Abstract
The survival of time-reversal symmetry in the presence of strong multiple scattering lies at the heart of some of the most robust interference effects of light in complex media. Here, the use of time-reversed light paths for imaging in highly scattering environments is investigated. A common-path Sagnac interferometer is constructed that is able to detect objects behind a layer of strongly scattering material at up to 14 mean free paths of total attenuation length. A spatial offset between the two light paths is used to suppress non-specific scattering contributions, limiting the signal to the volume of overlap. Scaling of the specific signal intensity indicates a transition from ballistic to quasi-ballistic contributions as the scattering thickness is increased. The characteristic frequency dependence for the coherent modulation signal provides a path length dependent signature, while the spatial overlap requirement allows for short-range 3D imaging. The technique of common-path, bistatic interferometry offers a conceptually novel approach that could open new applications in diverse areas such as medical imaging, machine vision, sensors, and lidar.
Highlights
The development of new methods capable of detecting objects on length scales from millimeters to hundreds of meters is of great importance for technology areas ranging from biomedical imaging to autonomous vehicle navigation
In optical coherence tomography (OCT), recovery of ballistic signals is possible through many mean free paths while the imaging distances are generally on the millimeter scale [5,6]
The method makes use of a common-path Sagnac interferometer consisting of two counterpropagating light paths, where the target object forms part of the interferometer
Summary
The development of new methods capable of detecting objects on length scales from millimeters to hundreds of meters is of great importance for technology areas ranging from biomedical imaging to autonomous vehicle navigation. In long-range bistatic lidar, similar principles of spatially offset excitation and detection paths are used to distinguish signals from different atmospheric regions [20] These techniques have in common that only a single, unidirectional path from illumination to detection is used. It is found that the scaling of target signal with attenuation length follows a less than exponential trend, indicating that the technique, next to providing the ballistic component, has a sensitivity to non-ballistic scattered light travelling in the same direction as the ballistic beam. The use of this signal for imaging and ranging is critically discussed
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