François Parnet

Full characterization of dichroic samples from a single measurement by circular polarization orthogonality breaking

We report a novel method to unambiguously determine the magnitude and orientation of linear dichroism in a simultaneous way. It is based on the use of a dedicated dual-frequency dual-polarization coherent source providing two orthogonal circularly polarized modes at the output. We show that the interaction of such a beam with dichroic media gives rise to a beatnote signal whose amplitude and phase enable the full determination of the diattenuation coefficient and axis orientation, respectively. The application of this method to polarimetric imaging provides single-shot sample characterization by its diattenuation coefficient and optical axis angle, with potential applications in biomedical imaging.

Orthogonality-breaking sensing model based on the instantaneous Stokes vector and the Mueller calculus.

Polarimetric sensing by orthogonality breaking has been recently proposed as an alternative technique for performing direct and fast polarimetric measurements using a specific dual-frequency-dual-polarization (DFDP) source. Based on the instantaneous Stokes-Mueller formalism to describe the high-frequency evolution of the DFDP beam intensity, we thoroughly analyze the interaction of such a beam with birefringent, dichroic, and depolarizing samples. This allows us to confirm that orthogonality breaking is produced by the sample diattenuation, whereas this technique is immune to both birefringence and diagonal depolarization. We further analyze the robustness of this technique when polarimetric sensing is performed through a birefringent waveguide, and the optimal DFDP source configuration for fiber-based endoscopic measurements is subsequently identified. Finally, we consider a stochastic depolarization model based on an ensemble of random linear diattenuators, which makes it possible to understand the progressive vanishing of the detected orthogonality-breaking signal as the spatial heterogeneity of the sample increases, thus confirming the insensitivity of this method to diagonal depolarization. The fact that the orthogonality-breaking signal is exclusively due to the sample dichroism is an advantageous feature for the precise decoupled characterization of such an anisotropic parameter in samples showing several simultaneous effects.

Orthogonality breaking through few-mode optical fiber

Polarization sensing and imaging through optical fibers is a technological challenge motivated by promising applications for in vivo, in situ polarimetric endoscopy for biomedical diagnosis. Among the recent approaches proposed to solve this issue, the depolarization/dichroism sensing by polarization orthogonality breaking (DSOB) technique was shown to perform remotely through single-mode optical fibers for depolarization/diattenuation measurements. In this article, we investigate the applicability of such a technique in slightly multimode waveguides. Through theoretical modeling and numerical simulations, we evidence the conditions required for the polarization orthogonality to be preserved after propagation in a few-mode fiber, notably in terms of detection geometry of the spatial modes. Original experiments realized in few-mode fibers both in transmission and reflection configurations are also reported and validate the theoretical predictions. These results allow us to analyze the influence of the experimental parameters, such as detection geometry, sample tilt, or fiber length, on orthogonality preservation and on the measurement dynamics of the DSOB technique in slightly multimode waveguides.

Free-space active polarimetric imager operating at 1.55 μm by orthogonality breaking sensing.

We report the design and optimization of an active polarimetric imaging demonstrator operating at 1.55 μm that is based on the orthogonality breaking technique. It relies on the use of a fibered dual-frequency dual-polarization source raster scanned over the scene. A dedicated opto-electronic detection chain is developed to demodulate the optical signal backscattered at each location of the scene in real time, providing multivariate polarimetric image data in one single scan with limited acquisition time. We experimentally show on a homemade scene that contrast maps can be built to reveal hidden dichroic objects over a depolarizing background, as well as their orientation. Finally, experiments through air turbulence illustrate the benefit of such an imaging architecture over standard polarimetric techniques requiring multiple image acquisitions.

Active infrared polarimetric imaging demonstrator by orthogonality breaking sensing

We report the design of a free-space active infrared polarimetric imaging demonstrator operating at 1.55 μm and based on a non-conventional approach: the orthogonality breaking sensing technique. Relying on the illumination of a scene with a specific light source, the imager offers an original tradeoff between image acquisition time (~ 1 s) and polarimetric consistency in comparison to standard polarimetric imagers such as division of time or division of amplitude systems. We will illustrate the capability of such an imager to enhance the visibility of hidden objects on homemade scenes.

Generation of a coherent light beam with precise and fast dynamic control of the state and degree of polarization

Recent developments of polarized light sources with tunable state and degree of polarization (SOP and DOP) inherently provide a temporally incoherent beam, which makes them unsuitable for applications like interferometry. We present a method for generating a coherent beam with full, precise, and independent control of the SOP and DOP. Our approach is based on an imbalanced dual-frequency dual-polarization light source. We demonstrate that it offers three different working regimes, respectively providing perfectly depolarized light, DOP modulated light, or fully polarized light with a deterministic SOP trajectory. A simple implementation of this versatile approach is described and experimentally validated.

Polarimetric Imaging by Orthogonality Breaking: From Single-mode to Few-mode Fiber Polarimetric Endoscopy?

Polarimetric imaging by orthogonality breaking is a recent novel approach to perform real-time polarimetric measurements. Particularly suited in principle to polarimetric endoscopy applications, we investigate how this technique could be used to perform images through single-mode and few-mode fibers.