The purpose of this paper is to investigate the impact of advanced immersion lithography process for the development of polarization optics at pixel level on CMOS image sensors. In the first part of this paper, we use Bloch formalism to define regimes that depend on the number of propagative Bloch modes within the structure. The presented analysis gives estimations of required features size to operate in NIR and visible range. The second part of this paper present optical characterization of silicon lamellar grating made on 300 mm wafer using advanced immersion lithography. Characterization results are discussed with respect to optical simulations and reconstructed grating profile is compared to patterning features estimated during first part.
Division of Focal plane imagers have recently been developed for polarimetric imaging. Those sensors use a grid composed of four different pixels with four different polarizers engraved on them. Four of these different pixels form a superpixel which enables the estimation of the linear Stokes vector with a single acquisition. Those sensors are particularly sensitive to the spatial variations of the scene. Therefore, if such variations are non-negligible compared with the measurement noise, the estimation of the state of polarization is corrupted. We propose a method to map the superpixels in which the estimation can be trusted.
KEYWORDS: Signal to noise ratio, Wave plates, Polarization, Monte Carlo methods, Imaging systems, Polarimetry, Sensors, Calibration, Cameras, Signal processing
Polarimetric imaging can be done with a division of focal plane (DoFP) camera. This type of camera uses a grid of superpixels. Each superpixel consists of four neighbor pixels with four polarizers having different orientations in front of them. Thus, this kind of camera enables to estimate the linear Stokes vector in a single acquisition. Full stokes polarimetric imaging can be realized by adding a retarder in front of the DoFP camera and performing at least two acquisitions with two different values of retarder orientation. The effective retardance of the retarder depends on several parameters such as temperature and wavelength, which are not always controlled when using such a camera on the field. Therefore, this retardance may not be known precisely, and using a retardance value different from the true one will lead to a bias in estimating the Stokes parameter S3, which contains the information about circular polarization. This bias may become greater than the estimation standard deviation due to noise and thus have a significant impact on estimation. We demonstrate that thanks to measurement redundancy, it is possible to calibrate this retardance directly from the measurements, provided that three acquisitions instead of two are performed and the signal to noise ratio is sufficient. This autocalibration totally cancels the bias and yields a Stokes vector estimation variance identical to that obtained with the true value of the retardance. We study the practical conditions under which this method can be applied, perform experimental validation of its performance, and propose a criterion to decide if it can be applied depending on the acquired measurements.
We report on the development and field trials of an active polarimetric imager in the SWIR domain. Polarization states are controlled for both emission and analysis. Based on past experience, we focus on Orthogonal State Contrast (OSC) imaging for which two images with orthogonal polarizations are needed. An important feature of the imager is the use of two InGaAs imaging detectors mounted orthogonally on a polarization beam splitter. This allows the synchronous imaging with the two orthogonal polarizations and the real time acquisition of OSC images at video frame rate without temporal artefacts. The demonstrator has been operated during field trials with static and moving scenes. These trials were mainly aimed at the detection of man-made objects (weapons, vehicles …) in complex scenes at up to a few hundreds of meters. Along with the presentation of some example of results, we discuss different representation modes of the polarimetric information.
An enhancement of contrast between healthy and neoplastic zones in Mueller matrix images of excised cervical tissue was demonstrated in our prior work for the visible wavelength range. In this paper we present the statistical analysis of Mueller polarimetric data for the diagnostics of high grade cervical intraepithelial neoplasia. The results of linear and non-linear post-processing compressions of the full Mueller matrix are discussed and compared in terms of diagnostic performance. The final goal of these studies is to estimate and compare the diagnostic usefulness of 16 polarimetric measurements required for the reconstruction of complete Mueller matrix of a sample, while looking for an optimal design of future imaging protocols.
We compare four passive polarization imaging configurations by quantitatively assessing their target detection performance for different kinds of noise. Through closed-form expressions we determine the gain of these configurations compared to intensity imaging in the case of target/backrgound discrimination. For three of these configurations we show that a minimum amount of polarimetric contrast between the target and the background is required to outperform intensity imaging. We show that the only configuration that has always better performance requires to use a polarizing beamsplitter and assumes that the main source of perturbation is the background shot noise. This work has interesting perspectives for imaging architecture design.
We describe an active polarimetric imager with laser illumination at 1.5 µm that can generate any illumination and analysis polarization state on the Poincar sphere. Thanks to its full polarization agility and to image analysis of the scene with an ultrafast active-contour based segmentation algorithm, it can perform adaptive polarimetric contrast optimization. We demonstrate the capacity of this imager to detect manufactured objects in different types of environments for such applications as decamouflage and hazardous object detection. We compare two imaging modes having different number of polarimetric degrees of freedom and underline the characteristics that a polarimetric imager aimed at this type of applications should possess.
For the sake of polarimetric accuracy, polarization imaging systems based on liquid crystal modulators often work at one given wavelength due to the strong chromatic properties of the liquid crystal retarders. This often requires the use of narrowband filters which reduces the amount of light in the system and thus the signal-to-noise ratio. For applications where the main parameter of interest is the target/background discriminability rather than polarimetric accuracy, spectral filtering may not be the best option. In this work, we investigate the impact of broadening the spectrum of the light entering the system on the discriminability performance of passive and active polarimetric systems. Through simulations, we show that broadening the bandwidth of the illumination can increase the contrast between two regions, as the increase of light flux compensates for the loss of polarimetric precision. Moreover, we show that taking into account the chromatic characteristics of the components of the imaging system can further enhance the contrast. We validate these findings through experiments in passive and active configurations, and demonstrate that the illumination bandwidth can be seen as an additional parameter to optimize polarimetric imaging set-ups.
We describe an experiment that allows distant users to perform a labwork using Erbium Doped Fiber Amplifier (EDFA) in order to understand the basics physics and engineering involved. The EDFA and the measurement instrumentation are specially designed so as to allow for remote control through the web. The purpose of the project can then be distant learning for students from developing countries which cannot afford this kind of high-cost equipment.
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