## 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, 2]. 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], Mach-Zehnder interferometers and modulators [4, 5, 6], arrayed waveguide structures [7, 8, 9, 10] 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, 12, 13]. More accurate corrections to CMT for slab waveguides have been investigated by Chiang [14], and Payne [15], 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.

As a test structure which highlights both the applicability and shortcomings of CMT, we will consider the multi-waveguide coupled-array structure [16] which consists of a number of directional couplers parallel to one another, as is often used in arrayed waveguide gratings and multi-element lasers and amplifiers, and which require an accurate estimate of the coupling coefficients to prevent imaging and phase errors [17]. This structure, also called the “multi-slot waveguide,” has been recently fabricated and demonstrated by us in SOI [18]. We show that, because of the narrow waveguide widths allowed by the high index contrast, multi-waveguide structures can reveal significant next-to-nearest-neighbor coupling and other deviations from the conventional picture of modal coupling, and we find the “critical” waveguide-to-waveguide separation distance at which such terms become significant.

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

If many slot waveguides are arranged in a parallel array, as would be encountered in the cross-section of an arrayed waveguide grating or coupler, or coupled-waveguide laser, then their modes can often be adequately described by supermode theory [16], which is one of the fundamental predictions of coupled-mode theory (CMT), and hence can be a test of the applicability of CMT to SOI photonics. The first part of this section will briefly describe an *ab initio* numerical algorithm we have encoded in MATLAB to calculate the modes accurately. The next part of this section presents the analysis of the modes based on supermode theory. In the following sections, we will compare the predictions of CMT with the FDFD calculations.

#### 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], the vectorial wave equation,

where **E**
_{⊥}(*x*,*y*) is the transverse electric field vector, is written in matrix form as

and,

$${P}_{\mathrm{yx}}{E}_{x}=\frac{\partial}{\partial y}\left[\frac{1}{{n}^{2}}\frac{\partial \left({n}^{2}{E}_{x}\right)}{\partial x}\right]+\frac{{\partial}^{2}{E}_{x}}{\partial y\partial x},\phantom{\rule{3.5em}{0ex}}{P}_{\mathrm{yy}}{E}_{y}=\frac{\partial}{\partial y}\left[\frac{1}{{n}^{2}}\frac{\partial \left({n}^{2}{E}_{x}\right)}{\partial y}\right]+\frac{{\partial}^{2}{E}_{y}}{{\partial x}^{2}}+{n}^{2}{k}^{2}{E}_{y}.$$

In this formulation the displacement vectors *n*
^{2}
*E _{x}* and

*n*

^{2}

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

_{y}#### 2.2. Coupled mode theory and its predictions

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

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

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

where *n*
^{2}
_{s} (*x*,*y*) corresponds to the cladding. Thus, *n*
^{2}
_{s} (*x*,*y*) + Δ*n*
^{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,

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

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

where,

*N* equations are formed by multiplying Eq. (8) by *𝓔 _{j}*

^{*}(

*j*= 1,2,…,

*N*), and integrating each of these equations over

*x*and

*y*,

We define the modal overlap integrals as follows:

with the normalization

*I _{jl}* 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,

$$\phantom{\rule{12em}{0ex}}=\left(\begin{array}{ccccc}{\beta}^{2}& 0& 0& \cdots & 0\\ 0& {\beta}^{2}& 0& \cdots & 0\\ 0& 0& {\beta}^{2}& \cdots & 0\\ \vdots & \vdots & \vdots & \ddots & \vdots \\ 0& 0& 0& \cdots & {\beta}^{2}\end{array}\right)\left(\begin{array}{c}{A}_{1}\\ {A}_{2}\\ {A}_{3}\\ \vdots \\ {A}_{N}\end{array}\right).$$

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

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,

If the waveguides are identical and equally spaced, *M* can be further simplified by setting *β*
^{2}
_{1} = *β*
^{2}
_{2} … = *β*
^{N}
_{1} ≡ *β*
^{2}
_{0} and also, *I*
_{l,l+1} = *I*
_{l-1,l} ≡ *I,κ*
_{l,l+1} = *κ*
_{l-1,l} ≡ *κ*. However, even if the waveguides are identical and equally spaced, *κ*
_{11} and *κ _{NN}* are not equal to

*κ*

_{22},

*κ*

_{33},…,

*κ*

_{N-1N-1}. In fact, Eq. (10b) shows that for those waveguides at the edges (

*l*= 1 and

*I*=

*N*) there are approximately only half as many contributing terms as the other waveguides: there are no waveguides to the left of the

*l*= 1 waveguide, and there are no waveguides to the right of the

*l*=

*N*waveguide, whereas all the other waveguides have contribution terms from both the left and right halves of their modal profiles.

We define *κ*
_{self} ≡ *κ*
_{22}, *κ*
_{33},…, *κ*
_{N-1N-1}, *κ*
_{self,edge} ≡ *κ*
_{11} and *κ _{NN}*, and

*δκ*

_{self}=

*κ*

_{self}-

*κ*

_{self,edge}.

To first order in the perturbation *δκ*
_{self}, the eigenvectors are

$$\times \underset{\underset{n\ne m}{m=1}}{\overset{N}{\Sigma}}\frac{1}{{\beta}^{{2}^{\left(m\right)}}-{\beta}^{{2}^{\left(m\right)}}}\left[\mathrm{sin}\frac{\mathrm{m\pi}}{N+1}\mathrm{sin}\frac{\mathrm{n\pi}}{N+1}+\mathrm{sin}\frac{\mathrm{mN\pi}}{N+1}\mathrm{sin}\frac{\mathrm{nN\pi}}{N+1}\right]\mathrm{sin}\frac{\mathrm{ln\pi}}{N+1}$$

where *m* is the modal number and *l* indicates which high-index rib waveguide (or low-index slot) is being described. [The expression for the eigenvalues is written later, Eq. (22).]

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,

Under nearest neighbor coupling, the magnetic field will also obey the scaling relationship of Eq. (13). If it can be assumed that ∣*∂H _{z}*/

*∂*∣ ≪ ∣

_{y}*∂H*/

_{y}*∂*∣, then

_{z}*H*and

_{y}*E*are related by

_{x}so that, within the high index ribs, we expect that Eq. (13) describes the scaling relationship of the peak electric field amplitudes. Although the peak electric field amplitudes of the entire modal profile are found not in the high-index regions, but in the low-index regions (just inside the core-cladding boundary), they obviously satisfy the same scaling law, as can be seen in Fig. 3.

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

CMT offers valuable physical insight into how waveguides couple—in particular, the structure of matrix *M* in Eq. (11) is revealing—but the quantitative predictions of CMT are in error in high-index-contrast SOI structures at short separation distances. To obtain a numerically-accurate picture of modal coupling, we propose a new extension of CMT, which we call “numerically assisted” CMT, to use the simulation results of the FDFD algorithm to back-calculate the elements of the coupling matrix *M*. We can thereby check if the assumption of nearest-neighbor coupling is valid at short separation distances, and identify various other interesting coupling phenomena (e.g., non-Hermiticity of *M*) which have not been pointed out earlier.

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

- 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.
- Having obtained both the eigenvectors (peak amplitudes) and eigenvalues (propagation constants), we construct the (non-singular) matrix of eigenvectors
*A*(whose columns are the linearly-independent supermodes), and the diagonal matrix of eigenvalues, Λ = diag{_{mn}*β*^{2}_{m}}. - Next, we reassemble
*M*[see Eq. (11)] by using the matrix theorem cited in Ref. [23, Eq. (7), Lem. 1–2]: if the eigenvalues are distinct (which they are in this case),*M*can be reconstructed as follows:*M*=*A*Λ*A*^{-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 (*λ _{k}*) 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.

We assume that the measurements result in some small, uncorrelated errors in the eigenvalues (Δ*λ _{k}*) and eigenvectors (Δ

*u*). The inversion algorithm presented in the previous section can also be used with measured data. In this section we study the accuracy of the nearest-neighbor coupling and next-to-nearest-neighbor coupling coefficients in terms of Δ

_{k}*λ*and Δ

_{k}*u*.

_{k}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

For simplicity, in this paper, we assume that Δ*u _{k}* 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]. 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

$$\Delta {M}_{j-1j+1}=\underset{k=1}{\overset{N}{\Sigma}}\frac{2}{N+1}\mathrm{sin}\frac{\left(j-1\right)\mathrm{k\pi}}{N+1}\mathrm{sin}\frac{\left(j+1\right)\mathrm{k\pi}}{N+1}\Delta {\lambda}_{k}.$$

Assuming that Δ*λ _{k}* are uncorrelated identically-distributed random variables with mean

*E*[Δ

*λ*] and variance Var[Δ

*λ*], the mean and variance of the nearest-neighbor coupling and next-to-nearest-neighbor coupling coefficients can be calculated. Both Δ

*M*

_{jj+1}and Δ

*M*

_{j-1j+1}are zero-mean, since, for example,

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

$$\phantom{\rule{4em}{0ex}}=\mathrm{Var}\left[\mathrm{\Delta \lambda}\right]\left(\frac{1}{N+1}\right),$$

and the same result is obtained for Var[Δ*M*
_{j-1j+1}].

Summarizing the results,

$$\mathrm{Next}-\mathrm{to}-\mathrm{nearest}:E\left[\mathrm{\Delta \lambda}{M}_{j-1j+1}\right]=0,\phantom{\rule{1em}{0ex}}Var\left[\mathrm{\Delta \lambda}{M}_{j-1j+1}\right]=\mathrm{Var}\left[\mathrm{\Delta \lambda}\right]/\left(N+1\right).$$

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.)

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.

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], 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),

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

Figure 5 shows the calculations of this ratio using the above algorithm. The exponential fit describes the TE polarization much better than it does the TM polarization, indicating that some of the central assumptions of CMT are starting to fail for the TM polarization at short distances. The next section will describe another symptom of the failure of CMT, obtained by looking at the eigenvalues, i.e., the propagation constants, of the supermodes.

#### 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

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]) 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]. 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.

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.

#### 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*
_{12} ≠ *κ*
_{21} and *κ*
_{13} ≠ *κ*
_{31}, etc.

The reason for this asymmetry is that the fields within the individual waveguides are no longer centered between the dielectric boundaries. As shown in Fig. 7. the modal profile starts to deviate in the location of its maxima and minima. For example, the peaks of the field in the outermost ribs are skewed and no longer centered in the middle of the dielectric boundaries, and can even reach the boundaries of the high-index and low-index regions. It is no longer accurate to read off the peak amplitudes of the supermode in order to write the eigenvectors *A*
^{(m)} in Eq. (13)—doing so would result in asymmetric *M* matrices.

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.

For the fundamental mode, Fig. 8(a-c), the summation of the fields associated with the first (blue) and second (green) waveguides results in the peak shifting towards the center of the five waveguide structure, which qualitatively agrees with the FDFD simulation. But the FDFD result for the fifth mode shows a shift of greater magnitude, now towards the outer edge of the waveguide structure, indicating that CMT no longer accurately predicts the modal profile of the supermode. TM polarized modes start to shift at a larger separation, due to the field discontinuities at the waveguide boundaries and electric field enhancement in the cladding regions.

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.

These calculations suggest that at small separation distances, CMT of high-index contrast waveguides should consider both polarization components of the electric field. Mathematically, the dimensionality of the basis set (of eigenvectors) depends on the separation distance. Practically, one should pay careful attention to where the power flow is concentrated in waveguide couplers at short separation distances, since both polarization components of the electric field (with different coupling lengths due to different modal effective indices) play a role in the exchange of power between the constituent waveguides.

## 6. Conclusion

In this paper, we have studied the validity of coupled-mode theory CMT for high-index contrast (e.g., silicon-based) optical waveguiding structures, in particular directional couplers and multi-slot waveguides. We have used a fully vectorial finite-difference frequency-domain (FDFD) algorithm to obtain modal profiles and effective indices of the supermodes in a non-perturbative way. When the modal profiles can be “discretized” to read off peak amplitudes within each of the waveguide cores, a theorem from matrix algebra can be employed to solve the “inverse problem”—to reconstruct accurately the matrix of coupling coefficients *M* (a procedure we have called numerically-assisted coupled-mode theory, NA-CMT). The NA-CMT framework can be used to find out when the nearest-neighbor-coupling approximation breaks down.

Aside from the inversion procedure, the results of FDFD calculations also directly address this problem. We have presented the eigenvalue fanout graphs for three different silicon-based material platforms and indicated the separation distances at which the CMT predictions break down. We have also shown that there are significant deviations in the modal shape as the waveguide separation distance decreases and the state of polarization of the field may change between quasi-TE or quasi-TM to a strongly-hybridized polarization.

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-*μ*m 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 *μm*^{2} 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]