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

 

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?

 

Please post a schematic of what you have tried so far.

ak

 

What is your transmission medium? Is it air or optical fiber?

 

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?

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!

 

What is your transmission medium? Is it air or optical fiber?

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!

 

Please post a schematic of what you have tried so far.

ak

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 !

 

100 feet.
open air.
visible light spectrum.
Amplitude Modulation.

You are going to have a major issue of noise.

 

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

 

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!

"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?

 

A local school had a data link between buildings. They had problems with rain and snow...

 

Absorption and scattering are well known behaviors in the UHF and microwave bands.

 

Absorption and scattering are well known behaviors in the UHF and microwave bands.

The rain and snow were interfering with a light beam, not an RF signal...

 

And so you think light and RF are different? Light is certainly scattered and absorbed by moisture.

 

And so you think light and RF are different? Light is certainly scattered and absorbed by moisture.

No, but he probably thinks that light and "the UHF and microwave bands" are different.

 

The higher you go in frequency the more problems you have with those factors.

 

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

Digital coded data would fair better.

 

Schematic required!!!! My Crystal ball is being fixed polished and calibrated at the moment. .....

 

How much bandwidth is required?

 

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

ak