state key lab of advanced optical communication …spm.pku.edu.cn/summerschool2014/summerschool...

1 Shanghai Jiao Tong University

Yikai Su

State Key Lab of Advanced Optical Communication Systems and Networks ,

Department of Electronic Engineering, Shanghai Jiao Tong University, China

[email protected]

System applications of silicon photonic

ring resonators

mailto:[email protected]


Motivation

Electronic processing Optical processing in silicon

photonics

Complexity (# of units) High Low

Line width 10’s nm >100 nm

Power mW - W mW - W

Speed Gb/s Gb/s-Tb/s

Optical processing may be

desired in some high-speed

applications


Parameters of digital differentiator

Filter A/D DSP chip D/A Filter

memory I/O

Realization of digital differentiator using DSP

TMS320C6455 DSP ADC:MAX109

Speed:2.2 Gs/s

Power dissipation:6.8 W

Size:734.4 mm2

DAC:MAX5881

Speed:4.3 Gs/s

Power dissipation:1160 mW

Size:11 mmx11 mm

DSP:TMS320C6455

Speed:

1.2 GHz clock rate; 9600MIPS (16bit)

Size:

0.09-um/7-level Cu Metal Process (CMOS)

BGA package: 24*24 mm2

Power dissipation:1.76 W


Optical processing using ring resonator

SEM photos of a silicon microring

resonator

250-nm thickness

450-nm width

Buffer layer: 3-µm silica

Mode area: ~ 0.1µm2

Air gap : ~100 nm

Silicon 250nm

Silica buffer layer 3μm

Silicon handing wafer

525 μm

Signal processing functions:

• Slow light (JSTQE 08)

• Fast light (OE 09)

• Wavelength conversion (APL 08)

• Format conversion (OL 09)

• Optical differentiation (OE 08)


Outline

Tunable delay in silicon ring resonators

• Optically tunable buffer for diverse modulation formats at 5-Gb/s

• Optically tunable phase shifter for 40-GHz microwave photonic

signal

Signal Conversions and Switching

• Dense wavelength conversion and multicasting in a resonance-

split silicon microring

• Wavelength selective switching

• Format conversions (NRZ to FSK, NRZ to AMI)

• Optical temporal differentiator

Concentric rings for bio-sensing

Conclusions


Outline




signal








Conclusions


Recent experiments on slow-light delay

in silicon nano-waveguides

Schemes Footprint

(mm2)

3dB Band

width Duration/Delay

Max storage

capacity (bits) Publication

SRS ~100GHz 3ps/ 4ps 1.3 Opt. Express

14(2006)

cascaded

microring

resonator

(APF / CROW)

0.09

0.045

54GHz

--

50ps/510ps

200ps/220ps

10 at 20bps

1 at 5bps

Nature Photonics

1(2007)

photonic crystal

(PC) ~260MHz 1.9ns/1.45ns <1

Nature Photonics

1(2007)

photonic crystal

coupled

waveguides

(PCCW)

12nm 0.8ps/40ps LEOS 2007

• Continuous tuning was not demonstrated

• Data format was limited to non return-to-zero (NRZ)


Tuning signal delay in resonator-

based slow-light structure

Tunable group delay is important for

implementing a practical buffer

Single microring-resonator is a basic building

block of the resonator-based slow-light structure

Tuning methods:

• Electro-optic effect by forming a p-i-n structure

• Thermo-optic effect by implanting a micro-heater

• MEMS actuated structure


Partial

coupling Input DI

More

coupling

Resonance

Incoming light is partially coupled into the ring

The signal in the ring interferes with the input

light after one round-trip time

Only the signal of resonance can be coupled

into the ring

Ring resonator


Slow light

Group delay

Also see the animation


Tunable slow-light in silicon ring resonator

Slow-light principle:

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

-8

-6

-4

-2

0

× 10-4Normalized frequency detuning

Norm

ali

zad

tra

nsm

issi

on

(d

B)

(a)

0

1

2

3

4

5

6

Ph

ase

sh

ift

(rad

)

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.50

20

40

60

80

100

120

140

× 10-4Normalized frequency detuning

Del

ay (

ps)

(b)

Δθ/Δω

= group delay

=> Slow light


When a pump light is injected into the microring resonator, the

absorbed energy is eventually converted to the thermal energy

and leads to a temperature shift

Ad T T P

dt CV

The refractive index changes with the temperature

4 11.86 10n k T

k K

τ- thermal dissipation time

ρ-density of the silicon

C-thermal capacity

V-volume of the microring

Kθ-thermo-optic coefficient

Temperature tuning

No need of additional procedure in the fabrication, very low

threshold in tuning


Silicon microring used in the experiment

SEM photos of the silicon microring resonator with a radius of 20 μm

250-nm thickness

450-nm width

Buffer layer: 3-µm silica

Mode area: ~ 0.1µm2

Air gap : 120 nm

Silicon 250nm

Silica buffer layer 3μm

Silicon handing wafer

525 μm

1552.6 1552.7 1552.8 1552.9 1553.0 1553.1

-8

-6

-4

-2

0

experimental data

curve fitting

wavelength (nm)

No

rma

lize

d t

ran

smis

sio

n(d

B)

~8-dB notch depth

~0.1-nm 3-dB bandwidth


Vertical coupling

Gold grating coupler to couple light between the

single mode fiber (SMF) and the silicon waveguide

The gold grating coupler is designed to support

TE mode only

Measured fiber-to-fiber coupling loss: ~20dB

The technique was invented by Ghent

SEM photo of the gold grating coupler


Experimental setup

A dual-drive MZM is used when

generating RZ-DB and RZ-AMI

CW laser

PPG

PCSingle drive

MZM

PC

EDFA EDFA

EDFA

RF

Single drive

MZM

BPF

BPF Attenuator

PRBS

Oscilloscope

PM

Coupler

PC

Attenuator

DPSK demodulation

generation of RZ / CSRZ signal

Fangfei Liu et al., IEEE JSTQE May/June 2008


Continuous Tuning of 5-Gb/s Non-

return-to-zero (NRZ) signal

-30 -20 -10 0 10 20

0

30

60

90

120 1G

5G

10G

Dela

y (

ps)

Pump power (dBm)

(a)

0 500 1000 1500 20000.0

0.5

1.0

1.5

2.0

2.5

Inte

nsity (

a.u

.)Time (ps)

-30.7dBm

4.8dBm

13.7dBm

(c)

Delay versus the pump power Delayed waveforms

(b)

Maximum delay of ~100 ps


-40 -30 -20 -10 0 10 20

0

30

60

90

120 1G

5G

De

lay (

ps)

Power (dBm)

(a)

0 100 200 300 400 5000.00

0.25

0.50

0.75

1.00 -37.0dBm

3.2dBm

13.6dBmIn

ten

sity

/ a

. u

Time / ps

(c)

Return-to-zero (RZ) signal

5Gb/s

5G RZ

eye diagram

Maximum delay of 80 ps

for 5-Gb/s RZ signal

Delay versus the pump power

Qiang Li et al., IEEE/OSA J. Lightw. Technol.,

Vol 26, No. 23, 2008


5-Gb/s carrier-suppressed RZ

(CSRZ) signal

Eye diagrams and waveforms for the 5-Gb/s CSRZ signal


0 0

CSRZ is used in long haul


5-Gb/s RZ-Duobianry (DB) and RZ-

Alternating-Mark-Inversion (AMI) signals

RZ-DB

RZ-AMI



RZ-DB is good for dispersion

uncompensated system in

metro

RZ-AMI is tolerant to

nonlinear impairments


Delay comparisons

Formats NRZ RZ CSRZ RZ-DB RZ-AMI

Delays (ps) 100 80 95 110 65

Optical spectra

the narrower,

the larger delay

Qiang Li et al., OSA Slow and Fast Light Topic Meeting, 2008


Resonator-based slow-light structures :

Single channel side-coupled integrated spaced sequences of resonators (SCISSOR)

Double channel SCISSOR

Coupled resonator optical waveguides (CROW)

Larger delay with cascaded rings

Single channel SCISSOR

double channel SCISSOR

CROW


Optically tunable microwave

photonic phase shifter

Operation principle

Op

ical

spec

tru

m

Frequency

20G20G

10dB

Ein Eout

E(0)E(L)

-3 -2 -1 0 1 2 3

-6

-5

-4

-3

-2

-1

0

Normalized frequency detuning

Norm

ali

zed

tra

nsm

issi

on

(d

B)

0

1

2

3

4

5

6

Ph

ase sh

ift (rad

)

× 10-4

(b)

(a)

(c)

The two tones of the microwave optical signal experience different phase shifts, resulting in group delay change


Experimental setup

20-GHz microwave photonic signal

Temperature tuning

Silicon

microring

Q. Li et al., ECOC 2008, paper P2.12


40GHz result – phase shift

0.0 0.5 1.0 1.5 2.0

0.2

0.4

0.6

0.8

1.0

Norm

ali

zed

in

ten

sity

(a.u

.)

Time (ns)

-4.6rad

Maximum phase shift:

-4.6 rad

Qingjiang Chang et al., IEEE Photon. Technol. Lett，vol. 21, no. 1, Jan. 2009


6 8 10 12 14 16-5

-4

-3

-2

-1

0

(a)

Pump power (dBm)

Ph

ase

sh

ift

(ra

d)

4 6 8 10 12 14 160

1

2

3

4(b)

Pump power (dBm)

Ph

ase

sh

ift

(ra

d)

Phase shift vs. pump power

Continuous tuning based on thermal nonlinear effect by changing the control light power


Outline




signal








Conclusions


Signal conversions in mode-split ring

The transmission function of the ring resonator is given by:

0

0 0 0 0 0 00 0

1 11 ( )

2( ) ( )

2 2 2 2 2 2

t

i e

u i e u i e

s

s Qj j

Q Q Q Q Q Q

Mode a is split into two resonance frequencies, ω0-ω0/(2Qu) and

ω0+ω0/(2Qu). The resonance-splitting is determined by the mutual coupling

factor Qu.

si st

ab

ω0 - the resonance frequency

QE - coupling quality factor

QL – intrinsic quality factor

Qu – coupling quality factor

Side wall roughness in E-beam

results in two resonance modes:


Observation of mode splitting

Resonance-splitting

Motivation: shift the resonance to convert signals by

using free carrier dispersion (FCD) effect

Ziyang Zhang et al., CLEO/QELS 2008

Tao Wang et al., JLT 2009


Experimental results – dense

wavelength conversion of 0.4nm

nm

1. Signal light is originally set at

the resonance -> ‘0’

2. Resonance is shifted when

pump is ‘1’

3. Signal light off resonance ->

‘1’ -> wavelength conversion

4. Inverted case can be realized

pump signal

Qiang Li et al., App. Phy. Lett., 2008


Conversions of 2 wavelengths ->

wavelength multicasting

By setting the signal wavelengths

properly, non-inverted and inverted

multicasting can be implemented

Wavelength multicasting

s1 s2 p

FSR

Qiang Li et al., App. Phy. Lett., 2008


Format conversion- NRZ to FSK

500μW/div

2.5ns/div

FSK Eye diagram

5dB/div

0.5nm/div

FSK Spectrum

s1 s2 p

Input NRZ signal

demodulated signal: upper sideband

demodulated signal: upper sideband

500μW/div

500ps/div

Fangfei Liu et al., APOC 2008


Reconfigurable optical add-drop multiplexers

Ref: E. Basch, et.al, JSTQE, vol. 12, no. 4, 2006.

R

O

A

D

M

n

o

d

e

flexible management

reconfigurable provisioning

Increased capacity

Multiple services

ROADMs

Exchanging node

WDM optical network


Wavelength selective switch Wavelength selective switches (WSSs) are core components in ROADMs.

Ref: P. Colbourne and B. Collings, OFC 2011

ROADM node

Function of WSS

Flexible and reconfigurable

allocation of wavelength

channels among various

routes


Previous works on WSS

WSS based on MEMS WSS based on liquid crystal

Ref: 1. Joseph Ford, et.al, JLT, vol. 17, no. 5, 1999.

2. Yasuki Sakurai, et.al., PTL, vol. 23, no. 14, 2011.


Evolution of WSSes

Ref: P. Colbourne and B. Collings, OFC 2011

Developing trends of WSSs

1. Low-cost

2. Miniaturization

Merits of silicon devices

1. On-chip integration

2. Reduced sizes

Free-space

WSS

Discrete

wavelength filter

Liquid-crystal

WSS

On-chip

WSS

Mini

WSS


WSS using silicon microring resonators

Ref: Douwe Geuzebroek, et. al., PTL, vol. 17, no. 2,

(2005)


Proposed structure

silicon MRR with nested pair of subrings (NPS)


Simulated transmission spectra

λ1 λ2

λ1: blocked at OUT1 and routed to OUT2

λ2: blocked at OUT2 and routed to OUT1


Device fabrication

248-nm DUV lithography, ICP etching process, on SOI wafer

Thermal-optic micro-heaters are fabricated along the NPS to tune

the central wavelengths of the split resonances.


Measured spectra with one NPS

Switch off λ1 0 V Switch off λ2 0.4 V Switch off λ4 1.0 V Switch off λ3 0.7 V


Demonstration of dynamic channel-routing

10 Gb/s NRZ input signal


10-Gb/s NRZ signal from OUT1 port


10-Gb/s NRZ signal from OUT2 port


BER performances

1.0 dB and 0.8 dB power penalties at OUT1 and OUT2, respectively.


Optical temporal differentiator

:

In the critical coupling region

(QL = QE), the transfer function

of the microring resonator is:

0

0

2( ) ( )

QT j

A typical function for a first-

order temporal differentiator


Experimental results

50ps/div 50ps/div 50ps/div 50ps/div

50ps/div 50ps/div 100ps/div 100ps/div

100ps/div 50ps/div100ps/div 100ps/div

(a-i) (a-ii) (a-iii) (a-iv)

(b-i) (b-ii) (b-iii) (b-iv)

(c-i) (c-ii) (c-iii) (c-iii)

10G 5G

Gaussian

Sine

Square

Input Output Input Output

Fangfei Liu, et al., Opt. Express 2008


Format conversion- NRZ to AMI

0 400 800 1200 1600 20000.0

0.2

0.4

0.6

0.8

1.0

(a)

Nor

mal

ized

am

plit

ude

(a.u

.)

Time (ps)

0 400 800 1200 1600 20000.0

0.2

0.4

0.6

0.8

1.0(b)

Nor

mal

ized

am

plitu

de (

a.u.

)

Time (ps)

10G NRZ 10G AMI

A microring is a high pass filter

NRZ + high pass filtering => AMI

Qiang Li et al., Chin. Opt. Lett., Vol 7, No. 2, 2009


How to build an ultra-high-speed all-

optical differentiator?


80-G optical differentiator using a ring

resonator with 2.5-nm bandwidth

Radius: 20 μm

Bandwidth : 2.5 nm

Resonance wavelength: 1551.73nm


Measurement setup


80-Gb/s differentiation result

G. Zhou et al., Electron. Lett. 2011


Future work: 160-G differentiation

Design of new ring resonator: critical coupling, large 3-dB bandwidth

One possible design:

• Large bandwidth: small diameter and high loss

• Critical coupling: long coupling length

B3dB=5nm


Comparison of optical and

electronic differentiators

Species Speed Size Power dissipation

Silicon ring 80 Gbps or

higher

20 μm (radius) < 1 mW

Digital

differentiator

a few GHz mm2 a few W

All-optical differentiator: (1) ultra-high speed

(2) compact structure

DSP based: configurable; can fulfill more than one function


Differential equation solver

1 1

2 2

y xy

t t

d d

d d

Differential equations are widely employed in virtually any field of

science and technology:

• Physics

• Biology

• Chemistry

• Economics

• Engineering

All constant-coefficient linear differential equations can be modeled

with finite number of:

• Differentiators

• Couplers/Subtractors

• Splitters

• Feedback branches


Optical differential equation solver

output port

input port

optical

differentiator

+

-

optical input

signal x optical output

signal y 1

2

1 1

2 2

y xy

t t

d d

d d


Outline




signal








Conclusions


Silicon microring for bio-sensing

DNA probe is attached to the ring After hybridization:

The effective index changes around the

waveguide results in resonance shift

Problems with the single ring:

limited sensing area

not easy to control the notch depth (air gap

between the ring and the straight waveguide)

DNA probe DNA hybridization


Proposal: concentric rings

Single ring concentric ring

Two samples

Field

distribution

The field is evenly distributed among the two concentric rings,

thus increasing the sensing area


Enhanced notch depth

Blue: single ring

Red: double rings

Enhanced notch depth, easier detection of resonance shift

More rings? Xiaohui Li, et al., Applied

Optics 2009


Conclusions

Silicon ring resonators with nano-scale SOI

waveguides can perform many functions:

• Tunable delay

– Digital: different modulation formats at 5 Gb/s

– Analog: 40-GHz microwave photonic signal

• Signal conversions

– Dense wavelength conversion and multicasting

– Format conversions

– Optical temporal differentiator

• Concentric rings for sensitive bio-sensing

state key lab of advanced optical communication …spm.pku.edu.cn/summerschool2014/summerschool...

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