This paper introduces a new technique for performing binary adaptive optics using optical components only, without the help of any electronic, or optoelectronic device. In this technique, the interferogram produced by the interference of a reference plane wave and the distorted wave modulates a light-driven crystal. The modulated light-driven crystal may produce pupil-plane only-phase or only-amplitude binary masks to mitigate phase aberrations. This new setup is more energy efficient and keeps the previous advantages as the capability of working unsupported. These features make it suitable for applications in hard-to-reach or hazardous locations such as satellites, underwater or contaminated places. The paper includes a simple experimental validation.
New discoveries in astronomy require the use of ever larger telescopes. These telescopes require segmented apertures, since current technology does not provide for the fabrication of monolithic mirrors with diameters much larger than 8 m. As a result, the point spread function (PSF) of the system presents a diffraction pattern characteristic of the segmentation geometry, and this complicates the task of detecting faint structures with high spatial resolution. In this work, we investigate the use of amplitude filters at the pupil plane to decrease the intensity of this diffraction pattern. It is sufficient if this diffraction pattern is reduced below the residual halo that remains after adaptive optics compensation, so that the intensity distribution that surrounds the PSF core is limited by scattering rather than diffraction. We show different kinds of filters that allow us to achieve this goal. We analyze their performance, taking into account some unavoidable errors in the telescope operation such as imperfect phasing of the telescope segments and gaps between them. Finally, we discuss the application of the technique considering errors in the adaptive optics correction.
There are two main ways to address the wavefront sensing problem with modal wavefront sensors. First, the slope approximation that estimates a series of local first derivatives of the wavefront, as in the Hartmann-Shack sensor. Secondly, the curvature approximation, that estimates a series of second derivatives of the wavefront in different areas, as in the curvature sensor. It has been demonstrated that optical differentiation can be used as a useful first derivative wavefront sensor. Here we present a complete review of this new sensor along with a novel procedure to estimate the curvature of the wavefront phase using optical differentiation. This sensor consists of a telescopic system located in one of the arms of an interferometer and a phase step in the other arm. A variable amplitude transmission mask is placed at the focal plane of the telescopic system to perform the second derivative of the incoming field. A detailed description of the set-up and the mask is presented. The main advantages of this sensor are high resolution and easily adjusting of the sampling of the wavefront so allowing its use in high resolution wavefront sensing.
KEYWORDS: Point spread functions, Transmittance, Polarizers, Optical storage, Confocal microscopy, Diffraction, Super resolution, Photonics, Image processing, Control systems
The light intensity distribution near focus of an optical system can be modified by pupil-plane masks. In fact, in some relevant fields, such as optical data storage or confocal microscopy, there is an increasing interest in the design of pupil masks for controlling the Point Spread Function (PSF). These techniques present a great advantage for educational purposes, since they deal with current technology but at the same time illustrate basic diffraction principles. In this work, we introduce a simple low-cost set up that allows the dynamical control of the PSF and can be easily implemented in any educational centre.
An increasing number of both civilian and military applications require the motion description of translating targets from a sequence of frames acquired through long atmospheric paths. These images are randomly distorted, due to atmospheric turbulence, although adaptive optics systems can partially compensate for this distortion in real time. In these adverse conditions, a velocimetry technique that is based on the spatio-temporal Fourier transform of a series of images presents several advantageous features. In those cases where the target is very dim or an additional processing time reduction is needed, low-light-level images are recorded. Consequently, we have developed a simulation algorithm that generates atmospherically distorted low-light-level images corresponding to different atmospheric conditions and different degrees of compensation. In this paper, simulated low-light-level images are used to analyze the technique accuracy for estimating the object velocity for several atmospheric conditions and for different correction degrees.
The appearance of commercial spatial light modulators (SLM) opens new ways for teaching some optical phenomena. There are possible applications in a great variety of fields: interferometry, diffraction theory, simulation and compensation of random media, Fourier Optics, etc. In this paper, we propose the use of low cost liquid crystals displays (LCDs) as SLMs to perform some interesting optical experiments. The liquid crystal SLMs are extracted from a commercial video projector. This is one of the cheapest ways to obtain a SLM. For phase modulation, it requires the calibration of the system, because the manufacturers do not provide the physical specifications of the LCDs. This work is quite instructive since many different aspects are involved in the calibration process. Finally, we show an experiment using this setup, which demonstrates that the proposed SLM is an easy-to-use and flexible tool to show some well-known optical phenomena.
The resolution limit attainable by large ground-based telescopes is limited by the refraction index fluctuations of the turbulent atmosphere. To overcome this limitation Adaptive Optics systems measure and compensate atmospheric-induced distortions. When working in the visible, only a partial compensation is possible. We analyze the image formation process in these conditions. We provide a model for the wave front statistics, which allows a simple estimate of the instantaneous PSF. New deconvolution techniques could be developed by using this approximation.
Random aberrations due to atmospheric turbulence determine the angular resolution of ground-based telescopes. Adaptive optics systems compensate the wavefront degradation before detection. Although systems with a large number of subapertures in the wavefront sensor and of actuators in the deformable mirror provide the best results, they are complicated and expensive. In contrast, simpler adaptive optics systems, (less than one actuator per atmospheric coherence diameter), compensate partially the wavefront distortions, having great potential application. The statistics of the image plane light intensity in partial compensation have been described using the Rician distribution. In order to achieve a more complete description of the phenomena, we describe the photon statistics in the whole image plane using the Poisson transform of the Rician distribution. When there is no compensation, the photon statistics follow a Bose-Einstein distribution. In partial compensation the PSF is composed by a bright core, where the photon statistics follow a Laguerre distribution, surrounded by a speckled halo with Bose- Einstein statistics. Some special cases have been studied and useful approximations have been derived. Theoretical results fit well with simulated values. This description of the light statistics as a function of the compensation may be used to extract more information about the object.
Many optical systems work on the basis of the correclion of the incoming wave-front. This correction has to be
as complete as possible. In actual experiments, when working in the visible, only a partial compensation is
attainable. In this case correction can be quite good at the image center where the light intensity remains almost
constant, depending on the degree of correction performed over the wave-front. The aim of this paper is to
analyze the intensity statistics of the light in the image center as a function of the number of Zernike
polynomials corrected when adaptive optics systems are used.
The spatio-temporal Fourier transform is usually applied to determine the velocity of an object from a series of standard light intensity frames. In this paper the technique is extended to also determine the object acceleration. Although this technique is useful in standard illumination conditions, we have applied it to experimental low-light-level images, which require a shorter processing time.
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