Re: laser through pinhole
- From: Sam Goldwasser <sam@xxxxxxxxxxxxxxxxxxx>
- Date: 03 Jul 2007 08:12:38 -0400
don@xxxxxxxxxxxxxx (Don Klipstein) writes:
In <1183432694.578391.285120@xxxxxxxxxxxxxxxxxxxxxxxxxxx>, Frogwatch wrote:
On Jul 2, 8:06 pm, d...@xxxxxxxxxxxxxx (Don Klipstein) wrote:
In article <1183417971.498468.124...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
anurag agrawal wrote:
I am trying to assemble a setup to measure the Point Spread Function
of an Optical System. I got a pinhole of 5 microns and need at least 1 mW
output through the pinhole. My boss got me a commercial laser pointer of
about 2mW power and it obviously does not have a good beam quality. Is
there anyway I can get that much power through the pinhole using the
pointer
Use a convex lens to focus down the beam. To get a waist size of 5
microns, the beam of a usual red laser has to converge to and diverge from
the waist at something like 200 milliradians. This means the focal length
of the lens should be not much more than 5 times the diameter of the beam
going into the lens.
- Don Klipstein (d...@xxxxxxxxx)
Use a microscope objective although I doubt you will get 1 mW through
the hole with a commercial laser pointer. You might do better with a
small HeNe laser. Laser pointers have poor beam quality.
I think more like using an ocular with focal length in/near the 10-16 mm
range, or one of the small lenses used in oculars and lower magnification
microscope objectives. I think that gets to be a good time to hack a
cheap toy quality telescope or microscope. Otherwise borrow an ocular
with focal length near/around 10-16 mm. Other than that, Anchor Optics
(probably about $6 for a small lens, plus shipping charge that I have
found to be on the high side and appearing to me to have added to it some
percentage of the merchandise price, and maybe a minimum order).
Probably needs to be a shorter focal length.
4 * wavelength * (lens focal length)
Spot Diameter = --------------------------------------
pi * (beam diameter)
or
(spot Diameter) * pi * (beam diameter)
focal length = ----------------------------------------
4 * wavelength
The typical green laser pointer has a 1 mm diameter beam. To obtain a
beam spot diameter less than 5 um requires an f/ number of about 7 mm.
And, the focused spot diameter should be less than 5 um to get most of
it through the 5 um pinhole so a 3 or 4 mm lens would be better. Try
the lens from a CD or DVD optical pickup. The focal length of a CD lens
is around 4 mm, a DVD lens is even shorter.
Note that you may not even need the pinhole. Once the beam has been
focused by the lens, it is effectively a point source whether there is
a pinhole or not.
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