laser FM detector

A moving speaker cone won't be able to make a very good signal for you to work with.

If you take a sample of the unmodulated laser with a beam splitter you can send that light to the photodetector along with the modulated light. Interferance at the photodetector could then turn a phase modulation into an AM modulation at the photodetector.

How can a photodiode change the frequency of a signal from a signal generator?

John

Is this supposed to be FMing the lasr freq? All it's going to do, is change the angle of reflection of the beam, changing to strength of the beam hitting the detector, no?

A simple photodetector responses to the strength of the signal, not the freq.

I think we're all stumped.

If you are doing FM by doppler, look what deviation you can get. If you have a speaker with 1 mm peak-peak excursion, and it is working at

1 kHz, you will have 1 m/sec peak velocity. The relative change in frequency will be 1 m/sec divided by the speed of light, or about one part in 300 million times the frequency of the light. Since photodiodes don't respond at light frequencies, the slope detection has to take place before the photodiode, which would require an extremely selective optical filter. The only chance you would have of doing it would be to detect the signal before the photodiode with the unmodulated laser beam, as MooseFet suggested.

--
John

Actually, 1 mm peak excursion --> 1 m/sec peak velocity.

-- John

A simple way of doing optical communication is to connect a small audio amplifier across the battery in a laser pointer, using a capacitor in series with one of the leads. This produces mostly AM, which you detect with a photodiode.

It's quite possible to do the measurement you're talking about, but you'll need either an interferometer, which isn't too hard to make but will require some fairly significant care in alignment.

Put a little spot of retroreflecting tape on the speaker cone where the laser beam hits--it'll increase your signal by at least 40 dB electrical, which is a big help.

Another possibility is using an optical lever--mount a small mirror on the edge of the speaker cone so that it tilts when the cone moves, and put the photodetector at a distance, positioned so that half the beam hits it. Then speaker vibrations will cause the beam to move partly on and off the detector. You can improve this very much by using two photodiodes next to each other and subtracting the photocurrents (i.e. wire them in series between + and - bias supplies--with the right polarity!--and take the output from the middle).

Cheers,

Phil Hobbs

You use a thin one and work at too long of wavelengths for the detector. If you apply a forward current to the PIN junction, the carrier density in the diode changes its index of refraction and speed of light. By modulating the density, you can phase modulate the signal passing through. If you integrate the modulation signal, you have frequency modulation.

I hope this helps :)

I think I haven't made myself clear on that. I know you got the gist on what I'm trying to do. But the thing here is I'm not trying to do a "true" frequency modulation (since it's utterly difficult in my configuration), but a sort of "emulation" of it. The frequency change happens through the change of distance of the laser beam. After that beam passes through a prism, the beam changes the angle.

Probably if the laser beam distance is short, then we wouldn't see much difference in angle, but what if it's about a hundred meters? (Sometimes, I wish I can draw diagrams, so you guys might know what I'm trying to do. Heh.)

Yes, but the photodetector (in my case a diode) is there to [supposedly] change the frequency of a pre-made signal using the strength of the beam signal it gets. Beam strength =3D change in frequency (not the frequency itself). That's what I *wanted* to happen.

But what happens after the beam hits the photodiode? You still don't have a signal after that, right? They're just photocurrents.

Using a photodiode to modulate a laser is news to me. If I follow you correctly, you are making the laser appear to move back and front, i.e. appear to change the distance to the receiver. How great is this modulation in terms of distance?

When I first read this post, I had envisioned a laser beam moving from side to side, falling on a photodector with a tapered mask painted on it. As the beam moves, the position on the mask changes, and thus the output voltage due to the attenuation of the mask. Think of the mask as a V.

How do laser printers move their spot around?

I quite don't get what you're saying here, but if there's much distance from the reflected laser (from a prism of some sorts) to the photodetector, the laser would appear to move back and forth even wider. And the photodiode doesn't modulate the laser. The photodiode [supposedly] modulates a pre-made signal according to the position of the laser to it.

That's what I was planning to do with the photodiode: put a "V" mask on it.

I have absolutely no idea. I don't own one, and I haven't seen how it operates.

I'll just put clearer, straight-forward question, since I feel that we're getting off-topic here:

Is there a way to make an FM signal using an AC output and a photodiode? (AC output for the carrier, photodiode for changing the carrier frequency.)

Generally, but not always, they use a rotating mirror and the progression of the drum used to transfer the image to the paper. There are other techniques.

You had what sounded half way viable with your first paragraph - FM modulation of a light beam via a Doppler shift caused by moving a mirror - then you just need a way to demodulate it and recover the intelligence. And someone gave you a way to do that . . .

Photo diodes are for converting light to electrical signals, that's all they do.

Now you seem to be off on a different tangent combining some radio terms and trying to apply them to a/the laser transmission scheme.

You could amplitude modulate the laser with a sub carrier then modulate the sub carrier with an FM signal to carry the intelligence - that would work and use the usual radio techniques for signal handling.

Can you use an AC output to make and FM signal - yes. Can you use a photodiode to do it - no (unless perhaps you mean a "laser diode" when you say photodiode) Then there are ways to modulate it with FM but not directly since the laser diode is tuned to one wavelength.

--
Yes.

Use a VCO to generate the carrier, and get the deviation you need by
amplifying the photodiode output as required.

http://www.national.com/an/AN/AN-20.pdf#page=1

http://www.national.com/an/AN/AN-74.pdf#page=1

http://focus.ti.com/lit/ds/symlink/cd74hc4046a.pdf

That's the purpose of the interferometer

No, you have a detected signal, because the speaker vibration moves part of the beam from the + photodiode to the - photodiode. It isn't an FM detector, but it does detect speaker displacement optically.

Cheers,

Phil Hobbs

Oh now I see what Moosefet meant. That was baffling. I though he meant the photodiode could reflect the beam and modulate it.

This is what makes thing confusing. I thought the goal was to send a signal on the laser beam, i.e point to point communications. Now if the goal is to send information via a laser beam, then we have a different situation. Bear with me here.

Lets label the areas point A and point B. At point A, there is a speaker and a mirror. At point B, there is a laser system. Now I see how to make the laser system. Modulate the laser intensity at a high frequency (a few MHz), i.e. put a carrier on the laser. Split the beam. Sense the direct laser energy with one photodiode, and the reflected laser energy with another photodiode. The wiggling of the speaker will phase modulate the reflected signal.