Single Reference Beam Research Articles

Motion-artifact-free single shot two-beam optical coherence elastography system.

The assessment of the biomechanical properties of the skin using various imaging techniques has been used as a diagnostic tool in dermatology. Optical coherence elastography (OCE) is one of the techniques that allows for the measurement of elastic properties. OCE relies on measuring tissue displacements induced by external sources. Measuring the tissue's mechanical properties in vivo using OCE is often challenging due to bulk tissue movement. This study aimed to develop an OCE system that allows for minimizing the effects of bulk tissue movements. To achieve this, we designed a two-beam OCE system that simultaneously measures the tissue displacement at two locations on the sample. This allows for cancelling the effect of the tissue bulk movement, which is common to both measurement points. We used a piezoelectric transducer to generate surface acoustic waves (SAW) in the sample. The velocity of the excited waves, which is proportional to the rigidity of the sample, was measured by calculating the phase delay of the SAW at two locations on the sample. Simultaneous measurement at two locations was achieved by dividing a single light beam into two by focusing on the sample at two different locations. The two beams travel different optical path lengths, and the reflected signals were depth encoded in a single optical coherence tomography scan using a single reference beam. The system was characterized using different tissue-mimicking phantoms and the skin of healthy volunteers at the wrist and the palm. We achieved an approximately 50-fold increase in phase sensitivity measurement. We designed a simple two-beam OCE system that effectively minimizes the effect of tissue movement. We believe that the presented system will find immediate applications in the clinic to monitor the progression of systemic sclerosis disease.

Neural Representation of Linear Accelerator Beam Data from a Single Reference Dataset for Fast Commissioning and Quality Assurance

<h3>Purpose/Objective(s)</h3> We hypothesize that linear accelerator (Linac) beam properties including lateral profiles and percentage depth dose (PDD) can be modeled through a neural representation network using a single vendor-provided reference beam dataset. Such beam model can be used to predict clinical Linac beam data distributions from sparse beam measurements, which has the potential of supporting fast commissioning and routine quality assurance (QA) in the clinic. <h3>Materials/Methods</h3> A sinusoidal neural representation network that consists of multilayer perceptron and periodic activation function was used to learn a mapping between 2D coordinates (1D spatial locations x field sizes) during beam scanning and the corresponding percentage dose. The network was trained using a vendor-provided 6 MV reference beam dataset only and evaluated using clinical beam datasets that consist of ten 6MV beam profiles at 10 cm depth and seven 6MV percentage depth dose (PDD) curves collected from multiple clinics. Only beam data with 30 × 30 cm² field size were assumed available and used to update the network weights. Beam data with 10 × 10 cm² were then predicted by the network and compared to the ground truth of clinically acquired data by calculating both the relative mean absolute error and the 1D gamma passing rate with a passing criteria of 1%/1 mm. <h3>Results</h3> Compared to vendor-provided reference beam data, the network-predicted beam data showed improved consistency with the clinically acquired data. The averaged mean relative absolute error and averaged gamma passing rate between network-predicted dose profile/reference beam dose profile and the clinical measurements were 1.9%/2.5% and 95%/90% respectively. The averaged mean relative absolute error and averaged gamma passing rate between network-predicted PDD/reference beam PDD and the clinical measurements were 0.6%/0.5% and 100%/97% respectively. <h3>Conclusion</h3> Neural representation network is capable of modeling inherent Linac beam properties and predicting clinical beam data accurately from sparse beam measurements. The proposed beam model has the potential of simplifying the clinical beam scanning process with much reduced beam data collection.

Single-shot multiwavelength phase unwrapping using a single reference beam and a monochromatic image sensor

We propose single-shot multiwavelength phase unwrapping using a single reference beam and a monochromatic image sensor. A single beam containing multiple wavelengths passes through a reference arm and acts as reference waves. Multiple-wavelength information is multiplexed in the space domain on the monochromatic image sensor and is separated in the spatial frequency domain by utilizing the spatial frequency difference of interference fringes at multiple wavelengths. From a single hologram, multiwavelength phase unwrapping is conducted and an unwrapped phase image is obtained. Both transmission and reflective single-shot multiwavelength digital holography systems using a monochromatic image sensor can be constructed with a simple optical setup employing the proposed technique. Single-shot multiwavelength phase unwrapping by the proposed technique is numerically simulated and experimentally demonstrated.

Some Progress on Interferometry Techniques Applied to Particle Measurements

The aim of this paper is to present some new developments in particle characterization. More precisely, the paper will be focused on the description of some progresses in the characterization of particles by interferometric methods which are generally considered as the most accurate ones. Three standard configurations will be presented and discussed: i) Interference between scattered light from a particle and a single reference beam (holography), ii) Interference between different kinds of light scattered by a single particle (ILIDS), iii) Interference between the light scattered from different particles. A special attention will be devoted to the possibility to obtain 3D information on the particle location.

Combined dual-beam and reference beam LDV for vehicle inertial navigation system

In order to adapt to the complicated environment of ground for the vehicle, a combined dual-beam and reference beam LDV is presented. It is made up of a single dual-beam subsystem and a single reference beam subsystem. Each optical head detects its own backscattering, and then the signal processing unit chooses one of the signals from the two detectors according to the validity of signal to extract the Doppler frequency and calculate the vehicle's velocity. Measurement results of combined LDV show good agreement with data obtained using DZL-1 electronic speedometer. The relative measurement error is less than 1.5%, which shows the reliability of the technique and potential for it to be applied to inertial navigation system.

Simultaneous multidimensional deformation measurements using digital holographic moiré

This paper proposes an elegant technique for the simultaneous measurement of in-plane and out-of-plane displacements of a deformed object in digital holographic interferometry. The measurement relies on simultaneously illuminating the object from multiple directions and using a single reference beam to interfere with the scattered object beams on the CCD plane. Numerical reconstruction provides the complex object wave-fields or complex amplitudes corresponding to prior and postdeformation states of the object. These complex amplitudes are used to generate the complex reconstructed interference field whose real part constitutes a moiré interference fringe pattern. Moiré fringes encode information about multiple phases which are extracted by introducing a spatial carrier in one of the object beams and subsequently using a Fourier transform operation. The information about the in-plane and out-of-plane displacements is then ascertained from the estimated multiple phases using sensitivity vectors of the optical configuration.

Formulation of off-axis single reference beam and two angle views for digital holography

Photo-plates used in classical holography have several advantages, mainly a highresolution and the ability to overlap several holograms. Conversely, they sufferfrom chemical processing and delays in getting the useful signal. In contrast tofilm holography, digital holography offers quick and easy access to the desiredinformation but suffers from the low resolution of the CCD sensors and their smalldetecting areas. This is why many holographers were limited to the in-line scheme,because of its simplicity and ability to overcome this resolution problem. Butit still suffers from several problems when recording particles such as depth offocus, superposition of twin images and others. This is why, in spite of a morecomplex optical arrangement, we propose an appropriate off-axis scheme and atheoretical formulation that enables us to record simultaneously two hologramson the same CCD sensor from two angle views using a single reference beam.Furthermore, such a scheme, simpler than other off-axis proposals, provides spatiallyseparate reconstruction of the two views and overcomes the above cited problems. Asthe aim of our development is to measure the influence of concentration of solidparticles on their velocity for mixing flows, we show here examples without extendedanalysis.

Recording schedule for partially coherent hologram multiplexing in a photorefractive medium

The exposure schedule for partially coherent hologram multiplexing, in which data pages are multiplexed by multiple signal beams and a single reference beam, is investigated in detail for the case of a pi/2 phase-shifted photorefractive medium. We found that the optimum recording schedule for partially coherent multiplexing cannot be determined by the classical recording schedule theory because of time-constant errors induced by partially coherent interaction between a reference beam and self-diffraction signal beams. To overcome the issue, we derive a modified recursion equation that accounts for the time-constant errors, and we also propose a novel iterative recording-schedule correction algorism for finding the optimum solution. In the calculation with hologram multiplicity of 30 and photorefractive coupling strength of 3.0, we could successfully obtain a flat diffraction-efficiency profile after the second recursion.

Polarization analysis with digital holography by use of polarization modulation for single reference beam

Digital holography is applied to analyze the spatial distribution of the polarization state of light transmitted through anisotropic objects by successive hologram recording using polarization switching of the reference wave by an optical fiber Faraday rotator. For a preliminary object to confirm the principle of polarization analysis, a quarter wave plate (QWP) is used, and the distribution of the polarization ellipse over the object surface is analyzed for various azimuths of the QWP. By inserting a polarizer into a part of the object wave after the object as a reference area, a phase drift of the reference waves caused by polarization switching and/or environmental disturbances can be evaluated and compensated for by using the reference area. The principal axis and ellipticity are calculated and compared with their theoretical values. A good agreement between the experimental and theoretical values is observed. For an application, a photoelastic effect induced in a stressed PMMA specimen is also investigated.

Multiplexed interferometer for ultrasonic imaging applications

A multiplexed laser interferometer is presented that allows for the simultaneous detection of acoustic waves from a number of points on the surface of a specimen. Phase gratings are used to create an array of laser beams that are directed to the specimen. The beams are collected using a lens system, combined with a single reference beam in a photorefractive crystal, and imaged on separate photodetector elements. Detection of surface acoustic waves scattered from a defect in an aluminum specimen is demonstrated using a nine-element receiver. Focusing of the array interferometer, by applying appropriate phase shifts between the detected signals, is also shown. The wide bandwidth and noncontact nature of this receiver make it well suited for phased array detection and ultrasonic imaging applications.

A Bragg grating based fibre optic reference beam laser Doppleranemometer

An in-line single mode optical fibre reference beam laser Doppleranemometer that utilizes the reflectivity of a Bragg grating as the source ofthe reference beam has been demonstrated. Tuning the emission wavelength of alaser diode across the bandwidth of a fibre Bragg grating (FBG) providescontrol of the amount of power in the reference beam, and thus a means foroptimizing the signal to noise ratio.

Optical matched-filter and phased-array detection of surface acoustic waves

A method to optically measure surface acoustic displacements simultaneously over an array of detection points is described. Optical phase gratings are used to create a detection array of laser beams that is directed to the specimen. The scattered beams from the detection array are then collected and combined with a single reference beam in a photorefractive crystal in a multiplexed two-wave mixing configuration. The output beams of the resulting multiplexed photorefractive interferometer are imaged onto either a single photodetector (matched-filter scheme) or to separate photodetector elements (phased-array scheme). The optical matched filter requires less electronic overhead. By using different object beam footprints, it is possible to configure optical matched-filter receivers that selectively detect SAWs (broadband, narrow-band, chirped, etc.) with enhanced sensitivity. The responses of equispaced and chirped array receivers to propagating SAWs are discussed. The more general phased-array configuration, although requiring greater electronic overhead, provides time-domain signals from each element for postprocessing. This system can therefore be synthetically focused by applying appropriate phase shifts between the detected signals to create images of the acoustic scattering region. Both these multiplexed optical detection schemes, with their high spatial resolution and subnanometer displacement sensitivities, have applications in nondestructive testing and materials characterization.

Comparative measurement in speckle interferometry using holographically generated reference wave by single reference beam technique

Comparative digital speckle pattern interferometry can be used for direct optical comparison of two nominally identical objects (master and test) and to display the result of the comparison in the form of an interference pattern related to the difference in deformation, shape or refractive index change of the two objects. Reconstructed virtual images of the master object in its initial and final states are used as references in the comparison process. The master interferogram can be recorded by two reference beams and the virtual images can be reconstructed separately. On the other hand, if the master interferogram is recorded by a single reference beam, the wavefronts of the initial and final states of the master object are reconstructed simultaneously. The difference correlation fringes that coexist with the sum correlation fringes are recorded and analyzed using both computer-simulated speckle patterns and real experiments. Theoretical aspects of this technique are discussed.

Polarization recording and reconstruction in Disperse Red 1–doped cellulose acetate

Cellulose acetate films doped with the azo dye Disperse Red 1 were prepared by dip coating. This matrix provides holograms of comparable efficiency for gratings recorded by intensity and polarization modulations. We used this matrix to store and successfully reconstruct the polarization state of an elliptically polarized object beam. A single plane-polarized reference beam was used for both recording and reconstruction.

Holographic storage using shift multiplexing

We demonstrate theoretically and experimentally a new multiplexing method for volume holographic storage using a single reference beam that is composed of multiple plane waves or is a spherical wave. We multiplex the holograms by shifting the recording material or the recording/readout head. The volume properties of the recording medium allow selective readout of holograms stored in successive overlapping locations. High storage densities can be achieved with a relatively simple implementation by use of the new method.

The Use of Holographic Phase Shifting in Assessing Unbonds in GRP Plates

During holographic inspection of glass fiber reinforcesd plastic (GRP) laminated plates, perturbation in the fringe patterns not only reveals the presence of an unbond but also enables estimation of its depth. The accuracy of estimation depends, among other factors, on the accuracy in determining the fringe orders within the boundary of perturbation. This study shows that phase shifting using a single reference beam enables determination of fractional fringe orders at any point on the test surface. When applied to the assessment of the detected unbond, this simple technique gives a more accurate estimation of the unbond depth than conventional holography.

Statistical Model for Noise Gratings Recorded in Volume Holograms

Abstract A statistical model is used to study the formation of noise gratings in holograms and the mechanism that causes radiation scattering in them. The theoretical model presented describes noise scattering in terms of diffraction due to the high number of spurious gratings formed during the recording stage by the interference of a single reference beam with its own scattered radiation. Numerical results calculated from the theory showing the dependence of noise grating efficiency on exposure are compared with experimental results obtained with bleached emulsions, and good agreement is found.

Difference holographic interferometry (DHI): Single reference beam technique

Difference holographic interferometry can be used for a direct optical comparison of two nominally identical objects (master and test) and to display the result of the comparison in the form of an interference pattern related to the difference in deformation, shape or refractive index change of the two objects. Reconstructed real images of the master object in its initial and final states are used for holographic illumination of the test object in both its states. The master interferogram can be recorded by two reference beams and the real images can be reconstructed separately. On the other hand, if the master interferogram is recorded by a single reference beam, the wavefronts of the initial and final states of the master object can only be reconstructed simultaneously. The theory of this new technique is discussed and its application for differential measurements of refractive index changes is presented.

Multichord, near infrared interferometers for the CTX and ZT-40M experiments

A versatile, multichord CO2 interferometer design with a wide dynamic range is adapted to several different experiments. The systems are double pass, Mach–Zehnder, heterodyne, interferometers [that employ a single germanium Bragg cell and 2n+1−1 (n is an integer) 50% beamsplitters to produce 2n interferometer chords], and a single reference beam with matched optical path lengths. Heterodyne signals detected with room-temperature HgCdTe photoresistors are demodulated with digital phase detectors that can track several fringes per microsecond. The real-time electron density readouts provide a considerable savings in ADC channels and computation time when compared with quadrature phase detection schemes. High reliability is achieved with a temperature-controlled wave guide CO2 laser operating on a single mode at intermittent duty. The CTX instrument has been operating with eight chords since March 1984. The ZT-40M instrument, which is under construction, employs a second wavelength to correct for room vibrations in the 0.01 fringe density range.

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.