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Thread: modulating a laser with analog signal at high speed!

  1. #1
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    Laser Warning modulating a laser with analog signal at high speed!

    heyy,
    i am doing a project in which i am trying to transmit an analog video signal (NTSC) from a camera through a laser. i have designed the circuit but i am stuck with the modulation of the laser..
    i want to that if the lasers that have been designed to transmit a digital signal at high speed (e.g in 100,s of Mbps), can they be used to transmit an analog waveform (video signal in my case)??

  2. #2
    mixedgas's Avatar
    mixedgas is offline Creaky Old Award Winning Bastard Technologist
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    Yes, you need a Bias Tee at the sending end and a DC restorer and amplifier at the receiving end. The Bias T allows you to turn the laser on with a bit of constant current so its response is somewhat linear. The DC restorer circuit makes sure the sync pulse is correctly below ground level. A fiber video sender is available as a off the shelf module set. Nearly any small diode laser and some LEDs will do this.

    Google "video over fiber"

    Steve
    Last edited by mixedgas; 01-20-2011 at 15:31.

  3. #3
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    Post thnkx mixedgas

    yes Bias tee is one of the solution but i am using a buffer amplifier instead and my circuit is such that it performs the similar function of a bias tee without using one. i can show you my design if you are intrested.
    at the moment i have designed it such that the laser diode is biased at about 20mA RMS current. the problem is that the waveform has a maximum frequency component of about 3.75Mhz and i am not sure the diode that i am currently using can modulate it that fast cz its a locally available laser diode.
    i do have a better performance diodes, like VCSEL's and etc, that are designed for Tx of digital waveform at very high data rates but i am not sure if they can be used for Tx a video waveform as in my case (i.e 1Vp-p signal with max freq component at abt 4Mhz). can sumbody help me on tht plz??

  4. #4
    mixedgas's Avatar
    mixedgas is offline Creaky Old Award Winning Bastard Technologist
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    Most diodes can handle almost 1 Ghz with a proper driver. What is the part number and manufacturer?

    Steve

  5. #5
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    i have got a number of diodes.. OPV310(optek),OPV314(OPTEK),ADL80Y01TL(LASER COMPONENTS)
    can you tel me one more thing.. the laser diodes that have a built in Automatic power control (APC) circuitry, can they be used for this purpose? i noe thats a silly question since a diode with built in APC will always try to normalize the current waveform hense distorting the waveshape but still i want to confirm if i am correct with this concept??

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    As Steve mentioned, a DC clamp is required to ensure the V sync pulse is at the correct DC level.

    I designed a video DC clamp many years ago, let me see if I can dig it up for you.
    This space for rent.

  7. #7
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    Default need help

    can someone plzz help me with the circuit of vdo transmission over laser media.....pretty urgent....
    email id:adi_rattlesnake316@yahoo.co.in
    will be greatful if helped
    thank u

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