By John G. Webster (Editor)
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For most practical systems with pulse duration 84 DYE LASERS large compared to , the time variations of the laser intensity are slow compared to the spontaneous or stimulated processes and a steady-state approximation is justified. Under this condition, Eq. (3) may be expressed as (13) 1 dI g0 SI = − I dz 1 + I/IS 1 + I/IS (5) Here, g0 ≡ σeff N(σ0pIp τ − σ0 /σeff ) σ0p Ip τ + 1 with eff ϵ e Ϫ 12, is the small-signal gain coefficient that describes the single-passage amplification of a weak light beam (I Ӷ IS) incident along the z direction according to I(z) ϭ I(0) exp(g0z).
Duarte (16) provides a detailed discussion on design considerations for prism beam expanders in narrow linewidth dye laser systems. This linewidth is still too large compared with linewidths of atomic and molecular transitions in vapor samples, especially when such samples are available in the form of collimated beams. In vapor samples, the frequency or wavelength of the directed laser source as seen by the randomly moving atoms DYE LASERS is Doppler shifted to different extents depending on the distribution of velocity components in that direction.
For an efficient laser dye, Qf should be close to 1. 93 for Rhodamine 6G). An important nonradiative de-excitation process that reduces Qf is intersystem crossing (ISC) of dye molecules from singlet to triplet states. Because the triplet to singlet radiative transitions are forbidden, the lowest triplet state is a metastable state that relaxes slowly to the ground state (T ȁ 10 Ȑs to 100 Ȑs). Thus, ISC removes dye molecules from the lasing cycle, accelerates photochemical degradation of dyes (since molecules with unpaired electrons are more reactive), and, more important, introduces a loss for laser photons by T1 Ǟ T2 absorption, which may have a substantial overlap with the 83 emission spectrum (Fig.
50.Quantum Electronics by John G. Webster (Editor)