New laser ultrasound technique can remotely image the inside of a person
news.mit.edu
news.mit.edu
This is different than ultrasound, which generates its own sound waves that are then reflected by the body back to the device.
It's novel, but it doesn't solve the problem of being "non-invasive"; you still need a sensor stuck to the body to pick up the sound waves.
>The researchers tested this idea with a laser setup, using one pulsed laser set at 1,550 nanometers to generate sound waves, and a second continuous laser, tuned to the same wavelength, to remotely detect reflected sound waves.
1. They did use different frequencies- 1540 nm and 1550 nm.
2. The measuring was done with doppler interferometry, so beats would not impact measurements. The sensor is only responding to the difference between two signals, not the intensity.
3. 10 nm wavelength shift is a much larger difference than anything they'd be measuring, so the only thing that would actually even "show up" (as noise) would be light at the same wavelength as the measuring laser. Any constant noise at the same wavelength as the measuring laser will be filtered out. Only slight frequency deviations would come through- like the kind from vibrations shaking the laser.
> For the LUS system reported here, the 1550 nm LDV was specifically selected to maximize permissible optical backscatter from the skin while maintaining safety.
> Thus, the 1540 nm optical source was selected to maximize the converted acoustic source amplitude while remaining within the safety limits.
They wanted lasers close in the range of 1500 nm, but just over that wavelength has better safety limits. I am pretty certain those lasers were just chosen because that was what was commercially available in the narrow range of frequencies they wanted. The source laser is a Q-switching laser[2], which is needed to produce those pulses.
It doesn't particularly matter what wavelength they used (~2000 nm also would have worked); the doppler (aka self-mixing[3]) technique they used is very highly tolerant to noise.
[1]: https://www.nature.com/articles/s41377-019-0229-8
[2]: https://en.wikipedia.org/wiki/Q-switching
[3]: https://en.wikipedia.org/wiki/Self-mixing_laser_interferomet...
https://en.m.wikipedia.org/wiki/Optical_coherence_tomography
It’s already a common diagnostic tool used for patients who are at risk for glaucoma. By creating a 3D cross section of the eye’s retinal structures, it lets physicians detect early damage from the disorder, hopefully before any vision loss.
This is oddly related to what I was just watching. It was a SciShow episode that discussed how phonons which can be generated (come from??) sound waves. The phonons may be affected by gravity in the reverse way than all other matter and may even repel matter.
Phonons it seems are not just sound. The MIT website explains "phonon is just a fancy word for a particle of heat."
Anyway it seemed somewhat related especially "The resulting mechanical vibrations generate sound waves that travel back up". But I guess the researchers in the article may not have meant the sound waves literally go up.
Perhaps something similar could be done to reduce noise, using an empty patch of ground nearby the target.