This paper describes a new methodology we have developed for the optical simulation of CMOS image sensors. Finite Difference Time Domain (FDTD) software is used to simulate light propagation and diffraction effects throughout the stack of dielectrics layers. With the use of an incoherent summation of plane wave sources and Bloch Periodic Boundary Conditions, this new methodology allows not only the rigorous simulation of a diffuse-like source which reproduces real conditions, but also an important gain of simulation efficiency for 2D or 3D electromagnetic simulations. This paper presents a theoretical demonstration of the methodology as well as simulation results with FDTD software from Lumerical Solutions.
This paper presents a new FDTD-based optical simulation model dedicated to describe the optical performances of CMOS image sensors taking into account diffraction effects.Following market trend and industrialization constraints, CMOS image sensors must be easily embedded into even smaller packages, which are now equipped with auto-focus and short-term coming zoom system. Due to miniaturization, the ray-tracing models used to evaluate pixels optical performances are not accurate anymore to describe the light propagation inside the sensor, because of diffraction effects. Thus we adopt a more fundamental description to take into account these diffraction effects: we chose to use Maxwell-Boltzmann based modeling to compute the propagation of light, and to use a software with an FDTD-based (Finite Difference Time Domain) engine to solve this propagation.We present in this article the complete methodology of this modeling: on one hand incoherent plane waves are propagated to approximate a product-use diffuse-like source, on the other hand we use periodic conditions to limit the size of the simulated model and both memory and computation time. After having presented the correlation of the model with measurements we will illustrate its use in the case of the optimization of a 1.75µm pixel.
In this paper, we present the results of rigorous electromagnetic broadband simulations applied to CMOS image sensors as well as experimental measurements. We firstly compare the results of 1D, 2D, and 3D broadband simulations in the visible range (380nm-720nm) of a 1.75µm CMOS image sensor, emphasizing the limitations of 1D and 2D simulations and the need of 3D modeling, particularly to rigorously simulate parameters like Quantum Efficiency. Then we illustrate broadband simulations by two proposed solutions that improve the spectral response of the sensor: an antireflective coating, and the reduction of the optical stack. We finally show that results from experimental measurements are in agreement with the simulated results.
Aims. We present the first lab and sky validation of spectro-polarimetric equipment put at the combined focus of an optical longbaseline interferometer. We tested the polarimetric mode designed for the visible GI2T Interferometer to offer spectropolarimetric diagnosis at the milliarcsecond scale. Methods. We first checked the whole instrumental polarization in the lab with a fringe simulator, and then we observed α Cep and α Lyr as stellar calibrators of different declinations to tabulate the polarization effects throughout the GI2T declination range. Results. The difference between both linear polarizations is within the error bars and the visibilities recorded in natural light (i.e. without the polarimeter) for calibration purposes are the same order of magnitude as the polarized ones. We followed the α Cep visibility for 2 h after the transit and α Lyr for 1.5 h and detected no decrease with hour angle due to the fringe pattern smearing by instrumental polarization. Conclusions. Differential celestial rotation due to the dissymetric Coudé trains of the GI2T is well-compensated by the field rotators, so the instrumental polarization is controlled over a relatively wide hour angle range (±2 h around the transit at least). Such a polarimetric mode opens new opportunities especially for studies of circumstellar environments and significantly enhances both the potential of an optical array and its ability for accurate calibration.
Stray light in an optical system is unwanted parasitic light that may degrade performance. It can originate from different sources and may lead to different problems in the optical system such as fogging, ghost images for imagers, or inaccurate measurements for time of flight applications. One of the root causes is the reflectivity of the sensor itself. In this paper we present a new optical simulation methodology to analyze the stray light contribution due to the sensor reflectivity by coupling electromagnetic simulation (to calculate the pixels’ bidirectional reflectance distribution function, also named BRDF) and ray-tracing simulation (for stray light analysis of the camera module). With this simulation flow we have been able to reproduce qualitatively red ghost images observed on different sensors in our laboratory.
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