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-   -   Voltage Controlled Variable Delay? (http://www.physicsforums.com/showthread.php?t=26035)

 Pfft May17-04 11:11 AM

Voltage Controlled Variable Delay?

How can I introduce a voltage controlled variable delay in the path of a mono-chromatic light beam? That is, how can I phase modulate the light?

 faust9 May17-04 11:37 AM

So, are you just trying to turn a light on/off with a slight delay based on a voltage input? How accurate does this need to be? PIC's, AVR's H8's and many other microcontrollers have built in 8bit ADCs which could easily be used as voltage controlled delay circuits. A 12 series (tiny 8 pin 12C671) PIC running with the internal oscillator could implement a simple delay using a handfull (less than 5) components.

 Pfft May17-04 03:33 PM

The source of the light is a laser and will not be controlled. What I need is a device which will vary the velocity of the laser beam passing through. That device will be voltage controlled.

Such a device might be a first surface mirror made with a piezo electric crystal such as quartz. As the voltage is varied, the thickness of the mirror will vary and the path length of the reflected laser beam will vary. The net effect will be a phase shift at the distant receiver. There are mechanical problems with such a solution, and I would prefer to identify some liquid crystal which will vary the velocity of the light beam while passing through.

Pfft

 enigma May17-04 05:05 PM

I'm pretty sure you can rig up a 555 timer to be variable time with voltage.

 Averagesupernova May17-04 08:22 PM

You want to phase modulate the atual frequency of the light? In other words, the color? Or is the laser light already pulsed and you want to vary the phase of the pulses mid-stream?

 Pfft May18-04 01:06 AM

The frequency of the light wave, the color if you wish, will vary during the shift. Once the shift has been completed, the "color" would return to the original value. This is a direct parallel to phase modulation of a radio wave.

Think of it as two optical fibers passing through the room. You cut one fiber and insert the device. In the next room, the lights from the two fibers are compared and the phase difference is noted. The color change is only there while the phase is being shifted. Once shifted, if the shift equals one wave length, it would be impossible to detect the change.

 enigma May18-04 01:24 AM

Uhm... yeah... disregard my previous post. I misread what you were asking for.

 Averagesupernova May18-04 12:16 PM

What is the frequency of modulation?

 Pfft May19-04 02:39 AM

Quote:
 Quote by Averagesupernova What is the frequency of modulation?
My face is red!

Ten megahertz for starters. (Sorry, I made a mistake in calculating the delay. Let me get some sleep, and I will run it through again.)
Incidentally, the light source is continuous, not pulsed.

 Njorl May19-04 08:02 AM

30 microseconds?!?!?!

That is nuts for light.

You'd need to physically switch light into different delay lines of optical fiber.

You could use a planar waveguide imaging splitter with an absorption modulator on each output. Hook up a variable length fiber to each output. Only allow light to pass through the channel with the appropriate delay. It would not be true phase modulation. It would have digitization error. You're not going to get that kind of variable delay electro-optically, or with any single channel, physical device.

Just looking at your reqs (30 microsecond delay at 10 megahertz) makes me think you don't really know what you need.

Njorl

 Pfft May19-04 03:30 PM

I am very very sorry for the error. The delay is not anything like 30 microsecs. I was horrified when I reviewed my message.

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