1. You say I should take an inrfared LED or an infrared laser, and pulse it on and off with a square wave, not sine wave at maybe 400HZ. This beam should be aimed at an old buried metal target. Correct?
Square wave, 50% duty cicle, from a 555. Yes, and his very precisse. No much distance with common IR leds. When you pick a beep in movement, you search for the precisse point.
2. If I understand your idea correctly for sending a beam to the long time buried metal, then my question is: How do I measure phase shift? Do I look for a change in the phase of the transmitted light beam frequency? or do I look for a diffenrence between the transmitted and the reflected beam? I would think a reflected beam will show very close to the same time as the beam sent when the target is at close distance.
I think occurs a variation in height between the tone present in a receiver (this is filtrate as an interference in ANY sensitive system like a milivoltmeter). YOU DON'T NEED IR leds receiver. Also this filtrates in FM reveiver, and the signal of the target is directly in the beam or beams, no in the telescopic antenna, for example. But today maybe is not possible in this band (FM).
3. In order to measure a phase shift, I would think it is necessary to use a very fast IC to see the small shift in a light beam time. Am I correct? Do we need to use methods similar to radar ranging techniques? What kind of circuit do you use to see the phase change?
Maybe you're correct in this, but never I use sophisticated ICs. Don't know if this is or not similar to radar. The height of the tone changes. A simple adjustable comparator with audio generator.
4. Also, has this phenomena been seen in different light frequencies besides the usual infrared diodes and lasers? Do you know of any experiments with the red lasers or the 532nm high power green lasers?
No experiment with this. Once I use a common (cheap) Chinese laser pointer modulated at very low freq., and found button at 20 cm depth. Maybe the modulated frequency isn't the most important regarding the wavelenght of IR and laser in microwave regions. Metals reflects microwaves, and sure emission in vecinity, this is, IR.
|