Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!agate!shelby!siegman@sierra.Stanford.EDU
From: siegman@sierra.Stanford.EDU (Anthony E. Siegman)
Newsgroups: sci.electronics
Subject: Re: lasers
Keywords: lasers
Message-ID: <335@sierra.stanford.edu>
Date: 17 Oct 89 01:22:14 GMT
References: <749@carroll1.UUCP>
Sender: siegman@sierra.STANFORD.EDU (Anthony E. Siegman)
Reply-To: siegman@sierra.UUCP (Anthony E. Siegman)
Organization: Stanford University
Lines: 61

In article <749@carroll1.UUCP> dtroup@carroll1.UUCP (David C. Troup  -  Skunk Works : 2600hz) writes:
>
>	Being a recent buyer of a 3mw helium-neon laser, I have a few questions
>	about the operation of the laser. 
>
>	oWhy when viewing the lasers red 'dot' on a wall, does it appear to
>	have little 'dot's in the illuminated area? It looks kinda 3-d 
>	(no flames about this being a 3-d world! :-) )

This is called "speckle".  It's a diffraction effect which results
because the wavefront hitting the rough wall is extremely "coherent",
meaning in this situation "highly planar".  The light coming off the
illuminated spot in the wall is in the form of narrow "needles", or
"antenna lobes", which your eye inteprets as bright spots.  Try moving
your eye and see which way the "dots" move; one way means you're
slightly near-sighted, the other means you're slightly far-sighted.

Ordinary light does exactly the same thing, except the "needles" are
different for every wavelength in the ordinary light, and for every
direction of arrival of the ordinary light, and so the "dots" are all averaged out and you don't see them.

Try blowing up the spot on the wall to an inch or so in diameter with
a simple lens, and put some printed material in the spot.  Look at it,
holding your head very still.  After a second or so the print will go
out of focus, because your eye involuntarily tries to focus on the
speckle.  Jiggle your head slightly, or jiggle the paper slightly; the
print will come back in focus.  Put the spot not on a white wall, but
on the side of a flat glass container containing milk; there should be
no speckle, because the "roughness" of the milk is moving.

>
>	o What will happen if I go from 12v dc to 24v dc? What will happen to
>	the power in the beam?
>

You'll probably blow up the power supply.  The power supply is
probably optimized for optimum current through the laser tube at 12
volts input; increasing the tube current will NOT increase the power
output.  (In technical terms, increased current pumps atoms out of the
upper level of the laser transition to still higher levels faster than
it pumps atoms into the upper level.)

>	o How can I defeat difraction on the beam over long distances?
>

There is NO way to defeat diffraction -- NONE.  Putting the beam
through a good telescope and expanding its diameter at the output of
the telescope will mean it will have a smaller angular spread in the
far field; this is the same as using a big dish antenna instead of a
small dish antenna on your microwave radar transmitter; you get a
narrower beam angle.  But nothing "difeats diffraction".

>	o anyone have any interesting projects for something like this?
>
>	thanks!
>
>
>-- 
>"We got computers, we're tapping phone lines, knowin' that ain't allowed"__         _______  _______________    |David C. Troup / Surf Rat                  
>    _______)(______   |         |dtroup@carroll1.cc.edu : mail             
>________________________________|414-524-6809______________________________