Self-Mixing Differential
Laser Vibrometer
Michele Norgia
Dipartimento di Elettronica e Informazione, Politecnico di Milano, Italy
Guido Giuliani, Silvano Donati
Optoelectronics Group - Dipartimento di Elettronica, Università di Pavia, Italy
[email protected]
Optoelectronics Group
Dipartimento di Elettronica
Università di Pavia
Outline
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Conventional Laser Doppler Vibrometry (LDV)
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Self-mixing interferometry
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high dynamic range vibrometer
high-sensitivity vibrometer
high-sensitivity differential vibrometer
Conclusions
Optoelectronics Group
Dipartimento di Elettronica
Università di Pavia
Laser Vibrometry
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It is a well-established technique that allows contacless
measurement of the vibration of a remote non-cooperative
target (rough surface)
Conventional scheme: LDV (Laser Doppler Velocimetry)
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a Michelson interferometer with velocity read-out
Commercial instruments performance
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Velocity:
Frequencies:
dynamic range:
few µm/s to 1000 mm/s
from 0.01 Hz to few MHz
100 dB
Optoelectronics Group
Dipartimento di Elettronica
Università di Pavia
LDV - Scheme
CC
λ/4
λ/4
Target
PBS
He-Ne laser
BS
PD2b
Beam Expander
PBS
λ/4
DET. 1
PD1a
PD2a
PBS
PD1b
„
DET. 2
Large number of components:
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lenses, polar./non-polar. beamplitters, waveplates, PDs
some LDVs also use an acousto-optical modulator
Optoelectronics Group
Dipartimento di Elettronica
Università di Pavia
Self-mixing interferometry - 1
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Conventional techniques are based on external interferometers
Interferometer: passive optical system, read by laser light
TARGET
LASER
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Self-mixing
The laser diode source is part of the interferometer
Reference path and beamsplitter are removed
TARGET
E0
LD
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ER
Light “samples” the target and is back-injected into the LD cavity
A mixing with lasing light occurs Î an interferometric signal is
superimposed to the power emitted by the LD
Optoelectronics Group
Dipartimento di Elettronica
Università di Pavia
Self-mixing interferometry - 2
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Extremely simple and compact optical set-up
s
LD
MONITOR PD
Interferometric waveform depends on backscatter strength
target
displacement
low
backscatter
interferometric
signals
moderate
backscatter
[20mV/div] [10mV/div] [1.2µm/div]
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LENS
DIFFUSIVE
TARGET
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Moderate backscatter: triangularshaped interferometric signal with
hysteresis
Optoelectronics Group has pioneered
interferometric applications of selfmixing
S.DONATI, G.GIULIANI, S.MERLO, IEEE J. QUANTUM ELECTRON.,
1995
Time [2 ms/div]
Optoelectronics Group
Dipartimento di Elettronica
Università di Pavia
High dynamic range vibrometer - 1
„
Self-mixing interferometer with triangular-shaped signal
LD
MONITOR PD
LENS
DIFFUSIVE
TARGET
Target
Targetdisplacement
displacement
Self-Mixing Signal
Time [2ms/div]
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Principle:
fringe counting
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Output signal:
displacement (not velocity…)
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Resolution:
λ/2
Optoelectronics Group
Dipartimento di Elettronica
Università di Pavia
[20mV/div] [1.2µm/div]
s
High dynamic range vibrometer - 2
Down
Trans-Z
Amplifier
Derivative
Up-Down
Counter
Polarity
Discrimination
Up
SELF-MIXING
SIGNAL
300 ns
DERIVATIVE
100 µs/div
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Resolution:
Max. Target speed:
Operating distance:
0.2 µs/div
400 nm (or 40 nm with averaging)
up to 1 m/s
0.1 ÷ 2 m
Optoelectronics Group
Dipartimento di Elettronica
Università di Pavia
High dynamic range vibrometer - 3
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Max. Vibr. ampl.:
26 mm p-p
Max. velocity:
0.15 m/s
Operation on rough surfaces
Working distance:
6-60 cm
Vout:
1 mV/ µm
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Ultra-compact optical head:
12 mm dia. - 60 mm length
Optoelectronics Group
Dipartimento di Elettronica
Università di Pavia
High-sensitivity vibrometer - 1
Self-mixing interferometer with triangular-shaped signal
s
LD
MONITOR PD
LENS
„
Target
Targetdisplacement
displacement
DIFFUSIVE
TARGET
Self-Mixing Signal
Time [2ms/div]
Locking to half-fringe
λ/2
S-M
SIGNAL
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t
Φ = 2ks
Goal: improve the λ/2
resolution to measure
sub-nm vibrations
t
Optoelectronics Group
Dipartimento di Elettronica
Università di Pavia
[20mV/div] [1.2µm/div]
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High-sensitivity vibrometer - 2
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Implementation: electronic feedback loop
ELECTRONIC
FEEDBACK LOOP
TARGET
DISPLACEMENT
MONITOR
PD
DIODE
LASER
VOLTAGE-CONTROLLED
CURRENT SOURCE
TRANS-Z
AMPLIFIER
LP
FILTER
VIBROMETER
OUTPUT SIGNAL
A
-
Optoelectronics Group
Dipartimento di Elettronica
Università di Pavia
High-sensitivity vibrometer - 3
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Sensitivity:
100 pm/√Hz
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Max. Vibration ampl.:
100 µm p-p
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Small-signal bandwidth:
70 kHz
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More than 100 dB dynamic range
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Operation on all rough surfaces
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Vout:
10 mV/ µm
LOUDSPEAKER TESTS
20 V
20 V
4 µm
10 µm
Optoelectronics Group
Dipartimento di Elettronica
Università di Pavia
High-sensitivity vibrometer - 4
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Application examples (FFT spectra)
Car body with the engine rotating at 2100 rpm
100 µm
A
B
C
10 µm
10 µm
RMS Displacement
RMS Displacement
100 µm
1 µm
100 nm
10 nm
1 nm
100 pm0
AC/AC 50 Hz Transformer; 3 A load
A = 13 Hz (suspension)
B = 35 Hz (engine 1st harmonic)
C = 70 Hz (engine 2nd harmonic)
20
40
60
80 100 120 140 160 180 200
1 µm
100 nm
10 nm
1 nm
100 pm
0
Frequency [Hz] Res BW = 0.49875 Hz
Optoelectronics Group
50
100
Frequency [Hz]
Dipartimento di Elettronica
Università di Pavia
150
High-sensitivity differential vibrometer - 1
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Motivation: measurement of micro- and gross-slip regimes
of two metal samples put into contact
Goal: develop an accurate model of the contact
hysteresis (relative displacement vs. tangential force)
Application: friction damping of vibrations of gas turbine
blades
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Collaboration: Dip. di Meccanica - Politecnico di Torino
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Approach:
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TWO high-sensitivity Self-Mixing vibrometers
+
High-accuracy electronic subtraction
Optoelectronics Group
Dipartimento di Elettronica
Università di Pavia
High-sensitivity differential vibrometer - 2
Normal Force
TARGET #2
(follower)
Tangential Force
Actuating Force
(sinusoidal)
LASER #2
d
LASER #1
TARGET #1
(master)
s
SPECIFICATIONS
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Optoelectronics Group
Differential noise equivalent
displacement:
20 nm RMS (with B = 10 kHz)
Common-mode
displacement:
up to 60 µm peak-to-peak
Small-signal bandwidth:
20 kHz
Dipartimento di Elettronica
Università di Pavia
High-sensitivity differential vibrometer - 3
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Noise Equivalent Differential Displacement:
100 pm/√Hz
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Max. Vibration (per channel):
100 µm p-p
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Small-signal bandwidth:
70 kHz
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Vout:
10 mV/ µm
Optoelectronics Group
Dipartimento di Elettronica
Università di Pavia
High-sensitivity differential vibrometer - 4
single-cycle
hysteresis
Measurement of
different regimes:
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micro-slip
gross-slip
Optoelectronics Group
averaged
Dipartimento di Elettronica
hysteresis cycles
Università di Pavia
Conclusion
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Three new types of laser diode vibrometers based on
self-mixing configuration have been demonstrated:
c high dynamic range - standard resolution
d high-sensitivity - high resolution
e high-sensitivity - high resolution - differential
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Reduced part-count and cost
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Many applications can be devised
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Standard and custom instruments will be supplied by a
Spin-off Company
contact:
[email protected]
Optoelectronics Group
Dipartimento di Elettronica
Università di Pavia