Self-calibrating common-path interferometry.

In this study, a simultaneous dual-wavelength phase-shifting interferometry system using two reference arms was built, and a simple phase retrieval method for the system with blind phase shifts was proposed using five phase-shifted interferograms. First, the unknown phase shifts were controlled independently to separate the information of single wavelength, which can be calculated directly by using the Euclidean matrix norm algorithm. Second, a wrapped phase of a single wavelength was obtained using a three-step phase-shifting algorithm. Finally, a continuous phase of a synthetic wavelength was obtained. The simulation and experimental results indicated the feasibility and greater accuracy of the proposed method. Moreover, the strict requirement of the phase shifter was eliminated because of the use of arbitrary and unknown phase shifts, thereby making experimental operation easier.

Transmission optical coherence tomography (tOCT) is gaining interest for imaging biological samples; however, so far, quantitative phase information has not been directly obtained. This study introduces SOPHAST-OCT, an approach for integrated 2D phase recovery in Off-Axis Full-Field Swept-Source tOCT. It uses phase-shifting to leverage OCT’s inherent phase changes between recorded interferograms. SOPHAST-OCT demonstrates promising phase agreement with the simulated ground truth phase. This approach represents a first step toward quantitative imaging in tOCT. Full Text: PDF References Y. Park, C. Depeursinge, G. Popescu, "Quantitative phase imaging in biomedicine", Nature Photon 12, 578 (2018). CrossRef K. Alm, H. Cirenajwis, L. Gisselsson, A.G. Wingren, B. Janicke, A. Mölder, et al., in Holography, Research and Technologies (IntechOpen, 2011). CrossRef W. Drexler, J. Fujimoto, editors , Optical Coherence Tomography: Technology and Applications, 2nd ed. (Springer Cham, 2015). CrossRef W. Krauze, P. Ossowski, M. Nowakowski, M. Szkulmowski, M. Kujawińska, "Enhanced QPI functionality by combining OCT and ODT methods", Quantitative Phase Imaging VII, 11653, 19 (2021). CrossRef B.C. Wilson, S.L. Jacques, "Optical reflectance and transmittance of tissues: principles and applications", IEEE J. Quantum Electron. 26, 2186 (1990). CrossRef J. van der Horst, A.K. Trull, J. Kalkman, "Deep-tissue label-free quantitative optical tomography", Optica 7, 1682 (2020). CrossRef P. Hosseini, Y. Sung, Y. Choi, N. Lue, Z. Yaqoob, P. So, "Scanning color optical tomography (SCOT)", Opt. Expr. 23, 19752 (2015). CrossRef P. de Groot, "Phase Shifting Interferometry" in Optical Measurement of Surface Topography, edited by R. Leach (Springer, Berlin, Heidelberg, 2011). CrossRef M. Wojtkowski, A. Kowalczyk, R. Leitgeb, A.F. Fercher, "Full range complex spectral optical coherence tomography technique in eye imaging", Opt. Lett. 27, 1415 (2002). CrossRef Q. Zhang, S. Zhong, J. Lin, W. Chen, M. Luo, J. Zhong, Y. Yu, Z. Peng, S. Cheng, "Full-range Fourier-domain optical coherence tomography based on Mach–Zehnder interferometer", Opt. Lasers Engin. 124, 105794 (2020). CrossRef R. Juarez-Salazar, C. Robledo-Sanchez, F. Guerrero-Sanchez, A. Rangel-Huerta, "Generalized phase-shifting algorithm for inhomogeneous phase shift and spatio-temporal fringe visibility variation", Opt. Express 22, 4738 (2014). CrossRef R. Juarez-Salazar, C. Robledo-Sánchez, C. Meneses-Fabian, F. Guerrero-Sánchez, L. M. Arévalo Aguilar, "Generalized phase-shifting interferometry by parameter estimation with the least squares method", Opt. Lasers Engin. 51, 626 (2013). CrossRef M.A. Herráez, D.R. Burton, M.J. Lalor, M.A. Gdeisat, "Fast two-dimensional phase-unwrapping algorithm based on sorting by reliability following a noncontinuous path", Appl. Opt. 41, 7437 (2002). CrossRef Z. Wang, A.C. Bovik, "A universal image quality index", IEEE Signal Proc. Lett. 9, 81 (2002). CrossRef

Subject of study. The shift direction of interference fringes depending on the sign of the local phase shift introduced to one of the interfering waves was investigated in terms of two-beam interferometry. Aim of study. This study aimed to validate the fact that two-beam interferometry based on a single interferogram enables definitive determination of the phase-shift sign using the shift direction of the interference fringe. Method. Positions of the interference fringes and their shifts were calculated considering the phase distribution of interfering waves in the observation plane. The Mach–Zehnder interferometer was used to experimentally confirm the obtained theoretical results. Two closely adjacent substrates were used as objects. A subwavelength layer of a standard material was deposited on a part of the surface of one substrate, and a layer of metamaterial was deposited on the part of the second substrate. Main results. Expressions determining the direction and amplitude of the shift of interference fringes depending on the sign and amplitude of the local phase shift in one of the two interfering waves with a plane or spherical front were obtained. Strict rules of interference fringe shift depending on the sign of the phase shift were formulated based on the calculations. Results of the experiment using the Mach–Zehnder interferometer with different combinations of beams with spherical and plane wavefronts confirmed the theoretical calculations. Practical significance. Classic interferometric methods require significant time and the application of complex experimental procedures to determine the sign of the phase shift. The application of the results obtained in this study enables the determination of the sign of the introduced local phase shift based on a single interferogram without additional time expenditures. Thus, the method investigating the metamaterial layers of a subwavelength thickness is significantly simplified.

A direct phase shift extraction and wavefront reconstruction method in two-step generalized phase-shifting interferometry (GPSI) with arbitrary unknown phase shift is proposed. In this method the unknown phase shift α can be extracted by a determinate formula directly without iteration or additional judgment of its correct value from two or more phase shift solutions as necessary before. By an appropriate formula of GPSI the complex object field in the recording plane can be calculated and then the object wavefront in the original object plane obtained. This method is applicable for GPSI of any frame number K≥2 and for both the amplitude and phase objects. Computer simulations have shown that the phase shift extraction errors are below 0.01 rad in a wide range of 0.4 rad<α<2.6 rad and the computation time is greatly reduced by a factor of about 20 compared with the previous method. The effectiveness and accuracy of this method are also verified by optical experiments.

Phase-shifting shadow moire is an efficient technique for three-dimensional (3-D) shape measurement, but the technique is always suffered from the error of phase shifter. A three-frame random phase-shifting algorithm is proposed to relax the requirements of repeatability and accuracy of the phase shifter in a shadow moire system. The proposed method can act as a bridge between the three phase-shifting analysis methods and two-shot phase-shifting methods. The value of introduced arbitrary and unknown phase shift can be calculated by a determinate formula. The algorithm has the advantages of being free from the disturbance of background elimination and less computation. In addition, the proposed method is immune to nonsinusoidal fringe patterns and can be implemented easily in many applications. Both simulation and optical experimental results verify the feasibility of this method.

A novel method is presented to extract phase distribution from phase-shifted interferograms with unknown tilt phase shifts. The proposed method can estimate the tilt phase shift between two temporal phase-shifted interferograms with high accuracy, by extending the regularized optical flow method with the spatial image processing and frequency estimation technology. With all the estimated tilt phase shifts, the phase component encoded in the interferograms can be extracted by the least-squares method. Both simulation and experimental results have fully proved the feasibility of the proposed method. Particularly, a flat-based diffractive optical element with quasi-continuous surface is tested by the proposed method with introduction of considerably large tilt phase shift amounts (i.e., the highest estimated tilt phase shift amount between two consecutive frame reaches 6.18λ). The phase extraction result is in good agreement with that of Zygo's MetroPro software under steady-state testing conditions, and the residual difference between them is discussed. In comparison with the previous methods, the proposed method not only has relatively little restrictions on the amounts or orientations of the tilt phase shifts, but also works well with interferograms including open and closed fringes in any combination.

Multimodal optical microscopy combines different optical and physiological principles working together to obtain rich perspectives from a single biological event. In this paper, the recent advances in multimodal microscopy systems based on off-axis digital holography and transport-of-intensity equation are reviewed. Both methods used the common-path configuration, and realized simultaneous quantitative phase and fluorescence imaging in 3D space. Feasibility checks with fluorescent micro-beads and live plant cells are presented. The results show that both methods are capable of fast acquisition of the quantitative phase and fluorescence information with digital refocusing ability.

Principal component analysis based simultaneous dual-wavelength phase-shifting interferometry

Digital holographic Microscopy is a non-destructive and label-free method that provides quantitative phase information in biological and industrial applications. High coherence light sources, such as lasers, are commonly used in digital holographic microscopes. Parasitic -interference fringes and speckle noise in high-coherence sources as well as complex configurations reduce the accuracy of the phase measurements. In this paper, a common-path and low-coherence digital holographic microscopy with a Fourier transform-based spectroscopy is introduced. The low-coherence source used here is an LED and the common path configuration is used based on splitting the wavefront by Fresnel biprism. Reconstruction of the hologram is analyzed using the Fourier method. In addition, The spectral line shape of the LED is obtained simultaneously with the Fourier transform of the visibility of the recorded fringes. The ability to simultaneously perform quantitative phase imaging and Fourier transform spectroscopy makes this system unique in the real-time study of biological samples in micron size.

The application of structured illumination for enhanced resolution requires accurate knowledge of phase shifts in the sinusoidal illumination. This work proposes a method to estimate random, unknown phase shifts and subsequent image reconstruction.

This paper combines virtual simulation technology to improve the traditional foreign language teaching mode. Aiming at the problems of traditional phase-shift shadow Moiré profiler, a three-step quadrature-phase demodulation technique using unknown phase shift was developed. Then, the paper compares the proposed method with the GS method using MATLAB simulation. It can be seen from the simulation and demodulation results that the two methods obtain similar results, thus proving the equivalence of the two methods.

Quantitative phase microscopy (QPM) is a label-free imaging technique often employed for long-term, high-contrast imaging of live bio-samples. Yet, QPM is not specific to a certain subcellular organelle. As a remedy, fluorescence microscopy can visualize specific subcellular organelles once being labeled with fluorescent markers. In this paper, a high-resolution phase/fluorescence dual-modality microscopic imaging method based on structured light illumination is proposed. In a dual-modality microscopic system, periodic stripes are generated by a digital micromirror array (DMD), and are used as the common illumination for both modalities. For QPM imaging, the holograms of the sample under structured light illuminations from different directions and phase shifts are recorded, from which a quantitative phase image with resolution enhancement can be reconstructed via a synthetic aperture procedure. Furthermore, a numerical approach is proposed to compensate for the environmental disturbances that often challenge aperture synthesis of phase imaging. This method determines each time the phase distortions caused by environmental disturbances through using the spectrum of the 0th order of the structured light illumination, and the phase distortions are removed from the phase distributions of the waves along the 0th and the ±1st diffraction orders. Resolution enhancement of QPM imaging is realized by synthesizing the spectra of all the waves along different diffraction orders of the structured light illuminations of different orientations. With phase images, three-dimensional shapes, inner structures, or refractive index distributions of transparent and translucent samples can be obtained. For fluorescence imaging, intensity images (morie patterns) of the sample under different structured light illuminations are recorded. The spectra along different diffraction orders are separated by using a phase shifting reconstruction algorithm, and are shifted to their original positions, forming a synthesized spectrum that is much broader than the spectra of raw intensity images (NA-limited spectra). An inverse Fourier transform on the synthesized spectrum yields a super-resolution fluorescence image of the sample. With the reconstructed fluorescence images, specific subcellular organelles labeled with fluorescent markers can be visualized. The combination of quantitative phase microscopy and fluorescence microscopy can obtain multidimensional information about the sample. In this dual-mode imaging system, the spatial resolution of quantitative phase imaging and fluorescence imaging are 840 nm and 440 nm, respectively. The proposed dual-mode microscopy imaging technique has been demonstrated for imaging fluorescent beads, fly wings, spring/rice leaves, mouse tail transection, and fluorescence-stained SiHa cells. We envisage that this method can be further applied to many fields, such as biomedicine, industry, and chemistry.

Astrocytes are star-shaped glial cells whose elaborate soma-process architecture supports neuronal functions, regulates ions as well as water homeostasis, and contributes to brain structural organization. This paper presents a quantitative imaging technique for comprehensive morphological analysis of astrocytes, including their somata and branches, using a custom-built common-path quantitative phase microscope. The microscope employs a common-path off-axis configuration, splitting the sample beam using a grating, and a dual-lens system combined with a spatial filter to produce a reference beam required for hologram generation. This microscope enables a label-free, high-temporal-stability, and real-time 3D visualization of biological samples by measuring optical path length difference (OPD). Astrocyte cells were imaged, and their morphometric properties were quantified in near real time using the developed microscope. The reconstructed optical path length difference (OPD) maps were used to perform multistage deep learning segmentation of entire astrocyte cells, and the corresponding soma and branches were segmented using an optical thickness-based clustering approach. The segmented soma and process regions were compared as well as correlated by their phase-derived and geometrical morphological parameters. To the best of our knowledge, this is the first report of using the proposed digital holographic approach with an AI model to segment and measure the morphometrics of astrocyte cells, including their somata and branches separately.

We present a Nd:YAG CW laser based second-harmonic interferometer with 60 &mu;m spatial resolution. The interferometer is sensitive to the phase shift between fundamental and second harmonic radiation when passing through a dispersive medium. The device performance is tested by measuring the dispersion induced phase shift of laser etched polymeric films resulting in a sensitivity down to 7×10⁻³ Rad for a detector acquisition time of 300 &mu;s. These results demonstrate the feasibility of high spatial resolution second-harmonic interferometry, and an outlook is given for its use as a novel quantitative phase sensitive imaging technique.

With its rapid development, the terahertz technology is widely used in radar, imaging, remote sensing and data communication. As one of terahertz wave devices, the terahertz phase shifter has become a research hotspot. The existing phase shifters have the disadvantages of large volume, high power consumption and small phase shifting. In the present work, a tunable terahertz phase shifter with liquid crystal and vanadium dioxide is proposed. It is composed of an upper vanadium dioxide embedded metal layer, a liquid crystal, a lower vanadium dioxide embedded metal layer, and a silicon dioxide substrate in sequence from top to bottom. The liquid crystal is sandwiched between the upper and lower vanadium dioxide embedded metal layer. The phase of the device can be controlled by both the phase transition characteristics of vanadium dioxide and the birefringence of liquid crystal. By changing the external applied temperature, the conductivity of vanadium dioxide is changed, and the phase of the device shifts accordingly. Likewise the refractive index of the liquid crystal changes under different externally applied voltages. Finally, the phase of the proposed device can be effectively controlled in a terahertz range by both externally applied temperature and voltage. The phase shift characteristics of the device are analyzed by using software CST studio. The results verify that the terahertz phase shifter can achieve a maximum phase shift of 355.37° at &lt;i&gt;f&lt;/i&gt; = 0.736 THz and a phase shift is larger than 350° in a range of 0.731–0.752 THz (bandwidth 22 GHz). Therefore, compared with the traditional phase shifter, this kind of phase change material-metasurface composite structure provides a new idea for flexibly manipulating the terahertz beam. And it is expected to be widely used in terahertz imaging, terahertz wireless and other fields.