Fig. 1. The receiver plane formulation (left) states the performance of the heterodyne lidar systems in the degree of coherence of the backscattered radiation and its match with the local oscillator field. The target plane formulation (middle) condenses the problem of calculating lidar efficiency to one of computing the transmitted and backpropagated fields along the propagation path and estimating the overlap function of the two irradiances. The target plane formulation allows taking account of the effects of angular beam misalignment Δϑ (right) in heterodyne systems in the presence of atmospheric turbulence (see the text for further details).

Fig. 2. Coherent solid angle [in decibels, 10 log₁₀(ΩCOH)] as a function of range R for a 2-µm collimated system by use of the simulation of beam propagation in turbulent atmosphere. Different diameter apertures and misalignment angles are considered. The level of turbulence has typical moderate-tostrong daytime value, $C_n^{\mathit{2}}$ =10^-12 m^-2/3 . The monostatic (upper dashed line) and bistatic (lower dashed line) ideal system geometries are also shown.

Fig. 3. Similar to Fig. 2 but for weaker-turbulence conditions, $C_n^{\mathit{2}}$ =10^-13 m^-2/3 . Although the effects of refractive turbulence on the coherent lidar performance are less apparent than for those presented in Fig. 2, the importance of the disturbances is still pronounced for any range R and most misalignment angles.