Junwei Min

Parallel two-step phase-shifting point-diffraction interferometry for microscopy based on a pair of cube beamsplitters

Parallel two-step phase-shifting point-diffraction interferometry for microscopy based on a pair of cube beamsplitters is proposed. The first 45°-tilted cube beamsplitter splits object wave into two parallel copies: one copy is filtered by a pinhole in its Fourier plane to behave as reference wave, while the other one remains unchanged as object wave. The second cube beamsplitter combines the object and reference waves, and then split them together into two beams. Along with the two beams, two parallel phase-shifting interferograms are obtained in aid of polarization elements. Based on the proposed configuration, slightly-off-axis interferometry for microscopy is performed, which suppresses dc term by subtracting the two phase-shifting holograms from each other. The setup is highly stable due to its common-path configuration, and has been demonstrated to be suitable for measuring moving objects or dynamic processes.

Autofocusing of digital holographic microscopy based on off-axis illuminations

An auto-focusing method for digital holographic microscopy has been proposed by employing two off-axis illumination beams. When specimens are illuminated by two plane waves in different directions, it is found that the farther the reconstruction plane is from the image plane, the wider the two reconstructed images are separated from each other. Thus, the image plane can be determinated by finding the minimum of the variation between the two reconstructed object waves on both the amplitude and phase distributions. The feasibility of the proposed method is demonstrated by the corresponding simulation and experiment.

Autofocusing based on wavelength dependence of diffraction in two-wavelength digital holographic microscopy.

An autofocusing method for two-wavelength digital holographic microscopy (TWDHM) based on the wavelength dependence of the diffraction process is proposed. Red and green lights are employed for the illumination of the TWDHM, and the generated holograms are recorded simultaneously by a color CCD camera. Due to the wavelength dependency of the diffraction process, the farther the reconstruction plane is from the image plane, the larger the difference is between the red and green light distributions. Thus, the image plane can be determined by finding the minimum of the variation between the red and green lights on their amplitude distributions. The feasibility of the proposed method is demonstrated by simulation and experiment.

Dual-wavelength slightly off-axis digital holographic microscopy

We propose dual-wavelength digital holographic microscopy with a slightly off-axis configuration. The axial measurement range without phase ambiguity is extended to the micrometer range by synthesizing a beat wavelength between the two wavelengths with separation of 157 nm. Real-time measurement of the specimen is made possible by virtue of the high wavelength selectivity of the Bayer mosaic filtered color CCD camera. The principle of the method is exposed, and the practicability of the proposed configuration is demonstrated by the experimental results on a vortex phase plate and a rectangular phase step.

Parallel two-step phase-shifting digital holograph microscopy based on a grating pair

An optical configuration for parallel two-step phase-shifting digital holographic microscopy (DHM) based on a grating pair is proposed for the purpose of real-time phase microscopy. Orthogonally circularly polarized object and reference waves are diffracted twice by a pair of gratings, and two parallel copies for each beams come into being. Combined with polarization elements, parallel two-step phase-shifting holograms are obtained. Based on the proposed configuration, two schemes of DHM, i.e., slightly off-axis and on-axis DHM, have been implemented. The slightly off-axis DHM suppresses the dc term by subtracting the two phase-shifting holograms from each other, thus the requirement on the off-axis angle and sampling power of the CCD camera is reduced greatly. The on-axis DHM has the least requirement on the resolving power of the CCD camera, while it requires that the reference wave is premeasured and its intensity is no less than 2 times the maximal intensity of the object wave.

Single-shot parallel four-step phase shifting using on-axis Fizeau interferometry

The purposes of the paper are threefold: (1) to show the possibility to perform parallel phase-shifting Fizeau interferometry by using a quarter waveplate with high flatness as a reference, (2) to present a comparative study between the phase-shifting algorithm and the off-axis geometry in surface microtopography measurement, and (3) to show the advantages of using the proposed common path Fizeau interferometry over the quasi-common path Michelson interferometry in terms of accuracy in measurement. The compelling advantage of the proposed parallel phase-shifting Fizeau interferometric technique is the long-term stability that leads to measuring objects with a high degree of accuracy.

Parallel phase-shifting interferometry based on Michelson-like architecture

In this paper, we present a new scheme for parallel phase-shifting interferometry that employs a Michelson-like architecture and a simple polarization unit to generate two phase-shifting interferograms with phase shift of π/2 at a single camera exposure. The parallel phase-shifting unit is built with simple optical components, and the distance between the parallel interferograms can be adjusted conveniently. Phase reconstruction is performed by using an algorithm developed for two-step phase-shifting interferometry. The practicability of the proposed configuration and the reconstruction method is demonstrated by experiments.

Intrinsic optical torque of cylindrical vector beams on Rayleigh absorptive spherical particles

The intrinsic optical torque of a focused cylindrical vector beam on a Rayleigh absorptive spherical particle is calculated via the corrected dipole approximation. Numerical results show that, for the radially polarized input field, the torque is distributed in the focal plane strictly along the azimuthal direction anywhere except at the focus. This shows a completely different property from what is observed in the focusing of a circularly polarized beam, where a strong axial torque component arises. For other cylindrically polarized input fields, the torque tends to align itself along the radial direction, as the polarization angle (the angle between the electric vector and the radial direction) changes from 0° to 90°. When limited to considering the torque at the equilibrium position, we find that only for those input fields with polarization angles larger than 50°, the particle experiences a nonzero torque at its equilibrium position. This is verified by showing quantitatively the effects of the polarization angle on the magnitude and orientation of the torque at the equilibrium position.

Parallel on-axis phase-shifting holographic phase microscopy based on reflective point-diffraction interferometer with long-term stability

Parallel on-axis two-step phase-shifting reflective point-diffraction interferometry for holographic phase microscopy based on Michelson architecture is proposed. A cube beamsplitter splits the object wave into two copies within the two arms. The reference wave is rebuilt by low-pass filtering with a pinhole-masked mirror. Both object and reference waves are split into two beams by a grating in a 4f imaging system; thus, two interferograms with quadrature phase-shift can be acquired simultaneously with the aid of polarization elements. The approach has the merit of nanometers-scale phase stability over hours due to its quasi-common-path geometry. It can make full use of camera spatial bandwidth while its temporal resolution is as fast as the camera frame rate. Phase imaging on microscale specimen is implemented, and the experimental results demonstrate that the proposed approach is suitable for investigating dynamic processes.

LED-based digital holographic microscopy with slightly off-axis interferometry

An LED illuminated Linnik-type digital holographic microscope (DHM) for high-quality phase imaging is presented by the adoption of slightly off-axis two-step blind-phase-shifting interferometry (TB-PSI). Slightly off-axis interferometry lowers the requirement on the angle between the object and the reference waves as well as the requirement on the resolving power of the charge-coupled device (CCD) camera. In addition, the apparatus is cost-effective and offers ease of alignment. The phase-shifting DHM is simply implemented by mechanically moving the reference mirror while disposing of a precise phase modulator such as a piezoelectric transducer (PZT). The phase shift between the two interferograms is extracted by Fourier transformation analysis, and then the phase image is reconstructed. The performance of the TB-PSI used in the scheme is analyzed. The phase imaging for nanostructured specimens is conducted, and the results demonstrate the feasibility of the scheme. The phase noise is reduced by 73% when compared to the result obtained with coherent illumination.