Self-Mixing Differential Laser Vibrometer - AIVELA · Self-Mixing Differential Laser Vibrometer...

Dipartimento di ElettronicaUniversità di PaviaOptoelectronics Group

SelfSelf--Mixing DifferentialMixing DifferentialLaser VibrometerLaser Vibrometer

Michele NorgiaMichele NorgiaDipartimento di Elettronica e Informazione, Politecnico di MilanDipartimento di Elettronica e Informazione, Politecnico di Milano, Italy o, Italy

Guido GiulianiGuido Giuliani, Silvano Donati, Silvano DonatiOptoelectronics Group Optoelectronics Group -- Dipartimento di Elettronica, UniversitDipartimento di Elettronica, Universitàà di Pavia, Italydi Pavia, Italy

[email protected]@unipv.it


OutlineOutline

Conventional Laser Doppler Vibrometry (LDV)Conventional Laser Doppler Vibrometry (LDV)

SelfSelf--mixing interferometrymixing interferometryhigh dynamic rangehigh dynamic range vibrometervibrometerhighhigh--sensitivitysensitivity vibrometervibrometerhighhigh--sensitivitysensitivity differentialdifferential vibrometervibrometer

ConclusionsConclusions


Laser VibrometryLaser Vibrometry

It is a wellIt is a well--established technique that allows contacless established technique that allows contacless measurement of the vibration of a remote nonmeasurement of the vibration of a remote non--cooperative cooperative target (rough surface)target (rough surface)

Conventional scheme: LDV (Laser Doppler Velocimetry)Conventional scheme: LDV (Laser Doppler Velocimetry)

a Michelson interferometer with velocity reada Michelson interferometer with velocity read--outoutCommercial instruments performanceCommercial instruments performance

Velocity:Velocity: few few µµm/s to 1000 mm/sm/s to 1000 mm/sFrequencies:Frequencies: from 0.01 Hz to few MHzfrom 0.01 Hz to few MHzdynamic range:dynamic range: 100 dB100 dB


LDV LDV -- SchemeScheme

Large numberLarge number of components:of components:lenses, polar./nonlenses, polar./non--polar. beamplitters, waveplates, PDspolar. beamplitters, waveplates, PDssome LDVs also use an acoustosome LDVs also use an acousto--optical modulatoroptical modulator

He-Ne laser

BS

PBS

λ/4

CC

PBS

PBS

λ/4

λ/4

Beam Expander

Target

PD1a

PD1b

PD2b

PD2aDET. 1

DET. 2


SelfSelf--mixing interferometry mixing interferometry -- 11

ConventionalConventional techniques are based on techniques are based on externalexternal interferometersinterferometersInterferometer: passive optical system, read by laser lightInterferometer: passive optical system, read by laser light

LASERTARGET

SelfSelf--mixingmixingThe laser diode source is The laser diode source is partpart of the interferometerof the interferometerReference path and beamsplitter are removedReference path and beamsplitter are removed

LD

TARGET

E0

ER

Light Light ““samplessamples”” the target and is backthe target and is back--injected into the LD cavityinjected into the LD cavityA mixing with lasing light occurs A mixing with lasing light occurs an an interferometric signalinterferometric signal is is superimposed to the powersuperimposed to the power emitted by the LDemitted by the LD


SelfSelf--mixing interferometry mixing interferometry -- 22

Extremely Extremely simplesimple and and compact compact optical setoptical set--upup

Interferometric waveform depends on Interferometric waveform depends on backscatter strengthbackscatter strength

s

LDMONITOR PD DIFFUSIVE

TARGETLENS

[20m

V/d

iv]

[10m

V/d

iv]

Time [2 ms/div]

[1.2µm

/div

]targetdisplacement

interferometricsignals

lowbackscatter

moderatebackscatter

Optoelectronics Group has pioneered Optoelectronics Group has pioneered interferometric applications of selfinterferometric applications of self--mixing mixing S.DONATI, G.GIULIANI, S.MERLO, IEEE J. QUANTUM ELECTRON., S.DONATI, G.GIULIANI, S.MERLO, IEEE J. QUANTUM ELECTRON., 19951995

Moderate backscatter: Moderate backscatter: triangulartriangular--shapedshaped interferometric signal with interferometric signal with hysteresis hysteresis


High dynamic range vibrometer High dynamic range vibrometer -- 11

SelfSelf--mixing interferometer with triangularmixing interferometer with triangular--shaped signalshaped signal

Principle:Principle: fringe countingfringe counting

Output signal:Output signal: displacementdisplacement (not velocity(not velocity……))

Resolution:Resolution: λλ/2/2

s


TARGETLENSTarget displacement

[20m

V/di

v][1

.2µm

/div

]

Self-Mixing SignalTime [2ms/div]

Target displacement



Resolution:Resolution: 400 nm (or 40 nm with averaging)400 nm (or 40 nm with averaging)Max. Target speed:Max. Target speed: up to 1 m/sup to 1 m/sOperating distance:Operating distance: 0.1 0.1 ÷÷ 2 m2 m

Trans-ZAmplifier

Up

Derivative PolarityDiscrimination

Up-DownCounter

Down

SELF-MIXINGSIGNAL

DERIVATIVE

100 µs/div 0.2 µs/div

300 ns



UltraUltra--compact optical head:compact optical head: 12 mm dia. 12 mm dia. -- 60 mm length60 mm length

Max. Vibr. ampl.:Max. Vibr. ampl.: 26 mm 26 mm pp--ppMax. velocity:Max. velocity: 0.15 m/s0.15 m/sOperation on rough surfacesOperation on rough surfacesWorking distance:Working distance: 66--60 cm60 cmVVoutout:: 1 mV/ 1 mV/ µµm m


HighHigh--sensitivity vibrometer sensitivity vibrometer -- 11

SelfSelf--mixing interferometer with triangularmixing interferometer with triangular--shaped signalshaped signal

Locking to halfLocking to half--fringefringe

s


TARGETLENS

Target displacement

[20m

V/di

v][1

.2µm

/div

]

Self-Mixing SignalTime [2ms/div]

Target displacement

S-MSIGNAL

Φ = 2ks

t

t

λ/2

Goal: improve the Goal: improve the λλ/2 /2 resolution resolution to measure to measure subsub--nm vibrationsnm vibrations



Implementation: electronic feedback loopImplementation: electronic feedback loop

DIODELASER

-A

LPFILTER

ELECTRONICFEEDBACK LOOP

VOLTAGE-CONTROLLEDCURRENT SOURCETRANS-Z

AMPLIFIER

MONITORPD

TARGETDISPLACEMENT

VIBROMETER OUTPUT SIGNAL



Sensitivity:Sensitivity: 100 pm/100 pm/√√Hz Hz

Max. Vibration ampl.:Max. Vibration ampl.: 100 100 µµm pm p--pp

SmallSmall--signal bandwidth:signal bandwidth: 70 kHz70 kHz

More than 100 dB dynamic rangeMore than 100 dB dynamic range

Operation on all rough surfacesOperation on all rough surfaces

VVoutout:: 10 mV/ 10 mV/ µµm m

20 V

10 µm

LOUDSPEAKER TESTS

20 V

4 µm



Application examples (FFT spectra)Application examples (FFT spectra)

0 20 40 60 80 100 120 140 160 180 200100 pm

1 nm

10 nm

100 nm

1 µm

10 µm

100 µm

Frequency [Hz] Res BW = 0.49875 Hz

RMS

Disp

lace

men

t

Car body with the engine rotating at 2100 rpm

A B C

A = 13 Hz (suspension)B = 35 Hz (engine 1st harmonic)C = 70 Hz (engine 2nd harmonic)

0 50 100 150100 pm

1 nm

10 nm

100 nm

1 µm

10 µm

100 µm

Frequency [Hz]R

MS

Dis

plac

emen

t

AC/AC 50 Hz Transformer; 3 A load


HighHigh--sensitivity differential vibrometer sensitivity differential vibrometer -- 11

MotivationMotivation: measurement of micro: measurement of micro-- and grossand gross--slip slip regimes regimes of two metal samples put into contactof two metal samples put into contact

GoalGoal: : develop an accurate model of the contact develop an accurate model of the contact hysteresis hysteresis ((relative displacementrelative displacement vs. vs. tangential forcetangential force))

ApplicationApplication: : friction damping of vibrations friction damping of vibrations of of gas turbine gas turbine bladesblades

CollaborationCollaboration: Dip. di Meccanica : Dip. di Meccanica -- Politecnico di TorinoPolitecnico di Torino

ApproachApproach: : TWO highTWO high--sensitivity Selfsensitivity Self--Mixing vibrometersMixing vibrometers

++HighHigh--accuracy electronic subtractionaccuracy electronic subtraction



s

LASER #1

LASER #2

d

TARGET #2(follower)

TARGET #1(master)

Normal Force

Actuating Force(sinusoidal)

Tangential Force

s

LASER #1

LASER #2

d

TARGET #2(follower)

TARGET #1(master)

Normal Force

Actuating Force(sinusoidal)

Tangential Force

SPECIFICATIONSSPECIFICATIONS

Differential noise equivalent Differential noise equivalent displacement:displacement:20 nm RMS (with B = 10 kHz)20 nm RMS (with B = 10 kHz)

CommonCommon--mode mode displacement:displacement:up to 60 up to 60 µµm peakm peak--toto--peakpeak

SmallSmall--signal bandwidth:signal bandwidth:20 kHz20 kHz



Noise Equivalent Differential Displacement:Noise Equivalent Differential Displacement: 100 pm/100 pm/√√Hz Hz

Max. Vibration (per channel):Max. Vibration (per channel): 100 100 µµm pm p--pp

SmallSmall--signal bandwidth:signal bandwidth: 70 kHz70 kHz

VVoutout:: 10 mV/ 10 mV/ µµm m



single-cyclehysteresis

averagedhysteresis cycles

Measurement of Measurement of different regimes:different regimes:

micromicro--slipslipgrossgross--slipslip


ConclusionConclusion

Three new typesThree new types of laser diode vibrometers based on of laser diode vibrometers based on selfself--mixingmixing configuration have been demonstrated:configuration have been demonstrated:

high dynamic range high dynamic range -- standard resolutionstandard resolutionhighhigh--sensitivity sensitivity -- high resolutionhigh resolutionhighhigh--sensitivity sensitivity -- high resolution high resolution -- differentialdifferential

Reduced partReduced part--count and costcount and cost

Many applications can be devisedMany applications can be devised

Standard and custom instruments will be supplied by a Standard and custom instruments will be supplied by a SpinSpin--off Companyoff Company

contact: contact: [email protected]@unipv.it

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