Fiber Optic Infrastructure Application Guide · Fiber Optic Infrastructure Application Guide ...
LINEAR MICROWAVE FIBER OPTIC LINK SYSTEM...
Transcript of LINEAR MICROWAVE FIBER OPTIC LINK SYSTEM...
LINEAR MICROWAVE FIBER OPTIC
LINK SYSTEM DESIGN
John MacDonald & Allen Katz
1
Linear Photonics, LLC
3 Nami Lane, Unit C-9, Hamilton, NJ 08619
609-584-5747
www.linphotonics.com
OUTLINE
• Analog (not Digital)
• Some General Applications
• Microwave Link Overview
– Intensity Modulators & Modulation
– Photoreceivers
2
– Photoreceivers
– Link/System Considerations/Comparisons
– Link Noise & Linearity
• Performance Improvement (Linearization)
– Electrical Predistortion
– Optical Linearization Example
• Conclusions
Digital or Analog?
• Majority of fiber optics communications use DIGITAL
signals (ON & OFF)
– Fast switching and low pulse distortion determine link performance
• Certain applications not suited to digital:
– Bandwidth too high to be effectively digitized
– System complexity better suited toward simpler modulation (size,
3
– System complexity better suited toward simpler modulation (size,
weight, power constraints)
• Whatever the system, the primary distinction between digital
and analog is linearity– Analog/Microwave links depend upon low distortion to achieve high
performanceDIGITAL = NONLINEAR
ANALOG = LINEAR
• RADIO OVER FIBER DIRECTLY MODULATES RF/MICROWAVE SIGNALS ON AN OPTICAL CARRIER.
• THIS ELIMINATES THE NEED UP AND DOWN COVERTERS FOR THE DISTRIBUTION OF MICROWAVE SIGNALS AND TAKES ADVANTAGE OF THE LOW LOSS OF OPTICAL FIBERS.
Microwave Link Applications
• A MACH ZEHNDER MODULATOR (MZM) IS OFTEN EMPLOYED TO AMPLITUDE MODULATE THE LIGHT CARRIER.
RF RFRF RF
Microwave Link Applications
• Phased Array Communications and Radar
– Narrowband RF feeds directly to antenna arrays
– Lower weight, lower complexity
– True Time Delay beamsteering
• Antenna and Signal Remoting
5
• Antenna and Signal Remoting• Direct-RF over longer distances (many km)
• Reliable alternative to wireless in fixed services
• Electronic Warfare / SIGINT / ELINT
– Fiber-Towed Decoys provide very high bandwidth
– Secure Comms (EMI hard)
• Connection to passive sensors (listening)
• Remoting personnel from active sensors (protection)
Microwave Link
• We mean:
“Characterized by an RF-input and an RF-
output”
– Necessarily has a method of modulating and
6
– Necessarily has a method of modulating and
demodulating an optical carrier, and some fiber
in between
– Today’s technology dominated by:
• Intensity modulation of semiconductor lasers
• Photodetection using PIN or avalanche photodiodes
Intensity Modulation
• Direct
– Laser diode is modulated directly
• External
– Laser source drives a separate optical
7
– Laser source drives a separate optical
component
RFRF
Direct Intensity Modulation
Bias
Modulating
Signal
SemiconductorLaser
0
2
4
6
8
10
12
0 20 40 60 80 100 120 140 160
Bias Current (mA)
Optical Power
(mW)
Slope Efficiency
ηL
IL
im
8
ITH
)()( mTHLLm iIIiP +−=η
IL
�Simplest to Implement
�Most Cost Effective
�Highly Linear
�Limited in Bandwidth (> 12 GHz)
Packaged Laser Limitations
• Low Cost Commercial Laser Transmitters utilize
packaged laser diodes– Widespread use in CATV, RF-over-Fiber
• Thermal and Frequency Limitations– Frequency Response Limited by package parasitics– Frequency Response Limited by package parasitics
– Thermal Operation limited by changing gain slope
– TEC-cooled “butterfly” packages solve thermal problem
– At expense of even lower bandwidth (more package effects)
– Uncooled “TO-Can” packages can operate to ~ 4 GHz
– Slope change limits thermal range for stable link gain to ~ 0 to 50 C
Cooled “butterfly” laserFrequency Response < 3 GHz
High Current (TEC)
Low Reliability (TEC)
Uncooled “TO” laserFrequency Response < 4 GHz
Limited Thermal Range
• A CW optical signal is intensity modulated
via a field-dependent optical medium
• Microwave modulation speeds can be
achieved with 2 major methods:
External Modulation
10
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
• 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 (EOM)
11
• Mach-Zehnder Modulator (MZM)
Vm (RF + Bias)
OpticalPropagation
• Propagation constant of the beam in
the lower leg is retarded due to
transverse electric field – experiences
less phase shift.
• Optical intensity (power) is
modulated by the applied RF signal
Diffused Optical Waveguide on LiNbO3 substrate
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:
Im = max output intensity
typically 3 to 4 dB below input
Vm = bias voltage
Mach-Zehnder (MZM)
12
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 Caused by adding 2 signals of differing phase
• 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 No even-order nonlinearity (need voltage tracking to maintain)
Electro-Absorption (EAM)
• 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
13
confinement, and instantaneous change in absorption
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
Modulation Summary
TYPE COMPLEXITY SIZE
WEIGHT
POWER
PRACTICAL
MODULATION
FREQUENCY
LINEARITY COST
DIRECT Low: one optical
component
(laser)
Lowest > 12 GHz
Degrades
> ~ 4 GHz
Best 2nd and
3rd-order
performance
Lowest
ELECTRO- Moderate: Similar to 40+ GHz Poorest Higher,
14
ELECTRO-
ABSORPTION
Moderate:
requires separate
source laser and
small modulator
Similar to
direct mod
40+ GHz
Effective linear
bandwidth limited
to a few 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
Very wide band
Well-defined
(sin curve).
Operation at
quadrature
provides 2nd-
order null.
Highest
Angle Modulation
• Experimental – Frequency modulated system (voltage control of laser
frequency)
– Phase modulated system (half of MZM)
• Advantages
15
• Advantages
- Very high linearity
- Wide bandwidth
• Problems
- Detection
- Cost
Photodetection
• P-I-N diodes are most common
iout
VDC
Iave
PRPI ⋅=)(
R = responsivity (amps/watt)
16
– Intrinsic bandwidth limited by diode capacitance
– Package and launch considerations may also limit performance
• 25 GHz bandwidth from lateral PIN
• 50+ GHz from waveguide PIN
Photoreceiver Response
17
MPR-series Photoreceiver
O/E Response and Return Loss of MPR0020 photoreceiver
Fiber
• Linear Microwave Links primarily employ Single-Mode Fiber
• Two Primary Wavelength Bands
– 1300 nm: -0.5 dB/km
– 1550 nm: -0.25 dB/km
18
– 1550 nm: -0.25 dB/km
• Source of Distortion
– Linear: Dispersion
� Can correct
– Nonlinear:
� Usually not an issue at low optical power (< 10 mW)
Quantitative Link Measures
• Gain/flatness
– Broadband links are lossy
– Gain Slope / Ripple important to system design
• Noise Figure
– Generally higher than the link loss– Generally higher than the link loss
• Third-Order Intercept
– Intercept of fundamental and 3rd-order IMD curves
• Spur-Free Dynamic Range (SFDR)
– Power Range over which the intermodulation distortion is below the noise floor
[ ]31743
23 IIPFSFDR +−= 3/2
HzdB ⋅
Performance Limiting Factors
• Linear Factors• Microwave Launch
• Laser diode and EAM diode are low-impedance
• Fano’s Rule
• Packaging
• MZM functionality dependent on interaction length of optic and electric fields
20
electric fields
• Traveling-Wave Launch with RF termination is inherently inefficient
• Inherent Bandwidth• Limited by device capacitances; smaller devices = lower power
• Noise
• Nonlinear Factors• Intermodulation Distortion
Link Examples: Bandwidth/Flatness
15 GHz Electroabsorption Transmitter
MPR0118 18 GHz Receiver
Broadband Mach-Zehnder Transmitter
40 GHz Waveguide PIN Receiver
25 GHz Electroabsorption Transmitter
MPR0020 Receiver
25 GHz Electroabsorption Transmitter
APR0020 Post-Amplified Receiver
Typical (Nonlinearized) Performance
Link Type
Low Freq
Gain
Operational
BW
Slope over
BW Input IP3 Noise Fig SFDR3
Optical
Budget
dB GHz dB dBm dB dB Hz^2/3 dB
Direct -32 3 -1 36 40 113 7
NB EAM -35 15 -3 23 40 105 7
EOM
(Vpi = 4 V)-24 30 -5 25 40 110 8
WB EAM -30 30 -3 18 40 101 3
22
• Performance relative to 0 dBmo incident receiver power (optical budget equal
to typical TX optical output power, or optical attenuation Tx to Rx).
• Results shown are typical for broadband links to the bandwidth indicated.
Narrowband microwave links can achieve higher performance.
• 1 dB decrease in optical attenuation results in 2 dB increase in RF Gain.
• 1 dB decrease in optical attenuation results in NF reduction of from 0 to 2 dB,
depending on if link is RIN, shot, or thermal noise limited.
WB EAM -30 30 -3 18 40 101 3
Performance Requirements
– Communications
• Multi-carrier, complex waveforms, moderate dynamic range, high bandwidth
– Requires very high linearity, lower drive levels
– Radar
• Mostly single-carrier, simple waveforms, lower dynamic range, lower bandwidth
23
– Requires moderate linearity, can drive to higher levels
– Distinction between transmit and receive side
– EW
• Ultra-wide bandwidth drives need for high dynamic range
IMPROVEMENTS IN LINEARITY/SFDR WILL BROADEN APPLICATION OPPORTUNITIES FOR
MICROWAVE FIBER OPTICS
Performance Improvement
• SFDR is useful quality measure
– Tells minimum signal levels for interference-free operation
[ ]31743
23 IIPFSFDR +−=
3/2HzdB ⋅
SFDR
• Avionics platforms are targeting broadband >120
• To improve SFDR must decrease noise figure (NF) and/or increase intercept (IP3)
– 3 dB decrease in NF yields 2 dB increase in SFDR3
– 3 dB increase in IP3 yields 2 dB increase in SFDR3
• Or, a 3 dB improvement in IMD yields 1 dB SFDR3
3
3/2HzdB ⋅
Understanding Link Noise
• Link noise results from 3 sources (if no optical amplifiers)
– System Thermal Noise
• Low-Level incident optical power (< 0 dBmo)
• Noise Figure varies 2:1 with optical power
– Receiver Shot Noise– Receiver Shot Noise
• Due to random photon arrivals, with quantized energy
• Mid-Level incident optical (1-3 dBmo)
• Noise Figure varies 1:1 with optical power
– Laser Relative Intensity Noise (RIN)
• Due to undesired transitions in the source laser
• High-Level incident optical (> 4 dBmo)
• Noise Figure independent of optical power and min for the link
Link Noise
• Total output noise is • Gain maintains 2:1 • Resulting noise figure
Output Noise of F/O Link
-180
-170
-160
-150
-140
-15 -10 -5 0 5 10 15
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
-70
-60
-50
-40
-30
-20
-10
0
-15 -10 -5 0 5 10 15
Optical Receive Power (dBmo)
Ga
in (
dB
)
Noise Figure of F/O Link
0
10
20
30
40
50
60
70
-15 -10 -5 0 5 10 15
Optical Receive Power (dBmo)
No
ise
Fig
ure
(d
B)
• Total output noise is
related to optical
received power:
2:1 in RIN region
1:1 in shot region
0 in thermal region
• Gain maintains 2:1
relationship with optical
drive (assuming linear
receiver)
• Resulting noise figure
is 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
Noise Reduction
• Drive the photoreceiver at high optical levels to maintain RIN limit
– Photoreceiver linearity may be problematic at high optical levels
• Other methods of reducing RIN and shot noise include balanced
detection
Bias
RIN Cancellation
RIN noise from the laser source is completely
cancelled (ideally), as long as optical phase is
temporally coherent.
RF Out
BalancedModulatori.e. MZM
SourceLaserandRIN
Intensity envelopes areout of phase; RIN iscommon-mode
RF Modulation
Result is shot-noise limited performance; increasing
optical power can always improve noise
figure.
Limitations arise due to:
1) The coherence length of the laser
source: the average length over which
the optical signal remains coherent. It
is limited by the laser linewidth.
2) The ability to maintain constant RF
envelope length in dual-fibers over
long distances will limit the frequency
bandwidth of cancellation
Understanding Linearity
• Sources of nonlinearity:
– Modulator
• Even and Odd order distortions caused by nonlinear transfer response
– Fiber medium (some examples:)
• Stimulated Brillouin Scattering (SBS) causes nonlinear noise spectrum and reduction in gain at high optical drive levelsreduction in gain at high optical drive levels
• Four wave mixing causes crosstalk in WDM systems
• Fiber Dispersion limits the length of transmission at higher frequencies
– Linear process, but causes gain nulling when AM sidebands cancel
– Photoreceiver
• Clipping and intermodulation at high optical levels
• For links of several km or less, at optical levels below ~ 10 mW, MODULATOR NONLINEARITY DOMINATES
Linearity Improvement
• Linearization reduces nonlinear distortion; increases dynamic range
– Techniques include optical, electrical, and combinatorial approaches
• All necessarily involve additional processing in the • All necessarily involve additional processing in the form of additional circuitry/components
• Electrical: aim is to cancel distortion products by providing conjugate distortion inputs
– Operates in RF domain
– Predistortion, Feedforward
• Optical: generally more complex
– Operates in optical domain – inherently wide-band
Optical & Electro-Optical Linearization
• Dual Series MZM Modulators
– Proper biasing of series MZMs may result in linearization of sinusoidal transfer
• Has been shown to approximate ideal limiter response
• Inherently wideband• Inherently wideband
• Difficult to tune/align
• Feedforward
– Nonlinear response is sampled (electrically) and re-injected to fundamental path in order to cancel undesired frequency products
• Limited bandwidth due to need for electrical delay in feedforward path
Electrical Predistortion
• Predistortion Linearization has long history in
Broadcast Power Amplifiers; SSPAs, TWTAs,
Space and Ground Station equipment
– Generally less complex than optical or combinatorial
systemssystems
• 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
Electrical Predistortion
• PERFORMANCE OF LINK IS PRIMARILY LIMITED BY THE DISTORTION INTRODUCED BY OPTICAL MODULATION.
• PREDISTORTION (PD) LINEARIZATION ELIMINATES THIS DISTORTION BY GENERATING A FUNCTION WITH OPPOSITE MAGNITUDE AND PHASE OF THE MODULATOR
Wideband Predistorter
• Functions from 1.5 to > 20 GHz
• Target: ∆Gain = 2.5 dB
• ∆Ф < 5 degrees (Ach. to 13 GHz)
• ∆Ф < 5 degrees (Ach. to 13 GHz)
• Flatness ± 0.5 dB (Ach. to 12 GHz)
• Feel can achieve <1GHz to <30GHz
• LPL generic predistorter is very broadband
Linearized Microwave Link
Preamp PredistorterPostamp/
Equalizer
MZM
1550 nm
Source Laser
MPR0020
Photoreceiver
LINEARIZER NONLINEARIZED LINK
RF IN RF OUT
34
• Nonlinearized MZM Link:
– Commercial modulator biased at quadrature
– 20 GHz flat receiver driven at 0 dBmo
• Linearizer:
– Includes broadband gain stages
– Predistorter is single-chip GaAs circuit (proprietary design)
• Signal levels adjusted to match gain expansion of predistorter to gain compression of MZM
– Postamp stage includes slope equalizer to match levels over frequency
LINEARIZER NONLINEARIZED LINK
LINEARIZED LINK
MZM Linearization
• Demonstration of IMD improvement from
predistorting an MZM link
MZM Microwave Link
Linearized IMD Products
120
Non Linearized
Linearized
• Results at 8 GHz
• Measured improvement
>14 dB (minimum)
0
20
40
60
80
100
0.01 0.10 1.00
Optical Modulation Index
Th
ird
-Ord
er
IMD
(d
Bc
)
21 dB IMD improvement
(yields 5 dB SFDR3)
>14 dB (minimum)
from 4 to 12 GHz
Predistortion Linearizer Performance
Pout
• Linearization Results of EAM Link at 14 GHz
• Non-Linearized� 4 dB Gain Compression at
Ref. Input Power
0-12
Input Power Backoff (IPBO) in dB
Gain Pout Gain
Pout
Input Power Backoff (IPBO) in dB
0-20
• Linearized� Predistortion linearizer
effectively compensates the
gain compression
Predistortion Linearizer Performance
• Intermodulation Distortion Improvement EAM– Measured at 6 dB IPBO
Non-Linearized Linearized
• 15 dB improvement in IMD equates to 5 dB
improvement in SFDR3
THIRD ORDER
AM COMPRESSION TERMS
F1 F2
FOR WB OPERATION (> OCTAVE BW) FOR WB OPERATION (> OCTAVE BW) FOR WB OPERATION (> OCTAVE BW) FOR WB OPERATION (> OCTAVE BW) ---- EVEN & ODD EVEN & ODD EVEN & ODD EVEN & ODD ORDER DISTORTION MUST BE CONSIDEREDORDER DISTORTION MUST BE CONSIDEREDORDER DISTORTION MUST BE CONSIDEREDORDER DISTORTION MUST BE CONSIDERED
Multi-Octave Problem
• IM AND HARMONIC DISTORTION A PROBLEMIM AND HARMONIC DISTORTION A PROBLEMIM AND HARMONIC DISTORTION A PROBLEMIM AND HARMONIC DISTORTION A PROBLEM• 2F1, F22F1, F22F1, F22F1, F2----F1, 2F2F1, 2F2F1, 2F2F1, 2F2----F1 AND 2F1F1 AND 2F1F1 AND 2F1F1 AND 2F1----F2 PRODUCTS OF MOST CONCERNF2 PRODUCTS OF MOST CONCERNF2 PRODUCTS OF MOST CONCERNF2 PRODUCTS OF MOST CONCERN• MOST PREDISTORTERS CORRECT ONLY ODD ORDER DISTORTIONMOST PREDISTORTERS CORRECT ONLY ODD ORDER DISTORTIONMOST PREDISTORTERS CORRECT ONLY ODD ORDER DISTORTIONMOST PREDISTORTERS CORRECT ONLY ODD ORDER DISTORTION
F2-F1 2F1-F2 2F2-F1 2F1
F1 F2
FREQUENCY
EMISSION LIMIT
The Multi-Octave Linearization
• MZMs produce minimal 2nd harmonic distortion with bias voltage control
• Predistorters can generate 2nd in addition to 3rd
order nonlinearities
39
order nonlinearities
– 2nd order terms may worsen performance for > octave bandwidth
– Even terms not in the proper phase to cancel
• Developed predistorters that generate only 3rd-order components and operate over multi-octave bandwidth
Multi-Octave Linearizer
• A multi-octave broadband with even order cancelation
- operating from 1 to 20 GHz
- with single linearizer providing both IM and
harmonic distortion correction using push-pull NLG
Balu
n
NLG
NLG
Pre-Distortion Linearizer Circuitry
NLG
NLG
AttenB
alu
n
Hitting the Target
• Starting from COTs-style MZM link with detected power = 0 dBmo and SFDR = 105:
– Increase detected power to RIN limit
• Improve NF by ~ 6 dB
– Predistortion linearization
• Improve IMD by 21 dB (IIP3 by 10.5 dB)• Improve IMD by 21 dB (IIP3 by 10.5 dB)
– Result: SFDR = 105 + 2/3(16.5) = 116
• If system supports RIN cancellation, gain another ~ 10 dB noise figure reduction
– Result: SFDR = 105 + 2/3(26.5) = 122
• 120 dB SFDR target within reach using straight-detect analog links
Optical Linearization Example
• Optical Feedforward Coupled linearization of
Mach-Zehnder modulator
– Third-order cancellationOMI
0.2(OMI)^3
split
Vrf
MZMbiased at Vpi/2
AC coupled
-0.5sin(piOMI/2)
a2
MZMbiased at Vpi/2
a1
0.2(a2)(OMI)^3
opticaloutput
delay
Potential for even greater cancelation (> SFDR)
Description
• First MZM generates distortion products
• Amplitudes of distorted detected outputs are:
• 2-tone 3rd-order amplitudes (IMDs) were found by eval. Fourier Series of the output
• Note that Fundamental and IMD products are always out of phase
⋅−=
2sin
2
1 OMIV fund
π 32.0 OMIVIMD ⋅=
• Note that Fundamental and IMD products are always out of phase
• RF signal is delayed and added to the distorted output• Level is set to “just cancel” the carriers of the detected signal, leaving just the
distortion
• Distortion products are re-modulated, and summed with the first modulator
output.• Summation must be noncoherent
• Dual lasers or sufficient delay
Modeled results with A2 = 4/π
7.9588
IMb OMI( )
IMD 1 OMI,( )
IMD 0.6 OMI,( )
80
60
40
20
0
Non-Linearized, Single MZM
A1=1
A1=0.6
Two-Tone
Third-order IMD
100
10.1 OMI
0 0.2 0.4 0.6 0.8 1100
0.554957
0.047167
0.5 sinπ OMI.
2
.
a 1 OMI, 5,( )
a 0.6 OMI, 5,( )
10.1 OMI
0 0.2 0.4 0.6 0.8 10
0.1
0.2
0.3
0.4
0.5
0.6
Two-Tone
Fundamental Transfer Function
Note: OMI is per-carrier
Non-Linearized, Single MZM
A1=1
A1=0.6
Conclusion
The key components and system concepts associated with linear photonic links have been introduced
And some of the latest advances in linear photonics presented:
• High performance Direct Modulation links have been produced for Ku band and beyond
• The ability of LPL’s predistortion linearization technology to bring link IMD level ~20 dB lower than presently available systems and increase SFDR 6 level ~20 dB lower than presently available systems and increase SFDR 6 to 8 dB and possibly more
• Linearized photonic links that function over multi-octave bandwidths has been demonstrated
• The feasibility of producing octave bandwidth electrically linearized fiber optic transmitters with overall SFDR > 120 dB⋅Hz2/3 has been shown.
These results show the technology capability to develop high dynamic range optical links for: Avionics, EW, ECCM, Radar, Antenna remoting, Phased arrays, and Point-to-point commercial and government communications systems.