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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.

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17 TH IEEE/LEOS Conference Puerto Rico, 7-11 th November, 2004


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slide1

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

17TH IEEE/LEOS Conference

Puerto Rico, 7-11th November, 2004

research motivation

Connector area

Laser – Detector array

Optical waveguides

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

the bent taper

b

y

Linear Taper

Bend Taper

y

z

c

b

c

r

z

θ

x

a

a

The Bent Taper
  • In a “bent taper” the lateral dimension, a, tapers linearly with respect to angle, θ to the final width, b
  • In a “linear taper” the lateral dimension, a, tapers linearly with respect to the – z axis to the final width, b

c

x

co ordinate transform

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

Solid line: Index of transformed guide

Dashed line: Step-index guide

simulation technique

z

z

Simulation Technique
  • 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

  • BPM paraxial limitations are altered
  • Significant reduction of the simulation area/time

(B) Transformed straight taper

(A) Bent Taper

A1

A2

R

θ

w

physical parameters

y

z

c

c

b

r

θ

x

a

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

Bent Taper

VCSEL fundamental mode

lateral misalignment

VCSEL

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

Transmittance (dB)

Field axial misalignment (μm)

angular misalignment

VCSEL

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

Transmittance (dB)

Field rotational misalignment (degrees)

φ

comparison with linear tapers
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

Lateral offset FWHM (μm)

Max. normalized power (dB)

Solid lines: Bent taper

Dashed lines: Linear taper

Taper ratio (a/b)

Solid line: Bent taper

Dashed line: Linear taper

Angular rotational

FWHM (degrees)

Taper ratio (a/b)

conclusions
Conclusions
  • 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 FWHMθdegrades from 100 to 20

Acknowledgements

  • Xyratex Ltd. for financial support and useful discussion
  • Frank Tooley for useful discussion