Digital equalization of time-delay array receivers on coherent laser communications

© [2017 Optical Society of America.]. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modifications of the content of this paper are prohibited.Field conjugation arrays use adaptive combining techniques on multi-aperture receivers to improve the performance of coherent laser communication links by mitigating the consequences of atmospheric turbulence on the down-converted coherent power. However, this motivates the use of complex receivers as optical signals collected by different apertures need to be adaptively processed, co-phased, and scaled before they are combined. Here, we show that multiple apertures, coupled with optical delay lines, combine retarded versions of a signal at a single coherent receiver, which uses digital equalization to obtain diversity gain against atmospheric fading. We found in our analysis that, instead of field conjugation arrays, digital equalization of time-delay multi-aperture receivers is a simpler and more versatile approach to accomplish reduction of atmospheric fading.Peer ReviewedPostprint (author's final draft

Limits to the information gain from lidar measurements

© [2015 Optical Society of America.]. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modifications of the content of this paper are prohibited.Measurements over the return signal are an integral part of lidar remote sensing by which we gather information about the characteristics of specific targets. But how much information is gained by performing a given lidar measurement? By defining Shannon's mutual information of a lidar observation, here we consider the bits of information content on the measurement and describe mathematically the capacity of lidar estimates to represent a corresponding property in the target. For heterodyne Doppler lidars in particular, we have found simple analytical formulas that consider the information gain in mean-frequency estimates.Peer ReviewedPostprint (published version

Statistical model for fading return signals in coherent lidars

Postprint (published version

Performance of coherent lidar receivers using atmospheric compensation techniques

Postprint (published version

Limits to the representation capacity of imaging in random media

The information capacity of an image in the atmosphere, ocean, or biological media does not grow indefinitely with increasing light power but has well defined limits. Here, the exact effects of the propagation of light in random inhomogeneous media are elucidated and upper bounds to the capacity of image pixels to represent a corresponding point in the object are described.Peer ReviewedPostprint (published version

Graph-based model for adaptive simulation of beam propagation in turbulent media

A graph-based approach uses a triangular adaptive mesh for simulating the propagation of light beams through the atmosphere. In this approach, the atmospheric turbulence and the beam wavefront are signals in a graph, with vertices representing an irregular distribution of signal points and edges between vertices showing their relationships. The adaptive mesh provides a better representation of the spatial variations in the beam wavefront, resulting in increased accuracy and resolution compared to regular meshing schemes. The adaptability of this approach to the propagated beam characteristics makes it a versatile tool for simulating beam propagation in various turbulence conditions.FUNDING: Funding. Agencia Estatal de Investigación (PID2020-118410RB-C21).Peer ReviewedPostprint (published version

Approaching fundamental limits to free-space communication through atmospheric turbulence

We have determined the optimal beams for free-space optical transmission through atmospheric turbulence. These are stochastic eigenmodes derived analytically from a canonical turbulence model, assuming known turbulence statistics. Under weak or strong turbulence, using these modes as transmit and receive bases minimizes signal degradation by turbulence, and minimizes the complexity of any signal processing method employed to compensate for turbulence. These modes can be mapped to/from single-mode waveguides by fundamentally lossless modal multiplexers and demultiplexers. Adaptive optics can be replaced by adaptive multi-input multi-output signal processing, enabling compensation of fast fluctuations of both phase and amplitude.Peer ReviewedPostprint (author's final draft

Optical Doppler shift with structured light

When a light beam with a transverse spatially varying phase is considered for optical remote sensing, in addition to the usual longitudinal Doppler frequency shift of the returned signal induced by the motion of the scatter along the beam axis, a new transversal Doppler shift appears associated to the motion of the scatterer in the plane perpendicular to the beam axis. We discuss here how this new effect can be used to enhance the current capabilities of optical measurement systems, adding the capacity to detect more complex movements of scattersPostprint (published version

Field conjugation adaptive arrays in atmospheric coherent optical links

Postprint (published version

Understanding the performance of atmospheric free-space laser communications systems using coherent detection

Postprint (published version