Coherent Receivers: A New Paradigm For Optical Components. ECOC Market Focus September 20, 2010

Transcription

1 Photonic Integrated Circuit Based Coherent Receivers: A New Paradigm For Optical Components G. Ferris Lipscomb ECOC Market Focus September 20, 2010

2 Agenda Advanced Coding Schemes Use Phase Encoding To Allow Multiple Bits Per Symbol DQPSK Coherent (DP QPSK) Phase Decoding Requires es High Performance Interferometers e ete Within Receivers Photonic Integration Is Well Suited To Mass Produce The Required Precision Optical Systems PIC DQPSK Demodulators PIC 90 Hybrids For Coherent Systems Hybrid Photonic Integration Based Integrated Receivers Limit Skew Between Optical Paths High Performance In A Compact Package

3 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 40/100Gbps System Co s Captive Production (Line Cards) Huawei ALU Huawei Infinera Ciena ALU Huawei Cisco/CoreOptics Infinera MSA Form Factor NeoPhotonics Mintera/Oclaro Oclaro Cisco/CoreOptics Vendors Finisar Opnext Yokogawa Finisar Opnext Finisar Teleoptix Opnext JDSU Opnext Mintera/Oclaro Acacia Multiple Techniques Have Been Used For 40 Gbps Transmission A Consensus Has Emerged That Coherent Transmission (DP-QPSK) Will Dominate 100 Gbps Many Advanced d Techniques For 40 and 100 Gbps Employ Phase Shift Keying (PSK) The Transmitted Information Is Encoded In The Phase Rather Than The Intensity

4 Coherent Adds Texture To The Optical Field Transmitter Traditional Receiver One Bit Per Symbol Transmitter Pol. + Phase Dynamic Reference Receiver Four Bits Per Symbol

5 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 Dimensions 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 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 Intensity Levels For the Photodetectors Producing All These Projections With The Proper Timing and Fidelity Is Critical Photonic Integration ti Enables The The Close Coupling Required For Coherent Systems

6 Advanced Transmission Requires Close Coupling Of Active And Passive Functions Active In Traditional 10G Systems Active And Passive Functions Are Separate λ1 10G λ2 10G λn-1 10G λn 10G Passive AWG AWG λ1 10G λ2 10G λn-1 10G λn 10G Active Advanced Transmission Approaches Require Active And Passive Functional Integration Active+Passive Active+Passive CoMix Tx λ1 100G Passive λ1 100G CoMix Rx λ2 10G λ2 10G DQPSK Tx λn-1 40G λn 10G AWG AWG λn-1 40G λn 10G DQPSK Rx

7 Change Phase To Intensity Modulation By Comparing The Signal To A Reference Signal Tunable Laser Phase Modulation Phase Demodulation Transmitter Optical Fiber (Polarization Uncontrolled) Receiver Reference Signal Simplest: In Phase = Out Of Phase = 0 Optical Phase Detectors Are Also Known As Interferometers Modulators Are Often Phase Modulators Inside A Mach-ZehenderInterferometer e e 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 The Coherence Length Must Be Long Compared To The Bit Length

8 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 nm PICs Are Also Being Used To Make Demodulators For Advanced Transmission 7

9 DQPSK Transmission Tunable Laser PBS Modulator PBS TDC Modulator DLI 90 Hybrid PDs PDs DQPSK Receiver TIA TIA ta DMUX Dat FEC Data In Data MUX Drivers Data Out 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 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 Requires Active Polarization Analysis And Separation

10 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) The Demodulator Creates Optical Bits That Can Be Directly Detected t Since The Polarization In The Optical Fiber Is Uncontrolled, The Polarization Dependent Wavelength (PDW) Of The Demodulator Is Critical PIC DQPSK Demodulators Achieve PDW Values Of <200MHz Skew Between Paths Is Critical Δτ 1-bit I α E Δφ =(2m 0.25)π τ0 + E τ1 2 I α E τ0 -E τ1 2 Δτ 1-bit Δφ =(2m+0.25)π Q α E τ0 + ie τ1 2 Q α E τ0 - ie τ1 2

11 Dual Polarization Coherent (DP-QPSK) Tunable Laser PBS Modulator Modulator Modulator PBS PBS Modulator LO 90 Hybrid 90 Hybrid PDs PDs PDs PDs TIA TIA TIA TIA ADC ADC ADC ADC DSP Data a DMUX Data Out Data In Data MUX Drivers 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

12 Signal Recovery For 40 and 100 Gbps Coherent The Signal Is Split Into Two Orthogonal Polarizations Not The Input Polarizations Each Polarization Is Compared To The Local Reference And The I and Q Components Extracted Differential Detection Can Be Used To Improve Signal to Noise The Output Intensities Represent The Full Electric Field Of The Signal The bits Are Not Directly Represented Digital Processing Can Be Used To Remove elinear Transmission sso Impairments And Extract The Data Stream 90 Hybrid PBS LO S+L S-L S+jL S-jL S+L S-L 90 Hybrid S+jL S-jL Ix Qx Iy Qy

13 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 Uncorrected Path Length Errors Of 1mm Can Cause Bit Error Rates Of 10-5 Parametric Simulation Integrated Receiver Channels Matched Discrete Receiver (Fiber Connected) 1mm (5ps) fiber mismatch

14 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 Active 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 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 And The Degree Of Integration Needed.

15 40G & 100G Integrated Coherent Receiver (ICR) All Functional Elements Are Integrated Into A Compact Package Skew Limited To Under 1 ps

16 Conclusions Advanced Coding Schemes Use Phase Encoding To Allow Multiple Bits Per Symbol DQPSK Coherent (DP QPSK) Phase Decoding Requires es High Performance Interferometers e ete Within Receivers Photonic Integration Is Well Suited To Mass Produce The Required Precision Optical Systems PIC DQPSK Demodulators PIC 90 Hybrids For Coherent Systems Hybrid Photonic Integration Based Integrated Receivers Limit Skew Between Optical Paths High Performance In A Compact Package