I am currently constructing a laser microphone using 5mW red laser pointer source based on the idea below, not directly output to a speakers or headphones, but the signals must be fed into computer for further analysis.
I am not satisfied with the detection area being too small and I have a 1V solar cell by now since it has been suggested that solar cell could be another choice as laser detector too. I just take in the raw output from the solar cell and fed into laptop using a 3.5mm aux audio cable.
The problem I am currently facing is the high noise content and 50Hz harmonics spikes when I check with the spectrum of recorded audio (sometimes it working really well as clear audio can be heard)but I am thinking of better and much more robust alternatives for my setup.
Photodiodes has come into my mind but I saw that they are not suitable in daylight conditions. I would have to use optical lens to concentrate the laser beam onto the photodiode surface.
So my question is If I am going to use solar cell as laser detector, what can I do to improve the responsivity and reduce the inherent noise? OR If I am going to use photodiode as main detector, what kind of photodiode which has fast responsivity, optical setup should I use, and any other possible issues I should take care of?
P/S : I do aware of the 50Hz noise may come from powerline interference but it still exists even it does not connected to AC power sources. I also make sure I turned off the lights and perform recording at dark environment. I did use high pass passive filters consists of resistors and capacitors which have cutoff at 92Hz. The effect is not really that good but at least it can attenuate the 50Hz noise.
Ok it has been a month but I here to say that not be able to recover the audio signals from modulated laser beam. The laser beam itself is modulated, but even with offline processing using the bandpass filter, I could not obtain any results after demodulation.