Davies W. de Lima Monteiro

Designing the Response of an Optical Quad-Cell as Position-Sensitive Detector

This paper focuses on 2-D position-sensitive detectors based on the quad-cell architecture, and presents design solutions to compensate the usual nonlinear response of a conventional quad-cell to radially symmetric light spots. It has been observed that by changing the quad-cell perimeter geometry and by adding a properly sized central region with a different sensitivity, the output response can be linearized. The simulation results presented herein will enable one to estimate the most appropriate quad-cell design for the requirements of a given application.

FPN Attenuation by Reset-Drain Actuation in the Linear-Logarithmic Active Pixel Sensor

The three FET CMOS active pixel sensors (APS) operating in the linear-logarithmic mode is one of the most efficient wide-dynamic-range imagers. However, the quality of the image generated at the focal-plane array is often compromised by fixed-pattern noise (FPN) between pixels. The classical correlated double sampling (CDS) technique is used to reduce FPN in imagers operating in the linear mode. But in the complementary linear-logarithmic mode CDS does not work properly and alternative techniques must be applied to reduce FPN. The ordinary alternative techniques increase either the complexity of the pixel or its external circuitry. In order to avoid these problems a new technique was devised to reduce FPN that can be applied to the basic three FET APS architecture. With the purpose to assert the efficacy of the proposed technique a small array was fabricated in a standard 0.35 μm CMOS technology. Experimental results show that the proposed technique is able to reduce FPN quite steadily within the whole illumination range used to test the array. And therefore, the signal-to-noise-and-distortion ratio (SNDR) of the array is also improved within the whole range of operation.

Evaluation of the full operational cycle of a CMOS transfer-gated photodiode active pixel

In this paper we evaluate the full operational cycle of the active-pixel circuit for CMOS image sensors in which four field-effect transistors are suitably combined with an ordinary photodiode; meaning a vertical p-n junction, as opposed to photogates, and with no custom pinned layer. Although this circuit topology itself is not novel, this evaluation intends to shine new light on the use of regular photodiodes. They became largely in disuse in four-transistor image chips under earlier process and design circumstances because the fourth transfer-gate FET did not hold a constant signal long enough and not for a fixed time. Our aim is to investigate the full underlying operational regimen of the transfer gate to propose that a regular photodiode might again be a choice to consider in the four-transistor pixel configuration, not only in conventional imaging but also in dedicated optical applications. The use of an ordinary p-n junction photodiode is advantageous as it offers full compatibility with even elementary mainstream CMOS processes. This investigation resorts to experiments and models both with fabricated integrated pixels and with a macro-pixel circuit implementation.

Silicon micro-optics for smart light control

We present an overview of the results of our recent research in the field of adaptive optical components based on silicon microtechnologies, including membrane deformable mirrors, spatial light modulators, liquid-crystal correctors, wavefront sensors, and both spherical and aspherical micro-optical components. We aim at the realization of adaptive optical systems using standard-technology solutions.

Arrays of spherical micromirrors and molded microlenses fabricated with bulk Si micromachining

We have extended the technology of fabrication of optical spherical mirrors by using single-mask bulk micromachining to fabricate highly-uniform spherical arrays of micro-mirrors and to mold polymer-on-glass microlenses. The arrays fabricated feature 100% optical fill factor and very high field uniformity of optical characteristics of individual micro-mirrors (lenses). The technology is specially suitable for the fabrication of uniform arrays of spherical mirrors with small numerical apertures for use in Hartmann-Shack wavefront sensors. Optical tests with the hexagonal array of molded microlenses with pitch of 300μm and focal length of ~30mm demonstrated that the contribution of microlens imperfections into the wavefront reconstruction error does not exceed λ/50 rms.

Alternative methodology for intraocular lens characterisation.

To present an alternative methodology to characterise intraocular lenses (IOLs) using an optical setup with a Hartmann‐Shack wavefront sensor.

Voltage Mode FPN Calibration in the Logarithmic CMOS Imager

The CMOS active pixel sensor (APS) operating in the logarithmic mode is the most common and useful CMOS wide-dynamic-range imager. Notwithstanding, fixed-pattern noise (FPN) between pixels compromises the quality of the image generated by the focal-plane array. Classical techniques as correlated double sampling do not work properly in this mode, and alternative techniques must be applied in order to calibrate FPN. The alternative techniques require either complex pixel circuitry, or external memory and software level calibration. Purposefully to improve image quality at reduced circuitry complexity, a new calibration technique is proposed that can be applied directly to the basic three-FET APS circuit. The efficacy of the proposed technique was experimentally verified with a small pixel array fabricated in a standard 0.35-

Response linearization of a 2D optical position-sensitive detector

\mu \text{m}

Fast-sampling multipixel detector for a heterodyne interferometer with angstrom precision

CMOS technology. The experimental results show a steady FPN attenuation within the whole tested illumination range and the improvement of the signal-to-noise and distortion ratio of the array.

Analysis of the Optical Quality of Spherical and Aspheric Intraocular Lenses in Simulated Nanophthalmic Eyes

This paper presents an alternative layout for a two-dimensional optical position-sensitive detector, namely a quad-cell, where the sensitivity (i.e. quantum efficiency) of the central region is higher than that of the peripheral region. Simulation results show that this approach improves the linearity of the quad-cell response at the central region, which is relevant in applications where high position resolution is demanded. A numerical model provides indication to an optimal spot-radius range and experimental results confirm good linearity for a spot within this range.