LASER modulation

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ameyambekar

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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 ....
 
Here is a simple PWM transmitter using a 555 and simple receiver to send and receive an audio signal.

To got 1.5 to 2 km you'd like need a more powerful laser and perhaps some optics to focus the beam.
 
ameyambekar said:
thanx for the help ,but can you give me some more info about LASER modulation tech ....
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Lasers are typically modulated by simply turning them off and with whatever digital pulse code you want to use (PCM, PPM, PWM, etc.). You design a driver to switch the laser on and off at it's rated current with the driver input being the pulse code.

At the receive end you detect the laser on/off signal with a photo detector and amplifier, and decode the signal to recover the original signal information.
 
crutschow said:
But they are they able to use lasers to commuicate over tens of kilometers free space (see this for example).
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The link I posted to Clint Turner's work shows two-way voice/data communication over a 173mi path. Clint presented a paper at IEEE, and blew away the previous attempts using lasers.

I go hiking with Clint and the other folks pictured in his article. I will be flying Clint and his demo to Boise for **broken link removed**later in the month.
 
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That was a great read, thanks for the link.

Did I read that right, he used no carrier frequency and just DC modulated the LED with the audio? Wow. That sounds a little masochistic.
 
Clint uses Fresnel lenses, about 18" in diameter. Aligning the transmitter and receiver is a non-trivial problem.

The secret to the long paths is the low noise detector, the wide beam, the relatively broad frequency spectrum of the LED light vs the laser.
 
also you may want to check out convergance of the beam and how it reacts w/ heat (waves), air desitys (atmospheric conditions), just to name a few
 

Yeah I saw the photos of the transciever construction, very cool. And read about the DX trials across the salt plains. Definitely worth the read, thanks for the link.

I'm still stumped why an RF guy didn't modulate the LEDs with a carrier and use a high gain bandpass receiver tuned to the carrier freq, it's a close analogue to an RF application and I thought it would have been instinctive?
 
AFIK, he did try modulating the beam in the 100s of kHz to low MHz range, which was in turn FM or AM modulated with the voice, but it didn't substantially improve the SNR.
 
Thanks Mike that's very interesting. I just would have assumed FM modulation and band pass receiver would have really improved the SNR since the noise would typically be amplitude based.

Maybe his carrier freq was too high, it's hard to modulate high power LED cleanly at those freq's I would have used a nice low freq maybe 30kHz.

Anyway that makes his project even more impressive that he got those distances with only an audio modulation of the LED brightness!
 
Most of my fellow amateur radio operators that have played with long distance laser comms have found it not very good after a few 10's of kms. What they have found is that non coherent light from a standard LED or light globe does better than the coherent light from a laser.
Here's a youtube to the lates escapades by some amateurs in the UK thay have just set (6th April 2011) a new UK distance record using red Luxeon LEDs of 111km !! cool!!

https://www.youtube.com/watch?v=gD0OJrYFIVA


Dave
VK2TDN
 
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Mr RB said:
Did I read that right, he used no carrier frequency and just DC modulated the LED with the audio? Wow. That sounds a little masochistic.
Click to expand...

Actually, I did a little at-home project kinda like that. All i did was transmit audio signals through an LED onto an LDR which was hooked up to a 386 amplifier. worked pretty well. Kinda fun, too. Havent done a laser yet...yet *evil grin*
 
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