Three-dimensional imaging of microstructures by an improved compact digital holographic microscope (ICDHM) with dual wavelength

An improved compact digital holographic microscope (ICDHM) is developed for three-dimensional imaging of microstructures. This system is based on lensless magnification using a diverging wave. A point source generated by a long working distance microscope objective is located into the cube beam-splitter to get a higher numerical aperture (NA) of the system. The lateral resolution and the field-of-view of the system are confirmed with a calibration experiment. For the case of the optical path lengths (OPL) of object with step pattern larger than the wavelength, the traditional phase unwrapping algorithms cannot be unequivocally determinate, resulting in a 2π phase ambiguity. To solve this problem, dual wavelength phase unwrapping method was integrated into ICDHM, which extends the measuring capability of ICDHM over several microns of range. The experimental results demonstrate that the developed system is well suitable for the measurement of MEMS and Micro systems samples with high resolution.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Published versio

Single-shot quantitative phase microscopy with the transport-of-intensity equation

We present a single-shot experimental configuration for quantitative phase microscopy recovery based on the transportof- intensity equation (TIE). The system can simultaneously capture two laterally separated images with different amounts of defocus using only one digital camera. The defocus distance can be adjusted by varying the free space propagation transfer function on a phase only spatial light modulator. The intensity derivative along optics axis can thus be estimated optimally. In contrast to the state of the art techniques, this configuration requires no mechanical moving parts. Furthermore its single-shot property allows potential application for measuring fast moving objects or dynamic processes. Validation experiments are presented.Published versio

Noninterferometric single-shot quantitative phase microscopy

We present a noninterferometric single-shot quantitative phase microscopy technique with the use of the transport of intensity equation (TIE). The optical configuration is based on a Michelson-like architecture attached to a nonmodified inverted transmission bright field microscope. Two laterally separated images from different focal planes can be obtained simultaneously by a single camera exposure, enabling the TIE phase recovery to be performed at frame rates that are only camera limited. Precise measurement of a microlens array validates the principle and demonstrates the accuracy of the method. Investigations of chemical-induced apoptosis and the phagocytosis process of macrophages are then presented, suggesting that the method developed can provide promising applications in the dynamic study of cellular processes.Published versio

Compound common-path digital holographic microscope

Digital holographic microscopy provides 3D quantitative phase imaging that is suitable for high resolving investigations on reflective surfaces as well as for transmissive materials. An optical configuration for a digital holographic microscope and a method for digital holographic microscopy are presented. A cube beam splitter in the optical path, with a small angle between the optical axis and its central semi-reflecting layer, both split and combine a diverging spherical wavefront emerging from a microscope objective to give off-axis digital holograms. Since the object wave and the reference wave go the same way to the CCD camera, it is called common-path digital holographic microscopy. When a plane numerical reference wavefront is used for the reconstruction of the recorded digital hologram, the phase curvature introduced by the microscope objective together with the illuminating wave to the object wave can be physically compensated. A compound digital holographic microscope (with reflection mode and transmission mode) has been build up based on this unique feature. Results from surfaces structures on silicon wafer and micro-optics on fused silica demonstrate applications of this compound digital holographic microscope for technical inspection in material science.Published versio

Boundary-artifact-free phase retrieval with the transport of intensity equation II: applications to microlens characterization

Boundary conditions play a crucial role in the solution of the transport of intensity equation (TIE). If not appropriately handled, they can create significant boundary artifacts across the reconstruction result. In a previous paper [Opt. Express 22, 9220 (2014)], we presented a new boundary-artifact-free TIE phase retrieval method with use of discrete cosine transform (DCT). Here we report its experimental investigations with applications to the micro-optics characterization. The experimental setup is based on a tunable lens based 4f system attached to a non-modified inverted bright-field microscope. We establish inhomogeneous Neumann boundary values by placing a rectangular aperture in the intermediate image plane of the microscope. Then the boundary values are applied to solve the TIE with our DCT-based TIE solver. Experimental results on microlenses highlight the importance of boundary conditions that often overlooked in simplified models, and confirm that our approach effectively avoid the boundary error even when objects are located at the image borders. It is further demonstrated that our technique is non-interferometric, accurate, fast, full-field, and flexible, rendering it a promising metrological tool for the micro-optics inspection.Published versio

Phase retrieval in arbitrarily shaped aperture with the transport-of-intensity equation

Phase is not easy to detect directly as intensity, but sometimes it contains the really desired information. The transport-of-intensity equation (TIE) is a powerful tool to retrieve the phase from the intensity. However, by considering the boundary energy exchange and the whole energy conversation in the field of view, the current popular Fast Fourier transform (FFT) based TIE solver can only retrieve the phase under homogeneous Neumann boundary condition. For many applications, the boundary condition could be more complex and general. A novel TIE phase retrieval method is proposed to deal with an optical field under a general boundary condition. In this method, an arbitrarily-shape hard aperture is added in the optical field. In our method, the TIE is solved by using iterative discrete cosine transforms (DCT) method, which contains a phase compensation mechanism to improve the retrieval results. The proposed method is verified in simulation with an arbitrary phase, an arbitrarily-shaped aperture, and non-uniform intensity distribution. Experiment is also carried out to check its feasibility and the method proposed in this work is very easy and straightforward to use in a practical measurement as a flexible phase retrieval tool.Published versio

A new phase error compensation method in digital holographic microscopy

In this paper we present a new method to compensate for phase aberrations and image distortion with recording single digital hologram in digital holographic microscopy. In our method, tilt is removed from the abberrated phase map first. Then an area of interest (AOI) is generated by flood filled algorithm. By fitting AOI with discrete orthogonal Zernike polynomials, error phase map in the form of a series of Zernike polynomials is obtained. Final result can be calculated by subtracting the error phase map from the abberrated phase map. Through applying our method in microlens testing, phase aberrations and image distortion introduced by microscope objective are well suppressed.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Published versio