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MICROWAVE OVER FIBERlinphotonics.com/website/wp-content/uploads/2013/09/Rochester-IEEE-JCM.pdf ·...
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MICROWAVE OVER FIBERApplications and Performance
March 31, 2010
Rochester IEEE JCM 20101
John A. MacDonald
Vice President of Engineering
Linear Photonics, LLC
Linear Space Technology, LLCHamilton, NJ 08619
609-584-5747
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OUTLINE
1. MICROWAVE LINKS: ANALOG / DIGITAL
2. INTENSITY MODULATION & DETECTION
3. PRACTICAL LINK STRUCTURES
– DIRECT MODULATION
– EXTERNAL MODULATION
– PHOTORECEIVERS
– TRANSMISSION MEDIUM
4. PERFORMANCE LIMITATIONS
– LINEAR EFFECTS
– NONLINEAR EFFECTS
5. NONLINEAR PERFORMANCE IMPROVEMENT
6. SYSTEM TRADE-OFFS
7. ANALOG / DIGITAL REPRISE
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Analog / Digital
• The bulk of fiber optic communications uses digital
modulation
– Fast switching and low pulse distortion determine link fidelity
• Certain applications not suited to digital:
– If bandwidth too high to be effectively digitized
– If system complexity is better suited toward simpler modulation (size, – If system complexity is better suited toward simpler modulation (size,
weight, power constraints)
– Examples to follow…
• Primary distinction between digital and analog is linearity
– Analog/Microwave links depend upon low distortion to achieve high
fidelity
• I will show later why everything is really analog
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Microwave Link Applications
• Phased Array Radar/Comms
– Narrowband RF feeds directly to antenna arrays
– Lower weight, lower complexity
– True Time Delay beamsteering
• Antenna and Signal Remoting
– Direct-RF over longer distances (many km)
– Reliable alternative to wireless in fixed services– Reliable alternative to wireless in fixed services
• Electronic Warfare / SIGINT / ELINT
– Secure Comms (EMI hard)
• Connection to passive sensors (listening)
• Remoting personnel from active sensors (protection)
– Towed Decoys provide very high bandwidth
• Space-based
– Mass
– Deployed fiber: can be radiation hard; less thermally sensitive
• Time and Frequency Distribution
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Microwave Link
Characterized by an RF input and an RF output
– Necessarily has a method of modulating and
demodulating an optical carrier
– Today’s Microwave Links dominated by
• Intensity modulation of semiconductor lasers
• Envelope detection using PIN or APD photodiodes
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Intensity Modulation
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Intensity Detection
Detection of Intensity-modulated optical signal can be performed using semiconductor photodiode
(photoelectric effect).
A photon (power) recombines to generate an electron (current)
For a P-I-N diode, the best case is one e- for every p
For an Avalanche Photodiode (APD), generally > 1 e- per p due to secondary recombinations: Gain (and Noise)
7
In either case, the ensemble average is called the Responsivity (amps/watt)
The electric current:
η = detector quantum efficiency
q = electron charge
The complex RF output spectrum: hω = photon energy
• The input square-law is reversed: RF output is linear with RF input
• Optical Phase information is lost
))](Re(21[)( tmRti +⋅∝
)()( tmFM ∝ω
DC term (ave power)
ωη
hq
R =
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Shot Noise
One side-effect of photoelectric effect is quantum shot noise
In particle terms, photons arrive randomly with Poisson distribution; each generates a shot of current. This gives
rise to a mean-square shot noise current density in W/Hz:
q = electron charge
R = photodiode responsivity
qRPi 22 =
8
R = photodiode responsivity
P = average optical power
z = load impedance
This signal is Gaussian (white) over all microwave frequencies and may limit the performance of the link.
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Other Modulation Formats
• Modulation need not be “Intensity”
– If m(t) is imaginary then “Phase Modulation” of the optical carrier
• Constant Intensity: p(t) = 1
• Difficulty in efficient/stable phase detection
– Coherent Detection
• Intensity modulated signal mixed with optical LO at receiver• Intensity modulated signal mixed with optical LO at receiver
– Optical Amplitude and Phase are preserved
– Requires additional filtering, higher laser stability
– Up/Down Converting Links
• Modulate twice:
– Sum and Difference signals
• A Multitude of approaches
– Focus here on Intensity Modulation
9
)()(Re21)( 21 tmtmtp +∝
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OUTLINE
1. MICROWAVE LINKS: ANALOG / DIGITAL
2. INTENSITY MODULATION & DETECTION
3. PRACTICAL LINK STRUCTURES
– DIRECT MODULATION
– EXTERNAL MODULATION
– PHOTORECEIVERS
– TRANSMISSION MEDIUM
4. PERFORMANCE LIMITATIONS
– LINEAR EFFECTS
– NONLINEAR EFFECTS
5. NONLINEAR PERFORMANCE IMPROVEMENT
6. SYSTEM TRADE-OFFS
7. ANALOG / DIGITAL REPRISE
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Practical Intensity Modulation
• Direct
– Laser diode is modulated directly
• External
– Laser source drives a separate optical – Laser source drives a separate optical
component
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Direct Modulation
Bias
ModulatingSignal
SemiconductorLaser
0
2
4
6
8
10
12
0 20 40 60 80 100 120 140 160
Bias Current (mA)
Optical Power
(mW)
Slope EfficiencyηL
IL
im
12
ITH
)()( mTHLLm iIIiP +−=η
IL
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• A CW optical signal is intensity modulated
via a field-dependent optical medium
• µwave / mm-wave modulation speeds can
be achieved with 2 major methods:
External Modulation
be achieved with 2 major methods:
– Electro-Optic Modulation
• Field-dependent change in optical index (electo-
optic effect)
– Electro-Absorption Modulation
• Field-dependent change in optical attenuation
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• Mach-Zehnder Interferometer
– Index along propagation axis is dependent on
applied field (modulating signal)– Electro-Optic effect can be realized in Lithium Niobate
(LiNbO3), InP, and other crystal structures, i.e. KDP (KH2PO4)
Electro-Optic Modulation
(LiNbO3), InP, and other crystal structures, i.e. KDP (KH2PO4)
14
Vm (RF + Bias)
OpticalPropagation
• Propagation constant of the beam in the lower leg is retarded due to applied electric field – experiences less phase shift.
• Optical intensity (power) is modulated by the applied RF signal
Diffused Optical Waveguide on LiNbO3 substrate
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0
0.5
1
-15 -10 -5 0 5 10 15
Vm
Inte
nsi
ty
Vπ
π
φπ
V
VPVP m
mm2
cos)( 2 +=
ex. Vπ = 5 V
Optical Output Power:
Pm = max output intensity
typically 3 to 4 dB below input
Vm = bias voltage
Mach-Zehnder
15
Vmex. Vπ = 5 V
φ = 0
Vm = bias voltage
φ = phase offset (shift from origin)
[ ]tOMIV
tVm ωπ cos12
)( ⋅+=
Quadrature Analog CW:
OMI = Optical Modulation Index
• Optical Output power follows cos2 function o Vector phase summation
• Vππππ is DC voltage that causes 180° phase rotationo Depends on crystal physics and electrode length o Corresponds to “min” and “max” output powero Digital Modulation: variation between min and max
• Analog Modulation: Bias at Quadrature (shown)o Results in linear intensity modulationo Slope = 1 at quadrature pointo Even order distortions are balanced (zero)
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• Absorption of optical signal dependent on applied bias
– Transmission follows exponential relationship with applied field
• Exponential f(Vm) is dependent on device length, carrier confinement, and instantaneous change in absorption
• Generally not as linear as MZM
Electro-Absorption
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0
0.25
0.5
0.75
1
0 0.5 1 1.5 2 2.5 3
Reverse Bias (V)
Relative Transmission
)()( Vmf
m eVP−=
N
P
ia
Vm
Pi
n
Pout
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• P-I-N diodes are most common
Photodetection
iout
VDC
Iave
PRPI ⋅=)(
R = responsivity (amps/watt)= ηq / hω
– Intrinsic bandwidth limited by diode capacitance
– Package and launch considerations may also limit
performance
• ~30 GHz bandwidth from lateral PIN
• ~100 GHz from waveguide PIN
17
= ηq / hω
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Transmission Medium
• Fiber is Optical Waveguide
• Fiber has very low loss
• High-fidelity Microwave Links
require Single-Mode fiber
cladcore nn >
require Single-Mode fiber
– The “single” mode is a hybrid:
• Nonzero E and M fields in direction
of propagation
• Two polarization modes
• Gives rise to mode dispersion
18
image from Wikipedia
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Singlemode
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Link Gain
22
G DLηη∝
Transmitter Photo-Receiver
Optical Attenuation(OA)
Direct Modulation: 2OA
G ∝
22
22
OAV
PG DL
π
η∝
Direct Modulation:
External (MZM): PL = Laser Carrier Power
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Direct Mod Intrinsic Gain
Single-frequency match at ωo:
Zin RL
Cj
Rs
Zout
HL(w) HD(w)
m(w)ηL
ηD
OA
D
L
WA
AW
=
=
)/(9.0
)/(25.0
η
ηTypical:
Z
Rochester IEEE JCM 2010 20
222
22
4 OARRCG
LSjo
DL
o ω
ηηω
=
Ls
j
D
RR
pFC
WA
=Ω=
=
=
)(5
)(2.0
)/(9.0η
out
soD
L
outoL
Z
RH
R
ZH
=
=
)(
)(
ω
ω
Then Link Gain at ωo is: Short Link (OA << 1):
)(64
)(5.1644
10
1
dBG
dBG
GHz
GHz
==
==
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OUTLINE
1. MICROWAVE LINKS: ANALOG / DIGITAL
2. INTENSITY MODULATION & DETECTION
3. PRACTICAL LINK STRUCTURES
– DIRECT MODULATION
– EXTERNAL MODULATION
– PHOTORECEIVERS
– TRANSMISSION MEDIUM
4. PERFORMANCE LIMITATIONS
– LINEAR EFFECTS
– NONLINEAR EFFECTS
5. NONLINEAR PERFORMANCE IMPROVEMENT
6. SYSTEM TRADE-OFFS
7. ANALOG / DIGITAL REPRISE
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• Linear Factors– Microwave Launch
– Inherent Bandwidth• Limited by device and package reactances
– Noise
– Fiber Medium
Performance Limits
– Fiber Medium• Absorptive Loss
• Dispersion (Chromatic, Polarization Mode)
• Nonlinear Factors– Distortion (Harmonic, Intermodulation, …)
– Fiber Medium• Stimulated Brillouin Scattering
• Raman Scattering
• Four-wave Mixing
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Linear Impacts: Microwave Launch
Primary source of microwave loss is input/output matching
DC
10 GHz
RL
RS
C
RS
C
• Narrow band links can be reactively tuned• Limited Bandwdith
• Fano’s rule: (BW)(Reflection Coefficient) < c
• However: Many or Most links require Broadband• Must use lossy matching – affects link gain
RLCL
Forward Biased Laser Diode
• Small real resistance RL
• Junction Capacitance and Ohmic Resistance
Reverse Biased Photo Diode
• Large reactive impedance dominated by junction
capacitance
• Ohmic Resistance adds dissipative loss
CJ
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Linear Impacts: Noise
• Laser Relative Intensity Noise (RIN)
– Caused by spontaneous emission (laser not a perfect
oscillator) RIN = dB/Hz w/r to Optical Carrier
– Receiver detects as microwave noise
– Noise Power follows detection square-law: No ~ I2·RIN– Noise Power follows detection square-law: No ~ I2·RIN
• Shot Noise
– Noise power follows Optical power: No ~ I
• Thermal Noise
– Ubiquitous Johnson Noise: No ~ constant
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Output Noise of F/O Link
-180
-175
-170
-165
-160
-155
-150
-10 -8 -6 -4 -2 0 2 4 6 8 10
Optical Receive Power (dBmo)
No
ise
Po
we
r D
en
sit
y (
dB
m/H
z)
Thermal
Shot
RIN
Total
Gain of F/O Link
-60
-50
-40
-30
-20
-10
0
-10 -8 -6 -4 -2 0 2 4 6 8 10
Optical Receive Power (dBmo)
Ga
in (
dB
)
Noise Figure of F/O Link
0
10
20
30
40
50
60
-10 -8 -6 -4 -2 0 2 4 6 8 10
Optical Receive Power (dBmo)
No
ise
Fig
ure
(d
B)
Noise Figure
• Total output noise is • Gain maintains 2:1 • Resulting link noise
25
• Total output noise is related to optical received power (ORP):
2:1 in RIN region1:1 in shot region0 in thermal region
• Gain maintains 2:1 relationship with ORP
• Resulting link noise figure best at RIN limit. • Minimum value depends on laser RIN noise
Link Noise Figure and Dynamic Range vary with optical power
– defined in conjunction with the operational system
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Linear Impacts: Fiber Medium
• SM Fiber Attenuation
– Due primarily to Rayleigh (elastic) Scattering
– 0.25 dB/km (1550 nm)
– 0.5 dB/km (1310 nm)
• Chromatic Dispersion
– Wavelength dependent propagation velocity
– Sidebands arrive out-of-phase: gain nulling– Sidebands arrive out-of-phase: gain nulling
• Polarization mode Dispersion
– Orthogonal Polarization Modes different
propagation velocities
– Non-uniform through fiber length
• concentricity defects
• mechanical and thermal perturbations
• laser spontaneous emission
– Nondeterministic
– Affects pulsed (digital) systems
– Affects Time/Freq Distribution systems
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Chromatic Dispersion
Polarization Mode Dispersion
FAST AXIS
SLOW AXIS
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Nonlinear Impacts:
Modulator Transfer Function
0
2
4
6
8
10
12
0 20 40 60 80 100 120 140 160
Bias Current
Optical Power Direct Modulation
• Gain Compression when drive level peaks reach threshold
• Even and Odd order amplitude distortion
• Phase distortion due to laser chirp (FM to PM)• Laser wavelength = function(drive level, temperature)
Bias Current
0
0.5
1
-15 -10 -5 0 5 10 15
Vm
Inte
nsi
ty
MZM External Modulation
• Gain Compression when as drive level approaches peaks
• Primarily Odd order amplitude distortion
• Little phase distortion
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Linearity Measures
• Third-Order Intercept (IP3)
– (Imaginary) point where two-tone third-order intermodulation products (IMD3) are equal to the fundamental
• Spur-Free Dynamic Range
– Range of power over which the fundamental is above the noise floor and the IMD3 are below the noise floor
Pout
[ ])()(3)(1743
2)( 3
2
3 dBBWdBmIIPdBNFHzdBSFDR ++−=⋅
28
Pin
Output Noise Floor
IIP3SFDR-3
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Nonlinear Impacts: Fiber
• Stimulated Brilluoin Scattering (SBS)
– Vibrational/Acoustic oscillations generated by high energy photons
– Forward wave energy is converted to acoustic backward wave (phonons)
– Threshold Effect
– Reduced forward gain; Increased noise floor
• Stimulated Raman Scattering (SRS)
– Inelastic photon scattering Nonlinear fiber effects must be – Inelastic photon scattering
– Wavelength translation
– Reduction in gain
• Self-Phase Modulation
– AM-PM conversion of a single signal
• Cross-Phase Modulation
– AM-PM Transfer from one signal to another (WDM systems)
• 4-Wave Mixing
– Intermodulation Distortion (WDM systems)
29
Nonlinear fiber effects must be
considered during link design phase.
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Modulation Summary
TYPE COMPLEXITY SIZE
WEIGHT
POWER
PRACTICAL
MODULATION
FREQUENCY
LINEARITY COST
DIRECT Low: one optical
component
(laser)
Lowest < 12 GHz Poor 2nd and
3rd-order
performance
Lowest
ELECTRO- Moderate: Similar to 40+ GHz Poorest Higher,
30
ELECTRO-
ABSORPTION
Moderate:
requires separate
source laser and
small modulator
Similar to
direct mod
40+ GHz Poorest
Linearity, but
may have 2nd-
and 3rd-order
null operating
points
Higher,
comparable
to EOM
ELECTRO-
OPTIC (MZM)
Highest: requires
source laser,
large modulator,
plus optical and
electrical
controls for bias
locking
Highest 40+ GHz Well-defined
(sin curve).
Operation at
quadrature
provides 2nd-
order null.
Highest
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Closing the Link
LINK• Directly Modulated Laser / PIN Receiver / Postamplifier• Reactively Tuned 3.25 – 4.00 GHz
LINK• E-O Modulator (MZM) / Waveguide PIN Receiver• Broadband Response to 40 GHz
PHOTORECEIVERBroadband O/E Response to 20 GHzand Output Return Loss
31
Link Type
Centerband
Gain Input IP3 Noise Fig SFDR3dB dBm dB dB Hz^2/3
4 GHz
Direct Mod-20 32 28 118
20 GHz
MZM-25 31 38 111
30 GHz
EAM-30 18 40 101
40 GHz
MZM-30 25 43 104
“Typical” Link
Gain, Noise, Dynamic Range
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OUTLINE
1. MICROWAVE LINKS: ANALOG / DIGITAL
2. INTENSITY MODULATION & DETECTION
3. PRACTICAL LINK STRUCTURES
– DIRECT MODULATION
– EXTERNAL MODULATION
– PHOTORECEIVERS
– TRANSMISSION MEDIUM
4. PERFORMANCE LIMITATIONS
– LINEAR EFFECTS
– NONLINEAR EFFECTS
5. NONLINEAR PERFORMANCE IMPROVEMENT
6. SYSTEM TRADE-OFFS
7. ANALOG / DIGITAL REPRISE
32Rochester IEEE JCM 2010
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• Linearization Techniques
– Linearization improves nonlinear distortion, increases dynamic range
– Major techniques under study include optical, electrical, and combinatorial approaches
Performance Improvement
electrical, and combinatorial approaches
• Electrical: aim is to cancel distortion products
– Feedforward/Feedback: Inject out-of-phase distortion products to cancel
– Predistortion: Nonlinear circuits with opposing distortion characteristics
• Optical: generally more complex
– Operates in optical domain – inherently wide-band
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• Predistortion Linearization has long history in
broadcast power amplifiers; SSPAs, TWTAs,
klystrons, space and ground station equipment
– Generally much less complexity than optical or
combinatorial systems
Electrical Predistortion
combinatorial systems
• Does not rely on sampled waveforms
• Bandwidth is the major challenge
– The aim is to compensate for the gain and phase
compression of the nonlinear system by providing a
cascaded element function that has the opposite gain and
phase characteristic: gain and phase expansion
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Predistortion Linearization
-5
-4
-3
-2
-1
0
1
2
3
-5 -4 -3 -2 -1 0 1
1
2
3
4
Input Power
Output Power
Phase
35
• Nonlinear Device exhibits Gain and Phase Compression
-2
-1
0
1
-5 -4 -3 -2 -1 0 1
Input Power
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Predistortion Linearization
-5
-4
-3
-2
-1
0
1
2
3
-5 -4 -3 -2 -1 0 1
1
2
3
4
-5
-4
-3
-2
-1
0
1
2
3
4
5
-5 -4 -3 -2 -1 0 1
1
2
3
4
Input Power Input Power
Output Power Output Power
Phase Phase
36
• Nonlinear Device exhibits Gain and Phase Compression
• Precede it with another nonlinear device that exhibits gain and phase expansion, in
conjugate with the device to be linearized (the linearizer)
-2
-1
0
1
-5 -4 -3 -2 -1 0 1
-2
-1
0
1
-5 -4 -3 -2 -1 0 1
Input Power Input Power
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Predistortion Linearization
-5
-4
-3
-2
-1
0
1
2
3
-5 -4 -3 -2 -1 0 1
1
2
3
4
-5
-4
-3
-2
-1
0
1
2
3
4
5
-5 -4 -3 -2 -1 0 1
1
2
3
4
-5
-4
-3
-2
-1
0
1
2
-5 -4 -3 -2 -1 0 1
-1
0
1
2
3
Input Power Input Power Input Power
Output Power Output Power Output Power
Phase Phase Phase
37
-2
-1
0
1
-5 -4 -3 -2 -1 0 1
-2
-1
0
1
-5 -4 -3 -2 -1 0 1
-5
-4
-3
-2
-1
-5 -4 -3 -2 -1 0 1Input Power Input Power Input Power
• Nonlinear Device exhibits Gain and Phase Compression
• Precede it with another nonlinear device that exhibits gain and phase expansion, in
conjugate with the device to be linearized (the linearizer)
• The desired outcome is an ideal limiter
– The linearity of an ideal limiter cannot be improvedRochester IEEE JCM 2010
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MZM Linearized Link
(Predistortion)
5.7 dB1.5 dB
38
Non linearized @ 8 GHzP1 dB is 5.7 dB from saturationPhase compression rapidly above sat
Linearized @ 8 GHzP1 dB is 1.5 dB from saturationPhase nonlinearity < 1°
A measure of linearity is the distance between P1dB and Psat
(How close are we to an ideal limiter?)
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MZM Linearized Link
(Predistortion)MZM Microwave Link
Linearized IM D Products
60
80
100
120T
hir
d-O
rde
r IM
D (
dB
c)
Non Linearized
Linearized
39
0
20
40
0.01 0.10 1.00
Optical Modulation Index
Th
ird
-Ord
er
IMD
(d
Bc
)
MZM Microwave Link two-tone IMD measured at 8 GHz
> 20 dB improvement in IMD
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OUTLINE
1. MICROWAVE LINKS: ANALOG / DIGITAL
2. INTENSITY MODULATION & DETECTION
3. PRACTICAL LINK STRUCTURES
– DIRECT MODULATION
– EXTERNAL MODULATION
– PHOTORECEIVERS
– TRANSMISSION MEDIUM
4. PERFORMANCE LIMITATIONS
– LINEAR EFFECTS
– NONLINEAR EFFECTS
5. NONLINEAR PERFORMANCE IMPROVEMENT
6. SYSTEM TRADE-OFFS
7. ANALOG / DIGITAL REPRISE
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Trade-Offs, or
Why Use Fiber?
• What are the System Engineer’s Tradeoffs?
• Should I consider fiber?
– Tradeoff performance, cost, weight vs. other options
• If fiber, then what do I need to know?
– Direct Mod vs. Ex-Mod (Decision Needed)– Direct Mod vs. Ex-Mod (Decision Needed)
• Direct Mod (usually < 12 GHz)
– Weight, Cost, Loss, Dynamic Range, DC Power
– Often Comparing directly to Coax
• Ex-Mod
– Must evaluate system impact for linearity, G/T, etc
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Direct Mod Comparison to Coax
• At Direct Mod frequencies, choice of Fiber vs.
Copper often made on basis of performance as a
function of link length
• Primary System Trades include:
– Attenuation– Attenuation
– Noise Figure
– Weight
– Cost
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Direct Mod Comparison to Coax
Attenuation
0
5
10
15
20
25
30
35
40
45
50
0 100 200 300 400 500 600 700 800 900 1000
Distance (meters)
Att
enuation (dB
)
RG8 100 MHz
RG8 1 GHz
RG8 2 GHz
LDF6 100 MHz
LDF6 1 GHz
LDF6 2 GHz
Direct Mod Link
Noise Figure
0
5
10
15
20
25
30
35
40
45
50
0 100 200 300 400 500 600 700 800 900 1000
Distance (meters)
Nois
e F
igure
(dB
)
RG8 100 MHz
RG8 1 GHz
RG8 2 GHz
LDF6 100 MHz
LDF6 1 GHz
LDF6 2 GHz
Direct Mod Link
Distance (meters)
Weight
0
50
100
150
200
250
300
350
0 10 20 30 40 50 60 70 80 90 100
Distance (meters)
Weig
ht (p
ounds)
RG8
LDF6
Direct Mod Link
Cost
$0
$1,000
$2,000
$3,000
$4,000
$5,000
$6,000
$7,000
$8,000
$9,000
$10,000
0 100 200 300 400 500 600 700 800 900 1000
Distance (meters)
Cost ($
) RG8
LDF6
Uncooled Direct Mod
Cooled Direct Mod
Distance (meters)
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• Direct Mod Link attractiveness depends on system requirements.
– Crossover Length: when DM performance exceeds that of coax in various categories
Attenuation Noise Figure Weight Cost
RG8 110 m 180 m 30 cm 1800 m
Direct Mod Comparison to Coax
• Other factors may influence choice of Fiber vs. Copper:
– Linearity (link dynamic range is less than coax)
– EMI immunity (may trump performance)
– Safety, Reliability, etc.
RG8 110 m 180 m 30 cm 1800 m
Heliax LDF6 560 m 900 m 50 cm 100 m
Example: Crossover Lengths at 2 GHz
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Some Comments on Digital Comms
• Modulation Formats: same hardware as analog
– Direct
• Laser driven between threshold (off) and Imax (on)
– External
• MZM Vmin (off) to Vmax (on), range = Vπ• MZM Vmin (off) to Vmax (on), range = Vπ
• EAM 0 (on) to Vmax (off)
• Receiver Formats
– Emphasis on limited (square wave) output
• PIN & Transimpedance Amp (TIA) / Limiter
• Sensitivity: dBmo ave optical power for 10-12 BER
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It’s Really Analog
Power Spectral Density of 10 GB/s Random Binary Waveform (NRZ)
2
)sin()(
=
fT
fTTfSxx
π
π
0
20
Sxx f( )
1000.1 f
0.1 1 10 10020
15
10
5
0
GHz
Sxx(f)
(dB)
• High data rates require microwave design techniques
– Random binary waveform = Broadband microwave spectrum
– Requires broadband flat link response
– Gain Flatness, Noise, Linearity will impact Bit Error Rate
– Packaging, RF launch
– Ultra broadband waveforms
– Device capacitance and parasitics
GHz
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• Applications for fiber optic links employing microwave
analog modulation steadily increasing
– Improvements in semiconductor laser manufacturing
– Availability of low cost optical passives and fiber optic cable
• Microwave links useful alternative to coaxial or wireless in
Summary
• Microwave links useful alternative to coaxial or wireless in
many applications
– Performance
– Size, Weight, Cost
– EMI
• Practical intensity modulation links were presented
– Typical Performance, Limitations, and Methods of Improvement
Rochester IEEE JCM 2010 47