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Demonstration of Optical Orthogonal Frequency Division Multiplexing

Dr. Ali Setoodehnia1

Dr. Feng Huang1 Dr. Hong Li 2

Dr. Kamal Shahrabi1

email: asetoode@kean.edu

1. Technology Department, Kean University, US2. College of Technology, CUNY, Brooklyn, NY

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1. Overview of Carriers

2. Introduction to OFDM

3. Introduction to various realizations.

4. The possible realization and the proposed optical OFDM.

5. Why do we want to implement the proposed system

6. Possible implementations of the pulse shaper, advantageous and disadvantageous

7. The proposed system, channel equalization capability and security of information transmission.

8. Results

9. Conclusions, the education perspectives, and things we will like to do in the future.

Outline

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Overview of carriersOverview of carriers•In a single carrier system, a single fade or interferer can cause the entire link to fail

•In multi-carrier system, only a small percentage of the sub-carriers will be affected.

•The total signal frequency band is divided into N non-overlapping frequency sub-channels. Each sub-channel is modulated with a separate symbol and then the N sub-channels are frequency-multiplexed.

•Good to avoid spectral overlap of channels to eliminate inter-channel interference. However, this requires guard band which leads to inefficient use of the available spectrum.

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OFDMOFDM

Using the overlapping multi-carrier modulation technique, we save more of the bandwidth.

To realize the overlapping multi-carrier technique, we need to reduce crosstalk between sub-carriers, which means that we want orthogonality between the modulated carriers.

In OFDM, the carriers are linearly independent (i.e., orthogonal)

Applying the discrete Fourier transform (DFT) to parallel data transmission systems as part of the modulation and demodulation process.

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Concept of OFDMConcept of OFDM

•The OFDM signal, multiplexed in the individual spectra with a frequency spacing b equal to the transmission speed of each sub-carrier

•At the center frequency of each sub-carrier, there is no crosstalks from other channels. Therefore, if we use DFT at the receiver and calculate correlation values with the center of frequency of each sub-carrier, we recover the transmitted data with no crosstalk.

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Concept of OFDMConcept of OFDM

Each subcarrier has exactly an integer number of cycles in the interval T, and the number of cycles between adjacent subcarriers differs by exactly one. This property accounts for the orthogonality between the subcarriers

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Orthogonal Frequency Division Multiplexing and optical realization.

Why optical OFDM Channel equalization; Extremely large bandwidth (5 THz for 200fs optical pulse)

utilized by 100 MHz RF modulation bandwidth with date fusion technique (TDM).

Ultrafast pulse shaping techniques Ultrafast pulse:

Within the pulse envelop only a few circles of the carrier wave

Advantage of pulse shaping:

Arbitrary pulse synthesis; high speed communication, coherent control etc.

Introduction to OFDM and optical implementation

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Information transmission

•Tb/s transmission speed: free space/ fiber pigtailed•Benchmark imagetransmission•No bit error found

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Viewing the pulse shaper as a DSP instrument Viewing the pulse shaper as a DSP instrument

(Time to spatial dispersed frequency component)

FFT

AOMModulator

RF Mixer

(spatial dispersed frequency component to time again)

FFT

(1)

(2)

(3)

At (1), the input ultrafast pulse contains the whole spectrum at the same beam,

At (2), different colors are spread in spatial domain, the beam at different location will have less spectrum component; the corresponding pulse duration is longer.

At (3), combined into the same beam again, if there is no pattern put into the pulse, it will correspond to the input pulse; if there are any phase mask to the AOM, the pulse will spreading depends on the FT of the pattern imposed on the AOM

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Full use of the capacity of optical fiber capacity

TRAFFIC TYPE BIT RATE (Mb/sBIT RATE (Mb/s))

VoiceVoice 0.0640.064

DataData 0.01-100.01-10

High Fidelity AudioHigh Fidelity Audio 1.01.0

TeleconferencingTeleconferencing 1.51.5

Entertainment VideoEntertainment Video 50-15050-150

LOSS (dB/km)

BANDWIDTH

0.1

10.0

1.0

1 MHz 1 GHz 1 THz

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3

4

1. Wire Pair 2. Coax Cable1. Wire Pair 2. Coax Cable

3. Waveguide 4. Single Mode Fiber3. Waveguide 4. Single Mode Fiber

• Hybrid OOFDM/WDM/TDM gives flexible, cost-Hybrid OOFDM/WDM/TDM gives flexible, cost-effective solution to the opto-electronic bottleneck problemeffective solution to the opto-electronic bottleneck problem

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Available Spatial Light Modulators for pulse shapingAvailable Spatial Light Modulators for pulse shaping

Fixed Mask, Holographic, Real-time Holographic Phase and Amplitude; no pixels; no wire

LCM-Arrays Phase and Amplitude; pixels; multi-line wires

Deformable Mirror Phase; no pixels; multi-line wires

Acousto-optic modulator, best for our application:

higher update rate, high resolution.

Speed of modulation, bandwidth resolution, update rate, Oriented for different applications.

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Programmable Pulse Shaping Using Liquid CrystalModulator (LCM) Array

• Same 4f configuration as the AOM pulse shaper•Using the LCM array, multiple lines attached to the pixels•Most of time phase only or phase-and-amplitude (usingtwo sets of 4f system or two sets of LCM arrays.•Using the liquid crystal to change the polarization of lights therefore the phase of the input pulse.•Typically 128 pixels on 100 m centers; up to 512 pixels reported.•Reprogramming time > 10 ms•Low attenuation•Demonstrated to below 10 fs•Phase and amplitude response must be calibrated

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Chirped Pulse WDM of Bell Labs

Principles: A mode-lock laser ~150 fs, using fiber to stretch the pulse to ~30 ns and spectrum is spread in this time range, using fast EO modulator (8 GHz) to modulate the stretched pulse and create about 300 WDM channelsAdvantages: High density WDM, individual programmable bandwidth for each WDM channelDisadvantages: Strong time wavelength coupling

Chann

el 1

Chann

el 2

Chann

el 3

Chann

el 4

...

Chann

el N

Time

Wa

vele

ng

th

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Programmable Pulse Shapers: Movable and Deformable Mirrors

Using the same optical setup, instead of AOM or LCM, aMirror was placed in the center Fourier Plane.

• Pivoting Mirror provides a linear spectral phase shift, hencea delay!•Spectral phase only control•Reprogramming time ~ 1 msec•Low attenuation •Continuous spatial modulation,

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Array-Waveguide-grating Pulse Shaper

• A double arrayed waveguide grating with a spatial phase Filter forms a nearly integrated pulse shaper

•Has been demonstrated for fixed dispersion slope compensationFor 2*40 WDM channels in C and L bands simultaneously.

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Combine WDM and TDM

Synthesis of shaped fs optical pulses through shaped s RF pulsesArbitrary spectrum modulation (both phase and amplidue offers possibilities for any encoding scheme (ASK, PSK, FSK ...)High fidelity amplification achievable using standard techniques)()()(

)()()(

inout

inout

tMtEtE

MEE

It is also shown one can propagate the Acoustic wave in parallelwith the light wave.

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Immunity to EMINo Cross-Talk Between WiresDifficult to Tap - High SecurityLight Weight and Small Cable Size and compact systemNo Ground-Potential Difference Currents

Advantage of Ultrafast Communication

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Optical OFDM communicationOptical OFDM communication

Traditional optical point to point communication One or a few channels only, low-powered infrared

lasersAirFiber (Nortel Networks), claims to have a product that, when deployed

throughout a metropolitan area, creates a meshed architecture that can transmit data in up to four directions at 622 Mbit/s simultaneously in a distance range between 200 and 450 meters.

TeraBeam Corp (Lucent Technologies Inc.), can be used as a point-to-multipoint product that uses a hub-and-spoke architecture. it can achieve data rates of 100 Mbit/s. 1-2 Km.

Proposed optical point to point satellite communication OFDM functionality: Power equalization, Channel

Add-Drop, etc. A compact system

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Traditional Multiplexing Methods

TDMA WDMA WD-OFDM

Time

Cha

nnel

1C

hann

el 2

Cha

nnel

3C

hann

el 4 ...

Cha

nnel

N

WavelengthC

hann

el 1

Cha

nnel

2C

hann

el 3

Cha

nnel

4 ...

Cha

nnel

N

Time

A: accessing

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Why Wavelength DomainWhy Wavelength Domain

–Optical Bandwidth(5 THz) v.s. Electronic Speed Optical Bandwidth(5 THz) v.s. Electronic Speed (~1GHz)(~1GHz)

O/E Interface needs High Ratio (10O/E Interface needs High Ratio (1044) Data Compression) Data Compression

Implementation of WD-OFDMImplementation of WD-OFDM

–Spectral Encoder (Spread Time)Spectral Encoder (Spread Time)

Amplitude Only (Optical Codes, PPM)Amplitude Only (Optical Codes, PPM)

Phase Only (Binary Codes)Phase Only (Binary Codes)

CDMA as the coding scheme-WD-CDMA

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New Protocol WD-OFDM

Pulse Shaping WDM: hundreds time increase of Date Transmission rate (DTR) combined with TDM

Pulse Shaping CDM: hundreds time increase of Channel number combined with TDM

High Spectrum Efficiency

Time

Wa

vele

ng

th

Wavelength

Inte

ns

ity

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Experimental results

0 10 20 30 40 50 600

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Channel

Bit

Err

or

Ra

te

BER for the proposed transmissionBER for the nomal transmissionTwo channel turned off

dteiSS ti )()(

dteiNN ti )()(

))()(()( NSC

))()(()( iNSiC ))()(()( NSc

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Experimental results

0 10 20 30 40 50 600

0.5

1

1.5

2

2.5

3

3.5

4

Channel

Bit

Err

or

Ra

te

BERX100 for the proposed transmissionBER for normal transmissionchannel transmission pattern

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Conclusions and the following works

Components level Introduction to various femtosecond pulse shaping

techniques Channel Equalization from OFDM

System level Optical OFDM communication and information

transmission Following work Developing demo system for education and external

funding purpose.

Novel techniques like optical OFDM and Pulse Shaping

can achieve 2Tb/s with commercially available components:

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Applications of AOM spectral encoder

Components level Channel Equalization Tunable Dispersion Compensation Adaptive phase feedback

System level DWDM/TDM Network

Spectrum phase and amplitude control implemented via diffractionfrom a modulated traveling acoustic wave.

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Typical Experimental Setup

Ultrafast FL

AWG PulsePickerDriver

PC

Scope

GPIB

(GPIB)CCDScope(Or OSA)

PCI

RF circuit

AWG: Arbitrary waveform generator, OSA: optical spectrum analyzer(HP 71451B)

IMRA