Quad Cells Research Articles

A high performance integrated readout circuit for wavefront sensors

This work presents an integrated pixel topology that promises to offer superior performance in a Hartmann–Shack Wavefront Sensor (WFS) with an orthogonal array of Quad Cells serving as Position-Sensitive Detectors. The readout integrated circuit for each photodiode is fully compliant to any standard CMOS microelectronics technology and is advantageously tolerant to high background illumination levels whereas maintaining both high linearity and high sensitivity. To assess the operation of this pixel on the focal-plane array of the WFS, we developed a computational platform encompassing a full detection chain comprising wavefront sampling, photodetection, electronic circuitry and wavefront reconstruction. It couples an algorithm written in C to SPICE (Simulated Program with Integrated Circuits Emphasis). The platform is technology agnostic and flexible, enabling easy modification to represent different detection or reconstruction methods. The results obtained with the proposed pixel have been compared to those obtained with a conventional pixel in CMOS image sensor.

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.

Numerical Simulation of Plane Cases of Gas Dynamics by Discrete Element Method

This article presents the methods for solving the gas dynamic cases with different finite difference schemes of the discrete element method. Scheme analysis was performed by a test case, which made possible to assess the used computational scheme properties for plane flow propagation in straight channel with direct and reflected shock waves. Mesh discretization was performed with quad and triangular cells. Results showed the high quality of the computational algorithm, which incorporates at the Euler stage parameters at the side boundaries calculated with the equations of gas properties of small decay.

Detection Statistics in Geiger-Mode Avalanche Photodiode Quad-Cell Arrays With Crosstalk and Dead Time

The detection statistics of Geiger-mode photodetector subarrays with a combination of reset-time blocking loss and optical crosstalk are investigated. Closed-form expressions are obtained for the means and covariances of the numbers of counts in $2 \times 2$ subarrays (quad cells) used in Shack–Hartmann wavefront sensors. The predicted wavefront sensing precision is compared with that obtained with a charge-coupled device-based wavefront sensor with readout noise. The results of the theory are also used to predict photon transfer curves for the Geiger-mode device and these are compared with experiment.

Measuring the centroid gain of a Shack–Hartmann quad-cell wavefront sensor by using slope discrepancy

Shack-Hartmann wavefront sensors (SH WFS) are used by many adaptive optics (AO) systems to measure the wavefront. In this WFS, the centroid of the spots is proportional to the wavefront slope. If the detectors consist of 2 x 2 quad cells, as is the case in most astronomical AO systems, then the centroid measurement is proportional to the centroid gain. This quantity varies with the strength of the atmospheric turbulence and the angular extent of the beacon. The benefits of knowing the centroid gain and current techniques to measure it are discussed. A new method is presented, which takes advantage of the fact that, in a SH-WFS-based AO system, there are usually more measurements than actuators. Centroids in the null space of the wavefront reconstructor, called slope discrepancy measurements, contain information about the centroid gain. Tests using the W. M. Keck Observatory AO system demonstrate the accuracy of the algorithm.

High-speed wavefront sensor compatible with standard CMOS technology

Abstract This paper addresses the design, implementation and performance of an integrated Hartmann (–Shack) wavefront sensor suitable for real-time operation and compatible with standard CMOS technology. A wavefront sensor can be used to detect distortions in the profile of a light beam or indirectly in that of an optical component. Such a sensor can also be coupled to a deformable mirror to enable the compensation of the detected distortions in a light beam. These adaptive optical systems find more and more applications in astronomy, industry and have recently been introduced in medical setups. We use the Hartmann wavefront-sensing method, in which a light beam is sampled into a number of sub-beams, which are projected on a plane. The displacements of the light spots on this plane are used to estimate the profile of the associated arbitrary wavefront. Usually, a conventional image sensor registers the light spots and an image-processing algorithm computes their displacements from a reference grid. However, this approach restricts the operational speed of the sensor because it relies on the frame-transfer rate of the imager and on the rather slow data-reduction algorithm. The novel contribution of this work is that, to render faster operation, we introduce a Hartmann wavefront sensor that uses an addressable matrix of CMOS integrated position-sensitive detectors, namely quad cells. Each sampled light spot is associated with a quad cell such that direct information about the spot-centroid displacement is available. Each particular application has its own requirements and the developed sensor, which should be used with light sources in the visible spectrum, shall perform well for applications involving enough light (>0.5 mW beams), and there where lower-order wavefront aberrations are expected (

Hybrid curvature and gradient wave-front sensor

A new wave-front sensor for adaptive optics that gives signals for the curvature and the two gradient components of the wave front simultaneously is proposed. The sensor uses quad cells placed at the foci of an array of astigmatic lenslets, giving three intensity-normalized differential signals. The parameters of a single subdetector are optimized to maximize the curvature signal. The performance of such a sensor designed for a membrane curvature mirror is analyzed. The good match between sensor and mirror is exhibited by an almost diagonal response matrix between the mirror actuator controls and the curvature signals.

A Quadtree-based adaptive Cartesian/Quad grid flow solver for Navier-Stokes equations

Quadtree-based adaptive Cartesian/Quad (quadrilateral) grid generator, grid adaptor and flow solver have been developed and fully demonstrated in this study. Given the definition of geometries (cubic B-splines and straight line segments), geometrically-adaptive, body-fitted Quad grids are generated automatically. These grids are then overlapped with a large Cartesian cell which is sub-divided recursively until the Cartesian cells which are intersected by the outer boundaries of the Quad grids have similar grid resolutions to the Quad cells. The final computational grid is then automatically produced through cell cutting. A Quadtree data structure has been used to store the grids. As a result, grid refinement and coarsening are trivial to accomplish. A cell-centered, second-order accurate viscous flow solver has been developed. The flow solver supports arbitrary control volumes. Solution-based grid adaptations are carried out after converged solutions are obtained on a given grid. Grid independent inviscid and viscous solutions have been obtained through automatic grid adaptations. The overall solution procedure integrates grid generation, grid adaptation and flow solver into an automated computing environment to achieve maximum efficiency and accuracy with minimum human and computer resources.