一种大型望远系统波前检测的方法

 对传统S-H波前检测方法进行了改进的基础上,设计了一套适应于大型望远系统的波前检测方案。利用五角棱镜光束转向系统形成大型望远系统各子孔径准参考光束,实现依次扫描各子孔径;并用改进的S-H波前传感器实现依次探测各子孔径的波前。理论分析证明了由五角棱镜的制造误差和运动误差引起的波前倾斜误差在微变情况下,可通过波前修正方法予以修正,可有效地再现出望远系统自身波前误差。该方案可适用于大口径望远系统的波前检测。     

Validity of wave-front reconstruction and propagation of ultrabroadband pulses measured with a Hartmann-Shack sensor

Wave-front reconstruction for ultrabroadband laser pulses is verified by use of a Hartmann-Shack sensor. We estimate the accuracy of numerical wave-front propagation by comparing numerical with experimental results and verify that wave fronts of ultrabroadband laser pulses from a hollow fiber can be propagated correctly by a single polychromatic wave-front measurement to a place where detection is not practicable, e.g., inside a vacuum chamber or laser focus.     

自适应光学系统校正后实际分辨率评价指标

为了评价地基大口径望远镜对空间目标自适应光学系统校正后的实际分辨率,提出了可分辨单元数比指标。结合1.23 m自适应光学望远镜的观测数据,对可分辨单元数比的物理意义、适用范围、误差影响因素以及大气弥散的影响进行了分析。结果表明：可分辨单元数比能全面反映望远镜系统对空间目标自适应校正后的实际分辨率,且物理意义更直观;适用于所有可获取波前特征的望远镜自适应校正后实际分辨率的评估;其误差主要依赖于波前探测器的测量误差;大气弥散对其影响可忽略。     

Adaptive-optics correction of a stellar interferometer with a single pyramid wave-front sensor

We investigate the pyramid wave-front sensor's capability to reconstruct in a closed loop, with only one sensor, the wave fronts on both apertures of a stellar interferometer, including the differential piston: The method consists of placing the principal element of this sensor, namely, a refractive-square-based pyramid, in a combined focal plane of the interferometer instead of using two sensors, one for each individual telescope. We show that the sensor signals allow one to measure the wave-front aberrations on both apertures and the differential piston at the same time. The performance of an adaptive-optics loop is computed numerically for the Large Binocular Telescope in terms of differential-piston rejection under several conditions. The results show that atmospheric correction including differential-piston compensation is possible as long as the corrected Strehl ratio at the wave-front sensing wavelength is greater than 20%.     

Nearly diffraction-limited laser focal spot obtained by use of an optically addressed light valve in an adaptive-optics loop

We demonstrate correction of laser wave-front distortions by use of an adaptive-optical technique based on a light valve. The setup consists of an achromatic and adjustable-sensitivity wave-front sensor and a wave-front corrector relying on an optically addressed liquid-crystal spatial light modulator. Experimental results with strongly aberrated beams focused close to the diffraction limit are presented for the cw regime. Additional experiments with pulses and measurement of damage thresholds show that this approach is relevant for spatial phase correction of ultraintense laser pulses.     

基于AR模型搜索迭代算法的望远镜跟踪误差分析

在大口径光学望远镜观测星体目标时,其跟踪误差主要由风载引起的望远镜跟踪抖动误差和大气湍流引起的跟踪误差组成.建立了望远镜跟踪误差的简化分析模型,提出了一种采用AR模型迭代算法将风载引起的望远镜跟踪抖动误差和大气湍流引起的跟踪误差分离的新方法,并在1.8 m望远镜上进行了实验验证.结果表明,风载引起的望远镜跟踪抖动误差与风向和风速直接相关,实验结果与理论分析比较符合.     

Experimental validation of a technique to measure tilt from a laser guide star

We have experimentally demonstrated for what is believed to be the first time a method for sensing wave-front tilt with a laser guide star (LGS). The tilt components of wave fronts were measured synchronously from the LGS by use of a telescope with a 0.75-m effective aperture and from the star Polaris by use of a 1.5-m telescope. The Rayleigh guide star was formed at an altitude of 6 km and at a corresponding range of 10.5 km by projection of a focused beam at Polaris from the full aperture at the 1.5-m telescope. Both telescope mounts were unpowered and bolted in place, allowing us to reduce substantially the telescope vibration. The maximum value of the measured cross-correlation coefficient between the tilt for Polaris and the LGS is 0.71. The variations of the measured cross-correlation coefficient in the range from 0.22 to 0.71 are caused by turbulence at altitudes above 6 km, which was not sampled by the laser beacon but affected tilt for Polaris, the cone effect for turbulence below 6 km, residual mount jitter of the telescopes, and variations of the signal/noise ratio. The results support our concept of sensing atmospheric tilt by observing a LGS with an auxiliary telescope and indicate that this method is a possible solution for the tip-tilt problem.     

Correction of strong phase and amplitude modulations by two deformable mirrors in a multistaged Ti:sapphire laser

We describe a novel scheme consisting of two deformable bimorph mirrors that can free ultrashort laser pulses from simultaneously present strong wave-front distortions and intensity-profile modulations. This scheme is applied to the Max-Planck-Institut für Quantenoptik 10-TW Advanced Titanium-Sapphire Laser (ATLAS) facility. We demonstrate that with this scheme the focusability of the ATLAS pulses can be improved from 10(18) to 2x10(19) W/cm(2) without any penalty in recompression fidelity.     

Experimental validation of Fourier-transform wave-front reconstruction at the Palomar Observatory

Wave-front reconstruction with use of the Fourier transform has been validated through theory and simulation. This method provides a dramatic reduction in computational costs for large adaptive (AO) systems. Because such a reconstructor can be expressed as a matrix, it can be used as an alternative in a matrix-based AO control system. This was done with the Palomar Observatory AO system on the 200-in. Hale telescope. Results of these tests indicate that Fourier-transform wave-front reconstruction works in a real system. For both bright and dim stars, a Hudgin-geometry Fourier-transform method produced performance comparable to that of the Palomar Adaptive Optics least squares. The Fried-geometry method had a noticeable Strehl ratio performance degradation of 0.043 in the K band (165-nm rms wave-front error added in quadrature) on a dim star.     

Shack-Hartmann波前传感器在光学检验中的应用

介绍用于光学检测的Shack-Hartmann(S-H)波前传感器,以及相应的检测实验结果。在实验室内,通过与Zygo干涉仪的测量结果对比,得出传感器的测量精度优于/50 RMS(=632.8 nm)。在外场利用星光作为光源,对口径1 m、焦距11 m的望远镜进行了系统波像差检验,测量结果为0.39~0.46RMS之间,并随着俯仰角的增加而增大,主要像差形式为三阶像散。     

五角棱镜的光束转向误差对波前测量的影响

从分析五角棱镜的角度制造误差产生的光学平行差入手，推导出确定入射光线和棱镜调整的表达式；利用棱镜转动定理建立了五角棱镜的光输出平面波前与其角度制造误差和导轨引起的运动误差之间的关系；并用计算机模拟试验分析了五角棱镜的角度制造误差和导轨引起的运动误差产生的扫描光束转向的波前误差。指出此研究结果有利于五角棱镜的加工和棱镜调整，还有利于大口径望远系统波前检测过程中扫描光束转向的波前误差的修正。     

红外自适应系统中双波段望远镜与红外物镜设计

大孔径红外光学系统往往易受自重和环境温度影响造成像质恶化,引入自适应光学技术的红外自适应系统能够很好地解决该问题,为此设计了一个用于Hartmann-Shack波前检测的红外自适应光学系统。重点设计了10×可见光与中波红外双波段望远镜,物镜为卡赛格林反射物镜组,无需消色差,在可见光与中波红外2个波段实现了消色差目镜设计;还设计了红外成像中继光学系统,可实现100%冷光阑效率,并补偿望远镜在中波红外波段的残余像差,使最终设计的光学系统MTF接近衍射极限,达到了0.5以上,满足设计指标要求。     

Direct measurement of wave-front distortion induced during second-harmonic generation: application to breakup-integral compensation

The wave-front distortion of femtosecond laser pulses recorded with a Shack-Hartmann analyzer makes it possible to retrieve the nonlinear index of refraction of different glasses and the nonlinear phase shift induced during second-harmonic generation in beta-barium borate (BBO) crystal versus the phase mismatch. It is shown that the nonlinear phase shift induced in a 2-mm-thick BBO crystal allows compensation for up to a 2pi breakup-integral induced in a 4-cm fused-silica glass. The stability of the compensation is reported to be from 10 to 100 GW cm(-2).     

Hartmann wave-front scanner

A wave-front sensor is described that uses a programmable moving aperture to scan an incoming wave front. The position of the diffraction spot is recorded behind an objective lens with a two-dimensional sensor and gives an estimate of the local slope at the aperture position. Then the wave front is reconstructed by processing of the slope data. The device is basically a programmable Hartmann wave-front sensor. Compared with a microlens Shack-Hartmann wave-front sensor, its much longer focal length provides higher resolution, although real-time operation is lost. A practical implementation of the new scanner with a liquid-crystal television as the programmable aperture is presented.     

Measuring wind speeds and turbulence with a wave-front sensor

We analyze wave-front sensor (WFS) measurements taken with the 1.5-m telescope at the Starfire Optical Range in Albuquerque, N.M., of wind speeds in the turbulent atmospheric layers that cause seeing. The frozen-flow hypothesis suggests that atmospheric turbulence is located in thin horizontal layers and that turbulent features do not change over short time scales but are drawn along by the prevailing wind. Exploiting autocorrelation properties of the WFS data that result from these characteristics of atmospheric turbulence, we are able to measure the movements of individual layers. We also test the validity of the frozen-flow hypothesis.     

Programmable focal spot shaping of amplified femtosecond laser pulses

We describe the programmable spatial beam shaping of 100-kHz, 4-microJ amplified femtosecond pulses in a focal plane by wave-front modulation. Phase distributions are determined by a numerical iterative procedure. A nonpixelated optically addressed liquid-crystal light valve is used as a programmable wave-front tailoring device. Top-hat, doughnut, square, and triangle shapes of 20-microm size are obtained in a focal plane. Their suitability for femtosecond laser machining is demonstrated.     

Focal plane wave-front sensing algorithm for high-contrast imaging

High-contrast imaging provided by a coronagraph is critical for the direction imaging of the Earth-like planet orbiting its bright parent star. A major limitation for such direct imaging is the speckle noise that is induced from the wave-front error of an optical system. We derive an algorithm for the wave-front measurement directly from 3 focal plane images. The 3 images are achieved through a deformable mirror to provide specific phases for the optics system. We introduce an extra amplitude modulation on one deformable mirror configuration to create an uncorrelated wave-front, which is a critical procedure for wave-front sensing. The simulation shows that the reconstructed wave-front is consistent with the original wave-front theoretically, which indicates that such an algorithm is a promising technique for the wave-front measurement for the high-contrast imaging. Supported by the National Natural Science Foundation of China (Grant No. 10873024)     

Ultrashort-pulse wave-front autocorrelation

Combined spatially resolved collinear autocorrelation and Shack-Hartmann wave-front sensing of femtosecond laser pulses is demonstrated for the first time to our knowledge. The beam is divided into multiple nondiffracting subbeams by thin-film micro-optical arrays. With hybrid refractive-reflective silica/silver microaxicons, wave-front autocorrelation is performed in oblique-angle reflection. Simultaneous two-dimensional detection of local temporal structure and wave-front tilt of propagating few-cycle wave packets is demonstrated.     

Wave-front reversal of multifrequency continuous emission by Bragg scattering from phase regular inhomogeneities

The possibility of wave-front reversal of continuous multifrequency emission with a wide line spectrum (in particular, emission of lasers operating on cascade vibrational-rotational transitions of diatomic molecules, e.g., HF or CO) is analyzed; the reversal is based on Bragg scattering from a spatial phase hologram that is recorded in a resonance-absorbing medium by emission with a frequency different from any of the reversed-signal frequencies. The mechanism of formation of a spatial phase hologram and the features of optical Bragg scattering on such structures are analyzed. It is shown that a scattered wave-front is complex conjugate to one of the waves recording the phase grating. An optical system for wave-front reversal is proposed and the Bragg scattering efficiency is estimated for a particular example. The analysis indicates that the proposed system does not exhibit frequency selectivity and is basically suitable for wave-front reversal of emission with a broad line spectrum. The main problem is to choose a medium with resonance absorption at the center of a multifrequency emission spectrum and with complete transparency for the reversed signal.     

Brillouin-enhanced four-wave-mixing vector phase-conjugate mirror with beam-combining capability

It is often desirable to remove both wave front and polarization aberrations from an optical beam. Scalar phase conjugation, such as ordinary stimulated Brillouin scattering, can correct only for wave-front aberrations. We have developed a new geometry for Brillouin-enhanced four-wave mixing that performs vector phase conjugation to correct for both wave-front and polarization distortions. Results show a reduction in the depolarization losses from 50% to less than 2% of the total output energy. Coherent, variable, multiple-beam combination is achieved without need of nonreciprocal devices such as Faraday rotators.