Detection of Sound through Optical Beam Diversion

Airborne acoustic waves create travelling changes in the air's density, a.k.a 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 was first demonstrated in air by Dr. James N. Caron. (Caron, James N., et al. Materials Evaluation 58.5 (2000).) Since the discovery, the instrumentation has evolved such that highly resolved waveforms have been detected in both the ultrasonic and audio frequency ranges. GCLAD was first developed at the University of Delaware as an alternative method to detect ultrasound after it has been transmitted through a solid material, primarily for composite materials.  This technique, coupled with a pulsed laser, creates a laser-based ultrasound system.

Ultrasound
Laser Ultrasonics (or Laser-based Ultrasound) is the generation and detection of ultrasonic waves with lasers. When a laser pulse strikes a material, the material converts the light energy into thermal energy causing the material to locally expand near the impact point. This expansion creates the ultrasonic wave. There are several ways to detect the wave once it has passed through the material.  Typically these methods reflect a laser beam off of the surface of the material.  The reflected light is gathered in an interferometer which then converts that to an electrical signal.

These techniques have one major disadvantage, the system is inherently dependent on the optical qualities of the system.  They function really well on mirror-like surfaces, and not-so-well on others.  With GCLAD, the detection beam never touches the surface, so it works well on all types of materials.  GCLAD also requires no electro-optic stabilization, or fine-tuning.  A general setup is shown below.

 
Fig1: A typical GCLAD setup used for sensing ultrasound in materials. The deflection shown is greatly exaggerated.

Audible Sound
There is little difference between detecting ultrasound and audible sound as far as the concept is concerned.  (Caron, James N., et al.  Review of scientific instruments 69.8 (1998): 2912-2917.) 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.  Soon this site will have a link to a recorded message first played at an Acoustical Society of America meeting.

Fig 2: Several notes played on a bassoon and recorded by GCLAD	Fig 3: Fourier transform of the tones.  As evident in the sound wave, the low C-natural contains several overtones.

Underwater Detection

In recent years, we have been studying the use of this technique in a water environment.  (Caron, James N., and Pratima Kunapareddy, AIP Conference Proceedings. Vol. 1581. No. 1. American Institute of Physics, 2014.) Instead of 'gas-coupled', the acoustic wave is coupled to the laser beam with water.  This has advantages with sensitvity, especially with freqeuncies higher than 500 kHz.  It also allows the ulstround to be detected at different positions.  We built a C-scanning system, depicted in the diagram, that passes the beam through the water twice.  This provided three strong signals, a direct wave from the transducer to the beam, a reflected wave from the sample surface, and a transmiited wave. 

Fig4: An underwater GCLAD setup used for sensing ultrasound in materials. An X-Y scanner moves the material in front of the detection point.

The C-scans below were built up by recording a single trigger event at each position for the X-Y scanner.  The amplitudes of each wave crossing were recorded as a function of position, making the C-scans.  The material in this case was a polymer-carbon fiber composite.

Fig 5: Images from the C-scanning of a composite plate.  The first is an optical image.  The next three result from measuring the amplitude of the ultrasound wave as it crosses the laser beam.

Lately we have been investigating the sensing of ultrasound in super-heated fluids.  To meet this goal, we directed a laser beam through a beaker of tap water that was heated to boiling from below.  The graph below shows the ultrasound waves recorded from different types of events.

Fig 6: Ultrasound waveforms recorded in a boiling beaker by optical beam diversion (also called Liquid-coupled laser acoustic detection).