Realtime Active Depolarization Imaging by OrthogonaLIty BREaking – RADIO LIBRE

Measuring the depolarization properties of materials is a powerful sensing tool for numerous civil and military applications, ranging from biomedical diagnostic or non-destructive control, to active target detection/recognition. In this domain therefore, constant effort is done to increase sensitivity, robustness and versatility of imagers. Meanwhile, reduction of the complexity, cost and duration of such optical measurements is intensely sought after, by trying to exhibit the most discriminating polarimetric figures of contrast for a given application. Besides, a number of these applications would strongly benefit from the possibility of remote-sensing operation through optical waveguides (biomedical imaging, optical sensing in hostile environments, etc.). However, this issue is still an unsolved technological challenge since propagation through an optical fiber does not allow a perfect control of the polarization states. Such control is very stringent in most existing polarimetric sensing modalities.

In this context, the RADIO LIBRE project proposes to revisit the way of performing depolarization images using a novel concept of field “orthogonality breaking”. This approach provides a direct characterization of the depolarization strength of a material from a single measurement. This technique, based on a microwave photonics approach and patented by the project leader in 2011, uses a peculiar preparation of the illumination field state to enlighten the sample. The depolarizing strength of a sample is then directly retrieved from a single measurement of a RF electrical modulation signal appearing on a photodetector when the sample exhibits pure depolarization and/or optical diattenuation. This new technique is able to provide highly sensitive depolarization measurements at an unprecedented rate (< 1µs) and exhibits high robustness and wavelength agility, since based only on the detection of a RF electrical signal. In addition, it must be noted that this technique is well suited for endoscopic remote sensing, being inherently insensitive to fiber/endoscope birefringences.

The main objective of the RADIO LIBRE project is to go beyond single measurements by validating the possibility of performing real-time active depolarization imaging using this new technique, and to check the potential interests of this approach for defense applications (active polarimetric target detection/recognition) and for civil applications (endoscopic polarimetric). To this aim, two partners have gathered their own competences to build an exhaustive work program: Institut Fresnel’s skills in theory and modeling will be involved in the elaboration of a solid theoretical background on the physics of “orthogonality breaking” signatures (Task 1) and in the results analysis (Tasks 3 & 4). Then, the conception and development of the imager (Task 2), as well as the realization of the experiments in both configurations (free-space (Task 3) and fibred (Task 4)) will be conducted at the IPR level, which also leads the project (Task 0).

We believe that the concepts to be analyzed and demonstrated in RADIO LIBRE can lead to significant scientific results through peer-reviewed international publications, and can trigger new patents. This is of great importance in a situation where the market is still in search of technological breakthroughs to facilitate the diffusion of polarimetric imaging in general public applications as well as in more specific domains such as defense or biophotonics. In these latter applications, it is highly probable that real-time capacity, as well as spectral agility offered by the microwave photonics approach proposed will be regarded as strong assets by industrial partners when compared to more standard techniques. Lastly, the imaging technique developed is expected to demonstrate the feasibility of polarization-sensitive endoscopic imaging, which could be of great interest for diagnostic assistance in a number of pathologies.