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Non-Contact Ultrasonics From The Laboratory To The Plant Floor
  As someone working in nondestructive testing, you have often encountered situations where contact between the sensor and the inspected part is difficult or simply not possible. Laser-ultrasonics is a novel inspection technique that combines the precision of ultrasonic inspection to the non-contact nature of optical detection. The use of laser-ultrasonics has been demonstrated in a wide variety of applications ranging from inspection of aerospace composite materials, to coating analysis with laser generated surface waves, to in-plant wall thickness measurements of hot mechanical tubing.
  Until recently, access to commercial laser-ultrasonic equipment has been rather limited. Tecnar Automation Ltee (TECNAR) has taken the task to development and market laser-ultrasonic equipment for use in both laboratory and industrial environments. TECNAR has made recent advances to bring laser-ultrasonics out of the hands of laser experts and into to the hands of NDE operators.
  TECNAR has introduced the TWM, a compact fully integrated detection system that is easy to setup and to use. The TWM is the perfect tool to add a sensor to your testing equipment. Also, TECNAR has introduced the FPC, a robust detection system that has the fastest response time for laser-ultrasonic detection. The FPC is perfectly adapted for use on mobile target as well as for rapid scanning of stationary parts.
  TECNAR can also deliver complete laser-ultrasonic system for dedicated application. In particular, the LUT is a Laser-Ultrasonic Thickness gauge for on-line wall thickness and eccentricity measurement of hot seamless steel tube.
  Introduction to laser-ultrasonics
  Laser-ultrasonics is a combination of the precision of ultrasound measurements with the flexibility of optical systems. Ultrasonics is a well known non-destructive technique that can provide several parameters of interest for materials. The measurement of the time-of-flight of an ultrasonic wave within the bulk of a material can directly measure the thickness, provided that the velocity of sound in the material is known, or can determine the presence of defect within the bulk of the material. The method is used in several industries, such as for aircraft inspection or thickness gauging of metal, because it provides high accuracy measurements. Subtle changes in wall thickness are easily detected and quantified.
  Measurements of ultrasonic velocity, when the materials thickness is known, can also be used to determine internal temperature, phase changes (such as austenitic to ferritic transition), texture and porosity. Ultrasonic attenuation (i.e., the reduction in sound amplitude between successive ultrasonic pulse echoes) provides additional information on the materials internal structure, in particular, provides a direct method for grain size measurement in-situ of metallic samples.
  Laser-ultrasonics is a combination of two separate methods: laser generation of ultrasounds and laser detection of ultrasounds. Laser generation of ultrasounds is a technique that has been in use since the early development of pulsed lasers. Efficient laser generation of ultrasounds on metal is performed by sending a strong laser pulse onto the surface of the material, thus causing ablation or vaporization of a small quantity of the material at the point of impact of the laser, as shown in Figure 1.1. Following the ablation, a recoil force is generated, which is the source of a compression (longitudinal) ultrasonic wave. The compression wave propagates in a direction normal to the free surface of the material, i.e., the surface of impact of the laser beam, independently of the angle of incidence of the laser light. Hence, time-of-flight of the traveling ultrasonic pulse is always recorded in the direction normal to the surface.
Figure 1.1: Laser generation of ultrasound in ablation mode
  When ablation of the material is not possible, the power of the generation laser is reduced. The absorption of the laser light then only causes a sharp increase of the temperature of the material near the surface, as shown in Figure 1.2. The temperature gradient will result in the creation of stress near the surface from the resulting thermal expansion of the material. Strong compression (longitudinal) waves will propagate in a direction normal to the surface, while shear (transverse) waves will propagate at an angle dependant on the properties of the material. After a short delay, the thermal energy is diffused within the bulk of the materials, leaving the surface without any damage or noticeable mark.
Figure 1.2: Laser generation of ultrasound in thermo-elastic mode
  Laser detection of ultrasound is based on the demodulation by an optical interferometer of the laser light reflected or backscattered from the surface of the material. In laser detection, a single frequency, high-coherence length, laser light is focused on or near the point of impact of the generation laser beam on the surface of the material. Any surface motion at the point of impact of the detection laser is recorded on the reflected light as a frequency (or phase) variations (Doppler Effect), as shown in Figure 1.3. The ultrasonic surface displacements are therefore "encoded" in the phase of the reflected or backscattered laser light, called the signal beam.
Figure 1.3: Laser detection of ultrasounds
  The basis of optical detection of ultrasound consists of three steps: an ultrasonics-to-optics conversion, an optical phase modulation to intensity modulation process, and finally an optical intensity to electrical signal conversion. The ultrasonics to optics conversion, as described previously, is produced by the surface motion associated with the ultrasonic wave that induces a phase change on the light beam reflected or scattered by the surface.
  Since optical detectors are not sensitive to phase variations but rather to intensity variations, an optical phase-to-intensity conversion is required. An interferometric process performs the conversion. The process is achieved by combining a reference light beam with the signal light beam. The combination of the two beams will then “beat” as a function of the phase difference between both beams. Numerous interferometers have been used to produce the reference beam based on passive interferometric techniques, such as the confocal Fabry-Perot, the use of which has been validated in various industrial environments.

Laser-ultrasonics detection with FPC

  TECNAR’s FPC laser-ultrasonic detection unit is based on the confocal Fabry-Perot approach. The principle of detection is to use time delayed images of the light from the target to form the needed reference beam. A Fabry-Perot is an optical system consisting of two mirrors: the front mirror being partially reflective while the second is fully reflective. As shown in Figure 1.4, the signal beam incident to the Fabry-Perot is partially reflected by the front mirror and partially transmitted inside the cavity. The transmitted signal beam then propagates inside the cavity where it is completely reflected by the back mirror. This reflected beam is then also partially transmitted and partially reflected by the front mirror. Therefore, in front of the cavity we have the initial signal beam that interferes with multiple reflections from the cavity.
Figure 1.4: Principle of operation of the FPC
  If there is no phase variation as function of time in the input beam to the Fabry-Perot, then the output beams will add in phase if the cavity length is an integer number of the optical wavelength of the input beam (either 532 nm or 1064 nm depending on the model of the FPC). This can be observed by recording the output optical intensity of the Fabry-Perot as function of the length of the cavity, as shown in Figure 1.5. Each peak in the figure corresponds to a resonance mode of the cavity.
Figure 1.5: Principle of operation of the FPC
  The same result would be achieved if the optical wavelength of the signal beam is swept instead of the cavity length. Since phase variation as a function of time can be viewed as a frequency variation, if the cavity length is set to half-height amplitude (mid-amplitude of the peaks shown in the figure above), any frequency variations will cause a corresponding amplitude variation of the reflected beams intensity, which can then be recorded by a photodetector.
  For proper laser-ultrasonic operation, the cavity length of the Fabry-Perot must therefore be constantly stabilized at a predetermined distance, with respect to the optical wavelength of the detection laser. Any drift of the detection laser optical wavelength must be tracked and accounted for by the cavity of the Fabry-Perot. Also, ambient vibration and mechanical thermal expansion can cause small changes in the cavity length, which must be accounted for.
  TECNAR’s FPC is supplied with an internal electronic stabilization loop. The system uses a combination of the reflected beams to obtain a stabilization feedback signal. The feedback signal is sent to a PZT translator mounted at one end of the Fabry-Perot. The PZT translator moves the mirror to adapt the length of the cavity and compensate for any mismatch between the detection laser wavelength and the cavity length in order to achieve optimum sensitivity for laser-ultrasonic detection.

Laser-ultrasonics detection with TWM

  Recently, following developments made at the IMI-NRCC and the Laboratoire Charles Fabry of the Institut d'Optique, Research unit associated to the Centre National de la Recherche Scientifique - France, an active interferometer approach has been introduced based on two-wave beam mixing.
Figure 1.6: Principle of detection with photorefractive crystal
  In this approach, a pump light beam and a signal light beam, which carries the ultrasonic information as phase modulation of the laser light, interfere inside a photorefractive crystal. Through the photorefractive effect, an index grating is created inside the crystal that reproduces (records) the interference pattern. Because of the limited response time of the crystal, the grating will contain the static and slow motion of the target but not the ultrasonic (fast) motions. The grating created then refracts the two beams. Thus in the direction of the signal light beam, a reference light beam is created from the pump beam, with the same spatial structure as the signal light beam (as show in Figure 1.6). Such a process can be viewed as “real-time” holography. The signal light beam transmitted through the crystal interferes on the photodetector with the diffracted pump light beam (reference signal) to perform phase demodulation and to give a signal proportional to the surface displacement.
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Figure 1.7: Typical laser-ultrasonic signal from cold steel sample
  Once the phase information has been extracted for the signal beam, the resulting signal is an analog electrical signal representative of the motion of the surface of the material. Figure 1.7 shows a typical laser-ultrasonic signal recorded on a steel sample. The signal contains a strong initial pulse, the initial impact of the generation laser light called “surface signal”, followed by several pulses, the echoes resulting from the forward propagation and reflection from the back wall of the initial compression ultrasonic pulse.
  To know more about this system or to receive a quotation, please do not hesitate to contact us at:sales@tecnar.com.

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