TEMPO 1D/2D - Performance Update - Bossa Nova Technologies

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
info@bossanovatech.com
© 2006 Bossa Nova Technologies, LLC. All rights reserved.

Table of contents
1. TEMPO Update - Summary 3
2. New Features - Description 4
2.1. Calibrated Output 4
2.2. Background vibration Compensation 5
2.3. Laser Intensity Noise Rejection 7
2.4. Simultaneous In-plane & Out-of-plane detections 10
2.4.1. Principle of operation 10
2.4.2. Figures of Merit 12
3. APPLICATIONS: Example of Results 13
3.1. Thin sample: Pulse detection using TEMPO-1GHz 13
3.2. Thin sample: Lamb mode resonance measurement with TEMPO-2D 16
3.3. Thermoelastic generation in Thick Plate: Pulse detection with TEMPO-2D 17
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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
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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.
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(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 Bi 12 SiO 20 (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 f o ) the photodetector signal is linearly
proportional to the displacement. At low frequencies (below f o ) the photodetector signal is
proportional to frequency, or equivalently to velocity. The decrease in the signal below f o
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
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photorefractive crystal at frequencies below f o . Thus, f o 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).
1.2
Normalized amplitude
1
0.8
0.6
0.4
0.2
0
Response time = 2.1 ms
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 TEMPO-
120MHz. 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.
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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.
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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.
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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.
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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).
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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.
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The collecting optic has a high numerical aperture (F0.75), with a maximum collection
angle of 31 o . 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
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Out-of-plane displacement, TEMPO (out-of-plane) has NESD better than 2x10 -7 nm (W/Hz) 1/2
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 :
NESD OUT = 1.7 x10 -7 nm (W/Hz) 1/2
NESD IN = 11x10 -7 nm (W/Hz) 1/2
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.
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Figure 12: Description of the experiment using the TEMPO-1GHz.
Figure 13: Result recorded on an AlTi plate with 230m thickness.
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Figure 14: Result recorded on an AlTi plate of 265m thickness.
Figure 15: Result recorded on a Copper plate of 400m thickness.
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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 S 1 mode and of the A 2 mode as described by
Clorennec et al [*]. The resonance of the A 2 Lamb mode corresponds to the thickness shear
resonance ( F ⋅ d = 3⋅V
2 ), where V S is the shear wave velocity and d is the thickness.
2 S
20mm.
Figure 16: High frequency Lamb waves detected in a 1mm thin aluminum sheet, after propagation along
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 S 1 and A 2 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 TEMPO-
2D
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.
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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.
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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