Laser microphone help

[masnoob]

Hi everyone!
I am currently working on a laser microphone project, inspired by this: Laser Microphone



The diagram above is my entire setup. I am using a cheap 5mW red laser pointer (SYD1230, if that matters).
The idea is to shoot the beam towards vibrating surface (due to sound) and capture the reflected beam using the photodetector.
In order to save the hassle of adjusting and realignment of laser beams onto the detector surface, I have chosen solar cell - 1V rating, which has a nice surface area in exchange of response time due to its larger capacitance.
I just connect the solar cell output to passive RC high pass filter, and obtain the output using 3.5mm AUX cable into my laptop to record the audio signal using Audacity.
The audio I obtained is quite noisy in general due to 50Hz harmonics, and majority low frequency noise (maybe some unknown noise source included).
I try to filter the audio but the raw signal is way too noisy itself, even a sharp roll-off filter also not viable. So I am currently thinking of some way to improve the raw captured signal quality using laser PWM modulation, as suggested by someone else.
I have saw many projects using lasers as means of audio transmission, which is totally possible and audio quality is acceptable in my project.
Based on the idea, I have refer to circuit schematic found online, and tried to modified some parts of it.
I going to compare DC and triangular waveform using LM311N, and using the output as a switch to turn laser on and off in a carrier frequency rate of 15kHz.



(I use LED to represent laser pointer since I couldn't found the symbol in Multisim)


There are some problems arises here:

1: What happens when a PWM modulated laser shoots on the vibrating objects (windows or any possible reflective surface which vibrating at distinguishable frequency?)

2: If I am going for this setup, what are the possible means to demodulate the raw captured signal, i.e., to separate sound from 15kHz carrier signal?

3: As described by the

article​

here, I would still like to use solar cell as my detector. How can I possibly enhance the detection efficiency and reduce the distortion rate?

I was not going into those sophisticated equipment like mixers or anything else as I want to improve the raw signal to noise ratio without spending too much of my budget.

Thanks for looking at my long post

 

[diachi]

What is your audio source? Is it a speaker with a reflector or someone's window half a kilometer away or something in between? Could just be that there's a lot of noise from your audio source and there's not much you can do on the laser end to reduce that. Could be atmospheric noise if it's over a long distance, could be vibrations at the audio source. Need more details about what you're trying to do.

 

[Encap]

Here is an article with circuit diagrams of a system that apparently works pretty well http://web.mit.edu/6.101/www/s2017/projects/jalverio_Project_Final_Report.pdf

 

[masnoob]

What is your audio source? Is it a speaker with a reflector or someone's window half a kilometer away or something in between? Could just be that there's a lot of noise from your audio source and there's not much you can do on the laser end to reduce that. Could be atmospheric noise if it's over a long distance, could be vibrations at the audio source. Need more details about what you're trying to do.

Hi the audio source would be a speaker playing music behind a reflective surface (CD) attached to it. I doing it within 2 meters.

 

[masnoob]

Here is an article with circuit diagrams of a system that apparently works pretty well http://web.mit.edu/6.101/www/s2017/projects/jalverio_Project_Final_Report.pdf

I have saw that few months ago but I dont understand why author perform PWM modulation at the receiving side ? Not on the transmitting side. Plus it using photodiodes as detector unlike mine, so I could not judge whether his setup is compatible with mine

 

[Benm]

Using a carrier beam may be good to isolate your signal from background noise, especially when doing this over a large distance.

Basically this will help you isolate light reflections from other interfering light sources in the beampath, so not a bad idea.

Where it goes wrong is in your choice of carrier frequency: that should be much higher than any audio you are listening in on. If you want to do this, you may look at >1 MHz or so carrier frequencies, isolating your return signal from any natural noise.

In historic designs they used a steady laser to do these things, but in modern times it's not a challenge any longer: things like laser diodes and photodiodes/transistors are readily available that can operate at higher frequencies, a few MHz certainly not being a problem. Another upside is that you can control signal timing better if you want to isolate your spying operation devise against random noise put onto windows in a tempest scenario

 

[masnoob]

Using a carrier beam may be good to isolate your signal from background noise, especially when doing this over a large distance.

Basically this will help you isolate light reflections from other interfering light sources in the beampath, so not a bad idea.

Where it goes wrong is in your choice of carrier frequency: that should be much higher than any audio you are listening in on. If you want to do this, you may look at >1 MHz or so carrier frequencies, isolating your return signal from any natural noise.

In historic designs they used a steady laser to do these things, but in modern times it's not a challenge any longer: things like laser diodes and photodiodes/transistors are readily available that can operate at higher frequencies, a few MHz certainly not being a problem. Another upside is that you can control signal timing better if you want to isolate your spying operation devise against random noise put onto windows in a tempest scenario

Thanks for replying, I do understood the modulation does help us to isolate the signal content from background interference by mixing carrier frequency with content frequency. However if I use a carrier signal frequency of more than my computer sound card sampling rate of 44100Hz, I could not reconstruct the audio signal from it. Another thing I am interested is how does the laser beam get modulated when I shoot it to a reflective vibrating surface ? via AM or Doppler shift phase modulation?

 

[Immo1282]

Thanks for replying, I do understood the modulation does help us to isolate the signal content from background interference by mixing carrier frequency with content frequency. However if I use a carrier signal frequency of more than my computer sound card sampling rate of 44100Hz, I could not reconstruct the audio signal from it. Another thing I am interested is how does the laser beam get modulated when I shoot it to a reflective vibrating surface ? via AM or Doppler shift phase modulation?

It's possible to use a high frequency carrier signal if you filter it with electronics instead of software - design a low pass filter with a break frequency of 50 or so kHz and it'll kill your recieved carrier signal before it even gets to your sound card.

 

[masnoob]

It's possible to use a high frequency carrier signal if you filter it with electronics instead of software - design a low pass filter with a break frequency of 50 or so kHz and it'll kill your recieved carrier signal before it even gets to your sound card.

Ok thanks for reply, I did include RC filters before it goes in to sound card, will consider ur suggestion. Thanks for replying

 

[Anthony P]

There are several detector circuits that will work in "The Laser Cookbook" by gordon McComb. A phototransistor or large clear red LED makes good receiver. Place it at focal length of large diameter convex lens. 2 meters may be too close. Background noise will easily block reception (dishwasher, vacuum cleaner etc.). Use battery power, not line power. Keep in mind that there are a couple of errors in Cookbook. A 741 op amp takes dual power... V+,V-, and ground. Absolutely nothing should connect to V- except op amp power pin. Also, use a better quality audio amplifier than the 386.
All that being said... check local laws. I am pretty sure an IR diode set up is illegal. Any variation may be illegal without consent.

 

[Anthony P]

edit. PWM circuit is in "Cookbook". It WILL clean up reception greatly.

 

[paul1598419]

A laser reflected off a surface, like a window, will only move with the vibrations. This will not get you a laser modulated with these vibrations. You will need a detector that the beam can move onto and off of to get the modulation you are looking for. Since these vibrations are very small when compared to a mirror attached to a speaker cone, it will only move slightly, so you will have to take this into consideration. It may not turn out to be a workable idea for listening in on other people.

 

[logsquared]

I would try adding a sharp optical bandpass filter corresponding to the laser wavelength in front of the detector. This should help reduce ambient noise pick-up.

 

[Anthony P]

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.

 

[Singlemode Laser]

Hi, this is an interesting project and I think there is space for improvement. First the general scheme:

Measuring something at DC is in general a bad idea if the signal to noise ratio is bad. Moving to a high frequency by modulating your laser removes low frequency noise from your system. To do this you have to demodulate the received signal with a mixer. You can buy ones pretty cheap or solder a simple one yourself.

Besides having a much more sensitive setup, you don't have to worry about stray light (it is not modulated at your frequency in your setup as long your detector is not saturated. In this case an optical band pass filter or laser diode/photo diode combination will help.

Another important part is the right choice of the detector. A fast photo diode with a low current noise will do best.

This priceless book covers everything you will need in this project and far beyond:

https://www.google.com/url?sa=t&sou...FjACegQIBRAB&usg=AOvVaw1fAbBYkSL1QXLOp_9Q2oqM

Singlemode

 

[masnoob]

Hi, this is an interesting project and I think there is space for improvement. First the general scheme:

Measuring something at DC is in general a bad idea if the signal to noise ratio is bad. Moving to a high frequency by modulating your laser removes low frequency noise from your system. To do this you have to demodulate the received signal with a mixer. You can buy ones pretty cheap or solder a simple one yourself.

Besides having a much more sensitive setup, you don't have to worry about stray light (it is not modulated at your frequency in your setup as long your detector is not saturated. In this case an optical band pass filter or laser diode/photo diode combination will help.

Another important part is the right choice of the detector. A fast photo diode with a low current noise will do best.

This priceless book covers everything you will need in this project and far beyond:

https://www.google.com/url?sa=t&sou...FjACegQIBRAB&usg=AOvVaw1fAbBYkSL1QXLOp_9Q2oqM

Singlemode

Thanks for replying, actually I prefer to perform demodulation using software. The noise from solar panel itself is very terrible, which would mask all audible audio completely. I have tested the solar panel which under maximum ambient light exposure only produce 0.74V. 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.
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!