Kamil Liżewski

High-numerical-aperture microlens shape measurement with digital holographic microscopy

In this Letter, we introduce an algorithm that overcomes limitations in shape measurement by holographic microscopic methods in cases of micro-optical elements with high NA, such as microlenses. The presented algorithm provides a simple method for shape reconstruction from interferometrically measured phase. The algorithm is based on the analysis of local ray transition through the measured object. We develop algorithms for holographic configurations working in transmission and reflection. The accuracy of the developed algorithms is proved by experiments and extensive simulations. We present an experiment in a holographic Mach-Zehnder configuration where we have measured and successfully reconstructed the height distribution of spherical and cylindrical microlenses with NA up to 0.3.

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.

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°.

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.

Degeneration of Fraunhofer diffraction on bacterial colonies due to their light focusing properties examined in the digital holographic microscope system

The degeneration of Fraunhofer diffraction conditions in the optical system with converging spherical wave illumination for bacteria species identification based on diffraction patterns is analyzed by digital holographic methods. The obtained results have shown that the colonies of analyzed bacteria species act as biological lenses with the time-dependent light focusing properties, which are characterized and monitored by means of phase retrieval from sequentially captured digital holograms. This significantly affects the location of Fraunhofer patterns observation plane, which is continuously shifted across optical axis in time.

Hybrid and transflective system based on digital holographic microscope and low coherent interferometer for high gradient shape measurement

The most suited techniques for quantitative and accurate determination of the phase distribution in a phase photonic microstructures are based on the interferometry, especially the digital holography (DH) in microscopic configuration. However there is well known limitation of the coherent full- field interferometric measurements: the phase difference between the neighboring samples cannot be larger than 2π, or objects shape have to generate light that can be collected by used optical system. This limitation might be overcame by use of a well-known technique called low-coherence interferometry (LCI) which allows for absolute shape measurements with a nanometer resolution and does not have 2π limitation of coherent interferometric techniques. In this work a dual channel measurement system for characterization of a high numerical aperture objects is presented. The system combines functionalities of the LCI system based on Twyman-Green configuration and the DHM system based on Mach-Zehnder configuration. The DHM allows to measure sample in transmission while LCI setup provides reflective measurement data and, therefore, provides a more complete tool for topography characterization. In presented paper we focus on the measurement of high gradient objects were both methods fail if applied independently: the LCI gives measurement only in the object area of low NA while the DHM cannot provide absolute shape characterization due to limited NA of imaging system. The dual channel system extends capabilities of both methods. In our paper we present experimental results for topography measurement of high NA microlenses. The accuracy of the development method is discussed and both simulation and experimental data are provided.

Revealing features of different optical shaping technologies by a point diffraction interferometer

Almost hidden residual defects of a test surface can be revealed using high precision instrument such as a point diffraction interferometer (PDI). In general, PDI is engaged to display the figure of a surface or wavefront with subnanometer accuracy paying attention to low-frequency configurations. Such technique is suited to test EUV or X-ray optics. The tool described in the paper is able to map absolute profile deviations of several angstroms and therefore it provides a new vision of a surface under test of various quality, e.g. detects specific characteristics which immediately disclose either lapping or diamond turning has been used to form the substrate. Such inspection may help optimize the processes in early stage of shape forming before final configuring.

Optical methods for measurements of surface shape in optical components for high power laser beam forming

The paper presents modifications of full-field optical methods commonly used to test the surface quality of optical components used for forming a high power laser beam and tests of a final wavefront. The modifications in reference to surface measurements rely on implementation of the novel fringe pattern processing methods including the quality improvement of initial interferogram and analysis of a reconstructed phase based on Hilbert-Huang transform aided by the principal component analysis. Also the Point Diffraction Interferometer as the efficient tool for high quality measurements of elements with high NA is introduced. In reference to a wavefront quality measurements two solutions are discussed: the use of a lateral shear interferometer and the system employing Transport of Intensity Equation method. The pros and cons for both methods are discussed.

On accuracy of holographic shape measurement method with spherical wave illumination

This paper presents the study on the accuracy of topography measurement of high numerical aperture focusing microobjects in digital holographic microscope setup. The system works in reflective configuration with spherical wave illumination. For numerical reconstruction of topography of high NA focusing microobjects we are using two algorithms: Thin Element Approximation (TEA) and Spherical Local Ray Approximation (SLRA). In this paper we show comparison of the accuracy of topography reconstruction results using these algorithms. We show superiority of SLRA method. However, to obtain accurate results two experimental conditions have to be determined: the position of point source (PS) and imaging reference plane (IRP).Therefore we simulate the effect of point source (PS) and imaging reference plane (IRP) position on the accuracy of shape calculation. Moreover we evaluate accuracy of determination of location of PS and IRP and finally present measurement result of microlens object.