Spontaneous and Stimulated Emission
We have already mentioned the term ‘spontaneous radiation’ in Chap. 5. It was used as a synonym with the notion of the random–phase wave fields summation. In terms of statistics, radiation emitted at random phases is characterized by a spectral–angular distribution of the average power flow. The total field phase is also random in this context. At the same time, it is clear that any realization of the emitter ensemble could be, in a sense, coherent if capable of preserving the fixed correlation between individual emitters during a time interval sufficiently long. For instance, the process of the regular wave scattering by a fixed lattice of charged particles meets these conditions (see Sect. 5.2.2). Surely, the oscillation phase of each particle, prescribed by the wave under scattering, remains correlated with the particle location even if the latter is random. Therefore, a certain degree of coherence is inherent in the total radiation, emitted by this ensemble. Naturally, if the emitters are characterized by a regular spatial distribution, the effects of the radiation coherence are more expressive.
KeywordsSpontaneous Emission Phase Trajectory Free Electron Laser Radiation Emission Stimulate Emission
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