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Laser microphone help

masnoob

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Dec 21, 2018
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Using LED as sensor will greatly reduce ambient noise, unless you just have band-pass filters laying around. Take a clear LED and connect it oscilloscope probe. Ambient light gives almost no response. Then shine a laser on it and see a voltage of 1.5 V or more.
This is a fascinating project that I did about 30 years ago with HeNe source (and no google access). However, as Paul pointed out, I doubt that it has use as practical tool.
"Information Unlimited" sells kits for this device. I only mention this because I look forward to hearing LPF members opinions of this company.
Been using Red LED as sensors and they do provide some voltage however that would require optical setup which is a problem to me
 



Singlemode Laser

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Jun 25, 2017
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63
Thanks for replying, actually I prefer to perform demodulation using software.
This will only work if your DAC has a higher bandwidth than your modulation frequency. There is a reason why radio and optical telecommunication use modulation techniques at high frequencies.

The noise from solar panel itself is very terrible, which would mask all audible audio completely.
This noise is at low frequencies and you could get rid of it by using a FM or AM scheme.


I have tested the solar panel which under maximum ambient light exposure only produce 0.74V.
If you think this is your limitation, then I would consider something else and not stick to it. Ps: if you are not biasing your solar panel than your bandwidth is very bad anyways.

In my final year project I would like to prioritize the reception surface area by making it as large as possible without degrading the performance.
The horrible bandwidth of your solar panels will get even worse by increasing the area. For sound you would need to detect frequencies up to 40kHz. Don't limit yourself by poor detector choice which will be your bottlenecks.



I did consider using the photodiodes, phototransistors initially but I would want to avoid doing realignment over and over again. I also don't have any knowledge in choosing appropriate optical setup to capture the beam if I go with smaller photodetectors. Will definitely read the book u posted for ideas!
You are detecting diffuse light anyway. It's about your signal to noise ratio, not the signal.

You should first crunch some numbers describing your setup before you take a path.

Singlemode
 

paul1598419

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Sep 20, 2013
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While modulating your audio signal at a high frequency AM or FM scheme, my impression from the OP was this was not going to be detecting audio in that way, but to detect it as a vibration of a window pane. It is not setup as, for instance, a 44 MHz FM modulated laser beam traveling through the air. If this is what the OP is trying to do here, then yes, this is definitely the way to go. You will get a much better signal to noise ratio and better selectivity with random light around your detector. But, this is nothing new at all. Radio signals have used these techniques for decades for these very reasons.
 

masnoob

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Joined
Dec 21, 2018
Messages
9
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1
While modulating your audio signal at a high frequency AM or FM scheme, my impression from the OP was this was not going to be detecting audio in that way, but to detect it as a vibration of a window pane. It is not setup as, for instance, a 44 MHz FM modulated laser beam traveling through the air. If this is what the OP is trying to do here, then yes, this is definitely the way to go. You will get a much better signal to noise ratio and better selectivity with random light around your detector. But, this is nothing new at all. Radio signals have used these techniques for decades for these very reasons.
Initially I just wanted to do a simple setup without any modulation scheme, enhanced detectors or whatsoever. However from most of the answers I obtained from various sites they suggested to do modulation.
(I not sure did they take a look at the links I provided in the main post, which is a detailed article by Lucid Science on Laser Microphones)
 

Singlemode Laser

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Jun 25, 2017
Messages
303
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63
While modulating your audio signal at a high frequency AM or FM scheme, my impression from the OP was this was not going to be detecting audio in that way, but to detect it as a vibration of a window pane. It is not setup as, for instance, a 44 MHz FM modulated laser beam traveling through the air. If this is what the OP is trying to do here, then yes, this is definitely the way to go. You will get a much better signal to noise ratio and better selectivity with random light around your detector. But, this is nothing new at all. Radio signals have used these techniques for decades for these very reasons.

As far as I understand the OP wants to detect the doppler shift of a vibrating surface to reconstruct the audio spectrum. This shift can be detected by an unmodulated beam or a modulated one. Since the OP stated that his detector has a high dc level already from stray light, he needs to improve the sensitivity of his setup. I totally agree with you Paul, AM / FM has been out there for decades and this has a reason, it is just a great way to improve your setup and the OP need a good reason not to do so.

Singlemode
 

paul1598419

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Hmm. It hadn't occurred to me that a Doppler shift would would be present in the movement of a widow pane, especially if taken at an obtuse angle. If this does occur, then one would indeed be able to modulate the carrier frequency imposed on a laser beam. I would think to get the best result, the angle would have to be as small as possible from the incident beam. This is something I will have to study further as I have not seen this type of Doppler shift to a modulated beam and need to learn more about it. Thanks, Singlemode.
 

masnoob

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Dec 21, 2018
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its been more than a week and I would want to report my progress.
I can record the 15kHz modulated signal very clearly however I could not been able to recover a single audio due to the fluctuating carrier frequency. Even if I able to, the signals was totally corrupted with only high frequency sounds in it
 




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