. D. Meade, Photonic Crystals: Molding the Flow of Light, 2nd ed. (Princeton Univ. Press, 2008).
  6. R. Ramaswami and K. N. Sivarajan, Optical Networks: A Practical Perspective (Academic Press, London, 1998).
  7. C. Elachi, "Waves in active and passive periodic structures: A review," Proc. IEEE 64, 1666-1698 (1976). [CrossRef]
  8. S. Fan, J. N. Winn, A. Devenyi, J. C. Chen, R. D. Meade, and J. D. Joannopoulos, "Guided and defect modes in periodic dielectric waveguides," J. Opt. Soc. Am. B 12, 1267-1272 (1995). [CrossRef]
  9. A. Bamberger and A. S. Bonnet, "Mathematical analysis of the guided modes of an optical fiber," SIAM J. Math. Anal. 21, 1487-1510 (1990). [CrossRef]
  10. H. P. Urbach, "Analysis of the domain integral operator for anisotropic dielectric waveguides," J. Math. Anal. 27, (1996).
  11. K. Yang and M. de Llano, "Simple variational proof that any two-dimensional potential well supports at least one bound state," Am. J. Phys. 57, 85-86 (1989). [CrossRef]
  12. R. G. Hunsperger, Integrated Optics: Theory and Technology (Springer-Verlag, 1982).
  13. B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics (Wiley, 1991). [CrossRef]
  14. C.-L. Chen, Foundations for Guided-Wave Optics (Wiley, 2006). [CrossRef]
  15. 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, 1285-1290 (2002). [PubMed]
  16. S. Wilcox, L. Botten, C. M. de Sterke, B. Kuhlmey, R. McPhedran, D. Fussell, and S. Tomljenovic-Hanic, "Long wavelength behavior of the fundamental mode in microstructured optical fibers," Opt. Express 13 (2005). [CrossRef] [PubMed]
  17. S. Kawakami and S. Nishida, "Characteristics of a doubly clad optical fiber with a low-index inner cladding," IEEE J. Quantum Electron. 10, 879-887 (1974). [CrossRef]
  18. T. Okoshi and K. Oyamoda, "Single-polarization single-mode optical fibre with refractive-index pits on both sides of core," Electron. Lett.  16, 712-713 (80). [CrossRef]
  19. W. Eickhoff, "Stress-induced single-polarization single-mode fiber," Opt. Lett. 7 (1982). [CrossRef] [PubMed]
  20. J. R. Simpson, R. H. Stolen, F. M. Sears, W. Pleibel, J. B. Macchesney, and R. E. Howard, "A single-polarization fiber," IEEE J. Lightwave Technol. 1, 370-374 (1983). [CrossRef]
  21. M. J. Messerly, J. R. Onstott, and R. C. Mikkelson, "A broad-band single polarization optical fiber," IEEE J. Lightwave Technol. 9, 817-820 (1991). [CrossRef]
  22. H. Kubota, S. Kawanishi, S. Koyanagi, M. Tanaka, and S. Yamaguchi, "Absolutely single polarization photonic crystal fiber," IEEE Photon. Tech. Lett. 16, 182-184 (2004). [CrossRef]
  23. M.-J. Li, X. Chen, D. A. Nolan, G. E. Berkey, J. Wang,W. A. Wood, and L. A. Zenteno, "High bandwidth single polarization fiber with elliptical central air hole," IEEE J. Lightwave Technol. 23, 3454-3460 (2005). [CrossRef]
  24. P. Kuchment, "The Mathematics of Photonic Crystals," in Mathematical Modeling in Optical Science, G. Bao, L. Cowsar, and W. Masters, eds., Frontiers in Applied Mathematics, pp. 207-272 (SIAM, Philadelphia, 2001).
  25. D. R. Smith, D. C. Vier, T. Koschny, and C. M. Soukoulis, "Electromagnetic parameter retrieval from inhomogeneous materials," Phys. Rev. E 71, 036,617 (2005). [CrossRef]
  26. P. Kuchment and B. Ong, "On guided waves in photonic crystal waveguides," in Waves in Periodic and Random Media, vol. 339 of Contemporary Mathematics, pp. 105-115 (AMS, Providence, RI, 2003).
  27. B. Simon, "The bound state of weakly coupled Schrödinger operators in one and two dimensions," Ann. Phys. 97, 279-288 (1976). [CrossRef]
  28. L. D. Landau and E. M. Lifshitz, Quantum Mechanics (Addison-Wesley, 1977).
  29. H. Picq, "Détermination et calcul numérique de la premiére valeur propre d???opérateurs de Schrödinger dans le plan," Ph.D. thesis, Université de Nice, Nice, France (1982).
  30. E. N. Economou, Green???s functions in quantum physics (Springer, 2006).
  31. C. Cohen-Tannoudji, B. Din, and F. Lalo¨e, Quantum Mechanics (Hermann, Paris, 1977).
  32. D. ter Haar, Selected Problems in Quantum Mechanics (Academic Press, New York, 1964).
  33. E. Hewitt and K. Stromberg, Real and Abstract Analysis (Springer, 1965).
  34. J. D. Jackson, Classical Electrodynamics, 3rd ed. (Wiley, New York, 1998).
  35. J. A. Kong, Electromagnetic Wave Theory (Wiley, New York, 1975).
  36. S. G. Johnson, M. L. Povinelli, M. Solja??i??, A. Karalis, S. Jacobs, and J. D. Joannopoulos, "Roughness losses and volume-current methods in photonic-crystal waveguides," Appl. Phys. B 81, 283-293 (2005). [CrossRef]
  37. P. Yeh, A. Yariv, and E. Marom, "Theory of Bragg fiber," J. Opt. Soc. Am. 68, 1196-1201 (1978). [CrossRef]

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