RoFS-Pforzheim 2007 1
Prof. Bogdan GalwasProf. Bogdan GalwasWarsaw University of Technology Warsaw University of Technology
RoFS-Pforzheim 2007 2
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
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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)
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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
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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
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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
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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
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2. 2. OpticalOptical... ... Analog optical link Analog optical link ((33))
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
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2. 2. OpticalOptical... ... Analog optical link Analog optical link ((44))
;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
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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
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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
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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
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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.
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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
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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
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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
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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
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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:
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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.
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3. 3. Optical GenerationOptical Generation......Two Optical Two Optical CarriersCarriers
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.
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3. 3. Optical GenerationOptical Generation......Two Optical Two Optical CarriersCarriers
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.
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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
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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.
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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
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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.
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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
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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
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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
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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.
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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
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5. Example (60 GHz P-MP)
T/R T/R modulemodule
T/R T/R modulemodule
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
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5. Examples (60 GHz P-MP)
156 Mb/s 156 Mb/s DPSK DPSK
ModemModem
60 GHz 60 GHz Trans-Trans-ceiverceiver
Base StationBase Station
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
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5. Examples (60 GHz P-MP)
Base StationBase Station
PDPD
EAMEAM
156 Mb/s 156 Mb/s DPSK DPSK
ModemModem
60 GHz 60 GHz Trans-Trans-ceiverceiver
156 Mb/s 156 Mb/s DPSK DPSK
ModemModem
60 GHz 60 GHz Trans-Trans-ceiverceiver
λλ22
λλ11
EAM – Electro-absorption modulator, PD - Photodiode
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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
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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