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LASER modulation

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ameyambekar

New Member
hi!
im doing my final year project on LASERS . rough idea behind the project is to modulate the LASER beam by a CODE and send it across some distance .... can anybody tell me what is the tech to modulate the LASER.

also can anybody tell is it possible to send such code at a distance of 1.5 to 2 km and how dose the laser behave at such distances ....
 

Dr_Doggy

Well-Known Member
shine the laser out the window, how far does it go? if the beam is straight it goes on for miles (far enough to blind air planes)

If you are going to do this though you may want to modulate it @40khz (for several reasons)

Circuit would be similar to one that flashes LED's......
 

trash

Member
Sam's Laser FAQ is the bible of lasers.

Lasers are much like any other source of EM energy. One thing to consider first is what kind of laser you're modulating. A YAG is a little tricky to modulate where as a semiconductor laser is relatively simple.

By far the simplest modulation is OOK, On-Off Keying. Basically Morse code. It's a simple way to send information but it has limitations.

High speed semiconductor lasers used on optic fibres use AM. The mark and space are different amplitudes, the laser is never turned off.
So a Mark may be 90% the output power and the space 20%. If the laser is turned off, it has a time to recover. I'm not up on semiconductor laser physics so I don't know the finer details of why it takes longer to turn on and off.

So to modulate them, you can treat them like modulating an LED with a class A amplifier.

Now depending on where you're using the laser, you may find that the signal is swamped by sunlight or other ambient light.
A subcarrier is often used to help overcome this. Some examples are a TV remote control. It just doesn't use OOK to send data to the TV,
it modulates that data onto a 38kHz subcarrier which together modulate the laser. One could call it an AM^2 system.
Police LIDAR also use a very similar system. I can't remember the subcarrier frequency, but if you own a lidar detector, you can modulate an IR LED and change the frequency till the detector alerts. From memory it was close to the remote control subcarrier, about 32kHz.


With gas lasers like a HeNe, there are a few more options open to you, but the laser itself is harder and more dangerous for newbies to work with.
A couple of thousand volts is enough to discourage most people. Even I don't like modulating the current driving an HeNe tube. Things flash over and catch fire :)

You can RF modulate a HeNe like some other gas lasers. A coil is wound around the tube and the magnetic field modulates the current in the tube and the RF signal is modulated onto the beam. The RF signal is recovered with a standard optical receiver like a diode and then that RF signal is amplified and demodulated like a standard radio signal.

It is possible to FM modulate a laser. I've forgotten how they achieve it. I think it was using a semiconductor laser and while the laser had an inherent FM component even when it was amplitude modulated. To recover the FM signal they treated the signal much like an RF signal and mixed a local oscillator (another laser) with the incoming signal in a detector diode which was then demodulated with a ratio detector circuit etc.

2km with a laser is relatively simple since it's a line of sight path. Sunlight swamping your detector circuit will be the main problem.
Using hoods and filters is the easiest method to keep ambient light out. If you're using the link at night, it's much much worse. Street lights flood the detector and it's not just the light saturating the detector, but that light itself is also modulated with a 50Hz or 60Hz signal. The power line hum can be very annoying.
It can be removed with a 50Hz notch filter or a high pass etc.

You can test your circuit across an ordinary room using an LED and an IR detector diode.
You'll find much the same problems in the room at low power as you will with a laser across town.
 
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