introduction to photonic integrated circuits

Microphotonic Devices Group

Introduction to Photonic Integrated Circuits

Min-Hsiung Shih(施閔雄)

Research Center for Applied Sciences (RCAS)&

Taiwan International Graduated Program (TIGP)Academia Sinica, Taiwan

May 9, 2008


Optical communication and photonic integrated circuits

Property of light

Elements for optical communication system1) Lasers and amplifiers2) Waveguides and fibers3) Modulators4) Multiplexer and Demultiplexer elements5) Photodectors

Photonic crystal waveguide and bending structures

Outlines


Speed Limitation of Electrical Communications


Optical Communication System

Typical DWDM (Dense Wavelength Division Multiplexing) system


Elements for Photonic Integrated Circuits


Example of Photonic Integrated CircuitsPhotonic Integrated Circuitcombines tunable laser and optical modulator 10/09/2007

JDSU demonstrated a photonic integrated circuit (PIC) thatcombines a tunable laser and optical modulator, using a technology known as the Integrated Laser Mach Zehnder (ILMZ).The PIC will allow the company to develop smaller, higher performance and more cost-effective tunable solutions that supportfaster network speeds. Tunable lasers are a key element requiredor the deployment of agile optical networks (AON). Such networksare deployed by service providers to scale network infrastructuresand replace slow, manual operations with simplified, dynamicnetwork solutions that can quickly respond to fluctuating traffic traveling over fiber optic networks. Tunable lasers providedynamic reconfigurability by allowing network operators to switchfrom one wavelength to another on demand, easing the cost ofpurchasing, storing and managing spare devices for wavelengthmanagement. The chip includes a widely tunable laser and MachZehnder modulator on a single chip that is small enough to fit on thetip of a finger. It will be incorporated into full-band tunabletransponders and transceivers within compact packages, such asthe 300-PIN small form factor (SFF) and pluggable small formfactors (XFP) starting in 2008. This combination enables JDSU to support transmission speeds greater than 11.3 Gigabits persecond and is scalable to support 40Gbit/s networks.

From www.epn-online.com


Optical Waveguides

Geometry of a Rib (Ridge) waveguide in planar photonic circuits


Types of Optical Waveguides

• SEM image of a dielectric rib (ridge) waveguide

• The calculated mode profile

n2

n1

n3


Optical Fiber

• High index core surround by lower index cladding

• The numerical aperture NA

0

2/122

sin

)(

nNA

nnNA claddingcore

=

−=

α


Semiconductor Lasers

Edge-emitting laser

VCSEL

DFB laser

Si Raman laser


Optical Amplifiers - EDFA• Erbium doped fiber

amplifier (EDFA)• Advantages

1. Cover wide wavelength range

2. Large total output power (> 1000 mW)

3. Large dynamic power range

• Disadvantages1. Not linear over the

working range2. Total power keep

constant for channels


Optical Amplifiers - SOA

• Semiconductor optical amplifier (SOA)

• Advantages1. For larger range of

wavelength

2. Easy to integrate to other elements

• Disadvantages1. Lower output power(5-

10 mW)

2. Crosstalk influences


Different Types of Optical Modulators

E-O Modulator Resonant Microdisk

Mach-ZehnderStructure

Size Large Compact

> 10 GHz

Vπ High Low Low

High

Larger

Speed > 10 GHz ~ 1 GHz

Cost Low Low


Demultiplexer Elements

Prism•Not used in DWDM system



Diffraction Grating• Reflection light is not

homogeneous

• Not used in DWDM system



Array waveguide grating (AWG)• Array WGs make phase shift,

and different focus in 2nd cavity• Higher No. of channels (>64)• Small wavelength spacing• Temperature dependence &

higher cost

λ (nm)

λ (nm)


Multiplexer Elements

• Optical couplers1. Simple, low cost

2. High insertion loss

3. Never be a DeMUX


Multiplexer Elements

Cascaded Mach- Zehnders

• Easy integrated

• Can be used for MUX/DeMUX


Photodectectors in Photonic Circuits

Transfer optical signals to electrical signals

The opposite way to lasers


Photodectectors in Photonic Circuits

Spectral response of different detector materials For fiber communication (~λ=1300-1550nm), InGaAs and Ge are prefer materials for photodetectors


End……


Photonic Crystal Devices for Photonic Integrated

Circuits

Min-Hsiung Shih(施閔雄)

Research Center for Applied Sciences (RCAS)&

Taiwan International Graduated Program (TIGP)Academia Sinica, Taiwan

May 16, 2008


Elements for Photonic Integrated Circuits


Building Blocks for Photonic Integrated Circuits with 2D Photonic Crystals

Laser / Light Source Waveguide / Bend Modulator

Photodetector

λ1, λ2, λ3 …

λ4, λ5, λ6 …


Photonic Crystal Defect Laser Cavity

• Micro-Disk Lasers

• Photonic Crystal D3 Membrane Cavity

• Continuous-Wave (CW) Operation Photonic Crystal Laser Cavity under


III-V Gain Materials for Semiconductor Lasers


Micro-Disk Laser Cavity

~ 5 μm

SEM image of a micro-disk laser from angle view4 InGaAsP

QWs enbeded

240 nm


Micro-Disk Laser CavityWhispering-gallery resonant modes

of a micro-disk

1st order mode 2nd order mode


Micro-Disk Laser Cavity

Lasing spectrum

1.40 1.45 1.50 1.55 1.60 1.65 1.70

0.0

20.0p

40.0p

60.0p

80.0p

100.0p

120.0p200712QW12 MD1_a6incident power = 6 mWduty cycle 1.5 % pulsed width 30 nsOSA res 1 nm

Mon

itor V

alue

(W)

Wavelength (μm)

1.35 1.40 1.45 1.50 1.55 1.60 1.650.0

1.0x10-11

2.0x10-11

3.0x10-11

4.0x10-11

5.0x10-11

6.0x10-11

7.0x10-11

8.0x10-11

Δλ=108nm

Mon

itor V

alue

(W)

Wavelength (μm)

PL from 200712QW 12incident power = 6 mW

λ=1555nm72.94 pW

PL spectrumGain peak ~ 1550 nm


0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

In-Plane Propagation DirectionΓΚΜ

Nor

mal

ized

Fre

quen

cy (a

/λ0)

Γ

Membrane Photonic Crystal Defect Laser Cavity

Band Gap

Two dimensional triangular photonic crystal suspended membrane with r/a = 0.3, d/a=0.6 and dielectric constant ε=11.56

3D Band diagram of the photonic crystal membrane from finite-difference time-domain (FDTD) method


Finite-Difference Time-Domain Method Simulation

The highly localized field in photonic crystal structures couldn’t be solved analytically

The finite-difference time-domain (FDTD) method can simulate the evolution of the field which governed by Maxwell’s equation in real time space domain

Simulated Hz of a D3(19 missing holes) suspended membrane photonic crystal cavity by three dimensional

FDTD method


Quality Factor (Q) of CavityThe decay of energy in a cavity is expressed in term of the quality factor or Q.

Different notations1)1) Theoretical def. Theoretical def.

2)2) Experimental data in frequency domainExperimental data in frequency domain

3)3) Photon life time, Photon life time, ττpp

)()(2

noscillatioofcycleainlostenergyresonanceatcavitytheinstoredenergyQ π

=

2/1

0

ωωΔ

=Q

0ωτ Q

p =


D3 Membrane Photonic Crystal Cavity

~3 μm

240 nm InGaAsP


Lasing Data from D3 Membrane Photonic Crystal Cavity

The lasing spectrum (red) of a suspended membrane D3 photonic crystal laser cavity.

The resonance peaks of the spectrum match well in the predicted normalized frequency which have higher quality (Q) factors and inside the high gain region.

0.32 0.33 0.34 0.35 0.36 0.37 0.38 0.39

0.01

0.1

1

0.31 0.32 0.33 0.34 0.35 0.36 0.37 0.38 0.39100

1000

10000

Res

onan

ce M

ode

Qua

lity

Fac

tor

Normalized Frequency (a/λ 0)

12/15/2002 offseted -0.025 in normalized frequency


Suspended Membrane and Sapphire-Bonded Structure

Sapphire substrate, n= 1.7

InGaAsP layer

Air-dielectric-air structure has better confinement for the localized fields

The structure only laseunder pulse width conditions

Air-dielectric-sapphire structure has less confinement for the localized fields

This structure can lase under continuous wave (CW) conditions

Thermal conductivityAir : 2.5x10-5 W/cm·K and Sapphire : 5x10-1 W/cm·K


Quality Factor (Q) of Sapphire-Bonded Photonic Crystal Cavity

0.24 0.26 0.28 0.30 0.32 0.340

2,000

4,000

6,000

8,000

10,000

12,000

14,000

Qua

lity

Fact

or

a/λ

(a)

0.28 0.30 0.32 0.34 0.36 0.380

2,000

4,000

6,000

8,000

10,000

12,000

14,000

a/λ

(b)

D3 on Sapphire D3 on Air

0.24 0.26 0.28 0.30 0.32 0.340

2,000

4,000

6,000

8,000

10,000

12,000

14,000

a/λ0

D4 on Sapphire

0.24 0.26 0.28 0.30 0.32 0.340

2,000

4,000

6,000

8,000

10,000

12,000

14,000

a/λ0

D4 on Sapphire

(c)

D4

D3


Continuous Wave (CW) Operation of Sapphire-Bonded Photonic Crystal

Cavities

3.2 μm

SEM image of D4 sapphire-bonded cavity from angle view

1.57 1.58

-30

-20

-10

0

Out

put p

ower

(dB

)

Wavelength (μm)

0.0 0.5 1.0 1.5 2.0 2.50.0

0.2

0.4

0.6

0.8

1.0

Out

put P

ower

(a.u

.)

CW incident pumped power (mW)


Side-Mode Suppression Ratio (SMSR) of Sapphire-Bonded PhC Cavities

1.50 1.55 1.6010-12

10-11

10-10

10-9

10-8

(a)CW Condition

Inte

nsity

(a.u

.)

Wavelength (μm)

30 dB

1.50 1.55 1.6010-12

10-11

10-10 (b)

13 dB

Inte

nsity

(a.u

.)

Wavelength (μm)

Pulse Condition 8/800 ns

In fiber communication or CATV system, SMSR > 25 dB is considered a single mode light source.


Lasing Modes of Sapphire-Bonded Photonic Crystal Cavities

Since PhC is scalable, the lasing wavelength varied with PhC lattice constant.

The lasing wavelengths are separated into two groups, group A has normalized frequency ~ 0.252, and group B has ~ 0.263.370 380 390 400 410 420

1350

1400

1450

1500

1550

1600

1650

B

Lasi

ng W

avel

engt

h (n

m)

Lattice Constant (nm)

A0.252

0.263


Comparison of Lasing Spectrum and 3D-FDTD Calculation

Mode B lasing spectrum (red curve)from a PhC laser under CW pumping condition.3D FDTD Calculated quality factor (Q) spectrum (orange) in normalized frequency axis.The shift between two spectra is about 1 %.

0.24 0.25 0.26 0.27 0.28

10-11

10-10

10-9

10-8

0.24 0.25 0.26 0.27 0.28102

103

104

Normalized Frequency (a/λ)

B

AIntensity (a.u.)

Qua

lity

Fact

or (Q

)


Photonic Crystal Waveguides

Introduction to photonic crystal waveguides

Doubly-bent photonic crystal waveguides

Photonic crystal Mach-Zehnder structure


Introduction to Photonic Crystal Waveguide

Band Gap

ΓK

0.0 0.2 0.4 0.6 0.8 1.00.0

0.1

0.2

0.3

0.4

Nor

mal

ized

Fre

quen

cy (a

/λ0)

In-Plane Propagation Constant β (x π/a)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

In-Plane Propagation DirectionΓΚΜ

Nor

mal

ized

Fre

quen

cy (a

/λ0)

Γ


Types of Photonic Crystal Waveguides

Suspended Membrane

Oxide Substrate

Deeply Etched


Loss in Photonic Crystal Waveguides from FDTD

• The out-of-plane radiation loss of the waveguide is generally large for modes in the radiation light cone.

• Low loss transmission for suspended membrane and oxidized lower claddingstructures occurs at the vicinity of the Brillouinzone boundary.

projection of sapphire light cone

projection of air light cone

0.0 0.2 0.4 0.6 0.8 1.0

0.1

1

10

100

1000

10000

Thin Oxide Thick Oxide

Vert

ical

Rad

iatio

n Lo

ss (c

m-1)

In-plane Wave Vector β (π/a)

Deep Undercut Shallow Undercut

Deeply-Etched

3.0 μm

1.0 μm

From Wan Kuang, MPDG, USC


InGaAsP Membrane Photonic Crystal Waveguides

Suspended Membrane

InGaAsP

InP scarified layer

InP substrate


Fabrication of Photonic Crystal Membrane Waveguides

InP Substrate

InGaAsP 240 nm

SiNx layer 50 nm

PMMA 100 nm

E-Beam lithography

RIE CF4 Etching ECR etching

with CH4/H2/Ar HCl wet etching

at 0ºC

Remove all masks

lapping & cleaving


SEM Images of InGaAsP Membrane Photonic Crystal Waveguides

10 μm

10-15 periods


Transmission Through Straight Photonic Crystal Waveguides

Bandstructure and measured transmission of suspended membrane photonic crystal waveguide


Group Index of Propagated Mode in Photonic Crystal Waveguides

0.0 0.2 0.4 0.6 0.8 1.00.0

0.1

0.2

0.3

0.4

Nor

mal

ized

Fre

quen

cy (a

/λ0)

In-Plane Propagation Constant β (x π/a)

gg n

c=

∂∂

=βων

0.265 0.270 0.275 0.280 0.285 0.290 0.295 0.3000

10

20

30

40

50

60

1st band

2nd band

Gro

up in

dex

ng

Normalized Frequency

Group Indices of two guided bands


1480 1500 1520 1540 1560

4

6

8

10

Gro

up In

dex

Wavelength (nm)

r/a=0.30 r/a=0.33

0 5 10 15 20

Group Index

Spe

ctra

l Am

plitu

de

ng = 5.14

Comparison of the Predicted and Observed

Group Indices of Photonic Crystal Waveguides

Lng2

2λλ =Δ

Fabry-Perot oscillation period

Group indices of defect waveguide modes obtained from bandstructure

a= 420 nm, r/a = 0.315, Length = 292 μm


Fabry-Perot Oscillations from Doubly Bent Photonic Crystal Waveguides

LBend= 42.42 μm

31.92 μm

21.42 μm

Fabry-PerotRBend

RBend

Reflection from the two bends will lead to Fabry-Perot oscillations with a period that should be inversely proportional to LBend


1400 1450 1500 1550 16000.0

0.1

0.2

0.3

0.4

0.5

0.6

Tran

smis

sion

(a.u

.)

Wavelength (nm)

Theoretical Prediction of Transmission Through Photonic Crystal Waveguides by Finite Element

MethodCalculated Transmission

Straight waveguide

Bent waveguide

r/a = 0.3

74 nm


1460 1480 1500 1520 1540 1560 15800.0

0.2

0.4

0.6

0.8

1.0

Tran

smis

sion

(a.u

.)

Wavelength (nm)

Comparison of Transmission Measurement and Simulation

• Fabry-Perot oscillations were observed in measured transmission.

• The oscillation from the bent section can be identified by the period.

Measured result

r/a~0.31

Simulated result r/a=0.30


1480 1490 1500 1510 1520

Tran

smis

sion

( a.

u.)

Wavelength ( nm )

1500 1510 1520 1530 1540

Tran

smis

sion

( a.

u.)

Wavelength ( nm )

1500 1510 1520 1530 1540

Tran

smis

sion

( a.

u.)

Wavelength ( nm )

Reflectance Extracted from Measured Spectra

lengthLratiovalleytopeakK

KKR

LdB

::

1011 10

−−−

⎟⎟⎠

⎞⎜⎜⎝

⎛

+−

=⎟⎠⎞

⎜⎝⎛ α

Error-bars represent the range of αdBfrom 2 to 20 dB/mm.

LBend = 21.21 μm

1480 1490 1500 1510 1520

Tran

smis

sion

( a.

u.)

Wavelength ( nm )

LBend = 42.42 μm

1480 1500 1520 1540 1560

Tran

smis

sion

( a.

u.)

Wavelength ( nm )1500 1510 1520 1530 1540 1550

0.0

0.1

0.2

0.3

Wavelength ( nm )

Mea

sure

d R

efle

ctan

ce (a

.u.)


1520 1530 1540 15500.0

0.2

0.4

0.6

0.8

Wavelength ( nm )

Sim

ulat

ed R

efle

ctan

ce (a

.u.)

1520 1530 1540 15500.0

0.1

0.2

0.3

Wavelength ( nm )

Mea

sure

d R

efle

ctan

ce (a

.u.)

Simulated Reflectance

r/a > r/a ~ 0.3 r/a=0.30

Measured Reflectance

Comparison of Measured and Simulated Reflectance


Fabricate asymmetric Mach-Zehnder interferometers and characterize the transmission as a function of wavelength.

The Fourier transform of the transmission spectrum will contain information about the propagation coefficient of the photonic crystal waveguide mode.

The Basic Idea of Mach-ZehnderStructure


Photonic Crystal Mach-Zehnder

Structures in InP

InGaAsP/InP Membrane Structure


Mach-Zehnder Transmission Data

Wavelength (nm)

1550 1560 1570 1580 1590 1600 1610 1620

1550 1560 1570 1580 1590 1600 1610 1620

1550 1560 1570 1580 1590 1600 1610 1620

Inte

nsity

Inte

nsity

Inte

nsity

162015801560 1600

symmetric structure

path lengthdifference

75 μm

path lengthdifference121 μm

expect transmittedintensity to

contain oscillations that

go ascos(βΔL),

andβ= β(ng)


The Group Index of Photonic Crystal Waveguide

Three- Dimensional Finite Element Calculation


Fourier Transform of Transmission Spectra

Symmetric Mach-Zehnder

0 2000 4000 6000 8000 10000

No Dominant signals form the measured transmission


Mach-Zehnderwith Path Length Difference of 75 μm


0 2000 4000 6000 8000 10000

502 μm Interference between two branches with a propagation index of 6.7



Mach-Zehnderwith Path Length Difference of 121 μm

0 2000 4000 6000 8000 10000

637 μm

2230 μm

Interference between thetwo branches with a propagation index of 5.3

Fabry-Perot mode propagatingthrough the substrate


SummaryPhotonic crystal device, can be engineered by Photonic crystal device, can be engineered by changing the geometry, is a excellent candidate for changing the geometry, is a excellent candidate for dense photonic integrated circuits.dense photonic integrated circuits.

Photonic crystal defect laser cavities were obtained Photonic crystal defect laser cavities were obtained from from InGaAsPInGaAsP membrane.membrane.

Smallest continuousSmallest continuous--wave (CW) photonic crystal wave (CW) photonic crystal laser cavities was achieved on the sapphire laser cavities was achieved on the sapphire substrate.substrate.

Photonic crystal waveguides and its MachPhotonic crystal waveguides and its Mach--ZehnderZehnderstructure are demonstrated.structure are demonstrated.


Thank You


Propagation Loss Measurements using the Fabry-Perot Resonance Method

ΓK = 6.80~ 10.56 dB/mm (15.66 ~ 24.32 cm-1)

Fabry-Perot Spectrum of Stripe PCWG

1550.00 1575.00 1600.00 1625.00Wavelength ( nm )

Inte

nsi

ty

The Propagation Loss ΓK


Characterization Setup

introduction to photonic integrated circuits

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