The Transverse Structure of the Electromagnetic Field in a Plane-Parallel Resonator
In the present chapter we shall consider a practical optical resonator containing an active medium.† The radiation in such a resonator undergoes losses connected with incomplete reflection from the end mirrors and absorption in the host material. These losses, which can be described by the inactive absorption coefficient ϰ1, are offset by the pump energy. Unlike the previous chapter, we shall not assume that the coefficient ϰ1 and the pump power N are zero. We shall show that the existence of radiation losses and their compensation by the pumping lead to new phenomena which are absent in the model of an ideal optical cavity resonator. For example, the wave equation describing the electromagnetic field in a practical resonator is nonlinear, but the nonlinear wave equation is satisfied by a definite superposition of longitudinal and transverse modes, rather than by an individual mode or an arbitrary linear combination of modes (as was the case in the previous chapter); this superposition can include a large number of modes. This effect, which is conveniently called nonlinear interaction of modes, determines the most important characteristics of the laser radiation: the spectral composition, the angular divergence, the diffraction losses, and the generation kinetics.
KeywordsPump Power Active Medium Angular Divergence Transverse Mode Excited Atom
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