Advances in DWDM Transmission Technologies · PDF file1 | Infinera Confidential & Proprietary...

1 | Infinera Confidential & Proprietary

Advances in DWDM Transmission Technologies Geoff Bennett: Director, Solutions and Technology

Vinay Rathore: Snr. Director of Product Marketing


Some Background • Spectral efficiency

• The impact of fiber impairments as modulation rate increases

The elements of a modern transmission system: • Phase modulation

• Polarization multiplexing

• Coherent detection

• Advanced digital signal processing

The next steps in high speed transmission • Higher order modulation: 8QAM, 16QAM

• Nyquist DWDM technologies for the transmitter

• Flexible Grid DWDM

• “Faster than Nyquist” technologies

Agenda












Agenda


Total fiber capacity is a function of: • Data rate per wavelength

• Number of wavelengths in the fiber

Capacity and Reach are antagonistic • If you increase one, you tend to decrease the other

But the general trend of DWDM innovation is to increase the product of Capacity and Reach*

DWDM Capacity Scaling Technologies

*Emmanuel B. Desurvire. “Capacity Demand and Technology Challenges for Lightwave Systems in the Next Two Decades” JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 24, NO. 12, DECEMBER 2006


A History of DWDM Capacity x Distance Innovations

Emmanuel B. Desurvire. “Capacity Demand and Technology Challenges for Lightwave Systems in the Next Two Decades” JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 24, NO. 12, DECEMBER 2006

Laser technology evolves to fiber

“sweet spot”





“sweet spot”

EDFA innovation enables WDM – multiple waves

in a fiber





“sweet spot”


in a fiber

Chromatic Dispersion Management enables 2.5G to 10G evolution





“sweet spot”


in a fiber

Chromatic Dispersion Management enables 2.5G to 10G evolution Forward Error

Correction directly increases Cap*Reach





“sweet spot”


in a fiber


Correction directly increases Cap*Dist

Addition of L-Band could double fiber capacity*





“sweet spot”


in a fiber


Correction directly increases Cap*Dist

Addition of L-Band could double fiber capacity*

Raman amplification gives excellent distance boost


Large scale photonic integration • Makes the following technologies operationally scalable

Polarization muxing • Roughly 2x Capacity * Reach increase

Complex modulation • Increasing the “bits per symbol”

Coherent detection • Enables long haul transmission at 100Gb/s and beyond • Includes electronic CD and PMD compensation • >10x increase in Capacity * Reach

Soft Decision Forward Error Correction (SD-FEC) • Roughly 2x increase in Capacity * Reach vs HD-FEC

From fixed to flexible grid DWDM

Innovations that post-date this paper, but that are in production today


Super-channel: Multiple coherent carriers in a single line card, and brought into service at the same time

State of the Long Haul DWDM Market Today: Coherent, Flexible Grid Super-Channels

100G Line Card

100G Line Card

100G Line Card

100G Line Card

100G Line Card 500G Line Card

It’s practical to build (using PICs)…

Operationally scalable

The same or better optical performance as 100G


Laser

Continuous Wave

Laser Line Widths and Modulation Broadening

200GHz

100GHz

50GHz 25GHz

Telecom lasers have narrow line width


Laser

Continuous Wave

Laser Line Widths and Modulation Broadening

200GHz

100GHz

50GHz 25GHz

Telecom lasers have narrow line width

Modulating the carrier will broaden the width of the signal The faster you modulate, the broader the signal Modulator

0,1,1,1,0,0,0,0,1,1,0,1…

2.5Gb/s NRZ

10Gb/s NRZ

40Gb/s NRZ*

*This did not become a major implementation option


Why, at 40Gb/s and beyond, NRZ was not viable How long is a 1 or a 0?

2.5Gb/s 0.4 ns

The comparative duration of a symbol (bit)

0.1 ns 10Gb/s

0.025 ns 40Gb/s

0.001 ns 100Gb/s



2.5Gb/s 0.4 ns

The comparative duration of a symbol

0.1 ns 10Gb/s

0.025 ns 40Gb/s

0.001 ns 100Gb/s

Why is this so significant?

Because impairments like CD and PMD become

“proportionately worse” as the symbol duration

decreases



2.5Gb/s 0.4 ns


0.1 ns 10Gb/s

0.025 ns 40Gb/s

0.001 ns 100Gb/s

From 2.5G to 10G the industry had to learn to manage Chromatic Dispersion



2.5Gb/s 0.4 ns


0.1 ns 10Gb/s

0.025 ns 40Gb/s

0.001 ns 100Gb/s

At this symbol rate for NRZ, PMD become unacceptable, and compensation is difficult




2.5Gb/s 0.4 ns


0.1 ns 10Gb/s

0.025 ns 40Gb/s

0.001 ns 100Gb/s

At this symbol rate for NRZ, PMD become unacceptable, and compensation is difficult

NRZ uses a direct detector, which operates on a square law, so Rx compensation is “challenging”

Can we use “smart” compensation technologies?


We need to change to a different modulation technology












Agenda


Triple benefits of phase modulation…

Phase Modulation

Uses amplitude and phase to encode data, and is generally more tolerant of fiber impairments

We can use a coherent detector, which is linear, and thus allows us to compensate for linear

impairments in the receiver

We can use higher orders of phase modulation to encode more bits per symbol


Pol-Mux QPSK Modulation (Shows a single polarization here)

Im{Ex}

Re{Ex}

S1

This structure called a “Super Mach Zehnder”

S2

LD

Quadrature Phase Shift Keying

Q

I

90

Symbol constellation


QPSK Encodes 2 Bits Per Symbol

Q

I

Symbol constellation

Q

I

Q

I

Q

I

Q

I

01

11 10

00


Let’s add polarization: PM-QPSK

Q

I

X-Pol

Y-Pol

Q

I

X Polarisation

Y Polarisation

PM-QPSK carries 4 bits per “symbol” (A symbol is now the X and Y polarizations)


PM-QPSK at 100Gb/s

X and Y polarizations at the same wavelength 100Gb/s PM-QPSK is 25GBaud

But with FEC overhead this rises to between 28GBaud

and 35GBaud

How wide is the peak?

The goal is to fit 100Gb/s into a 50GHz frequency slot


Coherent Detectors

Use local oscillator • Highly discriminating receiver

• Low noise amplification

• Linear response


Coherent Detector In More Detail (For one wavelength only)

Incoming carrier (2 polarizations, each with 4 phase states)

ADC A/D Converter AMZ Adjustable Mach Zehnder DSP Digital Signal Processor LO Local Oscillator PD Photo Detector PBS Polarization Beam Splitter

LO

PD

PD

PD

PD

ADC

ADC

ADC

ADC

DSP

AMZ

AMZ

AMZ

AMZ

Optical Circuit Electronic Circuit

PBS

PBS



LO

PD

PD

PD

PD

ADC

ADC

ADC

ADC

DSP

AMZ

AMZ

AMZ

AMZ


PBS

PBS

ADC A/D Converter AMZ Adjustable Mach Zehnder DSP Digital Signal Processor LO Local Oscillator PD Photo Detector PS Polarization Splitter

1

Step 1: Separate the X and Y polarizations




LO

PD

PD

PD

PD

ADC

ADC

ADC

ADC

DSP

AMZ

AMZ

AMZ

AMZ


PBS

PBS


2

Step 2: Mix the two optical sources – signal and local oscillator




LO

PD

PD

PD

PD

ADC

ADC

ADC

ADC

DSP

AMZ

AMZ

AMZ

AMZ


PBS

PBS


3

Step 3: Generate a set of interference patterns in the AMZ array




LO

PD

PD

PD

PD

ADC

ADC

ADC

ADC

DSP

AMZ

AMZ

AMZ

AMZ


PBS

PBS


4

Step 4: Convert optical signals to analog electronic signals




LO

PD

PD

PD

PD

ADC

ADC

ADC

ADC

DSP

AMZ

AMZ

AMZ

AMZ


PBS

PBS


5

Step 5: Convert analog to digital and process



The Impact of Coherent Detection and Sophisticated Signal Processing

Chromatic dispersion is bad!

PMD is even worse!

Data rates limited to 10Gb/s

Distances limited to about 2,500km

We may need to install new types of fiber to scale capacity

Before After


The Impact of Coherent Detection and Sophisticated Signal Processing

Chromatic dispersion is bad!

PMD is even worse!

Data rates limited to 10Gb/s

Distances limited to about 2,500km

We may need to install new types of fiber to scale capacity

We love chromatic dispersion!

PMD…no problem!

Data rates of 100Gb/s or more per carrier

Transmissions distances of:

• 4,500km (QPSK)

• 10,000km (BPSK)

The older the fiber, the better! (sort of)

Before After












Agenda


Adding More Bits Per Symbol

QPSK

2 bits

Double this up for Pol-Muxing

PM-QPSK 4 bits



QPSK 16QAM

2 bits 4 bits


PM-QPSK 4 bits

PM-16QAM 8 bits

How wide is the peak?

50GHz

PM-QPSK: 100Gb/s PM-16QAM: 200Gb/s



QPSK 16QAM

2 bits 4 bits


PM-QPSK 4 bits

PM-16QAM 8 bits

Modulation order

PM BPSK

PM QPSK

PM 8QAM

PM 16QAM

C-B

and

Cap

acity (Tb/s)

5

10

15

20

Typ

ical

Rea

ch (

,00

0 k

m)

5

10

Reach drops dramatically using higher order modulation


Inter Symbol Interference (ISI) and Inter Carrier Interference (ICI)

0,1,1,1,0,0,0,0,1,1,0,1

ISI: Sequential symbols in the same carrier

0,1,0,1,0,0,0,0,1,1,0,1 1,1,0,1,0,0,1,0,1,0,0,0 0,0,0,1,0,0,0,0,1,1,1,1 0,1,1,1,0,1,1,0,1,0,0,1

ICI: Symbols in adjacent carriers

ICI


Evolving Transmitter Technology (Note: Showing one polarization for simplicity)

S1

S2

LD

90

Today: 100Gb/s PM-QPSK

28-35GBaud symbol rate

4,500km reach


Evolving Transmitter Technology (Note: Showing one polarization for simplicity)

S1

S2

LD

90

Today: 100Gb/s PM-QPSK


4,500km reach

Soon: 200Gb/s PM-16QAM


700km reach

DAC

DSP


What Functions do the DSP and DAC perform?

3 Important functions

1: Pulse Shaping

2: Pre-dispersion

3: Non-linear compensation


ICI

Without DSP and DAC

Signal from the side lobes has been intelligently incorporated

into the main pulse

1: Intelligent Pulse Shaping


2: Pre-dispersion

Tx Rx

1 or 0? We know a signal will be dispersed as it travels along the fiber


2: Pre-dispersion

Tx Rx

1 or 0?

Using DSP and DAC we can apply negative direction of dispersion

before transmission


Promising demonstrations of NL Compensation gains of about 2dB OSNR improvements

An array of complex techniques: • NPCC: Nonlinear Polarization Crosstalk Correction

• AFCPR: Adaptive Fee-Forward Carrier Phase Recovery

• RF Pilot waves

• DBP: Digital Back Propagation

• Tx Pulse Pre-dispersion (aka pre-compensation, pre-emphasis)

3: Non-Linear Compensation

In general some techniques are promising for practical implementation over the next few years


Pulse-Shaped QPSK Spectrum

Using a DAC, driven by a DSP in the transmitter it’s possible to “shape” the pulse intelligently

Pulses can be spaced “at the Baud rate”

• Example: 28GBaud signal could be spaced at just over 28GHz

• The additional spacing is known as the alpha, and the practical limit for alpha is 3-4%

50GHz

Terminology: “Nyquist DWDM”

Laser Mod

DSP DAC

Transmitter

Alpha


FTN uses channel spacing < Baud rate

• One approach is tight optical filtering

• DSP and DAC in Tx also demonstrated

• Creates a major ISI penalty (between symbols in the same channels, caused by insufficient B/W)

• Also creates an ACI penalty (between adjacent channels) to a lesser degree

Techniques used to recover ISI penalty

• Example: Partial Response Signaling (eg. Quadrature Duobinary, QDB) – dates back to 1970s in radio

• But the tighter the filtering, the higher the penalty

MLSE* can be used to partly recover ISI penalty

• *Maximum Likelihood Sequence Estimation

Ultimately not as efficient as Nyquist DWDM using high order modulation

50GHz

Terminology: “Faster Than Nyquist DWDM”

Optical filtering of pulses


From Fixed to Flexible Grid

200GHz

100GHz

50GHz 25GHz

10Gb/s NRZ

100Gb/s PM-QPSK

In a fixed grid world we choose a grid spacing (eg. 50GHz) and then engineer the pulses to fit We might even force them to fit by taking a hit on the optical reach, or the fiber capacity

Pulse shaped signals may end up with “wasted spectrum” between neighboring fixed channels


Flexible slots have a total width granularity of 12.5GHz, but a center frequency granularity of 6.25GHz

The flexible slot is defined by two numbers, n and m • There’s a reference frequency of 193.1THz, and all flexible slots are defined

with respect to this reference frequency (outlined in red)

• 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.694.1 Flexible Grid


Over the past 5 years we have seen a major adoption of coherent technologies in long haul optical networks • These have delivered C-Band capacity in the 9.5Tb/s range, with reach of around 4,500km

using fixed ITU grids

There will continue to be a tradeoff of reach vs fiber capacity as we move to flexible grid • PM-QPSK capacity is up to 12Tb/s and reach of 4,500km

• PM-16QAM capacity is up to 24Tb/s, but with reach of only 700km

• “Intermediate” modulations such as 8QAM deliver a good compromise

Going beyond QPSK will require advances in Tx compensation technologies – some of which are not yet at the commercial stage

Interim solutions, such as FTN, probably will not be as effective as a true high order modulation approach

If we look even further we can see (almost) another doubling of fiber capacity enabled by L-Band operation • Some operational challenges to iron out!

Summary


Thank You!

Q&A

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