What Does Spontaneous Emission Look Like?
It appears that Amplified Spontaneous Emission , A.S.E., the emission process which occurs when a population inversion exists in an extended distribution of atoms in the absence of a laser cavity, is singularly well named. Sources of A.S.E. can be c.w. or pulsed. It is found in some pulse systems that the output has a granular structure (Fig. 1), or what another literature  describes as speckles. Such a structure does not occur in c.w. sources of A.S.E., in certain pulsed A.S.E. systems or in lasers; it is, however, well known that laser light scattered from rough surfaces gives rise to the speckles mentioned above. We are able to explain why speckles are, or are not, seen. The explanation forces a re-examination of the nature of the radiation from a source of spontaneous emission, derives the van Cittert-Zernike theorem  in an alternative way and relates the predicted coherence area to the size of the speckle structure. The explanation may be extended to demonstrate why different frequencies appear in different parts of the A.S.E. output .
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