Basic image-processing operations by use of acousto-optics

We describe some basic optical image-processing operations with acousto-optic (AO) Bragg diffraction. Instead of using frequency-plane filters, we place an AO cell behind the object. We then realize experimentally one-dimensional edge enhancement, which utilizes a high-pass filtering effect in the undiffracted order from the AO cell. A numerical simulation compares well with the experimental results. With two AO cells oriented orthogonally to each other, a second-order mixed derivative operation, evident from the four-corner enhancement of a square, is also demonstrated.     

Correcting the aero-optical aberration of the supersonic mixing layer with adaptive optics: concept validation

We describe an adaptive optics (AO) system for correcting the aero-optical aberration of the supersonic mixing layer and test its performance with numerical simulations. The AO system is based on the measurement of distributed Strehl ratios and the stochastic parallel gradient descent (SPGD) algorithm. The aero-optical aberration is computed by the direct numerical simulation of a two-dimensional supersonic mixing layer. When the SPGD algorithm is applied directly, the AO cannot give effective corrections. This paper suggests two strategies to improve the performance of the SPGD algorithm for use in aero-optics. The first one is using an iteration process keeping finite memory, and the second is based on the frozen hypothesis. With these modifications, the performance of AO is improved and the aero-optical aberration can be corrected to some noticeable extent. The possibility of experimental implementation is also discussed.     

Adaptive optics performance model for optical interferometry

The optical interferometry community has discussed the possibility of using adaptive optics (AO) on apertures much larger than the atmospheric coherence length in order to increase the sensitivity of an interferometer, although few quantitative models have been investigated. The aim of this paper is to develop an analytic model of an AO-equipped interferometer and to use it to quantify, in relative terms, the gains that may be achieved over an interferometer equipped only with tip-tilt correction. Functional forms are derived for wavefront errors as a function of spatial and temporal coherence scales and flux and applied to the AO and tip-tilt cases. In both cases, the AO and fringe detection systems operate in the same spectral region, with the sharing ratio and subaperture size as adjustable parameters, and with the interferometer beams assumed to be spatially filtered after wavefront correction. It is concluded that the use of AO improves the performance of the interferometer in three ways. First, at the optimal aperture size for a tip-tilt system, the AO system is as much as ~50% more sensitive. Second, the sensitivity of the AO system continues to improve with increasing aperture size. And third, the signal-to-noise ratio of low-visibility fringes in the bright-star limit is significantly improved over the tip-tilt case.     

Optimal reconstruction for closed-loop ground-layer adaptive optics with elongated spots

The design of the laser-guide-star-based adaptive optics (AO) systems for the Extremely Large Telescopes requires careful study of the issue of elongated spots produced on Shack-Hartmann wavefront sensors. The importance of a correct modeling of the nonuniformity and correlations of the noise induced by this elongation has already been demonstrated for wavefront reconstruction. We report here on the first (to our knowledge) end-to-end simulations of closed-loop ground-layer AO with laser guide stars with such an improved noise model. The results are compared with the level of performance predicted by a classical noise model for the reconstruction. The performance is studied in terms of ensquared energy and confirms that, thanks to the improved noise model, central or side launching of the lasers does not affect the performance with respect to the laser guide stars' flux. These two launching schemes also perform similarly whatever the atmospheric turbulence strength.     

Halo characteristics and their influence on companion searches at the starfire optical range

To image extrasolar planets at their large contrast, high-resolution adaptive optics (AO) is needed to correct atmospheric seeing. The 1.5-m AO system at the Starfire Optical Range was used to confirm theoretical models. Halo levels were reduced by a factor of 4, on average, from 0.5 to 3.0 arc sec radius, which when combined with the increased Strehl ratio improved the gain by a factor of 80. Speckle lifetimes ranged from 5 to 30 ms at 0.3 arc sec, which is much longer than the 0.6-ms AO update time. These results show good agreement with predictions for current technology and reveal no limitations, in principle, to the detection of planets by use of AO systems with higher speeds and resolutions.     

Self-characterization of linear and nonlinear adaptive optics systems

We present methods used to determine the linear or nonlinear static response and the linear dynamic response of an adaptive optics (AO) system. This AO system consists of a nonlinear microelectromechanical systems deformable mirror (DM), a linear tip-tilt mirror (TTM), a control computer, and a Shack-Hartmann wavefront sensor. The system is modeled using a single-input-single-output structure to determine the one-dimensional transfer function of the dynamic response of the chain of system hardware. An AO system has been shown to be able to characterize its own response without additional instrumentation. Experimentally determined models are given for a TTM and a DM.     

Optimization of doubly Q-switched lasers with both an acousto-optic modulator and a GaAs saturable absorber

A doubly Q-switched laser with both an acousto-optic (AO) modulator and a GaAs saturable absorber can obtain a more symmetric and shorter pulse with high pulse peak power, which has been experimentally proved. The key parameters of an optimally coupled doubly Q-switched laser with both an AO modulator and a GaAs saturable absorber are determined, and a group of general curves are generated for what we believe is the first time, when the single-photon absorption (SPA) and two-photon absorption (TPA) processes of GaAs are combined, and the Gaussian spatial distributions of the intracavity photon density and the initial population-inversion density as well as the influence of the AO Q-switch are considered. These key parameters include the optimal normalized coupling parameter, the optimal normalized GaAs saturable absorber parameters, and the normalized parameters of the AO Q-switch, which can maximize the output energy. Meanwhile, the corresponding normalized energy, the normalized peak power, and the normalized pulse width are given. The curves clearly show the dependence of the optimal key parameters on the parameters of the gain medium, the GaAs saturable absorber, the AO Q-switch, and the resonator. Sample calculations for a diode-pumped Nd3+:YVO4 laser with both an AO modulator and a GaAs saturable absorber are presented to demonstrate the use of the curves and the relevant formulas.     

Improved tribo-mechanical behavior of CaP-containing TiO2 layers produced on titanium by shot blasting and micro-arc oxidation

The combination of shot blasting (SB) and micro-arc oxidation (or anodic oxidation—AO) in titanium surfaces was shown to provide enhanced conditions for cell differentiation and osseointegration than those provided by SB or AO alone. This study associated both methods aiming to attain titania layers on Ti with adequate tribo-mechanical features for bone implants. SB was performed using alumina particles, and titania layers were grown by AO using a CaP-based electrolyte. Mechanical properties and scratch resistance were characterized at nanoscale by instrumented indentation and nanoscratch, and correlated with morphological and microstructural changes (XRD, SEM, EDS, AFM, and profilometry). Analytical methods were employed to correct roughness and substrate effects on the indentation results. CaP-containing TiO₂ layers were produced on AO and SB + AO. The latter presented small pore size and inhomogeneous layer thickness and Ca/P ratios, caused by the non-uniform surface straining by SB that affects the oxide growth kinetics in the electrochemical process. Elastic modulus of SB + AO layer (37 GPa) were lower than the AO one (45 GPa); both of them were smaller than bulk Ti (130 GPa) and close to bone values. The hardness profiles of AO and SB + AO were similar to the substrate ones. Because of the improved load bearing capacity and unique layer features, the critical load to remove the SB + AO titania coating in scratch tests was three times as much or higher than in AO. Results indicate improved mechanical biocompatibility and tribological strength of anodic titania layers grown on sand blasted Ti surfaces.     

Investigation on sol–gel boehmite-AlOOH films on Kapton and their erosion resistance to atomic oxygen

To improve the atomic oxygen (AO) erosion resistance of Kapton, boehmite-AlOOH films were deposited on it by sol–gel method and AO exposure experiments were performed in a ground-based AO simulator. The results indicate that the AlOOH-coated samples show an improved AO resistance and their erosion yield is one order of magnitude less than that of pristine Kapton. Furthermore, the AlOOH-coated Kapton remained optically stable under AO exposure. The AlOOH film structure before and after AO exposure was analyzed by scanning electronic microscope, atomic force microscopy, X-ray photoelectron spectroscopy and X-ray diffraction. It was found that after AO exposure, the film structure tends to transfer from an octahedral coordination for AlOOH to an octahedral and tetrahedral mixed-coordination for γ-Al₂O₃. This implies that a more stable γ-Al₂O₃ structure could be formed in AlOOH film during AO exposure. The AO erosion mechanism of the coated Kapton was discussed.     

Effect of wavefront corrugations on fringe motion in an astronomical interferometer with spatial filters

Numerical simulations of atmospheric turbulence and adaptive optics (AO) wavefront correction are performed to investigate the time scale for fringe motion in optical interferometers with spatial filters. These simulations focus especially on partial AO correction, where only a finite number of Zernike modes are compensated. The fringe motion is found to depend strongly on both the aperture diameter and the level of AO correction used. In all the simulations the coherence time scale for interference fringes is found to decrease dramatically when the Strehl ratio provided by the AO correction is < or = 30%. For AO systems that give perfect compensation of a limited number of Zernike modes, the aperture size that gives the optimum signal for fringe phase tracking is calculated. For AO systems that provide noisy compensation of Zernike modes (but are perfectly piston neutral), the noise properties of the AO system determine the coherence time scale of the fringes when the Strehl ratio is < or = 30%.     

Rotation angle measurement based on white-light interferometry with a standard optical flat

We propose a simple white-light interferometric method of measuring a one-dimensional rotation angle with use of an optical plane parallel plate of standard refractive index. The phase change of the interference pattern of the interferometer during the rotation of the flat plate of known refractive index and thickness placed in one of the interferometer's arms is used for determination of the rotation angle. This method has been demonstrated for an accurate angle measurement over the angle range from 0° to 40° within a maximum uncertainty of 0.057°.     

Extracting one-dimensional wavelet features with a diffractive optical inner-product transform

A diffractive optical element (DOE) that performs a wavelet transform on one-dimensional signals is presented. The DOE performs an inner product between the input image and a set of wavelet basis functions. The input image is displayed in a spatial light modulator. It is constructed from a one-dimensional signal that is spread over the second dimension. Simulated results obtained by the use of elementary one-dimensional signals are presented.     

Investigation of magnesium fluoride crystals for imaging acousto-optic tunable filter applications

Results of an investigation of acousto-optic (AO) cells using single crystals of magnesium fluoride (MgF2) are presented. Two acousto-optic tunable filter (AOTF) cells for imaging application have been designed and tested in the visible and ultraviolet (UV) regions of the spectrum from 190 to 490 nm. The two imaging filters were developed by using the wide-angle AO interaction geometry in the (010) and (11 0) planes of the crystal. These filters were used to obtain spectral images at the shortest wavelengths achieved so far. Advantages and drawbacks of this crystal are discussed and photoelastic, acoustic, and AO properties of MgF2 are examined. The investigation confirmed that MgF2-based AOTF cells can be used in the deep UV region up to 110 nm.     

Effect of aberrations and scatter on image resolution assessed by adaptive optics retinal section imaging

The effect of increased high-order wavefront aberrations on image resolution was investigated, and the performance of adaptive optics (AO) for correcting wavefront error in the presence of increased light scatter was assessed in a model eye. An AO section imaging system provided an oblique view of a model retina and incorporated a wavefront sensor and deformable mirror for measurement and compensation of wavefront aberrations. Image resolution was quantified by the width of a Lorentzian curve fitted to a laser line image. Wavefront aberrations were significantly reduced with AO, resulting in improvement of image resolution. In the model eye, image resolution was degraded with increased high-order wavefront aberrations (horizontal coma and spherical) and improved with AO correction of wavefront error in the presence of increased light scatter. The findings of the current study suggest that AO imaging systems can potentially improve image resolution in aging eyes with increased aberrations and scatter.     

Spectrometer slit-power-coupling calculations for natural and laser guide-star adaptive optics

We use numerical calculations to examine the relation between adaptive optics (AO) turbulence compensation and power coupled through a spectrometer slit for both laser and natural guide-star AO systems. The AO system and observing parameters used are relevant to the Gemini-North 8-m telescope. For this study, we separate residual tilt from residual higher-order aberrations to isolate their relative effects under a variety of operating conditions. Our results demonstrate that slit-coupled intensity is not uniquely determined by system Strehl alone; we show that this is due to the differing effects of higher-order and tilt aberrations on the shape of the compensated point-spread function. For the Gemini spectrometer and AO system, the wider point-spread function halo associated with an added residual higher-order aberration reduces slit-coupled intensity more rapidly than a broad point-spread function core induced by residual tilt.     

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.     

Measuring nitric oxide in single neurons by capillary electrophoresis with laser-induced fluorescence: use of ascorbate oxidase in diaminofluorescein measurements

As a family of novel fluorescent indicators for nitric oxide (NO), the diaminofluoresceins (DAFs) have allowed real-time measurement of neuronal NO, an important gaseous neurotransmitter. However, the measurement of NO by the most commonly used NO sensor, 4,5-diaminofluorescein (DAF-2), is altered by two processes: the interaction of DAF-2 with intracellular dehydroascorbic acid (DHA) and the impact of ascorbic acid (AA) on the levels of N2O3, the intermediate product of the oxidation of NO that reacts with DAF-2. Similar AA/DHA effects are observed with other DAF probes, including DAF-FM and DAR-4M. To overcome these limitations, we use a specific enzymatic reaction to eliminate the confounding effect of AA on DAF quantitation of NO and then use capillary electrophoresis (CE) with laser-induced fluorescence (LIF) detection to distinguish the various reaction products. First, the enzyme ascorbate oxidase (AO) is used to catalyze the oxidation of AA to DHA. Next, CE-LIF separates the fluorescent products of the reaction of DAF-2 with NO and DHA. Control experiments, including standard mixtures and single neurons with added NO donor, successfully demonstrate the utility of this approach. This protocol is further tested with homogenates of the mouth area from the sea slug Aplysia californica, previously shown to be NO-positive, and individual nitric oxide synthase-containing buccal neurons from the freshwater snail, Lymnaea stagnalis. In each case, significant amounts of NO are detected. This AO DAF methodology is specific, effective, simple, and allows NO to be measured in single cells without detectable interference from other compounds.     

Optimization of peak power of doubly Q-switched lasers with both an acousto-optic modulator and a Cr4+-doped saturable absorber

A doubly Q-switched laser can obtain a shorter pulse with a stable repetition rate and high pulse peak power, which has been experimentally proved. By taking into account the Gaussian spatial distributions of the intracavity photon density and the initial population-inversion density as well as the influence of the acousto-optic (AO) Q switch, we introduce the coupled rate equations for a doubly Q-switched laser with both an AO modulator and a Cr(4+)-doped saturable absorber. These coupled rate equations are solved numerically. The key parameters of an optimally coupled doubly Q-switched laser are determined based on maximizing the peak power, which include the optimal normalized coupling parameter, the optimal normalized saturable absorber parameters, and the normalized parameters of the AO Q switch. The optimal normalized peak power, the corresponding normalized energy, and the normalized pulse width are also given, and a group of general curves are generated for the first time to our knowledge. The curves can give us a good understanding of the dependence of the optimal key parameters on the parameters of the gain medium, the saturable absorber, the AO Q switch, the resonator, and the spatial distributions of the intracavity photon density. The optimal calculations for a diode-pumped Nd(3+):YVO(4) laser with both an AO modulator and a Cr(4+):YAG saturable absorber are presented to demonstrate the use of the curves and the related formulas.     

Hysteresis correction in the curvature adaptive optics system

An enhancing Coleman-Hodgdon model is introduced to describe the hysteresis curves of the bimorph deformable mirror (DM). Hysteresis curves are measured from a bimorph DM and then experiment is set up for the correction of hysteresis. Finally, step response and transfer functions of a curvature adaptive optics (AO) system are compared in three cases: with DM hysteresis, without hysteresis, and with hysteresis but corrected. Simulation results show that the bandwidth of a curvature AO system is improved significantly under different loop gains after hysteresis of the DM is corrected.