Increasing spectral efficiency through Coherent Optical Signal

Increasing Spectral Efficiency through
Coherent Optical Signal Generation
 Introduction to Coherent Optical
 What are Coherent Optical techniques and how is it used
to increase spectral efficiency
 Test equipment used
– Intro to AWGs in Coherent Optical applications
– Introduction to Coherent Optical Analyzers
High Speed Communications
 Impact and Importance
– Exponential growth in bandwidth demand
– Long-haul data networks are struggling to keep
pace with video on demand, video conferencing
 Demands on Engineering Community
– Increasingly complex modulation schemes to
improve transmission efficiently
– Higher speed clock and data channels drive
tighter timing margins
 Opportunity for Innovation
– Wider bandwidth, higher resolution signal
generation capability
Introduction to Coherent Optical
The ever-increasing need for capacity in metro and long-haul networks has
resulted in the continuous improvement of the optical network infrastructure all
around the World. Over the years, capacity has been improved through the
combination of multiple mechanisms.
Installation of additional fiber optics cables.
Increase of the baud rate for a given link.
Improvement of the transmission characteristics of the fiber by reducing or
mitigating the effects of attenuation and dispersion.
Multiplex of multiple signals in a single fiber by assigning different
wavelengths to them.
Increase of the number of wavelengths transported by a single fiber by
reducing the distance between them.
Addition of FEC (Forward Error Correction) techniques to enable faster
connections in in lossy or dispersive environments.
Improving Spectral Efficiency
SE can be improved by
modulating both the amplitude
and the phase of an optical
Part of the optical power goes
directly to the carrier and does
not transport any information.
Carrier 3 is modulated using a
QPSK modulation so 2 bits are
transported by each symbol,
doubling the capacity of the
OOK-modulated channel in the
same bandwidth.
Capacity may be increased
through the usage of more
complex modulations such as
OFDM or baseband filtering.
Wavelengths 1 and 2 transport
28 Gbaud signals with 2 and 5
bits per symbol respectively.
In this WDM link, four different wavelengths share the
same fiber in a standard ITU 50GHz grid. Wavelength
4 is carrying a 10Gb/s signal using the traditional
intensity modulation (or On-Off Keying, OOK).
Optical Modulation Methods
 Presently, traditional OOK-based DWDM links carry up to160 10Gbps
channels (1.6 Tbps aggregated capacity) in a 25GHz ITU grid or up to
forty 40Gbps channels in a 100GHz ITU grid.
 Commercial success of 40Gbps OOK modulated channels has been
rather limited as it is only feasible at the expense of much higher cost
and complexity due to the electronics involved and the need to apply
powerful dispersion compensation techniques
One way to modulate the amplitude and the phase of a carrier is a quadrature
modulator. There, two baseband signals, called I (or In-phase ) or Q (Quadrature),
modulate in amplitude two orthogonal carriers (90° relative phase) so any state of
modulation can be accomplished, The same scheme may be implemented for optical
carriers by using two Mach-Zehnder Modulators (MZM) in an arrangement known as
“Super-MZM” cell.
Introduction to Optical Modulation Methods
0 1
0 1 1 0
Pure AM (OOK)
On-Off Keying
1 bit/symbol
Traditional 10G transmissions modulate the amplitude of the light,
a.k.a. or on-off keying (OOK). Direct detection is used in the receiver.
0 1 0 1 1 0
Pure PSK
Phase Shift Keying
1 bit/symbol
Coherent transmissions modulate the phase of
the light, the simplest case is phase shift keying.
Optical Modulation Methods continued
01 11 10 10 11 00
Typical QPSK
Quadrature Phase Shift Keying
2-bits/ symbol
By doubling the number of phase states
the bit/symbol rate is also doubled.
Optical Modulation Methods continued
Rotating the polarization of one QPSK signal, and combining it with a second QPSK
signal, doubles the bit/symbol rate again.
00 11 10 10 11 01
2-bits/ symbol
01 11 10 10 11 00
2-bits/ symbol
Dual-Polarization QPSK
4 bits/symbol
Other formats are also used such as Differential QPSK (DQPSK), 8-PSK,
Quadrature Amplitude Modulation (QAM) and Orthogonal Frequency
Division Multiplex (OFDM).
Coherent Optical Test System
 Researchers and engineers require adequate tools to validate,
diagnose, and produce their designs, prototypes, and products.
 The goal of Test & Measurement (T&M) manufacturers is to provide
the appropriate tools. Stimuli and Analysis equipment, capable of
generating and analyzing optical and electrical signals with enough
quality, repeatability, and accuracy, to test receivers and other
components, systems and sub-systems, even entire networks.
 This equipment must be able to generate perfect (“golden”) or
impaired signals and they must be capable of emulating the effects of
interconnections and transmission systems.
Coherent Optical Test System continued
Coherent Transmitter
Analysis Software
Coherent Receiver
OM1106 Analysis SW
PPG3204 32Gb/s Pattern Generator
OM5110 Multi-Format
Optical Transmitter
OM4106D Coherent Lightwave
Signal Analyzer
Fiber Optic
– or –
AWG70001A Arbitrary Waveform Generator
DPO73304D Digital Phosphor Oscilloscope
Polarization Multiplexed QPSK Integrated Transmitter
Integrated Dual Polarization Intradyne
Coherent Receivers
Replace input
signals with
reference signals
Replace ADC with
New OIF Agreement
IA OIF2009.033.06
Test overall:
 Path gains
 Cross talk
 Phase angles
At any frequency or wavelength
Coherent Detection Architecture
Frequency Downconversion
Coherent Detection
Introduction to AWGs for Coherent Optical Continued
 AWGs can produce a large variety of
distortions linear and non-linear, applied
to modulated signals.
 These can emulate issues in the
transmitter, the receiver, and even the
link or the network.
 The same capability can be used to
compensate for such distortions and
obtain better-quality signals from poorquality components or links.
Introduction to AWGs for Coherent Optical Continued
Complete Polarization Division Multiplexed transmitter emulation require 4
synchronized AWG channels to generate the Ix, Qx, Iy, and Qy baseband signals.
Adequate control of those 4 signals can be used to emulate any static or dynamic
SOP (State-Of-Polarization).
Introduction to AWGs for Coherent Optical Continued
Some modulation schemes may be generated by single channel instruments.
Uncorrelated I and Q baseband signals may be obtained by delaying one of them by
a integer number of symbol times. Here, the two complementary outputs of a
Tektronix AWG70000 series generator are used in such an arrangement, providing
for higher amplitude signals than those coming out from a power splitter fed by one of
the outputs.
Why do I need a Coherent Signal Analyzer?
 Understand and optimize optical networks employing advanced
– Measure constellation parameters, quadrature and modulator bias
values, symbol masks, EVM, signal and phase spectra, BER, Q vs.
decision threshold
– Save time, enable a wider range of users
 Transition from R&D to qualification and production environments
– Enable automation
 Test equalization and phase recovery algorithms
 Understand effects of bandwidth limitations
– At the transmitter, digitizer, and receiver
Measuring TX Constellation Imperfections: Q-factor
 Counts errors as decision
threshold is moved.
 Errors fitted to error function in “Qspace”
 → Plot, max-Q and optimum
decision threshold
Measuring TX Constellation Imperfections: Phase Angle
Example: Modulator Bias Adjustment
32 Gbaud Optical Signal Digitized with the DSA73304D
in 50Gs/s mode (~23 GHz BW)
 Coherent optical signal generation is one of the more demanding
applications for an AWG and Coherent optical test systems. The
requirements such as
Number of channels
Sampling rate
Record length
Timing and synchronization
These can be only met by the highest performance instruments. The
unique capability of generating ideal or distorted signals and the ease to
add new modulation schemes and signal processing algorithms without
the need to add any extra hardware make AWGs the ideal tool for
coherent optical communication research and development.
 OM4000 Series Analyzer and DPO70000 Series Oscilloscope
Oscilloscope best matched to application
Best coherent signal analysis algorithms (“designed for optical”)
Preferred user interface
Open architecture DSP based in Matlab
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