Focal Switch Research Articles

The on-axis intensity behavior of the diffracted Laguerre-Gaussian beam, LGp , by an opaque disk

In this work, we present a detailed study of the on-axis intensitydistribution of the Laguerre-Gaussian beam with no azimuthal index,diffracted by an opaque disk, by means of numerical simulation ofFresnel-Kirchhoff diffraction integral. We investigate the effect of theopaque disk size, the beam order, the beam waist, and the focal length.The results show that different focal phenomena may occur includingthe focal shift, focal split, and focal switch.
 

Focal shift and focal switch of broadband hollow Gaussian beams passing through an aperture-lens system

Focal shift and focal switch of broadband hollow Gaussian beams passing through an aperture-lens system

Influence of correlation function on focal switch of a partially coherent beam

It is known that the phenomenon of focal switch depends on the Fresnel number of the lens system, coherence, polarization and amplitude of the incident beam. In this paper, we investigate the influence of the correlation function of a partially coherent beam on focal switch. We study the intensity distribution of an elliptical multi-Gaussian correlated Schell-model (MGCSM) beam focused by a thin lens. It is found that focal switch occurs and we can effectively control focal switch by modulating the structure parameters of the correlation function. Our results provide a novel way for manipulating the focal switch of a partially coherent beam by engineering the structure of the correlation function.

Focusing properties of Lorentz-Gaussian beam with azimuthally-variant phase filters

Optical vortex attracts many researchers for its interesting properties and promising applications. Focusing properties of the linearly polarized Lorentz-Gaussian beam with azimuthally-variant phase filters were investigated by vector diffraction theory. Results show that the focal intensity distributions can be altered considerably by phase variation parameter of phase filter and the beam parameters, changing phase variation parameter of the phase mask, focal shift, focal switch and focal split may occur, simultaneously, the focal shift direction may change. Moreover, altering phase variation parameter will change the energy distributions of maximum intensity peak and other small intensity peaks. And novel focal patterns also evolve considerably, such as from only one peak to multiple peaks. The focal pattern evolution principle is different with different beam parameter. The tunable focal shift can be used to construct controllable optical tweezers. In practice, azimuthally-variant phase filters can be implemented through phase-only spatial light modulator, which can conveniently alter the wavefront phase distribution of the incident laser beam in the control of computer.

Variation of focal switch with spectrum of a broadband laser

Effects of the spectrum on focal switch of a broadband laser in a dispersion dual-focus system are presented in this paper. The numerical results show that the two maximum intensities of the broadband laser on the z-axis vary when the central frequency of the broadband laser shifts and the spectrum shape changes, and the variations affect the generation of the focal switch. It is also found that difference of the two maximum intensities tends to increase when the absolute value of central wavelength increases. According to the results in this paper, the generation of the focal switch can be controlled by choosing the shift of the central frequency, the bandwidth, the distance between the two lenses, and the spectrum shape of the broadband laser.

Focal switch of broadband laser in a dispersion dual-focus system

Focal switch effect of broadband laser in a dispersion dual-focus system with large Fresnel number is reported in this paper. Influences of the bandwidth on the intensity distributions and focal switch are illustrated by numerical examples. The results indicate that the bandwidth brings on the variations of two maximum intensities on the axis and thus results in the generation of focal switch. Fresnel number and the distance between two lenses also have influence on the focal switch. The bandwidth needed for the generation of the focal switch decreases when the Fresnel number and the distance between the two lenses increase. Narrow bandwidth can induce the focal switch in the system with large Fresnel number but the bandwidth needed for the generation of the focal switch increases rapidly when the Fresnel number decreases.

Focal switching of photochromic fluorescent proteins enables multiphoton microscopy with superior image contrast

Probing biological structures and functions deep inside live organisms with light is highly desirable. Among the current optical imaging modalities, multiphoton fluorescence microscopy exhibits the best contrast for imaging scattering samples by employing a spatially confined nonlinear excitation. However, as the incident laser power drops exponentially with imaging depth into the sample due to the scattering loss, the out-of-focus background eventually overwhelms the in-focus signal, which defines a fundamental imaging-depth limit. Herein we significantly improve the image contrast for deep scattering samples by harnessing reversibly switchable fluorescent proteins (RSFPs) which can be cycled between bright and dark states upon light illumination. Two distinct techniques, multiphoton deactivation and imaging (MPDI) and multiphoton activation and imaging (MPAI), are demonstrated on tissue phantoms labeled with Dronpa protein. Such a focal switch approach can generate pseudo background-free images. Conceptually different from wave-based approaches that try to reduce light scattering in turbid samples, our work represents a molecule-based strategy that focused on imaging probes.

Focal shift and focal switch of phase-lock partially coherent flat-topped array beams passing through an aligned and misaligned lens system with aperture

The focal shift and focal switch of phase-lock partially coherent flat-topped (PCFT) array beams passing through a lens system with aperture is studied in detail. It is shown that there is a focal shift for mention beam propagating through aligned optical system and the amount of the focal shift depend on optical system and beam characteristics. In addition, a new phenomenon called focal switch occurs for misaligned optical system, i.e. the focal shift experiences a sudden transition as the aligned optical system turns to misalign one. It is shown that, the necessary conditions for the focal switch to take place are also investigated.

Focal shift and focal switch of partially coherent flat-topped beams passing through an alignment and misalignment lens system with aperture

The focal shift and focal switch of partially coherent flat-topped (PCFT) beams passing through a lens system with an aperture are studied in detail. We have shown that a focal shift is also present for beams propagating through an aligned optical system and the amount of the focal shift depends not only on the radius aperture of the focusing system, but also on the spatial coherence and order of the flat-topped beam of the incident partially coherent light. A new phenomenon called ‘focal switch’ occurs for misaligned optical systems, i.e. the focal shift experiences a sudden transition as the aligned optical system becomes misaligned, and the influence of the spatial coherence, order of flat-topped beam and aperture size reduction, on the focal switch is investigated in detail. Finally, the necessary conditions for the focal switch to take place are investigated.

Focusing of concentric piecewise vector Bessel–Gaussian beam

The focusing properties of a concentric piecewise vector Bessel–Gaussian beam are investigated in this paper. The beam consists of three portions: the center circular portion and outer annular portion are radially polarized, while the inner annular portion is generalized polarized with tunable polarized angle. Numerical simulations show that the evolution of focal pattern is altered considerably with different Bessel parameters in the Bessel term of the vector Bessel–Gaussian beam. The polarized angle also affects the focal pattern remarkably. Some interesting focal patterns may appear, such as two-peak, dark hollow focus; ring focus; spherical shell focus; cylindrical shell focus; and multi-ring-peak focus, and transverse focal switch occurs with increasing polarized angle of the inner annular portion, which may be used in optical manipulation.

Focusing of radially polarized beam with radial cosine phase wavefront

Radially polarized beam has gained much interest recently due to its properties and applications. In this article, the focusing properties of radially polarized beam with radial cosine phase wavefront are investigated theoretically. Results show that when the radially polarized beam with radial cosine wavefront phase is focused, the focal pattern differs considerably with frequency parameter in the cosine function term. In the high numerical aperture focusing system, focal shift occurs, and novel focal patterns evolve considerably, for instance, from only one peak to two or multiple overlapping peaks. In addition, peak intensity ratio of radially polarized component to longitudinal polarized component in the focal region fluctuates smoothly for low-frequency parameter, then drops sharply, and comes back remarkably with increasing frequency parameter. Simultaneously focal shift increases slowly, and then decreases, suddenly, focal shift sign changes that results from focal switch phenomenon, and then fluctuates.

Focal shift and focusing properties generation by radial cosine phase masks

Focal shift and focusing properties of Gaussian beams induced by radial cosine phase masks are investigated. Results show that focal shift and the energy distribution among intensity peaks are controlled by two different parameters of the radial cosine phase mask. Increasing the value of frequency parameter in the cosine part of the phase mask, focal shift and focal switch may occur, simultaneously, the focal shift direction may change. Moreover, by altering frequency parameter or phase variation parameter of the phase mask will change the energy distributions of maximum intensity peak and other small intensity peaks. And novel focal patterns also evolve considerably, such as from only one peak to six of multiple peaks. The tunable focal shift can be used to construct controllable optical tweezers. In practice, the tunable phase mask can be implemented through liquid crystal spatial light modulator, which can conveniently alter the wavefront phase distribution of the incident laser beam in the control of computer.

Focal shift and focal switch of flat-topped Mathieu–Gaussian beams passing through an apertured lens system

By introducing a hard aperture function into a finite sum of complex Gaussian functions, an approximate analytical expression predominating the distribution of axial intensity for the flat-topped Mathieu–Gauss (FTMG) beams passing through a system with the aperture and lens separated has been derived. The focal shift and the focal switch effect of FTMG beams passing through the system is studied in detail. Numerical calculations have shown that the position of real focal plane is not coincident with the geometrical focus but is somewhat shifted toward the lens. The focal shift and focal switch of FTMG beams take place when the relative separation s/f = 1 by a suitable choice of beam parameter and truncation parameter, for example, the beam parameter is smaller than its corresponding critical value or the truncation parameter is between its two corresponding critical values.

Microscopy and its focal switch

Until not very long ago, it was widely accepted that lens-based (far-field) optical microscopes cannot visualize details much finer than about half the wavelength of light. The advent of viable physical concepts for overcoming the limiting role of diffraction in the early 1990s set off a quest that has led to readily applicable and widely accessible fluorescence microscopes with nanoscale spatial resolution. Here I discuss the principles of these methods together with their differences in implementation and operation. Finally, I outline potential developments.

Focal switch and dual focal switch of a Hermite–Gaussian beam illuminating a lens and sinusoidal amplitude grating separated system

Based on the Collins formula, analytical representation of a Hermite–Gaussian beam propagating through a lens and sinusoidal amplitude grating separated system is obtained. Focal shift occurring in such a system is studied in detail with numerical calculation examples. It is found that the focal switch and the dual focal switch can emerge depending not only on the effective Fresnel number, but also on the uniform transmittance amplitude and the spatial frequency of the grating and beam parameter of the illumination Hermite–Gaussian beam.

厄米-余弦-高斯光束通过像散透镜的焦开关

厄米-余弦-高斯光束通过像散透镜的焦开关

Focusing of hyperbolic-cosine-Gaussian beam with a non-spiral vortex

The focusing properties of the hyperbolic-cosine-Gaussian beam, which contains an optical vortex induced by a non-spiral phase plate, are investigated numerically. The phase plate alters phase distribution linearly in a half-section part of the hyperbolic-cosine-Gaussian beam, which results in one non-spiral optical vortex. Results show that the phase variation rate of the non-spiral phase plate influences focal intensity distribution considerably, and some novel focal patterns, such as line focal spot, “H”-shape focal spot, and intensity peaks array, may occur. Focal shift, focal split, and focal switch appear in focal evolution with increasing phase variation rate. The focal evolution differs for different parameters in cosh parts of the hyperbolic-cosine-Gaussian beam. For big parameters in cosh parts, the value of local intensity peaks is comparable to that of maximum intensity peaks.

Focusing properties of Gaussian beam by a nonspiral phase plate system

In this article, a nonspiral plate is investigated numerically by vector diffraction theory to observe the focusing properties of Gaussian beams. Both low- and high-numerical-aperture (NA) optical systems are considered in the investigation. It is found that the parameter of NA and the phase vary rate of the phase plate influence the focal intensity distribution considerably. When a nonspiral plate is used to provide linear phase variation on one half of the Gaussian beam, it may adjust the focal spot considerably and conveniently. Changing the vary rate of the phase plate or the parameter of NA can alter optical intensity distribution; some novel focal spots and focal switch may also occur.

Focal shift and focal switch of focused truncated elliptical Gaussian beams

It is shown that when a generalized astigmatic elliptical Gaussian beam (EGB) propagates in a focusing optical system with an elliptical aperture, the effect of focal shift and focal switch can appear. An analytical propagation expression for an EGB through an axially nonsymmetrical optical system with a thin lens and an elliptical aperture is derived. Some numerical simulations are illustrated for its focal shift and focal switch, and some physical explanations are also given.

Focal shift and focal switch of partially polarized Gaussian Schell-model beams passing through a system with the aperture and spherically aberrated lens separated

Based on the beam coherent-polarization (BCP) matrix approach and propagation law of partially coherent beams, the focal shift and focal switch of partially polarized Gaussian Schell-model (PGSM) beams passing through a system with the aperture and spherically aberrated lens separated is studied in detail. Our main attention is focused on the effect of spherical aberration and partial coherence on the focal shift and focal switch of PGSM beams. It is shown that for polarizer-free case there is no focal switch of PGSM beams, the focal shift of PGSM beams is closely related with spherical aberration coefficient C 4, auto-coherence length σ a, truncation parameter δ and relative position s/ f between the aperture and lens in general, and is independent of the cross-coherence length σ c. After inserting a polarizer the focal switch can take place. Numerical calculation results are given to illustrate how the spherical aberration and partial coherence affect the focal shift and focal switch of PGSM beams.