Generation of an elliptic hollow beam using Mathieu and Bessel functions

A new (to our knowledge) technique for the generation of a propagation-invariant elliptic hollow beam is reported. It avoids the use of the radial Mathieu function and hence is mathematically simpler. Bessel functions with their arguments having elliptic locus are used to generate the mask, which is then recorded using holographic technique. To generate such an elliptic beam, both the angular Mathieu function, i.e., elliptic vortex term, and the expression for the circular vortex are used separately. The resultant mask is illuminated with a plane beam, and the proper filtering of its Fourier transform generates the expected elliptic beam. Results with both vortex terms are satisfactory. It has been observed that even for higher ellipticity the vortices do not separate.     

Space-time modulation of an X-ray beam by ultrasonic superlattice

Space-time modulation of an X-ray beam is investigated by total external reflection on a YZ-cut of a LiNbO₃ crystal modulated by surface acoustic waves. It is shown that the X-ray diffraction is determined by the amplitude and the wavelength of the surface acoustic waves. Both experimental and theoretical studies have revealed the possibility of fabricating X-ray optical elements based on space-time modulation by controlling the wavelength and the pulse modulation of the surface acoustic waves. These experiments were carried out at CNRS (Grenoble, France) using a rotating copper anode     

Application of Bessel beam for Doppler velocity estimation

Limited-diffraction beams have a large depth of field and could be applied to medical imaging, tissue characterization, and nondestructive evaluation of materials. This paper reports the application of limited-diffraction beams, specifically, the Bessel beam, to Doppler velocity estimation. The Bessel beam has the advantage that velocity estimation is less subject to the depth of moving objects and the Doppler spectrum has distinct shoulders that increase the accuracy of velocity (both magnitude and Doppler angle) estimation in noisy environments. The shoulders of the Doppler spectrum might also help in solving the inverse problem, e.g., estimation of the velocity distribution in vessels     

Optical parametric oscillator pumped by a Bessel beam

We demonstrate operation of a KTP optical parametric oscillator (OPO) pumped by a Bessel beam for the first time to our knowledge. It is shown that the output of the OPO has a transverse profile, which is consistent with noncollinear phase-matching relations defined by a conical pump. The central spot and ring related to the pair of signal and idler beams were generated in the OPO. By adjusting the OPO cavity mirrors, we easily selected the lowest-order mode as well as the higher-order transverse modes in the central spot. Bessel beam pumping was shown to be useful, providing tubular beam coupling into OPO cavity modes. The OPO threshold pump energy was ~100 muJ in a 6-ns pulse.     

Generation of Bessel beam arrays through Dammann gratings

In this work we apply the Dammann grating concept to generate an equal-intensity square array of Bessel quasi-free diffraction beams that diverge from a common center. We generate a binary phase mask that combines the axicon phase with the phase of a Dammann grating. The procedure can be extended to include vortex spiral phases that generate an array of optical pipes. Experimental results are provided by means of a twisted nematic liquid crystal display operating as a binary π phase spatial light modulator.     

Generation of a Bessel beam of variable spot size

We report the generation of a zero-order Bessel beam of continuously variable spot size using a simple optical setup. We have used a pair of metal axicon mirrors to generate a hollow beam of variable dark diameter. This beam was subsequently focused by a convex lens to get a Bessel beam of variable spot size. We also studied the effect of a hollow-beam ring width on nondiffracting propagation range of the generated beam.     

Dynamic Effects in Microparticle Pull-Off Using an AFM

We report a series of measurements aimed at understanding the dynamics of microparticle detachment from surfaces. Microparticles were adhered to AFM cantilever tips and load/displacement curves were obtained while the particles were repeatedly attached and detached from the surface with frequencies ranging from 1 to 30 Hz. Particles ranging in size from 1 to 30 micrometers and composed of alumina and polystyrene were studied. For each particle studied, a decrease in the pull-off force was consistently observed with increasing measurement frequency, indicating a dynamic effect that is not accounted for by equilibrium adhesion models. We present a lumped mass model of the canitilever/particle/surface system to definitively rule out the possibility that bulk inertial effects within the particle or cantilever may be responsible at these time scales. Furthermore, we ensured dry conditions for each experiment making it unlikely that humidity effects, such as water bridge formation, could have been responsible. Although we have not pinpointed the physical nature of the dynamics, we offer the possibility that energy dissipation mechanisms at the particle/surface interface may cause dynamic effects during microparticle attachment or detachment on time scales similar to those of the present experiments.     

Dynamic Effects in Microparticle Pull-Off Using an AFM

We report a series of measurements aimed at understanding the dynamics of microparticle detachment from surfaces. Microparticles were adhered to AFM cantilever tips and load/displacement curves were obtained while the particles were repeatedly attached and detached from the surface with frequencies ranging from 1 to 30 Hz. Particles ranging in size from 1 to 30 micrometers and composed of alumina and polystyrene were studied. For each particle studied, a decrease in the pull-off force was consistently observed with increasing measurement frequency, indicating a dynamic effect that is not accounted for by equilibrium adhesion models. We present a lumped mass model of the canitilever/particle/surface system to definitively rule out the possibility that bulk inertial effects within the particle or cantilever may be responsible at these time scales. Furthermore, we ensured dry conditions for each experiment making it unlikely that humidity effects, such as water bridge formation, could have been responsible. Although we have not pinpointed the physical nature of the dynamics, we offer the possibility that energy dissipation mechanisms at the particle/surface interface may cause dynamic effects during microparticle attachment or detachment on time scales similar to those of the present experiments.     

超指向性扬声器的阵列设计与研究

超指向性扬声器的阵列设计要求阵列具有高指向性,并且无旁瓣.基于对线列阵和矩形平面阵的理论分析,以及均匀线列阵和矩形交错阵的理论分析和比较,提出了一种六边形交错阵列设计,计算机模拟和实测均表明该阵列3dB波束宽度小于5°主旁瓣比为8.3,满足设计要求.初步分析了传感器相位对换能器阵列指向性的影响,为挑选换能器阵元组成阵列提供了理论指导.     

Practical Bessel beam pumping of periodically poled optical parametric oscillators

Abstract

The pumping of periodically poled optical parametric oscillators (OPOs) by practically realizable Bessel beams is analysed. The effect of the deviation of practical Bessel beams from the ideal form is assessed, the influence of the particular structure of periodically poled materials is discussed, and the concept of phase-matching aperture is introduced. In a typical situation, the efficiency of a periodically poled lithium niobate OPO pumped by a practical Bessel beam has been measured; no efficiency advantage was found in comparison to the corresponding Gaussian beam pumped system or to that predicted by the standard Brosnan-Byer model.     

Excitation of surface-plasmon polaritons by use of a zeroth-order Bessel beam

We report an experimental result that shows the excitation of surface plasmon polaritons by use of a zeroth-order Bessel beam. From the viewpoint of energy efficiency, the zeroth order Bessel beam is ideal for the local excitation of surface plasmon polaritons on a metal film. We introduce an optical setup using an axicon element in order to carry out the excitation.     

Bottle beam from a bare laser for single-beam trapping

We demonstrate that a laser beam converging from a specific transverse mode is a bottle beam, as described in J. Opt. Soc. Am. B 20, 1220 (2003). To our knowledge, this is the first time that a bottle beam has been generated directly from a laser. By calculating the radiation forces on a dielectric Rayleigh sphere in the bottle beam, we show that the single beam can trap high-refractive-index particles at the multiple axial sites of intensity maxima, and it can confine low-index particles on a transverse plane within the bottle regions. Such a novel laser beam may have other applications.     

Focused high frequency needle transducer for ultrasonic imaging and trapping

A miniature focused needle transducer (<1?mm) was fabricated using the press-focusing technique. The measured pulse-echo waveform showed the transducer had center frequency of 57.5 MHz with 54% bandwidth and 14?dB insertion loss. To evaluate the performance of this type of transducer, invitro ultrasonic biomicroscopy imaging on the rabbit eye was obtained. Moreover, a single beam acoustic trapping experiment was performed using this transducer. Trapping of targeted particle size smaller than the ultrasonic wavelength was observed. Potential applications of these devices include minimally invasive measurements of retinal blood flow and single beam acoustic trapping of microparticles.     

Bessel function output from an optical correlator

We introduce a new filter for an optical correlator that produces a Bessel function correlation output. The advantage of this type of output is that the width of the central dark spot at the center of the correlator output is narrower than the width of the usual correlation spot. In addition, the dark spot is insensitive to changes in illumination intensity. We present experimental results with which we write the new filter onto a programmable magneto-optic spatial light modulator.     

Acousto-optic interaction of a Gaussian laser beam with an ultrasonic wave of cylindrical symmetry

We present experimental studies of the interaction between a narrow Gaussian laser beam and a standing cylindrical ultrasonic wave. As a theoretical approach, a Fourier-optics-based successive diffraction model is used. Depending on the ratio of the Gaussian laser beam diameter to the first nodal diameter of the cylindrical ultrasound, light refraction or diffraction is observed. We experimentally investigate the time-averaged light intensity as well as the modulation of light in the far field of light refraction-diffraction by a cylindrical ultrasound. It is revealed that significant focusing appears if the phase front of the incident light is curved. The focusing effects of the acousto-optic system depend on the width of the laser beam and curvature of the phase front. Finally, possible applications are discussed.     

Correction of amplitude distortions for truncated Bessel beam and SER estimation for 4ASK

Abstract

We apply amplitude corrections to a truncated Bessel beam that has propagated through turbulent atmosphere modelled by random phase screens. These corrections are realized via transmitting an unmodulated beam in parallel to the one carrying the 4 amplitude shift keying (ASK) modulated message signal. On the receiver side, the amplitude corrections are obtained by dividing the intensity of the unmodulated beam by its free space equivalence. The corrections are then used to restore the amplitude distortions of the beam carrying the 4ASK modulated message signal and in the determination of decision boundaries for the received 4ASK symbols. The success of the system is visually inspected by comparing the received intensity profiles before and after the application of corrections. Furthermore, simulation analysis of symbol error rate (SER) is made, where the proposed set-up is found to be quite insensitive to wavelength difference between the unmodulated and modulated beams. On the other hand, the difference in the structure constant values of these two beams seems to have profound effect on system performance.     

The Movement of an Ion-exchange Microparticle in a Weak External Electric Field

The electrokinetic motion of spherical particle suspended in the electrolyte solution under influence of external electric field is studied. Due to impermeability of particle’s surface for one kind of ion species the particle exhibit behavior different to well investigated dielectric particles. Under an assumption of a weak external electric field, we derive the analytical estimation of the particle’s velocity by means of a method of matched asymptotic expansions. The analytical analysis is complemented by numerical solution, which gives the distribution of ion’s concentrations, electric potential profiles and flows streamlines. The analytical results are successfully compared with the results of numerical simulation.     

Characterization of a periodically poled lithium niobate optical parametric oscillator pumped by a Bessel beam

Abstract

A Bessel beam pumped periodically poled lithium niobate optical parametric oscillator has been characterized. A slope efficiency of 5.4 ± 0.1% was obtained and a threshold pulse energy of 472 ± 12 μJ was measured, which corresponds to a threshold fluence of 0.207 ± 0.005 J cm^?2. The signal output was tuned over ～20 nm by changing the crystal temperature by 50°C. The FWHM bandwidth at each temperature was observed to be 0.3 ± 0.1 nm at each wavelength. The beam profiles of the Bessel pump and Gaussian signal beams were studied in both the near and far field. A beam divergence of 1.14 ± 0.02 mrad was measured.     

Radiation trapping in an optically pumped thick sodium target

The effect of radiation trapping in an optically pumped sodium target was measured with a Faraday rotation method. The experimental results were compared with calculations based on recent theoretical work, and good agreement was found between the two.