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In article <175@tiger.Princeton.EDU>, zabetia@tiger.Princeton.EDU (Mahboud Zabetian) writes:
> I am a junior here and this year my project involves making a high speed
> transmission link between two computers or a computer or a terminal.  I am
> planning on using infra red LEDs or a semiconductor laser.  The goal is to
> reach speeds in excess of 20000baud.  (I am not using fiber optics)
> 
> My problem right now is to figure out which is better, IR or Lasers.  Anybody
> know?  I hear IR has the drawback of being the same wavelength as the size
> of fog droplets, therefore fog will block transmissions.  How about lasers?
> What wavelengths can I use with lasers?

	Since you are referring to semiconductor lasers, such as gallium
arsenide injection laser diodes, you don't have much choice as to the
wavelength range of available products: 820 nanometers to somewhat less than
1.0 micron.  Infrared LED's are also available only in the same range.
	So the point is: the propagation of laser diodes and infrared LED's
will be at essentially the same wavelength.
	Fog will be a problem at this wavelength, because of simple optical
dispersion like that of visible light, not because of molecular absorption
per se.  IR absorption on a molecular level results from O-H bond vibrations,
which first manifest theselves at around 1.4 micron, with additional absorption
bands from about 2.8 to 3.5 microns, 5.8 to 6.4 microns, and 10.5 microns
and above. 
	For what it's worth, I have used a four-element array of infrared
LED's (Fairchild FPE104) with a single-element phototransistor (Fairchild
FPT100) to establish a 9,600 baud data link between two buildings at a
distance of 600 feet.  The link was reliable during any kind of rain, light
fog, and medium snowfall, but would go to hell during heavy fog or snow.
The problem one runs into is signal-to-noise ratio on the received signal.
A non-obvious problem - until one encounters it empirically :-) - is the
serious amount of noise caused by outdoor mercury vapor and high pressure
sodium lighting at night.  While the 120 Hz frequency of the lighting is
<< than the data carrier frequency, and can be filtered out, this is not
a trivial task because the lighting signal is so HUGE at night compared
to the data signal.  Finding a rooftop location and using some attempt
at optical collimation can significantly reduce the noise resulting from
lighting and other outside events.  However, my application was an experiment
to see what one "could get away with" under worse-case conditions (put the
receiver and transmitter outside two windows and aim :-) ).  In any event,
regardless of location, I believe that any NON-laser diode system is limited
to well under 800 feet for reliable operation; 600 feet is more realistic.
I never got around to trying a laser diode.

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
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