[Laser] Focusing and Magnifying

[Art]

At 05:13 AM 6/15/04, you wrote:
>I need to work out what is a good way to magnify the signals at the RX end
>and focus them down to a point.
>
>I found a place in Australia that sells large TV screen Magnifiers which are
>supposed to fit up to a 26" screen, and states that it enlarges the image up
>to 50%. Would this be OK for Laser DX? I pressume it could still be focused
>to a point, but I am unsure if this is the best thing to try.
>They say the larger light gathering area the better, this is the biggest
>I've seen.

These are serious light gathering tools, probably not needed for anything 
but the longest range contacts or unless you are attempting non-line of 
sight troposcatter type of contacts.


>As for focusing (collimating?) the laser beam at the TX end, is it not
>possible to focus the beam to a point at the target your aiming at ? As far
>as I know you cant, but I'm only new at this. The beam on my modified little
>laser pointer is pretty ordinary, its not real good at all I feel. Perhaps I
>need to find a better laser module, rather than a cheap pointer.

To focus means to bring it to a point. If the original facet on the laser 
diode is 1 micron by 1 micron, then that particular diode can be focused to 
the same size at any point in front of the facet opening.

At close range, this can be accomplished...out to a few inches or so.

In a perfect vacuum, this range can be nearly infinite, limited by the 
quality of the optics.

In the real world (our atmosphere), the scattering of light by the 
atmosphere is the limiting factor, which is why you can't focus to a pin 
point at 20 miles!

It's important to differentiate the difference between collimation and 
focusing. Collimation means to make the beam so that all the photons are 
parallel to each other. Perfect collimation would mean that the beam does 
not spread out at all as it passes through the air. Collimation means a 
focal length of infinity.

Focusing means to make the focal length of the optics some value less than 
infinity. As mentioned above, the atmosphere limits the maximum distance 
you can focus at.

Hope this helps.

Art

PS:On a related note, there are recent developments on the microlens 
technology which might impact us all. Someone has developed a microlens 
that has a diameter of 3 mm. By adjusting the bias voltage applied to the 
lens, one can make the focal distance vary anywhere from 6 mm to infinity. 
It uses a thin layer of water (electrically conducting) and a thin layer of 
oil (electrically insulating). A bias voltage applied changes the surface 
tension, hence the focal length. The manufacturer says the process is well 
suited for mass production and they are cheap to make. These should be very 
good for focusing lasers and for adaptive optics.