Re: Transmitting Video through Laser

Greetings Joey:

References:

1.Mad Science Archives Question: How can I transmit an analog video signal
through a laser?

htt p://www.madsci.org/posts/archives/mar2000/953774296.Eg.r.html

2.Mad Science Archives Question: How do I transmit a sound wave by a laser?

http: //www.madsci.org/posts/archives/aug98/897352142.Eg.r.html

3. Joseph C. Palais, Fiber Optic Communications, Prentice-Hall, 1998

4. Edmund Optics Catalog

http://www.edmundoptic s.com/US/OnlineCatalog/

I am assuming that you have read my answers to the questions in Reference
1 and 2 in the Mad Science Archives. The Reference to Edmund Scientific
Corp in Reference 1 is now obsolete and the new URL is presented in
Reference 4. of this note were you can obtain a copy of the catalog.

Reference 3. is an excellent, low cost book available in hard cover and
paperback form that addresses the circuits that you are interested in.
Indeed, as you wrote, an analog video laser link can be similar to an
analog audio link with the exception that a base band video link requires
about 1.5 MHz (1.5 million cycles per second) of bandwidth, which is about
100 times more than the bandwidth used in an audio link. Also, circuits
used for analog modulation in fiber optic systems can be used for through
the air analog laser communications links by using a lens to collimate the
laser diode light output into a diffraction limited beam.

In Reference 3. Palais suggests that for experimentation a simple NPN
transistor amplifier circuit can be used to modulate a laser diode (LD) by
replacing the collector load resistor with the LD. For a typical LD the
transistor should have a maximum current of about 800 ma at 5 volts and
have a beta of about 40. Of course the transistor should have a bandwidth
well beyond 1.5 MHz. Palais suggests that the base bias circuit should be
adjusted to set the LD light output to about one half of maximum intensity
with no video signal applied in order to give maximum linear dynamic
range.

For a receiver circuit Palais suggests that a photodiode connected to a
low noise operational amplifier (OPAMP) connected in a transimpedance
amplifier configuration gives the best overall performance results when
compared to bipolar, FET, and high impedance receiver circuits.

Depending on your video source, you probably will need an OPAMP to
condition the video signal source level to drive the LD modulator circuit.
The receiver OPAMP output should match the impedance of the input circuit
of the video display unit or TV set which usually are 50 or 75 ohms.

Once again I would suggest working with a high power LED in place of a
more expensive laser diode to breadboard and check out the transmitter and
receiver circuits. Laser diodes easily burn out during circuit
development. We call them the fastest, most expensive fuses in the world!

Best regards, Your Mad Scientist
Adrian Popa