Luciana P. Salles

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.

Response linearization of a 2D optical position-sensitive detector

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.

Impact of CMOS Pixel and Electronic Circuitry in the Performance of a Hartmann-Shack Wavefront Sensor

This work presents a numerical simulation of a Hartmann-Shack wavefront sensor (WFS) that assesses the impact of integrated electronic circuitry on the sensor performance, by evaluating a full detection chain encompassing wavefront sampling, photodetection, electronic circuitry and wavefront reconstruction. This platform links dedicated C algorithms for WFS to a SPICE circuit simulator for integrated electronics. The complete codes can be easily replaced in order to represent different detection or reconstruction methods, while the circuit simulator employs reliable models of either off-the-shelf circuit components or custom integrated circuit modules. The most relevant role of this platform is to enable the evaluation of the applicability and constraints of the focal plane of a given wavefront sensor prior to the actual fabrication of the detector chip. In this paper, we will present the simulation results for a Hartmann-Shack wavefront sensor with an orthogonal array of quad-cells (QC) integrated along with active-pixel (active-pixel sensor (APS)) circuitry and analog-to-digital converters (ADC) on a “complementary metal oxide semiconductor” (CMOS) process and deploying a modal wavefront reconstructor. This extended simulation capability for wavefront sensors enables the test and verification of different photosensitive and circuitry topologies for position-sensitive detectors combined with the simulation of sampling microlenses and reconstruction algorithms, with the goal of enhancing the accuracy in the prediction of the wavefront-sensor performance before a detector CMOS chip is actually fabricated.

Wavefront Sensor for spatial scan using the Hartmann-Shack method

This paper presents a simplified instrument to measure the profile of a refractive object contactlessly based on the Hartmann-Shack (H-S) method. The sensing device has been designed with a simple CMOS Position-Sensitive Detector (PSD) of the quad-cell (QC) type, comprising four Active-Pixel Sensor (APS) pixels. A collimated laser beam traverses the object and the resulting wavefront is shaped by its profile. We demonstrate the reconstruction of this wavefront, and therefore the profile of the object, by sequentially sampling the wavefront through a moving single aperture coupled to a converging lens. Notwithstanding, we propose a sequence of steps for instrument calibration, among them the focalization step, using a feature of the QC not yet explored. A target lens was used as a test structure and its sagitta was measured, presenting an average error of 4% with respect to the datasheet value.

Micrometric displacement measurement using CMOS 0.35µm technology Quad-Cell

Precise displacement estimation has high demands on the measurement system, but permits a wide range of applications. This paper presents a new methodology for precise micrometric displacement measurement utilizing a Quad-Cell (QC), Position Sensitive Detector (PSD) realized in a 0.35μm CMOS technology. Experimental results indicate high precision in the range of several micrometers for displacements of up to 2,400μm. Further, extracted curves permit the choice of the best trade-off between precision and maximum measureable displacement.

CMOS integrated aCtive-Pixel Sensor in cryogenic temperature

This paper will present experimental results of a slightly modified Active-Pixel Sensor (APS) circuit operating in cryogenic temperatures such as 77 K, 40 K, 20 K and 8.8 K. The optical sensor used was a silicon photodiode with its integrated electronics in a CMOS 0.35 μm technology. Active pixel circuitry with a small number of transistors is important and widely used in image sensors. A modified APS with five transistors was designed and tested under visible light at low temperatures in order to assess its hybrid further use with III-V infrared quantum detectors, which operate at these temperatures.

Computational test bench and flow chart for wavefront sensors

The wavefront reconstruction diagram has come to supply the need in literature of an ampler vision over the many methods and optronic devices used for the reconstruction of wavefronts and to show the existing interactions between those. A computational platform has been developed using the diagram’s orientation for the taking of decision over the best technique and the photo sensible and electronic structures to be implemented. This work will be directed to an ophthalmological application in the development of an instrument of help for the diagnosis of optical aberrations of the human eye.

Surface Texturing with Hemispherical Cavities to Improve Efficiency in Silicon Solar Cells

Improvement of solar-cell efficiency at a minimum possible cost addition is constantly sought, and this is often achieved at incremental percentage steps. Among a number of alternatives, antireflective coatings and surface texturing are the most prominent. This paper presents an alternative texturing method of crystalline silicon in an attempt to improve the efficiency of photon transmission through the surface and collection in the bulk. The method relies on anisotropic etching of bulk silicon and requires only a single oxide mask and two etching steps with a KOH or TMAH aqueous solution. The surface texture consists of smooth hemispherical cavities, which do not demand a lithographic mask or intricate technology processes to obtain the hemispherical cavities. This method can be applied to increase the profile area of the originally flat frontal surface exposed to light and consequently increase the effective width of the depletion region. The latter implies a higher probability of photon collection, contributing to the improvement of the conversion efficiency of the device. The textured nontilted silicon solar-cell transmittance under small solar incidence angles at dawn and sunset is improved compared to a flat surface, increasing the photocurrent.