MadSci Network: Engineering
Query:

Re: How do you talk with lasers?

Date: Thu Oct 29 16:13:42 1998
Posted By: Steve Czarnecki, senior technical staff member, Lockheed Martin
Area of science: Engineering
ID: 907001902.Eg
Message:

The conversion from analog signals to binary data and back to analog 
signals is handled easily these days via analog to digital (A/D) and 
digital to analog (D/A) converter chips.  I'll address the middle part of 
your question, on getting the binary data from point A to point B via 
light.

What you have described is, in principle, workable, and very close to how 
many typical fiber optic communication links work.  The main difference is 
that usually the amount of time allocated to a "1" and "0" is the same, 
except that the light source (usually a diode laser) is turned on for one 
symbol and off for the other.  That is, the binary data "11010001" might be 
sent as "on" "on" "off" "on" "off" "off" "off" "on".   Note that the 
connection between "1" and "on" is arbitrary; the opposite convention would 
work just as well.  

The fact that the light is being guided via fiber optics is implementation 
detail.  Systems have been built that transmit the light through air.  And 
it's not even necessary to use a laser; the laser is merely a bright light 
that can be switched on and off very quickly.  An LED (light-emitting 
diode) can be made to work, and even an incandescent light bulb can work 
(albeit at very low data rate).  One must be careful to manage the "signal 
to noise ratio" by ensuring that extraneous light is minimized so the 
receiver can easily decide whether the transmitter is "on" or "off".

The generic term for digitizing an analog signal and transmitting it as a 
series of binary pulses representing the digits is called "pulse-code 
modulation", or PCM.  In modern day jargon, this is what you get if you 
input an analog signal into an A/D converter and transmit the output over a 
serial line.  But what seems obvious in 1998 was a pretty radical idea back 
in the '40s and '50s when these concepts were invented.

The trick in making this scheme work is to synchronize the clocks at the 
receiver and transmitter.  In this way, the receiver knows when to expect a 
symbol; if it detects a sufficient number of photons, then it decides the 
transmitter laser was switched on, otherwise it must have been switched 
off.

Your method of using different length pulse is similar to a technique known 
as "pulse-width modulation", or PWM.  Practical systems transmit the pulses 
on a periodic basis in order to simplify the transmitter make 
synchronization of the receiver and transmitter easier. (Acutally, one of 
the benefits of PWM is that the D/A conversion is often performed simply by 
a low pass filter that provides a sort of running average signal level as 
the output.  This is often used as a way of controlling the power provided 
to heavy electric equipment).

If you want to experiment, you might try looking in electronic hobbyist 
handbooks at the local library or electronics supply store.  Suppliers such 
as Digi-Key or Radio Shack may sell experimenter's kit complete with diode 
laser or LED for the transmitter, fiber optics, and photodiode or 
phototransistor for the receiver, plus the necessary transmitter, receiver, 
and interface electronics (some simple digital logic components and op amp 
circuits).  The parts needed for a simple demo are cheap (a few dollars) 
and widely available.

Have fun!

Steve Czarnecki






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