Open-loop control of liquid-crystal spatial light modulators for vertical atmospheric turbulence wavefront correction

We present an open-loop adaptive optics (AO) system based on two liquid-crystal spatial light modulators (LCSLMs) that profit from high precision wavefront generation and good repeatability. A wide optical bandwidth of 300 nm is designed for the system, and a new open-loop optical layout is invented to conveniently switch between the open and closed loop. The corresponding control algorithm is introduced with a loop frequency (the reciprocal of the total time delay of a correction loop) of 103 Hz. The system was mounted onto a 2.16 m telescope for vertical atmospheric turbulence correction. The full width at half-maximum of the image of the star α Boo reached 0.636 arc sec after the open-loop correction, while it was 2.12 arc sec before the correction. The result indicates that the open-loop AO system based on LCSLMs potentially has the ability to be used for general astronomical applications.     

Calibration and precompensation of noncommon path aberrations for extreme adaptive optics

Noncommon path aberrations (NCPAs) are one of the main limitations of an extreme adaptive optics (AO) system. NCPAs prevent extreme AO systems from achieving their ultimate performance. These static aberrations are unseen by the wavefront sensor and therefore are not corrected in closed loop. We present experimental results validating what we believe to be new procedures of measurement and precompensation of the NCPAs on the AO bench at ONERA (Office National d'Etudes et de Recherches Aérospatiales). The measurement procedure is based on refined algorithms of phase diversity. The precompensation procedure makes use of a pseudo-closed-loop scheme to overcome the AO wavefront-sensor-model uncertainties. Strehl ratio obtained in the images reaches 98.7% at 632.8 nm. This result allows us to be confident of achieving the challenging performance required for direct observation of extrasolar planets.     

加工工艺对CPE/AO2246阻尼性能与形态结构的影响

利用氯化聚乙烯(CPE)与有机小分子2,2亚甲基-双(4-甲基-6-叔丁基苯酚)(AO2246)采用不同工艺制备了有机杂化阻尼材料,通过动态力学分析(DMA)、差示扫描量热(DSC)、扫描电镜(SEM)和红外光谱(FT-IR)考察了材料的动态力学性能和微观结构。研究结果表明,AO2246在基体CPE中的分散状态对体系的阻尼性能影响较大。在未经热压的CPE/AO2246体系中,AO2246以三维谷粒结晶存在,造成体系的损耗因子(tanδ)比基体的还要低;而经过热压工艺的CPE/AO2246体系,AO2246与CPE产生了有机杂化,使体系的tanδ峰值获得较大的提高,同时玻璃化温度向高温方向移动,改善了体系的阻尼特性。红外光谱图表明,高温热压工艺的AO2246分子间产生了氢键作用的新的吸收峰,从而进一步提高了体系的阻尼性能。     

High quality GaN film overgrown on GaN nanorods array template by HVPE

The high density vertically aligned GaN nanorods array was fabricated by thermal evaporation of GaN powder with the assistance of HCl gas. The GaN nanorods array was used as template for GaN film growth by hydride vapor phase epitaxy (HVPE). The full width at half maximum values (FWHM) of high-resolution X-ray diffraction (HRXRD) rocking curves for the GaN film with GaN nanorods array template are 247 arc sec (002 reflection) and 308 arc sec (102 reflection), while those for the GaN film without GaN nanorods array template are 292 and 369 arc sec, respectively. This result indicates a significant reduction of dislocation density in the overgrown GaN film with GaN nanorods array template. Photoluminescence spectra measurements reveal the compressive strain relaxation and an improvement in the quality of the overgrown GaN film with GaN nanorods array template as compared to the regrown GaN film without GaN nanorods array template, which is consistent with the trend observed by HRXRD.     

Weld microstructure refinement in a 1441 grade aluminium-lithium alloy

Clad 2 mm thick sheets of Russian 1441 grade Al-Li alloys were welded using a gas tungsten arc welding process (GTAW). Comparisons were made between the weld beads obtained under (i) continuous current (CC), (ii) pulsed current (PC), and (iii) arc oscillation (AO) conditions for their macro- and microstructural details. In the case of CC GTAW, sound welds could be produced only under a narrow range of welding parameters. Centre line cracks, which occurred in CC GTAW welds under certain conditions, were halted by switching to PC or AO conditions while the welding was in progress. Microstructural refinement was significant in the case of PC and AO GTA welding. This revised version was published online in November 2006 with corrections to the Cover Date.     

Impact on the earth, ocean and atmosphere

Several hundred impact craters produced historically and at times as early as 1.9 × 10⁹ years ago with diameters in the range 10⁻² to 10² km are observed on the surface of the earth. Earth-based and spacecraft observations of the surfaces of all the terrestrial planets and their satellites, as well as many of the icy satellites of the outer planets, indicated that impact cratering was a dominant process on planetary surfaces during the early history of the solar system. Moreover, the recent observation of a circumstellar disk around the nearby star, β-Pictoris, appears to be similar to our own hypothesized protosolar disk. A disk of material around our sun has been hypothesized to have been the source of the solid planetesimals from which the earth and the other planets accreted by infall and capture. Thus it appears that the earth and the other terrestrial planets formed as a result of infall and impact of planetesimals. Although the present planets grew rapidly via accretion to their present size (in 10⁷ years), meteorite impacts continue to occur on the earth and other planets.Until recently meteorite impact has been considered to be a process that was important on the earth and the other planets only early in the history of the solar system. This is no longer true. The Alvarez hypothesis suggests that the extinction of some 90% of all species, including 17 classes of dinosaurs, is associated with the 1 to 150 cm thick layer of noble-element rich dust which is found all over the earth exactly at the Cretaceous-Tertiary boundary. The enrichment of noble elements in this dust is in meteorite-like proportions. This dust is thought to represent the fine impact ejecta from a 10km diameter asteroid interacting with the solid earth. The Alvarez hypothesis associates the extinction with the physics of a giant impact on the earth.Using finite-difference techniques, cratering flow calculations are used to obtain the spatial attenuation of shock pressure with radius, r, along the impact axis for the impact of silicate rock and iron impactors on a silicate half-space at speeds of 5 to 45 km/sec. Stress wave attenuation is found to be represented by two regimes, if the peak pressure, P, is fitted to expressions of the form P oc r². At distances from 2.2 to 5.6 projectile radii into a silicate target, the constant, a, is on the order of −0.2. This low-attenuation rate impedance matching regime extends further into the target at the slower impact velocities. This occurs because of the slightly divergent flow associated with the penetration of a spherical projectile. For the near-field impact regime, an impact at 5 km/sec of an iron object with silicate surface will induce complete melting for silicate; the iron will remain solid. At 15 km/sec, partial vaporization occurs for both silicate and iron whereas at 45 km/sec, complete vaporization occurs in both materials. Similar calculations were conducted for a silicate meteoroid striking a silicate surface at velocities ranging from 5 to 45 km/sec. At greater radii in the far-field regime, the exponent, a, varies systematically from −1.45 to −2.15 for impacts of silicate onsilicate as the impact velocity is increased from 5 to 45 km/sec. For an iron projectile impacting at speeds of 5 to 45 km/sec, the exponent, a, varies from −1.67 to −2.95.Upon impact of a 10 to 30 km diameter silicate or water object onto a 5 km deep ocean overlying a silicate half-space planet at 30 km/sec, we find that from 12 to 15% of the incident energy is coupled into the water. In the gravity field of the earth, some 10 to 30 times the impactor mass of water is launched on trajectories which can achieve stratospheric heights. The amount of ejecta launched to stratospheric altitudes is similar to the 10¹ to 10² projectile masses which result from impact of objects on an ocean-free silicate half-space (land). In the case of impact directly onto a silicate-half-space, only ejecta launched on trajectories which would carry it to stratospheric heights, has an impactor to target mass ratio which matches the fraction (10⁻² to 10⁻¹) of extraterrestral material found in the platinum-metal-rich Cretaceous-Tertiary boundary layer.Oceanic impact results in impulsive-like giant tsunamis initially having amplitudes of 4km, representing the solitary waterwave stability limit in the deep ocean, and containing 10⁻² to 10⁻¹ of the energy of the impact.Calculation of the interaction of a 10km bolide with the atmosphere indicates that only some 8% of the energy is imparted to the air during initial passage through the atmosphere. However, upon impact with the earth 10¹to10² times the bolide mass of water or rock is ejected into the stratosphere, although, only 0.1 bolide masses are in <1μm particles. The vaporized, melted, and (<1mm) solid ejecta transfer up to 40% of their energy to the atmosphere. Using the results of a similarity solution for the flow of gas as a result of an explosion in exponential atmosphere it is found that the atmosphere above such a large energy source is entirely ejected at speeds exceeding the escape velocity of the earth. Using the similarity solution we have calculated the mass of atmosphere lost due to impacts of 1 to 5 km radius projectiles. No atmosphere is lost for surface sources with energies less than 10²⁷ ergs. Impact of objects in the energy range 10²⁷ to 10³⁰ ergs causes gas losses of 10¹¹ to 10¹⁴ kg or 10⁻⁸ to 10⁻⁵ of the total present atmospheric mass. Impact energies of greater than 10³⁰ ergs cause little increase in atmospheric loss.     

Decolorization of Acid Orange 7 solution by gas-liquid gliding arc discharge plasma

The decolorization of 180 microM aqueous solutions of Acid Orange 7 (AO7) by means of a non-thermal plasma technique (i.e., the gas-liquid gliding arc discharge, which is generated between at least two metal electrodes with AC high voltage) was investigated in this paper. The effects of the plasma treatment time and the type of feeding gas, including air, oxygen, nitrogen and argon of the dye removal were determined. It is found that the voltage cycles of the gas-liquid gliding arc discharge are characterized by a moderate increase in the tension which is represented by a peak followed by an abrupt decrease and a current peak in the half period (10 ms); the concentration of AO7 solution decreases exponentially to reach 58.9, 77.4, 89.1, 95.1 and 99% in 25, 50, 75, 100 and 125 min, respectively, and the ln(Ct/C0) varies linearly with the treatment time t, indicating that decolorization reaction follow first pseudo-order kinetics with a constant rate of 0.03327 min(-1) when air was used as feeding gas; the decolorization rate during the plasma treatment is the greatest for oxygen as the feeding gas, in turn followed by air and argon, and was the least when using nitrogen. The variations of pH and conductivity and the formations of hydrogen peroxide and ozone are measured.     

Optimization of metal heating in plasma-mechanical treatment

Results are presented of an experimental investigation of the process of energy transfer to the anode of an arc in application to plasma-mechanical treatment of components.Notation Q_y energy introduced per unit mass of the layer being cut, W/kg - Q integral energy flux in the anode, W - I arc current, A - L spacing between the plasmotron nozzle exit and the component, m - h spacing between the plasmotron nozzle exist and the cathode, m - d plasmotron nozzle diameter, m - G gas consumption, kg/sec - q energy flux density in the anode spot, W/cm² - r spacing from the center of the anode spot to the point at which the energy flux density is calculated, cm - q_m energy flux density at the center of the anode spot, W/cm² - k_q concentration coefficient, cm^–2 - S_o degree of heat propagation, m - thermal diffusivity factor, m²/sec - time, sec - k spacing between the plasmotron and the cutter, m - v cutting rate, m/sec - coefficient of energy transmission to the anode - N electrical power of the arc, W Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 47, No. 1, pp. 138–143, July, 1984.     

Degradation of gas-liquid gliding arc discharge on Acid Orange II

The effects of pH value, initial concentration of dye solution and temperature on the degradation efficiency of Acid Orange II (AO7) using gas-liquid gliding arc discharge were investigated. The influences of pH value and temperature on degradation efficiency were not apparent. Increasing initial solution concentration caused the decrease of degradation rate and the increase of absolute degradation quantity. Considering energy efficiency and absolute degradation quantity, the gas-liquid gliding arc discharge is fit for treating high concentration organic wastewater. A possible mineralization pathway was proposed through the analysis of intermediate products detected by gas chromatograph coupled with mass spectrophotometer (GC-MS) and ion chromatograph (IC). Hydroxyl radicals reacted with the azo linkage-bearing carbon of a hydroxy-substituted ring, leading to the cleavage of -C-N- and degradation of AO7. The solution biodegradability was significantly improved (BOD(5)/COD from 0.02 to 0.43). The toxicity of intermediate products was lower than that of the initial Acid Orange II.     

Open-loop correction of horizontal turbulence: system design and result

Adaptive optics systems often work in a closed-loop configuration due to the hysteretic and nonlinearity properties of conventional deformable mirrors. Because of the high-precision wavefront generation and nonhysteretic properties of liquid-crystal devices, the open-loop control becomes possible. Open-loop control is a requirement for advanced adaptive optics concepts. We designed an open-loop adaptive optics system with a liquid-crystal-on-silicon wavefront corrector. This system is simple, fast, and can save much more light compared to conventional liquid-crystal-based closed-loop systems. The detailed principle, construction, and operation are discussed. The 500 m horizontal turbulence correction experiment was done using a 250 mm telescope in the laboratory. The whole system can reach a 60 Hz correction frequency. Evaluation of the correction precision was done at closed-loop configuration, which is 0.2 lambda (lambda=0.633 microm) in peak to valley. The dynamic image under open-loop correction got the same resolution compared to closed-loop correction. The whole system reached 0.68 arc sec resolution capability at open-loop correction, which is slightly larger than the system's diffraction-limited resolution of 0.65 arc sec.     

Small-angle measurement by use of a single prism

A new method of angle measurement based on the internal reflection effect is proposed that uses a single right-angle prism. We measure the angular displacement between a laser beam and the prism by detecting the changes in reflectance as a function of the angle of incidence. We achieve high linearity of measurement by taking the inverse of reflectance as the output. The inverse of reflectance is obtained from the intensities of the reflected and the transmitted beams measured by two photodiodes. Experiments with a prototype device have demonstrated that angle measurement with a range of ?500 arc sec, a nonlinearity error of ?0.1%, and a resolution of 0.1 arc sec can be readily achieved. The measurement range can be further increased with some sacrifice of linearity.     

High accuracy, wide range, rotation angle measurement by the use of two parallel interference patterns

Based on the idea of measuring small rotation angles with a parallel interference pattern (PIP), a method is developed to measure large rotation angles accurately. Two parallel PIP's that have different periods are used to measure a rotation angle of an object. The measurement made with a small-period PIP provides a high accuracy, and the measurement made with a large-period PIP provides a wide range. An accurate measurement for wide-range angles is made by combining the two measured values. The accuracy of the phase detection is determined by the periods of two PIP's. Rotation angles from approximately -30 to 30 arc min can be measured with an accuracy of 0.2 arc sec. Analytical results are supported by experimental results.     

Velocity of the electric arc in a plasmatron discharge chamber

An experimental investigation of the velocity of a high-current arc with air injection in the discharge chamber of a coaxial sectioned plasmatron is described. The experiments showed that the velocity of the cathode spot on the electrode surface depends on the arc current and on the external magnetic field strength. The air flow rate in the plasmatron chamber was 7.1 g/sec.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 17, No. 3, pp. 570–574, September, 1969.     

Comparison of vibration mitigation controllers for adaptive optics systems

Vibrations are detrimental to the performance of modern adaptive optics (AO) systems. In this paper, we describe new methods tested to mitigate the vibrations encountered in some of the instruments of the Gemini South telescope. By implementing a spectral analysis of the slope measurements from several wavefront sensors and an imager, we can determine the frequencies and magnitude of these vibrations. We found a persistent vibration at 55 Hz with others occurring occasionally at 14 and 100 Hz. Two types of AO controllers were designed and implemented, Kalman and H∞, in the multiconjugate AO tip-tilt loop. The first results show a similar performance for these advanced controllers and a clear improvement in vibration rejection and overall performance over the classical integrator scheme. It is shown that the reduction in the standard deviation of the residual slopes (as measured by wavefront sensors) is highly dependent on turbulence, wind speed, and vibration conditions, ranging--in terms of slopes RMS value--from an almost negligible reduction for high speed wind to a factor of 5 for a combination of low wind and strong vibrations.     

Structure and Features of the Motion of a Cathode Spot on a Continuous Titanium Cathode

High-speed video filming by the VS-FAST-NG CCD-array-based video camera from the firm of Videoskan with speeds of 1000 and 5000 frames per second and exposure time of 1·10⁻³ and 2·10⁻⁴ sec, respectively, is conducted. It is established that the arc burns from two or three cathode spots for (1–1.2)·10⁻³ sec. The mean and local speeds of the group of cathode spots are determined. If there is no external magnetic field present, the mean speed is equal to 5–6 m/sec. If there is a magnetic field B = 0.005 T present, the mean speed is equal to 15–16 m/sec. __________ Translated from Izmeritel'naya Tekhnika, No. 10, pp. 42–44, October, 2005.     

Adsorptive removal of Auramine-O: kinetic and equilibrium study

Present study deals with the adsorption of Auramine-O (AO) dye by bagasse fly ash (BFA) and activated carbon-commercial grade (ACC) and laboratory grade (ACL). BFA is a solid waste obtained from the particulate collection equipment attached to the flue gas line of the bagasse fired boilers of cane sugar mills. Batch studies were performed to evaluate the influences of various experimental parameters like initial pH (pH(0)), contact time, adsorbent dose and initial concentration (C(0)) for the removal of AO. Optimum conditions for AO removal were found to be pH(0) approximately 7.0 and equilibrium time approximately 30 min for BFA and approximately 120 min for activated carbons. Optimum BFA, ACC and ACL dosages were found to be 1, 20 and 2g/l, respectively. Adsorption of AO followed pseudo-second order kinetics with the initial sorption rate for adsorption on BFA being the highest followed by those on ACL and ACC. The sorption process was found to be controlled by both film and pore diffusion with film diffusion at the earlier stages followed by pore diffusion at the later stages. Equilibrium isotherms for the adsorption of AO on BFA, ACC and ACL were analyzed by Freundlich, Langmuir, Dubinin-Radushkevich, and Temkin isotherm equations using linear correlation coefficient. Langmuir isotherm gave the best correlation of adsorption for all the adsorbents studied. Thermodynamic study showed that adsorption of AO on ACC (with a more negative Gibbs free energy value) is more favoured. BFA which was used without any pretreatment showed high surface area, pore volume and pore size exhibiting its potential to be used as an adsorbent for the removal of AO.     

CO2-shielded arc as a high-intensity heat source

The characteristics of a CO₂-shielded arc are studied to evaluate its potential as a novel heat source for material processing, with lower costs and higher productivity than that of the tungsten–inert gas (TIG) arc. A double-gas-shielded system, using both CO₂ and an inert gas, is employed for the arc torch; this minimizes consumption of the tungsten electrode and gives arc stability equivalent to an argon TIG arc for 1800 s operation. The arc voltage of the CO₂-shielded arc is about 19 V for an arc current of 150 A and an arc gap of 3 mm, which is much higher than the 12 V obtained for an argon TIG arc. The CO₂ constricts the arc, resulting in an increase in the maximum heat flux density at the anode surface by a factor of about 10 relative to the TIG arc. The penetration depth of stainless steel melted by the CO₂-shielded arc is much larger than that for the argon TIG arc. It is concluded that the greater heating power of the CO₂-shielded arc, which is due to the greater arc constriction, in turn a consequence of the greater specific heat of CO₂, should lead to a large increase in material processing productivity.     

Stress reduction in GaN films on (111) silicon-on-insulator substrate grown by metal-organic chemical vapor deposition

The GaN film was grown on the (111) silicon-on-insulator (SOI) substrate by metal-organic chemical vapor deposition and then annealed in the deposition chamber. A multiple beam optical stress sensor was used for the in-situ stress measurement, and X-ray diffraction (XRD) and Raman spectroscopy were used for the characterization of GaN film. Comparing the characterization results of the GaN films on the bulk silicon and SOI substrates, we can see that the Raman spectra show the 3.0 cm^− 1 frequency shift of E₂(TO), and the full width at half maximum of XRD rocking curves for GaN (0002) decrease from 954 arc sec to 472 arc sec. The results show that the SOI substrates can reduce the tensile stress in the GaN film and improve the crystalline quality. The annealing process is helpful for the stress reduction of the GaN film. The SOI substrate with the thin top silicon film is more effective than the thick top silicon film SOI substrate for the stress reduction.     

Small-angle measurement based on surface-plasmon resonance and the use of magneto-optical modulation

A new, to our knowledge, optical method for small-angle measurement based on surface-plasmon resonance (SPR) is presented. In this method the high sensitivity of the phase of SPR to the angle of incidence is employed to improve the resolution of the measurement of the angle. Small-angle measurement is performed by the monitoring of the phase shift resulting from the minute change of the angle of incidence with the use of magneto-optical modulation. The validity of this method is demonstrated, and a measurement resolution of 0.2 arc sec is achieved experimentally.     

System design considerations to improve isoplanatism for adaptive optics retinal imaging

Adaptive optics (AO) retinal images are limited by anisoplanatism; wavefront shape varies across the field of view such that only a limited area can achieve diffraction-limited image quality at one time. We explored three alternative AO modalities designed to reduce this effect, drawn from work in astronomy. Optical design analysis and computer modeling was undertaken to predict the benefit of each modality for various schematic eyes and various complexities of the imaging system. Off-axis performance was found to be limited by system parameters and not by the eye itself, due to the inherent off-axis characteristics of the eye's gradient index lens. This rendered the alternative AO modalities ineffectual compared with conventional AO but did suggest several methods by which anisoplanatism may be reduced by altering the design of conventional AO systems. Several of these design possibilities were explored with further modeling. The best-performing method involved the replacement of system lenses with gradient index versions inspired by the human eye lens. Mirror-based relay optics also demonstrated good off-axis performance, but their advantage was lost in regions of the system suffering from uncorrected higher-order aberration. Incorporating "off-the-plane" beam deviations ameliorated this loss substantially. In this work we also show, to our knowledge for the first time, that the ideal location of a single AO corrector need not lie in the pupil plane.