Re: [CAD_CAM_EDM_DRO] Re: Laser specs ? optics ?

turbulatordude wrote:

>Hi all,
>
>Anyone know where one can get optics for lasers ?
>
>Only some small lenses really. I am looking at the optics in laser
>pointers and it seems they do not create a very focused beam. more of
>a tightly aligned group of dots.
>
>

This is laser "speckle". It is a characteristic of ALL lasers, unless you
run the beam through a device called a spatial filter. The problem is
that the light is coherent, so every photon interacts with every other one
in the beam. Slight imperfections in the laser diode itself, the optics,
and turbulence in the air cause the photon paths to cross and either lead or
lag the rest of the beam by a fraction of a wavelength. That causes
interference, and you perceive it as speckle.

It is possible to focus a laser pretty tightly. I built a laser photoplotter
that lays down .001" raster stripes on Litho film. I can focus the beam well
enough that the raster lines don't merge together, so that must be finer than
.001" I use a 2 mm sphere lens almost in contact with the laser, on a plate
that can be moved around to center the sphere in front of the diode. I made
a VERY crude aperture from aluminum foil, and put that in front of the sphere
lens to block light coming around the edge of the lens. The aperture isn't
real small, probably 1 mm or so. I have a negative meniscus (double concave)
lens about 2" away from the laser. This reduces spot size, and probably isn't
needed if you don't need a .001" spot size. Then, I use a 14 mm microfiche
viewer lens as the objective, about 4" from the meniscus lens. The working
distance to the film is about 14 mm (no surprise). I defocus this setup
slightly until the raster lines merge together.

When working on this setup, there is practically no detectable speckle,
although the spot is pretty bright, which tends to obscure the speckle.
I run the laser with a pulse generator at a 1:1000 ratio, so it doesn't
harm my eyes. It is still pretty bright, 5 mW/1000 on a .001" spot is
still an illuminance of 5 Watts/sq inch!

But, I don't think it is possible to eliminate speckle over long path lengths
unless the path is in vacuum.

Jon