[Laser] laser pointer maximum distance

[John Matz]

A Start ...
As a few others have indicated, that is a hard question to answer.  A laser
pointer is intense enough and can be visible against a fairly noisy
background.  Even if your receive system only "sees" the desired source.
Range and performance all depend on the required receive signal.  Note that
errors will occur when the noise peak voltage is as big as the signal so you
can't tell them apart.  Higher data rate means wider receive bandwidth and
more noise.  Noise must be less than desired signal, but how much determines
the Bit Error Ratio you are going to get.  Desired Signal voltage some 2 to
3 times the rms noise voltage usually results in a very acceptable BER.
That means SNR of 6 to 10 dB is enough.  For 10 Mbps (Ethernet file
transfers), bandwidth required is often about 10 MHz.  For
keyboard-to-keyboard rates of a few tens of WPM, bandwidth required is more
like 10 Hz.  That is a million times narrower ... and lets though 60 dB less
noise ... and works with 60 dB less signal.  Ballpark figures ...
Transmitter power from a laser pointer is about 1 mW optical output.  For
audio-like bandwidths of a few khz or data less than 10 kbps, noise is about
in the middle, like 30 dB (1000 times) worse than low speed rates and 30 dB
less than at Ethernet rates.  Also many detector/amplifier bandwidths are in
that region too.  If we assume we can collect all the source's light falling
on a 1 sq cm area (with a small lens in front of a typical photodetector,
for instance), we anticipate a range of a few kilometers.  People have
realized ranges like 1 km at Ethernet rates to a few tens of km at very slow
data rates.

Hope that helps a bit.

John Matz KB9II

----- Original Message -----
From: ridwan arifin <orapatigenah at yahoo.com>
To: Free Space LASER Communications <laser at mailman.qth.net>
Sent: Wednesday, January 24, 2007 9:04 AM
Subject: [Laser] laser pointer maximum distance


i'm using a laser pointer ~670nm 4.5V for sinding
serial data. i wanna know the maximum distance that
ican reach with the laser. and the beam pattern....

thanx alot



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