fabrication of integrated structures for coupling vcsel to fibre

Chair of OptoelectronicsDGAO 2005 - Wroclaw

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Fabrication of integrated structures for coupling VCSEL to fibre

Denis Wohlfeld, K.-H. BrennerChair of Optoelectronics, University of Mannheim

Chair of OptoelectronicsDGAO 2005 - Wroclaw

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Outline

• Motivation: Coupling VCSEL to fibre

• Negative photoresist SU-8

• SU-8 bevelled structures - problems & solutions

• Results

• Conclusion

Chair of OptoelectronicsDGAO 2005 - Wroclaw

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Motivation

• 45° mirror + fibre alignment element

• Index matching between fibre and mirror (depending on final material)

• Multimode fibre Ø 125 m

• VCSEL diameter ~ 30 m

Coupling VCSEL to fibre

GlassGaAsChip

SU-8 strucutre + fibre

Current

Top MirrorLaser Cavity

Botton Mirror

GainRegion

Setup for optical coupling Vertical Cavity Surface Emitting Laser

10 m

Chair of OptoelectronicsDGAO 2005 - Wroclaw

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Outline

• Motivation: Coupling VCSEL to fibre

• Negative photoresist SU-8

• SU-8 bevelled structures - problems & solutions

• Results

• Conclusion

Chair of OptoelectronicsDGAO 2005 - Wroclaw

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SU-8 Negative Photoresist

• Epoxy based negative photoresist

• UV-exposure ~365nm- 400nm

• Refractive index of n = 1.69

CH2CHC2

O

O

HC - C- CH3 3

O

CH2CHC2

O

CH2

CH2CHC2

O

O

HC - C- CH3 3

O

CH2CHC2

O

H

3

Exposure

Development

StructureSU-8 molecule

Chair of OptoelectronicsDGAO 2005 - Wroclaw

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Outline

• Motivation: Coupling VCSEL to fibre

• Negative photoresist SU-8

• SU-8 bevelled structures - problems & solutions

• Results

• Conclusion

Chair of OptoelectronicsDGAO 2005 - Wroclaw

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SU-8 bevelled structures - problems & solutions

Limitation of exposure angle due to refraction - problem

• Angle of bevelled resist-structures limited by refraction ( α < 36° )

arcsin AirMax

SU -8

n

n

SU-8

Exposure Setup 1 Bevelled SU-8 structures

90

Chair of OptoelectronicsDGAO 2005 - Wroclaw

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SU-8 bevelled structures - problems & solutions

Limitation of exposure angle due to refraction - our approach

• Coupling prism for larger angles

0 10 20 30 40 50 600

20

40

60

80

Grad

Grad

( 45 ) 31.1

Exposure Setup 2

90

Chair of OptoelectronicsDGAO 2005 - Wroclaw

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SU-8 bevelled structures - problems & solutions

Reflections from index boundaries - problem

• Total reflection at up to 4 boundaries ( )

• Fresnel Equations:

Layer model

QuartzAirMaskAirSU-8AirIM3

2sin

sin arcsin

sinsin arcsin

1

e ee

e ee

n

ntS S Sn

nt

R T R

2sin

tan arcsin

sintan arcsin

1

e ee

e ee

n

ntP P Pn

nt

R T R

0 0 2 2

P S P Sr tR R T T

I I I I

Chair of OptoelectronicsDGAO 2005 - Wroclaw

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SU-8 bevelled structures - problems & solutions

Reflections from index boundaries - our approach

• IM1, IM2, IM3 index matching layers

• H2O „bath“ to achieve index matching

• H2O: n = 1.33, almost neutral to SU-8

• Total transmission: 84.1%

Layer model

QuartzIM1MaskIM2SU-8SubstrateIM3

1 2 0... NI T T T I

Chair of OptoelectronicsDGAO 2005 - Wroclaw

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SU-8 bevelled structures - problems & solutions

• Slanted arrangement

• Flexible angle adjustment

• Index matching at all layers

Reflections from index boundaries - our approach

Adjustment screw

Chair of OptoelectronicsDGAO 2005 - Wroclaw

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SU-8 bevelled structures - problems & solutions

Problem: non-planar resist-surface

• Spin coated (130m)

• Prebake

• SU-8 surface 130m - 230m

Mask diffraction

Chair of OptoelectronicsDGAO 2005 - Wroclaw

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SU-8 bevelled structures - problems & solutions

• Mask z = 0m • Diffraction pattern z = 100m

Diffraction Calculation - Rayleigh, Sommerfeld

100m

2 20

2

2,

2

iz nk kik r d ku r z u k e e

Chair of OptoelectronicsDGAO 2005 - Wroclaw

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SU-8 bevelled structures - problems & solutions

Possible approaches:

• Polishing

• Reduction of mask size

Mask diffraction

Chair of OptoelectronicsDGAO 2005 - Wroclaw

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Outline

• Motivation: Coupling VCSEL to fibre

• Negative photoresist SU-8

• SU-8 bevelled structures - problems & solutions

• Results

• Conclusion

Chair of OptoelectronicsDGAO 2005 - Wroclaw

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150m 150m

Quality of structure - Mask diffraction

Results

• Standard mask size • Reduced mask size

Chair of OptoelectronicsDGAO 2005 - Wroclaw

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Quality of structure - Angle

Results

• Angle 45° ± 1.0°

Bevelled SU-8 structure: side view

100m 100m

Mirror element: side view

Chair of OptoelectronicsDGAO 2005 - Wroclaw

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Deflection at the mirror

Results

•Top view • Bottom view

Chair of OptoelectronicsDGAO 2005 - Wroclaw

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Results

• Motivation: Coupling VCSEL to fibre

• Negative photoresist SU-8

• SU-8 bevelled structures - problems & solutions

• Results

• Conclusion

Chair of OptoelectronicsDGAO 2005 - Wroclaw

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Conclusion

• Bevelled SU-8 structures with UV possible

• Angle 45° ± 1.0°

• SU-8 structures can be used as masters for injection molding

• Next step: Final integration chip to fibre

Chair of OptoelectronicsDGAO 2005 - Wroclaw

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Acknowledgment

We acknowledge the support of the European Community-

Research Infrastructure Activity under the FP6

"Structuring the European Research Area" programme

(HadronPhysics, contract number RII3-CT-2004-506078).