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Thread: Information transmission using a modulated laser

  1. #1
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    Default Information transmission using a modulated laser

    Hi everyone,

    For a project i'm doing i'm going to try and set up a free space data transmission using a modulated laser. I'm pretty confident on the theory in terms of how this can work i'm just not too sure on how to interface such a system with a computer. I'm planning to take a computer file and convert it to a binary string (1s and 0s) and then use this to switch the laser on and off. Then as a reciever have a photo-diode to measure the laser pulses and generate an identical binary string. This can then be converted back to the specific file type.
    I'm really not very experienced in programming or computer interfacing of this sort so if anyone has any suggestions then they'd be really really appreciated.

    Thanks in advance

    Paul

  2. #2
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    I remember years ago, when drlava lived near Case in Cleveland, he had a red laser set up to transmit TOS Link from from one place in his room, bounced off a couple of mirrors, over to his receiver for a Dolby Digital audio stream. You could stick your hand in the beam and the music would stop!

    He's pretty good with driving and modulating laser diodes!

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  3. #3
    mixedgas's Avatar
    mixedgas is offline Creaky Old Award Winning Bastard Technologist
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    How fast? How far? For a few tens of feet and say 10 kilobaud baud this is easy. That entails just a photodiode and a comparator, with ON/OFF keying of the laser.

    After that distance, or for higher speeds, things get very interesting and complex, hardware wise


    Is this with a microprocessor with a UART? Or are you trying with a laptop or PC?


    Google Ronja and Data link or IRDA Standard.


    The faster your data rate, the smaller your photodiode area has to be.

    You also need to expand the beam with a collimator.

    At some point you will also learn about CRC, cyclic redundancy checks. Adding a bit of data to the message to confirm the data is correct.

    Then you end up learning about data slicers, the electronics at the receive end to decide when the laser is on or off. These are also known as constant fraction discriminators.

    This fellow has a great place for you to start. LEDs do make this far easier. Simply because the larger, incoherent, beam creates less noise at the detector.

    http://www.maxmcarter.com/lasrstuf/lasermodulator.html

    The current optical, free space, distance records are with LEDs and cheap Fresnel lenses.

    Steve

  4. #4
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    Thanks so much guys. I'm going to do some research on the new info you've given me and see what i can come up with. In terms of the actual experiment itself it will be pretty small scale. We're talking on the order of a few meters and it doesn't have to be anything super fast. It's for a university project and the breif we've been given is "Transmission of information through space using a modulated laser beam". So it's pretty open to interpretation as to what type of modulation, what information etc etc.

  5. #5
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    This may be more advanced than you want to get... but you can use gnuradio to build the dsp chain for digital data transmission and use the soundcard interfaces to interact with the laser/detector. Rates are all adjustable in the software. The nifty thing is there is demo code already written that allows you to create a tunnel interface to move IP data over the resulting link. You could "ping" over the laser.

    Back in the 80s heathkit offered an analog modulated HeNe laser and a companion solar cell / opamp receiver. It did amplitude modulation; you could drive it with a line-level output. I think I still have it in a closet. Ah, good times!
    Last edited by tribble; 01-19-2013 at 16:00. Reason: added IP info

  6. #6
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    Not sure how your french is, but this might be of interest...



    http://www.youtube.com/watch?v=QN1X6vXA-Y0
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  7. #7
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    paulhunn,

    I stumbled across this in another thread here at PL. Here is a simple analog modulating driver for laser diodes that you can build for a few bucks in parts; very simple, works to 1MHz. (!!)

    http://www.repairfaq.org/sam/laserdps.htm#dpsldd317

    Let us know how you make out!

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