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Multimode Laterally Tapered Bent Waveguide. Ioannis Papakonstantinou, David R. Selviah and F. Anibal Fernandez Department of Electronic and Electrical Engineering University College London. Outline Research Motivation Modelling Approach Results - Discussion. - PowerPoint PPT Presentation
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©UCL 2004
17TH IEEE/LEOS Conference
Puerto Rico, 7-11th November, 2004
Multimode Laterally Tapered Bent Waveguide
Ioannis Papakonstantinou, David R. Selviah and F. Anibal Fernandez
Department of Electronic and Electrical EngineeringUniversity College London
Outline
• Research Motivation
• Modelling Approach
• Results - Discussion
2©UCL 2004
Research Motivation• To minimise cost of connectors
between the laser-detector arrays and the backplane waveguides
• Passive alignment of the optical connectors
• A large amount of misalignment must be tolerated
• Tapered waveguide entrances seem ideal
• In a dense configuration of boards and connectors the waveguides are curved to avoid the neighbouring connector
• A bent taper conserves space
Optical Backplane
Optical waveguides
Laser – Detector array
Connector area
3©UCL 2004
The Bent Taper
• In a “bent taper” the lateral dimension, a, tapers linearly with respect to angle, θ to the final width, b
x
c
z
y
c
a
b
θ r
Bend Taper
a
c
b
y
zx
Linear Taper• In a “linear taper” the lateral dimension, a, tapers linearly with respect to the – z axis to the final width, b
4©UCL 2004
R
r
x
y
u
v
)(uneff
y ≡
Co-ordinate Transform• The transform u = r – R, v =
Rθ maps the bent taper to a straight taper
• The effective index of the structure is tilted in comparison with the usual step index guide
• The slope of the tilt depends on the radius of curvature
• For u > uo, ncladding > ncore. A bend is always lossy
• Index in the core is asymmetric resulting to asymmetric modes
2
2
222
2
2
222
4
11)()(
o
y
ooeff krkr
R
krnrn
ou
Solid line: Index of transformed guideDashed line: Step-index guide
5©UCL 2004
Simulation Technique
(A) Bent Taper (B) Transformed straight taper
)cos1(1
2
R
w
A
A
zz
• FD – BPM
• 3D – Mesh of 0.1 μm × 0.1 μm and 1 μm axial step
• (1,1) Padé Coefficients
• Full TBC boundary conditions
Benefits by using the transform with BPM
A. BPM paraxial limitations are altered
B. Significant reduction of the simulation area/time
θ
w
R
A2A1
6©UCL 2004
Physical Parameters• Channel waveguide with
initial dimensions a = 50 μm, c = 50 μm
• Dimension b varies from 25 μm to 2 μm
• Variable taper ratio (a/b): 2 < a/b < 25
• ncore = 1.54, ncladding = 1.5107. N.A = 0.3
• R > 20 mm to minimize bend losses
• Material intrinsic losses and scattering losses all ignored
• Launching field: Gaussian 7 μm 1/e width, TE – polarised, λ = 850 nm
VCSEL fundamental mode
z
y
x
c
c
a
b
θ r
Bent Taper
7©UCL 2004
Lateral Misalignment• Input Gaussian field is
translated along the x-axis
• Position 0 is at the centre of the guide
• Maximum transmittance NOT when the source is centred to the guide
• Coupling is better towards the outer side of the bend
• This is due to the asymmetric nature of the modes inside the bend VCSEL
Tra
nsm
ittan
ce (
dB)
Field axial misalignment (μm)
8©UCL 2004
φ
Angular Misalignment• Input field is positioned at
the maximum position on the x - axis
• Then it is rotated on the xz - plane
• As the taper ratio increases losses increase
• For < 3 dB losses we can tolerate just a few degrees of misalignment in any case
• Therefore angular misalignment might be more critical than translational
VCSEL
Tra
nsm
ittan
ce (
dB)
Field rotational misalignment (degrees)
9©UCL 2004
Comparison with Linear Tapers• FWHM of the lateral and
rotational misalignment graphs for bent and linear tapers are compared
• Linear tapers show higher insertion loss but better lateral misalignment tolerance
• Bent tapers show better angular misalignment tolerance
• All FWHM degrade as taper ratios increase
Taper ratio (a/b)
Taper ratio (a/b)
Late
ral
offs
et FWHM
(μ
m)
Max
. no
rmal
ized
pow
er (
dB)
Ang
ular
rot
atio
nal
FWHM
(de
gre
es)
Solid lines: Bent taperDashed lines: Linear taper
Solid line: Bent taperDashed line: Linear taper
10©UCL 2004
Conclusions
Acknowledgements
• Bent taper simulations using FD-BPM revealed:
• As taper ratio varies from 1 < a/b < 25 lateral misalignment FWHMx degrades from 50 μm down to 7 μm
• Proportionally angular misalignment FWHMθ degrades from 100 to 20
•Xyratex Ltd. for financial support and useful discussion
•Frank Tooley for useful discussion
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