1

Shigeru Nakamura

Green Platform Res. Labs., NEC Corporation

Silicon based integrated optical switching technology for telecom application

Oct. 22, 2013 ISUPT 2013

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Outline

・Application to optical path switches for next generation ROADMUltrafast network currently being upgraded will enable and also requireoptical layer flexibility with optical path switches.

・Our approach to optical switches with silicon photonics

・Demonstration on optical switch circuitsbased on silicon photonics device integration

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・Application to optical path switchesfor next generation ROADM

Page 4

Evolution toward next generation CDC-ROADM

Conventional ROADM Next generation ROADM

Static connection betweentransponders and lines

Transponders

AWGMany transpondersfor advancedmodulation format

"Optical switching"

Reconfigurable or dynamic connectionbetween transponders and lines

- Color-less- Direction-less- Contention-less

Higher flexibilityin optical layer

ROADM: Reconfigurable Optical Add Drop Multiplexer

2-degreetransmissionlines

Multi-degreetransmission lines

(CDC)

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CDC-ROADMs

• Any transponders can be used for setting up path at any wavelength for any direction.

• Each transponder can be used for multiple purposes. Multiple transponders can be used for single purpose.

• Increase in transponder usage efficiency. Reduction in transponder number.• Shared backup scheme leading to highly reliable and cost-effective failure recovery• Higher reconfigurability corresponding to unexpected traffic change

Configuration Functions

Colorless

…

λ1~λx

…

…

Directionless

λ1

…

λ2 λx

…

λ1

…

λ1 λ1λ1

ContentionlessCut-through partwith WSS

Transponder aggregatorpart

Benefitsl-tunabletransponders

Transmissionlines

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Several proposals for transponder aggregator

l-tunable transponders

Transponder aggregator part

Transmission lines

Cyclic AWG

Splitter Splitter Splitter Splitter Splitter Splitter Splitter Splitter

Selector Selector Selector Selector Selector Selector Selector Selector

K. Mizutani , M. Sakauchi and A. Tajima,ECOC2010 P3.11 (2010).

Cycl ic AWG

Cut-through part with WSS

Configuration of CDC-ROADM

Examples of transponder aggregator using silica waveguide devices

T. Watanabe, K. Suzuki, T. Goh, K. Hattori,A. Mori, T. Takahashi, T. Sakamoto, K. Morita,S. Sohma, S. Kamei, OFC2011 OTuD3 (2011).

Cy cli c AWG

Cy clic AWG

Contention-less

Direction-less

Color-less

Split & Select AWG & Matrix Switch

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Ultra-compact optical switch based on silicon photonics

l-tunabletransponders

Transmission lines

Configuration of CDC-ROADM

Transponder aggregator

WSS

Splitter Selector

Splitter Selector

Splitter Selector

Splitter Selector

Splitter Selector

Splitter Selector

Splitter Selector

Splitter Selector

8x8 split & select switch

Si ribwaveguide

Heater

Electrodepads

100 um

Input

Output(Bar port)

Output(Cross port)

Mach-Zehnder TO switch element

Si coreSiO2 cladding

Sharply bendedwaveguide

Efficient thermo-optical effect

Heating

Si core

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Optical switch

Electrical path switch

Opticaltransponder

IP router

Electrical packet switch

Server゙ Storage゙LAN

Server゙ Storage゙

Rooter･Switch

LAN

Data center

Wide area network

Home user

Business user

Rooter･Switch

Data center

Optical network node

AccessMobile backhaul

Higher network flexibilitycoping with unexpected traffic change or failure and being provided with low power consumption is a basic requirement.

Optical fiber

Optical switch providesflexibility or reconfigurabilityin the lowest layerwith the lowest power consumption

Many effort in many layerstoward improvement of flexibility

Basic expectation for optical switch

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・Our approach to optical switcheswith silicon photonics

Page 10

Silicon photonics

Conventionaldiscreteoptical devices

Large silicon wafer

Optical switch Optical transceiverSilicon photonics

Switchingelement

AWG Photo-detector

Modulator

Cost effectively developinghigh density, large scale optical device integration

- Ultra-small, low-power optical devices- Design rules for their integration- Use of sophisticated CMOS process

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SiO2 core Dimension: 5~7 um

Conventional SiO2 waveguide

SiO2 cladding Large core dimensionSmall index contrastWeak light confinementGentle waveguide bending (Curvature radius: 1~10 mm）

Si waveguide

Si core Dimension: 0.3~1 umSiO2 cladding Small core dimension

Large index contrastStrong light confinementSharp waveguide bending(Curvature radius: 10~100 um）

10~50mm

100~500mm

Comparison of AWG size

Ultra-small optical devices using Si waveguidesStrong light confinement into small core with high index contrast,thus sharp waveguide bending

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NW equipment IT equipment

Cost 1/10

Optical path switchfor wide area network

Optical transceiverfor interconnect

2cm0.25mm

3cm 1~10Tb/s

LSI

Size 1/10 Power 1/10

Chip-to-chip/on-chipoptical interconnect

What generation of process technology is applied?

Small volume Large volume

Silicon photonics application

Rakesh Kumar, "Fabless Semiconductor Implementation," 2008

Si CMOSProcess technology

Progress under Moore's law

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Selecting waveguide structure compatible with process

1 um

10 um

100 um

1 mm

10 mm

100 mm

1 um

10 um

100 um

1 mm

10 mm

100 mm

0.01 dB/cm

0.1 dB/cm

1 dB/cm

10 dB/cm

100 dB/cm

0.01 dB/cm

0.1 dB/cm

1 dB/cm

10 dB/cm

100 dB/cm

0.1 um

Ben

ding

Rad

ius

Pro

paga

tion

Loss

1 um 10 umWaveguide width

SiON SiO2Si-RibSi-Wire Loss∝σ2／ｄ4

Si sub.

SiO2 cladding

Ge-SiO2 core

5～7 umSi sub.

Si rib core

SiO2 cladding

Si sub.

Si wire core

SiO2 cladding

Fluctuation~10nm

Fluctuation~4nm

Si CMOS process technologyParameters representing size fluctuation・Roughness・Uniformity

Si waveguide technologySize fluctuation influences waveguide properties・Propagation loss・Polarization dependence・Phase error in interference devices

Loss s2/d4130nm

2001

90nm

2004

65nm

2007

45nm

2010

32nm

2013

22nm

2016

DRAM Half pitch

Crit

ical

dim

ensi

on U

nifo

rmity

(nm

)R

ough

ness

3s

(nm

)

1

10

100

130nm

2001

90nm

2004

65nm

2007

45nm

2010

32nm

2013

22nm

2016

DRAM Half pitch

Crit

ical

dim

ensi

on U

nifo

rmity

(nm

)R

ough

ness

3s

(nm

)

1

10

100

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Option in silicon optical waveguide cross section

Si sub.

Si wire core

0.2~0.5mm

Si sub.

Si ribSiO2 cladding

Si sub.

Si rib

21

22

21

2nnn

Refractiveindexcontrast

~1.5mm~5mm

40%5%1%Contrast of effective refractive indexes at rib and slab

10%

10

0.1

1

Opt

ical

dev

ice

size

(mm

)

SiO2 cladding SiO2 cladding

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AWG: Arrayed waveguide grating

TE

TM

Polarization independence

500 um

1575 1580 1585 1590 1595 1600 1605-60

-50

-40

-30

-20

-10

Tran

smis

sion

(dB)

Wavelength (nm)

-: TE-: TM

Optical spectraStructure

Input Output

Si rib waveguide

Si sub.

Si rib

~1.5mm

SiO2 cladding

8 inch wafer

248-nm lithography process

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Si ribwaveguide

Heater

Electrodepads

Silicon thermo-optical (TO) switch element

Si corewith cross section dimensionsof 0.3~1.5 um

SiO2 cladding

Heating

Si core

100 um

Input

Output(Bar port)

Output(Cross port)

Sharply bended waveguides

Efficient thermo-optical effect

Mach-Zehnder type TO switch element

Compact

Compact & low power

CompactLow driving powerTransparent

Highly suitablefor integrated functional devices

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-60

-50

-40

-30

-20

0 20 40 60 80 100

Tran

smis

sion

(dB

)

Power (mW)

Basic properties of Si TO switch element

0 20 40 60 80 1000.0

0.5

1.0

Inte

nsity

(a.u

.)

Time (ms)

~ 13 ms

0 20 40 60 80 1000.0

0.5

1.0

Inte

nsity

(a.u

.)

Time (ms)

~ 14 ms

100 um

TE

TM

Input

Output(Bar port)

Output on-off contrast

Time response

Switching time ~ 15 us

Easily controllableCross state without heatingBar state with heating

Polarization independence

Structure

Output(Cross port)

Si ribwaveguide

Heater

Electrodepads

Si rib waveguide

Bar port

Cross port

~25 dB

Si sub.

Si rib

~1.5mm

SiO2 cladding

8 inch wafer

248-nm lithography process

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・Demonstration on optical switch circuitsbased on silicon photonics device integration

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Silicon optical switch using split & select configuration

Silicon optical circuitincluding 152 switch elementsis formed within the areaof 16 x 12 mm.

1 x 8 selector switch

Gate part Selector part

Splitter Selector

Splitter Selector

Splitter Selector

Splitter Selector

Splitter Selector

Splitter Selector

Splitter Selector

Splitter Selector

8 x 8 split & select switch

TO switch element

Si ribwaveguide

Heater

Electrodepads

100 um

Requirements

- Small footprint- Low power- Low loss- Polarization independent- Wide wavelength range- High on-off contrast- Ambient temperature independent- ...

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Light propagation in silicon TO switch element

Constructiveat cross port

Destructiveat bar port

Ambient temperature insensitiveMach-Zehnder is symmetric.Without heating one arm

Destructiveat cross port

Constructiveat bar portIndex increase

Ambient temperature sensitive

With heating one arm

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Temperature characteristics of Si TO switch element

G. Cocorullo, F. G. Della Corte, and I. Rendina,Appl. Phys. Lett. vol. 74, pp. 3338 - 3340 (1999).

Temperature dependenceof Si thermo-optic coefficient (dn/dT)

RIdTdn

RV

dTdnP

dTdnT

dTdnn 2

2

Temperature dependenceof heater resistance (R)

R

Temp.

Refractive index change in Si when heated

Underconstantvoltage

Underconstantcurrent

Mach-Zehnder is symmetric, the arm lengths of Mach-Zehnder are the same.We still need to consider temp. characteristics when one arm is heated.

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Measured temperature characteristicsOutput (Bar port)

-30

-20

-10

0

10

0 20 40 60 80 100

Tran

smis

sion

(dB

)

Power (mW)

-30

-20

-10

0

10

2.7 2.8 2.9 3 3.1 3.2

Tran

smis

sion

(dB

)

Voltage (V)

-30

-20

-10

0

10

4 4.1 4.2 4.3 4.4 4.5

Tran

smis

sion

(dB

)

Voltage (V)

-30

-20

-10

0

10

11.5 12 12.5 13 13.5 14

Tran

smis

sion

(dB

)

Current (mA)

-30

-20

-10

0

10

16.5 17 17.5 18 18.5 19

Tran

smis

sion

(dB

)

Current (mA)

-：75C-：50C-：25C-： 0C

Si ribwaveguide

Heater

Electrodepads

100 um

Input

Output(Bar port)

Output(Cross port)

As a function of heater power

As a function of heater current

As a function of heater voltage

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Light propagation in silicon TO switch element

Constructiveat cross port

Destructiveat bar port

Ambient temperature insensitiveMach-Zehnder is symmetric.Without heating one arm

Destructiveat cross port

Constructiveat bar portIndex increase

Ambient temperature sensitiveparticularly when constant current drive is used.

With heating one arm

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1 x 8 selector switch part

Gate part Selector part

Configuration of 1 x 8 selector switch

1 mm

1 m

m

Adding gate part and using configuration that light blocking is doneby two-stage gate elements, 1x8 selector switch part becomes tolerantto ambient temperature change

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Temperature independent, high extinction switching

1570 1580 1590 1600 1610 1620-90

-80

-70

-60

-50

-40

-30

-20 Filename: 110927-1F01->F05,F07 : 110927C1-C4,D1-D4

Tran

smis

sion

(dB

)

Wavelength (nm)1570 1580 1590 1600 1610 1620

-100

-90

-80

-70

-60

-50

-40

-30

Tran

smis

sion

(dB

)

Wavelength (nm)

-：TE-：TM -：75C

-：50C-：25C-： 0C

Measured transmission spectra showing on-off contrast

For different polarization

40dB 40dB

High extinction ratio (>40 dB)Wide wavelength range (>50 nm)Polarization independentAmbient temperature independent under constant current drive

At different ambient temperature

S. Nakamura, S. Takahashi, I. Ogura, J. Ushida, K. Kurata, T. Hino, H. Takeshita, A. Tajima, M.-B. Yu, and G.-Q. Lo, OFC2012, OTu2I.3 (2012).

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Splitter Selector

Splitter Selector

Splitter Selector

Splitter Selector

Splitter Selector

Splitter Selector

Splitter Selector

Splitter Selector

Toward silicon optical switch module

1 x 8 selector switch

Gate part Selector part

8 x 8 split & select switch 8x8 optical paths can beset up using 152 TO MZswitch elements, whichare integrated in the areaof 12 mm x 16 mm.

Optical switch circuit part Opticalin/out part

Silicon optical switch chipOptical fiber array

M. Tokushima, et al., Appl. Phys. Express vol. 5, 022202, (2012).

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Conclusion

Silicon thermo-optical switch element

Integration of silicon thermo-optical switchestoward next generation ROADMs

Using a devised configuration of 1 x 8 selector switch, high extinction, temperature independent optical switching is achieved.

8 x 8 split & select type optical switch- Small & Low power- Polarization independent (less than 0.8 dB)- Wide wavelength range (over 50 nm)- High on-off contrast (over 40-45 dB)- Ambient temperature independent (over the range of 0 - 75 C)- Low loss

Internal loss excluding splitter loss (less than 5 dB)Coupling loss - In progress

- Packaging - Near future