Performance analysis of a heterodyne lidar system incorporating a multimode optical waveguide receiver

A theoretical/numerical performance analysis of a heterodyne lidar system incorporating a multimode optical waveguide receiver has been performed. The performance parameters of interest are the coupling and mixing efficiencies of the lidar receiver, as they relate to the overall system carrier to noise ratio (CNR). The coupling efficiency relates the ability of the lidar receiver optics to couple light returning from a distant object into the multimode optical waveguide receiver. The energy thus received is then mixed with a local oscillator (LO) field in an evanescent wave coupler to generate the desired intermediate frequency (IF) signal current. The efficiency with which this mixing occurs is very strongly influenced by the receiver waveguide geometry, and thus the constituent modal structures of the guided LO and signal beam fields. Initial results indicate that for a symmetric square waveguide supporting seventy-five distinct modes, the coupling and mixing efficiencies can approach 78% and 23%, respectively. These results rival the performance characteristics of many free space and single mode waveguide lidar receiver designs, while higher efficiencies are expected for more realistic multimode waveguides supporting many more modes, and for lidar receivers constructed from circular (possibly graded index) multimode optical fiber waveguides