Dr. Nicolas Fraval Profile

Dr. Nicolas Fraval

at Evosens

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Proceedings Article | 16 February 2012 Paper

Novel resistive electrode structure for liquid crystal modal lens shifting

Nicolas Fraval, Frédéric Berier, Olivier Castany

Proceedings Volume 8252, 82520Q (2012) https://doi.org/10.1117/12.909233

KEYWORDS: Electrodes, Liquid crystals, Wavefronts, Cameras, Finite element methods, Image sensors, Resistance, Atrial fibrillation, Oxygen, Plasma etching

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Miniaturization of image sensors and increase of their resolution creates a demand for new miniature optical components providing classical functions in optics, like auto-focus (AF) and optical image stabilization (OIS). Several routes have been explored in order to realize AF and OIS functions in miniaturized components, for example electro-wetting liquid lenses, MEMS components and liquid crystal (LC) lenses. Our recent attempt to provide OIS with LC lenses showed the possibility to shift an image but limitations appeared. The sectorization of the electrodes generates aberrations due to the discontinuities of the electric field at the junction between two sectors. To overcome these limitations, we propose a new structure featuring a resistive electrode. This structure consists in substrates with thin electrodes joined with a ring-shaped resistive electrode (10kΩ/sq.) made of PEDOT-PSS and etched by oxygen plasma. A high resistivity layer (10MΩ/sq.) is then coated on the optical aperture and the cell is assembled like a classical LC modal lens. With this electrode structure, we succeeded to linearize the electric potential between the electrodes and reduce aberrations of the resulting wavefronts. First we simulated the lens by finite elements method to study the impact of the ring-shaped resistive electrode and to calibrate the physical parameters of each components (metallic electrodes, ring-shaped electrode, high resistivity layer, LC...). Then, we realized lenses and we characterized them in terms of focus, deviation angle and aberrations.

Proceedings Article | 14 February 2011 Paper

Liquid crystal lens auto-focus extended to optical image stabilization for wafer level camera

Nicolas Fraval, Frédéric Berier

Proceedings Volume 7930, 793009 (2011) https://doi.org/10.1117/12.875575

KEYWORDS: Electrodes, Liquid crystals, Switching, Cameras, Wavefronts, Wafer-level optics, Semiconducting wafers, Monochromatic aberrations, Atrial fibrillation, Glasses

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Miniaturization and reduction of production cost of optical components in consumer electronics leads to wafer level optics. This miniaturization, associated with the increase of CMOS sensors resolution, generates new needs such as auto-focus (AF) and optical image stabilization (OIS) in order to reduce the blurring caused by hand jitter. In this paper, we propose a wafer scale technology to perform AF and introduce OIS functionality. We managed to create a tunable focal lens by filling with nematic liquid crystal (LC) an assembly of two glass substrates coated with circular hole patterned chromium electrodes and resistive transparent layers of Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS). When a voltage with tunable magnitude and frequency is applied to the electrodes, the resistive layer creates a non-uniform voltage distribution from the edge to the center of the aperture which depends on electrical parameters of PEDOT-PSS and LC. The resultant electric field generates a gradient orientation of the nematic director which allows to focus light polarized along the director. It is also possible to shift the optical axis of the lens by dividing the hole patterned electrodes in several sectors and to apply different voltages on each sectors. The principle of the shifting effect has been demonstrated but its magnitude has to be increased by using more adapted electrode structure to ensure the OIS function. Finally, we characterised the dynamical behaviour of the lens in both focus and shifting modes.

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