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


MPR0020 Receiver


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


Ga

in (

dB

)

Noise Figure of F/O Link

0

10

20

30

40

50

60

70

-15 -10 -5 0 5 10 15


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.

linear microwave fiber optic link system...

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