Demonstration of a vectorial optical field generator with adaptive close loop control.

Generation of vectorial optical fields with complex spatial distribution in the cross section is of great interest in areas where exotic optical fields are desired, including particle manipulation, optical nanofabrication, beam shaping and optical imaging. In this work, a vectorial optical field generator capable of creating arbitrarily complex beam cross section is designed, built and tested. Based on two reflective phase-only liquid crystal spatial light modulators, this generator is capable of controlling all the parameters of the spatial distributions of an optical field, including the phase, amplitude and polarization (ellipticity and orientation) on a pixel-by-pixel basis. Various optical fields containing phase, amplitude and/or polarization modulations are successfully generated and tested using Stokes parameter measurement to demonstrate the capability and versatility of this optical field generator.

Vectorial optical fields: recent advances and future prospects

Based on the Fraunhofer diffraction theory and the properties of dove prism, the coordinate relationships among the four spatial light modulator (SLM) sections in a vectorial optical field generator (VOF-Gen) are derived and experimentally verified. Taking the coordinate system of the first SLM section as reference, the coordinate displacements between the first SLM section and the subsequent ones are measured through employing the specially designed four-quadrant patterns with varying shifts of the corresponding cross point. Complex optical field could be accurately generated by combining the derived coordinate relationships and pre-compensation of the measured coordinate displacements. A typical complex optical field is generated and the experimental results demonstrate the validity of the proposed transverse alignment method for the VOF-Gen.

We demonstrate a technique for the generation of vectorial optical fields based on a single pass of light onto a reflective twisted nematic spatial light modulator (TN-SLM). Using a beam splitter and a quarter-wave plate, a polarization rotator is implemented. A curve of the behavior of this polarization rotator with the twisted nematic modulator in use proves a modulation up to 177 deg, making this setup suitable for engineering of cylindrical vector fields. A mathematical justification and an experimental demonstration of the generation of these beams and their quality are presented. The experimental results show that despite the presence of a small component of elliptical polarization, a twisted nematic modulator can be used to achieve local polarization control. A series of vector fields with variations of the parameters of topological charge, phase offset, and quadratic phase are engineered, analyzed with a linear polarizer, and captured. The proposed setup represents a simple approach for polarization manipulation using a TN-SLM and few optical elements, achieving polarization purity greater than 94%.

view Abstract Citations (21) References (42) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Interstellar Circular Polarization and the Dielectric Nature of Dust Grains Chlewicki, Grzegorz ; Greenberg, J. Mayo Abstract The relationship between interstellar linear and circular polarization is reexamined. The derivation of optical constants for relevant materials is described, emphasizing general constraints on the electromagnetic response of an optical medium (Kramers-Kronig relations) which also provide the basis for the subsequent analysis of interstellar polarization. The formulas that relate the observed polarization to the properties of individual grains are summarized. It is shown that the apparent upper limit on the visual absorptivity of grains derived from earlier Mie theory calculations results from a violation of the Kramers-Kronig relations in the assumed optical constants of the particles. The analysis of circular and linear polarization in the interstellar medium is presented, and the implications of this analysis for grain models are addressed. Publication: The Astrophysical Journal Pub Date: December 1990 DOI: 10.1086/169474 Bibcode: 1990ApJ...365..230C Keywords: Circular Polarization; Dielectrics; Interstellar Extinction; Interstellar Matter; Optical Properties; Interstellar Gas; Linear Polarization; Mie Scattering; Astrophysics; INTERSTELLAR: GRAINS; POLARIZATION full text sources ADS |

Active unsteady flow control experiments were performed on a natural laminar flow airfoil at . The goal of this study was to control boundary-layer separation across the trailing-edge region of the airfoil in off-design conditions. Active control of separation was achieved using a series of blowing slots at the location. An adaptive closed-loop controller was developed based on the empirical mode decomposition algorithm. This controller was capable of automatically identifying the frequencies of natural instabilities in the flowfield, which were then used to set the driving frequencies of the flow control system. The airfoil performance with closed-loop frequency control was compared against a canonical open loop actuation. The differences in flowfield characteristics between these two cases were investigated in detail using particle image velocimetry measurements across the trailing-edge region of the airfoil. Closed-loop frequency control was observed to be associated with more effective alleviation of flow separation, having a shear layer closer to the surface of the airfoil in comparison with actuation. The empirical mode decomposition algorithm was also used to simultaneously perform closed-loop control of the actuation frequency and amplitude to achieve a desired value of .

Developing closed-loop brain stimulation systems can benefit the treatment of neurological and neuropsychiatric disorders and facilitate brain functions. Current designs of closed-loop controllers have used time-invariant linear models of brain activity to devise non-adaptive controllers. However, unmodeled nonlinear dynamics can happen during real-time closed-loop control, leading to nonlinear uncertainty in the brain activity model. Current non-adaptive controllers cannot track the nonlinear model uncertainty and are not robust to noise, both of which can compromise their control performance. Here, within an ℒ1 adaptive control framework, we develop a new discrete-time robust and adaptive closed-loop control algorithm that addresses a general form of nonlinear model uncertainty. We conduct Monte Carlo simulations to validate the robust and adaptive control algorithm and show that it significantly outperforms existing closed-loop control algorithms. Our results can facilitate future designs of precise and safe closed-loop brain stimulation systems to treat neurological and neuropsychiatric disorders and modulate brain functions.

In this paper we investigate the effect of an adaptive fuzzy closed-loop power control (CLPC) scheme on the performance of the DS/CDMA-based mobile satellite communication systems with a low earth orbit (LEO) satellite constellation in which the long round trip delay (RTD) and fading effect according to elevation angle should be considered. Since the RTD between mobile terminals (MT) and earth station (ES) via satellite in LEO is on the order of 10-20 ns and fading is more severe, a conventional closed-loop power control with fixed step size is much less effective than it is on a terrestrial channel. In order to cope with this performance degradation, we propose a closed-loop power control scheme by adaptive fuzzy rule-based step size decision method which is based on fuzzy logic control (FLC). So, in this paper, we compare the performance of the adaptive fuzzy closed-loop power control with that of the conventional fixed step closed-loop power control, we show that the proposed method makes better performance.

In this paper, a closed-loop control has been developed for the exoskeleton robot system based on brain-machine interface (BMI). Adaptive controllers in joint space, a redundancy resolution method at the velocity level, and commands that generated from BMI in task space have been integrated effectively to make the robot perform manipulation tasks controlled by human operator's electroencephalogram. By extracting the features from neural activity, the proposed intention decoding algorithm can generate the commands to control the exoskeleton robot. To achieve optimal motion, a redundancy resolution at the velocity level has been implemented through neural dynamics optimization. Considering human-robot interaction force as well as coupled dynamics during the exoskeleton operation, an adaptive controller with redundancy resolution has been designed to drive the exoskeleton tracking the planned trajectory in human brain and to offer a convenient method of dynamics compensation with minimal knowledge of the dynamics parameters of the exoskeleton robot. Extensive experiments which employed a few subjects have been carried out. In the experiments, subjects successfully fulfilled the given manipulation tasks with convergence of tracking errors, which verified that the proposed brain-controlled exoskeleton robot system is effective.

Under illumination of a solid absorbing target by short laser pulse the acoustic waves are excited both inside the target and in the surrounding air. When the surface of the target is plane the shape of acoustic signal in the air depends on the intensity distribution in the illuminating laser beam. In this report the results of restoration of two-dimensional intensity distribution of pulse laser beam by method of acoustic tomography are presented. As the target a plane metal plate was used. Measurements of acoustic signals were carried out by nine microphones placed around the illuminating beam not far from the target. Two-dimensional intensity distribution of laser beam obtained by acoustic methods are in good agreement with data obtained by usual photometric method.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

This article presents the implementation of closed-loop simple adaptive control on fixed-wing micro air vehicle dynamics to improve flight performance characteristics. It is known that to retain the micro air vehicle system performance during the entire flight regime is difficult due to model uncertainties, large parameter variation and wind disturbances compared to flight velocity. An adaptive controller can adapt to the uncertainties but the complexity involved in their implementation is high due to unavailability of required sensor information and computational resources on a micro air vehicle platform. Lack of flight test results in the open literature incorporating adaptive control so far can be partially attributed to this complexity. In this case, adaptive control architecture is implemented in such a way that only the uncertainties in the system dynamics are taken care of by the adaptive control and desired nominal plant performance is achieved by the basic controller. The proposed adaptive controller architecture is implemented in real flight test, and improvement of tracking performance over a proportional–integral–derivative controller is demonstrated which illustrates superior performance to conventional architectures. The proposed design approach can be implemented easily to an existing system, and system performance can be enhanced in the presence of unmodelled and uncertain system dynamics.

This paper proposes an indirect adaptive fuzzy neural network (FNN) controller with state observer and supervisory controller for a class of uncertain nonlinear dynamic time delay systems. First, the approximate function of unknown time delay system is inferred by the adaptive time delay FNN system. Next, a state observer is designed to estimate the unknown system states and the indirect adaptive fuzzy controller is constructed. Finally, the closed loop controller is obtained by incorporating the supervisory controller with the indirect adaptive fuzzy controller. Therefore, if the system tends to unstable, i.e., error dynamics is larger than a prescribed constraint which is determined by designer, the supervisory controller will activate to force the state to be stable. The free parameters of the indirect adaptive FNN controller can be tuned online by observer-based output feedback control law and adaptive laws by means of Lyapunov stability criterion. The resulting simulation example shows that the performance of nonlinear time delay chaotic system is fully tracking the reference trajectory. Meanwhile, simulation results show that the adaptive control effort of the proposed control scheme is much less due to the assist of the supervisory controller.

Four-petal Gaussian beam is a special type of Gaussian beam, and its propagation properties are widely used in micro optics, optical communication and splitting technology. Recently, the generations and the properties of different types of hollow beams have become a hot research topic, such as research on hollow optical vortex beams. The Gyrator transform can be used to fulfill the mode conversion of laser beam. In this paper, based on the Gyrator transform, the analytical expression of four-petal Gaussian beam passing through such a transform system is derived, and the intensity distribution and the corresponding phase distribution associated with the transforming four-petal Gaussian beam are analyzed by numerical simulations. It is found that the four-petal Gaussian beam can be transformed into rectangular hollow beam by Gyrator transform, under the appropriate conditions of the beam order, the beam parameter, the transform angle of Gyrator transform, and the waist width. For the beam order n=m=3, the transform angle of Gyrator transform = 0.4133, the beam parameter K=30, and the waist width = 0.9, the rectangular hollow optical vortex beams can be obtained. Under such conditions, the maximum intensities appear in the four corners, and they are almost uniform on the four sides. The effects of the beam parameters, the transform angle, and the beam order on the distributions of intensity and phase of the rectangular hollow beam are analyzed in detail. The numerical results show that for the beam parameter K10, the rectangular hollow beam always is obtained, and for a lager beam parameter, the intensity distribution of the rectangular hollow beam is more uniform. Different beam order generates different type of hollow beam. For example, for n=m = 2, = 1.2, K = 30, and = 0.5409, a new strange circular hollow beam with solid circular nucleus can be obtained. The transform angle of Gyrator transform has a significant effect on the energy distribution of the hollow beam. When the transform angle changes in a small range, the uniformity of the intensity distribution of the rectangular hollow beam is lost. The bigger the transform angle change, the more serious the loss of uniformity of the hollow beam intensity is. The size of the hollow beam bright ring is determined by the waist width of the four-petal Gaussian beam: the larger the waist width, the smaller the bright ring is. The results further enriches the applications of Gyrator transform system and the four-petal Gaussian beam in the beam shaping.

Compact vectorial optical field generator based on a 10-megapixel resolution liquid crystal spatial light modulator

This paper addresses an adaptive rigid-body pose-tracking control problem with unknown system parameters. The proposed control is based on a noncertainty equivalence (NCE) adaptive principle applied to the configuration manifold of rigid-body motion, i.e., special Euclidean group SE(3), and its tangent bundle TSE(3). The controller drives the system to follow an arbitrary reference trajectory with bounded time derivatives. The system states are composed of the elements of SE(3), consisting of rotations and translations, and the six-dimensional vector of angular and translational velocities in Euclidean space. Almost-global-asymptotic stability of the system is demonstrated using a Morse–Lyapunov stability analysis. Then, the performance of the proposed controller is verified and is compared to the results obtained by a certainty-equivalence (CE) adaptive control applied to exponential coordinates associated with the linear space of the Lie algebra . It is shown that the TSE(3)-based NCE adaptive control design can eliminate the performance degradation due to the cancellation of uncertain parameter effects that would otherwise appear when the CE adaptive controller is used. As a result, the proposed NCE-based adaptive pose-tracking controller recovers the closed-loop pose-tracking controller performance with the system parameters fully known to the controller.