1 | © 2015 Infinera Confidential & Proprietary Next Generation Coherent Technologies Geoff Bennett: Director, Solutions and Technology A Review of High-Speed Coherent Transmission Technologies for Long-Haul DWDM Transmission at 100G and Beyond Geoff Bennett, Kuang-Tsan Wu, Anuj Malik, Soumya Roy, and Ahmed Awadalla IEEE Communications Magazine, October 2014

2 | © 2015 Infinera Confidential & Proprietary A high level summary of optical transmission evolution 2010 Pre-Coherent Era First Coherent Era 2015 Second Coherent Era Gb/s 9.5Tb/s 12, 18, 24Tb/s Simple intensity modulation Direct detection No real signal processing HD-FEC Single wave transponders Fixed grid Phase modulation Coherent detection Receiver-based DSP 1 st gen SD-FEC Super-channels emerge Flexible grid emerges Advanced phase modulation Coherent detection Rx and Tx-based DSP 2 nd gen SD-FEC Super-channels dominate Flexible grid dominates

3 | © 2015 Infinera Confidential & Proprietary The Coherent Receiver (First Generation)

4 | © 2015 Infinera Confidential & Proprietary How does a coherent detector work? In commercial DWDM systems Demodulator Digital Signal Encoded as Phase Changes Photodetect Amplify Digitize Compensate Coherent Detector Local Oscillator Local Oscillator

5 | © 2015 Infinera Confidential & Proprietary  To a first approximation linear impairments can be characterized by a transfer function describing: Chromatic Dispersion PMD Tx and Rx filtering  As the signal propagates, noise is accumulated Amplified Spontaneous Emissions, Shot Noise, DRB, etc. Solving for Linear Impairments Linear equalizer transfer function Linear transfer function describing effects of CD and PMD Forzati et al: Non-linear compensation techniques for coherent fibre transmission. Proc. of SPIE-OSA-IEEENol

6 | © 2015 Infinera Confidential & Proprietary  In other media the solution to noise is to increase signal strength  But optical fiber is a non-linear medium The non-linear component of refractive index changes with the intensity of the optical power Refractive index at a given power level ( I ) given by: The Challenge of Compensating for Noise n(I) = n 0 + n 2. I n 0 Linear refractive index n 2 2 nd order non-linear refractive index Kerr Effect

7 | © 2015 Infinera Confidential & Proprietary  Self Phase Modulation  Cross Phase Modulation  Four Wave Mixing Non-Linear Effects Pulse “compression” Non-linear interaction of 1 and 2 causes the appearance of 3 and 4 If symbols in adjacent channels “line up” their power level can exceed the NL threshold 

8 | © 2015 Infinera Confidential & Proprietary The Coherent Transmitter (Second Generation)

9 | © 2015 Infinera Confidential & Proprietary Tx DSP and DAC Are Key Additions S1 S2 90 DAC DSP Laser 4 Important functions 1: High order modulation 2: Pulse Shaping 3: Pre-dispersion 4: Non-linear compensation

10 | © 2015 Infinera Confidential & Proprietary  Higher order modulation has “fewer photons per bit”, and requires higher symbol resolution Tx DSP and DAC Functions: Higher Order Modulation QPSK16QAM 2 bits 4 bits A given symbol has the same absolute optical power limit, but 16QAM is carrying twice the number of bits The resolution between symbol states is much tighter for 16QAM Result:16QAM has 5X shorter reach vs QPSK, for only 2X increase in spectral efficiency

11 | © 2015 Infinera Confidential & Proprietary Without DSP and DAC Tx DSP and DAC Functions: Intelligent Pulse Shaping With DSP and DAC: signal from the side lobes has been intelligently incorporated into the main pulse We can decrease channel spacing, and increase total fiber capacity, without a reach penalty ICI Too close a channel spacing will result in a reach penalty

12 | © 2015 Infinera Confidential & Proprietary Pulse-Shaped QPSK Spectrum  Shaped pulses can be spaced “at the Baud rate” Eg: 32GBaud signal could be spaced at just over 32GHz The additional spacing is known as the alpha, and the practical limit for alpha is 3-4% before OSNR penalties are incurred 50GHz Terminology: “Nyquist DWDM” LaserMod DSPDAC Transmitter Alpha

13 | © 2015 Infinera Confidential & Proprietary Fixed Grid Coherent Transmission PM-QPSK 100Gb/s 50GHz In 1 st generation coherent, 100G PM-QPSK fits efficiently into 50GHz grid spacing These fixed grids are spaced so that individual carriers can be optically added or dropped in ROADM We could choose to drop these carriers

14 | © 2015 Infinera Confidential & Proprietary PM-QPSK 100Gb/s 50GHz Pulse Shaped Carriers What can we gain if: We have narrower carriers? and… We don’t need to optically add/drop? We now become limited by the fixed grid plan

15 | © 2015 Infinera Confidential & Proprietary Moving to Flexible Grid Let’s forget about fixed grids We can move the carriers to “Nyquist” spacing If we need to optically add/drop we create a guard band at the ends of this super- channel The width of this super-channel can be flexible, but not totally flexible because of the need to manage optical spectrum Super-Channel Example shows a 12 carrier super-channel

16 | © 2015 Infinera Confidential & Proprietary We can create sliceable super-channels These carriers are generated on the same line card…so does that mean they have to start and terminate at the same points? Super-Channel 1Super-Channel GHz 1.2T PM-QPSK 1.2 Tb/s Super-Channel 162.5GHz 400G PM-QPSK 162.5GHz 400G PM-QPSK 162.5GHz 400G PM-QPSK 3 x 400 Gb/s Super-Channels (3 x = 487.5GHz) 12 x 100 Gb/s Individual Channels 50GHz 100G PM-QPSK 50GHz 100G PM-QPSK 12 X No…they can be sliced

17 | © 2015 Infinera Confidential & Proprietary  Flexible slots have a total width granularity of 12.5GHz, but a center frequency granularity of 6.25GHz  193.1THz reference frequency, and all flexible slots are defined with respect to this reference frequency (outlined in red) The flexible slot is defined by two numbers, n and m The center of a given flexible slot is n x 6.25GHz from the reference frequency, and is m x 12.5GHz total width ITU-T G Flexible Grid

18 | © 2015 Infinera Confidential & Proprietary For Flexible Grid: Super-Channels are Mandatory Key point: To achieve the claimed fiber capacity in next generation coherent systems, super- channels are mandatory This is a super-channel Whether it’s based on one card or many Whether it’s based on PICs or discretes Super-Channel

19 | © 2015 Infinera Confidential & Proprietary Tx DSP and DAC Functions: Pre-dispersion Tx Rx 1 or 0? We know a signal will be dispersed as it travels along the fiber Using DSP and DAC we can apply negative direction of dispersion before transmission Dispersion behavior can be tightly controlled In reality we would divide up the job of dispersion compensation between Tx and Rx

20 | © 2015 Infinera Confidential & Proprietary  Promising demonstrations of NL Compensation gains of >1dB in “real-world” OSNR improvements  An array of complex algorithms: NPCC: Nonlinear Polarization Crosstalk Correction (for XPM) AFCPR: Adaptive Fee-Forward Carrier Phase Recovery RF Pilot waves DBP: Digital Back Propagation, Perturbed DBP (for SPM) Tx Pulse Pre-dispersion (aka pre-compensation, pre-emphasis) Tx DSP and DAC Functions: Non-Linear Compensation Commercial implementation over the next few years

21 | © 2015 Infinera Confidential & Proprietary Solving for Non-Linear Effects: This is not a trivial excercise! Forzati et al: Non-linear compensation techniques for coherent fibre transmission. Proc. of SPIE-OSA-IEEENol Each equation represents the complex envelope, A, of two orthogonal polarization components of the electric field  2 : attenuation coefficient  2 : dispertion parameter  : nonlinear coefficient z: propagation direction t: time

22 | © 2015 Infinera Confidential & Proprietary Next Generation Coherent: The End Result Extended C-Band capacity increases from 9.5Tb/s to 12Tb/s using PM- QPSK. PM-8QAM delivers 18Tb/s, and PM-16QAM delivers 24Tb/s. Tx Pulse Shaping Most applicable on submarine links, but allows for a 5X increase in Chromatic Dispersion compensation. Dispersion management Algorithms are still evolving. Longer term expectation is to deliver around a 1dB OSNR improvement. Non-linear compensation Seems to be saturating out at about 12dB NGC (about 1dB improvement) Next Gen SD-FEC

23 | © 2015 Infinera Confidential & Proprietary Summary: Next Generation Coherent Technologies Outlined three distinct eras in optical transmission We’re in transition to the Second Coherent Era Flexible Grid and Super-Channels become mandatory

24 | © 2015 Infinera Confidential & Proprietary Thank You! Geoff Bennett