GRATING AND PRISM COUPLERS - RADIATION-PATTERN AND M-LINE

Prism or grating couplers are known to exhibit m-lines in the far-fiel d and zeros in the near-field. Despite the fact that a lot of papers h ave been devoted to these couplers, there is some confusion concerning the possible link between the m-line and zero: is the former a conseq uence of the latter? One of the objects of this paper is to clarify th is point. It is the opportunity to consider, in addition to the near-f ield and far-field patterns, the question of the intermediate region c onstituted by the Fresnel zone and to give a criterium of the far-fiel d pattern.