Re: laser with 4 dot pattern
From: Sam Goldwasser (sam_at_saul.cis.upenn.edu)
Date: 06/14/04
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Date: 14 Jun 2004 08:03:18 -0400
rrllff@yahoo.com (Helpful person) writes:
> Louis Boyd <boyd@apt0.sao.arizona.edu> wrote in message news:<cajej8$49u$1@oasis.ccit.arizona.edu>...
> > miso wrote:
> > > I'm a bit baffled here as none of the responses address my 4-dot
> > > problem, or google screwed up the thread. If google screwed it up,
> > > perhaps someone can repost what was relevant to my 4-dot problem.
> >
> > The first few made an attempt but got sidetracked.I think there is some
> > confusion as to how your "4-dot" pattern is arranged. Perhaps a better
> > explanation or a link to a photo of the pattern would help.
>
> A posible explanation for your 4 dot problem is that your laser in not
> lasing in the fundamental mode. It's been years since I worked with
> laser cavities (about 35) so maybe someone else can better describe
> the likelihood of this occuring in a modern HeNe laser.
I addressed the problem directly in my first post but maybe it didn't
go through:
I show:
Minimum e/2 c/2L Supply Nominal
Output Beam Diver- Mode Opr/Strt Tube Tube Size Model
Power Diam gence Spacing (Rb=75K) Current Diam/Lgth 05-LHx-
-----------------------------------------------------------------------------
10 mW .65 mm 1.24 mR 341 MHz 2.64/10 kV 6.5 mA 37/440 mm 991
This should be a single TEM00 beam. There is usually no way to get multiple
mode spots from these lasers even with misalignment.
> Yeah, it fires up just fine, but I get 4 distinct "dots", each of
> descending intensity. Any chance this can be caused by something
> simple, like perhaps dust or a particle in the tube housing.
You mean in a row? This is caused by multiple reflections off the output
mirror front and rear surfaces. Usually, just cleaning will help unless
the antireflection coating was damaged. These are called ghost beams
and sometimes there is no way to eliminate them entirely. On some laser
heads, there is also a protective glass plate in addition which could
be dirty or damaged.
Clean the output surface(s) with alcohol and some lens tissue
> My supply is 2450V at 6.5ma, a bit weak for the tube.
Not by much if it's a -991.
If it runs without sputtering or flickering, the power supply is running
it OK, though the power supply may be overloaded and fail prematurely if
the voltage spec is too low.
--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
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