Axial Defocusing Research Articles

Annular multi-focal-phase mask multiplexing based large depth of field imaging by interferenceless coded aperture correlation holography

Extending depth-of-field (DOF) of the imaging system without modifying the structure and sacrificing imaging performances of the optical system is of great significance to broaden the capability and application of the imaging system. In this paper, the interferenceless coded aperture correlation holography(I-COACH) is developed to be a large-depth incoherent imaging system by employing an annular multi-focal coded phase mask (AM-CPM). Based on the analyses of axial defocus characteristics in I-COACH, the defocus compensation function is defined, the AM-CPM is designed and multiplexed on the system optical pupil, which plays the role of a gradual lens. In AM-CPM, multi-annular zones with different focal lengths are used to compensate different axial defocus aberrations and adjacent annular zones have symmetric axial defocus aberration correction capability according to the imaging characteristics of the system. The simulations and experimental results fully demonstrate that the axial point spread function distribution of the system obtained by AM-CPM is continuous and the development method enables the extension of the DOF of the I-COACH system by only single exposure point spread hologram. This solution is expected to provide great potential in the field of microscopic imaging and other fields of that based on I-COACH system.

Molecule transfer into mammalian cells by single sub-nanosecond laser pulses.

A rapid, precise, and viability-retaining method for cytoplasmic molecule delivery is highly desired for cell engineering. Routine methods suffer from low throughput, lack of selectivity, requirement of helper compounds, predominant endosomal delivery, and/or are restricted to specific molecule classes. Photonic cell manipulation bears the potential to overcome these drawbacks. Here we investigated mammalian cell manipulation by single sub-nanosecond laser pulses. Axial beam waist positioning close to a cell monolayer induced culture vessel damage and zones of cell ablation. Cells at margins of ablation zones exhibited uptake of membrane-impermeant fluorophores and GFP expression plasmids. Increasing Rayleigh-length and beam waist diameter reduced the sensitivity to axial defocusing and resulted in robust molecule transfer. Serial application of single pulses focused over a moving cell monolayer yielded quantitative molecule transfer to cells at rates up to 40%. Our results could be basic to spatially and temporally controlled single laser pulse-mediated marker-free high throughput cell manipulation.

Single-Shot Autofocusing of Microscopy Images Using Deep Learning

We demonstrate a deep learning-based offline autofocusing method, termed Deep-R, that is trained to rapidly and blindly autofocus a single-shot microscopy image of a specimen that is acquired at an arbitrary out-of-focus plane. We illustrate the efficacy of Deep-R using various tissue sections that were imaged using fluorescence and brightfield microscopy modalities and demonstrate snapshot autofocusing under different scenarios, such as a uniform axial defocus as well as a sample tilt within the field-of-view. Our results reveal that Deep-R is significantly faster when compared with standard online algorithmic autofocusing methods. This deep learning-based blind autofocusing framework opens up new opportunities for rapid microscopic imaging of large sample areas, also reducing the photon dose on the sample.

Analytic formulas for calculation of axial defocus in microscopy

The paper analyses a possibility of determination of axial defocus of lighting point, which is imaged by an optical system without aberrations, from a knowledge of intensity distribution or an amount of light (amount of energy – fractional total energy) detected in a region around a point's image centre. Analytic formulas for calculation of the defocus are derived, and they are verified by numerical examples, simulations and experiment. Outputs show that the method can evaluate the axial depth of the particle up to several micro-metres with accuracy in a range of fraction of percents.

Phase error analysis of displaced-axis dual reflector antenna for satellite earth stations

In a classical Cassegrain geometry, the sub-reflector partly blocks reflected rays from the main reflector. Feed antenna of the subreflector also blocks some of the radiated energy towards main reflector. In a properly designed displaced-axis dual-reflector antenna (DADRA) system based on axially-displaced ellipse (ADE) sub-reflector configuration, these blockage losses are minimized. Thus, with its high gain and low return-loss, DADRA system is a strong candidate for large scale earth station antennas. In general, to achieve maximum gain using a single-reflector system, the phase center of the feed antenna should be placed at the focus of the paraboloidal reflector. However, phase center of a feed antenna moves with the change of frequency. Phase error loss is not avoidable, especially in wide band applications. In this work, phase error losses due to axial defocusing of a DADRA system designed for satellite earth station are analysed in Ku-band. The system is fed by a corrugated horn. Phase center stability of the feed over a wide frequency band is exhibited resulting in negligible loss levels. The results are compared with that of a single-paraboloidal reflector fed by the same horn. Measured and simulated patterns of DADRA system are presented.

Experimental Demonstration of Reflectarrays Acting as Conic Section Subreflectors in a Dual Reflector System

This paper experimentally demonstrates the use of a microstrip reflectarray as a low-profile planar substitute to a conic section subreflector (hyperboloidal type) in a symmetric dual reflector system at Ku-band. At first, a brief discussion on the simulation and measurement techniques utilized in the paper is provided. A nominal dual reflector Cassegrain system is synthesized through simulations where a feed horn is used to illuminate the hyperboloidal subreflector. Next, a flat metallic subreflector is placed at the subreflector location. This is a critical task as it shows the importance of phase compensation. Due to the flat subreflector, the feed is defocused from the image of the focus and creates phase aberration, leading to beam bifurcation, pattern degradation, and performance deterioration of the dual reflector system. A planar microstrip patch-type subreflectarray is then designed to mimic a hyperboloidal subreflector. Ray tracing is applied to the subreflector-feed system to calculate the phase needed to compensate for the axial defocusing of the feed. A prototype subreflectarray based on the ray-optics approach is fabricated. Radiation pattern measurements and back-projection holographic diagnostics demonstrate that the subreflectarray acts as a hyperboloidal subreflector and restores the antenna performance with a well-defined main beam and low side lobes.

Focusing limit of a cyclotron: Axial betatron instability against beam dynamics approach

In an isochronous relativistic cyclotron, axial defocusing of a beam caused by the radial growth of the isochronous magnetic field is compensated by the azimuthal field gradient introduced by sectors. The focusing capabilities of sectors set the maximum ion energy obtainable from the machine. Usually, the focusing limit of a machine is determined by using the criterion for axial beam instability evolving from the equations of betatron oscillations. The obtained value of the focusing limit is approximate because the equations of betatron oscillations it originates from are approximate as well. The accurate value of the focusing limit is obtained by simulating accelerated beam dynamics in the extraction region. It is shown that the focusing limit of a cyclotron resulting from the two methods could differ for more than 9%. The suggested third method for focusing limit computation relies on the beam dynamics simulation along the critical equilibrium orbit rather than the acceleration orbit and thus it is less time consuming although equally accurate.

Indoor Signal Focusing by Means of Fresnel Zone Plate Lens Attached to Building Wall

A simple inexpensive on-wall Fresnel zone plate (FZP) lens for indoor focusing of microwave signals is studied. It consists of concentric metal rings mounted on the outside of an exterior building wall. In our theoretical and empirical work the on-wall FZP lens is illuminated normally by a plane or spherical wave, of vertical or horizontal polarization, but other, more general incidence situations can be treated by similar means. The scalar quasi-optical focusing theory of the free-space zone plate has been modified and used for design and analysis of one-, two-, and three-ring on-wall FZP lenses. It is found that the presence of the wall does not change the FZP lens focusing efficiency significantly, but it has a strong axial defocusing effect. A 2-GHz FZP lens assembly consisting of three metal rings made out of thin antimosquito mesh has a focusing efficiency of about 15 dB (measured) and 14 dB (calculated), and axial defocusing of about 0.22 m. Some variations of on-wall/on-roof FZP lenses and their feasible applications in the microwave/millimeter-wave communication links are also discussed.

Indoor signal focusing by use of Fresnel zone plate

A simple Fresnel zone plate for indoor focusing of 2 GHz band signals is proposed and studied. It consists of concentric metal rings mounted on the outside of an exterior wall. A three-ring zone plate has a through-wall focusing efficiency of 15 dB (measured) and 14 dB (calculated), and axial defocusing of 0.22 m.

Comments on "The analysis of deployable umbrella parabolic reflectors"

In a recent paper (see abstr. B28664 of 1972), Ingerson and Wong show, using geometrical relations, that the entire deployable reflector can be thought of as being made up of an infinite number of parabolas. They give an expression for the focal length of this infinite collection of parabolas and then state incorrectly that `The infinite collection of parabolas has a continuous distribution of foci. Thus, if a primary feed is placed in focus for one parabola, it is out of focus for the rest of the collection'. This statement is evidently the result of improper application of geometrical optics (surface normals are not taken into account) and provides a wrong physical picture of the focusing and the axial defocusing in the umbrella parabolic reflectors

Remarks on the concept of equivalent parabolas for Cassegrain antennas

In Cassegrain antennas, the whole focusing system is usually replaced by an equivalent paraboloid of focal length mf, where f is the focal length of the paraboloid and m is the magnification of the Cassegrain system. Here, it is shown that this equivalent paraboloid is useful only for feeds near the Cassegrainian focus. The discussion of series expansions of the phase in the system aperture yields severe differences between the Cassegrain system and the prime focus-fed equivalent paraboloid for larger radial or axial defocusing.