Off-axis Configuration Research Articles

Electromagnetic scattering by a uniaxial anisotropic sphere located in an off-axis Bessel beam

Electromagnetic scattering of a zero-order Bessel beam by an anisotropic spherical particle in the off-axis configuration is investigated. Based on the spherical vector wave functions, the expansion expression of the zero-order Bessel beam is derived, and its convergence is numerically discussed in detail. Utilizing the tangential continuity of the electromagnetic fields, the expressions of scattering coefficients are given. The effects of the conical angle of the wave vector components of the zero-order Bessel beam, the ratio of the radius of the sphere to the central spot radius of the zero-order Bessel beam, the shift of the beam waist center position along both the x and y axes, the permittivity and permeability tensor elements, and the loss of the sphere on the radar cross section (RCS) are numerically analyzed. It is revealed that the maximum RCS appears in the conical direction or neighboring direction when the sphere is illuminated by a zero-order Bessel beam. Furthermore, the RCS will decrease and the symmetry is broken with the shift of the beam waist center.

Digital Gabor and off-axis particle holography by shaped beams: A numerical investigation with GLMT

In particle field holography, the laser beam is more or less astigmatic, and the particles are usually inhomogeneous. Holograms of homogeneous and multilayered spherical particles illuminated by shaped beam with Gabor and off-axis configurations in both near and far field are investigated on the basis of the generalized Lorenz–Mie theory (GLMT). Reconstruction of the simulated holograms are demonstrated using fractional Fourier transform. Effects of recording parameters, e.g. beam shape (circular or elliptical Gaussian beam) and particle location (before, at or after the beam waist) and inner refractive index gradient using multilayer model on the hologram pattern were evaluated. For the off-axis holography, special attentions are devoted to the Moiré effect reduction using a Gaussian beam as the reference wave and to the two series of concentric rings phenomenon using the Debye series. The code developed in this work enable to investigate the effects of the incident beam, reference wave, particle inner refractive index gradient, and CCD orientation on the formation and reconstructed image of the hologram separately.

Absolute measurement of small-amplitude vibrations by time-averaged heterodyne holography with a dual local oscillator

We report a demonstration of the measurement of the ratio between an optical modulation sideband component and the nonshifted light component by time-averaged heterodyne holography in off-axis and frequency-shifting configuration, through coherent frequency-division multiplexing with a dual optical local oscillator. Experimental results of sinusoidal vibration sensing are reported. This technique enables absolute measurements of subnanometric out-of-plane vibration amplitudes.

Overview of techniques applicable to self-interference incoherent digital holography.

Self-interference incoherent digital holography (SIDH) retrieves the complex hologram from the object illuminated by the incoherent light. Supported by the adaptive optic feature, SIDH is readily applicable to the ocular imaging to investigate the human retinal cells. Considering the practical issues, issues related to resolution, phase-shifting, and contrast should be addressed to implement the viable SIDH system which is capable of recording the holographic information of human retinal cells under the incoherent illumination. Super resolution image reconstruction technique can be directly applied to SIDH to enhance the resolution of the system without any change of configuration. We present the improved way to incorporate the phase-shifting itself into the lateral shift required by the super resolution technique. To deal with the phase-shifting issue, we present an arbitrary phase shift retrieval algorithm which can reduce the number of phase-shift and accept the blind phase-shift. The single-shot imaging is also possible by adopting the off-axis configuration of SIDH. We will provide the detailed procedures to retrieve the complex hologram using the proposed arbitrary phase shifting algorithm and the off-axis configuration.

Short-time Fourier transform laser Doppler holography

We report a demonstration of laser Doppler holography at a sustained acquisition rate of 250 Hz on a 1 Megapixel complementary metal-oxide-semiconductor (CMOS) sensor array and image display at 10 Hz frame rate. The holograms are optically acquired in off-axis configuration, with a frequency-shifted reference beam. Wide-field imaging of optical fluctuations in a 250 Hz frequency band is achieved by turning time-domain samplings to the dual domain via short-time temporal Fourier transformation. The measurement band can be positioned freely within the low radio-frequency (RF) spectrum by tuning the frequency of the reference beam in real-time. Video-rate image rendering is achieved by streamline image processing with commodity computer graphics hardware. This experimental scheme is validated by a non-contact vibrometry experiment.

STABLE AND UNSTABLE EQUILIBRIA OF UNIFORMLY ROTATING SELF-GRAVITATING CYLINDERS

The equilibrium configurations of self-gravitating uniformly rotating isothermal cylinders in contact with a heat bath and their stability is studied by recently derived analytical techniques. The known critical temperature Tc obtained by Katz and Lynden-Bell is found to be a stability threshold with respect to axially symmetric perturbations. We provide the almost explicit expression of negative specific heat solutions whose densities are sharply concentrated either near the symmetry axis or near some off-axis filaments as [Formula: see text]. The critical angular frequency observed numerically in literature is found to be the threshold value for the existence of these off-axis filaments. This is in striking contrast with the static case analyzed by Katz and Lynden-Bell where equilibrium configurations are found only if T > Tc and no negative specific heat equilibria exists at all. Metastability of the free energy's relative maximizers for T ≤ Tc is also discussed. Those off-axis configurations were predicted in the study of negative temperature states for guiding-centre plasmas and vortex systems.

Digital holographic microscopy through a Mirau interferometric objective

Digital Holographic Microscopy (DHM) typically uses either Michelson or Mach–Zehnder interferometers as interferometric tools to attain digital holograms. These interferometers need not only good optical alignment in order to compensate the spherical aberration, but also a special optical path difference compensation system when a low coherence illumination source is used. A Mirau interferometric objective appears as an alternative to overcome these difficulties and achieve reduced quadratic aberration in DHM. We show experimental results of the feasibility to perform DHM using this kind of objective; the tests conducted were in-line and off-axis configurations. In addition, we show not only the unique feature of the refocusing capability of DHM in a NBS 1963A resolution card showing its corresponding amplitude and phase images, but also a profile phase comparison of a 4.2μm high micro lens using interferometry and DHM, extending the depth of focus of the microscope objective as proof of the proposal. As far as we know, this device has not been used in DHM.

Confocal microscopy on the beamline: novel three-dimensional imaging and sample positioning

Confocal microscopy, a technique that has been extensively applied in cellular biological studies, may also be applied to the visualization and three-dimensional imaging of protein crystals at high resolution on synchrotron beamlines. Protein crystal samples are examined using a commercially available confocal microscope adapted for cryogenic use. A preliminary test using a custom confocal design adapted for beamline use is also presented. The confocal optics configuration is compatible with nonlinear imaging techniques such as two-photon excited fluorescence imaging and second harmonic generation. The possibilities of this method are explored using two modes: fluorescence and reflection confocal. In fluorescence mode, small amounts of dye are introduced into the crystal through soaking or growth conditions. Under such conditions, protein crystals are easily resolved from salts and amorphous precipitates, which do not generally take up dye. Reflection mode, which does not require dye, still exhibits greater resolution and sensitivity to surface detail than conventional wide-field microscopy as a result of the confocal optics configuration. The inherent three-dimensional nature of the method means that on-axis sample views (along the direction of the X-ray beam) can be reconstructed from an off-axis configuration, simplifying the beamline setup and providing uniquely detailed views of cryogenically cooled crystals.

Comparison of coaxial and off-axis nozzle configurations in one step process laser cladding on aluminum substrate

Laser cladding of aluminum bronze alloy has been studied in order to increase the wear resistance of aluminum parts. Aiming a more productive and controlled process, this study analyzes the blown powder process, with two possibilities of blowing configurations that are off-axis and coaxial nozzle, and process parameters, such as axial motion speed, mass flow and power modulation. Dilution, catchment efficiency and hardness were analyzed for the used process parameters and nozzle configurations.

SLM-based off-axis Fourier filtering in microscopy with white light illumination

In various microscopy applications spatial light modulators (SLMs) are used as programmable Fourier filters to realize different optical contrast enhancement methods. It is often advantageous to use the SLM in off-axis configuration, where the filtered image wave is sent into the first diffraction order of a blazed grating superposed to the phase mask on the SLM. Because of dispersion this approach is, however, typically limited to spectrally narrowband illumination. Here we suggest a method involving a grating for pre-compensation, which allows one to use spectrally broadband (even thermal) light in SLM-based Fourier filtering. The proposed approach is demonstrated by multicolor imaging of amplitude and phase objects, such as a resolution target, onion epidermal cells and human epithelial cheek cells.

Picosecond Nd:YLF five-passes laser amplifier with 20 mJ pulse energy

We have demonstrated an all-diode-pumped Nd:YLF laser amplifier that produces 20.8 mJ pulses of 12.9 ps duration at 10 Hz repetition rate. The laser system consists of a mode-locking oscillator, a regenerative amplifier and a five-pass amplifier. The small-angle off-axis five-pass configuration combining with the gradual expansion of the laser beam cross section makes the amplifier compact and effective.

Full off-axis red-green-blue digital holographic microscope with LED illumination

We developed a new full off-axis red-green-blue (RGB) digital holographic microscope with an LED illumination. A decisive advantage of the use of LED illumination is a large image quality improvement due to its partially coherent nature. The off-axis configuration enables the fast recording of the holographic data in each spectral channel. The digital holographic refocusing and the optical phase map computation are successfully demonstrated. The multiwavelength operation provides a significant improvement of the collected information for colored samples.

Simulation of EAST off-axis neutral beam heating and current drive

The capability of off-axis neutral beam heating and current drive has been investigated with NUBEAM for Experimental Advanced Superconducting Tokamak (EAST). Three different approaches to realize off-axis Neutral Beam Injection (NBI) have been studied. Simulation results for on- and off-axis NBI are reported. The effects of the alignment of NBI relative to the magnetic field pitch on off-axis neutral beam heating and current drive are observed and discussed qualitatively. By comparing the numerical results, a most favorable off-axis NBI configuration is recommended. The capability to control sawtooth is also investigated by comparing locations of the q=1 rational surface and the peak of the fast ion density profile.

Dual-wavelength slightly off-axis digital holographic microscopy

We propose dual-wavelength digital holographic microscopy with a slightly off-axis configuration. The axial measurement range without phase ambiguity is extended to the micrometer range by synthesizing a beat wavelength between the two wavelengths with separation of 157 nm. Real-time measurement of the specimen is made possible by virtue of the high wavelength selectivity of the Bayer mosaic filtered color CCD camera. The principle of the method is exposed, and the practicability of the proposed configuration is demonstrated by the experimental results on a vortex phase plate and a rectangular phase step.

Optimized off-axis cylindrical mirror-focused line-scanning system for optical coherence tomography imaging applications

The parameters of an off-axis cylindrical mirror-focused line-scanning system were studied to optimize the flatness of the 2 mm scan field. The scanning system parameters included the beam size, the distance between the scanning and the focusing mirror, the angle between the incident beam and the reflected beam, the optical scan angle, and the effective focal length of the cylindrical mirror. Because of the off-axis line-scanning system configuration, the scanning could be carried out either in the tangential (Y-scan) or in the sagittal (X-scan) plane. A 53 nm spectral bandwidth light source was used to evaluate the imaging performance of the scanning system. Since reflective optics is employed in this work for focusing, the scanning system could be used with a higher spectral bandwidth light source for optical coherence tomography applications. The effect of the angle between of the incident and reflected beams, the distance between the mirrors, the focal length of the cylindrical mirror and the scanning directions, on the flatness of the scan field were studied. It was proved that the sagittal scanning is least sensitive to variations in scanning system parameters and thus provides maximum flexibility in design.

Acoustic radiation torque of arbitrary shaped waves

In this work, a general expression of the acoustic radiation torque produced over an object by an arbitrary shaped beam is presented. The object is immersed in an inviscid fluid. To obtain the expression, the stress tensor of the angular momentum is integrated over a farfield virtual sphere, which encloses the object. The incident and scattered acoustic fields are represented through the partial wave expansion in the spherical coordinates. After performing the integration, the radiation torque is given in terms of the beam-shape and the scattering coefficients, which come from the incident and scattered partial wave series, respectively. The method is applied to compute the torque upon a fluid sphere by a vortex (first-order) Bessel beam in both on- and off-axis configurations. It is shown that the torque only arises on absorbing spheres. In this case, the angular acceleration and velocity are obtained from the torque. It is found that the angular acceleration may reverse its direction depending on the wave frequency. In conclusion, the presented theory might be useful for describing the particle dynamics of a sphere subjected to acoustic vortex beams.

Single‐shot, simultaneous incoherent and holographic microscopy

In this work, we demonstrate single-shot, simultaneous recording and subsequent retrieval of one incoherent and two holographic (intensity and phase) images from the same camera frame. Demultiplexing of incoherent and holographic signals in the spatial frequency domain is made possible by carrier frequency modulation and spatial oversampling intrinsic to the off-axis digital holographic configuration. In particular, we show applications to combined fluorescence and digital holographic microscopy, as well as combined bright field and holographic second harmonic generation microscopy.

Microscale experimental investigation of failure mechanisms in off-axis woven laminates at elevated temperatures

Off-axis woven laminates fabricated from carbon fiber and a high glass transition temperature thermosetting resin were subjected to tensile static and fatigue loading at temperatures ranging from room temperature up to 205 °C. The damage mechanism prevalent to these specimens was investigated by post-mortem examination using a scanning electron microscope. During most of their life fatigue specimens had accumulated minimal damage which consisted of matrix cracks, transverse bundle cracks and intra-ply delamination. Just before failure fiber bundles began to straighten out and rotate towards the loading direction. This behavior led to large elongation and necking of the specimens before fracture. Overall, the matrix-dominated material behavior and fiber reorientation due to the off-axis configuration had a far greater influence on the fracture morphology than the gradual accumulation of damage due to fatigue loading. It was also found that damage formation was strongly influenced by the type of applied loading and the test temperature.

Digital holographic microscopy with pure-optical spherical phase compensation

Telecentric architecture is proposed for circumventing, by the pure-optical method, the residual parabolic phase distortion inherent to standard configuration of digital holographic microscopy. This optical circumvention produces several important advantages. One is that there is no need for computer compensation of the parabolic phase during the phase map recovering procedure. The other is that in off-axis configuration, the spatial frequency useful domain is enlarged. The validity of the method is demonstrated by performing quantitative measurement of depth differences with high axial resolution.

Video-rate laser Doppler vibrometry by heterodyne holography

We report a demonstration video-rate heterodyne holography in off-axis configuration. Reconstruction and display of a one megapixel hologram is achieved at 24 frames per second, with a graphics processing unit. Our claims are validated with real-time screening of steady-state vibration amplitudes in a wide-field, noncontact vibrometry experiment.