Laser ultrasonics
Encyclopedia
Laser-ultrasonics uses lasers to generate and detect ultrasonic
waves. It is a non-contact technique used to measure materials thickness, detect flaws and materials characterization. The basic components of a laser-ultrasonic system are a generation laser, a detection laser and a detector.
power lasers. Common lasers used for ultrasound generation are solid state
Q-Switched
Nd:YAG
and gas laser
s (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)
fs lasers are used often in a pump-probe configuration with the detection system (see picosecond ultrasonics
).
Common detection techniques include: interferometry
(homodyne or heterodyne or 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 the 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).
It is interesting to note that under ideal circumstances most detection techniques can be considered theoretically as interferometers and, as such, their ultimate sensitivities are all roughly equal. The reason for this is that in all these techniques the interferometry is used to linearize 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 and 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, because the shot noise limited SNR is proportional to the square root of the total detection power this has limited effect and it is easy to reach damaging power levels before getting 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
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.
Ultrasound
Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Ultrasound is thus not separated from "normal" sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is...
waves. It is a non-contact technique used to measure materials thickness, detect flaws and materials characterization. The basic components of a laser-ultrasonic system are a generation laser, a detection laser and a detector.
Ultrasound generation by laser
The generation lasers are short pulse (from tens of nanoseconds to femtoseconds) and high peakpower lasers. Common lasers used for ultrasound generation are solid state
Solid-state laser
A solid-state laser is a laser that uses a gain medium that is a solid, rather than a liquid such as in dye lasers or a gas as in gas lasers. Semiconductor-based lasers are also in the solid state, but are generally considered as a separate class from solid-state lasers .-Solid-state...
Q-Switched
Q-switching
Q-switching, sometimes known as giant pulse formation, is a technique by which a laser can be made to produce a pulsed output beam. The technique allows the production of light pulses with extremely high peak power, much higher than would be produced by the same laser if it were operating in a...
Nd:YAG
Nd:YAG laser
Nd:YAG is a crystal that is used as a lasing medium for solid-state lasers. The dopant, triply ionized neodymium, typically replaces yttrium in the crystal structure of the yttrium aluminium garnet , since they are of similar size...
and gas laser
Gas laser
A gas laser is a laser in which an electric current is discharged through a gas to produce coherent light. The gas laser was the first continuous-light laser and the first laser to operate "on the principle of converting electrical energy to a laser light output...
s (CO2
Carbon dioxide laser
The carbon dioxide laser was one of the earliest gas lasers to be developed , and is still one of the most useful. Carbon dioxide lasers are the highest-power continuous wave lasers that are currently available...
or Excimers
Excimer laser
An excimer laser is a form of ultraviolet laser which is commonly used in the production of microelectronic devices , eye surgery, and micromachining....
) . 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
Emissivity
The emissivity of a material is the relative ability of its surface to emit energy by radiation. It is the ratio of energy radiated by a particular material to energy radiated by a black body at the same temperature...
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)
fs lasers are used often in a pump-probe configuration with the detection system (see picosecond ultrasonics
Picosecond ultrasonics
Picosecond ultrasonics is a type of ultrasonics that uses ultra-high frequency ultrasound generated by ultrashort light pulses. It is a non-destructive technique in which picosecond acoustic pulses penetrate into thin films or nanostructures to reveal internal features such as film thickness as...
).
Ultrasound detection by laser
Ultrasound 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
Interferometry
Interferometry refers to a family of techniques in which electromagnetic waves are superimposed in order to extract information about the waves. An instrument used to interfere waves is called an interferometer. Interferometry is an important investigative technique in the fields of astronomy,...
(homodyne or heterodyne or 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 the 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).
It is interesting to note that under ideal circumstances most detection techniques can be considered theoretically as interferometers and, as such, their ultimate sensitivities are all roughly equal. The reason for this is that in all these techniques the interferometry is used to linearize 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 and 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, because the shot noise limited SNR is proportional to the square root of the total detection power this has limited effect and it is easy to reach damaging power levels before getting 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
CHOTS
Corporate Headquarters Office Technology System was a restricted electronic mail and office administration system used by the Ministry of Defence of the United Kingdom....
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.