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Problems with beaming

To Lee,

>There are two problems really. First of all; there is the inverse square
>law, the intensity (and therefore the power) of radiation decreases as the
>inverse square of the distance. I have not seen anyone's calculations here
>take into account the amount of power that must be generated HERE in order
>to provide a reasonable amount of power THERE. 

Yes, but lasers keep a tight beam and thus their diametre (cross-section)
does not increase with distance and the energy-density (per surface) stays
the same.
So I don't see a problem here.

>The second problem involves another aspect of basic optical diffraction
>theory; the Rayleigh criterion defines the required diameter of the laser
>transmitter. For yellow light, a 400 km aperture transmitter would be
>required to just fill a light sail at Tau Ceti, 11 light years away.

The criterion of Rayleigh states that two wavelengths l and l+dl are just
resolved in the n-th spectral order when the maximum of one spectral-line
falls upon the first minimum of the other.
I don't see what resolvement has to do with beaming power, could you explain
what I missed?
(What formula did you use?)