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Published online by Cambridge University Press: 12 April 2016
In a conventional laser operating in the near ultra-violet, optical or infra-red spectral bands the photon energies, not exceeding lOeV, are closely matched to the electronic or molecular energy levels of neutral and weakly ionised atoms. Consequently typical photon energies (~ eV), and transition lifetimes (~ ns) closely match the characteristics of fast electrical circuitry feeding a weakly ionised discharge which may be used to pump either directly or indirectly the laser medium.
In a X-ray laser operating at about 10Å, photon energies are about 1 keV, and lifetimes about 10−14s (l0fs). In consequence the power required to pump the laser must be expected to increase rapidly as the wavelength decreases. The gain per unit length is given by:
where ζ is the line shape factor, A the spontaneous transition probability, λ the wavelength, and Δν the width of the line, and (n2,g2) and (n3,g3) the population density and statistical weight of the lower and upper laser states respectively. The total power loss per unit area, p, of the medium must exceed that emitted by spontaneous decay of the laser transition.