IAW: d = lambda / (2 * NA), and since an expanded beam can focus again from a wider angle it results in a larger NA (= n sin max_angle).
http://en.wikipedia.org/wiki/Diffraction-limited_system is about optical imaging system, but the same applies for a laser I would say.
So the main answer: diffraction
-- So the main answer: diffraction
On Fri, Mar 30, 2012 at 5:25 AM, Nathan McCorkle <nmz787@gmail.com> wrote:
I've read some equations and searched google, but I'm still not
understanding WHY expanding the beam and shortening the focal length
allows light to be focused into a smaller spot. I know Feynman would
probably tell me I shouldn't ask WHY... but I still wonder
The equations here are intelligible, but I'm not finding any
explanations.... to me it seems like you'd have more play with a
longer focal length (i.e. minute variations past the focus would be
less impact as the divergence is slower)
http://www.kellerstudio.de/repairfaq/sam/laserioi.htm#ioicdf
http://specialoptics.com/pdf/wp_laser_beam_expander_theory.pdf
Simon, care to physicize me?
--
Nathan McCorkle
Rochester Institute of Technology
College of Science, Biotechnology/Bioinformatics
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