Radiation by Many Excited Atoms Between Mirrors
Theories of laser generally assume that the interaction between the atoms or molecules of the active laser medium, and the electromagnetic field can be accounted for by considering at most a few modes of the field as determined by the laser mirrors. Examples of this kind are furnished by the work of Lamb[l], Scully and Lamb, Fleck, Haken, and many others. These theories are very successful in describing the properties of real lasers, and they provide the basis for extensions to theories of mode locking, harmonic production, and a variety of other effects. It has remained somewhat of a puzzle, however, why the restriction to a few modes does work so well, especially when the cavities used in practice are so far from closed and do not come close to providing a complete set of modes. As seen from the center of a typical He-Ne laser used in an instructional laboratory, for example, the mirror occupies only 1% of the total solid angle. There is radiation into free space modes, and its intensity can be used as a measure of population inversion. There is, therefore, finite coupling to all modes of the radiation field.
KeywordsExcite Atom Active Laser Medium Einstein Coefficient Single Mode Approximation Real Laser
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