Optoacoustics with Laser-Ultrasound

Conventional ultrasound measurements hold the drawback that the sample has to be in direct contact with the measurement apparatus (e.g. water jet, coupling agent). In Laser-Ultrasound the ultrasonic wave is transmitted via light. 
 
This method presents an ideal complementary technique to Optical Coherence Tomography (OCT) as OCT cannot be applied for the determination of the inner structure of substrates showing very low optical penetration depths. There it is being absorbed in a layer near the surface, leading to heating, thermal expansion and finally to emission of an ultrasonic wave (see figure).
 

Principle

Generelly, Opto-Acoustics stands for the generation of sound through light. In the particular case described here, Laser-Ultrasound, pulsed light (a few femto- to picoseconds in length) of a laser hits the substrate. The laser pulse is absorbed at the surface and the consequent heating of the substrate causes an ultrasonic pulse which propagates through the inside of the substrate (see figure).
 

Innovation / Benefit

Excitation of ultrasonic impulses and detection of the echo using light (interferometer) aims at contactless ultrasound measurement. Thereby e.g. also hot samples can be investigated using ultrasound.
 
Anisotropy
Quality control of composites via contactless determination of fiber orientation within the material.
  
Applicable for companies manufacturing composites with epoxy or metal matrix, but also for the production of deep drawing sheet made of steel or aluminium.
 
3-D Image Reconstruction
3-D ultrasound images of product parts passing by are recorded contactless at high speed. Thereby it is possible, in contrast to conventional image processing, to not only monitor the outer shape of a product, but also to determine the inner structure, e.g. possible pores or other material defects on-line and to allow for immediate reaction in the production process.
Applicable for all companies manufacturing security relevant parts which so far have to be examined using e.g. x-ray techniques.