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8/25/2019
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Advanced Computation:Computational Electromagnetics
Beam Propagation Method (BPM)
Outline
• Overview• Formulation of 2D finite‐difference beam propagation method (FD‐BPM)
• Implementation of 2D FD‐BPM• Formulation of 3D FD‐BPM• Alternative formulations of BPM
• FFT‐BPM• Wide Angle FD‐BPM• Bi‐Directional BPM
Slide 2
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Slide 3
Overview
Geometry of BPM
Slide 4
z
x
longitudinal directionTran
sverse
direction
BPM is primarily a “forward” propagating algorithm where the dominant direction of propagation is longitudinal.
The grid is computed and interpreted as it is in FDFD. The algorithm and implementation looks more like the method of lines than it does FDFD.
BPM is implemented on a grid. There are no layers.
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Example Simulation of a Coupled‐Line Filter
Slide 5
This animation is NOT of the wave propagating through the device. Instead, it is the sequence of how the solution is calculated.
Slide 6
Formulation of 2D Finite‐Difference
Beam Propagation Method
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Starting Point
Slide 7
We start with Maxwell’s equations in the following form.
yzxx x
x zyy y
y xzz z
EE Hy zE E Hz xE E Hx y
yzxx x
x zyy y
y xzz z
HH Ey zH H Ez xH H Ex y
Recall that we have normalized the grid according to
0x k x 0y k y 0z k z
Recall that the material properties potentially incorporate a PML at the x and y axis boundaries (propagation along z).
y xxx xx yy yy zz zz x yx y
s s s ss s
y xxx xx yy yy zz zz x yx y
s s s ss s
Reduction to Two Dimensions
Slide 8
Assuming the device is uniform along the y direction,
zEy
yxx x
x zyy y
y x
EH
zE E Hz xE Ex y
zz zH
zHy
yxx x
x zyy y
y x
HE
z
H H Ez xH Hx y
zz zE
0y
and Maxwell’s equations reduce to
yxx x
x zyy y
yzz z
EH
zE E Hz x
EH
x
yxx x
x zyy y
yzz z
HE
zH H Ez x
HE
x
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Two Distinct Modes
Slide 9
We see that Maxwell’s equations have decoupled into two distinct modes.
E Mode
x zyy y
yxx x
yzz z
H H Ez x
EH
zE
Hx
H Modex z
yy y
yxx x
yzz z
E E Hz x
HE
zH
Ex
Slowly Varying Envelope Approximation
Slide 10
Assuming the field is not changing rapidly, we can write the field as
eff eff, , , ,jn z jn zE x z x z e H x z x z e
Substituting these solutions into our two sets of equations yieldsE Mode
eff eff eff
eff eff
eff eff
, , ,
, ,
, ,
jn z jn z jn zx z yy y
jn z jn zy xx x
jn z jn zy zz z
x z e x z e x z ez x
x z e x z ez
x z e x z ex
H Mode
eff eff eff
eff eff
eff eff
, , ,
, ,
, ,
jn z jn z jn zx z yy y
jn z jn zy xx x
jn z jn zy zz z
x z e x z e x z ez x
x z e x z ez
x z e x z ex
eff
eff
x zx yy y
yy xx x
yzz z
jnz x
jnz
x
eff
eff
x zx yy y
yy xx x
yzz z
jnz x
jnz
x
Not a good approximation to make for metamaterials and photonic crystals!
Better for lenses, waveguides, etc.
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Matrix Form of Differential Equations
Slide 11
Each of these equations is written once for every point in the grid. This large set of equations can be written in matrix form as
E Mode H Modeeff
eff
x zx yy y
yy xx x
yzz z
jnz x
jnz
x
eff
eff
hxx x z yy y
yy xx x
ex y zz z
djndz
djn
dz
hh D h ε e
ee μ h
D e μ h
eff
eff
x zx yy y
yy xx x
yzz z
jnz x
jnz
x
eff
eff
exx x z yy y
yy xx x
hx y zz z
djndz
djn
dz
ee D e μ h
hh ε e
D h ε e
Wave Equation for E‐Mode
Slide 12
We can reduce the set of three equations to a single equation. This is the matrix wave equation.
eff
eff
Hx yy y
yy xx x
Ex y zz
xz
z
xdjndz
djn
dz
D ε e
ee μ h
D e μ
h h
h
h
eff
21 2
eff eff2
11 1eff eff
2 0
H Ezz xx yy y
y y H Exx x zz x y xx
yxx y xx y
yy y
djndz
d dj n n
d djn jn
dz d
dz d
z
z
μ DD εe e
e eμ D μ D e μ ε I
μ I e
e
μ I e
1
1effxx
zz x
y
Ez
x
y
jnz
μ D
μ I
h
e
e
h
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Small Angle Approximation (1 of 2)
Slide 13
When this is the case, we can make the “small angle approximation.”2
2y yd d
dz dz
e e
also called the Fresnel approximation
In many cases, the envelope of the electromagnetic field does not change rapidly as it propagates.
Rapid Variation Slow Variation
Small Angle Approximation (2 of 2)
Slide 14
Given the small angle approximation2
2y yd d
dz dz
e e
We can drop the term from our wave equation.2
2y
z
e
2
2yd
dze
1 2eff eff
1 2eff eff
1 2eff
eff
2 0
2 0
2
y H Exx x zz x y xx yy y
y H Exx x zz x y xx yy y
y H Exx x zz x xx yy y
dj n n
dz
dj n n
dz
d j ndz n
eμ D μ D e μ ε I e
eμ D μ D e μ ε I e
eμ D μ D μ ε I e
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Different Finite‐Difference Solutions
Slide 15
How do we approximate the z‐derivative with a finite‐difference?
First, we write our matrix wave equation more compactly as1 2
effeff
2
y H Ey xx x zz x xx yy
d j ndz n
e
Ae A μ D μ D μ ε I
1
eff1
11
eff1 1
1
eff
Forward Euler2
Backward Euler2
Crank-Nicolson2 2
i iy y i
i y
i iy y i
i y
i i i iy y i y i y
jz n
jz n
jz n
e eA e
e eA e
e e A e A e
This is a standard finite‐difference evaluated at i+0.5
This term is interpolated at i+0.5.
Computationally simpler, but solution is nonlinear and only first‐order accurate at best. Can be unstable.
The Crank‐Nicolson method is second‐order accurate and unconditionally stable.
Forward Step Equation
Slide 16
1 11
eff
11
1eff eff
2 2
4 4
i i i iy y i y i y
i iy i i y
jz n
j z j zn n
e e A e A e
e I A I A e
Solving our new equation for leads to1iye
We now have a way of calculating the field in a following slice based only the field in the previous slice.
Backward waves are ignored in this formulation.
Note: We need to make a good guess for the value of neff.
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Slide 17
Implementationof 2D FD‐BPM
Grid Schemes for BPM
Slide 18
Periodic Structures Finite Structures
PBCPBC
No BC Needed!
No BC Needed!
No BC Needed!
No BC Needed!
DirichletBCDiric
hlet
BC
PMLPM
L
Spacer
Spacer
No spacer needed!
No spacer needed!No spacerneeded!
No spacerneeded!
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The Effective Refractive Index, neff
Slide 19
Recall the slowly varying envelop approximation.
eff eff, , , ,jn z jn zE x z x z e H x z x z e
The BPM does not calculate neff. We must tell BPM what is neff.
How do we know neff without modeling the device? We have to calculate it or estimate it.
Techniques:
1. For plane waves and beams, calculate the average refractive index in the cross section of your grid to estimate the longitudinal wave vector.
2. For waveguide problems, calculate the effective index of your guided mode rigorously and use that in BPM.
Compute field in next plane
1 1i i i i e P e
Compute source at first plane
Ey(:,1) = ?;
Compute matrix derivative operators
and e hx xD D
Build deviceon grid
Calculate optimized grid
Block Diagram of BPM
Slide 20
Define simulation parameters
Compute propagator P
1
1 1eff eff
1 2eff
4 4i i i i
i i i ih ei xx x zz x xx yy
j z j zn n
n
P I A I A
A μ D μ D μ ε I
Post processDone?
Loop over z
BPM can handle modes, beams, plane waves, etc., very easily.
Extract slice & diagonalize. and i iμ ε
yes
no
Chooseneff
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Slide 21
Formulation of 3D Finite‐Difference
Beam Propagation Method
Starting Point
Slide 22
We have the same starting point as with 2D FD‐BPM.yz
xx x
x zyy y
y xzz z
EE Hy zE E Hz xE E Hx y
yzxx x
x zyy y
y xzz z
HH Ey zH H Ez xH H Ex y
Recall that we have normalized the grid according to
0x k x 0y k y 0z k z
Recall that the material properties potentially incorporate a PML at the x and y axis boundaries (propagation along z).
y xxx xx yy yy zz zz x yx y
s s s ss s
y xxx xx yy yy zz zz x yx y
s s s ss s
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Slowly Varying Envelope Approximation
Slide 23
Assuming the field is not changing rapidly, we can write the field as
eff eff, , , ,jn z jn zE x z x z e H x z x z e
Maxwell’s equations become
Not a good approximation to make for metamaterials and photonic crystals!
Good for lenses, waveguides, etc.
eff
eff
yzy xx x
x zx yy y
y xzz z
jny z
jnz x
x y
eff
eff
yzy xx x
x zx yy y
y xzz z
jny z
jnz x
x y
yzxx x
x zyy y
y xzz z
EE Hy zE E Hz xE E Hx y
yzxx x
x zyy y
y xzz z
HH Ey zH H Ez xH H Ex y
Matrix Form of Differential Equations
Slide 24
Each of these equations is written once for every point in the grid. This large set of equations can be written in matrix form as
eff
eff
yey z y xx x
exx x z yy y
e ex y y x zz z
djn
dzdjndz
eD e e μ h
ee D e μ h
D e D e μ h
eff
eff
yhy z y xx x
hxx x z yy y
h hx y y x zz z
djn
dzdjndz
hD h h ε e
hh D h ε e
D h D h ε e
eff
eff
yzy xx x
x zx yy y
y xzz z
jny z
jnz x
x y
eff
eff
yzy xx x
x zx yy y
y xzz z
jny z
jnz x
x y
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Eliminate Longitudinal Components
Slide 25
We solve the third equation in each set for the longitudinal components ez and hz.
1 e ez zz x y y x h μ D e D e 1 h hz zz x y y x e ε D h D h
We now substitute these expressions into the remaining equations.
1eff
1eff
ye h hy zz x y y x y xx x
e h hxx x zz x y y x yy y
djn
dzdjndz
eD ε D h D h e μ h
ee D ε D h D h μ h
1eff
1eff
yh e ey zz x y y x y xx x
h e exx x zz x y y x yy y
djn
dzdjndz
hD μ D e D e h ε e
hh D μ D e D e ε e
Rearrange Terms
Slide 26
We rearrange or remaining finite‐difference equations and collect the common terms.
1 1eff
1 1eff
e h e hxx x zz y x yy x zz x y
y e h e hy xx y zz y x y zz x y
d jndzd
jndz
e e D ε D h μ D ε D h
ee μ D ε D h D ε D h
1 1eff
1 1eff
h e h exx zz y x yy x zz x y x
y h e h exx y zz y x y zz x y y
d jndzd
jndz
h D μ D e ε D μ D e h
hε D μ D e D μ D e h
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Block Matrix Form
Slide 27
We can now cast these four matrix equations into two block matrix equations.
1 1eff
1 1eff
e h e hxx x zz y x yy x zz x y
y e h e hy xx y zz y x y zz x y
d jndzd
jndz
e e D ε D h μ D ε D h
ee μ D ε D h D ε D h
1 1eff
1 1eff
h e h exx zz y x yy x zz x y x
y h e h exx y zz y x y zz x y y
d jndzd
jndz
h D μ D e ε D μ D e h
hε D μ D e D μ D e h
1 1
eff 1 1 e h e hx zz y yy x zz xx x x
e h e hy y y xx y zz y y zz x
d jndz
D ε D μ D ε De e hP Pe e h μ D ε D D ε D
1 1
eff 1 1 h e h ex zz y yy x zz xx x x
h e h ey y y xx y zz y y zz x
d jndz
D μ D ε D μ Dh h eQ Qh h e ε D μ D D μ D
Matrix Wave Equation
Slide 28
We derive the matrix wave equation by combining the two block matrix equations. First, we solve the first block matrix wave equation for the magnetic field term.
1eff
x x x
y y y
d jndz
h e eP
h e e
Second, we substitute this into the second block matrix equation.
1 1eff eff eff
22
eff eff eff2
2
2
x x x x x
y y y y y
x x x x x
y y y y y
x
d d djn jn jndz dz dz
d d djn jn ndz dz dz
ddz
e e e e eP P Q
e e e e e
e e e e ePQ
e e e e e
ee
2eff eff2
x x
y y y
dj n ndz
e eI PQe e
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Small Angle Approximation
Slide 29
Assuming the fields to not diverge rapidly, then the variation in the field longitudinally will be slow. This means
2
2x x
y y
d ddz dz
e ee e
This means we can drop the term from our wave equation.2
2x
yz
ee
also called the Fresnel approximation
2
2x
y
ddz
ee
2eff eff
2eff
eff
2
12
x x
y y
x x
y y
dj n ndz
d ndz j n
e eI PQe e
e eI PQ
e e
Explicit Finite‐Difference Approximation
Slide 30
We explicitly approximate the z‐derivative with a finite‐difference.
eff
1 1 1
eff
12
12 2
i i i i i i
ddz j n
z j n
e Ae
e e A e A e
This is a standard finite‐difference evaluated at i+0.5
First, we write our matrix wave equation more compactly as
2eff
eff
1 2
x
y
d ndz j n
ee Ae e A I PQe
, 2eff ,
, x ii i i i i
y in
ee A I PQe
This term is interpolated at i+0.5.
Note: This is called the Crank‐Nicolson scheme because it is a central finite‐difference.
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Forward Step Equation
Slide 31
1 1 1
eff
1
1 1eff eff
12 2
4 4
i i i i i i
i i i i
z j n
z zj n j n
e e A e A e
e I A I A e
Solving our new equation for leads to1ie
We now have a way of calculating the field in a following slice based only the field in the previous slice.
Backward waves are ignored in this formulation.
Note: We need to make a good guess for the value of neff.
Slide 32
Alternative Formulationsof BPM
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FFT‐BPM
Slide 33
The FFT based BPM was the first BPM. It was essentially replaced by FD‐BPM because FFT‐BPM has the following disadvantages:
• Simulations were slow (FFTs are computationally intensive)• Discretization in the transverse dimension must be uniform • No transparent boundary condition could be used• Very small discretization widths were not feasible• Polarization cannot be treated• Inaccurate for high contrast devices• Propagation step must be small
one layer
propagate plane waves
propagate plane waves
Introduce material phase in real‐space.
Algorithm to Propagate One Layer1. FFT the fields to calculate plane
wave spectrum.2. Add phase for one half of layer
to plane waves according to their longitudinal wave vector.
3. Inverse FFT at mid‐point to reconstruct the real‐space field.
4. Introduce the phase due to the materials in the layer.
5. Repeat steps 1 to 3 for the second half of the layer.
M. D. Feit, J. A. Fleck, Jr., “Light propagation in graded-index optical fibers,” Applied Optics, Vol. 17, No. 24, December 1978.
Wide Angle FD‐BPM – Recurrence Formula
Slide 34
The wave equation before the small angle approximation was made was:
2
2eff eff2
12 0y y yxx xx yy yzz
j n nz z x x
This can be written more compactly as2
2eff eff2
12 0 y y y xx xx yyzz
j n A A nz z x x
We can rearrange this differential equation to derive a recurrence formula for the derivatives.
eff
eff
211
2
y y
Aj n
zj n z
eff
1eff
211
2m
m
Aj n
zj n z
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Wide Angle FD‐BPM – Padé Approximant Operators
Slide 35
We initialize the recurrence with
1
0z
0th Order (Small Angle)
eff
0 eff
1eff
21 21
2
Aj n Aj
z nj n z
1st Order (Wide Angle)
eff eff
12eff0eff
2 21 11
42
A Aj n nj Az
nj n z
2nd Order (Wide Angle)2
3eff eff eff
22eff1eff
2 2 81 11
22
A A Aj n n nj Az
nj n z
The numerator and denominator are polynomials of the operator A of orders N and D respectively.
This leads to the Padé Approximate operators denoted as Padé(N, D)
Wide Angle FD‐BPM – Implementation
Slide 36
Previously we solved by setting . This was the small angle
approximation.
For wide angle BPM, we instead solve where N(A) and D(A) are the
polynomials of A.
This equation is usually implemented with finite‐differences using the “multi‐step” method.
2
eff2 2 0y y
yj n Az z
2
2 0y
z
yy
N Aj
z D A
G. Ronald Hadley, “Wide-angle beam propagation using Padé approximant operators,” Optics Lett., Vol. 17, No. 20, October 1992.
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Bi‐Directional BPM
Slide 37
The beam propagation method inherently propagates waves in only the forward direction.
It is possible to modify the method so as to account for backward scattered waves.
This is accomplished in a manner similar to how we derived scattering matrices.
By the time BPM is modified to be bidirectional and wide‐angle, it approaches being a rigorous method. The implementation, however, is tedious. At this point, use the method of lines which is fully rigorous and has a simpler implementation.
Hatem El-Refaei, David Yevick, and Ian Betty, “Stable and Noniterative Bidirectional Beam Propagation Method,” IEEE Photonics Technol. Lett, Vol. 12, No. 4, April 2000.
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