arXiv subjects

Jigme Zangpo

Publications and source records attributed to Jigme Zangpo.

5 recordsLinked to original sources

arXiv2026

Comparative analysis of wavenumber response in phase contrast and spiral phase imaging systems for plasma diagnostics

Phase contrast imaging (PCI) has been used for decades to study plasma density fluctuations, but its wavenumber response $k$ is constrained by the phase plate groove width and beam waist. Spiral phase contrast imaging (SPCI) with a spiral phase plate may offer broader sensitivity, even though its output signal is quadratic, because it has no constraint except at the central singularity, i.e., $k = 0$. In this work, we numerically compare the wavenumber response of both techniques using two distinct models: (i) static square phase objects with scale lengths $R$ ranging from 5 to 25 mm, and (ii) a time-evolving, anisotropic, multi-scale turbulence field with a Kolmogorov-like spectrum. For static square objects, PCI exhibits a lower cutoff at $k_{\text{min}} \approx 0.1$ mm$^{-1}$, while SPCI produces measurable signals down to $k_{\text{min}} \approx 0.007$ mm$^{-1}$ via the autocorrelation of the gradient spectrum. For the plasma-like turbulence model, PCI retains its lower cutoff at $k \approx 0.1$ mm$^{-1}$. In contrast, SPCI produces measurable signals down to $k \approx 0.007$ mm$^{-1}$. These results suggest that SPCI provides low-wavenumber information below the PCI cutoff, offering complementary diagnostic information for multi-scale plasma turbulence studies.

physics.plasm-ph↗

arXiv2026

Phase-edge imaging using q-plate shifts for faster and simpler microscopy

We present a simplified method for isolating the edges of a phase object from the edges of an amplitude object using a 4f system with an off-axis q-plate. Instead of the four off-axis shifts of the q-plate required in previous work, we need only two shifts (along +/- x) combined with linear polarizers at 45 degrees and 135 degrees. The number of measurements is reduced by half, potentially doubling the acquisition speed. We derive the theoretical basis, showing that the resulting intensity corresponds to the phase gradient squared, with amplitude-object contributions eliminated. Experiments on two phase-amplitude object samples demonstrate amplitude-edge reduction up to 97.6% and correlation coefficients up to 0.78 (sample 1) and 0.75 (sample 2). In overlapping regions, the phase edge is partially recovered; full recovery would require additional processing such as inverse filtering. This research is useful for biological imaging applications where fast and simple phase-edge isolation is desired.

physics.optics↗

arXiv2025

Single-pixel edge enhancement of object via convolutional filtering with localized vortex phase

Microscopy is an essential tool in imaging research, and the edge-enhanced microscope by using the vortex filter is of particular interest as an optical information processing that highlights amplitude and phase edges of object in all directions. The application of this technique is not limited to the visible range, but edge enhancement of object in invisible wavelength is also crucial for near-infrared fluorescence and electronic circuit inspection through silicon semiconductors. One disadvantage of near-infrared imaging is that digital cameras such as CCD and CMOS become much more expensive than cameras for the visible spectrum. As an cost-effective method to implement invisible edge enhancement, the Fourier single-pixel imaging has already been proposed without using a camera, but using a single-pixel detector. However, this method requires 3 or 4 times more single-pixel measurements due to the three-phase or four-phase shift to detect optical complex amplitude in Fourier domain. In response, we propose a method for single-pixel edge enhancement of object via convolutional filtering with a localized vortex phase, eliminating the extra single-pixel measurements required by the phase-shifting method. Our simulation results show that the correlation coefficient between the ideal edges of an object and the edge enhanced by our proposed method is 0.95, indicating that our method is effective way to detect the edges. This novel and effective approach for enhancing and detecting the edges of object can be valuable in various invisible imaging applications.

physics.optics↗

arXiv2024

Isolation of phase edges using off-axis q-plate filters

Edge-enhanced microscopes with q-plate have been attracted more attention to enhance the edges of phase-amplitude objects in biological sample due to their capability for all-directional edge enhancement, while differential interference-contrast microscopy enhances edges in only one-direction. However, the edge-enhanced microscopes cannot distinguish the edges of phase and amplitude objects, as both edges are equally enhanced. This study introduces a novel method for isolating the edge of a phase object from an amplitude object using an off-axis q-plate filter in a 4f system. Herein, we combined off-axis q-plates with four different displacements to isolate the phase object edge from the amplitude object. To demonstrate the proposed method, we conducted experiments using two distinct samples. The first sample comprised a phase test target surrounded by an aperture, and the second sample involved an overlap between the phase test target and a white hair with non-zero transmittance. In the samples, the isolated phase object edge is in good agreement with the theoretical expectations, and the amplitude object edge was reduced by approximately 93%. The proposed method is a novel and effective approach for isolating the edge of a phase object from an amplitude object and can be useful in various biological imaging applications.

physics.optics↗

arXiv2023

Edge-Enhanced Microscopy of Comlplex Object using Scalar and Vectorial Vortex Filtering

Recently, $4f$ system containing a q-plate has been used to perform edge detection and enhancement of amplitude and phase objects. However, only few studies have concentrated on edge enhancement of phase-amplitude objects. Here, we experimentally verified the functional difference between scalar and vectorial vortex filtering using an onion cell, the experimental results agree well with theoretical analysis. We verified our experimental results through numerical simulation. Although vectorial vortex filtering successfully enhanced the edges of phase and amplitude objects in the phase-amplitude object, they are indistinguishable due to the equal enhancement of the edges of the phase and amplitude objects. To address this, we propose a method to isolate the edge of the phase object from the edge of the amplitude object using off-axis beam illumination. We theoretically calculated the isolation of the edge of the phase object from the amplitude object, and verified via numerical simulations.

physics.optics↗