laser reflected frequency

[Noodles141]

What i would like to know is if a laser is reflected off say a piece of glass and you had a sensor reading the reflection. how would the frequency change if say the glass was vibrating slightly. And if the frequency wasn't changing something must have changed with regards to the reflected beam of light, what would that change be?

 

[kubeek]

Generally I would say that the direction of the reflected beam will change.
If the beam is perpendicular to the surface, then you get interference.
you probably want to look at this site http://williamson-labs.com/laser-mic.htm

 

[DerStrom8]

Yup, laser mics work on this principle, but it has to be extremely precise. It's also not very clear-there's a lot of interference, but it is possible.

 

[Noodles141]

Kubeek thanks for the link i'm sure it will come in handy when i start building, should be able to do it with a bit of luck and hard work. Just a fun project i thought i might take up

 

[JMW]

Yes it will change, and it is measurable and usable. Doppler laser for police departments use a similar technology for the speed guns, but much lower frequency than audio.

 

[Mr RB]

I think it would be way more sensible to use AM detection of the received laser light. Especially at a "hobby" level.

 

[unclejed613]

for doppler or FM you need an RF source to modulate the beam, 100Mhz for instance. a PLL connected to the receive side gives you an error signal output on the VCO control voltage, which can be slowly varying DC for a doppler speed radar, or audio for a laser mic. it's very easy to get a reference signal, because the RF for the transmitter is generated locally. this is a bit more complicated than just tapping the audio off of the photodetector (AM demodulation of the light beam). in the AM detection method, you take advantage of the movement of the target changing the deflection of the beam, and therefore the amount of light returning from the target. narrow optical filters between the target and the photodetector will reduce the amount of noise from unwanted light sources (sunlight, fluorescent lights, etc...)

 

[Noodles141]

Do you think i could use any old laser to do this, with just knowing the frequency it puts out and using that and one of the detection method you guys suggested to make it work. wanna make it as simple as possible so its some what portable if possible but if its not i wouldn't mind, the main point is to get the idea to work, thats whats important to me.

 

[kubeek]

Normal laser pointer doesn´t put out any frequency, it just makes light at a specific wavelength (color) but that doesn´t matter unless you´re into the interferometry approach.
For FM and doppler you need to modulate the laser, for AM you just measure the difference.
See bkraz´s video on this, with some practical advice. https://www.youtube.com/watch?v=1MrudVza6mo