Sagi Mathai 1

Si WDM Modulator ArraySi WDM Modulator Arrayfor FWH-OCDMAfor FWH-OCDMA

Sagi Mathai, Xin SunProf. Tsu-Jae King, Prof. Ming C. Wu

EECS DepartmentUniversity of California, Berkeley

OCDMA ReviewApril 6th, 2005

Sagi Mathai 2

FWH-OCDMA All Si TransmitterFWH-OCDMA All Si Transmitter

EncodedOutput

Multi-WavelengthSource Input

Drop Ports for Feedback Control

Wavelength Selective Microring Modulator

Array

Sagi Mathai 3

Free-Carrier Plasma EffectFree-Carrier Plasma Effect

Carrier Concentration (cm-3) Carrier Concentration (cm-3)

PNPN 1818 100.6105.8,

8.01822 105.8108.8, PNPNn

Ref: Irace, et.al., Silicon Photonics, Topics in Appl Phys, vol 94, pp 361-392, 2004.

1012 1013 1014 1015 1016 1017 101810-6

10-5

10-4

10-3

10-2

10-1

100

101

102

1012 1013 1014 1015 1016 1017 101810-9

10-8

10-7

10-6

10-5

10-4

10-3

Absorption Coefficient Change Refractive Index Change

(

cm-1)

-n

Electrons

Holes

Electrons

Holes

Sagi Mathai 4

Modulation MechanismModulation Mechanism

• Index of refraction can be tuned by injecting or depleting carriers in the microring optical waveguide

• Shifting the index of refraction will shift the microring resonant frequency and thus its transfer curve

• The resulting modulation in resonant frequency will cause intensity modulation on the optical carrier

si

sd

n0

n=0

OPTICAL FREQUENCYRESPONSE

SCHEMATIC(TOP VIEW)

Modulated Output

st

|st/si|2

0

Sagi Mathai 5

Wavelength Channel DistributionWavelength Channel Distribution

100 GHz

FSR = 500 GHz0 5 10 15 20 25 30

0

200

400

600

800

1000

Radius (µm)

FS

R (

GH

z)

4 WAVELENGTH CHANNELS

FSR = 500 GHz corresponds to R = 24 µm

FREE SPECTRAL RANGE

Sagi Mathai 6

Quality Factor and RC Parasitic LimitsQuality Factor and RC Parasitic Limits

5 10 15 20 25 30200

250

300

350

400

450

500

Radius (µm)

f 3d

B (

GH

z)

103 104 1050

5

10

15

20

25

30

Quality Factor

f 3d

B (

GH

z)

QUALITY FACTORLIMITED BANDWIDTH

RC LIMITEDBANDWIDTH

RL=50 0=1.55 µm

RC Parasitics do not limited the bandwidth

2.5 Gb/s switching speed requires Q = 80,000

10 Gh/s switching speed requires Q = 20,000

Sagi Mathai 7

Transfer Function at ResonanceTransfer Function at Resonance

1013 1014 1015 1016 1017-25

-20

-15

-10

-5

0si

sd

st

i

t

s

slog20

Carrier Density (cm-3)

POWER TRANSMISSION

2 = 0.063 = 0.54 cm-1

Target 10 dB Extinction Ratio

Sagi Mathai 8

Carrier Transport SimulationCarrier Transport Simulation

0.0 0.5 1.0 1.5 2.0 2.5 3.0-7.0x10-6

-6.0x10-6

-5.0x10-6

-4.0x10-6

-3.0x10-6

-2.0x10-6

-1.0x10-6

0.0

tOFF

=0.042nstON

=0.051ns

n

Time (ns)

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.01E10

1E11

1E12

1E13

1E14

1E15

1E16

Ho

le C

on

cen

trat

ion

(cm

-3)

Driving Voltage (V)

SWITCHING DYNAMICS CARRIER CONCENTRATION

Sagi Mathai 9

Previous Results on Passive Si Microdisk Previous Results on Passive Si Microdisk ResonatorsResonators

• Microdisk resonators have been fabricated on Si

• Optical performance characterized

• Optical Q > 100,000 was demonstrated

Sagi Mathai 10

Si Microring Modulator SchematicSi Microring Modulator Schematic

InputPort

DropPort

TransmitPort

AddPort

P-typeRegions

N-typeRegion

SOI

Sagi Mathai 11

Cross SectionCross Section

B

A

C D

TOP VIEW

A-B CROSS SECTION C-D CROSS SECTION

N-typeDoping

C D

BOX

0.45 m

0.45 m

P-typeDoping

A B

0.2 m

0.05 m

BOX

N-typeDoping P-type

Doping

Sagi Mathai 12

BPM Waveguide SimulationsBPM Waveguide Simulations

Width = 0.45 mRib height = 0.2 m

Slab height = 0.05 m

Radius = 24 m

STRAIGHT WAVEGUIDE CURVED WAVEGUIDE

Oxide Oxide

Oxide Oxide

SiSi

Sagi Mathai 13

Mask LayoutMask Layout

Input Port

Drop Port

Transmit Port

Add Port

RF and DC Biasing Pads

MicroringResonator

250 microns

Sagi Mathai 14

(1) Waveguide Dry Etching (2) Hydrogen Anneal (3) P+ Implant

Fabrication ProcessFabrication Process

(4) N+ Implant(5) Recrystallization/DopantActivation Anneal

(6) Ni-Silicide

(7) Passivation/Via (7) Contact Pads/Interconnect

Sagi Mathai 15

Berkeley Microlab CapabilityBerkeley Microlab Capability

• Complete 0.35μm CMOS on 6” wafers

• 0.35μm Deep UV Lithography: ASML 5500/90 Stepper

• Device group demonstrated FIN FET with 60 nm gate length (King, Hu, Bokor)

Sagi Mathai 16

ECTL PC /2 Plate Polarizer

Bias-TDC Voltage

SourceRF

Source

EDFA

Free-SpaceOptical Bench

Oscilloscope

ESA

BERT

IR CameraVOA

ElectricalCharacterization

OSA

DUT

RFAmp

PD

Si-Modulator TestbedSi-Modulator Testbed

Sagi Mathai 17

SummarySummary

• Reviewed preliminary study on an all Si based microring modulator array

• Exploit SOI technology and CMOS compatible fabrication process

• Built testbed for device characterization

• 1st generation device target– 2.5 Gb/s

– Single wavelength channel

– Free spectral range ~ 0.5 THz

• Next generation device– 10 Gb/s

– Q = 20,000

– 4 wavelength channels (100 GHz spacing)

– Low power (~µW)