Abstract :
The paper presents, as far as possible in non-mathematical language, the concepts of modes currently in use. Some of the concepts are modified or generalized and a unified description is obtained. The term `proper-mode¿ is reserved for a certain mathematical concept, which is usually concomitant with the concept of the eigen function, whereas the term `quasi-mode¿ is reserved for a field configuration which can be supported by a physically realizable guiding structure. In the description given, the radiation field which must always accompany a guided wave is viewed in a slightly novel light, and a bridge is constructed between the concept of modes guided in closed waveguides and those guided by open structures. The paper concludes with a discussion on the existence of guided waves, and it is demonstrated that the existence or non-existence of a mode is entirely a matter of subjective opinion (implicit in the definitions). Nevertheless, it is shown that all quasi-modes are a physical reality and that they are useful in practical applications.
