3 ms·

How does it help to have the sensing laser at the same wave length as the pulsing laser, if the pulsing laser is triggering sound waves which have a different w

How does it help to have the sensing laser at the same wave length as the pulsing laser, if the pulsing laser is triggering sound waves which have a different wavelength?

TeMPOraL 7y ago
I'm guessing in order to achieve the same body penetration characteristics on RX and TX.
jcims 7y ago
If they had two lasers of similar frequencies they would get interference beats/aliases in the data. If they use the exact same frequency (eg by splitting the source) it shouldn’t show up. Should allow for less stable/expensive laser sources too.
- hwillis 7y ago
  > If they had two lasers of similar frequencies they would get interference beats/aliases in the data. 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.
hwillis 7y ago
The source laser was 1540 nm, the sensing laser was 1550 nm.[1] > 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 https://www.nature.com/articles/s41377-019-0229-8 [2]: https://en.wikipedia.org/wiki/Q-switching https://en.wikipedia.org/wiki/Q-switching [3]: https://en.wikipedia.org/wiki/Self-mixing_laser_interferometry https://en.wikipedia.org/wiki/Self-mixing_laser_interferomet...

How does it help to have the sensing laser at the same wave length as the pulsing laser, if the pulsing laser is triggering sound waves which have a different wavelength?

TeMPOraL 7y ago

I'm guessing in order to achieve the same body penetration characteristics on RX and TX.

jcims 7y ago

If they had two lasers of similar frequencies they would get interference beats/aliases in the data. If they use the exact same frequency (eg by splitting the source) it shouldn’t show up. Should allow for less stable/expensive laser sources too.

hwillis 7y ago
> If they had two lasers of similar frequencies they would get interference beats/aliases in the data. 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.

hwillis 7y ago

The source laser was 1540 nm, the sensing laser was 1550 nm.[1] > 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 https://www.nature.com/articles/s41377-019-0229-8 [2]: https://en.wikipedia.org/wiki/Q-switching https://en.wikipedia.org/wiki/Q-switching [3]: https://en.wikipedia.org/wiki/Self-mixing_laser_interferometry https://en.wikipedia.org/wiki/Self-mixing_laser_interferomet...