Performance evaluation of a sensorless adaptive optics multiphoton microscope

A wavefront sensorless adaptive optics technique was combined with a custom‐made multiphoton microscope to correct for specimen‐induced aberrations. A liquid‐crystal‐on‐silicon (LCoS) modulator was used to systematically generate Zernike modes during image recording. The performance of the instrument was evaluated in samples providing different nonlinear signals and the benefit of correcting higher order aberrations was always noticeable (in both contrast and resolution). The optimum aberration pattern was stable in time for the samples here involved. For a particular depth location within the sample, the wavefront to be precompensated was independent on the size of the imaged area (up to ～360 × 360 μm²). The mode combination optimizing the recorded image depended on the Zernike correction control sequence; however, the final images hardly differed. At deeper locations, a noticeable dominance of spherical aberration was found. The influence of other aberration terms was also compared to the effect of the spherical aberration.     

利用相位差异技术恢复宽带白光图像

研究了用于扩展目标成像的相位差异技术。该技术利用焦面和离焦面上的两幅图像之间的相位差异来估算波前相位畸变,同时对采集的图像进行恢复。设计了白光目标成像实验系统,用宽带白光作光源,在实验室搭建实验平台,模拟无穷远处目标和波前相位畸变。采用直角梯形分光棱镜将光路分为两个通道,在一台相机上同时采集两个通道像面上的图像,消除了相机不同产生的差异,极大地抑制了噪声对图像恢复的影响,分光后系统抗两个通道独立的高斯白噪声的能力为10%。实验结果显示恢复后的图像分辨率提高了19%,表明该技术是适合光电成像系统的图像恢复技术。     

微变形反射镜主要性能测试研究

微变形反射镜(MEMS-DM)是用于自适应光学中波前校正的重要元件。测试实验中对37单元微变形反射镜的光学影响函数矩阵进行了推导和全面测量,从而验证了其叠加性。由光学影响函数推导出了微变形反射镜的控制电压矩阵,利用电压矩阵校正了变形镜的初始面形。最后,对微变形镜校正波前畸变能力进行了测量和评估,得出关于优化微变形镜设计相关方面的一些结论。     

Phase Retrieval with a Diffractive Micro Lens Array

Multi-plane phase retrieval is a well established technique for reconstructing both, amplitude and phase of an object wave. This standard technique works best, if the intensity of the object wave changes rapidly along the optical axis. For slowly varying intensities, the iterative procedure may not converge at all. To overcome this limitation we combined the standard technique with a periodic phase element. We demonstrate that a binary diffractive micro lens array with overlapping aperture significantly improves the convergence of phase retrieval and thus the quality of reconstruction. Thus multi-plane phase retrieval can be applied for both rough and smooth phase distributions.     

Measurement of specimen-induced aberrations of biological samples using phase stepping interferometry

Confocal or multiphoton microscopes, which deliver optical sections and three‐dimensional (3D) images of thick specimens, are widely used in biology. These techniques, however, are sensitive to aberrations that may originate from the refractive index structure of the specimen itself. The aberrations cause reduced signal intensity and the 3D resolution of the instrument is compromised. It has been suggested to correct for aberrations in confocal microscopes using adaptive optics. In order to define the design specifications for such adaptive optics systems, one has to know the amount of aberrations present for typical applications such as with biological samples. We have built a phase stepping interferometer microscope that directly measures the aberration of the wavefront. The modal content of the wavefront is extracted by employing Zernike mode decomposition. Results for typical biological specimens are presented. It was found for all samples investigated that higher order Zernike modes give only a small contribution to the overall aberration. Therefore, these higher order modes can be neglected in future adaptive optics sensing and correction schemes implemented into confocal or multiphoton microscopes, leading to more efficient designs.     

200单元硅基单压电变形镜的设计与测试

针对天文望远镜对变形镜的需求,设计了200单元级硅基单压电变形镜并对其进行了性能测试。首先,通过有限元方法对单压电变形镜进行结构建模,分析比较了电极排列形式和边界支撑方式对变形镜性能的影响。然后,制备了3款200单元变形镜样机(环形简支,环形固支,六边形简支)。最后,利用干涉仪测量了变形镜的影响函数,并通过激光多普勒测振仪测量其频响特性。仿真与实验结果表明:致动器在50V电压下变形量可达1μm,固支条件下谐振频率在1kHz左右,对前100项Zernike像差具有良好的校正能力。该变形镜在低成本天文自适应光学系统中具有应用潜力。     

极小孔径衍射波前测试分析

研制点衍射干涉仪时,小孔的形貌和装调误差会对衍射波前质量产生影响,故本文研究了采用相位复原对小孔衍射波前质量进行测评的方法。设计并搭建了衍射波前测试装置,对光纤衍射波前进行了测评实验。为了验证测试方法的有效性,分别对采集的衍射图像与重构的衍射图像,以及测试获得的振幅分布与理论计算获得的振幅分布进行了对比。结果显示。采集图像与重构图像之间的差异为0.32%,验证了采用相位复原技术进行衍射波前测试方法的有效性。同时,对干涉方法测量与相位复原结果进行了对比,二者差异仅为0.001 4λRMS。     

137单元变形镜性能测试及校正能力实验

自制了一个137单元分立促动器的连续镜面式变形镜,用于自适应光学相关技术的研究。首先,利用Zygo激光干涉仪对变形镜的静态性能进行了测试,包括面形测试、单个促动器的响应测试和耦合测试;然后,针对变形镜的校正能力,分别进行了Zernike多项式拟合和展平测试,并利用自适应光学实验平台实验验证了变形镜的校正能力;最后,利用搭建的快速响应测量平台测试了变形镜及其驱动电路的动态响应性能。测试及实验表明,137单元变形镜静态展平后的面形RMS值优于λ/50,PV值优于0.18λ（λ=632.8 nm）;能够对前7阶Zernike多项式进行较好的拟合和校正;变形镜及其高压驱动器的动态响应优于1 kHz;该变形镜能使系统的Strehl ratio从小于0.1提高到优于0.9,明显改善了系统的成像能力。     

Adaptive correction of depth-induced aberrations in multiphoton scanning microscopy using a deformable mirror

We demonstrate adaptive aberration correction for depth‐induced spherical aberration in a multiphoton scanning microscope with a micromachined deformable mirror. Correction was made using a genetic learning algorithm with two‐photon fluorescence intensity feedback to determine the desired shape for an adaptive mirror. For a 40×/0.6 NA long working distance objective, the axial scanning range was increased from 150 mm to 600 mm.     

Simultaneous phase and amplitude extraction from a single defocused image of a homogeneous object

We demonstrate simultaneous phase and amplitude extraction from a single defocused image of a homogeneous object. Subject to the assumptions explicitly stated in the derivation, the algorithm solves the twin‐image problem of in‐line holography and is capable of analysing data obtained using X‐ray microscopy, electron microscopy, neutron microscopy or visible‐light microscopy, especially as they relate to defocus and point projection methods. Our simple, robust, non‐iterative and computationally efficient method is applied to data obtained using an X‐ray phase contrast ultramicroscope.     

点衍射波前位相的测评

提出一种检测点衍射干涉仪关键部件针孔所产生的衍射光学波前的方法。介绍了点衍射波前的产生原理,分析了小孔质量状态、照明光路调整状态与波前各个像差分量之间的关系。基于信息光学基础理论,采用傅里叶变换和迭代算法,采集针孔衍射图像并进行计算分析,实现对衍射波前的位相复原以获得波前信息。阐述了测试方法的理论依据和计算公式,应用研制的位相复原分析计算软件测试并分析了实际采集的点衍射图像,通过15次的迭代,输出的位相值逐渐收敛,图像误差因子下降到0.12。目前,该方法已用于对针孔的筛选和针孔照明系统的装调中,实验结果证明了该测试方法的可行性。     

Magnetic domain imaging with a scanning near-field optical microscope using a modified Sagnac interferometer

We report on the combination of a scanning near-field optical microscope and a modified Sagnac interferometer for magnetic-domain imaging in the reflection mode. The Sagnac interferometer is used for detection of the magneto-optical Kerr effect. Since the interferometer is inherently insensitive to polarization changes caused by topography effects, magnetic-domain imaging is not limited to samples with flat surfaces. In this way, it is possible to image magnetic bits written on the tracks of a magneto-optical disc that has a rather pronounced surface profile.     

Imaging of soft and hard materials using a Boersch phase plate in a transmission electron microscope

Using two levels of electron beam lithography, vapor phase deposition techniques, and FIB etching, we have fabricated an electrostatic Boersch phase plate for contrast enhancement of weak phase objects in a transmission electron microscope. The phase plate has suitable dimensions for the imaging of small biological samples without compromising the high-resolution capabilities of the microscope. A micro-structured electrode allows for phase tuning of the unscattered electron beam, which enables the recording of contrast enhanced in-focus images and in-line holograms. We have demonstrated experimentally that our phase plate improves the contrast of carbon nanotubes while maintaining high-resolution imaging performance, which is demonstrated for the case of an AlGaAs heterostructure. The development opens a new way to study interfaces between soft and hard materials.     

基于PI迟滞模型的单压电变形镜开环控制

压电非线性迟滞导致压电变形镜的开环控制精度及闭环工作带宽降低,限制了其在自适应光学系统中的应用。为克服迟滞影响,提出采用PI迟滞模型描述单压电变形镜的迟滞非线性特性,实现单压电变形镜的高精度开环控制。首先建立PI迟滞数学模型,利用最小二乘法辨识PI迟滞模型的权值,计算出PI逆模型的权值和阈值,从而获得消除迟滞后的变形镜控制电压;接着搭建了基于哈特曼波前传感器的自适应光学测试平台,采用单压电变形镜的环形致动器进行离焦面形的开环控制实验。实验结果表明,经过迟滞消除后变形镜的电压-变形迟滞由9.3%降低到1.2%,离焦面形的开环重构精度提高70%以上,证明该迟滞模型可有效应用于单压电变形镜的开环控制。     

相位检测法定镜自适应校正技术

为保持傅里叶变换光谱仪在动镜运动和外界震动等恶劣条件下动镜和定镜间的准直性,采用了相位检测法检测准直性误差的定镜自适应校正技术。介绍了相位检测法的基本原理和实现自适应校正的途径以及自适应校正系统的基本结构。采用高速压电倾斜镜作为驱动补偿元件,分析了相位检测精度对校正精度的影响。实验结果表明,采用自适应校正后系统干涉效率明显提高。     

A scanning near-field optical microscope having scanning electron tunnelling microscope capability using a single metallic probe tip

A new microscope system that has the combined capabilities of a scanning near-field optical microscope (SNOM) and a scanning tunnelling microscope (STM) is described. This is achieved with the use of a single metallic probe tip. The distance between the probe tip and the sample surface is regulated by keeping the tunnelling current constant. In this mode of operation, information about the optical properties of the sample, such as its refractive index distribution and absorption characteristics, can be disassociated from the information describing its surface structure. Details of the surface structure can be studied at resolutions smaller than the illumination wavelength. The performance of the microscope is evaluated by analysing a grating sample that was made by coating a glass substrate with gold. The results are then compared with the corresponding SNOM and STM images of the grating.     

Orientation independent retardation imaging using quantitative polarized phase microscopy

Simultaneous optical phase and retardation measurement of a birefringent specimen is demonstrated independently of a priori knowledge of the optic axis orientation. The two‐dimensional retardation distribution in both magnitude and angle of the fast axis orientation is uniquely determined from transverse phase images recorded with a bright field transmission microscope using light polarized at a minimum of three different polarization orientations. This approach opens a new possibility for stain‐free phase and orientation‐independent retardation characterization of samples using only one polarizer without needing other additional optical elements traditionally used in polarimetric measurements. Microsc. Res. Tech. 2012. © 2012 Wiley Periodicals, Inc.     

基于单压电变形镜的可控环形焦斑整形

为了实现可控环形焦斑的整形,提出了一套基于单压电变形镜的整形方法。首先结合波前衍射理论和随机并行梯度下降算法模拟迭代出环形焦斑整形所需的调制相位,进而利用波前传感器探测光束的波前信息,控制变形镜重构目标光斑对应的调制相位,实现聚焦光斑的整形。搭建了一套基于62单元单压电变形镜的光斑整形实验平台,采用焦平面上的CCD记录远场聚焦光斑。实验结果表明,该方法实现了对不同直径(0.32,0.4,0.6 mm)和宽度(0.05,0.08,0.1mm)环形焦斑的整形,可有效应用于激光束整形。     

Linear phase imaging using differential interference contrast microscopy

We propose an extension to Nomarski differential interference contrast microscopy that enables isotropic linear phase imaging. The method combines phase shifting, two directions of shear and Fourier‐space integration using a modified spiral phase transform. We simulated the method using a phantom object with spatially varying amplitude and phase. Simulated results show good agreement between the final phase image and the object phase, and demonstrate resistance to imaging noise.