Laser Ultrasonics

Laser Ultrasonics

Laser Ultrasound (LUS) is a method to generate and detect ultrasonic signals remotely with the aid of lasers, without requiring direct physical contact between probe and sample (see ref. [1]).

Motivation

Standard ultrasonic techniques have the disadvantage of requiring direct contact between the measuring device and the sample. If testing is performed manually, a coupling agent is usually applied between sample and measuring head. For automatic testing a water-jet is used to couple sample and transducer, or the sample is placed into a bath of water. On hot samples (e.g. glowing steel), or other devices where contact measurement is prohibited by the test specifications, laser ultrasound permits testing of the device. Another advantage of laser ultrasound is the enhanced resolution. Due to the optical excitation and detection, measurement frequencies of more than 500 MHz can be achieved with our current setups.

Principles

Ultrasonic waves are generated on the surface of a sample by exposing a small spot to a short and focused laser pulse. Depending on the power density, ultrasonic waves are generated by ablation or by thermoelastic expansion. The type of wave generation influences the propagation direction and energy of the ultrasonic waves [2, 3, 4]. Note that the material itself is the emitter of ultrasonic waves. Thus the propagation direction is independent from the incidence angle of the exciting laser beam. Consequently, at sample areas which are difficult to access (and are therefore illuminated under shallow incident angles), the ultrasonic waves are emitted in the same way as under perpendicular incidence.

Abbildung 4

Laser based detection of ultrasound

Illuminating a point on the sample with a cw laser, allows the detection of ultrasonic waves. The ultrasonic waves lead to small displacements in the order of 10 nm and surface speeds of several meters / second.

These displacements lead to a Doppler shift (i.e. change of the wave length), which can be demodulated, for example, by a Fabry-Pérot interferometer [2, 5, 6, 7]. Alternatively the surface displacements can, for example, be measured with a photorefractive crystal interferometer. In both interferometric techniques the resulting laser light is detected with a high frequency photodiode.

A laser ultrasonic measuring system has two main advantages: (i) the remote generation and detection of ultrasound; and (ii) the high bandwidth of the detectable frequencies. The temporally short excitation pulse (analogue to a Dirac delta pulse), as emitted by a nanosecond (or picosecond) laser, leads to the generation of high frequency ultrasonic waves, which can reach the GHz region. Conventional ultrasonic systems only generate ultrasonic waves at one specific frequency. The frequency is thereby dependent on the capacity and design of the transducer and usually lies below 40 MHz. Detection of the ultrasonic waves with a cw laser system is limited only by the bandwidth of the photo detector. Thus, detection in the GHz range is possible.   

Pulse-Echo technique

Thickness measurements with bulk waves are performed by generating ultrasonic waves which propagate in the material. When these bulk waves reach the opposite surface a surface motion can be detected.

Also, the wave is partially reflected at the material/air interface and propagates back into the sample, echoing between the samples' surfaces. By measuring the arrival times, or the temporal distance between the echoes, the sample thickness can be evaluated.

Left: Measurement on a 150 µm thick metal sheet; Right: Measurement of a 4 mm thick aluminum block

Resolution

The maximum achievable resolution is limited by the damping at high frequencies within the sample. However, due to the higher bandwidth (up to 500 MHz), the achievable resolution is usually several times better than that achieved with conventional ultrasonic techniques employing piezo-electric transducers.

Measurement with Surface Acoustic Waves

The characterization of thin films can be achieved with the aid of surface acoustic waves. If a film is deposited on a substrate, the ultrasonic waves propagate (depending on their acoustic frequencies), to a greater or lesser extent in the bulk or in the film. If bulk and film have different acoustic properties, different acoustic frequencies have varying surface acoustic velocities. Thus, the surface acoustic waves are dispersive.

By analyzing the dispersion curve, the thickness of the film can be calculated. Furthermore, evaluation of Young’s modulus, Poisson ratio, and density of the film (and/or the substrate) is possible. With this method, films with thicknesses as small as several nanometers can be characterized.

Abbildung 9

Determination of Anisotropies

Anisotropy can be determined with the aid of surface acoustic waves. Measurements are performed at different angles and the resulting curves give information about compression or elongation of the material. Applications include quality control of composite materials by remote determination of the fiber orientation. Another field of application is the characterization of deep drawn steel or aluminum sheets.

Literature

  1. T. Dehoux, K. Ishikawa, P. H. Otsuka, M. Tomoda, O. Matsuda, M. Fujiwara, S. Takeuchi, I. A. Veres, V. E. Gusev, and O. B. Wright:
    Tracking picosecond coherent phonon pulse focusing inside a sub-micron object,
    (Submitted to Nat. Sci. Rep.)

  2. C. Grunsteidl, T. W. Murray T. Berer and I. A. Veres:
    Inverse characterization of plates using zero group velocity Lamb modes,
    (Submitted to Ultrasonics)

  3. C. Grunsteidl, T. W. Murray and I. A. Veres:
    Experimental and numerical study of the excitability of ZGV-Lamb waves by laser-ultrasound,
    J. Acoust.Soc. Am. 138(1), 242-250(2015)

Industrial Applications

Carbon fiber reinforced plastics

Delaminations in CFRP materials are often a big problem as errors cannot be identified by inspection of the sample-surface. By using Laser Ultrasonics technology we are able to solve this problem in a non-destructive way!

Fiber directions in composite materials

Do you want to gain a deeper insight and understanding of interior properties of your fiber-reinforced plastics? Are you interested in the anisotropies of your components or do you need to determine the orientation of the fibers inside a CFRP-injection moulded component?
We can help with our Terahertz-Technology (THz), OCT technology, and Laser Ultrasonics Technology.

Online quality assurance during the welding process

With our patented Laser Ultrasonics technology we are able to understand the welding seam inspection inline in all welding processes. Through integrated and contactless measurements (that allow high-resolution measurements on the hot weld), the testing of the welding seam is completed within seconds after the welding has been done.

Spot welding

With Laser Ultrasonics technology, welded connections (e.g. spot welds) can be tested directly in the process.

Foundry process – casting ingot

In foundry processes it is possible to save a lot of time, money, and energy if you are able to detect already developing cracks during the on-going casting process.
Our patented Laser Ultrasonics technology can achieve this! Solidification cracks / hot cracks, surface cracks, and core cracks can reliably be detected in cast ingot (round and flat ingot).

Sealing or bonding processes

Do you operate sealing processes e.g. for food packaging? With our Laser Ultrasonics or (depending on the material) OCT technology we are able to measure the sealing seams inline in the process. We provide similar testing methods for bonding processes, e.g., hot melt adhesives for packaging.

Sheet metal & pipes

Laser Ultrasonics technology can also be used for the material characterization of sheet metal (as an example). An advantage of this is that the material only has to be accessible from one side and such measurements can often be performed directly in the manufacturing process. This enables inline wall thickness measurements of pipes during the production process.

LUS technology coupled with innovative data analysis is a highly flexible answer to your questions related to the different aspects (sheet metal thickness, elastic properties, anisotropies) of different materials (steel, aluminium, brass, copper).

Solder connections

The Laser Ultrasonics technology is also applicable for the testing of adhesion and bonding layers, e.g., solder joints. As the high-frequency ultrasonic waves are influenced at the interfaces, an image of this critical inner zone can be generated.

3D printing / generative manufacturing / additive manufacturing

In recent years the rapid development and quality improvements in the field of 3D printing for plastic and metal components has enabled many applications, including the serial production of critical components. Using the OCT inspection technique (for plastics) or Laser-Ultrasonics Technology (LUS, for metals) allows to monitor the quality of the products directly during the printing process. This gives the possibility to intervene immediately to correct occurring faults inline.

Analytics for paper & the paper industry

Paper is an extremely diverse material and its characterization requirements are relevant for production, processing, and application. We can support you in many ways with our testing methods (IR & Raman Spectroscopy, THz Spectroscopy, OCT, and Laser-Ultrasound), for example, in chemical analytics (directly in the production process) and the characterization of surfaces and fibers for both absorbency and anisotropy.