Interference of Sub-millimetre Waves over 450 m Path Difference

Abstract

SUB-MILLIMETRE waves are attractive for metrology over long distances, because they are not much scattered by fog or cloud¹ and can be specularly reflected from ordinary machined surfaces. A device using interference over a long path difference would give a simple method of measuring small movements of a distant body and could be developed for the precise measurement of distances. Interference between beams which have traversed a large path difference requires that the radiation be confined to a narrow bandwidth, and to this end we were interested in finding the effective line width of the continuous-wave 337µ maser². A preliminary experiment was carried out by beating two masers together as shown in Fig. 1, using a Putley indium-antimonide detector³ as the nonlinear mixing element.