video transmission via laser diode

Discussion in 'General Electronics Chat' started by sesmd (stan), Apr 27, 2015.

  1. sesmd (stan)

    Thread Starter New Member

    Sep 30, 2014
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    Greetings to all interested - I have been trying to build a system where I can send a ccd camera acquired
    real-time image via a modulated laser diode beam. I've successfully built the transmitter part - which intensity
    modulates a red laser diode. However, I'm having trouble obtaining adequate frequency response at the
    receiver end. I've chosen a new approach - using a red LED as the beam detector (setup reverse biased @ 5V),
    driving a FET-NPN BJT cascode amp. However, I'm having trouble with cascode amp biasing. I am mainly
    self-taught, so my knowledge is spotty, and I have very little experience using software circuit design tools.
    I welcome any help and/or pointers from anyone interested in such a project.
    Thanks in advance to all contributions!

    stan
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    I have some questions:
    1. How is the beam from the laser diode modulated?
    2. What datarate are you using for the digital information?
    3. Why did you choose a reversed biased LED as a detector?
    4. Can you show us a schematic diagram so we can see how the amplifier is biased?
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Please post a schematic of what you have tried so far.

    ak
     
  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    What is your transmission medium? Is it air or optical fiber?
     
  5. sesmd (stan)

    Thread Starter New Member

    Sep 30, 2014
    8
    1
    Thanks for your interest!
    I am analog modulating the laser diode, using a video signal to drive a simple MOSFET amp, adjusting bias and gain, such
    that the laser maintains adequate current for coherent beam production. I believe the approximate video frequency is
    about five MHz. A red LED matched to a red laser diode (by trial) work well together, and a LED as a sensor has a
    very fast response rate. Reverse biasing allows using a greater bias across the LED, with greater voltage variation in response
    to laser illumination. Being new to this site, I'll have to see what means I have to post a schematic that is generally accessible
    to all... I'll post as soon as I work this out.

    Thanks again!
     
  6. sesmd (stan)

    Thread Starter New Member

    Sep 30, 2014
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    I'm projecting the laser diode across an air gap of about 100 ft., between two buildings, I use a one inch double convex lens to focus the
    beam on the LED sensor. The eventual plan is to have the LED / cascode circuit as a unit at the receiver end, feeding a video signal i n
    by coax.

    Thank for your interest!
     
  7. sesmd (stan)

    Thread Starter New Member

    Sep 30, 2014
    8
    1
    As soon as I figure software/ file format compatible for general viewing, I will post xmitter and rcvr schematics of what I have
    so far, and describe function (or lack thereof).

    Thanks !
     
  8. Ramussons

    Active Member

    May 3, 2013
    557
    92
    100 feet.
    open air.
    visible light spectrum.
    Amplitude Modulation.

    You are going to have a major issue of noise.
     
  9. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Commercial products for this have been around since the 70's, and every one I've ever seen uses FM modulation of the optical beam. Back in my night shift days we built one and quickly learned why. Besides the mammoth noise issue, you have two nonlinear transducers whose nonlinearities do not compliment or cancel out each other. FM is not simple or easy, and we cheated by using the FM modulator and demodulator in two videotape machines. The difference in results is huge.

    ak
     
    kubeek likes this.
  10. Papabravo

    Expert

    Feb 24, 2006
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    "Analog Modulating" is not a very useful or precise phrase. I don't understand what you mean by that, can you elaborate?
    Did you perhaps mean Amplitude Modulate?
     
  11. RichardO

    Well-Known Member

    May 4, 2013
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    A local school had a data link between buildings. They had problems with rain and snow...
     
  12. Papabravo

    Expert

    Feb 24, 2006
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    Absorption and scattering are well known behaviors in the UHF and microwave bands.
     
  13. RichardO

    Well-Known Member

    May 4, 2013
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    The rain and snow were interfering with a light beam, not an RF signal...
     
  14. Papabravo

    Expert

    Feb 24, 2006
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    And so you think light and RF are different? Light is certainly scattered and absorbed by moisture.
     
  15. WBahn

    Moderator

    Mar 31, 2012
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    No, but he probably thinks that light and "the UHF and microwave bands" are different.
     
  16. Papabravo

    Expert

    Feb 24, 2006
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    The higher you go in frequency the more problems you have with those factors.
     
  17. Sensacell

    Well-Known Member

    Jun 19, 2012
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    Agreed that you will have troubles with noise, non-linearity and offsets.

    Digital coded data would fair better.
     
  18. sheldons

    Active Member

    Oct 26, 2011
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    Schematic required!!!! My Crystal ball is being fixed polished and calibrated at the moment. .....
     
  19. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    How much bandwidth is required?
     
  20. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Full-bandwidth NTSC video is 4.2 MHz without an audio subcarrier. This sits comfortably on 10 MHz FM (or higher).

    ak
     
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