  The growing use of photonic integrated circuits in lidar sensors has made the
speckle and phase distribution of the reflected optical power relevant for
coupling into the guided-modes of these systems. Current models for lidar
coupling into photodetectors do not account for such effects, so here we derive
an alternate model that does handle the free-space loss in lidar sensors with
guided-mode receivers. This model is both presented and
experimentally-validated for the idealized situation of Lambertian scattering
from targets at normal incidence, for which we observe a close agreement
between the loss predictions of the model with experiment.
