On the coupling of waves in nondissipative and dissipative systems

The interaction between two waves in nondissipative and dissipative systems is analyzed with the general principles of coupled mode theory. It is shown that the character of the interaction is determined by the product C
12
C
21
of the coupling coefficients. The extreme cases of purely real and purely imaginary values of C
12
C
21
correspond to the nondissipative and dissipative systems, respectively. The results of the analysis are applied to the case of acoustic amplification.