Frequency-modulated continuous-wave laser detection and ranging (FMCW LADAR) measures the range to a surface through coherent detection of the backscattered light from a frequency-swept laser source. The ultimate limit to the range precision of FMCW LADAR, or any coherent LADAR, to a diffusely scattering surface will be determined by the unavoidable speckle phase noise. Here, we demonstrate the two main manifestations of this limit. First, frequency-dependent speckle phase noise leads to a non-Gaussian range distribution having outliers that approach the system range resolution, regardless of the signal-to-noise ratio. These outliers are reduced only through improved range resolution (i.e., higher optical bandwidths). Second, if the range is measured during a continuous lateral scan across a surface, the spatial pattern of speckle phase is converted to frequency noise, which leads to additional excess range uncertainty. We explore these two effects and show that laboratory results agree with analytical expressions and numerical simulations. We also show that at 1 THz optical bandwidth, range precisions below 10 μm are achievable regardless of these effects.
Coherence and Statistical Optics
Original Manuscript: April 17, 2014
Revised Manuscript: June 25, 2014
Manuscript Accepted: June 30, 2014
Published: August 8, 2014
Esther Baumann, Jean-Daniel Deschênes, Fabrizio R. Giorgetta, William C. Swann, Ian Coddington, and Nathan R. Newbury, "Speckle phase noise in coherent laser ranging: fundamental precision limitations," Opt. Lett. 39, 4776-4779 (2014)