1. Optical Modulation Schemes
By: Seyed Reza Ehsani

2. In this presentation More Capacity Modulation Types
Limitation of more bit rate
New Modulation Scheme
Block Diagram
Comparison
Cost Study for higher bit rates.

3. Need for Speed & More Capacity : 40 and 100 Gigabit Ethernet

4. Evolution of optical and Ethernet standards
100 Gb/s data rate standards:
IEEE (100 GbE client interface),
100GBASE-LR4
ITU-T (OTU4 framing),
OIF standardized the
PM-QPSK transceiver
implementation
Data port speed and transport channel capacity evolution

5. Why do we need Complex Modulation?
Optical transmission is about:
• Sending high data rates
• Over very long distances
• For very little money
Our biggest problem in optical fiber:
• Loss
• Dispersion
• Modal dispersion
• Chromatic dispersion
• Polarization mode dispersion
• Non-linear effects
• Self phase modulation
• Cross phase modulation
• Four wave mixing
If you stress any one of these variables, the others will respond
For a given modulation type, the magnitude of these impairments scales roughly with the square of the symbol rate

6. More capacity More bit rate (TDM) More optical channel (WDM)
Increase bit rate (OOK Modulation.)
Direct Modulation (Limit up to 2.5G)
External Modulation (Limit up to 10G)
New Modulation Scheme
More optical channel (WDM)
There is challenge between high bit rate with less channel spacing.

7. Modulation Types

8. Modulation Formats ABC
A transmitted signal s(t) can be described as:
q(t) – pulse shape, Ak – symbols containing the information
NRZ-OOK 1
RZ-OOK + -
RZ-DPSK
NRZ-DPSK
CSRZ-OOK

10. Modulation Techniques:
1- Direct or internal modulation by varying the injection current of laser diode.
Simple but has some limitations:
- limited modulation speed up to 2.5 Gbps
- limited output power
- chirp.
2- External modulation
changing the intensity of the beam after it leaves the light source.

11. EX. Mod. a) Electro-Optic (EO) Mach-Zehnder
Based on the electro-optic effect, that is the refractive index change of certain materials due to the applied electric field.
Mach-Zehnder
The Mach–Zehnder (MZ) modulator consists of a Y-splitter junction, one or two phase modulators, two waveguides, and a Y-combiner junction.

12. b) Electro-Absorption Modulator (EAM)
The electro-absorption (EA) modulator is an on–off optical device made with InGaAsP.
The EAMs control the light intensity by changing the absorption coefficient as a function of the electric field.
The EAMs are based on the reverse-biased PIN structure.

13. On–Off Keying
On/Off Keying (OOK) may interpret the presence of a signal as a “1”, and the absence of a symbol as a “0”
A binary OOK pulse train,
the spectrum of the square modulating pulse,
the spectrum of the modulated signal

14. NRZ Versus RZ
OOK – NRZ: The logic “one” is lighted for the full period (T = 1/f),
OOK – RZ: The logic “one” is lighted for a fraction of the period (such as ⅓ or ½),
NRZ modulation utilizes the full period as compared with RZ, which is a fraction of it,
The energy within a NRZ bit is much more than the energy in a RZ bit, thus the NRZ signal can propagate to longer distances than the RZ
OOK modulation can be used in both coherent and direct detection. However, coherent detection requires phase stability.

15. NRZ Modulation: 1bit per symbol
Simple modulation technique
Easy to implement
Low power use
But very sensitive to fiber impairments
as bit-rate increases
• This is what we’re talking about with the “square” relationship
Increasing power will trigger non-linear effects

16. Return-to-zero (RZ) implementation
Methods to generate optical return-to-zero (RZ) signals:
Modulating a semiconductor laser with an RZ data signal,
Generating an optical pulse train and modulating it with a non-return-to-zero (NRZ) data signal,
Pulse carving of an optical NRZ signal by a Mach-Zehnder modulator (MZM).

19. Phase-Shift Keying
Modulation of a light beam (the carrier) by changing the phase of the carrier (by 180 degrees) at the transition from logic one to logic zero and vice versa
For multilevel PSK, the change may be in increments of 45 degrees (8 levels).As a consequence, PSK requires a coherent carrier.
Electro-refraction modulators (ER) directly control the phase of an optical wave upon the application of a voltage.

20. Using Phase to Apply a Signal

21. Mach Zehnder Modulator as a Phase Modulator
A single Mach Zehnder Modulator, and the resulting phase constellation

22. Frequency-Shift Keying
Modulation of a light beam by changing the frequency of the carrier at the transitions between logic “zero” and logic “one”;
wideband FSK: When the deviation is large, , the
spectral bandwidth approaches , ;
narrowband FSK: When the deviation is narrow, , the spectral bandwidth approaches 2B, ;
FSK is achieved with electro-acoustic Bragg modulators or with DFB semiconductor lasers.

23. Duo-Binary Modulation
The duo-binary modulation case is an amplitude modulation case;
Duo-binary modulation reduces the high frequency components of the signal compared to IM (Intensity Modulation) so it has higher tolerance to chromatic dispersion, and hence can reach longer distances;
It combines two successive binary pulses in the digital stream to form a multilevel electrical signal. At the transmitting end, if the bit stream is Xk, the output of the duo-binary encoder is Yk = Xk + Xk–1,
xk: –1 –1 1 – –1 –1 –1 1 –1
yk: – –2 –

24. Optical Duo-Binary
The electrical method can be extended to optical binary using the symbols 0 and 1;
The duo-binary levels may represent either optical power in three different power levels, one of three frequencies, one of three phases, or one of three polarization states;
Power intensity suffers attenuation and, with additive noise. It may be suitable in certain short-haul and fiber-to-home applications;
Three frequencies per channel implies that for spectral efficiency a channel must be sliced into three sub-spectral ranges, which may be possible in CWDM;
In three phases case, the three symbols (–2, 0, +2) or (0, 1, 2) may represent a phase shift of –90, 0, and +90 degrees. Thus, in optical communications, this duo-binary version corresponds to a phase-shift-keying (PSK) method

25. ODB Modulation (Optical Duo-Binary)
First generation 40G modulation scheme
Phase & Amplitude based modulation
• Requires MZ modulator
• Can use simple, direct detection
Much more tolerant of dispersion
Limited reach
Widely used by 1st Gen 40G
• Stratalight, Mintera

26. ODB implementation Types
Optical duobinary (ODB) format can be implemented by delay and add operations of two signals D(t) and (t-δ)
ODB can be generated using drive signals in the following formats:
D(t) + (t-δ) is used to drive MZM and δ = T. The delay corresponds with the bit period T (conventional ODB);
D(t) and (t-δ) on push-pull MZM, δ ≈ 0.7T (fractional bit delay ODB) or T (full bit delay ODB);
D(t) and (t) on push-pull MZM while both signals are Bessel low pass filtered (PSBT);
Optically demodulating an NRZ-DPSK signal with a delay and add MZ differential interferometer (DI) to obtain ODB.

27. Implementation of optical duo-binary modulation
every symbol is a combination of the current bit being sent and the previous bit being received
The MZM (Mach Zehnder Modulator) is driven by the modulated signal and the inverted signal
The duo-binary bit stream provides the control signal to a MZM modulator;
When the amplitude is 1, the phase is 0; when it is 0, the phase is –90; and
when it is 2, the phase is +90 degrees.

29. BPSK optical modulation format
Q (quadrature, imaginary part)
Traditional OOK format.
Information is coded in the amplitude.
There are two symbols. I
(in-phase or real part)
BPSK modulation format is used in coherent detection
Q (quadrature, imaginary part)
In BPSK (binary phase shift keying),
the information is coded in the phase instead of amplitude.
Number of symbols is still two! I
(in-phase or real part)
BPSK time domain waveform
This technique will reduce the OSNR requirement,
but not the channel bandwidth requirements.

30. DPSK: 1bit per symbol
Data is superimposed on carrier wave and phase-shifted
Differential technique allows phase slips to be ignored
Useful for 40 Gbps
PMD issues limit reach at 100 Gbps
Used by OpNext & Mintera, and their OEMs

31. DPSK Time domain waveform
Data
Stream
Delayed
Result for
direct
detection
By one
bit period
In a DPSK signal, a phase change by p reflects a zero. If the phase does not
change from one bit to the next, this is interpreted as a 1
It is a good choice though for speeds up to 40 Gbps for long-haul and ultra-
long-haul

32. Differential phase shift keying (DPSK) modulation implementations
In DPSK line coding, the bits are encoded by the phase differences between successive bits;
Phase to intensity conversion of a DPSK signal using 1-bit delay interferometer and balanced photoreceiver
Modulation configuration using MZM modulator for NRZ-DPSK generation

35. State-of-Polarization Shift Keying
This method varies the polarization of light between two orthogonal states,PT and PII;
It assumes that over the optical path, there is no strong birefringence or polarizing devices that may alter or shift one of the polarization states.
SoPSK method can be used in multilevel (two, four, many) applications. However, the polarization dispersion mode (PMD) is a limiting factor.
Polarization division multiplexing can double the signal capacity of a wavelength

36. Limitation of more bit rate: Ex. From 10Gbps to 40Gbps
Electronic
Bandwidth increase by 4
Power dissipation
Footprint
Economic
40G system price expected to be 2.5~3 than 10G.

37. Limitation of more bit rate: Ex. From 10Gbps to 40Gbps
Optic
OSNR Lower by 6dB
CD tolerance goes down by 16
PMD tolerance goes down by 4
Nonlinear effect of fiber
Shorter transmission range
More linewidth with factor 4 (LW=2 X Bit Rate)

38. 40G & 10G linewidth

39. Approaches to 100G Pre-standard approaches to 100G transmission:
10 Transmitters & Receivers carrying 10G per wavelength
10 optical channels consumed
2 Transmitters & Receivers carrying 50G per wavelength
1 optical channels consumed (internal 2:1 optical mux)
OIF standardized single-carrier DP-QPSK approach
1 Transmitters & Receivers carrying 100G per wavelength
1 optical channels consumed
Multiple component suppliers
Multiple module suppliers
Speed up/Optimize 100G Ecosystem
Coherent Polarization-Multiplexed DQPSK (CP-DQPSK) modulation
(Cisco CRS 1-Port 100 Gigabit Ethernet Coherent DWDM)
providing 100-Gbps capacity transmission over existing 10/40-Gbps networks,
up to 3000 km, thus enabling Ultra Long Haul (ULH) networks

40. Modulation formats for 100G
Binary amplitude modulation
Technologies using multi-level modulation formats
(RZ-) DQPSK format and direct detection
RZ-DPSK-3ASK modulation format and direct detection
PM-DQPSK (DP-DQPSK) with polarization demux and direct detection
OP-FDM-RZ-DQPSK and direct detection
PM-QPSK (DP-QPSK) and coherent detection
PM-OFDM-QPSK (DP-OFDM-QPSK) and coherent detection

41. The 100 Gbaud Binary Amplitude Modulation
For channel rates higher than 10 Gb/s the continuous light of a DFB-laser is modulated utilizing external Mach–Zehnder modulators (MZM) or electro-absorption modulators (EAM)
For the realization of 100 Gb/s OOK systems the performance of high-speed electronic and opto-electronic components as well as integration and packaging technologies have to be pushed to current technology limits
To obtain binary 100 Gb signal data, signal electronic 2:1 multiplexer are realized
At a given bitrate OOK systems are in general most sensitive towards signal distortions at fiber transmission like chromatic dispersion (CD) and polarization mode dispersion (PMD).

42. The 100 Gb/s multi-level modulation formats
Multi-level formats (coding of several bits in one symbol) enable a reduction of the symbol rate of the system
This lead to the expense of an increased transmitter and receiver complexity
Multi-level coding reduces the optical bandwidth consumption of the channel
This coding enables WDM transmission with a narrower DWDM channel spacing

43. Quadrature PSK: 2bits per symbol
1,1
0,1
1,0
0,0
Advanced modulation, 4 phase states = 2 bits
More bits per symbol
Doubles the line rate compared to OOK

44. This is QPSK...
QPSK modulation can be obtained by using a single embedded MZ-I/Q-modulator which is driven by two binary electrical modulation signals at the in-phase and quadrature-phase modulators
Precoded Data
Laser
Precoded Data
This structure called a “Super Mach Zehnder”

45. Differential Quadrature Phase Shift Keying (DQPSK)
To avoid phase ambiguity in QPSK at the receiver side due to phase shifts induced by the fiber, just like in BPSK, a differential variation of QPSK can be used
Splits the stream into two data channels
each equivalent to 50Gbps for a 100Gbps line rate
Allows 100 Gbps transmission on fibers not significantly PMD-impaired
In order to retrieve the two initial data streams at the receiver there is a need for electronic pre-coding in the transmitter to generate appropriate I and Q modulation signals

46. Schematic setup of the opto-electronic components for DQPSK format
(RZ-) DQPSK format
DQPSK signals can be achieved by using:
a single embedded MZ-I/Q-modulator
a cascade of two phase modulators for the modulation of the optical phase by 0..pi/2 and by 0..pi/4 applying binary modulation signals
a single phase modulator driven by an electrical 4-level modulation signal
Schematic setup of the opto-electronic components for DQPSK format

47. DQPSK
Symbol
Phase 00 01
/2 10  11
-/2

49. High order amplitude/phase modulation
IM-DD is very susceptible to fiber impairments, such as CD and PMD, as the data rate increases beyond 10 Gb/s.
So, alternative modulation techniques such as DPSK and DQPSK modulation formats are investigated.
In the case of DPSK, there is a significant advantage in the required optical signal to noise ratio (OSNR) as compared to IM-DD
By encoding more amplitude/phase changes in the carrier, it is possible to increase the number of bits carried in each symbol, and the sensitivity to fiber impairments relates to the symbol rate

50. Quadrature Amplitude Modulation
Considering QAM modulation schemes for 400 Gbps and higher data rates
The constellation points of higher order QAM lie further apart than in pure PSK schemes like BPSK or 8-PSK
QAM Less susceptible to noise and
distortions, which results in a lower BER
In a -QAM scheme, the -constellation
points represent a series of n bits each,
usually distributed in a square lattice
4-QAM may be different than QPSK,
but the resulting constellation diagram
is the same.
8-QAM/8-PSK
constellation diagram

51. High order modulation using a super Mach Zehnder structure
A series of nested MZM components, known as a Super Mach-Zehnder, can be used to generate all of the amplitude/phase modulation techniques shown on the right of Figure
For speeds of 400 Gbps and beyond, however, 16-QAM is preferable because of its easier implementation and better OSNR performance

52. Amplitude- and phase-shift keying
In APSK, as its name implies, both amplitude and phase are modulated

53. RZ-DPSK-3ASK modulation format
A combination of mixed ASK-modulation and phase modulation
In this scheme, the 2.5 bits are coded in one symbol
Due to limited extinction ratios of the ASK modulated levels, the OSNR tolerance is also limited, finally strongly limiting the transmission reach
Schematic setup of the opto-electronic components for DPSK-3ASK transmission format.

54. Polarization Multiplexing
A further reduction of the symbol rate can be achieved by applying polarization division multiplexing
By selectively transmitting modulated signals using polarization mutilplexed (PM) carriers we can effectively double the spectral efficiency of a given modulation technique while using the same PM receiver
In this case a “symbol” is the combination of amplitude/phase states and polarization states, which can be referred to as a ‘dual-pol symbol’

55. PM-QPSK, 4 bits per “symbol”
Two Polarizations

56. PM-QPSK TX
A more complex modulator is needed consisting of two embedded
MZ-I/Q-modulator which modulate each half of the laser light

57. PM-QPSK Transmitter
PBC Polarization Beam Combiner
Schematic of 100Gb/s, single carrier, PM-QPSK transmitter, and the resulting phase constellations on each polarization state

59. PM-DQPSK (DP-DQPSK) Polarization Multiplexing for DQPSK:
Split the DQPSK modulated waves into respective polarizations
Effective symbol rate is 25 Gbps for 100 Gbps line rate
Improves performance of signal with impaired fiber links
PMD compensation required only on PMD-impaired routes
Support 100G DWDM transmission with 50 GHz channel spacing.

60. Main features of 100 Gb/s modulation formats.

62. Coherent Technologies
The high order modulation used by coherent technologies offers much greater spectral efficiency than IM-DD.
Coherent technologies solve the problem of chromatic and polarization mode dispersion suffered by IM-DD systems above 10G
Coherent technologies allow at least a factor of five increase in total fiber capacity
The technology enablers for DWDM capacity

63. Coherent Implementation
Expectation:
High order amplitude/phase modulation
Polarization multiplexing
Coherent detection using a local oscillator laser in the receiver
High-speed ADCs and sophisticated digital signal processing in the receiver

64. Coherent Detection?
Physically
• A detection technique that is based on the phase properties of the carrier
• If you are using a phase-based detector, you could claim to be implementing coherent detection
Practically
• The market has now come to expect a “coherent detector” to make use of sophisticated, digital signal processing (DSP) algorithms
A direct detector operates on a square law principle, in which the output of the detector is proportional to the intensity
Coherent detection is a linear process and linear equalization can be employed to effectively compensate for CD and PMD

65. Two very different functions in the detector
Separate the polarization components
Create interference against a reference
laser (local oscillator)
Separate the phase components
PD & A/D conversion
Phase state extraction
Compensate for local oscillator instability
Compensate for static CD
Compensate for dynamic PMD
Signal processing

66. A Coherent Detector Schematic
Optical Circuit Electronic Circuit
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
PS Polarization Splitter

67. Evolution of Modulation Formats
Coherent PM-QPSK
Same as PM-DQPSK on the transmitter side
Coherent receiver is employed
incoming signal is coupled with a local oscillator and detected.
Coherent receivers have superior sensitivity over incoherent detection
Reduces the need for dispersion compensation
Less sensitive to PMD
Requires development of digital signal processing ASICs

68. PM-QPSK & Coherent Detection
The phase signals are converted from the optical domain into the electronic domain
using a series of balanced photodetectors
Schematic of a single-carrier coherent detector including polarization demux

69. PM-QPSK RX

70. Block Diagram: PM-QPSK / DP-QPSK
PM-QPSK= Polarization Multiplexing-Quadrature Phase Shift Keying DP-QPSK=Dual Polarization-Quadrature Phase Shift Keying PBS=Polarization Beam splitter Cube

71. Optical orthogonal frequency division multiplexing (O-OFDM)
OFDM was originally developed for multichannel data transmission
The concept of OFDM is the division of a high bit rate data stream into several low bit rate streams by generating several sub-carriers and modulating each low bit rate stream on to a sub-carrier.
When the spacing between adjacent sub-carriers is higher or equal to the sub-carrier baud rate, O-OFDM is commonly referred as Sub-Carrier Multiplexing (SCM).
Three approaches to SCM

72. Externally modulated sub-carrier based implementation
One implementation for generating two-sub-carrier OFDM signals is to use an IQ-modulator
First transmitter scheme for two-sub-carrier OFDM generation
Another implementation for generating two-sub-carrier OFDM signals

73. Orthogonal polarization optical FDM-DQPSK (OPFDM-DQPSK)
In the case of two-sub-carrier OFDM implementation, NRZ-DQPSK or RZ-
DQPSK modulation scheme can be applied to each sub-carrier after sub-
carrier separation;
The two sub-carrier DQPSK or RZ-DQPSK tributaries can be then orthogonally
polarization-combined

74. OP-FDM-RZ-DQPSK
To eliminate the fast automatic optical polarization demultiplexer, alternatively, the two polarizations can be used to carry two optical carriers
The two frequency locked optical carriers (FDM), obtained by carrier suppressed RZ-carving, are splitted by an optical filter, modulated by DQPSK modulators and combined with orthogonal polarization (OP)
Due to the separation of two optical carrier in two polarizations only 100 GHz channel spacing is supported

75. 100G constellation diagrams…
Various modulation schemes – PSK, QAM, …
NRZ vs. RZ (duty cycle)
Multi-carrier
OSNR optimum for (DP-) QPSK (coherent)
QPSK Re Im
6PSK Re Im
8PSK Re Im
8QAM Re Im
Bipolar
6ASK Re Im Re Im
9QAM Re Im
16QAM
16QAM Re Im
16PSK Re Im Re Im
DP-QPSK Re Im
DP-8PSK Re Im
DP-16QAM

76. Characteristics of modulation formats at 100 Gbps without polarization multiplexing

77. System tolerances of 100 Gb/s modulation formats

78. OPTICAL TECHNOLOGIES FOR CHANNEL RATES BEYOND 100 GB/S
Techniques to construct a channel
with a data rate beyond 100 Gb/s:
a) increase symbol rate;
b) increase bits per symbol;
c) use more optical carriers.

79. Modulation formats for systems beyond 100 Gb/s
The major focus is on multi-level modulation format based on M-QAM (quadrature-amplitude modulation) and coherent reception;
Recently QAM scheme together with polarization multiplexing is utilized to achieve a channel rate of 200 Gb/s with 16 QAM.
In an M-QAM or -QAM signal, m bits are transmitted in a single time slot or symbol,
Adding polarization multiplexing to make PM QAM format, 2 x m bits are transmitted per symbol
The major target is to maximize their spectral efficiency
Two types of modulation formats are:
Single carrier modulation formats
Multi carrier modulation formats – optical OFDM transmission

80. Dual Carrier PM-QPSK Structure
The primary advantage of a dual carrier implementation is that the opto-electronics
only need to operate at 12.5GBaud, plus overhead
Schematic of 100Gb/s, dual carrier, PM-QPSK transmitter, and the resulting phase constellations on each polarization state

81. SUPER CHANNELS
Super- channel is a channel that uses more than one optical carrier
All subcarriers in a super-channel are routed as a group, allowing the subcarriers to be spaced closer together
With this technique no attempt is being made to limit the bandwidth of each subcarrier and there is considerable spectral overlap
Spectral characteristics for: a) a WDM super-channel; b) an all-optical OFDM super-channel; c) a Nyquist WDM super-channel; d) an OFDM super-channel.

82. Comparison of modulation formats 100G and beyond

83. Overview on single carrier M-QAM options

84. ADVA Tests on PM-DQPSK w/ WSS
Transmitted over ~540 km (6 spans) with Wavelength-Selective Switches
OSNR: 22.6 dB
WSS, EDFAs and DCMs from current ADVA product line CW
28 Gb/s Data A
90 km SSMF
DCM 6X PC
PBS
WSS
28 Gb/s Data B
28 GHz Clock
EDFA
90° T+ T-
BERT R
(I)
(Q)
Q = 3.83
Q =3.48
X Polarization
Q = 3.09
Q =3.24
Y Polarization

85. Carrier Core Trans/Muxponder
High-performance network interface
PM-QPSK with coherent detection (OIF standard)
50 GHz channel spacing Re Im
PM-QPSK
Long-Haul capability
>1500 km reach
Dispersion (CD, PMD) compensation
Strong FEC
G.709-compliant mapping, multiplexing
Services
100GbE (IEEE802.3ba), OTU4 (G.709)
10 x 10GbE / STM-64 / OC-192
4 Slots
4 Slots

86. 100G DP-QPSK transmitters and 100G coherent receivers
Optical component integration

87. Spectral efficiency
vs. SNR