Catálogo de publicaciones - libros

Interferometer-based adaptive optics has an advantage of direct measurement of the wavefront profile. Nevertheless the majority of adaptive optical systems, realized so far, use other types of wavefront sensors, such as Hartmann sensors. Interferometric sensors have two problems: (1) a source of a coherent reference wave should be present and (2) in many cases it is impossible to reconstruct the wavefront unambiguously from a single interferogram, due to the illposed nature of the phase unwrapping problem. In the case of an adaptive optical system with a limited number of degrees of freedom, one can expect that the ambiguity of the solution will be partly or even completely eliminated by looking for the wavefront reconstruction only in the existing basis. We report on the expected performance of interferometric adaptive optical system with a 37- channel membrane deformable mirror, based on a computer model of a complete system.

A combination of a Hartmann-Shack sensor and a standard farfield measurement on one single detector is proposed. The technique is fast, manages without moveable parts, thus permitting a very compact design. It is not only suited for characterisation of the wavefront distribution, but may also be considered for determination of the important parameters beam width, beam divergence and beam propagation ratio M of partially coherent laser beams. First results indicate that a fairly thorough beam characterisation including spatial coherence, propagation characteristics and beam quality can be achieved with this method.

High resolution wavefront sensors are devices with a great practical interest since they are becoming a key part in an increasing number of applications like extreme Adaptive Optics. We describe theoretically a novel wavefront sensor, which basically consists of a telescopic system with a linearly increasing amplitude mask placed at the intermediate common focal plane. This sensor offers high resolution and an easy adjustment of the sampling and of the dynamic range. The parameters and performance of the new sensor are discussed, and a comparison with commonly used Hartmann-Shack sensors is carried out. Our sensor presents several advantages. Resolution is higher, and consequently a larger number of modes can be estimated for reconstructing the wavefront, and the dynamic range and sampling can be easily adjusted. Furthermore, we show that a proper election of the mask parameters allows an acceptable performance even in adverse photon noise conditions.

Kestrel Corporation has extensive experience using distorted grating wavefront sensors (DGWFS) in a number of applications. The DGWFS has previously been demonstrated in the visible range (400–700 nm) by Kestrel and others. An experimental system was built in a laboratory environment to show that the DGWFS could recover wavefront characteristics of a midwave infrared (MWIR) laser. This paper describes the theory of the DGWFS and the experimental procedures implemented to run the system with the MWIR laser. The sensitivity to the type of gratings employed will be addressed. The results of sensitivity, dynamic range and thermal noise measurements will be discussed.

The aim of this work was to determine the relative performance of a Shack-Hartmann (SH) wavefront sensor and a distorted grating wavefront curvature sensor (DGWFS) when used to measure the aberrations in the human eye. Previous work carried out by Kestrel and others suggests that the DGWFS is able to successfully reconstruct wavefronts in severely scintillated conditions in which SH sensors typically fail to give a good reconstruction. The poor performance of conventional SH sensors in scintillated conditions prevents their use in ophthalmic aberrometers with human subjects who have medical conditions such as cataracts. This limitation substantially restricts the percentage of the population that can take advantage of emerging technology enabled by having accurate aberration data for the anterior segment. The SH sensor utilized has a novel dithered reference source which mitigates scintillation problems. However, the DGWFS potentially offers a simpler, lower cost and more robust solution.

The measurement of the deviations of a test sample from its ideal shape is the key to quality assurance in fabrication processes of the optical industry. Here, interferometers are often used as standard tools. However, in recent years Shack-Hartmann sensors have been introduced in various applications with competitive performance. We show two typical application examples, the null test of a sphere where the highest accuracy is obtained and a non null test of an aspherical surface. In the latter case the Shack-Hartmann sensor is superior to an interferometer due to its higher dynamic range.

We describe existing concepts for CMOS-based Hartmann-Shack wavefront sensors and propose the next step for a further miniaturization of adaptive optics by integrating the modal wavefront reconstruction on chip. In conventional Hartmann-Shack wavefront sensing a CCD camera is placed on the focal plane of a microlens array. The spot pattern in the focal plane is captured, analyzed by image processing and the wavefront is often decomposed into orthogonal modes, usually expressed in coe.cients of Zernike polynomials. The single-chip modal wavefront reconstructor would include all these processing steps in a single chip. We propose to use already existing CMOS-based wavefront sensor concepts and add a hardware arti.cial neural network for modal wavefront reconstruction.

We assess, in this paper, the use of CMOS technology (Complementary-Metal-Oxide-Semiconductor) for the fabrication of fast wavefront sensors based on the Hartmann-Shack technique. We briefly recapitulate the core of this technology and we point out its pros and cons with respect to the sensor performance. Focusing on fast operation, we compare conventional image sensors with custom layout approaches that make use of position-sensitive detectors (PSDs). We also present the results obtained with three different CMOS sensor concepts implemented so far:

Heidelberg1: 0.6 μm — AMS — 16 × 16-PSDs alternate — winner-take-all digital readout

Heidelberg2: 0.35 μm — AMS — 8 × 8-PSDs chessboard-like — winner-take-all / resistive ring [2]

Delft1: 1.6 μm — DIMES — 8 × 8-PSDs quad cell — passive pixels [3, 4] The aim is to identify the practical capabilities of standard CMOS technology in wavefront sensing.

Phase-Diversity is an algorithm for reconstruction of wavefront phase from data corresponding to images of the input wavefront intensity on two planes normal to the direction of propagation and located at different positions along the axis of propagation. These planes are generally described as symmetrically placed about the image plane, but can equally well be symmetrically placed about the system input pupil. In this case the phase diversity algorithm becomes essentially the same as the wavefront curvature algorithm. For reconstruction of the wavefront phase the inverse problem is presented in terms of the di.erential Intensity Transport Equation and solved either iteratively or through use of Green’s functions. Here we will explore what other aberrations, other than defocus, can be used in a generalised phase diversity wavefront reconstruction. The possible advantages of this approach will be considered.

Some early results demonstrating the performance of the Generalised Phase Diversity Wavefront Sensor were presented. In these computer simulations we would seek to validate the theoretical analysis that we have previously published and to explore the optimisation of the sensor for various forms of wavefront error. Consideration would be given to the extent to which optimisation that exploits information about the wavefront decreases the chance to detect other wavefront characteristics.