[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Problems with beaming

A laser beam with wavelength lambda that has been formed by a
primary objective mirror with an aperture diameter D will di-
verge in the far field (never mind, that's where you want to
work) due to diffraction.  I don't have the explicit relations
at home, but I believe that the angular width of a "diffraction-
limited" beam at half maximum is the same as that used in the
Rayleigh criterion: 1.22 lambda/D.

This width is further broadened due to (1) thermal effects in
the lasing medium, by a factor called "beam quality" that is in
the range 1.1-1.3, depending on the type of laser, and (2) beam-
pointing instability called "jitter," usually about one-third
the diffraction-limited beam divergence, that enters in a root-
sum-squares way.  The overall angular beam width of a contempo-
rary laser "weapon" is of the order of a microradian.  A factor
of ten reduction (narrowing) in beam width by the time an inter-
stellar mission would be undertaken would not be unreasonable to

Regards, Rex