Partially Coherent Illumination Based Point-Diffraction Digital Holographic Microscopy Study Dynamics of Live Cells

This paper presents a partially coherent point-diffraction digital holographic microscopy (PC-pDHM) prototype and demonstrates its application in label-free imaging of the dynamic processes of live cells. In PC-pDHM, the light scattered by a rotating diffuser is coupled into a multi-mode fiber, and the output light is used as the partially coherent illumination (PCI), which reduces the speckle noise significantly in PC-pDHM. A polarization-grating is used to remold the object and the reference waves, and the fringe contrast of the generated hologram can be adjusted by changing the polarization of the illumination wave. Using the PC-pDHM prototype, transparent samples and notably the dynamic processes of live cells were imaged with high contrast and in a label-free manner, discovering the pathological mechanisms of biology in the cellular and sub-cellular levels.

Extension and Limits of Depolarization-Fringe Contrast Roughness Method in Sub-Micron Domain

To guarantee quality standards for the industry, surface properties, particularly those of roughness, must be considered in many areas of application. Today, several methods are available on the market, but some damage the surface to be tested as they measure it by contact. A non-contact method for the precise estimation of sub-micron roughness values is presented, which can be used as an extension of existing roughness measurement techniques to improve them further considering the depolarized light reflected by the sample. This setup is based on a Michelson interferometer, and by introducing a quarter-wave plate on a half part of the reference mirror, the surface roughness can be directly derived by measuring the fringe contrasts. This article introduces a simple model describing the intensity distortions resulting from the microscopic roughness in divided interferograms when considering depolarization. This work aimed to extend the measurement range of the technique developed in a previous work, in which depolarization effects are taken into account. For verification, the experimental results were compared with the fringe contrast technique, which does not consider the depolarization of the scattered light, especially regarding the extended wavelength interval, highlighting the limits of the technique. In addition, simulations of the experiments are presented. For comparison, the reference values of the sample roughness were also generated by measurements with a stylus profiler.

Determination of the Spatial Resolution in the Case of Imaging Magnetic Fields by Polarized Neutrons

One of the most important parameters characterizing imaging systems (neutrons, X-rays, etc.) is their spatial resolution. In magnetic field imaging, the spatial resolution depends on the (magnetic) resolution of the depolarization of spin-polarized neutrons. This should be realized by different methods, but they all have in common that a spin-polarizing and spin-analyzing system is part of the resolution function. First a simple and useful method for determining the spatial resolution for unpolarized neutrons is presented, and then methods in the case of imaging with polarized neutrons. Spatial resolution in the case of polarized neutron imaging is fundamentally different from ‘classical’ spatial resolution. Because of π-periodicity, the shortest path along which a spin-flip can occur is a measure of ‘magnetic’ spatial resolution. Conversely, the largest detectable magnetic field (B-field) within a given path length is also a measure of magnetic spatial resolution. This refers to the spatial resolution in the flight direction of the neutrons (Δy). The Δx and Δz refers to the spatial resolution in x- or z-direction; however, in these cases a different method must be used. Therefore, two independent methods are used to distinguish longitudinal and lateral spatial resolution, one method to determine the modulation transfer function (MTF) by recording the frequency-dependent fringe contrast of magnetic field images of a coil (longitudinal spatial resolution), and the second method, to observe the fringe displacement at the detector as a function of magnetic motion, provided that the accuracy of the motion is much better than the pixel size (at least half the pixel size) of the detector (lateral spatial resolution). The second method is a convolution of the fringe pattern with the pixel array of the detector.

Production of good quality holograms by the THz pulsed vortex beams

In this paper, we discuss the quality of holograms based on the calculation of the modulation depth. It’s shown that the terahertz (THz) pulsed vortex beams play a vital role in holography filed, where two lasers with frequency difference have used. The THz vortex beams give new regions of larger frequency detuning and an important value of the modulation depth and fringe contrast (MDFC ratio) for obtaining the best holograms contrary to the Gaussian beams for which the best holograms are realized for small frequency detuning. The particular cases such as, Gaussian beam and single cycle pulses are deduced from our result. Numerical simulations are also presented to study the dependence of the MDFC ratio on the frequency detuning for THz vortex beams and Gaussian beams. This research could be beneficial in holographic interferometry, and it will firmly establish as a tool for scientific and engineering studies.

Analysis of the modulation depth of some femtosecond laser pulses in holographic interferometry

On the basis of the modulation depth in the interference and holographic processes, we discuss in this paper the quality of holograms generated by some femtosecond laser pulses of two different colors. The modulation depth in terms of the fringe contrast (MDFC ratio) of Higher-order sh- and ch-Gaussian temporal profiles (shnGTP and chnGTP) are investigated in detail. It’s shown that, when we use two lasers having a very large frequency detuning, the shnGTP exhibit more precise results than the Gaussian beams for holography. shnGTP give new areas of frequency detuning to realize the very significant value of the MDFC ratio for obtaining better holograms, which is impossible with Gaussian beams. It also permits flexibility in the variable frequency difference and pulse duration for good quality holograms, and in the case of the chnGTP, null regions develop and frequency bands are observed favoring the formation of the holograms. The numerical simulations are presented to illustrate and discuss the influence of the frequency detuning, the beam order and the controller parameter of the waves on the modulation depths. The propose theory will be a good basis for the development of some new experiments on the holographic interferometry and it will certainly be very useful for the specialists of the studied femtosecond laser pulses.

Analysis of the Noise Reduction Characteristics of a Diffuser with Fizeau Interferometry

To solve the coherent noise problem of an interference image, the method of a rotating diffuser was adopted to change the coherence of a beam to reduce the noise of the interference system. The relationship between the speed of the diffuser and the signal-to-noise ratio (SNR) of the fringe contrast system was simulated to obtain the diffuser control parameters needed for the best interference fringe state. The fringe contrast of each image and the SNR of the system were analyzed. The results showed that the increased speed of the diffuser reduced the contrast of the interference image to a certain extent, but the increased speed also effectively improved the SNR and facilitated the subsequent interference image processing. Due to the coherent noise in the interferometric system, the method of the rotated diffuser reduced the coherence of the light beam to suppress the noise of the interference image. By analyzing the coherent noise reduction characteristics of the rotated diffuser with different surface roughnesses, the relationship between the surface roughness and the noise contrast for different rotation speeds was simulated, and the effective roughness range with the noise reduction effect was selected. A noise reduction system was built based on Fizeau interference, and the noise contrast of the interference image was collected and calculated. The effective range of σh/λ was 0.2–0.5 when the rotation speed was 10 r/s, while the effective range of σh/λ was 0.4–0.6 when the rotation speed was 100 r/s. The experimental results showed that the surface roughness and wavelength ratio σh/λ of the rotated diffuser increased when the noise contrast tended toward 1, but the effective range of the surface roughness decreased with the increase of the rotational speed of the diffuser.