Self-Mixing Differential Laser Vibrometer

Self-Mixing Differential Laser Vibrometer Michele Norgia e Informazione, Politecnico di Milano, Italy Guido Giuliani,, Silvano Donati -,, Italy guido.giuliani@unipv.it

Outline Conventional Laser Doppler Vibrometry (LDV) Self-mixing interferometry high dynamic range vibrometer high-sensitivity vibrometer high-sensitivity differential vibrometer Conclusions

Laser Vibrometry It is a well-established established technique that allows contacless measurement of the vibration of a remote non-cooperative target (rough surface) Conventional scheme: LDV (Laser Doppler Velocimetry) a Michelson interferometer with velocity read-out Commercial instruments performance Velocity: Frequencies: dynamic range: few µm/s to 1000 mm/s from 0.01 Hz to few MHz 100 db

LDV - Scheme CC λ/4 λ/4 Target PBS He-Ne laser BS PD2b Beam Expander DET. 1 PBS PD2a λ/4 PD1a PBS Large number of components: PD1b DET. 2 lenses, polar./non-polar. beamplitters, waveplates, PDs some LDVs also use an acousto-optical optical modulator

Self-mixing interferometry - 1 Conventional techniques are based on external interferometers Interferometer: passive optical system, read by laser light LASER TARGET Self-mixing The laser diode source is part of the interferometer Reference path and beamsplitter are removed TARGET E 0 LD E R Light samples the target and is back-injected into the LD cavity A mixing with lasing light occurs an interferometric signal is superimposed to the power emitted by the LD

Self-mixing interferometry - 2 low backscatter moderate backscatter Extremely simple and compact optical set-up MONITOR PD LD LENS Interferometric waveform depends on backscatter strength target displacement interferometric signals Time [2 ms/div] [1.2µm/div] [20mV/div] [10mV/div] s DIFFUSIVE TARGET has pioneered interferometric applications of self- mixing Moderate backscatter: triangular- shaped interferometric signal with hysteresis S.DONATI, G.GIULIANI, S.MERLO, IEEE J. QUANTUM ELECTRON., 1995

High dynamic range vibrometer - 1 Self-mixing interferometer with triangular-shaped signal s MONITOR PD Principle: LD LENS DIFFUSIVE TARGET fringe counting Target displacement Target displacement Self-Mixing Signal Time [2ms/div] [20mV/div] [1.2µm/div] Output signal: displacement (not velocity ) Resolution: λ/2

High dynamic range vibrometer - 2 Down Trans-Z Amplifier Derivative Polarity Discrimination Up-Down Counter Up SELF-MIXING SIGNAL DERIVATIVE 300 ns 100 µs/div 0.2 µs/div Resolution: 400 nm (or 40 nm with averaging) Max. Target speed: up to 1 m/s Operating distance: 0.1 2 m

High dynamic range vibrometer - 3 Max. Vibr. ampl.: Max. velocity: Operation on rough surfaces Working distance: V out : 26 mm p-p 0.15 m/s 6-6060 cm 1 mv/ µm Ultra-compact optical head: 12 mm dia. - 60 mm length

High-sensitivity vibrometer - 1 Self-mixing interferometer with triangular-shaped signal MONITOR PD LD LENS Locking to half-fringe fringe S-M SIGNAL λ/2 s Φ = 2ks DIFFUSIVE TARGET t Target displacement Target displacement Self-Mixing Signal Time [2ms/div] Goal: improve the λ/2 resolution to measure sub-nm vibrations [20mV/div] [1.2µm/div] t

High-sensitivity vibrometer - 2 Implementation: electronic feedback loop ELECTRONIC FEEDBACK LOOP MONITOR PD TARGET DISPLACEMENT DIODE LASER TRANS-Z AMPLIFIER - VOLTAGE-CONTROLLED CURRENT SOURCE LP FILTER A VIBROMETER OUTPUT SIGNAL

High-sensitivity vibrometer - 3 Sensitivity: 100 pm/ Hz Max. Vibration ampl.: 100 µm m p-pp Small-signal bandwidth: 70 khz More than 100 db dynamic range Operation on all rough surfaces V out : 10 mv/ µm LOUDSPEAKER TESTS 20 V 20 V 4 µm 10 µm

High-sensitivity vibrometer - 4 RMS Displacement 100 µm 10 µm 1 µm 100 nm 10 nm Application examples (FFT spectra) Car body with the engine rotating at 2100 rpm A B C 1nm A = 13 Hz (suspension) B = 35 Hz (engine 1st harmonic) C = 70 Hz (engine 2nd harmonic) 100 pm 0 20 40 60 80 100 120 140 160 180 200 Frequency [Hz] Res BW = 0.49875 Hz RMS Displacement 100 µm 10 µm 1 µm 100 nm 10 nm 1 nm AC/AC 50 Hz Transformer; 3 A load 100 pm 0 50 100 150 Frequency [Hz]

High-sensitivity differential vibrometer - 1 Motivation: : measurement of micro- and gross-slip 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 Collaboration: : Dip. di Meccanica - Politecnico di Torino Approach: TWO high-sensitivity Self-Mixing vibrometers + High-accuracy electronic subtraction

High-sensitivity differential vibrometer - 2 Normal Force TARGET #2 (follower) LASER #2 Tangential Force d Actuating Force (sinusoidal) TARGET #1 (master) s LASER #1 SPECIFICATIONS Differential noise equivalent displacement: 20 nm RMS (with B = 10 khz) Common-mode mode displacement: up to 60 µm m peak-to to-peak Small-signal bandwidth: 20 khz

High-sensitivity differential vibrometer - 3 Noise Equivalent Differential Displacement: 100 pm/ Hz Max. Vibration (per channel): 100 µm m p-pp Small-signal bandwidth: V out : 70 khz 10 mv/ µm

High-sensitivity differential vibrometer - 4 single-cycle hysteresis Measurement of different regimes: micro-slip gross-slip slip averaged hysteresis cycles

Conclusion Three new types of laser diode vibrometers based on self-mixing configuration have been demonstrated: high dynamic range - standard resolution high-sensitivity - high resolution high-sensitivity - high resolution - differential Reduced part-count and cost Many applications can be devised Standard and custom instruments will be supplied by a Spin-off Company contact: guido.giuliani@unipv.it