Efficient coherent radiation discrimination by using super-étalons

Abstract

In many applications, the discrimination of coherent radiation against an incoherent background is required. Several approaches based on various forms of Fabry-Perot étalons have been investigated.1 Because of the intrinsic physical limitations of the étalons, the discrimination achievable with those devices is typically low. In this paper a new approach for achieving much higher discrimination is presented. The new device consists of multiple layers of thick films, referred to as super-étalons. The average optical thickness of the layers is chosen to be greater than the coherence length of the background light but less than that of the coherent light of interest. Thus, the phenomenon of multiple-beam interference cannot take place for background light. The reflectance is determined by the incoherent super-position of various orders of reflected beams from the layer interfaces. However, multiple beam interference still takes place for coherent radiation because of the long coherence length. The reflectance is determined by the coherent superposition of various orders of reflected beams from the layer interfaces. As a result, the transmittance for coherent light is dramatically different from that for incoherent light. New laser sensor concepts based on the super-étalons are discussed.