Active polarimetric imaging by orthogonality breaking is an alternative polarimetric imaging method developed at the Institut FOTON, Rennes. By illuminating a sample with a dual-frequency dual-polarization (DFDP) beam whose polarizations are orthogonal, it is possible to characterize its diattenuation and the orientation of the anisotropy in a single acquisition. However, this technique is not sensitive to other polarimetric effects such as birefringence or pure depolarization and requires a detection/demodulation chain that introduces non-linearity effects and does not allow results to be obtained quantitatively. In this paper, after a presentation of the orthogonality-break imaging system, we will detail the calibration/correction protocol which is now implemented to take into account the effects of non-linearities. Then, we will show that it is possible, by adding a polarimetric analysis module, to make this method sensitive to the main polarimetric effects. The results obtained on a simulated operational scene will be presented.
Since long, optical intensity modulation/demodulation techniques have encountered numerous applications in telemetry, free-space communications or optical characterization of scattering media. Upgrading those techniques to a full-field, real-time imaging modality can allow massive multiplexing, an essential asset not only for 3D imaging or optical communications, but also for imaging in turbid media (medical diagnosis, underwater vision, imaging in colloids, or navigational aid for safe transports). In this context, we have recently proposed a new concept of Full-field All-optical Snapshot Technique for QUADrature demodulation imaging (FAST-QUAD), whose capacity in real-time image demodulation have been demonstrated up to frequencies of 500 kHz, without requiring any synchronization between the receiver and the intensity-modulated source(s) in the imaged scene.
This technique relies on an all-optical architecture, at the heart of which is an electro-optical crystal and appropriate polarization optics components, making it possible to spatially multiplex four transmission « gates » in quadrature to each other (0°, 90°, 180°, 270°), addressing four sub-images detected on the same single standard sensor (CCD/CMOS). This setup behaves as a quadrature lock-in detection circuit, well-known in the electronics field, but in the optical domain and in a massively spatially multiplexed way, using the acquisition time of the camera as a low-pass integrator. This optical module can therefore be inserted in front of any camera, and allows the number of electronics components to be minimized. This property provides FAST-QUAD with a major asset, as its operating frequency is fully and continuously tunable in the RF range, which allowed us to establish an experimental proof-of-concept between 0 Hz (DC) and 500 kHz on the first prototype built in the laboratory. We will detail the instrumental conception of this prototype as well as the calibration/processing pipeline developed. Experimental validation results and examples of application of the FAST-QUAD approach will also be presented.
We report an in-depth experimental characterization and analysis of an infrared active polarimetric imaging system based on the orthogonality breaking polarization-sensing approach. We first recall the principle of this laser scanning polarimetric imaging technique, based on the illumination of a scene by means of a dual-frequency dual-polarization light source. The experimental design is then described, along with measurements on test scenes with known polarimetric properties used to validate/calibrate the imaging system and to characterize its optical properties (sensitivity and resolution). The noise sources that temporally and spatially affect the quality of the orthogonality breaking data are then investigated. Our results show that the raw temporal signals detected at a given location of the scene are perturbed by Gaussian fluctuations, and the spatial information contained in the images acquired through raster scan of the scene are dominated by speckle noise, which is a common characteristic of active polarimetric imaging systems. Finally, the influence of the source temporal coherence on the images is analyzed experimentally, showing that orthogonality breaking acquisitions can still be performed efficiently with a low-coherence source.
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.
We show the use of a simplified snapshot polarimetric camera along with an adaptive image processing for optimal detection of a polarized light beacon through fog. The adaptive representation is derived using theoretical noise analysis of the data at hand and is shown to be optimal in the Maximum likelihood sense. We report that the contrast enhancing optimal representation that depends on the background noise correlation differs in general from standard representations like polarimetric difference image or polarization filtered image. Lastly, we discuss a detection strategy to reduce the false positive counts.
We report the experimental validation of a snapshot computational degree of polarization imaging technique, based on local analysis of the statistics of a single speckle image acquisition. The applicability of this imaging technique is demonstrated on various samples, and it precision is analyzed and compared with theoretical predictions. Then, we theoretically study the ability of this approach to discriminate samples with various depolarization degrees while sharing similar reflectance properties. We quantitatively compare the detection performances of this approach with standard with standard polarization imaging strategies and evaluate the increase in spatial resolution required to share similar detection efficiency.
Optimally enhanced vision of a polarized lightmark in obscured weather conditions (fog, haze, cloud) is reported when imaged over long distances (above 1 km) using a snapshot polarimetric camera. We derive and experimentally validate an optimal adaptive polarimetric representation, whose expression is shown to depend on the correlation of the noise fluctuations in the two orthogonal polarimetric images. We quantitatively compare the gain (experimental and theoretical) in contrast with respect to standard intensity imaging, and standard polarimetric representations. Lastly, we discuss efficient implementation strategies for automated detection in real-time in obscured weather conditions.
A novel technique is proposed to unambiguously determine the magnitude and orientation of linear dichroism. It relies on the use of a dual-frequency dual-polarization coherent source emitting two orthogonal circularly polarized modes at the output. The interaction of such beam with dichroic media is shown to give rise to a beatnote signal in the radiofrequency range. The amplitude and phase of such beatnote makes it possible to fully determine the magnitude and orientation angle of the diattenuation. We also report the application of this method to polarimetric imaging, with promising perspectives in biomedical imaging. Indeed, it provides a direct characterization of dichroic sample orientation, showing uniform estimated dichroism magnitude, whatever the orientation of the sample.
In this paper, we first report the recent achievement of a mid-infrared supercontinuum fiber laser source in our
laboratory. Using fluoride fibers, we have generated a wavelength supercontinuum covering the whole 2-3.5μm range,
and delivering a power spectral density of 0.3 mW/nm on a large spectral range. Experimental results are presented. This
source can open opportunities for broadband remote sensing of multiple gas species in the atmosphere, especially above
3 μm, where numerous organic compounds have strong absorption signatures. Therefore, we consider a simple
Supercontinuum Laser Absorption Spectroscopy (SLAS) experiment, and we develop a numerical case study above
3 μm, involving a multi-component gas mixture. We first describe a method for modelling noisy spectroscopic signals.
Then we consider the inverse problem, and attempt to perform identification and quantitative estimation of the gas
mixture. After showing the inapplicability of a direct multi-linear regression, we focus on processing methods that use
complexity penalization principles, and show that they can address efficiently the identification/estimation problem.
Among various penalization criteria, those based on Minimum Description Length (MDL) approaches are shown to
perform particularly well. Finally, we apply these methods to preliminary experimental spectroscopic signals obtained
with supercontinuum sources in our laboratory.
Active polarimetric imagery is a powerful tool for accessing the information present in a scene. The polarimetric images obtained can indeed reveal polarizing properties of the objects that could not appear using conventional imaging systems.1,2 The degree of polarization is a key quantity to define the way the scene polarizes or depolarizes the light. Its estimation3 from images obtained with illumination by coherent light is perturbed by speckle noise. Using the definition developed by Goodman,4 we propose in this paper to perform both a theoretical and a numerical experimental study on the estimation of the degree of polarization. We first present that one can estimate the degree of polarization with only two intensity measures, whereas four measures are used in classical studies. This method may have interesting practical applications. Indeed, the use of only two images could reduce the costs and could improve the acquisition time. The quantification of the loss of precision in the estimation of the degree of polarization is analyzed when one uses two images instead of four. This analysis is based on statistical studies considering the speckle noise in the data. The precision on the estimation of the degree of polarization is compared using either the electric field (that needs four measurements) or only two intensity measures. The theorical results are validated on simulated polarimetric data considering different situations described by different polarization matrices.
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