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Optical Surface-Mount Packaging and Guided-Mode Coupling

Shogo Ura Kyoto Institute of Technology, Japan

December 4, 2015 The University of Texas at Arlington, Texas

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Outline

1.  Introduction High-density optical interconnects

2.  Typical I/O couplers 45˚ mirrors, grating couplers

Application potential for WDM system

3.  High-density vertical coupler Grating coupler with cavity-resonator integration

4.  Trials for functional coupler for advanced packaging Guided-mode resonance for stable VCSEL coupling

5.  Summary

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Paradigm shift for future LSI

Size shrinking Increase of Tr

Paradigm shift

Super global wiring High density massive signal transmission > 20 Tbps

LSI

LSI

LSI

Increase of power dissipation

Future SiP

Small feature size

Jumping production cost

Optimized packaging of multiple LSIs

On-board optical interconnection

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Appl. Roadmap on Optical Interconnects

A. F. Benner et al. @IBM, IBM J. Res. & Dev. 49, 755 (2005)

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High-density optical interconnect module

High density optical wiring: 20 Tbps in 10mm width

VCSEL array PD array

I/O couplers I/O couplers

Electrical signals Electrical signals

A key issue: how to couple optical signals to optical waveguides

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Outline

1.  Introduction High-density optical interconnects

2.  Typical I/O couplers 45˚ mirrors, grating couplers

Application potential for WDM system

3.  High-density vertical coupler Grating coupler with cavity-resonator integration

4.  Trials for functional coupler for advanced packaging Guided-mode resonance for stable VCSEL coupling

5.  Summary

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Embedded 45˚ mirrors

VCSEL

Guiding core

PCB 45-degree mirror

VCSEL

Guiding core

PCB 45-degree mirror

Basic configuration

Coupling with lens

Vertical coupling Easy design

Larger alignment tolerance

NA matching Higher efficiency

NA mismatching

Lens integration Cost Size

Pros

Cons

Pros

Cons

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Embedded 45˚ mirrors Machining with taper blade

Low cost

Not applicable to micron size

Line machining No localization

Simple processes Pros

Cons

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Embedded 45˚ mirrors

Pros

Cons

Lithography Localization High throughput Low cost

Liquid emersion exposure

Applicable to micron size

Special setup

Tilt exposure

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Fabrication of embedded micromirror

Liquid immersion exposure is necessary for realizing an exposure angle of 45˚ in resist.

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Coupling by embedded micromirror

Input coupling efficiency : 75%

Output coupling efficiency : 60% J. Inoue et al. @ Kyoto Inst. Tech., J. Lightwave Tech. 30, 1563 (2012)

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Grating couplers (GCs)

θa Λ

θs

Grating coupler

Guiding core

Substrate

Air radiation

Substrate radiation

Guided wave

Grating vector

: Wave-vector size in the air

: Wave-vector size in the substrate

: Propagation vector of the guided mode

Air radiation by 1st order diffraction

Substrate radiation by 1st order diffraction

€

K =2πΛ€

β = Nk€

k =2πλ

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Coupling characteristics of GC Vertical coupling

Maximum efficiencies are limited by reciprocal theorem

Input coupling

No directional preference in excitation

Output coupling

Distributed Bragg reflection due to 2nd-order diffraction

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Coupling characteristics of GC Substrate radiation suppression

Optimization of buffer layer thickness

Efficiency enhancement by interference effect

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Free-space-wave OADM coupler

TE0: Transmission mode TE1: I/O coupling mode

FGC: Vertical coupling FS wave TE1

DBR: Wavelength selective TE0 TE1

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Free-space-wave OADM coupler Preliminary experimental results of 8-ch WDM optical interconnects consisting of 16 FGC/DBR pairs

K. Kintaka et al. @ AIST, IEEE Lightwave Technol. 20, 1398 (2010)

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Free-space-wave couplers

Required coupling characteristics and couplers

Vertical coupling

High efficiency

Planar process

Grating couplers 45˚ Mirrors

Small aperture

Yes

Under study

Low efficiency

Not available

OADM function

Available

Complicated Feasible

Available

Yes Under study

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Outline

1.  Introduction High-density optical interconnects

2.  Typical I/O couplers 45˚ mirrors, grating couplers

Application potential for WDM system

3.  High-density vertical coupler Grating coupler with cavity-resonator integration

4.  Trials for functional coupler for advanced packaging Guided-mode resonance for stable VCSEL coupling

5.  Summary

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Cavity-resonator-integrated GIC

High-efficiency vertical coupling with small aperture can be realized even if grating coupler itself has low coupling coefficient

Cavity-resonator-integrated grating in/out coupler (CRIGIC)

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Design example of CRIGIC

Coupling wavelength 842 nm

TE0 mode index 1.489

Reflectivity 98.8%

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Fabrication & characterization CRIGIC of 20µm aperture

Theoretical Experimental

GC w/o cavity resonator needs 500µm aperture to give 60% efficiency.

K. Kintaka et al. @ AIST, Opt. Lett. 35, 1989 (2010)

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Free-space-wave couplers

Vertical coupling

High efficiency

Planar process

Cavity-resonator-integrated grating I/O couplers

Small aperture

Yes

OADM function

Available

Feasible

Available

Yes

Required coupling characteristics and couplers

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Outline

1.  Introduction High-density optical interconnects

2.  Typical I/O couplers 45˚ mirrors, grating couplers

Application potential for WDM system

3.  High-density vertical coupler Grating coupler with cavity-resonator integration

4.  Trials for functional coupler for advanced packaging Guided-mode resonance for stable VCSEL coupling

5.  Summary

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Surface mount package of VCSEL

CRIGIC: Aperture size ~ 4 µm Single-mode VCSEL: Aperture size ~ 4 µm

Alignment tolerance for high-efficiency coupling: ~ deep sub-micron

Automatic lateral alignment is strongly desired.

High efficiency input coupling to single-mode waveguide

No good for practical applications

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Concept of advanced packaging Control of VCSEL operation by

a component integrated in photonic circuit board

A larger-aperture half-VCSEL and a smaller integrated mirror may adjust an emission point of VCSEL to mirror position

and relief the alignment problem. Wavelength-selective mirror may provide

wavelength stabilization required in WDM applications.

A problem is mismatch in wavelength between vertical resonance mode and mirror reflection.

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New integrated mirror/coupler

may serve as the external mirror for simultaneous realization of wavelength selectivity (stabilization of fixing), wavelength adjustment of vertical cavity mode, and guided-mode excitation, by small aperture.

CRIGM consists of a guide-mode resonance (GMR) filter, a bottom mirror, and a waveguide cavity.

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Guided-mode resonance (GMR) filter

A wavelength filter of a simple structure consisting of a grating on a waveguide

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Reflection characteristics of GMR filter

Fano resonance of interaction of a continuous state (radiation mode) and a discrete state (guided mode)

Large wavelength dependence of reflection phase attractive for future optical packaging of VCSEL

Narrowband reflection spectrum

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GMR filter with bottom mirror

Anomaly reflection in resonance is given by a superposition of direct reflection from the top surface, radiation from the excited and accumulated guided wave, and reflection from the bottom high reflection layer.

Normal reflection without resonance is given by a superposition of direct reflection from the top surface and reflection from the bottom high reflection layer.

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Calculation model

TE0 guided mode index : 1.861 @ λ = 1.54 µm

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Reflection spectra calculated by RCWA

Low reflectance indicates formation of vertical cavity. Cavity modes readily loses energy.

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Reflection spectra calculated by RCWA

Cavity modes formed by normal reflection

Fabry-Perot part Ordinary FP resonance between Au layer and grating layer. Mode wavelength proportional to tb. Wide branch due to low finesse.

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Reflection spectra calculated by RCWA

Cavity mode formed by anomaly reflection

GMR part Lower dependence of wavelength on tb due to variation of reflection phase. Two anomaly points A & B.

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Reflection spectra calculated by RCWA

A: small reflection from GMRF

Reflectance is determined by high reflection layer.

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Reflection spectra calculated by RCWA

B: ~100% reflectance from GMRF

High reflectance is from GMR filter.

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Reflection spectra calculated by RCWA

Near B: High reflectance from GMRF

High Q of vertical cavity results in large loss and low reflectance.

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Reflection spectra calculated by RCWA

Reflection spectrum @ tb = 3.62 µm

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Reflection spectra calculated by RCWA

Large wavelength dependence of reflection phase

Reflection spectrum @ tb = 3.37 µm

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Vertical cavity resonance mode

€

2la2πλ−ϕGMRM (λ) −ϕDBR = 2πm (m : Integer)

Wavelength λ of the vertical cavity mode

€

ϕGMRM (λ) : Reflection phase of GMRM

€

ϕDBR : Reflection phase of DBR

€

dλdla

≅laλ

+λ4π

dϕGMRM(λ )dλ

⎛

⎝ ⎜

⎞

⎠ ⎟ −1

Dependence of cavity-mode wavelength λ on air-gap length la

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Calculated reflection spectra

Air-gap-length insensitive wavelength-stabilized cavity mode

High reflectance

T. Kondo et al. @ Kyoto Inst. Tech, J. Opt. Soc. Am. A. 32, 1454 (2015)

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Cavity resonator integration Guided mode resonance filter needs large aperture

for accumulating sufficient guide-mode power to cancel/enhance reflection/transmission.

Aperture miniaturization

Waveguide cavity consisting of a pair of DBRs

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Fabrication of CRIGM

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Measured reflection phases

J. Inoue et al. @ Kyoto Inst. Tech, Opt. Lett. 39, 1893 (2014)

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Summary

1.  A grating coupler (GC) is a powerful coupler for a single mode waveguide,

but is not good at vertical coupling and

needs much larger aperture size in comparison to channel waveguide.

2.  Integration of waveguide cavity resonator provides

high-efficiency vertical coupling by small aperture enabling high density.

3.  Concept of advanced surface-mount packaging of VCSEL is introduced.

4.  Guided-mode resonance is discussed as a unique external cavity mirror

integrated in a single mode waveguide.

This configuration would provide wavelength-stabilization, high-density,

and larger tolerance in VCSEL packaging.

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Acknowledgment Kyoto Institute of Technology Dr. J. Inoue, K. Nishio, Y. Imaoka, J. Ohmori, A. Horii, K. Shinoda, T. Kobayashi, S. Murata, T. Ito, Y. Kita, T. Ogura, T. Kondo

National Institute of Advanced Industrial Science and Technology Dr. K. Kintaka

Osaka Prefecture University Prof. H. Kikuta, K. Kameda, Y. Minamino, S. Oue, D. Yamashita

The University of Texas at Arlington Prof. R. Magnusson

Fujitsu Laboratories Dr. K. Yokouchi, Dr. M. Katoh, Dr. A. Sugama