RoFS-Pforzheim 2007 1 Prof. Bogdan Galwas Warsaw University of Technology

RoFS-Pforzheim 2007 1

Prof. Bogdan GalwasProf. Bogdan GalwasWarsaw University of Technology Warsaw University of Technology


R-o-F R-o-F – Basic Structure of – Basic Structure of

SystemSystem Data transmission between Central Station and Base Station via fiber link Data transmission between Base Station and Terminal via radio link

Mobilewave

Terminal

Mobilewave

Terminal

BaseStatio

n

DataInput

Fiber

Central

Station

DataOutpu

tOptical

Transceiver

Data Output

Optical Transceiver

Data Input

Radio Radio SystemSystem

Fiber linkFiber link


R-o-F R-o-F – Basic Structure of – Basic Structure of SystemSystem

Central Station transmits optical carriers

(fO) modulated at RF (fC & data) over fiber

links toward remote base stations

Photodiode PD converts the optical signal

into an electrical RF signal (fC,2fC... nfC &

data)

RF signal is amplified and transmitted by

an antenna (fC,2fC... nfC & data)


2. Optical Transmission of μwave Signals

Outline of lectureOutline of lecture::

1. IntroductionIntroduction

4. Optical- μwave Mixing

5. Examples

6. Conclusions

3. Optical Generation of μwaves


1. Introduction1. Introduction

Modulation frequency of of laser diodes LD is limited to the 40 GHz by the internal resonance between the electrons and photons. The push-pull principle solves partially these problems.

Two types of the external optical modulators are widely used: The electro-optic (EOM) ridge-type travelling wave LiNbO3 Mach-Zender modulators, Electro-absorption (EAM) optical modulators.

The new types of travelling-wave PIN photodectors have moved the bandwidth above 100 GHz . Special constructions of Metal-Semiconductor-Metal photodetectors have banwidth

above 300 GHz.

3 10 30

100 300f [GHz]

LD

EAM

EOM

MSMP-I-N


2. 2. OpticalOptical... ... Analog optical Analog optical link (1)link (1)

The simplest technique for the distribution of the RF signal modulated with date is an intensity modulation scheme via direct modulation of laser. We will discuss the overall gain G of the system.

Problem: microwave signal (fRF,PIN,POUT ) is transmitted by an analog optical link.

Fiber

Laser

fRF,PIN

WN

Photodetector

WO

fRF,POUT

fOPT,PT

L,=+j

fOPT,PR

;fP

fP

fP

fP

fP

fP

fP

fPG

RFIN

OPTT

OPTT

OPTR

OPTR

RFOUT

RFIN

RFOUT

Modulation Transmission Detection


2. 2. OpticalOptical... ... Analog optical link Analog optical link ((22))

Principle of operation of optical analog link with direct intensity modulation of laser optical power.

Gain of analog link: ;RSG 2D

2L

IL [mA]

PO

PT

[mW

]

SL [W/A] POPT(t)

t

t

I D

[A]

POPT[W]

RD[A/W]

t

t

ID [mA]

Attenuation by fiber is simply expressed: ;ePP Lf2

TROPT



An intensity modulation of laser optical power may be realised by external electrooptical modulator.

LaserPO

Fiber

fOPT,PT

L,=+j

fOPT,PR

Photodetector

WO

fRF,POUT

fRF,PIN

WN

I D

[A] RD[A/W]

POPT t

I(t)

t

V(t)

1

T

t

t

POPTSMZ[V-1]

V



;RV

PRSG 2

D2

202

D2MZ

The transmission of M-Z modulator can be described as:

Gain of analog link is proportional to the level of optical power P0 :

;V

Vcos1

2

TVT MAX

In the point of inflexion of the T(V) characteristic there is a long straight line section at V0 = V/2 and with a slope SMZ :

;

V2

T

V

VTS MAX

VVMZ


2. 2. OpticalOptical... ... Analog optical link (Analog optical link (55))

Photodetector current [mA]

Gain G[dB]

-10

-20

-300,01 0,1 1 10 10

0

0

10

20

30

High SL

laser

Typical link

External modulation

Laser modulation

Analog link with external electro-optical modulator offers high gain


2. 2. Optical Transmission of Optical Transmission of μμwavwaves (1)es (1)

DATASIGNAL

FIBER

LASERDIODE

OUTPUT

CARRIERREFERENCE

FIBERGAIN

LASERDIODE

PHOTO--DIODE

PHOTO--DIODE

a). A conventional FO link in which the data signal is up-converted by the MMW carrier reference before laser bias current modulation

b) The date signal and carrier are transmitted separately over different FO links. Separation of signals can significantly increase dynamic range

DATASIGNAL

PHOTO--DIODEFIBER

LASERDIODE

OUTPUT

CARRIERREFERENCE


2. 2. Optical Transmission of Optical Transmission of μμwavwaves es (2)(2)

FIBER

LASERDIODE

OUTPUT

CARRIERREFERENCE

GAIN

+ FILTER

DATASIGNAL

PHOTO--DIODE

FIBER

OUTPUT

GAIN

FIBER

PHOTO--DIODE

DATASIGNAL LASER

DIODE

CARRIERREFERENCE

LASERDIODE

PHOTO--DIODE

c) The photodiode is used as W mixer. This solution reduces numbers of elements and local oscillator power

d) The photo-detector output signal is filtered and carrier reference signal is separated, next amplified and directed to the W mixer


2. 2. Optical Transmission of Optical Transmission of μμwavwaves (3)es (3)

DATASIGNAL

OUTPUT

CARRIERREFERENCE

GAIN

FIBER

FIBER

LASERDIODE

LASERDIODE

PHOTO--DIODE

PHOTO--DIODE

EOMODUL.

EOMODUL.

e) It is also a conventional FO link in which the data signal is up-converted by the W carrier reference and external modulator is used

f) The structure of the circuit was discussed earlier, external modulator is also used.

DATASIGNAL

FIBER

OUTPUT

CARRIERREFERENCE

LASERDIODE

PHOTO--DIODE

EOMODUL.


2. 2. Optical Transmission of Optical Transmission of μμwavwaves es (4)(4)

Very interesting and professional system for transmission signal from 40-58 GHz millimeter-wave region by optical link.

LD - 1=1,3 m

5 GHz

DM

x 8 Mux Demux x 8

5 GHz 5 GHz

40-58 GHz

40-58 GHz

0-18 GHz

0-18 GHz

PD - 1=1,3 m

LD - 1=1,5 m

PD - 1=1,5 m

Amp

Amp


2. 2. Optical Optical ... ... Chromatic-Dispersion EffectChromatic-Dispersion Effect

Laser optical power is modulated to generate an optical field with the carrier and two sidebands

;tcosmJtcosmJtcosmJAE 2RF011RF010000T

If the signal is transmitted over fiber, chromatic dispersion causes each spectral component to experience different phase shifts depending on the fiber-link distance L, modulation frequency fRF, and dispersion parameter D[ps/nm.km]

;L...21

jLexpEE 2

02

2

00TR

0

At the PIN output the amplitude of the mm-wave power is given by:

;ff

LcDcos2

cosP2

0

RF212OUT


2. 2. Optical Optical ... ... Chromatic-Dispersion EffectChromatic-Dispersion Effect

PRF = 0 at frequency fTO, where N = 1, 3, 5...

Problem: The standard amplitude modulation of optical carriers generates double-sideband signals.

Due to the chromatic dispersion effects the sidebands arrived at the BS are phase shifted.

In consequence periodical fading of PFR is observed.

The techniques of Optical Single-Sidebands OSSB generation have been developed.

;LcD2/Nff 0TO


2. 2. OpticalOptical... ... Subcarrier Subcarrier MultiplexingMultiplexing

Selective Terminal

BaseStatio

n

Fiber

Central

StationOptical

TransceiverOptical

Transceiver

Data 1 – f1

M U X

f

N

Data N

Data 2

Data 1

f2f1

Selective Terminal

Data 2 – f2

Subcarrier multiplexing may be used for multichannel

transmission


3. 3. Optical GenerationOptical Generation......OOptical ptical mixmixinging

Signal fS

Intermediate

Frequency fIF

Coupler3dB, 1800

LocalOscillator

fLO

Photodetector The second signal, local oscillator, ELO, |ALO|, fLO i LO.

The signal directed to the photodetector:

Photocurrent I is proportional to the incident power P and detector’s sensitivity R :

- PS and PLO are the powers, is intermediate frequency.

The name of the process: optical mixing, optical heterodyning, photomixing, coherent optical detection.

;tf2cosPP2PPRRPI LOSIFLOSLOS

E E ES LO ;

;eAReeAReE SSS tf2jS

tf2jSS

Photodetector is responsive to the photon flux, is insensitive to the optical phase. Two optical signals (EM fields): the first signal:


3. 3. Optical GenerationOptical Generation......Two Optical Two Optical CarriersCarriers

Data

Laserf2

Laserf1

Coupler

0 f1, f2,

fopt

Carier& Data

Amp

0 f1- f2

fIF

One optical signal may be modulated by data.

The spectrum of optical signals must be “pure”, it is not easy to satisfy this condition .

Process of optical mixing may be used for generation of microwave frequency signal.

The simplest way is to use 2 lasers with frequency f1 and f2, to transmit the optical

signals by fiber to a photodiode and to extract the intermediate frequency fIF.



f1, f2,

Double-modeLaserf1 & f2

0

fopt

MicrowaveSignal

Amp

0 f1- f2

fIF

Dual-Mode DFB semiconductor laser for generation of microwave signal

The mode separation is adjusted to the desired value by proper choosing the grating

strength coefficient.

It is possible to construct a specially modified distributed feedback semiconductor

laser (DFB) in which oscillation occurs simultaneously on two frequencies,

for two modes.



TunableMaster Laser

fOPT nf

Fiber

PDSlave

Laser 1

f

SlaveLaser 2 fOPT +

10f

fOPT - 10f

20f

The slave laser 1 and laser 2 are synchronized for different sidebands: upper sideband fOPT + 10f, and lower sideband fOPT - 10f,

The frequency of output signal is equal to 20 f..

The spectral purity of the microwave signal may be really improved by synchronising the laser action.

The master laser is tuned by stable microwave source of frequency f.


3. Optical Generation of μwaves

M-Z Modulator

Date(fSi Bi)

fOPT

f0 fm

fm

Laser Nd:LiNbO

3

Laser inside microwave

cavity

Microwave Generator

fOPT

f0 (fPi Bi)

Laser on Nd:LiNbO3 electrooptical material placed inside microwave cavity changes its frequency of optical oscillation.

Optical transmitter with Nd:LiNbO3 laser with frequency modulated by Microwave Generator and with external Mach-Zehnder modulator


3. Optical GenerationGeneration of μwaves

PD

Amp

fREF, PIN

fOUT= n m fREF

POUT >> PIN

VCO

Ampx n

Frequency

Multiplier

mFrequen

cyDivider

PhaseDetector

Complex and universal circuit for optical controlling of frequency from millimetre-wave region.

It is possible to transmit reference frequency fREF and to control a frequency of VCO by Phase Detector and PLL system.

With using frequency multiplication process we can obtain every frequency from millimetre-wave region.


4. Optical- μwave Mixing (1)

RF

RF

PIN

Coplanar Line

Planar Optical Waveguide

POUT

a)

0 V

C

A

BV

b)

TMAX

T(V)

V0

Transmission of an optical power by Mach-Zehnder interferometer may be written as:

Above formula will be the the starting point for a theoretical analysis of nonlinear mixing processes.

;VV

Vcos121

TPP

V,VT RF00

MAXIN

OUTRF0N


4. Optical- μwave Mixing (2)

V1,f1

Fiber

POUT [W]

Photodiode

Laser

P0

Combiner

Filter

V2,f2

V0

M-Z Modulator

f1, f2, 2f1, 2f2, 2f1-f2, 2f2+f1, 2f2-f1

System to perform optical-microwave mixing process with the use of M-Z modulator

A combiner and bias circuit allow inputting the bias voltage and two alternating sine-form voltages into the modulator.

;tsinVtsinVV 2211RF The amplitude of the first of them, called also the signal, is small. The second

signal at the amplitude V2 plays role of a heterodyne and usually V2 >> V1.


5. Examples (1)

Optical link for transmitting the received signal to the base station.

DataM-Z Modulator

Laser Amp

Data.....

f1, f2,... fN

f

Fiber

Remote Antenna Receiver at Base Station

Amp


5. Examples (2)

Optical link for transmitting microwave signal to remote antenna.

DataM-Z Modulator

Laser

Antenna

Amp

Data.....

f1, f2,... fN

f

Fiber

Transmitter at Base Station

Receiver at Remote Antenna


5. Examples (3)

Fiber

100...200 m

Picocell

Millimetre-waveradio signals

Optical coupler

CentralStation

Radio-over-fiber system delivers the broad-band services to the customers by a radio


5. Examples (4)

Block diagram of the system which uses dense WDM

f0

MUX M-ZModul

OpticalCoupler & Filter

fIF1

LD11

fIF2

LD22

fIFN

LDN

N

Base Station

PD1

1

f0fIF1

xN

PD2

2

f0fIF2

Transponder 1

Transponder 2

xN

By using wavelength division multiplexing WDM techniques into the fiber access network each BS can be addressed by a different wavelength.


5. Examples (5)

Block diagram of base-station circuit with multiplication of carrier frequency for full-duplex, mm-wave fiber-radio network

Amp

Base-station

fC

fD,D

x N

Amp

Amp

Laser DFB

fD

Amp

Amp

WDM

2

1

Photodiode

Customer

Unit


5. Example (60 GHz P-MP)

T/R T/R modulemodule



E/O E/O systesyste

mm

T/R moduleT/R module156 Mb/s/60GHz 156 Mb/s/60GHz

TransceiverTransceiver

Point-to-multipoint radio-over-fiber full duplex system transmits data between computer

systems

Central StationCentral Station

Base StationBase Station

BSBS


5. Examples (60 GHz P-MP)

156 Mb/s 156 Mb/s DPSK DPSK

ModemModem

60 GHz 60 GHz Trans-Trans-ceiverceiver


Central StationCentral Station

LD – Laser diode, EAM – Electro-absorption modulator, EDFA – Fiber amplifier, DWDM Mux – Multiplexer,

PD Photodiode

EAMEAM EDFA

PDPD EDFA

LDLD

LDLD

λλ11

λλ22

DWDM MuxDWDM Mux


5. Examples (60 GHz P-MP)


PDPD

EAMEAM


ModemModem



ModemModem


λλ22

λλ11

EAM – Electro-absorption modulator, PD - Photodiode


5. Examples (125 GHz/10 Gb/s)

PDPD

EDFA EOMEOM LDLD

fM=62,5 GHz

fOPT

f0

fM

fOPT

2fM

EOMEOM

EDFA DATA

125 GHz 125 GHz ReceiverReceiver

DATA

TerminalTerminal

The last experimental

system


6. 6. Conclusions Conclusions

Photonic technology opens new possibilities to generate and to transmit the microwave signals, especially in millimeter-wave region

New wideband communication systems are developed on the basis of mm-wave and optical technologies

The gap between what is theoretically possible and what we experimentally demonstrated has narrowed considerably in the last decade

Documents

RoFS-Pforzheim 2007 1 Prof. Bogdan Galwas Warsaw University of Technology