Passive Heterodyne Detection Using Multiple Spatial Modes
We investigated the utility of passive heterodyne detection for the remote sensing of chemicals. We performed sensor simulations for a heterodyne receiver that operates with multiple spatial modes of radiation. We find that the heterodyne signals associated with distinct spatial modes of radiation can be detected in combination. We compare this to the well-known concept referred to as the “Siegman antenna theorem”, which is usually interpreted as limiting heterodyne receivers to a single spatial mode. We show that the heterodyne signal/noise ratio does not improve beyond the single-mode SNR. We discuss the possibility of performing imaging in the multi-mode configuration, using a multi-mirror array or an array heterodyne detector. We conclude that multiple-mode passive heterodyne sensing requires a bright source of radiation, since the combination of the modes does not boost the SNR. We discuss why passive heterodyne imaging is potentially feasible.