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Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 3 — Feb. 2, 2009
  • pp: 1583–1599
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Numerically-assisted coupled-mode theory for silicon waveguide couplers and arrayed waveguides

Michael L. Cooper and Shayan Mookherjea  »View Author Affiliations


Optics Express, Vol. 17, Issue 3, pp. 1583-1599 (2009)
http://dx.doi.org/10.1364/OE.17.001583


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Abstract

We investigate coupled-mode theory in designing high index contrast silicon-on-insulator waveguide couplers and arrayed waveguides. We develop and demonstrate a method of solution to the inverse problem of reconstructing the coupling matrix from the modal profiles obtained, in this case, from finite-difference frequency-domain field calculations. We show that whereas supermode theory provides a good approximation of the mode profiles, next-to-nearest-neighbor coupling becomes significant at small separation distances between arrayed waveguides. These distances are quantified for three different silicon-on-insulator material platforms. We also point out the phenomenon of field skewing and deformation at small separations.

© 2009 Optical Society of America

1. Introduction

Silicon-on-insulator (SOI) waveguides and optical on-chip circuitry rely on the high refractive index contrast between core (silicon, n = 3.5) and cladding (silicon dioxide, n = 1.45) materials to guide light in very compact structures and with small bending radii [1

1. F. Xia, L. Sekaric, and Y. Vlasov, “Ultracompact optical buffers on a silicon chip,” Nat. Photonics 1, 65–71 (2007). [CrossRef]

, 2

2. Q. Xu, D. Fattal, and R. G. Beausoleil, “Silicon microring resonators with 1.5-μm radius,” Opt. Express 16, 4309–4315 (2008). [CrossRef] [PubMed]

]. SOI photonics is one of the most active areas of ongoing research and large-scale integrated circuits are being designed and fabricated. In many of these proposed circuits, one of the most critical waveguide components is the directional coupler between two parallel waveguides, which is used in microring-based filters [3

3. F. Xia, M. Rooks, L. Sekaric, and Y. Vlasov, “Ultra-compact high order ring resonator filters using submicron silicon photonic wires for on-chip optical interconnects,” Opt. Express 15, 11934–11941 (2007). [CrossRef] [PubMed]

], Mach-Zehnder interferometers and modulators [4

4. A. S. Liu, R. Jones, L. Liao, D. Samara-Rubio, D. Rubin, O. Cohen, R. Nicolaescu, and M. Paniccia, “A highspeed silicon optical modulator based on a metal oxide-semiconductor capacitor,” Nature 427, 615–618 (2004). [CrossRef] [PubMed]

, 5

5. Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometer-scale silicon electro-optic modulator,” Nature 435, 325–327 (2005). [CrossRef] [PubMed]

, 6

6. W. M. J. Green, M. J. Rooks, L. Sekaric, and Y. A. Vlasov, “Optical modulation using anti-crossing between paired amplitude and phase resonators,” Opt. Express 15, 17264–17272 (2007). [CrossRef] [PubMed]

], arrayed waveguide structures [7

7. X. Liu, I. Hsieh, X. Chen, M. Takekoshi, J. I. Dadap, N. C. Panoiu, R. M. Osgood, W. M. Green, F. Xia, and Y. A. Vlasov, “Design and fabrication of an ultra-compact silicon on insulator demultiplexer based on arrayed waveguide gratings,” in Proceedings of the Conference on Lasers and Electro-Optics (CLEO, 2008), paper CTuNN1.

, 8

8. P. Cheben, J. H. Schmid, A. Delage, A. Densmore, S. Jannz, B. Lamontagne, J. Lapointe, E. Post, P. Waldron, and D. C. Xu, “A high-resolution silicon-on-insulator arrayed waveguide grating microspectrometer with sub-micrometer aperture waveguides,” Opt. Express 15, 2299–2306 (2007). [CrossRef] [PubMed]

, 9

9. K. Sasaki, F. Ohno, A. Motegi, and T. Baba,“Arrayed waveguide grating of 70×60 μm2 size based on Si photonic wire waveguides,” Elec. Lett. 41, (2007).

, 10

10. P. Dumon, W. Bogaerts, D. V. Thourhout, D. Taillaert, and R. Baets, “Compact wavelength router based on a Silicon-on-insulator arrayed waveguide grating pigtailed to a fiber array,” Opt. Express 14, 664–669 (2006). [CrossRef] [PubMed]

] etc.

Here, we investigate and quantify the limitations of coupled-mode theory (CMT) in designing high index contrast SOI couplers and arrayed waveguides. CMT is a simple and often reliable approach to the design of such structures, in which the coupling coefficient can be written down in terms of overlap integrals of the individual waveguide modes and the refractive index distribution n(x,y) in the cross-sectional plane [11

11. H. Kogelnik and C. V. Shank, “Coupled-mode theory of distributed feedback lasers,” Appl. Phys. 43, 2327–2335 (1972). [CrossRef]

, 12

12. A. Hardy and W. Streifer, “Coupled-mode theory of parallel waveguides,” J. Lightwave Technol. LT-3, 1135–1146 (1985). [CrossRef]

, 13

13. W. P. Huang, “Coupled-mode theory for optical waveguides: An overview,” J. Opt. Soc. Am. A 11, 963–983 (1994). [CrossRef]

]. More accurate corrections to CMT for slab waveguides have been investigated by Chiang [14

14. K. S. Chiang, “Coupled-zigzag-wave theory for guided waves in slab waveguide arrays,” J. Lightwave Technol. 10, 1380–1387 (1992). [CrossRef]

], and Payne [15

15. F. P. Payne, “An analytical model for the coupling between the array waveguides in AWGs and star couplers,” Opt. Quantum Electron. 38, 237–248 (2006). [CrossRef]

], among others (see references therein).

In this paper, we compare “exact” results of the coupling coefficients of directional couplers calculated using a fully-vectorial finite difference frequency-domain (FDFD) mode-solving computer program with predictions of CMT. We demonstrate how to solve the inverse problem of reconstructing the coupling matrix from the solutions of the FDFD program. The significance of these results lies, firstly, in the resulting simplification of the design process of high-index contrast SOI waveguide directional couplers and coupled-waveguide structures, providing graphs and rule-of-thumb estimates for the validity and invalidity of simple CMT instead of having to rely on time-consuming ab initio numerical simulations in every case. Secondly, our method of solution of the inverse problem can be easily applied to design couplers from a mode solver rather than a time-consuming propagation simulation.

2. Coupled-mode theory (CMT) of the modes of multi-slot waveguides

2.1. Finite-difference frequency-domain (FDFD) algorithm

In the finite-difference frequency-domain (FDFD) algorithm, the dielectric profile of the waveguide’s cross section is discretized on a rectangular grid. As developed by C.L. Xu et al. [20

20. C. L. Xu, W. P. Huang, M.S. Stern, and S. K. Chaudhuri, “Full-vectorial mode calculations by finite difference method,” IEE Proc.-Optoelectron. 141, 281–286 (1994). [CrossRef]

], the vectorial wave equation,

2E+(n2k2β2)E=[·E1n2·(n2E)]
(1)

where E (x,y) is the transverse electric field vector, is written in matrix form as

(PxxPxyPyxPyy)(ExEy)=β2(ExEy)
(2)

and,

PxxEx=x[1n2(n2Ex)x]+2Exy2+n2k2Ex,PxyEy=x[1n2(n2Ey)y]+2Eyxy,
PyxEx=y[1n2(n2Ex)x]+2Exyx,PyyEy=y[1n2(n2Ex)y]+2Eyx2+n2k2Ey.

In this formulation the displacement vectors n 2 Ex and n 2 Ey are both continuous across any dielectric discontinuity, and with the graded-index approximation, the central difference equations can be applied directly without any special treatment at the boundaries. Fully discretized versions of these operators can be found in Ref. [21

21. W. P. Huang and C. L. Xu, “Simulation of three-dimensional optical waveguides by a full-vector beam propagation method,” IEEE J. Quantum Electron. 29, 2639–2649 (1993). [CrossRef]

]. We have written a set of routines in MAT-LAB to calculate the modes and their effective indices for arbitrarily-shaped index profiles.

Fig. 1. (a) Refractive index profile in the transverse plane n(x,y) for an N arrayed-waveguide structure. (b) The same refractive index profile may be decomposed, mathematically, into the sum of parts, Δn2 i = n2 i -n2 s , each of which appears in integrals equation for the coupling coefficients. For the structures considered in this paper, h = 500 nm, w = 200 nm, and s varies over the range 50 nm to 1μm. For these waveguide widths and heights (similar to those in Ref. [18]), the polarization direction of the principal transverse component of the electric field is indicated for the (quasi) TE and TM modes.

2.2. Coupled mode theory and its predictions

To describe the modes of multislot waveguides, we begin with the wave equation [19

19. P. Yeh, Optical Waves in Layered Media (John Wiley & Sons, New York, 2005).

],

2E+ω2c2n2(x,y)E=0,
(3)

and consider each polarization in turn. (TE and TM polarization are defined in terms of as the major component of the electric field, which for the structure in question, are polarized vertically and horizontally as shown in Fig. 1.)

2.2.1. TE Polarization

Consider an array of N single mode waveguides, whose refractive index profile is shown schematically in Fig. 1a. As long as the waveguides are not too close to each other (i.e., greater than a separation distance which we will investigate and quantify in a subsequent section), the transverse mode profile of the multislot waveguide structure can be approximated by an expansion of the individual high index waveguide modes

Ê=ŷE(x,y)eiβz=ŷ[Σl=1NAl𝓔1(x,y)]eiβz.
(4)

As shown by Fig. 1, the relative dielectric coefficient distribution of the entire N waveguide structure n 2 (x,y) can be written as a sum of individual waveguide contributions, so that

n2(x,y)=ns2(x,y)+Σl=1NΔnl2(x,y)
(5)

where n 2 s (x,y) corresponds to the cladding. Thus, n 2 s (x,y) + Δn 2 l (x,y) would yield the dielectric coefficient profile of the l th waveguide in the absence of the others. Substituting the above two equations into the wave equation, we have,

(2+ω2c2[ns2(x,y)+Σl=1NΔnl2(x,y)]β2)[Σl=1NAl𝓔1(x,y)]=0.
(6)

The modes of the individual waveguides satisfy their respective eigenvalue equations,

(2+ω2c2[ns2(x,y)+Δnl2(x,y)]βl2)𝓔1(x,y)=0
(7)

and therefore, using Eq. (7), Eq. (6) can be written as,

Σl=1NA1(Δl+ω2c2Σm=1mlNΔnm2(x,y))𝓔1(x,y)=0
(8)

where,

Δ1βl2β2.

N equations are formed by multiplying Eq. (8) by 𝓔j * (j = 1,2,…,N), and integrating each of these equations over x and y,

Σl=1NA1(Δl∫∫𝓔j*𝓔1dxdy+ω2c2Σm=1mlN∫∫𝓔j*Δnm2(x,y)𝓔1dxdy)=0,j=1,2,,N.
(9)

Ijl=∫∫𝓔j*𝓔1dxdy,
(10a)
κjl=ω2c2Σm=1mlN∫∫𝓔j*Δnm2(x,y)𝓔1dxdy,
(10b)

with the normalization

∫∫𝓔j*𝓔1dxdy=1.
(10c)

Ijl is the overlap integral of the modes of two waveguides which are not orthogonal to each other (particularly in the case of small waveguide separation), and κ are the self-coupling and cross-coupling (exchange coupling) coefficients familiar from coupled-mode theory [19, p. 362].

Equation (9) can then be written in matrix form as an eigenvalue problem,

(β12+κ11Δ2I12+κ12ΔN1I1,N1+κ1,N1ΔNI1N+κ1NΔ1I21+κ21β22+κ22ΔN1I2,N1+κ2,N1ΔNI2N+κ2NΔ1I31+κ31Δ2I32+κ32ΔN1I3,N1+κ3,N1ΔNI3N+κ3NΔ2IN1+κN1Δ2IN2+κN2ΔN1I1,N1+κN,N1βN2+κNN)(A1A2A3AN)
=(β20000β20000β20000β2)(A1A2A3AN).
(11)

(The matrix on the left-hand side of the above equation will be referred to as M.)

Fig. 2. TE Polarization Ey: The modes of an N = 5 coupled waveguide array for λ = 1550 nm, calculated using coupled-mode theory (blue solid lines), and a finite-difference frequency-domain algorithm (black crosses). The coupled-mode theory calculations were done by using the effective index method, calculating the overlap integrals, solving Eq. (11), and reassembling the field. Waveguide height = 500 nm, width = 200 nm, separation = 200 nm, ncore = 3.47, and nclad =1.46. Under nearest neighbor coupling, the scaling relationship predicted by Eq. (13) adequately predicts the field amplitudes within each waveguide.

If we assume that only nearest neighbor coupling is significant, then the integrals in Eq. (10) are nonzero only when l = j -1, j, j +1. M takes the tridiagonal form,

M=(β12+κ11Δ2I12+κ1200Δ2I21+κ21β22+κ22000Δ2I32+κ32000ΔN1IN,N1+κN,N1βN2+κNN).
(12)

We define κ selfκ 22, κ 33,…, κ N-1N-1, κ self,edgeκ 11 and κNN, and δκ self = κ self - κ self,edge.

Fig. 3. TM Polarization Ex: The modes of an N = 5 coupled waveguide array, calculated using coupled-mode theory (blue solid lines), and a finite-difference frequency-domain algorithm (black crosses). The coupled-mode theory calculations were done by using the effective index method, calculating the overlap integrals, solving Eq. (11), and reassembling the field. Waveguide height = 500 nm, width = 200 nm, separation = 1μm, ncore = 3.47, and nclad =1.46. Under nearest neighbor coupling, the scaling relationship predicted by Eq. (13) adequately predicts the field amplitudes within each waveguide.

To first order in the perturbation δκ self, the eigenvectors are

Al(m)=(2N+1)1/2sinlmπN+1δκself(2N+1)3/2
×Σm=1nmN1β2(m)β2(m)[sinN+1sinN+1+sinmNπN+1sinnNπN+1]sinlnπN+1
(13)

For large N, the second term in the above expression, is smaller than the first by (N + 1)-1 and can be ignored, yielding a simpler expression. The progression of peak-amplitude values (in the high index regions) {A (m) l},l = 1,…,N matches with the numerical calculations shown in Fig. 2. However, we shall see that the agreement is good only at large separation distances between the individual waveguides.

2.2.2. TM Polarization

For the TM polarization (in which the electric field is normal to the waveguide/slot boundary), we again start with the wave equation only now defined in terms of the magnetic field, which we expand in terms of the individual waveguide modes,

H=ŷH(x,y)eiβz=ŷ[Σl=1NAlH1(x,y)]eiβz.
(14)

Ex=βε(x,y)ωHy,
(15)

3. Numerically-assisted CMT: The “Inverse Problem”

To develop NA-CMT, we use the following mathematical procedure, based on a matrix theorem previously developed for coupled-resonator structures [23

23. S. Mookherjea, “Spectral characteristics of coupled resonators,” J. Opt. Soc. Am. B 23, 1137–1145 (2006). [CrossRef]

].

  1. First, we solve for the supermodes using FDFD, which does not contain any of the limitations of nearest-neighbor CMT under investigation. The propagation constants of the supermodes are also obtained by this algorithm.
  2. Having obtained both the eigenvectors (peak amplitudes) and eigenvalues (propagation constants), we construct the (non-singular) matrix of eigenvectors Amn (whose columns are the linearly-independent supermodes), and the diagonal matrix of eigenvalues, Λ = diag{β 2 m}.
  3. Next, we reassemble M [see Eq. (11)] by using the matrix theorem cited in Ref. [23

    23. S. Mookherjea, “Spectral characteristics of coupled resonators,” J. Opt. Soc. Am. B 23, 1137–1145 (2006). [CrossRef]

    , Eq. (7), Lem. 1–2]: if the eigenvalues are distinct (which they are in this case), M can be reconstructed as follows: M = AΛA -1. The matrix is unique to within a similarity transformation, which does not affect the following step. An example is shown in Table 1. (Notice that κ 11 and κ 55 are approximately one-half of κ 22, κ 33, or κ 44, as discussed earlier.) The values of the reconstructed M matrix may be useful to design couplers in the strongly-coupled regime from the output of the FDFD mode-solver algorithm itself, without having to carry out time-consuming beam-propagation simulations.

4. Asymptotic accuracy of numerically-assisted CMT

In this paper, the eigenvalues (λk) and eigenvectors (A (k)) are obtained from a computer simulation. However, they may be obtained from measurements on fabricated structures, in order to test whether the intended coupling matrix was successfully obtained in practice. The experimental procedure to measure eigenvalues and eigenvectors could be similar to that used to image the modes of laser resonators.

First, we will carry out a simple theoretical estimation. We will assume that the coupling matrix is Hermitian. After some algebra, the error in any element of M can be written to first order as

ΔMij=Σk=1N[(uik+ujk)Δukλk+uikujkΔλk].
(16)

For simplicity, in this paper, we assume that Δuk is zero, i.e., the errors are only in the measured eigenvalues, since for identical arrayed waveguide structures, successive eigenvectors look quite different from each other and are easily distinguished [23

23. S. Mookherjea, “Spectral characteristics of coupled resonators,” J. Opt. Soc. Am. B 23, 1137–1145 (2006). [CrossRef]

]. We will also use the simpler form of the eigenvectors, retaining only the first term of Eq. (13), so that the errors in the reconstructed nearest-neighbor coupling and next-to-nearest-neighbor coupling coefficients are

ΔMjj+1=Σk=1N2N+1sinjkπN+1sin(j+1)N+1Δλk,
ΔMj1j+1=Σk=1N2N+1sin(j1)N+1sin(j+1)N+1Δλk.
(17)

Assuming that Δλk are uncorrelated identically-distributed random variables with mean Eλ] and variance Var[Δλ], the mean and variance of the nearest-neighbor coupling and next-to-nearest-neighbor coupling coefficients can be calculated. Both ΔM jj+1 and ΔM j-1j+1 are zero-mean, since, for example,

E[ΔλMjj+1]=E[Δλ]Σk=1N2N+1sinjkπN+1sin(j+1)N+1=0
(18)

because the summation vanishes as a consequence of the orthogonality of the eigenvectors (the sum is equal to u (j).u (j+1) = 0).

To calculate the variance, we see that

Var[ΔλMjj+1]=Var[Δλ](1N+1)2Σk=1N(cos2N+1cos2jkπN+1)2
=Var[Δλ](1N+1),
(19)

and the same result is obtained for Var[ΔM j-1j+1].

Summarizing the results,

Nearestneighbor:E[ΔλMjj+1]=0,Var[ΔλMjj+1]=Var[Δλ]/(N+1),
Nexttonearest:E[ΔλMj1j+1]=0,Var[ΔλMj1j+1]=Var[Δλ]/(N+1).
(20)

Numerical calculations, shown in Fig. 4 confirm Eq. (;20). (Numerical calculations show that the same relations are seen to hold in the case of non-identical waveguides, in which case the off-diagonal terms of the coupling matrix are not identical along the sub-diagonals, and also for slightly asymmetric matrices.)

Fig. 4. Error versus N: Exact eigenvalues of a tridiagonal symmetric matrix of size N were perturbed by values chosen from a uniform random distribution with variance chosen to be ten percent of the first eigenvalue. The variance and mean of the reconstructed nearest-neighbor coupling and next-to-nearest-neighbor coupling coefficients are plotted, calculated from a distribution of coupling matrices generated by 105 iterations, showing that Eq. (20) is a good predictor of the reconstruction accuracy.

These results show that the error in reconstructing the coupling coefficients decreases, rather than increases, as the number of inaccurately-measured eigenvalues increases. This results from (spectral) averaging: each reconstructed coupling coefficient averages over the entire spectrum of eigenvalues, and therefore, benefits from the law of averages. In contrast, directly measuring a coupling coefficient e.g., by a local near-field probe of the field in the coupling region, does not benefit from any ensemble averaging.

5. Discussion

5.1. Next-to-nearest-neighbor coupling

As Table 1 shows (calculated at one specific value of the waveguide separation distance), M contains useful information about non nearest-neighbor coupling. We can read off whichever coupling coefficients are needed: in particular, we calculate the ratio ∣κ 13/κ 12∣, i.e., the ratio of next-to-nearest-neighbor coupling coefficient to the nearest-neighbor coupling coefficient.

Table 1. An example of a reconstructed coupling (M) matrix from FDFD calculations of eigenmodes and eigenvalues. Si/SiO2, TE polarization, separation s = 350 nm, β 0 = 2.26128 (2π/λ). Although the nearest-neighbor coupling coefficients dominate, the self-coupling and off-tridiagonal coupling terms are non-zero.

table-icon
View This Table

First we estimate the expected dependency of this ratio of coupling coefficients to the edge-to-edge separation, s. Using Kuznetsov’s solution for the coupling coefficients of two slab waveguides [22

22. M. Kuznetsov, “Expressions for the coupling coefficient of a rectangular waveguide directional coupler,” Opt. Lett. 8, 499–501 (1983). [CrossRef] [PubMed]

], we observe that κ in both the TE and TM cases varies with s as κ ~ e -ps where p is the field decay length in the cladding. Therefore, the ratio κ 13/κ 12 for both polarizations has the following expression (to leading order),

Δ3I13+κ13Δ2I12+κ13κ13κ12=ep(2s+w)eps=ep(s+w)
(21)

i.e., the ratio of next-to-nearest-neighbor coupling coefficient to the nearest-neighbor coupling coefficient should fall off exponentially with increasing separation.

5.2. Eigenvalue fanout: effective index of the supermodes versus separation distance

Another way to evaluate the predictions of CMT is the theory behind the eigenvalues of Eq. (11), which predicts that the effective index of the m-th supermode is given by the equation

β2(m)=β02+κself+2(κ+Δ0I)cosN+12δκselfN+1(sin2N+1+sin2NmπN+1),
(22)

where β 0 is the propagation constant of a single waveguide in isolation. Note that for N = 5, the m = 3 supermode has the special property that the right-hand-side of the above expression , i.e., the index of that supermode does not change with the coupling coefficient κ. Hence, n (3) eff is only weakly dependent on the separation distance (through the self-coupling coefficients, κ 11,κ 22,…,κ 55).

To verify this prediction, Fig. 6 shows the effective index calculated by FDFD for each of the five supermodes at various separation distances in three different silicon-based material systems. These values of the effective index take into account coupling-induced frequency shifts (CIFS, [25

25. M. Popovic, C. Manolatou, and M. Watts, “Coupling-induced resonance frequency shifts in coupled dielectric multi-cavity filters,” Opt. Express 14, 1208–1222 (2006). [CrossRef] [PubMed]

]) because M itself results from a numerical calculation of the supermodes (and their eigenfrequencies), rather than individual waveguide modes and the propagation constants of isolated waveguides.

In the limit of large separation, the effective indices of all the supermodes tends to that of the single waveguide. As the separation distance is decreased, the coupling coefficients increase, and the effective indexes of the different modes separate [24

24. G. Lenz and J. Salzman, “Eigenmodes of multiwaveguide structures,” J. Lightwave Technol. 8, 1803–1809 (1990). [CrossRef]

]. The first three modes remain guided even as s shrinks to zero, since their effective indices are higher than that of the single waveguide. From Fig. 6, one can read off the waveguide separation distance at which conventional CMT is expected to fail, and more accurate design tools, such as FDFD calculations, should be used to accurately predict the coupling coefficients.

Fig. 5. Ratio of coupling coefficients for different separation distances extracted from Eq. (11), which was reconstructed using an algorithm described in the text. (a) TE Polarization An exponential fit expected from a simple nearest-neighbor-coupling theory holds throughout this regime. (b) TM Polarization At a separations less than 450 nm, the ratio deviates significantly from the predicted behavior. (c) TE Polarization The ratio of cross coupling coefficients show that the reconstructed coupling matrix M becomes asymmetric as the waveguide separation is reduced. (d) TM Polarization The asymmetry of the coupling matrix begins at a larger separation.

An interesting observation obtains from the m = 3 supermode: at a certain (small) waveguide separation, n (3) eff is no longer independent of s and begins to deviate substantially from a straight line, contrary to the prediction of Eq. (22). This deviation is much more pronounced in the case of the TM polarization.

Fig. 6. Left column: TE Polarization, and Right column: TM polarization. Effective index of the five supermodes for different separation distances with ncore = 3.47, and (a,b) nclad =1.46, (c,d) nclad = 1 (e,f) nclad = 2.05. For each case as the separation between the waveguides increases, the effective indexes of the modes converge to that of the single waveguide. These values are (a) neff = 2.36 and (b) neff = 1.66 for oxide cladding, (c) neff = 2.24 and (d) neff = 1.07 for air cladding, (e) neff = 2.56 and (f) neff = 2.26 for nitride cladding. The shaded regions indicate > 5% deviation of neff for the m=3 supermode from its theoretical value, which as discussed in the text, is predicted by CMT to be independent of the separation distance.

5.3. Field skewing and reshaping

At short separation distances, the reconstructed matrix M can become non-symmetric (non-Hermitian), although the eigenvalues remain stricly real as long as the mode is above cut-off. This can be seen in fact in the matrix written in Table 1 and Fig. 5(c,d): κ 12κ 21 and κ 13κ 31, etc.

Fig. 7. TM Polarization Ex: The field profile of the fifth eigenmode in the first waveguide. When the separation is decreased below 450 nm, the peak of the field in the high-index rib indicated by the dotted red line in (a) is no longer centered, and the mode shape is considerably altered, thereby changing both κ and n eff. Consequently, CMT can no longer accurately predict the mode coupling.

At a short separation distance of 80 nm, Fig. 8 shows the coupled-mode theory used to reconstruct the m = 1 and m = 5 supermode (plotted with continuous lines) and the supermode calculation of FDFD (with crosses). Note that in both cases the field is asymmetrically centered within the dielectric boundaries of the outer waveguides. Recall that CMT is based on writing the field as a summation of the scaled individual waveguide modes, Fig. 8(a,d), each of which is centered within its own core-cladding boundaries. At short separation distances, when, for example, there is a significant contribution of the (asymmetric) tail from the field in the second waveguide to the (symmetric) mode of the first waveguide, CMT itself predicts a lateral shift of the peak (of the sum) away from the exact center of the waveguide. The scaling relationships from Eq. (13) will enhance this effect for an multi-waveguide arrayed structure compared to a (twin-waveguide) directional coupler.

Fig. 8. TE Polarization Ey: Using the exact solution from a FDFD simulation of a single waveguide, the horizontal cross section is extracted and five copies are shifted from one another so that their separation corresponds to a waveguide separation of 80 nm. (a) These individual waveguide modes are scaled in accordance with Eq. (13) for the fundamental mode (m=1). (b) The sumation of the individual waveguide modes; superimposed is the FDFD solution of the entire five waveguide structure. (c) Zoomed in to just the first waveguide. CMT and FDFD show a shift of the mode towards the center of the waveguide structure. (d-e) The fifth mode, both CMT and FDFD show a shift towards the edge of the waveguide structure however FDFD shows a shift of greater magnitude.

Note also, as shown in Fig. 8(e), that FDFD predicts a different exponential decay constant of the field wings, compared to CMT. This is a fundamental failure of CMT in the sense that the eigenmode of the composite structure can no longer be written as the sum of modes of individual waveguides (in isolation from each other). Until a satisfactory theory of mode evolution in this strong-coupling regime can be obtained, we recommend that designers rely on direct numerical simulations to obtain field profiles.

5.4. Polarization hybridization

Alongside this phenomenon, we also observe that the polarization becomes strongly hybridized as the separation distance is reduced. Figure 9 shows the major and minor E field components at large and small separation distances, along with the cross section of the Poynting vector, which indicates power flow. Notice that at small separation distances, the polarization component that was previously negligible has become, in fact, the dominant one. Furthermore, the power is actually carried above and below the waveguide structure at the outer edges, rather than within the inner slots, contrary to the original intention of slot waveguides.

Fig. 9. TM Polarization: finite element simulations (COMSOL) of the transverse field amplitudes, Ex and Ey, and Power flow, Pz for a separation of 1 μm (first Row) and 100 nm (second row) for the fifth mode. For small separations the polarization becomes strongly hybridized and the power is carried at the outer edges.

6. Conclusion

We suggest that the terminology “strong coupling” in the context of waveguide directional couplers be used to describe this coupling regime, for which a simple theory is not yet available, rather than simply large values of the length-integrated coupling coefficient, which can approach unity even with small coupling coefficients per unit length for long couplers, as this latter regime has already been well studied.

Acknowledgment

The authors are grateful to the National Science Foundation for support (ECCS-0642603 and ECCS-0723055) and thank Jung S. Park and Mark A. Schneider for useful discussions.

References and links

1.

F. Xia, L. Sekaric, and Y. Vlasov, “Ultracompact optical buffers on a silicon chip,” Nat. Photonics 1, 65–71 (2007). [CrossRef]

2.

Q. Xu, D. Fattal, and R. G. Beausoleil, “Silicon microring resonators with 1.5-μm radius,” Opt. Express 16, 4309–4315 (2008). [CrossRef] [PubMed]

3.

F. Xia, M. Rooks, L. Sekaric, and Y. Vlasov, “Ultra-compact high order ring resonator filters using submicron silicon photonic wires for on-chip optical interconnects,” Opt. Express 15, 11934–11941 (2007). [CrossRef] [PubMed]

4.

A. S. Liu, R. Jones, L. Liao, D. Samara-Rubio, D. Rubin, O. Cohen, R. Nicolaescu, and M. Paniccia, “A highspeed silicon optical modulator based on a metal oxide-semiconductor capacitor,” Nature 427, 615–618 (2004). [CrossRef] [PubMed]

5.

Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometer-scale silicon electro-optic modulator,” Nature 435, 325–327 (2005). [CrossRef] [PubMed]

6.

W. M. J. Green, M. J. Rooks, L. Sekaric, and Y. A. Vlasov, “Optical modulation using anti-crossing between paired amplitude and phase resonators,” Opt. Express 15, 17264–17272 (2007). [CrossRef] [PubMed]

7.

X. Liu, I. Hsieh, X. Chen, M. Takekoshi, J. I. Dadap, N. C. Panoiu, R. M. Osgood, W. M. Green, F. Xia, and Y. A. Vlasov, “Design and fabrication of an ultra-compact silicon on insulator demultiplexer based on arrayed waveguide gratings,” in Proceedings of the Conference on Lasers and Electro-Optics (CLEO, 2008), paper CTuNN1.

8.

P. Cheben, J. H. Schmid, A. Delage, A. Densmore, S. Jannz, B. Lamontagne, J. Lapointe, E. Post, P. Waldron, and D. C. Xu, “A high-resolution silicon-on-insulator arrayed waveguide grating microspectrometer with sub-micrometer aperture waveguides,” Opt. Express 15, 2299–2306 (2007). [CrossRef] [PubMed]

9.

K. Sasaki, F. Ohno, A. Motegi, and T. Baba,“Arrayed waveguide grating of 70×60 μm2 size based on Si photonic wire waveguides,” Elec. Lett. 41, (2007).

10.

P. Dumon, W. Bogaerts, D. V. Thourhout, D. Taillaert, and R. Baets, “Compact wavelength router based on a Silicon-on-insulator arrayed waveguide grating pigtailed to a fiber array,” Opt. Express 14, 664–669 (2006). [CrossRef] [PubMed]

11.

H. Kogelnik and C. V. Shank, “Coupled-mode theory of distributed feedback lasers,” Appl. Phys. 43, 2327–2335 (1972). [CrossRef]

12.

A. Hardy and W. Streifer, “Coupled-mode theory of parallel waveguides,” J. Lightwave Technol. LT-3, 1135–1146 (1985). [CrossRef]

13.

W. P. Huang, “Coupled-mode theory for optical waveguides: An overview,” J. Opt. Soc. Am. A 11, 963–983 (1994). [CrossRef]

14.

K. S. Chiang, “Coupled-zigzag-wave theory for guided waves in slab waveguide arrays,” J. Lightwave Technol. 10, 1380–1387 (1992). [CrossRef]

15.

F. P. Payne, “An analytical model for the coupling between the array waveguides in AWGs and star couplers,” Opt. Quantum Electron. 38, 237–248 (2006). [CrossRef]

16.

E. Kapon, J. Katz, and A. Yariv, “Supermode analysis of phase-locked arrays of semiconductor lasers,” Opt. Lett. 10, 125–127 (1984). [CrossRef]

17.

A. Klekamp and R. Munzner, “Calculation of imaging errors of AWG,” J. Lightwave Technol. 21, 1978–1986 (2003). [CrossRef]

18.

S. H. Yang, M. L. Cooper, P. R. Bandaru, and S. Mookherjea, “Giant birefringence in multi-slotted silicon nanophotonic waveguides,” Opt. Express 16, 8306–8316 (2008). [CrossRef] [PubMed]

19.

P. Yeh, Optical Waves in Layered Media (John Wiley & Sons, New York, 2005).

20.

C. L. Xu, W. P. Huang, M.S. Stern, and S. K. Chaudhuri, “Full-vectorial mode calculations by finite difference method,” IEE Proc.-Optoelectron. 141, 281–286 (1994). [CrossRef]

21.

W. P. Huang and C. L. Xu, “Simulation of three-dimensional optical waveguides by a full-vector beam propagation method,” IEEE J. Quantum Electron. 29, 2639–2649 (1993). [CrossRef]

22.

M. Kuznetsov, “Expressions for the coupling coefficient of a rectangular waveguide directional coupler,” Opt. Lett. 8, 499–501 (1983). [CrossRef] [PubMed]

23.

S. Mookherjea, “Spectral characteristics of coupled resonators,” J. Opt. Soc. Am. B 23, 1137–1145 (2006). [CrossRef]

24.

G. Lenz and J. Salzman, “Eigenmodes of multiwaveguide structures,” J. Lightwave Technol. 8, 1803–1809 (1990). [CrossRef]

25.

M. Popovic, C. Manolatou, and M. Watts, “Coupling-induced resonance frequency shifts in coupled dielectric multi-cavity filters,” Opt. Express 14, 1208–1222 (2006). [CrossRef] [PubMed]

26.

E. Marcatili, “Improved coupled-mode equations for dielectric guides,” IEEE J. Quantum Electron. QE-22, 988–993 (1986). [CrossRef]

27.

H. A. Haus, W. P. Huang, S. Kawakami, and N. A. Whitaker, “Coupled-mode theory of optical waveguides,” J. Lightwave Technol. LT-5 , 16–23 (1987). [CrossRef]

OCIS Codes
(130.2790) Integrated optics : Guided waves
(230.7380) Optical devices : Waveguides, channeled
(230.4555) Optical devices : Coupled resonators

ToC Category:
Integrated Optics

History
Original Manuscript: October 8, 2008
Revised Manuscript: January 14, 2009
Manuscript Accepted: January 22, 2009
Published: January 27, 2009

Citation
Michael L. Cooper and Shayan Mookherjea, "Numerically-assisted coupled-mode theory for silicon waveguide couplers and arrayed waveguides," Opt. Express 17, 1583-1599 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-3-1583


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References

  1. F. Xia, L. Sekaric, and Y. Vlasov, "Ultracompact optical buffers on a silicon chip," Nat. Photonics 1, 65-71 (2007). [CrossRef]
  2. Q. Xu, D. Fattal, and R. G. Beausoleil, "Silicon microring resonators with 1.5-μm radius," Opt. Express 16, 4309-4315 (2008). [CrossRef] [PubMed]
  3. F. Xia, M. Rooks, L. Sekaric, and Y. Vlasov, "Ultra-compact high order ring resonator filters using submicron silicon photonic wires for on-chip optical interconnects," Opt. Express 15, 11934-11941 (2007). [CrossRef] [PubMed]
  4. A. S. Liu, R. Jones, L. Liao, D. Samara-Rubio, D. Rubin, O. Cohen, R. Nicolaescu, and M. Paniccia, "A highspeed silicon optical modulator based on a metal oxide-semiconductor capacitor," Nature 427, 615-618 (2004). [CrossRef] [PubMed]
  5. Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, "Micrometer-scale silicon electro-optic modulator," Nature 435, 325-327 (2005). [CrossRef] [PubMed]
  6. W. M. J. Green, M. J. Rooks, L. Sekaric, and Y. A. Vlasov, "Optical modulation using anti-crossing between paired amplitude and phase resonators," Opt. Express 15, 17264-17272 (2007). [CrossRef] [PubMed]
  7. X. Liu, I. Hsieh, X. Chen, M. Takekoshi, J. I. Dadap, N. C. Panoiu, R. M. Osgood,W. M. Green, F. Xia, and Y. A. Vlasov, "Design and fabrication of an ultra-compact silicon on insulator demultiplexer based on arrayed waveguide gratings," in Proceedings of the Conference on Lasers and Electro-Optics (CLEO, 2008), paper CTuNN1.
  8. P. Cheben, J. H. Schmid, A. Delage, A. Densmore, S. Jannz, B. Lamontagne, J. Lapointe, E. Post, P. Waldron, and D. C. Xu, "A high-resolution silicon-on-insulator arrayed waveguide grating microspectrometer with submicrometer aperture waveguides," Opt. Express 15, 2299-2306 (2007). [CrossRef] [PubMed]
  9. K. Sasaki, F. Ohno, A. Motegi, and T. Baba, "Arrayed waveguide grating of 70x60 μm2 size based on Si photonic wire waveguides," Electron. Lett.  41, 801-802 (2005).
  10. P. Dumon, W. Bogaerts, D. V. Thourhout, D. Taillaert, and R. Baets, "Compact wavelength router based on a Silicon-on-insulator arrayed waveguide grating pigtailed to a fiber array," Opt. Express 14, 664-669 (2006). [CrossRef] [PubMed]
  11. H. Kogelnik and C. V. Shank, "Coupled-mode theory of distributed feedback lasers," Appl. Phys. 43, 2327-2335 (1972). [CrossRef]
  12. A. Hardy and W. Streifer, "Coupled-mode theory of parallel waveguides," J. Lightwave Technol. LT-3, 1135-1146 (1985). [CrossRef]
  13. W. P. Huang, "Coupled-mode theory for optical waveguides: An overview," J. Opt. Soc. Am. A 11, 963-983 (1994). [CrossRef]
  14. K. S. Chiang, "Coupled-zigzag-wave theory for guided waves in slab waveguide arrays," J. Lightwave Technol. 10, 1380-1387 (1992). [CrossRef]
  15. F. P. Payne, "An analytical model for the coupling between the array waveguides in AWGs and star couplers," Opt. Quantum Electron. 38, 237-248 (2006). [CrossRef]
  16. E. Kapon, J. Katz, and A. Yariv, "Supermode analysis of phase-locked arrays of semiconductor lasers," Opt. Lett. 10, 125-127 (1984). [CrossRef]
  17. A. Klekamp and R. Munzner, "Calculation of imaging errors of AWG," J. Lightwave Technol. 21, 1978-1986 (2003). [CrossRef]
  18. S. H. Yang, M. L. Cooper, P. R. Bandaru, and S. Mookherjea, "Giant birefringence in multi-slotted silicon nanophotonic waveguides," Opt. Express 16, 8306-8316 (2008). [CrossRef] [PubMed]
  19. P. Yeh, Optical Waves in Layered Media (John Wiley & Sons, New York, 2005).
  20. C. L. Xu, W. P. Huang, M .S. Stern, and S. K. Chaudhuri, "Full-vectorial mode calculations by finite difference method," IEE Proc.-Optoelectron. 141, 281-286 (1994). [CrossRef]
  21. W. P. Huang and C. L. Xu, "Simulation of three-dimensional optical waveguides by a full-vector beam propagation method," IEEE J. Quantum Electron. 29, 2639-2649 (1993). [CrossRef]
  22. M. Kuznetsov, "Expressions for the coupling coefficient of a rectangular waveguide directional coupler," Opt. Lett. 8, 499-501 (1983). [CrossRef] [PubMed]
  23. S. Mookherjea, "Spectral characteristics of coupled resonators," J. Opt. Soc. Am. B 23, 1137-1145 (2006). [CrossRef]
  24. G. Lenz and J. Salzman, "Eigenmodes of multiwaveguide structures," J. Lightwave Technol. 8, 1803-1809 (1990). [CrossRef]
  25. M. Popovic, C. Manolatou, and M. Watts, "Coupling-induced resonance frequency shifts in coupled dielectric multi-cavity filters," Opt. Express 14, 1208-1222 (2006). [CrossRef] [PubMed]
  26. E. Marcatili, "Improved coupled-mode equations for dielectric guides," IEEE J. Quantum Electron. QE-22, 988-993 (1986). [CrossRef]
  27. H. A. Haus, W. P. Huang, S. Kawakami and N. A. Whitaker, "Coupled-mode theory of optical waveguides," J. Lightwave Technol. LT-5, 16-23 (1987). [CrossRef]

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