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Atomic transitions and electromagnetic waves

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Presentation on theme: "Atomic transitions and electromagnetic waves"— Presentation transcript:

1 Atomic transitions and electromagnetic waves
The interaction of an assembly of identical atoms with a radiation field spontaneous emission stimulated emission absorption

2 Transition rates the photon energy density per unit frequency The interaction of an assembly of identical atoms with a radiation field transition rates the population densities of levels 1 and 2 spontaneous emission stimulated emission absorption

3 Determination of the Einstein coefficients
We consider the case where the atoms are in a thermal equilibrium with a blackbody radiation field (Planck’s law) (total downward transition rate = total upward transition rate) (Boltzmann statistics)

4 Conclusions for the LASER
We need a large photon concentration, this is achieved in an optical cavity We need population inversion, i.e., The LASER principle is based on non-thermal equilibrium.

5 Induced transitions Induced transition rate per atom (stimulated emission, absorption): the spontaneous lifetime of the atom Including the spectral lineshape function leads to (for details, see Yariv-Yeh, page 227) (monochromatic field) intensity [watts/square meter] energy density [Joule/volume]

6 Absorption and amplification
Induced transition rate per atom The net power generated within a unit volume For plane wave propagating in z gain coefficient

7 Absorption and amplification
gain coefficient

8 Ytterbium-Doped Fiber Amplifiers

9 Ytterbium-Doped Fiber Amplifiers

10 Ytterbium-Doped Fiber Amplifiers
(for the signal power) (for the pump power)

11 Ytterbium-Doped Fiber Amplifiers
(for the signal power) (for the pump power)


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