Laser ultrasonics
Laser-ultrasonics uses lasers to
generate and detect ultrasonic waves. It is a non-contact
technique used to measure materials thickness, detect flaws and carry out
materials characterization. The basic components of a laser-ultrasonic system
are a generation laser, a detection laser and a detector.
Contents
- 1Ultrasound
generation by laser
- 2Ultrasound
detection by laser
- 3Ultrasonic
laser technique operation
Ultrasound generation by laser
The generation lasers are short pulse (from tens of
nanoseconds to femtoseconds) and high peak power lasers. Common lasers used for
ultrasound generation are solid state Q-Switched Nd:YAG and gas
lasers (CO2 or Excimers). The physical principle is
of thermal expansion (also called thermoelastic regime)
or ablation. In the thermoelastic regime, the ultrasound is
generated by the sudden thermal expansion due to the heating of a tiny surface
of the material by the laser pulse. If the laser power is sufficient to heat
the surface above the material boiling point, some material is evaporated
(typically some nanometres) and ultrasound is generated by the recoil effect of
the expanding material evaporated. In the ablation regime, a plasma is often
formed above the material surface and its expansion can make a substantial
contribution to the ultrasonic generation. consequently the emissivity patterns
and modal content are different for the two different mechanisms.
The frequency content of the generated ultrasound is
partially determined by the frequency content of the laser pulses with shorter
pulses giving higher frequencies. For very high frequency generation (up to
100sGHz) femtosecond lasers are used often in a pump-probe configuration with
the detection system (see picosecond ultrasonics).
Historically, fundamental research into the nature of
laser-ultrasonics was started in 1979, by Dewhurst and Palmer. They set up a
new laboratory in the Department of Applied Physics, University of Hull.
Dewhurst provided the laser-matter expertise and Palmer the ultrasound
expertise. Investigations were directed towards the development of a scientific
insight into physical processes converting laser-matter interaction into
ultrasound. The studies were also aimed at assessing the characteristics of the
ultrasound propagating from the near field into the far field. Importantly,
quantitative measurements were performed between 1979 and 1982. In solids,
the measurements included amplitudes of longitudinal and shear waves in
absolute terms. Ultrasound generation by a laser pulse for both the
thermoelastic regime and the transition to the plasma regime was
examined. By comparing measurements with theoretical predictions, a
description of the magnitude and direction of stresses leading to ultrasonic generation
was presented for the first time. It led to the proposition that
laser-generated ultrasound could be regarded as a standard acoustic
source. Additionally, they showed that surface modification can sometimes
be used to amplify the magnitude of ultrasonic signals.
Their research also included the first quantitative studies
of laser induced Rayleigh waves, which can dominate ultrasonic surface waves.
In studies beyond 1982, surface waves were shown to have a potential use in
non-destructive testing. One type of investigation included surface–breaking
crack depth estimations in metals, using artificial cracks. Crack sizing was
demonstrated, using wideband laser-ultrasonics. Findings were first reported at
a Royal Society meeting in London with detailed publications elsewhere.
Important features of laser ultrasonics were summarised in
1990.
Ultrasound detection by laser
For scientific investigations in the early 1980s, Michelson
interferometers were exploited. They were capable of measuring ultrasonic
signals quantitatively, in typical ranges of 20nm down to 5pm. They possessed a
broadband frequency response, up to about 50MHz. Unfortunately, for good
signals, they required samples that had polished surfaces. They suffered from
serious sensitivity loss when used on rough industrial surfaces. A significant
breakthrough for the application of laser ultrasonics came in 1986, when the
first optical interferometer capable of reasonable detection sensitivity on
rough industrial surfaces was demonstrated. Monchalin et al. at the
National Research Council of Canada in Boucherville showed that a Fabry–Pérot
interferometer system could assess optical speckle returning from rough
surfaces. It provided the impetus for the translation of laser ultrasonics into
industrial applications.
Today, ultrasound waves may be detected optically by a
variety of techniques. Most techniques use continuous or long pulse (typically
of tens of microseconds) lasers but some use short pulses to down convert very
high frequencies to DC in a classic pump-probe configuration with the
generation. Some techniques (notably conventional Fabry–Pérot detectors)
require high frequency stability and this usually implies long coherence
length. Common detection techniques include: interferometry (homodyne
or heterodyneor Fabry–Pérot) and optical beam deflection (GCLAD) or
knife edge detection.
With GCLAD,(Gas-coupled laser acoustic detection), a laser
beam is passed through a region where one wants to measure or record the
acoustic changes. The ultrasound waves create changes in the air's index of
refraction. When the laser encounters these changes, the beam slightly deflects
and displaces to a new course. This change is detected and converted to an
electric signal by a custom-built photodetector. This enables high sensitivity
detection of ultrasound on rough surfaces for frequencies up to 10 MHz.
In practice the choice of technique is often determined by
the physical optics and the sample (surface) condition. Many techniques fail to
work well on rough surfaces (e.g. simple interferometers) and there are many
different schemes to overcome this problem. For instance, photorefractive
crystals and four wave mixing are used in an interferometer to
compensate for the effects of surface roughness. These techniques are usually
expensive in terms of monetary cost and in terms of light budget (thus
requiring more laser power to achieve the same signal to noise under ideal
conditions).
At low to moderate frequencies (say < 1 GHz), the
mechanism for detection is the movement of the surface of the sample. At high
frequencies (say >1 GHz), other mechanisms may come into play (for
instance modulation of the sample refractive index with stress).
Under ideal circumstances most detection techniques can be
considered theoretically as interferometers and, as such, their ultimate
sensitivities are all roughly equal. This is because, in all these techniques,
interferometry is used to linearize the detection transfer function and when
linearized, maximum sensitivity is achieved. Under these conditions,
photon shot noise dominates the sensitivity and this is fundamental
to all the optical detection techniques. However, the ultimate limit is
determined by the phonon shot noise. Since the phonon frequency is many
orders of magnitude lower than the photon frequency, the ultimate sensitivity
of ultrasonic detection can be much higher. The usual method for increasing the
sensitivity of optical detection is to use more optical power. However,
the shot noise limited SNR is proportional to the square
root of the total detection power. Thus, increasing optical power has limited
effect, and damaging power levels are easily reached before achieving an
adequate SNR. Consequently, optical detection frequent has lower SNR than
non-optical contacting techniques. Optical generation (at least in the firmly
thermodynamic regime) is proportional to the optical power used and it is
generally more efficient to improve the generation rather than the detection
(again the limit is the damage threshold).
Techniques like CHOTs (cheap optical transducers) can
overcome the limit of optical detection sensitivity by passively amplifying the
amplitude of vibration before optical detection and can result in an increase
in sensitivity by several orders of magnitude.
Ultrasonic laser technique operation
Ultrasonic laser set-up
The "Laser Ultrasonic" technique is part of those
measurement techniques known as "non-destructive techniques or NDT",
that is, methods which do not change the state of measurand itself. Laser
ultrasonics is a contactless ultrasonic inspection technique based on
excitation and ultrasound measurement using two lasers. A laser pulse is
directed onto the sample under test and the interaction with the surface
generates an ultrasonic pulse that propagates through the material. The reading
of the vibrations produced by the ultrasounds can be subsequently measured by
the self-mixing vibrometer : the high performance of the instrument makes
it suitable for an accurate measurement of the ultrasonic wave and therefore
for a modeling of the characteristics of the sample. When the laser beam hits
the surface of the material, its behavior may vary according to the power of
the laser used. In the case of high power, there is a real "ablation"
or "vaporization" of the material at the point of incidence between
the laser and the surface: this causes the disappearance of a small portion of
material and a small recall force, due to compression longitudinal, which would
be the origin of the ultrasonic wave. This longitudinal wave tends to
propagate in the normal direction to the surface of the material, regardless of
the angle of incidence of the laser: this would allow to accurately estimate
the thickness of the material, knowing the speed of propagation of the wave,
without worrying about the angle of incidence. The use of a high power laser,
with consequent vaporization of the material, is the optimal way to obtain an
ultrasonic response from the object. However, to fall within the scope of
non-destructive measurements, it is preferred to avoid this phenomenon by using
low power lasers. In this case, the generation of ultrasound takes place thanks
to the local overheating of the point of incidence of the laser: the cause of
wave generation is now the thermal expansion of the material. In this way there
is both the generation of waves longitudinal, similarly to the previous case,
and the generation of transverse waves, whose angle with the normal
direction to the surface depends on the material. After a few moments the
thermal energy dissipates, leaving the surface intact: in this way the
measurement is repeatable an infinite number of times (assuming the use of a
material sufficiently resistant to thermal stresses) and non-destructive, as
required in almost all areas of application of this technology. The movement of
the object causes a shift in the phase of the signal, which cannot be
identified directly by an optical receiver: to do this it is first necessary to
transform the phase modulation into an amplitude modulation (in this case, in a
modulation of luminous intensity ). Ultrasound detection can therefore be
divided into 3 steps: the conversion from ultrasound to phase-modulated optical
signal, the transition from phase modulation to amplitude and finally the
reading of the amplitude modulated signal with consequent conversion into an
electrical signal.
Industrial applications
Well established applications of laser-ultrasonics are composite
inspections for the aerospace industry and on-line hot tube thickness
measurements for the metallurgical industry Optical generation and
detection of ultrasound offers scanning techniques to produce ultrasonic images
known as B- and C-scans, and for TOFD (time-of-flight-diffraction) studies. One
of the first demonstrations on small defects (as small as 3mm x 3mm) in
composites was demonstrated by Dewhurst and Shan in 1993, for which they
were awarded an outstanding paper award by the American Society for
Non-Destructive Testing in 1994. This was also the time when significant
developments on composite examinations were developed from the National
Research Council of Canada and elsewhere. A wide range of applications
have since been described in the literature.
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