Gas-coupled Laser Acoustic Detection

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Airborne acoustic waves create changes in the air's density. This is the general description of a sound wave. The changes in density also produce changes in the air's index of refraction. A light beam traveling through this section will deflect slightly from its intended path. A position-sensitive photodetector that receives the light beam could translate the acoustic signals into electrical signals without any mechanical components.

This is the concept of Gas-coupled Laser Acoustic Detection, or GCLAD. This type of detection has been demonstrated before as the light traveled through acoustic fields in liquids or solids, but Dr. James Caron was the first to demonstrate this property in air. Since the discovery, the instrumentation has evolved such that highly resolved waveforms have been detected in both the ultrasonic and audio frequency ranges.

Audible Sound There is little difference between detecting ultrasound and audible sound as far as the concept is concerned. Changes in the air's index of refraction caused by the sound waves deflect the beam of the light from its original path. To maximize sensitivity, a different photodetector is used, and we have developed some simple amplifiers. The technique has been used to record such acoustic waveforms as clapping, the human voice at normal, and musical instruments. Shown in the figure below is a sound wave produced by a bassoon, played by the inventor.

Fig 3: Several notes played on a bassoon and recorded by GCLAD Fig 4: Fourier transform of the tones. As evident in the sound wave, the low C-natural contains several overtones.

Liquid-coupled Laser Acoustic Detection, or LCLAD, offers another solution.  Using the essentially the same setup, the probe beam is sent through a water tank to sense acoustic waves in the water.  This method can be used for Ultrasound inspection of immersed parts, acoustic emission from bubbles, or whale sounds (maybe....have not tried it.)

Figure 4: A series of ultrasonic waveforms from bubbles sensed by LCLAD in a beaker of water during the nucleate stage boiling.

It is easy to envision some applications where this technology would be useful, such as remote sound recording, and directional recording. However, this concept has never fully taken root for fear that the lasers involved would be to expensive to justify. This need not necessarily be the case. Using improved electronics, and multi-path detection, the price tag can be reduced significantly.

Figure 5: An LCLAD ultrasound scan of a composite plate.  From left-to-right, the images are an optical image of the scan area, the amplitude of the wave refected off the first surface, the reflected wave from the second surface, and the transmitted wave.