Photonic Integrated Circuit BasedPhotonic Integrated Circuit Based Coherent Receivers:

A New Paradigm For Optical Components

G F i Li bG. Ferris LipscombECOC Market FocusSeptember 20, 2010

Agenda

• Advanced Coding Schemes Use Phase Encoding To Allow Multiple Bits Per Symbol–DQPSK–Coherent (DP QPSK)

• Phase Decoding Requires High Performance Interferometers ase ecod g equ es g e o a ce te e o ete sWithin Receivers

• Photonic Integration Is Well Suited To Mass Produce The Required Precision Optical Systemsq p y–PIC DQPSK Demodulators–PIC 90° Hybrids For Coherent Systems

• Hybrid Photonic Integration Based Integrated Receivers• Hybrid Photonic Integration Based Integrated Receivers–Limit Skew Between Optical Paths–High Performance In A Compact Package

Many Approaches Have Been Used For Next Gen. Line Side 40G and 100G Transmission

Category Duo‐Binary DPSK DQPSK Coherent(DP‐QPSK)

Speed 40Gbps 40Gbps 40Gbps 40/100GbpsSpeed 40Gbps 40Gbps 40Gbps 40/100Gbps

System Co’sCaptive Production 

Huawei ALU HuaweiInfinera

CienaALUHuawei

(Line Cards) Cisco/CoreOpticsInfinera

MSA Form Factor  NeoPhotonics Mintera/Oclaro Oclaro Cisco/CoreOpticsTransponder Vendors

FinisarOpnext

OpnextFinisarTeleoptix

YokogawaFinisarOpnextJDSU

OpnextMintera/OclaroAcacia

JDSU

• Multiple Techniques Have Been Used For 40 Gbps Transmission• A Consensus Has Emerged That Coherent Transmission (DP-QPSK) Will Dominate 100 Gbps

M Ad d T h i F 40 d 100 Gb E l “Ph Shift K i ” (PSK)• Many Advanced Techniques For 40 and 100 Gbps Employ “Phase Shift Keying” (PSK)• The Transmitted Information Is Encoded In The Phase Rather Than The Intensity

Coherent Adds ‘Texture’ To The Optical Field

Transmitter ReceiverTraditional

DynamicReference

One Bit Per Symbol

Transmitter Receiver

ReferencePol. + Phase

Four Bits Per Symbol

“Coherent” Techniques Open The Phase-Polarization Constellation For Use In Transmission

• Traditional Optical Coding Schemes Use Only Optical Power (Or Its Absence) To Carry Information– Only Expect 1 Bit Of Information Per Transmitted Symbol.– Receiver Expected To Decide If It Is Seeing A ‘1’ Or A ‘0’

• Encoding Information In Phase And Polarization Instead Of Just Power Opens Additional pDimensions – Symbol Constellations In Multi-dimensional Space Can

Encode Multiple Bits For Each Transmitted Signal– Receiver May Not Be Expected To Decide The Value Of The y

Data It Is Seeing• Optical Detectors Are Only Sensitive To Power• A Passive Optical Circuit Must Project The Optical

Signal Against A Reference to Produce Optical Signal Against A Reference to Produce Optical Intensity Levels For the Photodetectors

• Producing All These Projections With The Proper Timing and Fidelity Is CriticalPh t i I t ti E bl Th Th Cl • Photonic Integration Enables The The Close Coupling Required For Coherent Systems

Advanced Transmission Requires Close Coupling Of Active And Passive Functions

ActiveIn Traditional 10G Systems Active And Passive Functions Are Separate

Active

Passive λ1 – 10G

λ2 – 10G

λn-1 – 10G

λ1 – 10G

λ2 – 10G

λn-1 – 10GAWG AWG

Transponder

Transponder

Transponder

Transponder

Transponder

Transponder

λn – 10Gλn – 10G

p

Transponder

p

Transponder

λ1 100Gλ1 100G

Advanced Transmission Approaches Require Active And Passive Functional Integration

PassiveActive+Passive Active+Passive

λ1 – 100G

λ2 – 10G

λn-1 – 40G

λ1 – 100G

λ2 – 10G

λn-1 – 40GAWG AWG

CoMix Rx

DQPSK Rx

PassiveCoMix Tx

DQPSK Txλn – 10Gλn – 10G

Change Phase To Intensity Modulation By Comparing The Signal To A Reference Signal

Tunable Laser Phase Modulation

Transmitter Receiver

Phase Demodulation

Optical Fiber (Polarization Uncontrolled)

Tunable Laser Phase Modulation

Transmitter Receiver

Phase Demodulation

Optical Fiber (Polarization Uncontrolled)

Reference

Signal

Simplest: In Phase = “1” 180° Out Of Phase = “0”

• Optical Phase Detectors Are Also Known As Interferometers• Modulators Are Often Phase Modulators Inside A Mach-Zehender Interferometer

Simplest: In Phase = 1 180 Out Of Phase = 0

odu ato s e O te ase odu ato s s de ac e e de te e o ete• There Are Two Different Approaches To Obtaining The Reference Signal

– “Differential”: Compare The Signal To A Time Delayed Version Of Itself (DQPSK)“Coherent”: Compare The Signal To An External Reference Laser– Coherent : Compare The Signal To An External Reference Laser• The “Coherence Length” Must Be Long Compared To The “Bit Length”

Photonic Integrated Circuits (PIC)

• Photonic Integrated Circuit Based Interferometers Are Being Mass Produced• Arrayed Waveguide Gratings (AWGs) Are Precision Interferometers

–Random Path Length Fluctuations Are Less Than 10 nmRandom Path Length Fluctuations Are Less Than 10 nm• PICs Are Also Being Used To Make Demodulators For Advanced Transmission

7

DQPSK Transmission

Tunable Laser

Modulator

ModulatorPBS PBS TDC 90°

Hybrid

PDs

PDs

TIA

TIADLI

FEC

ta DMUX

Data MUX

Drivers

DQPSK Receiver

Data In

Data Out

Dat

Opto‐electronic

Electronic

• Two Bits per Symbol Are Encoded Using Quadrature Phase Points–Two Data Streams At 20 Gbps Each

Per Channel Dynamic Dispersion Compensation Is Often Required–Per Channel Dynamic Dispersion Compensation Is Often Required• The Reference Is Provided By A Time Delayed Copy Of The Signal• In Some Cases The Each Polarization Is Used To Carry A DQPSK Signal

• Four Data Streams At 10 Gbps Each• Four Data Streams At 10 Gbps Each• Requires Active Polarization Analysis And Separation

40Gb/s DQPSK Demodulator• The Reference Is Provided By A Time Delayed Copy Of The Signal• The Demodulator Uses A Pair Of Delay-Line Interferometers (DLIs)

Th D d l t C t O ti l “Bit ” Th t C B Di tl D t t d• The Demodulator Creates Optical “Bits” That Can Be Directly Detected• Since The Polarization In The Optical Fiber Is Uncontrolled, The Polarization

Dependent Wavelength (PDW) Of The Demodulator Is CriticalPIC DQPSK D d l t A hi PDW V l Of <200MH–PIC DQPSK Demodulators Achieve PDW Values Of <200MHz

• Skew Between Paths Is Critical

Δτ ∼ 1-bitΔφ =(2m – 0.25)π I α |Eτ0 + Eτ1|2

I α |Eτ0 - Eτ1|2

Δτ ∼ 1-bitΔφ =(2m+0.25)π

Q α |Eτ0 + iEτ1|2

Q |E iE |2Q α |Eτ0 - iEτ1|2

Dual Polarization Coherent (DP-QPSK)

Tunable Laser

Modulator

Modulator

ModulatorPBS PBS PBS

DSP

a DMUX

90°Hybrid

PDs

PDs

TIA

TIA

ADC

ADC

Data Out

Modulator

Data MUX

Drivers

LO

D

Data

90°Hybrid

PDs

PDs

TIA

TIA

ADC

ADC

Data In

Out

Opto‐electronic

Electronic

• Four Bits per Symbol Are Encoded Using Quadrature Phase Points And Two Polarizations–Four Data Streams At 28 Gbps Each

• The Reference Is Provided By A Separate Local Oscillator Laser• The Coherence Length Between The LO and The Signal Must Be Long

Compared To The Bit Period• Optical “Bits” Are Not Produced Directly By The 90° Hybrid But Must Be

Recovered Using Digital Processing

Signal Recovery For 40 and 100 Gbps Coherent

S+LIx

• The Signal Is Split Into Two Orthogonal Polarizations

90° HybridS-L

S+jL

S-jL

Ix

Qx– Not The Input Polarizations• Each Polarization Is Compared To The

Local Reference And The I and Q Components Extracted

S-jL Qx

PBS LOComponents Extracted• Differential Detection Can Be Used To

Improve Signal to Noise• The Output Intensities Represent The

F ll El t i Fi ld Of Th Si l

S+L

S-LIy

Full Electric Field Of The Signal– The “bits” Are Not Directly Represented

• Digital Processing Can Be Used To Remove Linear Transmission

90° Hybrid S+jL

S-jL Qye o e ea a s ss oImpairments And Extract The Data Stream

Impact of Skew On Coherent Transmission• In Coherent Transmission The In Phase And Quadrature Components Must Be Received

At The Same Time To Properly Decode The Bits• Path Length Differences In Fibers Lead To Skew Between Components Causing Errors• Path Length Differences In Fibers Lead To Skew Between Components Causing Errors• Uncorrected Path Length Errors Of 1mm Can Cause Bit Error Rates Of 10-5

Parametric SimulationIntegrated ReceivergChannels Matched

Discrete Receiver(Fiber Connected)1mm (5ps) fiber mismatch( p )

Hybrid Integration Combines Active & Passive Functions

• Allows Use Of The Best Material System For Each Function– PLC Integration For DWDM Passives– MEMS For High End Switching And VOA– Semiconductors For Active Lasers and Detectors– Highest Performance At The Lowest Cost

• Passive Devices Are Much Larger Than gActive Devices– Typical Wafer Size For PLC is 6 or 8 inches– Typical Wafer Size For InP is 2 or 4 inches

• Cost Of Wafer Processing Is Much • Cost Of Wafer Processing Is Much Higher For InP Than For PLC

• It Is Desirable Therefore To Mount Smaller InP Chips On Larger PLC Chips

• Issues Are Pick and Place, Sub-Micron Alignment And Reliable Attachment

• The Most Cost Effective Approach Depends On The Performance Required p qAnd The Degree Of Integration Needed.

40G & 100G Integrated Coherent Receiver (ICR)

• All Functional Elements Are Integrated Into A Compact Package• Skew Limited To Under 1 ps

Conclusions

• Advanced Coding Schemes Use Phase Encoding To Allow Multiple Bits Per Symbol–DQPSK–Coherent (DP QPSK)

• Phase Decoding Requires High Performance Interferometers ase ecod g equ es g e o a ce te e o ete sWithin Receivers

• Photonic Integration Is Well Suited To Mass Produce The Required Precision Optical Systemsq p y–PIC DQPSK Demodulators–PIC 90° Hybrids For Coherent Systems

• Hybrid Photonic Integration Based Integrated Receivers• Hybrid Photonic Integration Based Integrated Receivers–Limit Skew Between Optical Paths–High Performance In A Compact Package