John Payson wrote:

> Otherwise, I was wondering what would happen if you could use a pair
> of lenses of different focal lengths, seperated by the sum of their
> focal lengths (e.g. use lenses with 5mm and 50mm focal lengths, with
> 55mm between their centers).  It would seem that this would give a
> fairly collimated beam, but 10x the diameter of the original.

  Regarding the diode laser, it would appear that most people are quite
unaware of their construction.  They actually require a collimation lens
as they have an initial beam spread of some 30 degrees or more (i.e.,
aperture about F/1.5 to F/2).  This lens is generally attached to the
heatsink by a screw collet.

  If you *require* a wide parallel beam, you simply *do not assemble*
the small lens in the first place, but use instead a lens of the desired
diameter, say 1 or 2 inch, mounted at its corresponding focal length
(exactly!).  No fuss about adding a second and third lens at all!  OTOH,
this arrangement actually permits, should you choose, a degree of focus
of the distant image at *least* ten times better (diameter; thus 100
times better in intensity) than the small lens.

  But that is incidental.  *One* large lens allows complete control of
beam diameter and divergence.

  By contrast, a He-Ne laser inherently generates an *almost* collimated
beam without a lens, but an adjustable two-lens collimator such as a
pair of binoculars (or, impractically, a single lens at a ridiculously
long focal length of some tens of  metres) is required to project it any
*significant* distance (e.g., for communications).
--
  Cheers,
        Paul B. (Collector of cheap binoculars)