Analog Photonics
Outline
These slides answer the following questions:
1. What is photonics?
2. Why use photonics? and...
3. How?
4. Why use analog photonics?
Analog Photonics
Edward Ackerman
Photonic Systems, Inc.
Billerica, Massachusetts, USA
March 4, 2014
www.photonicsinc.com
www.photonicsinc.com
Analog Photonics: Question 1
2
Analog Photonics: Question 2
Why use photonics?
What is “photonics”?
One common answer: Exploit its potential bandwidth
Answer: the generation and manipulation of photons for
applications such as sensing, communication, or information
processing (analogous to electronics, which is the manipulation
of electrons for the same purpose)
Most work in photonics has been
within a 40-THz-wide band in the nearIR part of the EM spectrum
Clarification: Isn’t that the same as “optics”?
– Some might say it is the same; convention seems to say it isn’t
– Optics is, more generally, the study of light. To work with
photonics therefore requires understanding of optics.
– You’ll see lots of terms like these used almost interchangeably.
Some almost interchangeable adjectives:
• Optic vs Photonic vs Lightwave
• Electro-optic vs Opto-electronic
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3
Radio
Infra-red
(includes RF,
microwave and
millimeter-wave
bands)
(includes submillimeter and
THz bands)
Visible
UV
X-ray
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Typical Attenuation for Silica-Based Optical Fiber
Analog Photonics: Question 2, cont’d
Why use photonics?
Infrared
absorption tail
from lattice
transitions
5
–OH absorption
peaks
2
Fiber Attenuation (dB in 100 m)
Attenuation (dB in 100 m) in Coaxial Cables
A second answer: Preserve signal quality over long distances
(especially in optical fiber)
Compare these data to
attenuation in optical
fiber (next slide)
1
0.5
0.2
0.1
“Dry” fiber
0.05
Rayleigh
Scattering
0.02
0.01
“Near” infrared
0.005
0.002
600
800
1,000
1,200
1,400
1,600
1,800
2,000
Wavelength (nm)
400
http://www.procom.dk/procomlab/product-help/attenuation-versus-frequency
Frequency (MHz)
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300
Frequency (THz)
200
Source: C. Cox, Analog Optical Links, Cambridge University Press, 2004.
150
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Optical Fibers
Analog Photonics: Question 2, cont’d
Why use photonics?
A third answer: Enable small size and weight
(in optical fibers or integrated optical waveguides)
250 feet of typical coax vs optical fiber
125 m
Single-mode
optical fiber
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Fiber spool
Multi-mode
optical fiber
Polarization-maintaining
optical fiber
Source: C. Cox, Analog Optical Links, Cambridge University Press, 2004.
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Analog Photonics: Question 3
A Conventional Electronic Link
Baseband or intermediate
frequency (IF) signal
How to exploit the advantages of photonics
(again: huge BW, long distances, small size/weight)?
Baseband or intermediate
frequency (IF) signal
f
a. Start with the basic photonic building block: a signal “link”
fRF
b. Know the devices that are required, and
Electronic
Mixer
Local
Oscillator
(LO)
f
Electronic propagation
medium
c. Understand what link architectures can be used
fRF
f
Electronic
Mixer
f
fRF
Local
Oscillator
(LO)
f
Conventional Electronic Link
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A Photonic Link
Amplification in a Link
and required devices (at minimum)
Electronic (e.g. coaxial-cable) vs Photonic (e.g. fiber-optic)
Baseband or intermediate
frequency (IF) signal
Baseband or intermediate
frequency (IF) signal
f
fRF
Local
Oscillator
(LO)
fRF
f
fRF
Electronic
Mixer
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f
Electronic
Mixer
f
Electronic propagation
medium
fRF
Coax
Coax
Low-noise
LNA
Amplifier
(LNA)
f
Electronic
Mixer
f
Local
Oscillator
(LO)
fRF
10
LO
Electronic
Mixer
Low-noise
LNA
Amplifier
(LNA)
LO
Coax Link with RF Amplification
f
Optical
Fiber
Optical
Fiber
fopt
Optical
Source
Optical
Source
Photonic propagation
medium
Modulator
Modulator
Erbium-Doped
EDFA
Fiber Amplifier
(EDFA)
EDFA
Erbium-Doped
Fiber Amplifier
(EDFA)
Photodetector
Photodetector
Fiber-optic Link with Optical Amplification
fopt
Photonic Link
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Channelization in a Link
Analog Photonics: Question 3, cont’d
Electronic (e.g. coaxial-cable) vs Photonic (e.g. fiber-optic)
Signal 1
RF Combiner
Signal N
Coax
Coax
LNA
LO at fRF N
RF Splitter
LO at fRF 1
Signal 1
Filter at fRF 1
How to exploit the advantages of photonics
(again: huge BW, long distances, small size/weight)?
LO at fRF 1
a. Start with the basic photonic building block: a signal “link” 
Signal N
b. Know what devices are required. At minimum:
Filter at fRF N
a. Optical source
LO at fRF N
b. Modulator
Coax Link with RF Channelization
c. Photodetector
Signal 1
Optical Source
at fopt 1
Signal 1
Optical
Fiber
Optical
Fiber
c. Understand what link architectures can be used
Photodetector
Signal N
Signal N
Optical Source
at fopt N
EDFA
N-channel
N-channel
Wavelength-Division
WDMs
Multiplexers (WDMs)
Photodetector
Fiber-Optic Link with Optical Channelization
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Photodetector
14
Direct vs Coherent Detection of Light Intensity
p-i-n Photodiode
Optical illumination =
Aopt e

j opt t opt

“Direct” Detection
Input
Signal
Optical
Source
Intensity
Modulator
Output
Signal
Photodetector

j
t 

The quantity Aopt e opt opt is
modulated by the input signal
“Coherent” Detection
Input
Signal
Optical
Source 1
Amplitude, Phase,
Frequency, or
Intensity Modulator
Photodetector

j opt 1t opt 1
Either Aopt1 , opt1 , opt1 or Aopt1e
is modulated by the input signal
Photogenerated current  power of incident optical illumination
 Aopt e
http://www.allaboutcircuits.com/vol_3/chpt_3/12.html


j opt t opt 2
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
Optical
Source 2
Output
Signal
Therefore, the total quantity
Aopt1e

j opt 1t opt 1

 Aopt 2 e

j opt 2t opt 2

is modulated by the input signal
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Analog Photonic Link
Analog Photonics: Question 3, cont’d
Direct Modulation
How to exploit the advantages of photonics
(again: huge BW, long distances, small size/weight)?
RF
Input
Direct Modulation Photonic Link
RF
Output
a. Start with the basic photonic building block: a signal “link” 
b. Know what devices are required. At minimum:
a. Optical source
In direct modulation links, one device is
both the optical source and modulator
b. Modulator
c. Photodetector
DC
DC
c. Understand what link architectures can be used
RF
Output
RF
Input
Diode
LASER
or LED
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+
RFIn
FABRY-PEROT DIODE LASER
DISTRIBUTED-FEEDBACK
DIODE LASER (DFB)
VERTICAL-CAVITY
SURFACE-EMITTING
LASER (VCSEL)
•Simple structure
•High power available
•Major application:
CD players
•Cost: $10 - 500
•Low laser noise
•High linearity
•Major application:
CATV distribution
•Cost: $500 – 5,000
•Testable at wafer
level
•Circular, low-divergence
beam
•Major application:
LAN links
•Cost: $10 -500
OPTICAL SPECTRUM
OPTICAL SPECTRUM
ID
IL
18
Diode Lasers
Direct-Modulation / Direct-Detection Link
+
Photodetector
Optical Propagation Medium
(e.g., optical fiber)
RFOut
P0
PD
Photodiode
ID (mA)
PD (mW)
Diode Laser
SD (A/W)
SL (W/A)
PD (A/W)
IL (mA)
g  Current gain (loss ) 
Link gain (loss ) 
dPD dI D
 SL SD
dI L dPD
OPTICAL SPECTRUM
 
Output RF Signal Power
 10 log g 2
Input RF Signal Power
Source: C. Cox, “Optical transmitters,” in The Electronics Handbook, J. Whitaker, ed., CRC Press, 1996.
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Detector Slope Efficiency vs Bandwidth
FP – < 1100 nm
FP – ~1300 nm
FP – ~1550 nm
DFB – < 1100 nm
DFB – ~1300 nm
DFB – ~1550 nm
VCSEL – < 1100 nm
VCSEL – ~1300 nm
VCSEL – ~1550 nm
1
Theoretical Maximum = h c / q  = 0.802 W/A
Gray line indicates maximum
available at any given frequency
DERA ‘98
Bell Labs ‘91
Avalon ‘01
Alcatel ‘04
Chalmers ‘02
Chalmers ‘04
Tokyo Inst.
Tech. ‘95
U. Wurtzburg/
Thales/Technion ‘05
0.1
0
U. Michigan/
Bellcore ‘03
Samsung ‘02
Chalmers ‘03
UCSB ‘98 Sandia ‘96
Agilent ‘02
U. Ulm ’94, ‘96
Research Devices
Ortel ‘95
Sandia ‘97
Equivalent Device Slope Efficiency SD
at 1550 nm (A/W)
Equivalent Device Slope Efficiency SL
at = 1550 nm (W/A)
Directly-Modulated Semiconductor Laser Slope Efficiency
vs Bandwidth (Research Devices – not fiber coupled)
U. Michigan/Bellcore ‘97
GTE ‘85
UCSB ‘91
Ortel ‘04
National Chiao-Tung U. ‘03
UCSB ‘93
Bell Labs ‘89
U. Cincinnati/Agere /03
Thales ‘04
Rockwell ‘92
U. Wurtzburg ‘04 Bell Labs ‘92
U. Michigan/
Fraunhofer Inst. ‘94
Bellcore ‘97
Bell Labs ‘94
UCSB ‘93
Fraunhofer Inst. ‘06
Hanyang U. ‘04
Cornell U. ‘91 Technische Universitat Berlin/Politecnico di Torino ‘04
U. Michigan/Georgia Tech. ‘98
OKI ‘97
Fraunhofer Inst. ‘94
UCSB ‘97
U. Of Munich, UCB ‘09
10
20
30
40
1.25
1
0.75
0.5
0.25
0
Waveguide
(830 nm)
PIN
– ~1300 nm
UMR ‘97
Technische Universitat Berlin ‘96
Waveguide (1000 nm)
Navy ‘04
PIN
– ~1550 nm
Technische Universitat
Waveguide
(1300 nm)
UCSB ‘05
Waveguide
(1550
Berlin ‘99
Waveguide
PIN
– <nm)
1100 nm
Thomson ’96, ‘97
ETRI (Korea), 2010
MSM/Schottky (600 nm)
Bell Labs
Waveguide
PIN
–
~1300 nm
National Taiwan U. ‘05
‘86
NTT ‘00 NEC ‘99 NTT ‘03
Waveguide PIN – ~1550 nm
Fujitsu ‘91
TI ‘97 Epitaxx ‘87
Ortel ‘96 NTT ’91, ‘92
MSM/Schottky – < 1100 nm
UVA ‘09
Bilkent U./Boston U. ‘02
UMR ‘97
MSM/Schottky – ~1550 nm
GTE ‘85
NTT ‘02 Alcatel ‘04
Bell Labs ‘86
Navy ‘04
UVA ’06
Alcatel ‘00
Boston U./NIST ‘98
Army/Drexel ‘96 UVA ’08 UIUC ‘02
NTT ‘94 Technische Universitat Berlin ‘04
Infineon/IBM ‘04
UCSD ‘93
NTT ‘97
UCLA ‘98
U Texas/Navy ‘03 UCSB/Colorado State ‘95
BT
Labs
‘91
UCSB ‘96
National Taiwan/
Raytheon/DSC ‘01
Gray line indicates maximum
Bell Labs ‘85 UCSB ‘93
UIUC ‘04
UCLA ‘98
Navy ‘02
available at any given frequency
Stanford ‘91
UMR ‘97
UVA ‘06
NTT ‘05
TRW/UCSD ‘99
UCSD ‘03 UCLA/Bell Labs ‘00
NTT ‘03
UMR ‘97
Bell Labs ‘86
U. Michigan/MIT ‘91
Heinrich Hertz/UVA, ‘08
UCSD ‘92
Gerhard Mercator ‘98
UCLA/JPL/Bell Labs ‘96
UCSB ‘95
NTT ‘97
NTT ‘98
UCSB ‘98
Technische Universitat Berlin ‘98
UCSB ‘95
NTT ‘00
10
50
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Experimental Broadband Direct Modulation Link
Gain Results > –20 dB
30
Link Gain (dB)
20
10
0
–20
0.01
UCSB, 2003
UCB, 2004
MITLL, 1990
UCLA, 2004
Chalmers U., 2002
0.1
1
Frequency (GHz)
10
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C. Cox, et al., IEEE Trans. Microwave Theory Tech., vol. 54, February 2006.
Source: C. Cox, et al., IEEE Trans. Microwave Theory Tech., vol. 54, February 2006.
Sun, 1997
100
1000
3 dB Bandwidth (GHz)
3 dB Bandwidth (GHz)
–10
PIN
–<
1100
nm
PIN
(1500
nm)
Theoretical Maximum = q  / h c = 1.249 A/W
23
Source: C. Cox, et al., IEEE Trans. Microwave Theory Tech., vol. 54, February 2006.
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Analog Photonics: Question 3, cont’d
How to exploit the advantages of photonics
(again: huge BW, long distances, small size/weight)?
a. Start with the basic photonic building block: a signal “link” 
b. Know what devices are required. At minimum:
a. Optical source
b. Modulator
In external modulation links, separate devices
are the optical source and modulator
c. Photodetector
c. Understand what link architectures can be used
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Analog Photonic Link
Single-mode Dielectric Optical Waveguides
External Modulation
Typical Properties at 1550 nm
Fiber
RF
Input
Lithium Niobate
RF
Output
External Modulation Photonic Link
Polymer
DC
Photodetector
Modulator
RF
Input
Optical Propagation Medium
(e.g., optical fiber)
RF
Output
III-V Semiconductor
DC
Laser
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Figures provided by G. Betts, MIT Lincoln Laboratory
V
LiNbO3 or Polymer Mach-Zehnder
Electro-Optic Modulator
V
Semiconductor Mach-Zehnder
Electro-Optic Modulator
V
Electroabsorption
(in semiconductor)
V
V
Directional Coupler
Total Internal
Reflection
Figures provided by G. Betts, MIT Lincoln Laboratory
Semiconductor ElectroAbsorption Modulator
V
V
RFIn
RFIn
RFIn
Laser
Laser
Laser
PI
V
26
Transfer Functions of Optical Modulators
Examples of Modulator Designs
Mach-Zehnder Interferometric Modulator (MZ)
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PO
PI
PO
PO
PI
PI
LI
PO
PI
PO
PO
PI
LI
LI
Mode evolution
V
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V
V
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Fujitsu ‘02
NTT ‘91
EA – ~1300 nm
EA – ~1550 nm
Polymer MZ – ~1300 nm
Polymer MZ – ~1550 nm
LiNbO3 MZ – < 1100 nm
LiNbO3 MZ – ~1300 nm
LiNbO3 MZ – ~1550 nm
Semicond. MZ – ~1550
1
Kotura ‘09
NEC ‘96
PSI ‘07
France Telecom ‘93
CRL ‘94 Lumera ‘08
OKI ‘97
PSI ‘07
Fujitsu ‘06 OKI ‘97
OKI ’97 NTT ‘07
NGK ‘02
CRL ‘96
Navy/TRW ‘98 NEC ‘92
UCSD/Mitsubishi ‘96
UCLA/USC ‘01
UCSD ‘04
NTT ‘98
NTT ‘91
NG ‘07
Navy ‘00
NTT ‘94
Navy ‘98
HP ‘92
Hughes ‘83
Intel ‘04
Amphenol ‘87
UCSD/Navy ‘01
U.
Paris-Sud,
‘08
MITLL ‘96
U. Delaware ‘07
UCSD/Navy ‘94
MITLL ‘96
UCSD’97
GigOptix ‘10
GigOptix ‘09
France Telecom ‘91
HP ‘84
T-Networks ‘02
Navy ‘84
Navy ‘98
UCLA/USC ‘00
MIT ‘85
0.1
0.1
1
10
100
3 dB Bandwidth (GHz)
* with optical input power = 400 mW and electrode impedance = 50 (if not reported in literature)
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ID
V
RFIn
RFOut
P0
CW Laser
PD
PI
Mach-Zehnder Modulator
Photodiode
SD (A/W)
SM(W/A)
V
V
g  Current gain (loss ) 
Link gain (loss ) 
29
PD (A/W)
dPD dI D
P
 SM SD  I SD
dV dPD
V
 
Output RF Signal Power
 10 log g 2
Input RF Signal Power
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Amplifierless IM / DD Link Gains > – 20 dB
Link Gain vs Detector Current
(Input and Output at Same Frequency)
30
40
Gain Predicted
By Model
Externally
Modulated
Solid State
Laser*
20
10
PSI, 2007
20
Measured Gain
Link Gain (dB)
Link
(dB)
LINKRF
RF Gain
GAIN (dB)
30
Directly Modulated HighSlope-Efficiency Laser
0
-10
Directly Modulated LowSlope-Efficiency Laser
-20
PSI, 2007
PSI, 2007
Navy, 1999
10
Johns Hopkins, 2010
Navy, 1997
Navy, 2007
PSI, 2005
MITLL, 1990
L-3, 2007
UCSD, 2004
MITLL, 1990
0
-30
Navy, 2009
-40
0.001
*V = 0.65 V,  = 90°
0.01
0.1
1
10
–10 UMass/Drexel U, 2009
100
Navy, 1995
AVERAGE
DETECTOR
CURRENT
(mA)
Average Detector
Current
(mA)
–20
0.01
Source: C. Cox, Analog Optical Links, Cambridge University Press, 2004.
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UCSD 2004
0.1
1
Frequency (GHz)
30
C. Cox, et al., IEEE Trans. Microwave Theory Tech., vol. 54, February 2006.
Avanex ‘07
Navy ‘98
+
+
ID (mA)
10
External-Modulation / Direct-Detection Link
PD /PI
Gray line indicates maximum
available at any given frequency
MITLL ‘96
C. Cox, et al., IEEE Trans. Microwave Theory Tech., vol. 54, February 2006.
Equivalent Device Slope Efficiency SM
at  = 1550 nm* (W/A)
Electro-Optic Modulator Slope Efficiency *
vs Bandwidth (Research Devices)
L-3, 2008
T-Networks, 2004
Navy, 1997
10
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EIA 7030832
(Input and Output at Same Frequency)
Direct Modulation
External Modulation
30
PSI, 2007
Link Gain (dB)
20
PSI, 2007
PSI, 2007
Navy, 1999
10
Johns Hopkins, 2010
Navy, 1997
Navy, 2007
PSI, 2005
MITLL, 1990
MITLL, 1990
0
UCSD, 2004
–10 UMass/Drexel U, 2009
L-3, 2007
UCSB, 2003
UCB, 2004 Navy,
2009
MITLL, 1990
Navy, 1997 Chalmers U., 2002
Navy, 1995
UCLA, 2004
U Leeds, 2006
–20
0.01
Where Does the Link Gain Come From ?
DC
DC
RF In
Laser
Modulator
Photodetector
RF
Out
Small Signal Efficiency >1
Total Efficiency < 1
L-3, 2008 T-Networks, 2004
Navy, 1997
UCSD 2004
0.1
C. Cox, et al., IEEE Trans. Microwave Theory Tech., vol. 54, February 2006.
Amplifierless IM / DD Link Gains > – 20 dB
1
10
Frequency (GHz)
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Analog Photonics: Question 3, cont’d
34
Outline
How to exploit the advantages of photonics
(again: huge BW, long distances, small size/weight)?
These slides answer the following questions:
1. What is photonics?
2. Why use photonics? and...
3. How?
4. Why use analog photonics?
a. Start with the basic photonic building block: a signal “link” 
b. Know what devices are required. At minimum:
a. Optical source 
b. Modulator 
One common answer: in some antenna remoting situations, A/D
conversion at the antenna site is impossible or undesirable
c. Photodetector 
c. Understand what link architectures can be used 
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Photonic Remote Sensing
Digital vs Analog Links
SETI’s Allen Telescope Array
Hat Creek, California
LO
A/D
Analog
Signals
Digital
Data
Preselector
Filter
E/O
Transducer
Optical Fiber
O/E
Transducer
Digital Photonic Link
To Signal
Digital Processor
Data
• Downconverter and A/D size and power consumption are major concerns;
one set of this hardware is required at each element
• Requires phase synchronization of LOs at all the sensor elements
LO
Analog
Signals
E/O
Transducer
Preselector
Filter
Optical Fiber
O/E
Transducer
Analog Photonic Link
A/D
Analog
Signals
Digital
Data
To Signal
Processor
• Minimizes size and power consumption of componentry at sensor element; in a phased array, analog
signals can be combined and then processed by a single set of down-converter and A/D hardware
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38
Phased Array Antennas
Outline
Analog Architectures
Conventional All-Electronic Implementation
These slides answer the following questions:
1. What is photonics?
2. Why use photonics? and...
3. How?
4. Why use analog photonics?
Antenna
Elements
T/R Modules
(Element-level Beamforming:  Shifters,
Splitter/Combiners)
Subarraylevel RF
beamformer
RF-IF
Conversion
Analog-toDigital
Conversion
Processing and
Display
One common answer: in some antenna remoting situations, A/D
conversion at the antenna site is impossible or undesirable
A second answer: to perform higher processing functions in the
optical domain
Phased array antenna beamforming
RF-IF down-conversion and IF-RF up-conversion
A/D conversion
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Phased Array Antennas
Straightforward Optical Approach to True Time Delay (TTD)
Analog Architectures
Conventional Implementation w/Fiber-Optics
Benefits:
1.
Antenna
Elements
2.
Photonic
T/R Modules
Remoting of beamforming
and other functions
May enable other
functions to be performed
in optical domain
T2
Tn
• Conceptually straightforward, historically always
limited by switch loss
– Lowest switch “on” insertion loss: ~2.5 dB
– Consequently total RF-to-RF loss for 8-bit delay: 40 dB
Optical Fibers
Photonic
T/R Modules
T1
Subarraylevel RF
beamformer
RF-IF
Conversion
Analog-toDigital
Conversion
Processing and
Display
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– Electronic switch, wavelength tunable, fiber-optic prism, etc.
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Conformal Wide Band Array
Controlled by Photonics
•
•
•
•
• Implication: To date most fiber-optic TTD work has
focused on ways to avoid the optical switch
42
Wideband Photonic Beamforming Techniques
Fiber-optic realization of a Rotman lens
Conformal L-Band array
500 MHz bandwidth, ±60° scan
96 elements, 24 columns, 8 subarrays
Each subarray controlled by a 5-bit
photonic time shifter
from R. Sparks, et al., “Eight beam prototype fiber optic Rotman lens,” Proc. IEEE
Top. Meeting on Microwave Photonics, November 1999.
Courtesy of J.J.Lee,
Raytheon
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EIA 7030843
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44
Wideband Photonic Beamforming Techniques
Photonic Down-Converter
Delay determined by fiber dispersion and tunable laser
Principle of Operation
Spectrum below
100 GHz:
LO
RF
Optical
Source
IF
Optical
Filter
Modulator
Photodetector
IF
Modulator
If photodetector is
low-speed, you don’t
need the optical filter!
Optical
Spectrum:
from R. Esman, et al., “Microwave true time-delay modulator using fibre-optic
dispersion,” Electron. Lett., vol. 28, pp. 1905, September 1992.
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Example of Photonic A/D Converter
Analog RF
V(t)
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Summary
• We differentiated analog photonics from related
fields, such as optics or digital photonics
• We examined the basic building block of analog
photonics – i.e., the signal link
Comparator
Parallel
Digital
Word
Pulsed
Laser
* From datasheet for STMicroelectronics STW82100B RF downconverter (1.6 – 2.4 GHz)
** Projected using PSI’s well-established link performance models
– Components: optical source, modulator, photodetector
– Candidate architectures: direct modulation, external
modulation
• We explored some other things that can be done with
analog photonics
Sampler
Quantizer
Optics
– Phased array beamforming
– Up-/Down-conversion
– A/D conversion
Hold
Electronics
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