Download Microwave Photnics

Prof. Bogdan Galwas
Warsaw University of Technology
RoFS-Pforzheim 2007
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R-o-F – Basic Structure of System
 Data transmission between Central Station and Base Station via fiber link
 Data transmission between Base Station and Terminal via radio link
Data Output
Data
Output
Base
Station
Central
Station
Optical
Transceiver
Data
Input
Fiber link
Fiber
Optical
Transceiver
Data Input
Mobile
wave
Terminal
Mobile
wave
Terminal
Radio System
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R-o-F – Basic Structure of System
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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Outline of lecture:
1. Introduction
2. Optical Transmission of μwave Signals
3. Optical Generation of μwaves
4. Optical-
μwave Mixing
5. Examples
6. Conclusions
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1. 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.
LD
3
10
30
P-I-N
100
EAM EOM
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MSM
300
f [GHz]
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2. Optical... Analog optical link (1)
 Problem: microwave signal (fRF,PIN,POUT ) is transmitted by an analog optical link.
Fiber
fRF,PIN
fRF,POUT
fOPT,PT
WN
Laser
fOPT,PR
WO
L,=+j
Photodetector
 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.
POUT f RF  POUT f RF  PR f OPT  PT f OPT 
G

;
PIN f RF 
PR f OPT  PT f OPT  PIN f RF 
Modulation
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Transmission
Detection
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2. Optical... Analog optical link (2)
PR  PT e  2  f
 Attenuation by fiber is simply expressed:
POPT
[mW]
SL [W/A]
IL [mA]
t
POPT(t)
ID
[A]
RD[A/W]
OPT
L
ID
[mA]
POPT[W]
t
;
t
t
 Principle of operation of optical analog link with direct intensity modulation of
laser optical power.
 Gain of analog link:
2
2
G  SL R D ;
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2. Optical... Analog optical link (3)
 An intensity modulation of laser optical power may be realised by external
electrooptical modulator.
fRF,PIN
Fiber
WN
Laser
fOPT,PT
L,=+j
PO
T
1
fOPT,PR
V
WO
Photodetector
ID
[A]
POPT
SMZ[V-1]
fRF,POUT
RD[A/W]
POPT
t
I(t)
t
V(t)
t
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t
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2. Optical... Analog optical link (4)
 The transmission of M-Z modulator
can be described as:
 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 :
 Gain of analog link is
proportional to the level of
optical power P0 :
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TV  
SMZ 
TMAX
2

 V  
1

cos

 V  ;
  

T
TV 
  MAX ;
V VV
2V
2
P
G  S2MZ R 2D  02 R 2D ;
V
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2. Optical... Analog optical link (5)
 Analog link with external electro-optical modulator offers high gain
30
Gain G[dB]
20
External
modulation
10
0
High SL laser
-10
Laser
modulation
-20
-30
0,01
Typical link
0,1
1
10
100
Photodetector current [mA]
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2. Optical Transmission of μwaves (1)
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
DATA
SIGNAL
FIBER
PHOTO-DIODE
OUTPUT
CARRIER
REFERENCE
DATA
SIGNAL
CARRIER
REFERENCE
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LASER
DIODE
LASER
DIODE
LASER
DIODE
OUTPUT
FIBER
PHOTO-DIODE
FIBER
PHOTO-DIODE
GAIN
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2. Optical Transmission of μwaves (2)
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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DATA
SIGNAL
CARRIER
REFERENCE
LASER
DIODE
PHOTO-DIODE
FIBER
OUTPUT
GAIN
LASER
DIODE
FIBER
PHOTO-DIODE
DATA
SIGNAL
OUTPUT
+
LASER
DIODE
CARRIER
REFERENCE
PHOTOFIBER
-DIODE
FILTER
GAIN
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2. Optical Transmission of μwaves (3)
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.
LASER
DIODE
EO
MODUL.
FIBER
DATA
SIGNAL
LASER
DIODE
PHOTO-DIODE
CARRIER
REFERENCE
EO
MODUL.
FIBER
PHOTO-DIODE
DATA
SIGNAL
LASER
DIODE
OUTPUT
EO
MODUL.
FIBER
OUTPUT
GAIN
PHOTO-DIODE
CARRIER
REFERENCE
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2. Optical Transmission of μwaves (4)
40-58 GHz
0-18 GHz
LD - 1=1,3 m
PD - 1=1,3 m
0-18 GHz
40-58 GHz
Amp
Mux
x8
Demux
DM
x8
Amp
5 GHz
LD - 1=1,5 m
5 GHz
PD - 1=1,5 m
5 GHz
 Very interesting and professional system for transmission signal from
40-58 GHz millimeter-wave region by optical link.
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2. Optical ... Chromatic-Dispersion Effect
 Laser optical power is modulated to generate an optical field
with the carrier and two sidebands
ET  A0 J 0 m cos0 t  0   J1 m cos0  RF t  1   J1 m cos0  RF t  2 ;
 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]


 

1  2
2
  0  
  0   ... L;
E R  E T exp L  j0  
2
 
2 

 

0
 At the PIN output the amplitude of the mm-wave power is given by:
POUT
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
 f RF  
2  1   2 
 ;
 cos 
  cos LcD
 2 
 f 0  

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2. Optical ... Chromatic-Dispersion Effect
fTO  f0 N / 2LcD ;
 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.
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2. Optical... Subcarrier Multiplexing
Base
Station
Central
Station
Optical
Transceiver
Fiber
Selective
Terminal
Optical
Transceiver
Selective
Terminal
Data 2 – f2
M U X
f1
f2
fN
Data 1
Data 2
Data N
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Data 1 – f1
Subcarrier multiplexing may be
used for multichannel
transmission
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3. Optical Generation...Optical mixing
 Photodetector is responsive to the photon flux, is insensitive to the optical phase.
 Two optical signals (EM fields): the first signal:




E S  Re A Se j2 f S t  Re A S e j2 f S t  S  ;
 The second signal, local
oscillator, ELO, |ALO|, fLO i LO.
The signal directed to the
photodetector:
Local
Oscillator fLO
Photodetector
Coupler
3dB, 1800
Signal fS
E  ES  E LO ;
Intermediate
Frequency fIF
 Photocurrent I is proportional to the incident power P and detector’s sensitivity R :
I  RP  RPS  PLO  2 PS PLO cos2f IF t  S  LO ;
- PS and PLO are the powers, is intermediate frequency.
 The name of the process: optical mixing, optical heterodyning, photomixing, coherent
optical detection.
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3. Optical Generation...Two Optical Carriers
 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.
Coupler
Laser
Carier
& Data
Amp
f2
fIF
Data
Laser
f1
fopt
0
0
f1 - f2
f1, f2,
 One optical signal may be modulated by data.
 The spectrum of optical signals must be “pure”, it is not easy to satisfy this condition .
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3. Optical Generation...Two Optical Carriers
 It is possible to construct a specially modified distributed feedback semiconductor
laser (DFB) in which oscillation occurs simultaneously on two frequencies,
for two modes.
Microwave
Signal
Double-mode
Laser
f1 & f2
Amp
fIF
fopt
0
f1, f2,
0
f1- f2
 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.
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3. Optical Generation...Two Optical Carriers
 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.
f
fOPT - 10f
PD
Slave
Laser 1
Tunable
Master Laser
fOPT  nf
Slave
Laser 2
20f
Fiber
fOPT + 10f
 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..
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3. Optical Generation of μwaves
 Laser on Nd:LiNbO3 electrooptical material placed inside microwave cavity
changes its frequency of optical oscillation.
M-Z
Modulator
Date
(fSi  Bi)
fm
Microwave
Generator
Laser
Nd:LiNbO3
Laser inside
microwave
cavity
fOPT
f0  fm
fOPT
f0  (fPi  Bi)
Optical transmitter with Nd:LiNbO3 laser with frequency modulated by
Microwave Generator and with external Mach-Zehnder modulator
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3. Optical Generation of μwaves
 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.
Amp
fREF, PIN
Phase
Detector
m
Frequency
Divider
PD
VCO
fOUT= n m fREF
POUT >> PIN
xn
Frequency
Multiplier
Amp
Complex and universal circuit for optical controlling of frequency from
millimetre-wave region.
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4. Optical- μwave Mixing (1)
 Transmission of an optical power by Mach-Zehnder interferometer may be
written as:
TN V0 , VRF  

POUT
1
V  
 1  cos0 V0   RF  ;
PIN TMAX 2 
V  

 Above formula will be the the starting point for a theoretical analysis of
nonlinear mixing processes.
a)
POUT
b)
T(V)
TMAX
Planar Optical Waveguide
C
RF
PIN
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RF
A
B
Coplanar Line
0
V0
V
V
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4. Optical- μwave Mixing (2)
System to perform optical-microwave mixing process with the use of M-Z
modulator
M-Z Modulator
POUT [W]
Laser
P0
Fiber
V0
Photodiode
f1, f2, 2f1, 2f2, 2f1-f2, 2f2+f1, 2f2-f1
Combiner
V1,f1
Filter
V2,f2
 A combiner and bias circuit allow inputting the bias voltage and two alternating
sine-form voltages into the modulator.
VRF  V1 sin 1t  V2 sin 2 t;
 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)
Amp
Laser
M-Z Modulator
Remote Antenna
.....
Data
f
f1, f2,... fN
Fiber
Amp
Data
Receiver at Base Station
Optical link for transmitting the received signal to the base station.
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5. Examples (2)
.....
f
Data
Antenna
f1, f2,... fN
Laser
M-Z Modulator
Transmitter at Base
Station
Fiber
Amp
Data
Receiver at
Remote Antenna
Optical link for transmitting microwave signal to remote antenna.
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5. Examples (3)
Millimetre-wave
radio signals
Central
Station
Picocell
Fiber
100...200 m
Optical
coupler
Radio-over-fiber system delivers the broad-band
services to the customers by a radio
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5. Examples (4)
 By using wavelength division multiplexing WDM techniques into the fiber
access network each BS can be addressed by a different wavelength.
fIF1
fIF2
fIFN
LD1
LD2



Transponder 1 Transponder 2
1
2
N
LDN
M-Z
Modul
MUX
f0
Base Station
Optical
Coupler
& Filter
2
1
PD1
f0
xN
fIF1
PD2
f0
 

fIF2
xN
Block diagram of the system which uses dense WDM
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5. Examples (5)
Base-station
fD,D
Amp
Amp
Photodiode
1
xN
fC
Amp
Customer
Unit
WDM
2
Amp
fD
Amp
Laser DFB
Block diagram of base-station circuit with multiplication of carrier frequency
for full-duplex, mm-wave fiber-radio network
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5. Example (60 GHz P-MP)
Base Station
Central Station
T/R module
156 Mb/s/60GHz
Transceiver
E/O
system
T/R
module
T/R
module
BS
Point-to-multipoint radio-over-fiber full duplex
system transmits data between computer
systems
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T/R
module
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5. Examples (60 GHz P-MP)
λ2
Central Station
LD
λ1
156 Mb/s
DPSK
Modem
60 GHz
Transceiver
Base Station
LD
EAM
PD
EDFA
EDFA
DWDM Mux
LD – Laser diode, EAM – Electro-absorption modulator,
EDFA – Fiber amplifier, DWDM Mux – Multiplexer,
PD Photodiode
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5. Examples (60 GHz P-MP)
Base Station
λ1
λ2
PD
60 GHz
Transceiver
156 Mb/s
DPSK
Modem
60 GHz
Transceiver
156 Mb/s
DPSK
Modem
EAM
EAM – Electro-absorption modulator,
PD - Photodiode
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5. Examples (125 GHz/10 Gb/s)
PD
EDFA
125 GHz
Receiver
DATA
DATA
Terminal
EOM
EDFA
EOM
LD
2fM
fOPT
fM
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fM=62,5 GHz
f0
fOPT
The last
experimental
system
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6. 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
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