yikai su

1 Shanghai Jiao Tong University

Yikai SuState 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


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:MAX109Speed:2.2 Gs/s Power dissipation:6.8 W Size:734.4 mm2

DAC:MAX5881 Speed:4.3 Gs/sPower dissipation:1160 mWSize:11 mmx11 mm

DSP:TMS320C6455Speed: 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 thickness450-nm widthBuffer layer: 3-µm silicaMode 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 different modulation formats at 5-Gb/s rate

• Optically tunable phase shifter for 40-GHz microwave photonic signal

Signal Conversions

• Dense wavelength conversion and multicasting in a resonance-split silicon microring

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

• Optical temporal differentiator

Concentric rings for bio-sensing

Conclusions


Recent experiments on slow-light delay in silicon nano-waveguides

SchemesFootprint

(mm2)

3dB Band

widthDuration/Delay

Max storage capacity (bits)

Publication

SRS ~100GHz 3ps/ 4ps 1.3Opt. 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 <1Nature 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


PartiaPartial l couplicouplingng

InputInputDIDIMore More couplicouplingng

ResonanResonancece

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

Nor

mal

izad

tra

nsm

issi

on (

dB)

(a)

0

1

2

3

4

5

6

Ph

ase

shif

t (r

ad)

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.50

20

40

60

80

100

120

140

× 10-4Normalized frequency detuningD

elay

(p

s)

(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 Tk K

τ- thermal dissipation timeρ-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 thickness450-nm widthBuffer layer: 3-µm silicaMode 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)

Nor

mal

ized

tra

nsm

issi

on(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: ~20dBThe 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

Delay versus the pump power Delayed waveforms

(b)

Maximum delay of ~100 ps


0 100 200 300 400 5000.00

0.25

0.50

0.75

1.00 -37.0dBm 3.2dBm 13.6dBm

Inte

nsi

ty /

a.

u

Time / ps

(c)

Return-to-zero (RZ) signal

5Gb/s

5G RZeye 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

Maximum delay of 95 ps

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 spectrathe 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

sp

ectr

um

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

Nor

mal

ized

tra

nsm

issi

on (

dB

)

0

1

2

3

4

5

6

Ph

ase shift (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

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

shif

t (r

ad)

4 6 8 10 12 14 160

1

2

3

4(b)

Pump power (dBm)

Ph

ase

shif

t (r

ad)

Phase shift vs. pump power

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


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 Q j jQ 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.

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/div2.5ns/div

FSK Eye diagram

5dB/div0.5nm/div

FSK Spectrum

s1 s2p

Input NRZ signal

demodulated signal: upper sideband

demodulated signal: upper sideband

500μW/div500ps/div

Fangfei Liu et al., APOC 2008


Optical temporal differentiator

:

In the critical coupling region (QL = QE), the transfer function of the microring resonator is:

00

2( ) ( )

QT j

A typical function for a first-order temporal differentiator


Experimental results

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(b)

No

rmali

zed

am

pli

tud

e (

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 μmBandwidth : 2.5 nmResonance 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 12 2

y xy

t td dd 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 inputsignal x

optical outputsignal y

12

1 12 2

y xy

t td dd d


Silicon microring for bio-sensing

DNA probe is attached to the ringAfter 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

yikai su

Documents