An efficient iterative algorithm for computation of scattering from dielectric objects

doi:10.1364/OE.19.003304

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An efficient iterative algorithm for computation of scattering from dielectric objects

Shaolin Liao, N. Gopalsami, A. Venugopal, A. Heifetz, and A. C. Raptis »View Author Affiliations

Optics Express, Vol. 19, Issue 4, pp. 3304-3315 (2011)
http://dx.doi.org/10.1364/OE.19.003304

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Abstract

We have developed an efficient iterative algorithm for electromagnetic scattering of arbitrary but relatively smooth dielectric objects. The algorithm iteratively adapts the equivalent surface currents until the electromagnetic fields inside and outside the dielectric objects match the boundary conditions. Theoretical convergence is analyzed for two examples that solve scattering of plane waves incident upon air/dielectric slabs of semi-infinite and finite thicknesses. We applied the iterative algorithm for simulation of sinusoidally-perturbed dielectric slab on one side and the method converged for such unsmooth surfaces. We next simulated the shift in radiation pattern of a 6-inch dielectric lens for different offsets of the feed antenna on the focal plane. The result is compared to that of the Geometrical Optics (GO).

1. Introduction

Electromagnetic scattering of dielectric objects of arbitrary shape and size is difficult to simulate with reasonable speed and accuracy. There exist many computational methods such as Finite Difference Time Domain (FDTD) [1

A. Taflove and S. C. Hagness, Computational Electrodynamics: The Finite-Difference Time-Domain Method , 3rd. ed. (Artech House, 2005).

], Finite Element (FEM) [2

J. L. Volakis, L. C. Kempel, and A. Chatterjee, Finite Element Method Electromagnetics (IEEE Computer Society Press, 1998). [CrossRef]

], and Method of Moments (MoM) [3

R. F. Harrington, Field Computation by Moment Methods (Wiley IEEE Press, 1993). [CrossRef]

] which can provide accurate results, but the computation times become prohibitive for electrically large objects. For example, FDTD and FEM require generation of dense computational mesh of scattering object volume, with problem freedom or unknowns scaling as M³(we have assumed the number of mesh nodes M_x = M_y = M_z = M to be equal for all 3 dimensions). MOM is better in terms of the number of unknowns since it deals with boundary surface of the object instead of the volume of the object. However, MOM requires the inverse of the impedance matrix, which takes a computational complexity of N³ for N unknowns. The required memory scales as N². Although Multi-level Fast Multipole Method (MLFMM) can reduce the computational complexity to N log N [4

W. C. Chew, J. M. Jin, E. Michielssen, and J. Song, Fast and Efficient Algorithm in Computational Electromagnetics (Artech House Publisher, 2001).

], but its computational procedure is too complicated and still not so efficient for some ultra-large-scale object simulation. Other boundary element methods can only simulate some particular geometries efficiently [5

C. M. Kelso, P. D. Flammer, J. A. DeSanto, and R. T. Collins, “Integral equations applied to wave propagation in two dimensions: modeling the tip of a near-field scanning optical microscope,” J. Opt. Soc. Am. A 18(8), 1993–2001 (2001). [CrossRef]

–8

X. An and Z. Q. Lu, “An efficient finite element-boundary integral method solving electromagnetic scattering problems,” Microwave Opt. Technol. Lett. 51(9), 2065–2071 (2009). [CrossRef]

]. Finally, methods such as Physical Optics (PO) and Geometric Optics (GO) are generally faster but less accurate.

In this paper, we describe a new iterative algorithm based on finding the equivalent surface currents (both magnetic and electrical) on the dielectric surface. Fast and accurate convergence of the iterative algorithm is theoretically validated for plane wave incidence upon dielectric slabs of semi-infinite and finite thicknesses. We then applied the iterative algorithm on two examples: 1) sinusoidally-perturbed dielectric slab on one side and 2) a 6-inch dielectric lens with different offsets that we used for a passive millimeter wave radiometer [9

N. Gopalsami, S. Liao, E. R. Koehl, T. W. Elmer, A. Heifetz, H.-T. Chien, and A. C. Raptis, “Passive millimeter wave imaging and spectroscopy system for terrestrial remote sensing,” Proc. SPIE 7670, 767003 (2010). [CrossRef]

], both at the frequency of 150 GHz. Also, the radiation patterns for different offsets of the lens with respect to a horn antenna were obtained and compared with the GO results.

2. Equivalent Surface Current Model for Scattering Problem

Fig. 1 shows the simulation based on equivalent surface currents on an arbitrary dielctric surface. The equivalent surface currents are given by,

J_{s} = 2 \hat{n} \times H; M_{s} = - 2 \hat{n} \times E

(1)

where (E, H) are the total fields expressed by the sum of the incident (E_i, H_i) and scattered fields (E_sca, H_sca),

H = H_{i} + H_{sca}; E = E_{i} + E_{sca}

(2)

Fig. 1 The equivalent current approach to the scattering problem.

Now the potentials A and F are given by,

A = \int \int_{S} d s J_{s} \frac{{exp}^{- j k | r - r_{s} |}}{4 π | r - r_{s} |}

(3)

and

F = ɛ \int \int_{S} d s M_{s} \frac{{exp}^{- j k | r - r_{s} |}}{4 π | r - r_{s} |}

(4)

from which we can obtain the scattered electric and magnetic fields [10

C. A. Balanis, Advanced Engineering Electromagnetics , (John Wiley & Sons, 1989).

\begin{array}{l} E_{sca} = E_{A} + E_{F} \\ H_{sca} = H_{A} + H_{F} \end{array}

(5)

with the following definitions,

\begin{matrix} E_{F} (r) = - \frac{\nabla \times F (r)}{ɛ} \\ H_{F} (r) = - \frac{1}{j ω μ} \nabla \times E_{F} (r) = \frac{\nabla \times \nabla \times F (r)}{j ω μ ɛ} \end{matrix}

(6)

and

\begin{matrix} H_{A} (r) = \frac{\nabla \times A (r)}{μ} \\ E_{A} (r) = \frac{1}{j ω ɛ} \nabla \times H_{A} (r) = \frac{\nabla \times \nabla \times A (r)}{j ω μ ɛ} \end{matrix}

(7)

The calculation of the electromagnetic fields in Eq. (6) and Eq. (7) can be done efficiently through the Taylor-FFT algorithm developed by the author in [11

S. Liao and R. J. Vernon, “A fast algorithm for computation of electromagnetic wave propagation in half-space,” IEEE Trans. Antennas Propag. 57(7), 2068–2075 (2009). [CrossRef]

]. The problem now reduces to finding a way to obtain the equivalent surface currents (M_s, J_s), which is the main subject of this paper.

3. The Iterative Algorithm

The iterative algorithm aims at obtaining the equivalent surface currents J_s and M_s for the scattering problem. Fig. 1 shows that the total field of the scattering problem can be viewed as the sum of the incident and the scattered fields. One way to obtain the equivalent surface currents J_s and M_s is [12

S. B. Sorensen and K. Pontoppidan, Lens analysis methods for quasioptical systems , in The 2nd European Conference on Antennas and Propagation (EuCAP 2007), Edinburgh, UK, 11–16 Nov. 2007.

]: the scattered fields E_sca and H_sca outside the object are obtained when these surface currents radiate in a homogeneous medium ɛ₀; while the negative scattered fields inside the object E_o and H_o are obtained when the these surface currents −J_s and −M_s radiate in a homogeneous medium ɛ_r. In this way, J_s and M_s can be obtained as [12

S. B. Sorensen and K. Pontoppidan, Lens analysis methods for quasioptical systems , in The 2nd European Conference on Antennas and Propagation (EuCAP 2007), Edinburgh, UK, 11–16 Nov. 2007.

]

\begin{array}{r} J_{s} = \hat{n} \times H = \hat{n} \times [H_{i} + H_{sca}] = \hat{n} \times H_{o} \\ M_{s} = - \hat{n} \times E = - \hat{n} \times [E_{i} + E_{sca}] = - \hat{n} \times E_{o} \end{array}

(8)

The algorithm begins with initial guess of (J_s, M_s) through approximate incident fields E_i and H_i; then scattered fields (E_sca_,_k, H_sca_,_k) of the k^th iteration are calculated as follows,

\begin{matrix} E_{sca, k} = E_{sca, k - 1} + \hat{n} \times \frac{1}{Y} [H_{i, k} + H_{sca, k - 1} - H_{o, k - 1}] \\ H_{sca, k} = H_{sca, k - 1} - \hat{n} \times \frac{1}{Z} [E_{i, k} + E_{sca, k - 1} - E_{o, k - 1}] \end{matrix}

(9)

where

\bar{Y} = \frac{1}{2} [\frac{1}{η_{0}} + \frac{1}{η_{r}}]

\bar{Z} = \frac{1}{2} [η_{0} + η_{r}]

, with

η_{0} = \sqrt{\frac{μ_{0}}{ɛ_{0}}}

and

η_{r} = \sqrt{\frac{μ_{r}}{ɛ_{r}}}

being the wave impedances for vacuum and dielectric materials respectively; so we have the equivalent surface currents (J_s,k, M_s,k) of the k^th iteration according to Eq. (8),

\begin{array}{r} J_{s, k} = J_{s, k - 1} + \frac{1}{Z} [E_{i} + E_{sca, k - 1} - E_{o, k - 1}] \\ M_{s, k} = M_{s, k - 1} + \frac{1}{Y} [H_{i} + H_{sca, k - 1} - H_{o, k - 1}] \end{array}

(10)

The iteration continues until it converges for a given criterion. Fig. 2 shows the detailed procedures for the iterative algorithm, which are summarized below,

Make the initial guess of the total field (E₀, H₀): although there is no specific requirement of the initial guess, it is preferable to use reasonable value so that the iteration converges fast. The simplest guess is to begin with the incident field (E_i, H_i);
Update the equivalent surface currents (M_s,k, J_s,k) of the k^th iteration according to Eq. (10).
Update the scattered fields on both sides (inside and outside) of the dielectric object.
Correct the total field at k^th iteration (E_k, H_k) according to Eq. (2).
If the correction is small compared to some criterion, the iterative algorithm converges and go to step 6) below; otherwise, repeat step 2) to step 4) until the algorithm converges.
Calculate the far field pattern.

Fig. 2 The iterative algorithm implementation procedures (also see text for details).

4. Convergence Analysis of the Iterative Algorithm

Before we show the numerical results for the lens simulation, let us first give a general analysis of the convergence of the iterative algorithm and apply it to two basic examples: i.e., air/dielectric interface and dielectric slab with finite thickness. Both examples assume plane wave incidence.

4.1. General Analysis

Following Eq. (10), for plane wave incidence of arbitrary number of dielectric slabs, the iterative algorithm follows the correction procedure shown in Fig. 3, i.e., the k^th iterative equivalent surface currents M_s,k, J_s,k can be expressed using the previous ones,

[\begin{matrix} M_{s, k} \\ J_{s, k} \end{matrix}] = t [\begin{matrix} M_{i} \\ \frac{η_{0}}{η_{r}} J_{i} \end{matrix}] + \overset{=}{C} [\begin{matrix} M_{s, k - 1} \\ J_{s, k - 1} \end{matrix}]

(11)

where

\overset{=}{C} = [\begin{matrix} \overset{=}{α} & \overset{=}{β} \\ \overset{=}{θ} & \overset{=}{γ} \end{matrix}]; [\begin{matrix} M_{i} \\ J_{i} \end{matrix}] = [\begin{matrix} E_{i} \times \hat{n} \\ \hat{n} \times H_{i} \end{matrix}]

(12)

Fig. 3 The equivalent currents approach to the scattering problem.

From Eq. (11) we can obtain the convergence of the iterative algorithm by letting k → ∞,

\begin{matrix} [\begin{matrix} M_{s} \\ J_{s} \end{matrix}] = t \sum_{k = 0}^{\infty} {[\begin{matrix} \overset{=}{α} & \overset{=}{β} \\ \overset{=}{θ} & \overset{=}{γ} \end{matrix}]}^{k} [\begin{matrix} M_{i} \\ \frac{η_{0}}{η_{r}} J_{i} \end{matrix}] \\ + {[\begin{matrix} \overset{=}{α} & \overset{=}{β} \\ \overset{=}{θ} & \overset{=}{γ} \end{matrix}]}^{\infty} [\begin{matrix} M_{s, 1} \\ J_{s, 1} \end{matrix}] \end{matrix}

(13)

Eq. (13) can be solved by expressing the matrix in its eigenvalue-eigenvector form,

\begin{matrix} [\begin{matrix} \overset{=}{α} & \overset{=}{β} \\ \overset{=}{θ} & \overset{=}{γ} \end{matrix}] = \overset{=}{Q} [\begin{matrix} {\overset{=}{Λ}}_{1} & 0 \\ 0 & {\overset{=}{Λ}}_{2} \end{matrix}] {\overset{=}{Q}}^{- 1} \\ {[\begin{matrix} \overset{=}{α} & \overset{=}{β} \\ \overset{=}{θ} & \overset{=}{γ} \end{matrix}]}^{k} = \overset{=}{Q} [\begin{matrix} {\overset{=}{Λ}}_{1}^{k} & 0 \\ 0 & {\overset{=}{Λ}}_{2}^{k} \end{matrix}] {\overset{=}{Q}}^{- 1} \\ {[\begin{matrix} \overset{=}{α} & \overset{=}{β} \\ \overset{=}{θ} & \overset{=}{γ} \end{matrix}]}^{\infty} = \overset{=}{0} \end{matrix}

(14)

Eq. (13) converges if all the diagonal elements in matrices Λ̿_1,2 are smaller than unit 1, and the result is,

[\begin{matrix} M_{s} \\ J_{s} \end{matrix}] = t \overset{=}{Q} [\begin{matrix} \frac{1}{1 - {\overset{=}{Λ}}_{1}} & 0 \\ 0 & \frac{1}{1 - {\overset{=}{Λ}}_{2}} \end{matrix}] {\overset{=}{Q}}^{- 1} [\begin{matrix} M_{i} \\ \frac{η_{0}}{η_{r}} J_{i} \end{matrix}]

(15)

4.2. Semi-infinite dielectric slab

For a plane wave (assuming E_i = x̂E_i,x; H_i = ŷH_i,x) incident upon an air-dielectric (ɛ_r) interface, let us denote the total field of the (k − 1)^th iteration as (E_k₋₁, H_k₋₁) = (x̂E_k−1, ŷH_k−1). The equivalent surface currents are (we choose the surface normal to be n̂ = −ẑ),

\begin{array}{r} M_{s, k - 1} = - (- \hat{z}) \times E_{k - 1} = \hat{y} E_{k - 1}; \\ J_{s, k - 1} = (- \hat{z}) \times H_{k - 1} = \hat{x} H_{k - 1} \end{array}

(16)

Then the scattered field is given by,

\begin{array}{r} E_{sca, k} = \hat{x} [\frac{1}{2} E_{k - 1} - \frac{η_{0}}{2} H_{k - 1}]; \\ H_{sca, k} = \hat{y} [- \frac{1}{2 η_{0}} E_{k - 1} + \frac{1}{2} H_{k - 1}] \end{array}

(17)

\begin{array}{r} E_{o, k} = \hat{x} [\frac{1}{2} E_{k - 1} + \frac{η_{r}}{2} H_{k - 1}]; \\ H_{o, k} = \hat{y} [\frac{1}{2 η_{r}} E_{k - 1} + \frac{1}{2} H_{k - 1}] \end{array}

(18)

The updated equivalent surface currents are given by Eq. (11) and we find that α̿ = β̿ = θ̿ = γ̿ = 0, so

[\begin{array}{l} M_{s} \\ J_{s} \end{array}] = t [\begin{array}{l} M_{i} \\ \frac{η_{0}}{η_{r}} J_{i} \end{array}]

(19)

[\begin{matrix} E \\ H \end{matrix}] = t [\begin{array}{l} E_{i} \\ \frac{η_{0}}{η_{r}} H_{i} \end{array}]

(20)

as expected. Interestingly, for infinite dielectric slab, only one iteration is required to obtain the result.

4.3. Finite dielectric slab

Now let us extend the problem to the finite dielectric slab. In this case, we have surface currents on both left and right planes, with surface normals of n̂⁻ = −n̂⁺ = −ẑ(− and + denote the left and right sides respectively). Similarly, let us denote the total field of the (k − 1)^th iteration as follows

(E_{k - 1}^{\pm}, H_{k - 1}^{\pm}) = (\hat{x} E_{k - 1}^{\pm}, \hat{y} H_{k - 1}^{\pm})

. The equivalent surface currents are,

\begin{array}{r} M_{s, k - 1}^{\pm} = \mp \hat{y} E_{k - 1}; \\ J_{s, k - 1}^{\pm} = \mp \hat{x} H_{k - 1} \end{array}

(21)

Then the scattered field is given by,

\begin{array}{l} E_{sca, k}^{\pm} = \hat{x} [\frac{E_{k - 1}^{\pm}}{2} \pm \frac{η_{0}}{2} H_{k - 1}^{\pm} - (\frac{E_{k - 1}^{\mp}}{2} \pm \frac{η_{0}}{2} H_{k - 1}^{\mp}) {exp}^{\mp j k d}] \\ H_{sca, k}^{\pm} = \hat{y} [\pm \frac{E_{k - 1}^{\pm}}{2 η_{0}} + \frac{H_{k - 1}^{\pm}}{2} + (\mp \frac{E_{k - 1}^{\mp}}{2 η_{0}} - \frac{H_{k - 1}^{\mp}}{2}) {exp}^{\mp j k d}] \end{array}

(22)

\begin{array}{l} E_{o, k}^{\pm} = \hat{x} [\frac{E_{k - 1}^{\pm}}{2} \mp \frac{η_{r}}{2} H_{k - 1}^{\pm} + (\frac{E_{k - 1}^{\mp}}{2} \pm \frac{η_{r}}{2} H_{k - 1}^{\mp}) {exp}^{\mp j k_{r} d}] \\ H_{o, k}^{\pm} = \hat{y} [\mp \frac{E_{k - 1}^{\pm}}{2 η_{r}} + \frac{H_{k - 1}^{\pm}}{2} + (\pm \frac{E_{k - 1}^{\mp}}{2 η_{r}} + \frac{H_{k - 1}^{\mp}}{2}) {exp}^{\mp j k_{r} d}] \end{array}

(23)

where k and k_r are the wave vectors of the air and the dielectric.

From Eq. (10), we have,

[\begin{array}{l} M_{s, k}^{+} \\ J_{s, k}^{+} \\ M_{s, k}^{-} \\ J_{s, k}^{-} \end{array}] = t [\begin{array}{l} M_{i}^{+} \\ \frac{η_{0}}{η_{r}} J_{i}^{+} \\ M_{i}^{-} \\ \frac{η_{0}}{η_{r}} J_{i}^{-} \end{array}] + \overset{=}{C} [\begin{array}{l} M_{s, k - 1}^{+} \\ J_{s, k - 1}^{+} \\ M_{s, k - 1}^{-} \\ J_{s, k - 1}^{-} \end{array}]

(24)

where,

\overset{=}{C} = [\begin{matrix} 0 & 0 & - \frac{\frac{𝒫}{η_{0}} + \frac{𝒫_{r}}{η_{r}}}{2 \overset{=}{Y}} & - \frac{𝒫 + 𝒫_{r}}{2 \overset{=}{Y}} \\ 0 & 0 & - \frac{𝒫 + 𝒫_{r}}{2 \overset{=}{Z}} & - \frac{η_{0} 𝒫 + η_{r} 𝒫_{r}}{2 \overset{=}{Z}} \\ - \frac{\frac{𝒫}{η_{0}} + \frac{𝒫_{r}}{η_{r}}}{2 \overset{=}{Y}} & \frac{𝒫 + 𝒫_{r}}{2 \overset{=}{Y}} & 0 & 0 \\ \frac{𝒫 + 𝒫_{r}}{2 \overset{=}{Z}} & - \frac{η_{0} 𝒫 + η_{r} 𝒫_{r}}{2 \overset{=}{Z}} & 0 & 0 \end{matrix}]

The eigenvalues of matrix C̿ are,

[\begin{matrix} {\overset{=}{Λ}}_{1} & 0 \\ 0 & {\overset{=}{Λ}}_{2} \end{matrix}] = [\begin{matrix} r 𝒫_{r} & 0 & 0 & 0 \\ 0 & - r 𝒫_{r} & 0 & 0 \\ 0 & 0 & - r 𝒫 & 0 \\ 0 & 0 & 0 & r 𝒫 \end{matrix}]

(25)

where

r = \frac{η_{r} - η}{η_{r} + η}

is the reflection coefficient; 𝒫 = exp^−jkd and 𝒫_r = exp^−jk_rd. Since |r𝒫_r| = |r𝒫| ≤ 1, the iterative algorithm converges according to Eq. (15).

5. Numerical Results

We applied the iterative algorithm on two examples: 1) Gaussian beam propagation through a dielectric slab with sinusoidal shape on one side and 2) dielectric lens simulation with different offsets of feed horn on the focal plane.

5.1. Dielectric slab with sinusoidal shape on one side

Fig. 4 shows the schematic of the simulation: a Gaussian beam source is 1” away from the dielectric slab,

E_{x} (x, y) = {exp}^{- \frac{x^{2} + y^{2}}{w^{2}}}

(26)

with w = 5λ = 10 mm in the simulation. The sinusoidal shape is given by

z (x, y) = 10 λ + λ cos (\frac{k}{40} x) cos (\frac{k}{40} y)

(27)

Fig. 4 The application of iterative algorithm on dielectric slab with sinusoidal shape on one side.

Fig. 5 shows the convergence of tangential fields along x-direction on the sinusoidal surface of the dielectric slab after 7 iterations. As can be seen from the Fig. 5, the accuracy is up to −120 dB, good enough for most applications.

Fig. 5 E_// and H_// of outside (air) and inside (dielectric) on the sinusoidal surface of the dielectric slab along x-direction after 7 iterations. Both amplitude and real part are shown.

5.2. Dielectric lens simulation

Lens simulation is important to obtain accurate radiation pattern [13

J. P. Thakur, W.-G. Kim, and Y.-H. Kim, “Large aperture low aberration aspheric dielectric lens antenna for W-band quasi-optics,” PIER 103, 57–65 (2010). [CrossRef]

–15

A. V. Boriskin, G. Godi, R. Sauleau, and A. I. Nosich, “Small hemielliptic dielectric lens antenna analysis in 2-D: boundary integral equations versus geometrical and physical optics,” IEEE Trans. Antennas Propag. 56, 485–492 (2008). [CrossRef]

]. We apply the iterative algorithm on the simulation of a 6-inch dielectric lens at the frequency of 150 GHz for feed antenna at different offsets on the focal plane (see Fig. 6). The antenna pattern is approximated by a Gaussian beam as,

E_{x} (x, y) = {exp}^{- \frac{{(x - x_{o f f})}^{2} + y^{2}}{w^{2}}}

(28)

where x_off is the antenna offset.

Fig. 6 The application of iterative algorithm on horn-lens radiation pattern simulation.

The simulated lens is a thin spherical lens given by,

z (x, y) = \sqrt{R^{2} - x^{2} - y^{2}} - (R - h)

(29)

where R = 3.4″ is the radius of the lens and h = 1.8″ is the thickness of the lens. The focal length F is given by the lens maker’s formula,

F \sim \frac{R}{n - 1} = 2 R = {6.8}^{″}

(30)

with n = 1.5 for our simulation.

Fig. 7 shows the tangential component E_// of the electric field and the tangential component H_// of the magnetic field inside and outside the dielectric surface after the algorithm runs for 7 iterations. The corresponding radiation patterns are shown in Fig. 8, together with the GO results for comparison. The GO result is obtained by using Snell’s law and Fresnel’s law on the plane surface of the lens to obtain the field inside the lens; the obtained field then is propagated to the convex surface of the lens, where again the Snell’s law and Fresnel’s law are applied to obtain the transmitted field outside the lens.

Fig. 7 E_// and H_// of outside (air) and inside (dielectric) on the convex surface of the dielectric lens along x-direction after 7 iterations. Only amplitude is shown.

Fig. 8 Radiation patterns for different antenna offsets along x-direction: from 0λ to 4λ, 1λ increment: lines are results from iterative algorithm and dots are GO results.

6. Discussion

We compared the efficiency of our algorithm in terms of the memory costs and computational time to different computational electromagnetics methods, with result shown in Table 1. Without loss of generality, we chose methods of FDTD, FEM and MOM using MLFMM (MOM-MLFMM) for comparison. In Table 1, we have assumed the dimension sizes are equal in all 3 dimension, L_x = L_y = L_z = L, which also means the numbers of mesh cells are the same M_x = M_y = M_z = M. We also assumed that the boundary surface of the object is proportional to the square of dimension, i.e., S ∝ L² (e.g., a simple cube). It is straight forward that the memory for FDTD and FEM is at the order of 𝒪(M³) [1

A. Taflove and S. C. Hagness, Computational Electrodynamics: The Finite-Difference Time-Domain Method , 3rd. ed. (Artech House, 2005).

, 2

J. L. Volakis, L. C. Kempel, and A. Chatterjee, Finite Element Method Electromagnetics (IEEE Computer Society Press, 1998). [CrossRef]

]. The computation time for FDTD scales as 𝒪(M³ × M) = 𝒪(M⁴) since it takes ∝ M time step to propagate a pulse through the scattering object of length L [1

A. Taflove and S. C. Hagness, Computational Electrodynamics: The Finite-Difference Time-Domain Method , 3rd. ed. (Artech House, 2005).

]. The computation time for FEM using an iterative solver can be as low as 𝒪(M³) [2

J. L. Volakis, L. C. Kempel, and A. Chatterjee, Finite Element Method Electromagnetics (IEEE Computer Society Press, 1998). [CrossRef]

]. It is noted that the FDTD computation time is M times as much as that of the FEM. This is because FDTD can give 𝒪(M) frequency points after the time domain has been transformed to the frequency domain using FFT, while FEM only gives one frequency point. The memory for MOM-MLFMM can achieve the order of 𝒪(M² log M²) and the computation time using MLFMM scales as 𝒪(M² log M²) [4

W. C. Chew, J. M. Jin, E. Michielssen, and J. Song, Fast and Efficient Algorithm in Computational Electromagnetics (Artech House Publisher, 2001).

]. At last, the memory for our algorithm is 𝒪(M²). The computation time is 𝒪(M² log M²) when fast field propagation method like FFT-based method [11

S. Liao and R. J. Vernon, “A fast algorithm for computation of electromagnetic wave propagation in half-space,” IEEE Trans. Antennas Propag. 57(7), 2068–2075 (2009). [CrossRef]

] is used. Now let’s apply the above analysis to our two examples. In example 1, L_x = L_y = 127λ, L_z = 11λ. In FDTD and FEM, assuming the discretization size ΔL = 1/10λ, we have, M_x = M_y = 1270, M_z = 110. The memory for both FDTD and FEM is 𝒪(M_xM_yM_z) = 𝒪(1.8 × 10⁸). The computation time for FDTD and FEM are 𝒪(M_xM_yM_z × max (M_x, M_y, M_z)) = 𝒪(2.3 × 10¹¹) and 𝒪(M_xM_yM_z) = 𝒪(1.8 × 10⁸) respectively. The memory and computation time for MOM-MLFMM are the same, i.e., 𝒪(M_xM_y log (M_xM_y)) = 𝒪(2.3 × 10⁷). The memory and computation time for our algorithm are 𝒪(M_xM_y) = 𝒪(1.6 × 10⁶) and 𝒪(M_xM_y log(M_xM_y)) = 𝒪(2.3 × 10⁷) respectively. In example 2, we used the following discretization: M_x = M_y = 760, M_z = 230 for ΔL = 1/10λ. The result is shown in Table 1. Obviously, our algorithm is more efficient in memory requirement than or at least as efficient as other methods used here for comparison. In terms of computation time, our algorithm is at least as efficient as MOM-MLFMM (only a few iterations are needed: 7 in both of our examples) and is much simpler to be implemented.

Table 1 Memory and Computation Time (C. T.) for Different Methods

Methods	FDTD	Iterative FEM	MOM-MLFMM	our algorithm

Memory	M³	M³	M² log M²	M²
C. T.	M⁴	M³	M² log M²	M² log M²
Example 1: Memory	𝒪 (1.8 × 10⁸)	𝒪 (1.8 × 10⁸)	𝒪 (2.3 × 10⁷)	𝒪 (1.6 × 10⁶)
Example 1: C. T.	𝒪 (2.3 × 10¹¹⁾	𝒪 (1.8 × 10⁸)	𝒪 (2.3 × 10⁷)	𝒪 (2.3 × 10⁷)
Example 2: Memory	𝒪 (1.3 × 10⁸)	𝒪 (1.3 × 10⁸)	𝒪 (7.7 × 10⁶)	𝒪 (5.8 × 10⁵)
Example 2: C. T.	𝒪 (1.0 × 10¹¹)	𝒪 (1.3 × 10⁸)	𝒪 (7.7 × 10⁶)	𝒪 (7.7 × 10⁶)

In updating the surface currents (M_s, J_s) in Eq. (10), we have implicitly used the geometric optics approximation, which means our algorithm is more efficient for relatively smooth surface, i.e., the smoother the surface, the fewer the iterations it takes to converge. The rule of thumb of this is that good convergence could be obtained if each of the principal radius of curvature of the object surface is greater than a few wavelengths [16

S. Liao and R. J. Vernon, “On the image approximation for electromagnetic wave propagation and PEC scattering in cylindrical harmonics,” Prog. Electromagn. Res. 66, 65–88 (2006). [CrossRef]

7. Conclusion

We have developed an efficient iterative algorithm to simulate the electromagnetic scattering by dielectric objects of different shapes, including a dielectric lens. The new algorithm iteratively adapts the equivalent surface currents to match the boundary conditions inside and outside the dielectric objects. The iterative algorithm converges after just a few iterations. Theoretical convergence analysis has been done for plane wave incidence upon dielectric slab. The algorithm has been applied to two examples, i.e., dielectric slab with perturbation on one side and a 6-inch dielectric lens with different offsets of a feed antenna on the focal plane. A comparison has been made among our algorithm and other commonly used computational electromagnetics methods, which shows that our algorithm is more efficient or at least as efficient as the state-of-the-art method like MOM-MLFMM and our algorithm is straightforward and simpler to be implemented.

References and links

1.	A. Taflove and S. C. Hagness, Computational Electrodynamics: The Finite-Difference Time-Domain Method , 3rd. ed. (Artech House, 2005).
2.	J. L. Volakis, L. C. Kempel, and A. Chatterjee, Finite Element Method Electromagnetics (IEEE Computer Society Press, 1998). [CrossRef]
3.	R. F. Harrington, Field Computation by Moment Methods (Wiley IEEE Press, 1993). [CrossRef]
4.	W. C. Chew, J. M. Jin, E. Michielssen, and J. Song, Fast and Efficient Algorithm in Computational Electromagnetics (Artech House Publisher, 2001).
5.	C. M. Kelso, P. D. Flammer, J. A. DeSanto, and R. T. Collins, “Integral equations applied to wave propagation in two dimensions: modeling the tip of a near-field scanning optical microscope,” J. Opt. Soc. Am. A 18(8), 1993–2001 (2001). [CrossRef]
6.	Q. H. Liu, Y. Lin, J. Liu, J. H. Lee, and E. Simsek, “A 3-D spectral integral method (SIM) for surface integral equations,” IEEE Microw. Wirel. Compon. Lett. 19(2), 62–64 (2009). [CrossRef]
7.	M. Y. Xia, C. H. Chan, S. Q. Li, B. Zhang, and L. Tsang, “An efficient algorithm for electromagnetic scattering from rough surfaces using a single integral equation and multilevel sparse-matrix canonical-grid method,” IEEE Trans. Antennas Propag. 51(6), 1142–1149 (2003). [CrossRef]
8.	X. An and Z. Q. Lu, “An efficient finite element-boundary integral method solving electromagnetic scattering problems,” Microwave Opt. Technol. Lett. 51(9), 2065–2071 (2009). [CrossRef]
9.	N. Gopalsami, S. Liao, E. R. Koehl, T. W. Elmer, A. Heifetz, H.-T. Chien, and A. C. Raptis, “Passive millimeter wave imaging and spectroscopy system for terrestrial remote sensing,” Proc. SPIE 7670, 767003 (2010). [CrossRef]
10.	C. A. Balanis, Advanced Engineering Electromagnetics , (John Wiley & Sons, 1989).
11.	S. Liao and R. J. Vernon, “A fast algorithm for computation of electromagnetic wave propagation in half-space,” IEEE Trans. Antennas Propag. 57(7), 2068–2075 (2009). [CrossRef]
12.	S. B. Sorensen and K. Pontoppidan, Lens analysis methods for quasioptical systems , in The 2nd European Conference on Antennas and Propagation (EuCAP 2007), Edinburgh, UK, 11–16 Nov. 2007.
13.	J. P. Thakur, W.-G. Kim, and Y.-H. Kim, “Large aperture low aberration aspheric dielectric lens antenna for W-band quasi-optics,” PIER 103, 57–65 (2010). [CrossRef]
14.	Z. X. Wang and W. B. Dou, “Full-wave analysis of monopulse dielectric lens antennas at W-band,” Int. J. Infrared Millim. Waves 31, 151–161 (2010).
15.	A. V. Boriskin, G. Godi, R. Sauleau, and A. I. Nosich, “Small hemielliptic dielectric lens antenna analysis in 2-D: boundary integral equations versus geometrical and physical optics,” IEEE Trans. Antennas Propag. 56, 485–492 (2008). [CrossRef]
16.	S. Liao and R. J. Vernon, “On the image approximation for electromagnetic wave propagation and PEC scattering in cylindrical harmonics,” Prog. Electromagn. Res. 66, 65–88 (2006). [CrossRef]

OCIS Codes
(080.0080) Geometric optics : Geometric optics
(290.0290) Scattering : Scattering

ToC Category:
Scattering

History
Original Manuscript: November 22, 2010
Revised Manuscript: January 3, 2011
Manuscript Accepted: January 12, 2011
Published: February 4, 2011

Citation
Shaolin Liao, N. Gopalsami, A. Venugopal, A. Heifetz, and A. C. Raptis, "An efficient iterative algorithm for computation of scattering from dielectric objects," Opt. Express 19, 3304-3315 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-4-3304

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References

A. Taflove, and S. C. Hagness, Computational Electrodynamics: The Finite-Difference Time-Domain Method, 3rd. ed. (Artech House, 2005).
J. L. Volakis, L. C. Kempel, and A. Chatterjee, Finite ElementMethod Electromagnetics (IEEE Computer Society Press, 1998). [CrossRef]
R. F. Harrington, Field Computation by Moment Methods (Wiley IEEE Press, 1993). [CrossRef]
W. C. Chew, J. M. Jin, E. Michielssen, and J. Song, Fast and Efficient Algorithm in Computational Electromagnetics (Artech House Publisher, 2001).
C. M. Kelso, P. D. Flammer, J. A. DeSanto, and R. T. Collins, “Integral equations applied to wave propagation in two dimensions: modeling the tip of a near-field scanning optical microscope,” J. Opt. Soc. Am. A 18(8), 1993–2001 (2001). [CrossRef]
Q. H. Liu, Y. Lin, J. Liu, J. H. Lee, and E. Simsek, “A 3-D spectral integral method (SIM) for surface integral equations,” IEEE Microw. Wirel. Compon. Lett. 19(2), 62–64 (2009). [CrossRef]
M. Y. Xia, C. H. Chan, S. Q. Li, B. Zhang, and L. Tsang, “An efficient algorithm for electromagnetic scattering from rough surfaces using a single integral equation and multilevel sparse-matrix canonical-grid method,” IEEE Trans. Antenn. Propag. 51(6), 1142–1149 (2003). [CrossRef]
X. An, and Z. Q. Lu, “An efficient finite element-boundary integral method solving electromagnetic scattering problems,” Microw. Opt. Technol. Lett. 51(9), 2065–2071 (2009). [CrossRef]
N. Gopalsami, S. Liao, E. R. Koehl, T. W. Elmer, A. Heifetz, H.-T. Chien, and A. C. Raptis, “Passive millimeter wave imaging and spectroscopy system for terrestrial remote sensing,” Proc. SPIE 7670, 767003 (2010). [CrossRef]
C. A. Balanis, Advanced Engineering Electromagnetics, (John Wiley & Sons, 1989).
S. Liao, and R. J. Vernon, “A fast algorithm for computation of electromagnetic wave propagation in half-space,” IEEE Trans. Antenn. Propag. 57(7), 2068–2075 (2009). [CrossRef]
S. B. Sorensen, and K. Pontoppidan, Lens analysis methods for quasioptical systems, in The 2nd European Conference on Antennas and Propagation (EuCAP 2007), Edinburgh, UK, 11–16 Nov. 2007.
J. P. Thakur, W.-G. Kim, and Y.-H. Kim, “Large aperture low aberration aspheric dielectric lens antenna for W-band quasi-optics,” PIER 103, 57–65 (2010). [CrossRef]
Z. X. Wang, and W. B. Dou, “Full-wave analysis of monopulse dielectric lens antennas atW-band,” Int. J. Infrared Millim. Waves 31, 151–161 (2010).
A. V. Boriskin, G. Godi, R. Sauleau, and A. I. Nosich, “Small hemielliptic dielectric lens antenna analysis in 2-D: boundary integral equations versus geometrical and physical optics,” IEEE Trans. Antenn. Propag. 56, 485–492 (2008). [CrossRef]
S. Liao, and R. J. Vernon, “On the image approximation for electromagnetic wave propagation and PEC scattering in cylindrical harmonics,” Prog. Electromagn. Res. 66, 65–88 (2006). [CrossRef]

An, X.
Boriskin, A. V.
Chan, C. H.
Chien, H.-T.
Collins, R. T.
DeSanto, J. A.
Dou, W. B.
Elmer, T. W.
Flammer, P. D.
Godi, G.
Gopalsami, N.
Heifetz, A.
Kelso, C. M.
Kim, W.-G.
Kim, Y.-H.
Koehl, E. R.
Lee, J. H.
Li, S. Q.
Liao, S.
Lin, Y.
Liu, J.
Liu, Q. H.
Lu, Z. Q.
Nosich, A. I.
Raptis, A. C.
Sauleau, R.
Simsek, E.
Thakur, J. P.
Tsang, L.
Vernon, R. J.
Wang, Z. X.
Xia, M. Y.
Zhang, B.

IEEE Microw. Wirel. Compon. Lett.

Q. H. Liu, Y. Lin, J. Liu, J. H. Lee, and E. Simsek, “A 3-D spectral integral method (SIM) for surface integral equations,” IEEE Microw. Wirel. Compon. Lett. 19(2), 62–64 (2009). [CrossRef]

IEEE Trans. Antenn. Propag.

M. Y. Xia, C. H. Chan, S. Q. Li, B. Zhang, and L. Tsang, “An efficient algorithm for electromagnetic scattering from rough surfaces using a single integral equation and multilevel sparse-matrix canonical-grid method,” IEEE Trans. Antenn. Propag. 51(6), 1142–1149 (2003). [CrossRef]
S. Liao, and R. J. Vernon, “A fast algorithm for computation of electromagnetic wave propagation in half-space,” IEEE Trans. Antenn. Propag. 57(7), 2068–2075 (2009). [CrossRef]
A. V. Boriskin, G. Godi, R. Sauleau, and A. I. Nosich, “Small hemielliptic dielectric lens antenna analysis in 2-D: boundary integral equations versus geometrical and physical optics,” IEEE Trans. Antenn. Propag. 56, 485–492 (2008). [CrossRef]

Int. J. Infrared Millim. Waves

Z. X. Wang, and W. B. Dou, “Full-wave analysis of monopulse dielectric lens antennas atW-band,” Int. J. Infrared Millim. Waves 31, 151–161 (2010).

J. Opt. Soc. Am. A

C. M. Kelso, P. D. Flammer, J. A. DeSanto, and R. T. Collins, “Integral equations applied to wave propagation in two dimensions: modeling the tip of a near-field scanning optical microscope,” J. Opt. Soc. Am. A 18(8), 1993–2001 (2001). [CrossRef]

Microw. Opt. Technol. Lett.

X. An, and Z. Q. Lu, “An efficient finite element-boundary integral method solving electromagnetic scattering problems,” Microw. Opt. Technol. Lett. 51(9), 2065–2071 (2009). [CrossRef]

PIER

J. P. Thakur, W.-G. Kim, and Y.-H. Kim, “Large aperture low aberration aspheric dielectric lens antenna for W-band quasi-optics,” PIER 103, 57–65 (2010). [CrossRef]

Proc. SPIE

N. Gopalsami, S. Liao, E. R. Koehl, T. W. Elmer, A. Heifetz, H.-T. Chien, and A. C. Raptis, “Passive millimeter wave imaging and spectroscopy system for terrestrial remote sensing,” Proc. SPIE 7670, 767003 (2010). [CrossRef]

Prog. Electromagn. Res.

S. Liao, and R. J. Vernon, “On the image approximation for electromagnetic wave propagation and PEC scattering in cylindrical harmonics,” Prog. Electromagn. Res. 66, 65–88 (2006). [CrossRef]

Other

S. B. Sorensen, and K. Pontoppidan, Lens analysis methods for quasioptical systems, in The 2nd European Conference on Antennas and Propagation (EuCAP 2007), Edinburgh, UK, 11–16 Nov. 2007.
C. A. Balanis, Advanced Engineering Electromagnetics, (John Wiley & Sons, 1989).
A. Taflove, and S. C. Hagness, Computational Electrodynamics: The Finite-Difference Time-Domain Method, 3rd. ed. (Artech House, 2005).
J. L. Volakis, L. C. Kempel, and A. Chatterjee, Finite ElementMethod Electromagnetics (IEEE Computer Society Press, 1998). [CrossRef]
R. F. Harrington, Field Computation by Moment Methods (Wiley IEEE Press, 1993). [CrossRef]
W. C. Chew, J. M. Jin, E. Michielssen, and J. Song, Fast and Efficient Algorithm in Computational Electromagnetics (Artech House Publisher, 2001).

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