Self-Mixing Laser Diode Vibrometer with Wide Dynamic Range

Transcription

1 Self-Mixing Laser Diode Vibrometer with Wide Dynamic Range G. Giuliani,, S. Donati, L. Monti -, Italy

2 Outline Conventional Laser vibrometry (LDV) Self-mixing interferometry Self-mixing vibrometer Principle: locking to half-fringe fringe + active phase-nulling Prototype instrument Performance Conclusion

3 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) Basically a Michelson interferometer with velocity read- out rather than position Commercial instruments performance few µm/s to 1000 mm/s frequencies from 0.01 Hz to few MHz a single instrument hardly reaches 100 db dynamics

4 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

5 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

6 Self-mixing interferometry - 2 Extremely simple optical set-up s low backscatter moderate backscatter MONITOR PD LD LENS DIFFUSIVE TARGET Interferometric waveform depends on backscatter strength target displacement interferometric signals Time [2 ms/div] [1.2µm/div] [20mV/div] [10mV/div] 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

7 Self-mixing vibrometer: principle - 1 Self-mixing interferometer with triangular-shaped signal MONITOR PD Locking to half-fringe fringe S-M SIGNAL LD LENS λ/2 s Φ = 2ks DIFFUSIVE TARGET t Target displacement Target displacement Self-Mixing Signal Time [2ms/div] Compensation of slow environmental phase variations Vibration: s < λ/2 p-p [20mV/div] [1.2µm/div] t

8 Self-mixing vibrometer: principle - 2 Methods of phase compensation: LD on a PZT λ by I λ by T S-M SIGNAL Dynamic range can be expanded by phase-nulling technique active compensation of phase variations caused by target displacement φ = 2 (2π/λ) s2 2 (2π/λ 2 ) s λ = 0 t λ/2 Φ = 2ks t λ = (λ/s)( /s) s

9 Block scheme Servo- Feedback Loop s P SM P 0 + P I Self-Mixing Waveform φ P/I characteristic Interferometer λ LD Trans-Z Amplifier A LP Filter Voltage-Controlled Current Source V REF VIBRATION OUT Compensation Loop DC Offset + + -

10 Instrument- 1 L1 L2 TARGET Optical head 800 nm single-mode F-P F P LD collimating lens + focussing lens trans-z Z amplifier Electronic unit power supply LD driver feedback loop LASER DIODE OPERATING DISTANCE: 50 ± 8 cm

11 Instrument- 2 Experimental results Measured vibration waveforms (loudspeaker) BLUE: DRIVE WAVEFORM RED: VIBROMETER OUTPUT [10 mv/µm] 30 Hz SINE 3 Hz TRIANGULAR WAVE 10 Hz SQUARE WAVE 20 V 20 V 20 V 4 µm 4 µm 10 µm Vibration amplitude >> λ/2

12 Instrument- 3 Experimental results SMALL-SIGNAL SIGNAL FREQUENCY RESPONSE LINEARITY Response [db] Frequency [Hz] Vibration Signal [mv] Noise Level Target Vibration [nm]

13 Instrument- 4 Performance Normogram:

14 Instrument- 5 Performance Noise equiv. vibration: 100 pm/ Hz (shot-noise limited) Max. Vibration amplitude: 600 µm m p-p p (limited by electronic compensation circuit bandwidth ) Small-signal bandwidth: 70 khz More than 100 db dynamic range Operation on all rough surfaces

15 Instrument- 6 Performance Comparison with commercial LDVs and sensors

16 Instrument- 7 Speckle effects Efffects of transversal target translation (without speckle-tracking) Target: white paper Vibration amplitude: 100 µm m p-p p 100 Hz LD L1 L2 TARGET VIBRATION TRANSLATION Backscattered Light Intensity [a.u.] OPTICAL HEAD 1 Successful Measurement ( 87 % ) Target Transversal Translation [mm] 1 Successful Measurement ( 87 % ) Target Transversal Translation [mm]

17 RMS Displacement 100 µm 10 µm 1 µm 100 nm 10 nm Instrument- 8 Use Application examples (FFT spectra) Car body with the engine rotating at 2100 rpm A B C 1nm A = 13 Hz (suspension) B = 35 Hz (motor 1st harmonic) C = 70 Hz (motor 2nd harmonic) 100 pm Frequency [Hz] Res BW = 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 Frequency [Hz]

18 Applications Replacement of piezo-electric electric accelerometers Monitoring of soft, lightweigth structures Modal analysis Loudspeaker and PZT characterization MEMS testing

19 Conclusion A new type of laser vibrometer based on self- mixing effect in LD and phase-nulling technique has been demonstrated Wide dynamic range (>( 100 db) Sub-nm noise-equivalent equivalent-vibrationvibration Reduced part-count and cost the welcomes proposals for production and marketing of the instrument!!!

20 the welcomes proposals for production and marketing of the instrument!!!