Accurate shape measurement of focusing microstructures in Fourier digital holographic microscopy.

This paper proposes a measurement method of focusing objects with a high gradient shape of a small and large radius of curvature. The measurements are carried out on a Fourier digital holographic microscope with optimized illumination conditions maximizing the usage of the system's numerical aperture. The obtained fringe patterns are the result of interference of deformed spherical object and spherical reference waves. The key elements of the method are the aberration compensation and calibration procedures. They provide accurate reconstruction of the object wave and determination of the focus position of the sample. The shape is calculated in two steps. First, the object wave is reconstructed at the plane of the object focus using single or multiframe phase extraction algorithm and the specialized propagation method. The step includes compensation for spherical aberration. In the second step, the sample shape is computed with the local ray approximation approach. The proposed method is experimentally validated with measurements of challenging, high gradient shapes (convex, concave) of different radiuses of curvature.

Holographic tomography with object rotation and two-directional off-axis illumination.

A hybrid system of holographic tomography, which utilizes rotation of a sample and two-directional, off-axis illumination is proposed. The applied type of illumination brings two major benefits. First, it offers theoretical potential for the resolution improvement with respect to conventional tomography. Second, it enables effective, numerical compensation of the defocus error, which is achieved with an accurate, noise-immune autofocusing. Hence, the main practical obstacle of hybrid tomography is removed and its high-resolution potential is put into practice. The utility of the proposed concept is experimentally demonstrated with the tomographic measurement of a photonic crystal fiber.

Accurate approach to capillary-supported optical diffraction tomography.

A new holographic data processing path for accurate quantitative tomographic reconstruction of 3D samples placed in a cylindrical capillary is proposed. The method considers strong unintentional focusing effects induced by the inner cylindrical boundary of the vessel: 1) introduction of cylindrical wave illumination of a sample, and 2) object wave deformation. The first issue is addressed by developing an arbitrary illumination tomographic reconstruction algorithm based on filtered backpropagation, while the second by a novel correction algorithm utilizing the optical rays analysis. Moreover, the processing path includes a novel holographic method for correction of spherical aberration related to refraction at a planar surface. Utility of the developed data processing path is proven with numerical simulations and experimental measurement of a specially prepared test sample.

Digital holography with multidirectional illumination by LCoS SLM for topography measurement of high gradient reflective microstructures.

In this paper we present a method for topography measurement of high gradient reflective microstructures that overcomes the limited numerical aperture (NA) of a digital holographic (DH) system working in reflection. We consider a case when a DH system is unable to register the light reflected from the full sample area due to insufficient NA. To overcome this problem, we propose digital holography in a microscope configuration with an afocal imaging system and a modified object arm in the measurement setup. The proposed modification includes application of a spatial light modulator (SLM) based on liquid crystal on silicon (LCoS) technology for multidirectional plane wave illumination. The variable off-axis illumination enables characterization of the sample regions that cannot be imaged by the limited NA of a classical DH system utilizing on-axis illumination. In the proposed method, the final object topography is merged from a set of captured object waves corresponding to various illumination directions using a novel automatic algorithm. The proposed technique is experimentally validated by full-field measurement of a silicon mold with a high gradient of shape.

Absolute shape measurement of high NA focusing microobjects in digital holographic microscope with arbitrary spherical wave illumination.

In this paper a new high NA shape measurement technique working with an arbitrary spherical wave illumination is presented. The main contribution of this work are formulas, derived from exact reflection and refraction laws for both the reflection and the transmission configurations, which enable accurate shape calculations in systems with an arbitrary location of the illuminating point source. The proposed algorithms permit measurement of multiple samples of arbitrary shapes using a single hologram. An accuracy of this method is confirmed with numerical simulations, which show superiority of this approach over a standard procedure utilizing paraxial approximation. The method is validated experimentally using a reflective measurement of a microlens topography, whose NA in reflection is 0.7. Furthermore, a new measurement configuration is presented that extends the capabilities of transmission systems for characterization of high gradient shapes.

Noise suppressed optical diffraction tomography with autofocus correction.

We propose a novel tomographic measurement approach that enables a noise suppressed characterization of microstructures. The idea of this work is based on a finding that coherent noise in the input phase data generates an artificial circular structure whose magnitude is the highest at the centre of tomographic reconstruction. This method decreases the noise level by applying an unconventional tomographic measurement configuration with an object deliberately shifted with respect to the rotation axis. This enables a spatial separation between the reconstructed sample structure and the area of the largest refractive index perturbations. The input phase data defocusing that is a by-product of the introduced modification is numerically corrected with an automatic focus correction algorithm. The proposed method is validated with simulations and experimental measurements of an optical microtip.

High-precision topography measurement through accurate in-focus plane detection with hybrid digital holographic microscope and white light interferometer module.

High-precision topography measurement of micro-objects using interferometric and holographic techniques can be realized provided that the in-focus plane of an imaging system is very accurately determined. Therefore, in this paper we propose an accurate technique for in-focus plane determination, which is based on coherent and incoherent light. The proposed method consists of two major steps. First, a calibration of the imaging system with an amplitude object is performed with a common autofocusing method using coherent illumination, which allows for accurate localization of the in-focus plane position. In the second step, the position of the detected in-focus plane with respect to the imaging system is measured with white light interferometry. The obtained distance is used to accurately adjust a sample with the precision required for the measurement. The experimental validation of the proposed method is given for measurement of high-numerical-aperture microlenses with subwavelength accuracy.

Digital holographic microscope for measurement of high gradient deep topography object based on superresolution concept.

In this Letter, a novel concept based on superresolution technique that enables the measurement of high gradient and deep topography objects using digital holographic (DH) microscopy is introduced. The major problem of DH systems is limited NA that prohibits the metrological characterization of object features of high frequencies. The proposed technique has the ability to extend spatial frequency spectrum of the measured topography by applying multidirectional plane wave illumination, which is experimentally realized with a grating. The technique recovers sample topography from the set of object waves with different object spectra that are converted into a set of topographies by using an algorithm which takes into account refraction. Application of this novel approach is experimentally validated by characterization of high gradient topography objects with maximum angle of tangent 65°.

Holographic tomography with scanning of illumination: space-domain reconstruction for spatially invariant accuracy.

The paper presents two novel, space-domain reconstruction algorithms for holographic tomography utilizing scanning of illumination and a fixed detector that is highly suitable for imaging of living biomedical specimens. The first proposed algorithm is an adaptation of the filtered backpropagation to the scanning illumination tomography. Its space-domain implementation enables avoiding the error-prone interpolation in the Fourier domain, which is a significant problem of the state-of-the-art tomographic algorithm. The second proposed algorithm is a modified version of the former, which ensures the spatially invariant reconstruction accuracy. The utility of the proposed algorithms is demonstrated with numerical simulations and experimental measurement of a cancer cell.