Ultrasonics

In laser ultrasonics, lasers are used to generate as

well as detect ultrasonic waves. Ultrasonic waves

are used for non-destructive testing of materials

such as composites in aerospace and other

industries. Upon interaction with the material,

the resulting ultrasonic waves are optically

detected using lasers by techniques

such as Fabry-Pérot interferometry.

Laser ultrasonic technology is a technique in which a pulsed laser is used to not only generate ultrasonic waves but detect them as well. Most typically, laser ultrasonics is used to investigate flaws in composite materials in the aerospace industry, though the inherent beauty of laser ultrasonics is its applicability to any industry where such measurements are of value. The governing physical principle is straightforward: as the laser pulse strikes the surface of the material to be studied, a sudden, localized, and instantaneous thermal expansion event occurs. Should the laser power be sufficiently high as to induce boiling of the material, an ultrasound is generated via a recoil mechanism of the expanding material. At even higher laser powers – when the material is purely ablated – a plasma is formed, the expansion of which contributes greatly to ultrasonic generation.

 

 

 

IPG Photonics offers a broad range of pulsed fiber lasers applicable to the generation of ultrasonic waves at medium to high repetition rates, including:

1)     Nanosecond pulsed fiber lasers
2)     Pico- and femtosecond pulsed fiber lasers

Subsequent to the generation of the ultrasonic waves via lasers, the detection occurs, most often also with the aid of laser technology. As the ultrasonic wave is detected optically, common techniques include interferometry or Fabry-Pérot detection schemes. Most techniques make use of continuous or long pulse (tens of µs), though shorter pulses have also found their domain of applicability. Nevertheless, the principle is relatively the same: since the ultrasound waves generate a change in the index of refraction of the surrounding air, incident laser pulses will deflect slightly and thus change course. This detected change is converted into an electrical signal. At higher frequencies, the scheme of detection is somewhat different as the method relies on the detection of movement of the surface of the sample. Nevertheless, since the theoretical detection limit is determined by phonon frequency – many orders of magnitude less than photon frequency – the sensitivity of laser ultrasonics is quite high.

IPG's continuous wave (CW) and pulsed fiber lasers are a cost-effective and robust alternative suitable for use in both ultrasonics wave generation as well as detection in the aerospace industry.

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