Re: Beam Expansion Question


"Phil Hobbs" <pcdh@xxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:47011990.2060407@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Jem wrote:
Hi

The idea is to acheive a collimated beam from a collimating mirror.

Thanks


"Phil Hobbs" <pcdh@xxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:46FFC210.9070400@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Jem wrote:
I have a 532nm laser with a beam diameter of 1.2mm at the point where I
need to place the spatial filter. On the beams exit from the spatial
filter I need to expand the beam to 8" diameter at a distance of 22"

Question:

1. What magnification objective do I require to acheive this ?

2. What size pinhole do I require ?

If 1 & 2 don't apply, is it just impractical to acheive this sort of
beam diameter at such a short distance?

Many thanks for any help.


Do you want it to be collimated when you're done, or just spread out in
a cone?


Please bottom-post--it makes the discussion much easier to follow.

For a hobby project, I'd probably suggest using a Fresnel lens for the
objective. Since you started out talking about spatial filters, though, I
gather that you have a much more stringent requirement in mind.

When you say "collimating mirror", I presume you mean that you already own
a paraboloidal mirror, because you won't get good results from a spherical
mirror at f/2.8. That's going to be a painful thing to align, and will
need a pretty substantial mount. The coma and astigmatism of a mirror
rapidly become truly horrid as you move off axis, so with a mirror design
you'll need some sort of folding mirror to avoid blocking the collimated
beam with your laser and lens mount. For a low-performance design, you
might be able to use a large flat mirror with a hole drilled in it, but
keeping good wavefront fidelity with a thin mirror will be very tough, and
a thick one will be expensive and heavy.

Spherical mirrors are pretty easy to use, because all you have to worry
about is tilt and spacing--there's no unique vertex on a sphere, whereas
on a paraboloid, there is. Thus with a paraboloid there are 5 adjustments
you have to make (xyz and two tilts), and they all have to be right.
Furthermore, xy and tilt interact very strongly in a fast paraboloid like
that, so finding the right alignment is going to be hard unless you're
willing to make some serious test equipment. Amateur telescope makers do
this, but generally not at f/2.8. (I've never done it myself, so others
on this list can offer more details.)

Off-axis paraboloids avoid the obscuration problem, but are harder still
to align. For these reasons one usually tries to use lenses where
possible, but lenses that big don't grow on trees, unfortunately. A small
Cassegrain telescope would give you a nearly turnkey solution, and its
folded light path would allow you to use a smaller aperture, maybe f/5
instead of f/2.8. That's still pretty fast for a telescope, though--most
of them are f/8 or f/10. So it looks like you're going to have to build
your own mount, which is not a particularly simple undertaking.

Cheers,

Phil Hobbs

Thanks folks

Phil, sorry for the top post last time, I will try harder, honest :-)

All I want to do with this is make a simple Denisyuk single beam holography
setup using a (as near as the size of my breadboard will allow) collimated
beam instead of a diverging beam. I want to use the things that I have
laying around - I only have a small optical breadboard and I happen to have
an f/3 spherical collimating mirror. This is simply an experiment for me
and it's not really that critical that everything is absolutely spot on. I
was just wanting to compare the resulting hologram with those made with a
diverging beam. So far I have only ever made simple single beam reflection
holograms and am just wanting to expand my horizons somewhat before I spend
more money and move on to multi beam setups on a larger breadboard/optical
table

For me, the fun is in building the setups and making the holograms, I try
not to get too tied up in the theory if I can avoid it ;-)

Thanks again

Jem


.

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