In this work, a dielectric waveguide mode solver is presented considering a general nonreciprocal permittivity tensor. The proposed method allows us to investigate important cases of practical interest in the field of integrated optics, such as magneto-optical isolators and anisotropic waveguides. Unlike the earlier developed mode solver, our approach allows for the precise computation of both forward and backward propagating modes in the nonreciprocal case, ensuring high accuracy and computational efficiency. As a result, the nonreciprocal loss/phase shift can be directly computed, avoiding the use of the perturbation method. To compute the electromagnetic modes, the Rayleigh-Ritz functional is derived for the non-self adjoint case, it is discretized using the node-based finite element method and the penalty function is added to remove the spurious solutions. The resulting quadratic eigenvalue problem is linearized and solved in terms of the propagation constant for a given frequency (i.e., γ–formulation). The main benefits of this formulation are that it avoids the time-consuming iterations and preserves the matrix sparsity. Finally, the method is used to study two examples of integrated optical isolators based on nonreciprocal phase shift and nonreciprocal loss effect, respectively. The developed method is then compared with the perturbation approach and its simplified formulation based on semivectorial approximation.
© 2014 Optical Society of America
Dielectric waveguides are fundamental components of optoelectronic and microwave devices. They have been extensively studied in the past and different kinds of materials have been considered. To perform the waveguide modal analysis, several numerical methods are commonly used to compute the electromagnetic modes such as the finite element method (FEM) , the finite difference method (FDM) , the method of lines (MoL) , and the film mode matching (FMM) . Due to the possibility of using adaptive meshing, FEM shows several advantages. It usually provides a better approximation and requires less memory to store the stiffness matrix with respect to FDM, while it is more appropriate than MoL and FMM for modal analysis of graded index waveguides, such as ion-implanted waveguides , and in general for the analysis of waveguides with complex cross-section geometry and refractive index profiles . In addition, it is the most suited to solve deformation and stress problems in solids, like for the case of stress-induced effects in optical waveguides (e.g., silicon waveguides on silicon-on-insulator platform ).
In the finite element analysis, the solution can be numerically computed as a linear combination of basis functions which, in electromagnetism, are usually of two kinds: node elements (also called Lagrangian elements) [1, 8–14] and edge elements (also called Nédélec elements) [1,8,15–19]. The use of the edge elements shows mainly three important benefits: i) the spurious solutions can be effectively removed in several electromagnetic problem formulations, ii) the boundary conditions at material interface and conducting surface can be easily imposed, iii) there are no difficulties in treating conducting and dielectric edges and corners related to the field singularities [1,20]. On the other hand, the node elements are more efficient as regards to the storage requirements and the number of floating point operations (FLOPs). In addition, the solutions computed using node elements provide higher accuracy when extremely flat or elongated elements are used in the mesh . It is worth noting that in order to completely get rid of any spurious solutions introduced by the node elements within the range of interest of the guided modes, the penalty function can be added to the functional [8, 21].
In this work, the node elements have been used, since we do not consider waveguides with field singularities (e.g., by using magnetic-field formulation) and assume a zero-field condition on the border. However, the method can be implemented also with edge elements. The mode solver has been developed for straight waveguides, as the one shown in Fig. 1 where the cross-section is in the xy-plane and the light propagates only along the z-direction.
Using Cartesian coordinate system, we have considered the following relative permittivity tensor9], permitting to consider also lossy material. In addition, it includes the case presented by Lu and Fernandez  and allows us to investigate optical-isolators based on magneto-optical materials and anisotropic waveguides.
For the sake of clarity, let us consider the case of a magneto-optic garnet. Its permittivity tensor can be written as23, 24]. Note that when KMx or KMy are not zero, the forward and backward mode propagation constants have different values. More specifically, when KMx or KMy are purely imaginary numbers, the forward and backward modes differ only in the phase constant (i.e., the imaginary part of the propagation constant) and a nonreciprocal phase shift effect (NRPS) arises. By exploiting this effect, optical isolators have been designed using Mach-Zehnder Interferometer (MZI) configuration in order to generate constructive interference for forward light and destructive interference for backward light [25,26]. On the other hand, when the real part of KMx or KMy is not zero, also the propagation loss with respect to the two directions is different. Optical isolating function can be achieved combining the nonreciprocal loss (NRL) with the gain of an optical amplifier, which compensates for the loss of the forwarding wave, whereas keeps a large loss for the reverse direction .
While Mx and My are responsible for NRPS and NRL of the transverse magnetic (TM) and transverse electric (TE) modes, respectively, Mz gives rise to TE-TM mode coupling. In integrated optics Mz is usually negligible for several applications and the magnetization vector is all in the xy-plane. As a result, the permittivity tensor of Eq. (2) has the form described by Eq. (1).
Considering the previous hypothesis, the augmented Rayleight-Ritz functional with the penalty function has been derived. By using the node elements, we have computed the eigenvalue problem as a function of the angular frequency ω and propagation constant γ. The γ-formulation, where the frequency is provided as the input parameter and the propagation constant is the output eigenvalue, has been explicitly derived. This formulation avoids the time-consuming iterations necessary to evaluate the propagation constant at a given frequency .
By exploring two examples of optical isolators based on magneto-optical waveguides for silicon compatible platform, we show how the shift between forward and backward propagation constants can be directly computed instead of using the pertubation theory .
2. Mathematical model
2.1. Differential electromagnetic problem
In a linear, instantaneous and time invariant medium, the magnetic and electric fields can be written as a linear combination of harmonic wavesEq. (5), we used the relation for the speed of light in the vacuum and we omit the explicit dependence on the space variable r for the sake of simplicity. In a similar way, it is possible to derive an equivalent problem for E.
2.2. Variational formulation: non-self-adjoint problem
As it is known from the variational methods, it is possible to construct a functional linked to the differential problem such as the stationary point of that functional is the exact solution of the differential problem . In our case, the relative permittivity tensor defined in eq. (1) is neither symmetric (i.e., ) nor hermitian (i.e., ). As a result, the differential operator is not symmetric or self-adjoint . To compute the related functional an auxiliary problem is introduced, which is also called adjoint problem . Considering the original problem (5), its adjoint problem is30].
By considering the perfect electric conductor (PEC) or perfect magnetic conductor (PMC) boundary conditionsequation (5) is the stationary point of the functional 30]. A similar formulation can be derived for E and the electric adjoint field Ea.
2.3. Waveguide mode problem
In our work, we consider that the materials are non-magnetic (μr = 1). As a result, the normal and tangent component of H are continuous across any boundary separating two different media. For this reason, the formulation in term of the magnetic field is preferred. Moreover, because a mode is a transverse electromagnetic wave which propagates in the waveguide without sources, we set J = 0 and Ja = 0.
By considering an infinite straight waveguide, like the one shown in Fig. 1, the geometry is invariant along the z-axis and the electric and magnetic fields can be written as30–32]. To compute the functional in Eq. (8), we need to find the relationship between the direct and the adjoint fields. Equation (4c) and (4d) for the direct field are (12) have opposite sign with respect to the ones in the original problem (11). Using the following relations (12) we obtain the system (11). The previous relations allow us to link the components of the original fields with those of the adjoint ones. At this point, it is easy to compute the Rayleigh-Ritz functional (8). The relationship (13) is also verified when both εr and μr have the form of Eq. (1), which is the case of magnetized ferrites . In that case, the variational formulation with respect to E is preferred.
Let us note that from Eq. (8), the solution of the problem H and Ha belongs to the Sobolev space H(curl).
2.4. Interpolating function
Because the electromagnetic problem for a straight waveguide is independent of the direction of propagation (z-axis), only the transverse cross-section is discretized. Considering a generic mesh element e, the interpolating function on it can be written as
As we already mentioned, one of the drawbacks associated with the node element approach is the appearance of spurious or nonphysical solutions. In fact, when the magnetic field H solves the curl-curl equation (5), it also solves the constraintequations (5), the constraint should be taken into account using the augmented functional F̃(H, Ha) 21]. Equation (17) implies that H, Ha belong to the Sobolev space H(curl) ∩ H(div). The constant αp is a free parameter and it is usually chosen equal to 1.
2.5. Discretized functional
Using the approximation of the previous paragraph and introducing the tensor , we have calculated the functional (17). Note that the integral along the z-axis is neglected because the argument is independent of this variable. To make the computation clearer, let us consider the integrals on a generic element e with surface Se. In addition, we assume p is constant in each element. At this point, let us introduce a more compact matrix notation by defining the vectors9]. The matrix entry with index (i, j) depends only on the basis functions and on the element e. Note that the matrices Re, De, and Ee are symmetric.
From Eq. (17), we can see that the functional F̃(H, Ha) is the sum of three integrals. By computing the first one, we obtain
Similarly, we computed the second term
At this point we move from the local computation performed on the generic element e to the global one that defines the matrices associated to the discretized problem. The total functional is the sum of the contributions of all of the elements(26) are sparse, with very few non-zero elements.
2.6. Eigenvalue problems8]. In the first case, the propagation constant is provided as an input of the problem and the previous equation becomes Eq. (28). Vice versa, fixing the angular frequency ω, the Eq. (27) is a quadratic eigenvalue problem (QEP) [34, 35], where h is the eigenvector and γ its eigenvalue.
The number of rows/columns of the matrices in Eq. (28) is three times the number of the mesh nodes N̂. In the literature, different approaches have been derived in order to reduce the memory and computational efforts, e.g. the formulation in terms of the transverse magnetic field components. However, to the best of our knowledge, this formulation has been obtained only in the case of εxz = εyz = 0 [1, 8, 22, 36, 37], while its derivation is not straightforward in the more general case. Moreover, the full vectorial formulation preserves the matrix sparsity and provides a higher accuracy on the computation of the longitudinal component Hz, which is in fact directly evaluated . It is worth noting that an accurate value of all field components is important especially in silicon photonics, where the concepts of pure TE and TM modes are undermined due to the high index contrast.
An easy way to solve the QEP consists in transforming it into an equivalent generalized eigenvalue problem . By introducing the vectorEq. (28)) and the γ-formulation (Eq. (30)) have been transformed into a generalized eigenvalue problem
The eigenvector-eigenvalue pairs, which are the numerical solutions of the electromagnetic problem, can be found using Krylov methods. Such methods allow us to compute few eigenvalues of large sparse matrices, e.g. the eigenvalues which have the largest or the smallest magnitude. If we roughly know the eigenvalues we are interested in, we can shift the spectrum close to them and then apply the method, looking for the smallest magnitude eigenvalues. Called such approximation σ0, we subtract σ0Bv from either term in Eq. (31)(31) and (33) have the same eigenvectors, but shifted eigenvalues Eq. (35) it is useful to define the effective index as neff = β/k0 which can be easily compared with the refractive index of the waveguide.
2.7. Lossy case: fully complex formulation
The described mode solver formulation is fully complex and the propagation loss can be directly computed. In order to evaluate the radiation loss of the waveguide, the perfectly matched layer method (PML) can be used , and the parameters can be easily estimated using the method presented in [14, 41].
2.8. Lossless case: real formulation
Ordinarily, the field E and H have complex components. However, the problem can be simplified considering a particular case for the permittivity tensor(17) fully real [1, 9] and this halves the memory demand.
3. Numerical results
In this section, we are presenting some numerical results obtained by the described mode solver. The method has been fully programmed in MATLAB. The integrands in Eq.s (19) are polynomials over triangular elements and can be computed exactly . The routine eigs has been used to numerically solve the eigenvalue problems. It provides the reverse communication required by the Fortran library ARPACK [42,43]. ARPACK is a collection of Fortran77 subroutines designed to solve large scale eigenvalue problems. This software is based upon an algorithmic variant of the Arnoldi process called the implicitly restarted Arnoldi method (IRAM) [44,45]. In the ω–formulation, because both of the matrices in the generalized eigenvalue problem are symmetric, the IRAM is replaced by the implicitly restarted Lanczos method (IRLM) .
3.1. Nonreciprocal phase shift in lossless magneto-optical waveguides
Here, we present the modal analysis results of a nonreciprocal waveguide that has been used to perform silicon-based optical isolator for the TM mode using both Mach-Zender interferometer [26, 46, 47] and micro-ring resonator configurations [48, 49]. In the former photonic circuit, the device is designed to generate constructive interference for the forward light and destructive interference for the backward light, while in the latter the different propagation constant for the clockwise (CW) and the counterclockwise (CCW) modes results in a different resonant wavelength for the two directions.
The waveguide cross-section is shown in Fig. 2. The silicon waveguide is fabricated on a silicon-on-insulator (SOI) wafer, having refractive index nSi = 3.45 and nSiO2 = 1.46, respectively. In order to achieve NRPS, the top-layer of the waveguide is bonded with a cerium-substituted yttrium iron garnet (Ce:YIG) (nCe:YIG = 2.22, ) grown on a (Ca,Mg,Zr)-substituted gadolinium gallium garnet (SGGG), (nSGGG = 1.97).
The Ce:YIG is a magneto-optic (MO) garnet used in silicon photonics to perform optical isolators and circulators. By applying a static magnetic field along the horizontal axis (x-direction in Fig. 2), the Ce:YIG permittivity tensor results48–50], and then εyz = 7.65 · 10−3. However, in high quality Ce:YIG crystal, a Faraday-rotation coefficient as large as 4500°/cm can be measured .
By solving the curl-curl equation for the magnetic field, as described in section 2, we have computed the three components of the magnetic field and the phase constant, for the forward and backward propagating waves, respectively. The NRPS can be directly computed asFig. 3, where we assumed a 600nm × 215nm silicon waveguide cross-section, with a 380nm thick Ce:YIG layer above it. Those values maximize the NRPS for the TM mode, as it is explained in the following.
Because the value of the off-diagonal terms in the permittivity tensor are usually small (e.g., ), it is very important to optimize the waveguide cross-section in order to maximize Δβ and to achieve the highest isolation. In Fig. 4 we report the NRPS computed for different silicon and Ce:YIG layer thicknesses (hMO and hSi in Fig. 2), while we consider a 600nm wide silicon waveguide in order to guarantee the single mode regime (wSi in Fig. 2).
Note that NRPS is maximized when the maximum of |Hx|2 is located close to the boundary of the two materials, similarly to what happens using garnet double layers with opposite Faraday rotation values . The same calculation for the TE mode shows a negligible split between the phase constants of the propagating and counterpropagating modes.
Considering hMO = 380 nm, hSi = 215 nm and wSi = 600 nm, we have performed the modal analysis for λ within the range: [500nm, 3000nm]. Figure 5 shows the dispersion curves of the first ten modes with respect to the wavelength. As it is known, the higher the frequency (the lower the wavelength), the larger the number of modes that are confined in the structure. Moreover, the modes with an effective index higher than the refractive index of the cladding (1.97 for SGGG) are confined in the silicon-waveguide core and can propagate, while the ones which have neff < 1.97 are below cut-off and cannot propagate.
3.2. Finite element method vs. perturbation method
In the literature, the most used method for investigating the nonreciprocal effect is the perturbation method [28, 47, 52–55]. Such a method can be applied when the off-diagonal elements of the permittivity tensor are rather small, which in our case means εyz ≪ εxx, εyy, εzz. When this happens, εyz can be assumed as a small perturbation on the permittivity tensor and the shift between forward and backward propagation constants can be estimated by applying the perturbation theory . Using this approach, the electromagnetic fields are computed considering diagonal permittivity tensor and the phase constant difference is estimated like
The previous equation can be further simplified using the semivectorial approximation (i.e., Ex = 0 and Ez = j∂yEy/β for the TM mode and Ey = 0 and Ez = j∂xEx/β for the TE mode) [47, 53, 55]. Considering our example (TM mode), Eq. (44) can be rewritten as follows
To conclude our analysis, we compare the NRPS computed with our model with the ones calculated using Eq. (44) and Eq. (45). In Fig. 6(a), the percentage difference between Δβ and Δβpm is reported for different values of layer thickness. From that figure, we can easily see that the difference between the two methods is less than 3%. Similarly, a relative difference larger than 10% is computed between Δβ and Δβsv, as shown in Fig. 6(b). Such a big difference can be explained considering that the Ex component is neglected for the TM mode in Eq. (45). Indeed, in a high index contrast system, like silicon-photonics, the concepts of pure TE and TM modes is undermined and all three field components become non-negligible and comparable in magnitude.
Finally, in Fig. 7 we compare the three methods considering hMO = 500nm and varying the silicon thickness only.
3.3. Nonreciprocal loss in magneto-optical waveguides
Optical isolation based on nonreciprocal loss can be implemented by considering a magneto-optic material, which provides the nonreciprocal loss, and an optical waveguide amplifier, which compensates the forward propagation loss. Several configurations have been proposed for both indium phosphide [56–58] and silicon photonics technologies .
In this section, we present the modal analysis of a nonreciprocal loss waveguide for hybrid silicon photonics integrated circuit platforms. The silicon waveguide is defined on SOI wafers by etching 400nm deep trenches into a 700nm thick silicon layer . The nonreciprocal loss is induced by a thin film of iron (Fe) as schematically shown in Fig. 8. The iron layer is separated from the silicon waveguide by a thin titania (TiO2) layer to reduce and optimize the Fe-layer loss. The propagation loss for the forward light can be effectively compensated by bonding an InGaAsP multi quantum well (MQW) active layer above the silicon waveguide.
In our simulations, we assume a 30nm-thick layer of titania and we varied the thickness of the iron between 25nm and 100nm. The refractive indices of the TiO2 and Fe at 1550nm are assumed to be 2.1 and 3.17 + 5.27i, respectively . The nonreciprocal effect can be induced on the TE mode by applying a magnetic field along the vertical direction (i.e., the y-axis in Fig. 8). For this case, the Fe permittivity tensor results57, 61]. It is worth mentioning that, although iron is a ferromagnetic material at low frequency, at optical frequencies the magnetic susceptibility ceases to have any physical meaning and its magnetic permeability has been assumed equal to μ0 [23, 62, 63].
The results of the modal analysis are reported in Fig. 9, where the amplitude of the magnetic field components of the TE-mode are shown.
In Fig. 10(a), the propagation loss for both the forward and backward modes is reported for different values of the Fe-layer thickness. The thicker the iron layer, the larger the difference between the loss in the two directions and, as a consequence, the higher the optical isolation. On the other hand, a thicker Fe-film produces larger loss that needs to be compensated by a MQW layer. With our model, we directly compute the NRL and NRPS asFig. 10(b). It is worth noting that the longer the waveguide, the larger the nonreciprocal effect. As it can be seen from the figure, a 25nm-thick iron film will produce a NRL of about −7dB/mm, while a four times thicker layer generates an isolation of almost −12dB/mm. Those values are consistent with those reported in the literature [56, 57, 59].
Also in this case, a compared analysis with the perturbation method is performed. Let us note that in the lossy case, the formula (44) must be replaced by the more general one28]. The results of this comparison are reported in table 1.
From table 1, we can observe that the results of the two methods are in good agreement. In addition, the relative difference between the two methods is smaller than the ones computed in the previous case. This can be understood because the layer of iron is much smaller than the silicon waveguide and only a small percentage of the field is confined in it. This is in agreement with the basic assumption of the perturbation method.
It is worth noting that the proposed method allows us to directly compute the NRPS and the NRL for complex structures based on new materials characterized by higher Faraday rotation constants like, for example, the bismuth iron garnet (BiIG) , in which the perturbation method could be rather inaccurate.
We have presented a rigorous full vectorial mode solver based on the finite element method. Such a method allows us to study a generic magneto-optic and anisotropic lossy straight waveguide. After having computed the Rayleigh-Ritz functional for the non-self-adjoint case, the finite element method has been implemented using the node based second order shape functions. Furthermore, the penalty function has been introduced to remove the spurious solutions from the eigenvalue spectral region of interest.
Considering the discretized eigenvalue problem, the γ-formulation has been derived, avoiding the time-consuming iterations necessary in previous formulations for evaluating the mode propagation constants at a given operating frequency . This formulation allows speeding up simulations and improving the convergence of the eigenvalues at the same time. Moreover, all the modes and the propagation constants are computed at the same time and not one after the other as performed in the case of iterative approaches.
Concerning the magneto-optic materials the accuracy of the method has been compared with the perturbation method, the most used technique for investigating the nonreciprocal effect, providing satisfactory results. The mesh adaptivity of the finite element method allows us to efficiently compute the modes of graded index waveguides, like the anisotropic ion-implanted waveguides reported in , as well as the modes of high refractive index contrast slot waveguides [65,66]. The described mode solver can be effectively employed also to design microring-based optical isolators [49,67,68], where the mode of a bent waveguide with a large ring radius is approximated by a lateral shift of the straight waveguide mode [69, 70]. The presented approach provides then an important tool for integrated optical device modal analysis and design.
The author would like to thank Prof. Fabrizio Di Pasquale from Scuola Sant’Anna (Italy), Dr. Claudio Bonati from Istituto Nazionale di Fisica Nucleare in Pisa (Italy), Prof. Cornelis van der Mee, Prof. Giuseppe Rodriguez and Prof. Sebastiano Seatzu from University of Cagliari (Italy), Prof. John E. Bowers from University of California Santa Barbara (USA), and Dr. Ming-Chun (Jason) Tien from Intel for the useful discussions and their suggestions.
This work has been partially supported by the Tuscany Region through the project ARNO-T3 PAR FAS 2007–2013 and by the European Commission grant IRIS, project no. 619194 FP7-ICT.
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