MICROWAVE OVER FIBERlinphotonics.com/website/wp-content/uploads/2013/09/Rochester-IEEE-JCM.pdf · MICROWAVE OVER FIBER Applications and Performance March 31, 2010 Rochester IEEE JCM

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

[email protected]

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

2Rochester IEEE JCM 2010

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


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


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


Intensity Modulation


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 =


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.


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 +∝


OUTLINE






– PHOTORECEIVERS



– LINEAR EFFECTS






Practical Intensity Modulation

• Direct

– Laser diode is modulated directly

• External

– Laser source drives a separate optical – Laser source drives a separate optical

component


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


• 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


• 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


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)


• 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

16

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


• 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


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


Singlemode

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


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

==

==

OUTLINE






– PHOTORECEIVERS



– LINEAR EFFECTS






• 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


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


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


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


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


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


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

26

Chromatic Dispersion

Polarization Mode Dispersion

FAST AXIS

SLOW AXIS


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


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


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.


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


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


OUTLINE






– PHOTORECEIVERS



– LINEAR EFFECTS






• 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


• 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


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



-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


• 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



-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


• 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

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?)


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


OUTLINE






– PHOTORECEIVERS



– LINEAR EFFECTS






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


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



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)


• 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


• 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


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


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


• 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

Documents

MICROWAVE OVER FIBERlinphotonics.com/website/wp-content/uploads/2013/09/Rochester-IEEE-JCM.pdf · MICROWAVE OVER FIBER Applications and Performance March 31, 2010 Rochester IEEE JCM