Just to prevent possible confusion before it arises:
High intensity = large number of photons = high amplitude
High energy = high frequency
Copied from Chi Meson from physicsforums.com:
A photon does not travel "up and down" like a dolphin swimming. It just goes straight forward (through a vacuum). The "Up and down" is only the fluctuation of the strengths of the electric and magnetic fields at a point in space through which the photon travels.
Frequency is indicative of the energy of the photon. Protons endowed with high energy have high frequencies.
Now entering speculation on my part
I'm not sure about the relationship between dispersion and frequency. In vacuum, I don't think the frequency would matter one lick, actually. All that should matter is how clean your original beam is, and it will disperse at a constant rate thereafter.
It is possible there would be engineering challenges requiring a tradeoff between frequency and dispersion, though I doubt it, actually. (Background information: electrons closer to the nucleus of an atom have low energy, electrons orbiting far from the nucleus have high energy) Most lasers are created by exciting electrons in atoms to certain "above-normal" levels. When the electron drops back down to a lower energy state, the difference between the high and low energy levels is given off as a proton. Big energy difference = high frequency proton. Since the electrons exist at discrete energy states (orbit level 1, 2, or 5; never 1.5) then all the photons come off with the same frequency. I don't think that exciting electrons more would cause the light to come off at crazier angles, though I could be wrong.
Conclusion: I don't think there would be a tradeoff, but I'm not a physics major anymore. In my mind, the best laser weapon would be range adjustable, using lenses to ensure the focal point lies at the target.
This post has been edited by n64mon : 04 October 2009 - 01:06 PM