Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Laser Ultrasonic Receiver . TEMPO Serie Performance Update - Nov 2009 - Bossa Nova Technologies 606 Venice Blvd. Suite B Venice, CA 90291 USA Tel: (310) 577 8113 Fax: (310) 943-3280 www.bossanovatech.com [email protected] © 2006 Bossa Nova Technologies, LLC. All rights reserved. Table of contents 1. TEMPO Update - Summary 2. New Features - Description 3 4 2.1. Calibrated Output 2.2. Background vibration Compensation 2.3. Laser Intensity Noise Rejection 2.4. Simultaneous In-plane & Out-of-plane detections 2.4.1. Principle of operation 2.4.2. Figures of Merit 4 5 7 10 10 12 3. APPLICATIONS: Example of Results 13 3.1. 3.2. 3.3. 13 16 17 Thin sample: Pulse detection using TEMPO-1GHz Thin sample: Lamb mode resonance measurement with TEMPO-2D Thermoelastic generation in Thick Plate: Pulse detection with TEMPO-2D 2 1. TEMPO Update - Summary Over the past year, TEMPO has been significantly redesigned, integrating a number of improvements, including: • Motorized focusing (25 mm) with Manual or computer (USB connection) control. Computer controlled auto-focus can be implemented via the USB port. • Very versatile optical platform, allowing TEMPO to easily adapt to different performance requirement: • Small detection spot = ∅10m Standard detection spot = ∅75m • High Frequency Detection: Up to 1GHz • Simultaneous detection of the Out-of-plane and In-plane (horizontal) displacement: See TEMPO-2D • Rejection of the Laser Intensity Noise. Laser Intensity Noise is rejected in order to achieve shot-noise limited detection below 10MHz, • CALIBRATED Output. The CALIBRATED Output delivers a signal proportional to the surface displacement, and it is automatically normalized to 100mV for 1nm displacement. The CALIBRATED Output operates up to 90MHz. • Improved compensation of low frequency vibrations. The New compensation loop uses an internal automatic signal normalization scheme, which allows to optimize the compensation loop independently of the amount of light collected (signal strength). This leads to fast response with weak or strong signals and it avoids compensation loop instability (possible oscillation) that could be caused by very strong signal. A) B) Figure 1: View of the TEMPO electronic back panel: A) TEMPO-1GHz and B) TEMPO-2D 3 2. New Features - Description 2.1. Calibrated Output TEMPO allows for absolute amplitude measurement. A calibrated 1nm-amplitude modulation is induced in the path of the internal reference beam. By monitoring the strength of the interference signal corresponding to this internal 1nm-displacement vibration, we can directly convert the output voltage of the TEMPO signal to nanometer displacement value. For convenience, in order not to interfere with the measurement, the frequency of the internal reference vibration is set outside of the TEMPO detection bandwidth. Two modes of operations are possible: 1) With CALIBRATION set in "Auto mode", the Calibrated output signal is automatically set for a conversion of 100mVnm. 2) With CALIBRATION set in "Free mode", the Signal is not normalized and the output amplitude varies according to the amount of collected light. The conversion coefficient (to convert mV to nanometers) is continuously displayed and it is also available as a DC voltage. The DC voltage corresponds to a 1nm displacement. During scanning, the amount of collected light will vary from point to point. With calibration in "Auto mode", the CALIBRATED output of TEMPO is always normalized in order to maintain 100mVnm conversion. Automatic normalization is done with a feedback loop where an Automatic Gain Control (AGC) amplifier is controlled by the amplitude of the internal calibration signal. The AGC amplifier limits the CALIBRATED output to frequency below 90MHz. For measurement above 90MHz, absolute calibration is achieved in "Free mode" by recording the DC calibration value and by scaling (dividing) the HF signal after digital acquisition by the DC calibration value. Figure 2 illustrates the calibration features. Figure 2 shows result for C-scan done by scanning a thin metal plate glued on a piezo electric transducer. The scan surface is 0.4” x 0.4”. These are only out-of-plane measurements. The C-Scan is a 2D representation of the out-ofplane displacement at a given time. The large point in the center corresponds to the place where the piezo is glued. The other peaks are different modes propagating at the surface of the plate. Figure 2 shows a comparison among: (a) HF output signal not normalized, (b) HF output signal (a) normalized by dividing by the DC calibration value ("Free mode") and (c) the CALIBRATED Output signal ("Auto mode"). We see than the two normalized C-scans (b) & (c) are similar. The two normalized scans (b) & (c) are calibrated at 100mVnm. 4 (a) (b) (c) Figure 2: C-Scans of a thin metal plate glued on a piezo electric transducer vibrating. (a) Output without normalization, (b) Output (a) with a post processing normalization (scaling by the DC calibration value) and (c) Output with the automatic normalization provided by the TEMPO. 2.2. Background vibration Compensation TEMPO takes advantage of the high-efficiency and high-sensitivity of two-wave mixing in a Bi12SiO20 (BSO) photorefractive crystal. The corresponding response time for the photorefractive (PR) effect to record the dynamic hologram (to adapt to the change in the backscatter light) is around 2ms. This response time is given by the nature of the photorefractive material and the experimental setup (interference pitch, applied High-Voltage and amount of light intensity). A plot of the detector output at constant displacement versus the frequency is given in Figure 3. At high frequencies (above a cutoff frequency fo) the photodetector signal is linearly proportional to the displacement. At low frequencies (below fo) the photodetector signal is proportional to frequency, or equivalently to velocity. The decrease in the signal below fo provides the well-known insensitivity of the signal to low-frequency noise sources. The noise due to vibrations or turbulence is compensated by the adaptive two-wave mixing in the 5 photorefractive crystal at frequencies below fo. Thus, fo is often called the compensation frequency or compensation bandwidth. This parameter is dependant of the material properties and of the incident optical power. For TEMPO with a laser wavelength of 532nm, the compensation bandwidth is around 75 Hz (Figure 6) corresponding to a crystal response time of 2.1 ms (Fit of the experimental data). Normalized amplitude 1.2 1 Response time = 2.1 ms 0.8 0.6 0.4 0.2 0 0 500 1000 1500 2000 f (Hz) Figure 3: Frequency response of the photorefractive effect: - Theoretical fit; • Experimental. For measurement on thin samples, background vibrations can easily be picked-up by the thin sample (resonance of a membrane), leading to reduction in the system performance if the PR crystal can not compensate fast enough. To overcome this limitation, TEMPO includes an internal stabilization loop, which improves the compensation speed (the response time) of the crystal. This stabilization loop has been recently highly improved, showing much faster response time, Typically 1s (a X2 improvement is response speed compared to the initial measurement of the photorefractive effect). Data shown Figures 4-A and 4-B illustrates the improvement in response time that is achieved with "Compensation ON". These measurements were recorded for our TEMPO120MHz. A step displacement corresponding to the maximum displacement measurable by the interferometer (8) is applied to the sample and the DC output signal is displayed, showing how fast the interferometer adapts to this displacement (response time of the interferometer). With "Compensation OFF", the response is given by the response time of the photorefractive effect. It must be pointed out that the response time of the photorefractive effect is slightly dependent on the direction of the displacement. This is a well-known effect for photorefractive crystal with an applied DC high-voltage [Refxx]. With "Compensation ON", the response time is highly improved, with only about 100S to recover from a 66.5nm displacement step. In the measurements shown Figure-4, the Photorefractive two-wave mixing effect exhibited a "natural" response time of 3ms to ms. 6 Figure 4-A: Response (measured at DC output) to a negative displacement step of 66.5nm Figure 4-B: Response (measured at DC output) to a positive displacement step of 66.5nm 2.3. Laser Intensity Noise Rejection In theory the signal-to-noise ratio (SNR) of a laser-ultrasound interferometric detection system should increase with the square-root of the amount of light power received on the detector. This statement assumes that the interferometer is shot-noise limited and all the other noise sources (electronic noise, laser intensity noise and laser phase noise) are negligible. However in practice, as the power on detector increase, measurements become more and more sensitive to the laser intensity noise. The use of interferometers with high collection efficiency and the use of detection lasers with higher power are advantageous only if the intensity noise is kept below the shot noise level. 7 TEMPO now integrates a noise rejection circuitry [patent Pending] which rejects the Laser Intensity Noise (LIN) for frequencies below 10MHz. With single-frequency CW Lasers used for interferometric detection (and for ~1mW of light at 532nm on the detector): - Intensity Noise is dominant at lower frequencies: < 1MHz - Shot Noise dominates over LIN at Higher frequencies: > 10MHz - Electronic noise dominates for low power on detector at high frequency. The LIN rejection circuit is designed to reject the common intensity noise at frequencies up to about 10MHz. LIN Rejection is now included as a standard feature for the TEMPO Serie: - TEMPO-2D: - Out-of-plane output includes LIN rejection. - For In-plane detection, LIN is naturally rejected because of the differential process of the in-plane detection principle. - TEMPO-1GHz: - The LIN rejection is available at the CALIBRATED OUTPUT (Frequency bandwidth up to 90MHz) - The FULL-BANDWITH Output [1MHz - 1GHz] does not include the LIN rejection. - TEMPO-12MHz: - The LIN rejection is available at both the CALIBRATED Output [90MHz] and The FULL-BANDWITH Output [120MHz]. Examples of LIN rejection are shown Figure 5 and Figure 6. The laser from Figure 6 exhibited a strong noise frequency peak at 350kHz. With the LIN rejection, this noise frequency was reduced by more than 30dB. 8 Figure 5: Rejection of the Laser Intensity Noise (TEMPO-120MHz detection bandwidth). Comparison among the noise spectrums: 1) Electronic noise from the rejection circuit only, 2) Electronic noise from the photodetector (newFocus-12MHz), 3) output noise for 1mW without LIN rejection and 4) with LIN rejection. Figure 6: Rejection of the Laser Intensity Noise (TEMPO-20MHz detection bandwidth): Comparison between with & without Noise Rejection. 9 2.4. Simultaneous In-plane & Out-of-plane detections The TEMPO measures a phase shift that is proportional to a surface displacement along the direction of the probe beam (Z-axis). For most measurement, the probe beam is at (or near) normal incidence of the inspected surface. Thus, It is often referred as Out-of-plane measurement. Out-of-plane displacement is easily measured by interferometer, specifically if the surface is highly reflective (strong specular reflection). However, for measuring in-plane displacement the light backscattered at large angle (away from the specular reflection) must be used instead (Figure 7) because the directly backscattered light (specular reflection) doesn’t carry any in-plane displacement information. Figure 7: Schematic Principle for detecting In-plane and out-plane displacement 2.4.1. Principle of operation Adaptive interferometers based on two-wave mixing (TWM) in photorefractive crystal (PRC) are optimized to collect many speckles. High collection efficiency is achieved using a high numerical aperture collecting optic. TEMPO with its large aperture optical system is thus potentially well suited for simultaneous in-plane & out-of-plane detections. Beams backscattered at large angle carry more information about the in-plane, than beams in the center that are carrying only out-of-plane information (Figure 8). An important feature of an adaptive interferometer using TWM in PRC, is that multiple beams can be independently processed inside the same crystal without cross-talk issue. This feature is used to realize a multiplexed interferometer for simultaneous detection of multiple beams corresponding to the observation at different viewing angles of the same illuminated point. For detection of the two components, we use a similar layout than our standard TEMPO. The optical setup was adjusted in order to ensure that the entrance pupil (the collecting optic) is imaged on the detector and the single-element detector was replaced with a detector array (Figure 9). 10 Figure 8: Detection principle using a single, large aperture collecting lens. Figure 9: TEMPO-2D Optical Set for in-plane and out-of-plane detection. Each element of the detector array corresponds to a small area of the entrance pupil and thus corresponds to light backscattered along well defined incidences. Processing of the interference signals, as a function of the back-scattered angle, provides simultaneously inplane and out-of-plane displacements. The schematic of the electronic signal processing is shown Figure 10. 11 The collecting optic has a high numerical aperture (F0.75), with a maximum collection angle of 31o. The multi-detector is a linear-array with 16 elements. The collected light is focused on the linear array using a cylindrical lens. In this configuration, we detect the in-plane component along the orientation of the linear array. The light backscattered by the sample may not be uniformly distributed and each channel must be normalized before calculating the in-plane and out-of-plane. Normalization is achieved using automatic gain controlled (AGC) amplifiers monitoring the low frequency signal generated by an internal piezo-mirror in the path of the reference beam. After amplitude normalization, the signals are processed by pairs, with same incidence angles. For each pair, the two normalized signals are added to generate the elementary out-of-plane component and their subtraction gives the elementary in-plane component (Figure 10). Figure 10: Principle of the electronic demodulation circuitry for simultaneous in-plane and out-of-plane detection. 2.4.2. Figures of Merit A critical receiver parameter is the surface displacement sensitivity or minimum detectable displacement called the Noise-Equivalent Surface Displacement (NESD). NESD is the RMS surface displacement amplitude that can be detected at a signal-to-noise of unity for 1 W incident power and 1 Hz bandwidth. The units of NESD are nm (WHz)12. To determine the minimum detectable displacement, the NESD is multiplied by the square root of the bandwidth and divided by the square root of the power on the detector.For 12 -7 1/2 Out-of-plane displacement, TEMPO (out-of-plane) has NESD better than 2x10 nm (W/Hz) for f > 1 MHz. The noise spectrums for both the out-of-plane and in-plane outputs are shown Figure 11. For frequency above 1MHz we measured the NESD for Out-of-plane and In-plane : -7 1/2 NESDOUT = 1.7 x10 nm (W/Hz) -7 1/2 NESDIN = 11x10 nm (W/Hz) For the In-plane displacement, the sensitivity strongly depends on the spatial distribution of the backscattered light. Optimal conditions are achieved when uniform scattering occurs. The example of NESD measured in Figure 10 corresponds to near-optimal conditions. If the light scattering is not uniform, the noise level of the weakest signals will be amplified through the normalization process, resulting into a higher noise level for the in-plane output. Figure 11: Noise spectrum for the Calibrated In-plane Output and the Calibrated Out-of-plane output. 3. APPLICATIONS: Example of Results 3.1. Thin sample: Pulse detection using TEMPO-1GHz Examples of high-frequency measurement using the TEMPO-1GHz are shown below. In this experiment, generation was carried out with a MicroChip laser from Teem Photonics (PowerChip PNG), with a 300ps pulse duration. The experiment is described in Figure 12 and results are shown Figures 13, 14 and 15. 13 Figure 12: Description of the experiment using the TEMPO-1GHz. Figure 13: Result recorded on an AlTi plate with 230m thickness. 14 Figure 14: Result recorded on an AlTi plate of 265m thickness. Figure 15: Result recorded on a Copper plate of 400m thickness. 15 3.2. Thin sample: Lamb mode resonance measurement with TEMPO-2D Figure 16 shows an example of high-frequency Lamb wave detected with TEMPO-2D, in a 1mm thin aluminum sheet after propagation along 20mm. Generation was carried out with a line-focused pulsed laser beam. For this measurement, the measured in-plane direction is along the propagation direction. The signals were high-pass filtered with cut-off frequency set at 2MHz, in order to only visualize the higher frequencies. The high frequency vibration of the plate experiences low attenuation and the plate vibration is visible on both in-plane and outof-plane displacements for a long time, much longer than the time window showed Figure 16. We can see that the In-plane component carries higher frequency resonances compared to the out-of-plane component. When the detection and the generation are alignedsuperimposed then strong resonances are detected. Some Lamb modes exhibit an anomalous behavior at frequencies where the group velocity vanishes while the phase velocity remains finite. The zero group velocity (ZGV) leads to Sharp cw resonance and ringing effects. Figure 17 shows the fast Fourier transform computed on the first 50s of the signals. The spectrum of the out-of-plane signal (Figure 17-A) clearly shows the resonance of the S1 mode and of the A2 mode as described by Clorennec et al [*]. The resonance of the A2 Lamb mode corresponds to the thickness shear resonance ( F2 ⋅ d = 3 ⋅ V S 2 ), where VS is the shear wave velocity and d is the thickness. Figure 16: High frequency Lamb waves detected in a 1mm thin aluminum sheet, after propagation along 20mm. 16 Figure 17: Time signal and corresponding Spectrum of the high-frequency Lamb wave, with detection at epicenter. A) out-of-plane displacement and B) In-plane displacement On the spectrum of the in-plane signal (Figure 17-B) the S1 and A2 resonances are also visible. We also see many other resonances at higher frequencies, which were not visible on the out-of-plane displacement. These modes correspond to the higher order Lamb modes, demonstrating the wealth of information available with the simultaneous measurement of inplane and out-of-plane. [*] D.Clorennec, C. prada, D. Royer and T. Murray , “laser impulse generation and interferometer detection of Zero Group Velocity Lamb mode resonance”, Appl. Phys. Lett. 89, 2006 3.3. Thermoelastic generation in Thick Plate: Pulse detection with TEMPO2D An other example of In-plane measurement is shown below. Figure 18 describes the through transmission experiment. Generation was achieved in thermoelastic regime, with the laser beam focused along a line and detection was carried out on the other side. The sample was a thick aluminum plate: 12.7mm thickness. In a first experiment, we use a sample free of defects (no blind holes). The detection was scanned along a 50mm line. The B-scan results for both In-plane and out-of-plane displacements are shown Figure 19. The strong shear wave is clearly visible. Many reflected and converted wave arrivals are visible. After 12s, some reflections from the sample edges are visible. For these measurements, the calibration coefficient is 100mVnm for both In-plane and out-of-plane outputs. 17 Figure 18: Description of the measurement setup for the sample with Blind-holes. Two defects (blind holes as described on Figure 18) were introduced and the measurements were repeated. The result comparing the B-scan, without and with, defects are shown Figure 20. Reflection diffraction from the two defects are clearly identifiable on the Inplane B-scan. 18 Figure 19: B-scan results for through-transmission In-plane and Out-of-plane measurement on a 12.7mm thick aluminum plate (no defect) with laser line generation. 19 Figure 20: B-scan results for through-transmission In-plane and Out-of-plane measurement on a 12.7mm thick aluminum plate: A) without defect and B) with Defect (2 blind holes). 20