Separate Reference Beam Research Articles

TEST RESULTS OF REINFORCED CONCRETE CROSS-RIBBED MODEL OF SPAN STRUCTURE OF THE BRIDGE AND THEIR ANALYSIS

Presentation of samples of basic experimental data from research of spatial work of large-scale (scale 1: 4) reinforced concrete model of cross-ribbed span structure of the bridge and separate beams similar in structure to model beams, as well as methodological features and results of their analysis. During the tests, the model and separate (reference) beams were loaded with concentrated force, alternately at the intersections of the longitudinal and transverse ribs of the model. Deflections in the same nodes and support reactions of longitudinal beams were measured. A new methodological feature of this research was the determination of experimental bending moments in the model beams by direct comparison of the deflections of the model beams with similar deflections of separate (reference) beams. Depending on the location of the external load and the stiffness ratio, the load on the beams of the model is different. Taking into account an elastic-plastic stage of their work, the method of calculation of the maximum bending moments in the most loaded beams is offered in this paper.

A differential phase spectropolarimeter for measuring optical rotatory dispersion

This paper considers the features of polarimetric methods used for the analysis of optically active substances. It presents a design of a differential spectropolarimeter operating by analyzing the phase difference between two harmonical signals received when splitting a laser beam from a common source into separate reference and object beams. The expediency of using the Fourier transform to calculate the phase difference of differential spectropolarimeter signals is noted. The results of calculating the parameters of the measuring method and the results of an experimental study of reference polarimetric plates at various wavelengths are presented. The error in measuring the optical rotation angle in the wavelength range 405–780 nm did not exceed ±0.005°.

Measurement of Pure States of Light in the Orbital-Angular-Momentum Basis Using Nine Multipixel Image Acquisitions

The existing techniques for measuring high-dimensional pure states of light in the orbital angular momentum (OAM) basis either involve a large number of single-pixel data acquisitions and substantial postselection errors that increase with dimensionality, involve substantial loss, or require interference with a reference beam of known phase. Here, we propose an interferometric technique that can measure an unknown pure state using only nine multipixel image acquisitions without involving postselection, loss, or a separate reference beam. The technique essentially measures two complex correlation functions of the input field and then employs a recursive postprocessing algorithm to infer the state. We experimentally demonstrate the technique for pure states up to dimensionality of 25, reporting a mean fidelity greater than $90\mathrm{%}$ up to 11 dimensions. Our technique can significantly improve the performance of OAM-based information processing applications.

Tutorial: Common path self-referencing digital holographic microscopy

Quantitative phase imaging of cells provides important morphological information about them, leading to their characterization, comparison, and identification. The interference principle when applied to microscopy provides high-contrast quantitative phase images of otherwise transparent objects along with their thickness information. The two-beam off-axis geometry of interference microscopes, in which the light beam interacting with the object interferes with a separate reference beam, is preferred since it leads to single shot quantitative phase imaging methodologies. But these techniques lead to bulky setups, with lower temporal stability not suitable for the measurement of nanometer-level cell thickness fluctuations. Self-referencing interference microscopes manipulate a portion of the light beam interacting with the cells to act as the reference, leading to compact, temporally stable geometries ideal for the measurement of cell dynamics. Here we present an overview of our efforts in the development of self-referencing digital holographic microscopes and their use in quantitative phase imaging of cells.

Single-shot digital holographic microscopy for quantifying a spatially-resolved Jones matrix of biological specimens.

Field-based polarization measurements are essential for the completeness of information when exploiting the complex nature of optical responses of target objects. Here, we demonstrate digital holographic microscopy for quantifying a polarization-sensitive map of an object with a single-shot measurement. Using the image-splitting device generating four different copies of an object image and a separate reference beam of an off-axis configuration enables single-shot and multi-imaging capability. With the use of two polarization filters, four complex field images containing an object's polarization response are obtained simultaneously. With this method, we can construct a complete set of 2-by-2 Jones matrix at every single point of the object's images, and thus clearly visualize the anisotropic structures of biological tissues with low level of birefringence. This method will facilitate the high-precision measurements for fast dynamics of the polarization properties of biological specimens.

Multimodal computational microscopy based on transport of intensity equation.

Transport of intensity equation (TIE) is a powerful tool for phase retrieval and quantitative phase imaging, which requires intensity measurements only at axially closely spaced planes without a separate reference beam. It does not require coherent illumination and works well on conventional bright-field microscopes. The quantitative phase reconstructed by TIE gives valuable information that has been encoded in the complex wave field by passage through a sample of interest. Such information may provide tremendous flexibility to emulate various microscopy modalities computationally without requiring specialized hardware components. We develop a requisite theory to describe such a hybrid computational multimodal imaging system, which yields quantitative phase, Zernike phase contrast, differential interference contrast, and light field moment imaging, simultaneously. It makes the various observations for biomedical samples easy. Then we give the experimental demonstration of these ideas by time-lapse imaging of live HeLa cell mitosis. Experimental results verify that a tunable lens-based TIE system, combined with the appropriate postprocessing algorithm, can achieve a variety of promising imaging modalities in parallel with the quantitative phase images for the dynamic study of cellular processes.

Lensless 3D Digital Holographic Microscopic Imaging at Vacuum UV Wavelength

We demonstrate imaging of microscopic objects using a lensless in-line 3D digital holographic microscope at vacuum UV wavelength of 193 nm. The absence of microscope objective and a separate reference beam simplifies the setup and reduces the number of UV components required. We also discuss the space-bandwidth considerations in the design of such a system. Experimental results using human blood cells and latex beads are presented.

Characterization of a broadband pulse for phase controlled multiphoton microscopy by single beam SPIDER

We present what we believe to be a new version of spectral phase interferometry for direct electric field reconstruction (SPIDER) using only a single-phase and polarization controlled laser beam. Two narrow pulses and one broadband pulse are selected out of an ultrafast laser pulse by a polarization and phase control technique to generate second harmonic generation (SHG) signals, which are equivalent to a spectral shear interferogram in the conventional SPIDER method. The spectral phase of the broadband laser pulse is extracted analytically with double quadrature spectral interferometry (DQSI). An arbitrary spectral phase can be retrieved with great precision and compensated in situ at the sample position of a microscope. This new method requires no separate reference beam and is suitable for nonlinear optical microscopy with a phase controlled laser pulse.

Comparison of sensitivities of Moiré deflectometry and interferometry to measure electron densities in z-pinch plasmas

Pulsed laser probing using Moiré deflectometry and Normaski interferometry was performed simultaneously to diagnose the electron density in a wire z-pinch plasma. The Moiré deflectometer has a higher sensitivity while the Nomarski interferometer is able to probe higher densities. The advantages of each technique permitted us to measure a wider range of densities. This includes the low-density coronal plasma with density of an order of magnitude lower (1016 cm−3) than that which can be measured by interferometry. Furthermore, our optical probing presents a low response to mechanical vibrations as it is not necessary to use a separate reference beam as in a Mach–Zehnder interferometer. The experiments were performed on a 160 kA current generator at Imperial College, London, for three different discharge configurations in order to study the initial resistive heating stage of the wire z-pinch.

Theory of electronically phased coherent beam combination without a reference beam

The first theory for two novel coherent beam combination architectures that are the first electronic beam combination architectures that completely eliminate the need for a separate reference beam are presented. Detailed theoretical models are developed and presented for the first time.

Coherent gradient sensing interferometry: application in convective fluid medium for tomographic measurements

In this work, a measurement technique based on a single-beam coherent gradient sensing (CGS) interferometer is used to obtain line-of-sight projections of a 3-D temperature field in a convective fluid medium. Due to its inherent insensitivity to vibrations, simple and cost effective optical layout, and, most importantly, the absence of a separate reference beam, CGS is an attractive choice for measurements in convective fluids. In the present study, line-of-sight interferometric projections of a differentially heated water medium in a cylindrical cavity are presented for a cavity aspect ratio (diameter/height) of 3.9. The 3-D temperature field is then obtained using the line-of-sight interferometric projections with a 3-D tomographic reconstruction algorithm. The temperature field obtained shows the absence of a radial-symmetric pattern at a Rayleigh number of 1.6×105, which is in agreement with observations from the literature.

Resonant holographic interferometry for species concentration measurements with saturated anomalous dispersion

A double-exposure resonant holographic interferometry measurement technique for full-field, instantaneous species is presented. This technique utilizes tuned pulsed dye lasers, separate reference beams, and a corrective holographic optical element. The resulting interferogram fringes represent the species number density of interest with variations in the bulk refractive index due to the temperature field automatically subtracted. Expressions for relating resonant absorption with fringe shift are disclosed. The relations suggest that RHI is well suited for optically dense media and long path lengths, where absorption measurements can be difficult. A saturation related modification of the dispersion near a broadened resonant transition is described. The depopulation of the ground state due to saturation by the probe beam causes an asymmetric modification of the resonant refractive index, which when convolved over the probing beam does not affect the measured fringe shift as measured by RHI. Resonant interferometry is demonstrated as an accurate and quantitative measure of the concentration of sodium vapor in a low pressure cell. Furthermore, as a combustion diagnostic, RHI is demonstrated on a minor combustion species, OH, in a nonpremixed slot burner.

Wavefront correction using a self-referencing phase conjugation system based on a Zernike cell

A wavefront correction system was produced using a phase conjugating method based on a point diffraction interferometer, or Zernike cell. This has the main advantage that no separate reference beam is required. The PDI was constructed using an optically addressed liquid crystal spatial light modulator, which means that the system is also relatively easy to align, and insensitive to tip/tilt errors. We demonstrate phase correction of induced high spatial frequency aberrations, and show wavefront correction leading to a 3× improvement in Strehl ratio. We also discuss the utility of the spatial light modulator as a common-path point diffraction interferometer

Surface profiling using shearography

We review profile measurement methods using shearography. Traditionally, shearography implies the use of small image shearing, which yields first-order derivatives of displacements or of spatial coordinates. We demonstrate that the use of large image shearing is equivalent to holography, since displacement-related, or spatial coordinate-related, phase fringes are generated. Hence, the technique of shearography can appropriately be perceived as an optical technique that directly measures displacements (or spatial coordinates) and surface strains (or surface slopes). Unlike holography, shearography does not require special vibration isolation since a separate reference beam is not required; hence, it is a practical tool that can be used in the field/ factory environment. Examples of the use of small and large image shearing for surface profiling are given. Finally, a novel method for computing phase derivatives for the determination of curvatures of object surfaces from shearographic measurements is discussed.

An Interferometeric Technique Utilizing Holographic Optical Elements (HOEs) For Imaging Through Scattering Media

Imaging through scattering media as fog and smoke represents a very great problem in practice. Different holographic techniques have been demonstrated a successful solution for this problem, but their usefulness is limited in field conditions by the requirement of a separate coherent reference beam and perfect stability of the holographic recording apparatus. In this paper, an interferometric imaging technique that utilizes holographic optical elements (HOEs) as a diffraction grating is presented. The obtained results show a great improvement in the image quality over the known classical techniques.

Improved measurement accuracy in a long trace profiler: Compensation for laser pointing instability

Laser pointing instability adds to the error of slope measurements taken with the long trace profiler. (P.Z. Takacs and S. Qian, United States Patent 4884697, 1989). As with carriage pitch error, this laser pointing error must be accounted for and subtracted from the surface under test slope measurement. In the past, a separate reference beam was used to characterize the component of slope error from carriage pitch. However, the component of slope error from laser pointing manifests itself differently in the surface under test measured slope error and in the reference beam slope error. An analysis of angle error propagation is given, and the effect of these errors on measured slope is determined. A method is proposed for identifying these errors and subtracting them from the measured surface under test slope function. Separate measurements of carriage pitch and laser pointing instability isolate these effects, so that the effectiveness of the error identification algorithm may be demonstrated.

One-step rainbow holography using a spherical mirror

A simple one-step rainbow holographic method is proposed. The optical setup requires only a slit and a spherical mirror, without a separate reference beam. Also, due to the experimental arrangement, the resulting image can be viewed by reflection. The primary aberrations of this hologram are calculated, and its advantages are discussed.

Sequential hologram interferometry with diffusely illuminated objects

Two simple holographic multiplexing techniques for sequential hologram interferometry of a diffusely illuminated object are described. These techniques enable one to use the frozen-fringe hologram interferometry to study a quasi-continuous change in the object and so regain some of the advantages of real-time hologram interferometry. In the first method of multiplexing with a single reference beam, a small angle prism is used for varying the direction of the reference beam between successive exposures and a double-exposure hologram is recorded at each position of the prism. In the second method of multiplexing with multiple reference beams, a separate reference beam is used for recording each state of the object independently and the interference pattern between any two states is obtained by simultaneous reconstruction of two light fields. The advantage of this method over the previous one is that it is also suitable for dynamic studies. Some reconstructed interferograms of a diffusely illuminated transparent object under different stresses are presented as illustrations.

Imaging through scattering media by interferometric techniques

Imaging through scattering media such as fog is a problem with few known viable solutions. Holographic techniques have been demonstrated successfully in the laboratory, but their usefulness is often limited in field conditions by the requirement of a separate coherent reference beam. In this paper, an interferometric imaging technique that utilizes a grating interferometer is presented. Experimental results obtained with this technique show substantial improvement in image contrast over that obtained via direct imaging.

A new surface-wave acousto-optic time integrating correlator

A new realization of the acousto-optic time-integrating correlator has been constructed. The new device uses a surface-acoustic-wave delay line and is configured so that no separate reference beam is required for coherent detection. Instantaneous bandwidths of 30 MHz and integration times of 30 ms have been achieved. Broadband signals, such as pseudonoise biphase modulated waveforms, with signal-to-noise ratios of −40 dB have been detected. In addition, center frequency, bandwidth, and relative time-difference-of-arrival of these signals can be determined.