Exploring the Modulation of Laser Light on Solar Cells

[mishima]

Hi, we set up a cheap laser pointer bouncing off a mirror that was hot-glued onto a balloon stretched over the end of a coffee can. The reflected light hit a cheap 3x5 solar cell that was plugged into a stereo's microphone jack. A person could speak into the can and their voice would be heard on the stereo.

I don't really understand how this works. Is this AM? Solar cells produce a voltage based on the intensity of light that falls on their surface, so the laser's intensity must be changing with the vibrations of the mirror/balloon/sound. But why would intensity change by being reflected and bounced around? Is there some more complicated sort of interference going on?

Or is it FM? Do the vibrations add/subtract to the laser's frequency? Wouldn't that change the light's wavelength? Frequency is related to photovoltaic effect so this would make more sense...

 

[NascentOxygen]

Did you try this in a darkened room, so you could let the reflected beam shine onto the wall instead of the PV cell? How large was the pattern it drew on the wall as you spoke?

 

[mishima]

Large enough to see the circular shapes, lines, and patterns resembling lissajous figures. But the shape itself wouldn't be causing anything with the changes in voltage the solar cell outputs. I guess it just signs that there must be 2 frequencies interfering...or wait, lissajous figures are about phase differences not frequency...that must be it, phase modulation. The mirror is changing the phase of the laser in step with the vibrations.

 

[NascentOxygen]

Large enough to see the circular shapes, lines, and patterns resembling lissajous figures. But the shape itself wouldn't be causing anything with the changes in voltage the solar cell outputs.

I'd like to see that myself, it sounds an interesting investigation. Was the audio clear enough to be understood?

Taking account of the distances and the way the beam angles out, can you estimate whether the wavering pattern drawn by the laser was of sufficient size that it must have been spilling off the sides of the photocell?

 

[mishima]

The audio was very clear, but spotty and seemed to have too much bass. At times it would indeed spill over the sides of the cell; I don't recall that having an impact on the sound quality.

We tried a student's laser pointer and my presentation laser. My presentation laser apparently had a circuit that would PWM the laser to save battery, because a direct hit would give a steady audio tone (Morse code could be done). The student's laser did not have this feature (and sported a fancy rechargable Li-ion battery), and gave a low hum/buzz similar to just shining fluorescent lights on the cell.

We do not have nice optical equipment so our setup was crude, with ringstands and testtube holders. There would at times be vibrations. Also interesting, we did try roughly collimating it with a 5 cm collimating lens. Doing so would severely distort the signal present when no one was speaking, and the audio resembled feedback...but it too was spotty. The last thing we tried was connecting multiple cells together in series electrically, and in a 2x2 square physically (they wanted to go for distance). This seemed to have a positive impact on sound quality but we did not quantify it in any way.

 

[256bits]

It sounds like you have built yourself some sort of a laser listener, or laser microphone.
There must be some more info available on the modulation of the light when it hits a vibrating surface.

Bell used modulation of light way back in the 1800's to transmit audio and it worked for him too.

Radio electronics had an article on a laser listener, but they used a photo transistor rather than a silicon cell.

https://ia801704.us.archive.org/0/items/radio_electronics_1987-10/Radio_Electronics_October_1987.pdf
Page 39 of the PDF.

 

[Windadct]

This technique has been used (seriously) by spies, reflecting a laser off of a window, and then the receiver can use the reflected laser beam to "hear" what was being said in the room.

This is not AM or FM - the angle of the laser is being modulated - and the solar cell is picking up the movement enough to act as a transducer. You are not affecting the phase of the laser. As far as why- the solar cell is probably not that consistent of output across it's surface, also as the laser moves from one location to another, the NEW location likley has a slightly higher output V - and the longer the laser is in that location the V drops off a little. The input of the stereo is likly doing some filtering - by only seeing the ac ( signal) portion of the Cells output, and blocking the DC.

In the case of the presentation pointer - it is using a laser diode and a driver circuit that pulses the laser. The tone you hear is the frequency of the pulses used in the driver circuit.

Still -- pretty cool experiment..

 

[mishima]

256bits:
Thanks for that great article. They were shocked to see that people were doing this stuff in the 1980s, and want to try for long range using a receiver like in the back page.

Winadct:
I can just point the laser directly on the cell and get a low buzz, and I can "scratch" the cells surface with the dot to make a tearing noise. But I guess I don't understand how, if I'm not affecting the phase of the laser as you say, the audio is encoded onto the light. Are you talking about the geometric angle of reflection being modulated, not phase angle? If that were the case could I not replicate any sound by clever choice of pattern drawn onto the cell? I'm not doubting you just trying to understand what's really happening physically.

 

[Windadct]

The tearing noise is technically that.. noise - a random signal.

On the general concept I think you are getting my point - it is the movement of the light beam ( laser or otherwise) across the face of the solar cell - not a modulation in the typical AM or FM sense. The overall effect will not require a laser, you can try this by attaching a mirror to the balloon, and using a shaft of sunlight, and reflect it to the solar cell.

For example there is this project... Light Beam Receiver

 

[mishima]

That's interesting, I still don't really get it. On that same page through the link "Voice on a light beam" they use the term "amplitude modulation" in the "how it works" section. So I am rather confused.

I suppose what I'm confused about is what's happening on the solar cell's surface. It's the photovoltaic effect, all locations on the surface are not created identical. Scraping the surface with a laser dot outputs the noise (difference in surface locations) as an AC signal. I'm with you there, I believe.

What is the correlation between the sound being transmitted and the instantaneous position of the laser on the solar cell's surface? I don't understand how there can be one. I can move the mirror slightly to cause the center of the laser dot's oscillations to be 1 cm to the right of its previous and hear the same audio signal coming from the solar cell, for example.

If it is the geometric reflection angle being modulated, why is a certain Δθ a certain audio note? Why if I turn the mirror this specific way do I get a B flat, for instance. I'm overlooking something...I guess I need to forget about normal types of modulation.

 

[NascentOxygen]

While I could speculate on how the beam's movement across the cell surface gives rise to an audio signal, I think we should invite PV researcher @toutiao to give us an expert opinion. ☺

 

[Windadct]

With the Light Beam Receiver - it is not using a laser. The total light hitting the Solar Cell is being modulated, so the output of the solar cell is amplitude of a DC signal being modulated. - however the overall brightness of the light is not.

If you are using a laser - the same thing. Moving the laser beam on the balloon does not affect it's amplitude or modulate its wave / frequency.

One thing that may help - if you can find one is look at the "signal" from the solar cell with an oscilloscope. The solar cell is not as pure of a on / off device as you may think and the laser is a much more intense than the solar cell is designed for - so most of the cell only sees the ambient light, and a spot of the laser is "out of spec" - basically any change on a solar cell surface has an impact - temperature, brightness, etc. Just the act of moving the dot around on the surface is creating a changing output (signal) from the cell.