Interferometer System Research Articles

Characterization of a retroreflector array for 320-GHz interferometer system in Heliotron J.

A retroreflector array, composed of a cluster of small retroreflectors, is experimentally studied for application to a Michelson-type interferometer system in the fusion plasma experiment. Such a new-type reflector has the potential to be a vital and effective tool at a spatially limited location, such as on the vacuum chamber wall of plasma experimental devices. To investigate the effect of retroreflector array on the reflected beam properties, a tabletop experiment is performed with the retroreflector array composed of 4mm corner-cube retroreflectors and with a 320-GHz (λ ∼ 0.937mm) submillimeter wave source. An imaging camera is utilized to measure the submillimeter wave beam profile and is scanned perpendicularly to the beam propagation direction if necessary. The experimental result exhibits a diffraction effect on the reflected beam, resulting in the emergence of discrete peaks on the reflected beam profile, as predicted in the past numerical study; however, the most reflected beam power converges on the one reflected into the incident direction, resulting from a property as a retroreflector. Furthermore, the dependence of the reflected beam on the incident beam angle is characterized while fixing the detector position, and the retroreflection beam intensity is found to vary due to the diffraction effect. Such an undesired variation of beam intensity induced by the diffraction can be suppressed with a focusing lens placed in front of the detector in the practical application to an interferometer.

Development of the multi-chord CO2 interferometer on HFRC

A multi-chord interferometer system is established on the HUST Field Reversed Configuration (HFRC). Configured as a Mach-Zehnder heterodyne interferometer with seven probe chords distributed in the HFRC middle plane, the system is designed to estimate the integral density within the collisional-merging and compression region of the device. With a spatial resolution of 6 cm and a radial coverage range from -7 cm to 31 cm, it provides a density measurement range of 2 × 1018∼1 × 1021m−2. A 10.6 μm CO2 laser serves as the laser source, while an acoustic-optic modulator (AOM) generates a modulation frequency of 40 MHz corresponding to a temporal resolution of 25 ns. A laser interferometric reference chord and a fitting vibration compensation method are implemented to reduce frequency drift and vibration noise. Preliminary experimental results show that the phase resolution within 0.052rad has been achieved, which corresponds to a minimum measurable density of ∼2×1018m−2. Beyond the density profile measurements, the multi-chord interferometer system plays a pivotal role in observing instabilities of FRC plasmas.

Extended Coherent Flame Model applied to an optical single-cylinder engine fueled with ethanol

One way to achieve a fast track for the decarbonization of the transportation sector is through the usage of biofuels. Among the many biofuels available for transportation, ethanol is one of the most promising, especially when combined with direct-injection spark-ignited engine technologies. The present work aims to validate 3D Computation Fluid Dynamics (CFD) ethanol spray and combustion models with the calibration of specific model parameters using experimental data obtained with optical measurements. Focus is given on the investigation and determination of the Extended Coherent Flame Model parameters for hydrous ethanol turbulent spray combustion. To characterize the spray produced by the injector, measurements obtained with a Phase Doppler Interferometer system are used. Natural luminosity and in-cylinder pressure are acquired on a single-cylinder research engine with optical access. The work also considers results obtained from 1D and 3D CFD models to supplement the acquired experimental setup. From the comparison between experimental and numerical results, it comes out that a correction of the Extended Coherent Flame Model turbulence stretch parameter can be done according to a ratio of flow and combustion length scales obtained at the spark time. In this sense, an expression is proposed to allow the correction of such a parameter in different engine operating conditions. Accordingly, in-cylinder mean effective pressure calculated with 3D CFD simulations show a good agreement with the experimental data for all studied cases.

Enhanced resolution of edge enhancement in three-dimensional vortex imaging based on a modified Michelson interferometer

Edge enhancement is emerging as an essential technique for object recognition in recent years. The edge detection of three-dimensional (3D) imaging has been successfully demonstrated in Fresnel incoherent correlation holograph (FINCH) system; however, with a relatively low resolution which needs to be further improved to meet practical requirements. In this work, a modified Michelson interferometer system using a spatial light modulator (SLM) is proposed, which realizes 3D vortex imaging with high resolution and low background noise. Compared with the spiral phase modulated polarization multiplexing FINCH system, the resolution of this system has apparently improved from 50.80 lp mm−1 to 57.02 lp mm−1. The improvement is assumed to be ascribed to the inherently high luminous flux utilization configuration of the Michelson interferometer and the precision phase shifts. More importantly, the proposed system exhibits prominently improved performance of edge enhancement for different objects, including the standard resolution target and label-free biological living cells. The experimental results of the small watch parts also demonstrate its ability in 3D vortex imaging with edge detection. These results provide a promising strategy for high-quality 3D vortex imaging and edge enhancement, and might pave the way for future applications in the fields such as edge detection and pattern recognition.

Laser frequency noise correction in LFM-based interferometric fiber-optic hydrophone array.

In this paper, we propose a novel time-division multiplexed (TDM) array for a large-scale interferometric fiber-optic hydrophone system, in which we introduce a power-optimized reference probe and effectively reduce the additional white noise while correcting for light source frequency noise. Laser frequency noise usually introduces appreciable phase noise during demodulation of interferometric fiber-optic hydrophones. In the previous means, one would introduce an additional probe isolated from the environment in sensor array, and use it as a reference to calibrate the demodulation results of the other actual sensors. However, while correcting, the reference probe also introduces a large white noise. In our array, the echo of the reference probe is higher than the other sensors, thus solving this problem. The novel array design is applied to our previously proposed fiber-optic hydrophone based on a linear frequency modulated (LFM) light source. Experiments show that the deterioration of phase noise floor caused by additional white noise is improved from at least 3 dB originally to within 1 dB. This paper analyzes the factors that need to be concerned for the successful implementation of correction algorithms in hydrophone systems based on LFM sources. Particular focus is given to the impact of the power optimization of reference probe on the white noise and the corrected phase noise. Our proposal allows a significant relaxation of the demanding linewidth requirement for interferometric hydrophone. It is shown that laser with linewidth of 338.06 MHz can replace that with 1.417 kHz in our new system, while achieves the same demodulation noise floor.

Absolute calibration of a streaked optical pyrometer at nanosecond time scale with a luminescent concentrator

The Streaked Optical Pyrometer (SOP) is a visible diagnostic widely used to study the warm dense matter regime at high energy laser facilities, gas guns, or ion accelerators. It is usually coupled with a Velocity Interferometer System for Any Reflector (VISAR) diagnostic for simultaneous shock wave velocity, reflectivity, and temperature measurements to study the Equation of State (EOS) of materials. While VISAR is a well-mastered technology that provides velocity measurements with low relative uncertainties (close to percent), SOP diagnostics still suffer from high imprecision. In this article, we present a new calibration method in order to obtain absolute temperature measurements with reduced uncertainties. This approach is based on a novel light source: a Ce:YAG luminescent concentrator pumped by LEDs. This device produces enough optical power for calibration at the nanosecond sweep duration of the streak camera. As a demonstration, it has first been installed at the LULI facility and tested on quartz samples shocked at temperatures above 4000 K.

Error analysis of laser interferometric system for measuring radius of curvature

Spherical surfaces are essential components of optical systems and imaging devices. Moreover, precision spheres are calibration standards for many accurate instruments in dimensional and mass metrology. A spherical surface's main property is its radius of curvature, which can be measured using contact or non-contact methods. Interferometry is an accurate non-contact technique, but some error sources impact it. This study investigates seventeen error sources that affect a laser interferometric system for measuring the radius of curvature of a precision sphere. The measurements are obtained using a Fizeau laser interferometer (GPI-XP, Zygo) with phase-shifting capability and a displacement measuring interferometer (ZMI-1000, Zygo). A silicon–nitride precision sphere with a nominal radius of 12.49965 mm is dealt with in this study. One of the main contributions of this study is proposing three additional error sources: focal shift, optical distortion, and y-axis vibration. Besides, deadpath, nulling, and focal shift error sources contributed 70% of the total uncertainty budget. Also, to correlate measurement accuracy with the reference surface, three transmission spheres (f/3.3, f/1.5, and f/0.65) are employed; f/0.65 reported the most accurate radius measurement of 12.49922 ± 0.00089 mm. This study also investigates the dependence of the nulling error on the coverage factor that defines the tested surface area. The analysis of the measurement uncertainty and the optimum conditions that minimize the system's potential error sources are described in this work.

A New Optical Interferometric Biosensing System Enhanced with Nanoparticles for Alzheimer's Disease in Serum.

In this scientific work, we demonstrate, for the first time, a new biosensing system and procedure to measure specifically the total Tau (T-Tau) protein in serum, one of the most relevant biomarkers of Alzheimer's disease (AD). AD is a progressive brain disorder that produces neuronal and cognitive dysfunction and affects a high percentage of people worldwide. For this reason, diagnosing AD at the earliest possible stage involves improving diagnostic systems. We report on the use of interferometric bio-transducers integrated with 65 microwells forming diagnostic KITs read-out by using the Interferometric Optical Detection Method (IODM). Moreover, biofunctionalized silicon dioxide (SiO2) nanoparticles (NPs) acting as interferometric enhancers of the bio-transducers signal allow for the improvement of both the optical read-out signal and its ability to work with less-invasive biological samples such as serum instead of cerebrospinal fluid (CSF). As a result, in this paper, we describe for the first time a relevant diagnostic alternative to detect Tau protein at demanding concentrations of 10 pg/mL or even better, opening the opportunity to be used for detecting other relevant AD-related biomarkers in serum, such as β-amyloid and phosphorylated Tau (P-Tau), neurofilaments, among others that can be considered relevant for AD.

System of Laser Interferometers with a Large Spatial Difference to Study the Seismic-Deformation Oscillations of the Ea

The results of unique experiments on synchronous recording of oscillations of earth’s surface with three laser interferometers–deformographs spaced at a distance of 6740 km, obtained during 2016–2020, are considered: two 100-m laser deformographs (Fryazino, Moscow oblast) and one 18-meter (Karymshina site, Kamchatka krai). It is shown that frequency-stabilized and thermocontrolled lasers, as well as systems for registering shifts of the compensation and modulation type interferogram provide an absolute instrumental resolution of 0.1–0.01 nm.

System of Laser Interferometers with a Large Spatial Difference to Study the Seismic-Deformation Oscillations of the Earth

System of Laser Interferometers with a Large Spatial Difference to Study the Seismic-Deformation Oscillations of the Earth

Interferometer response characterization algorithm for multi-aperture Fabry-Perot imaging spectrometers.

In recent years, the demand for hyperspectral imaging devices has grown significantly, driven by their ability of capturing high-resolution spectral information. Among the several possible optical designs for acquiring hyperspectral images, there is a growing interest in interferometric spectral imaging systems based on division of aperture. These systems have the advantage of capturing snapshot acquisitions while maintaining a compact design. However, they require a careful calibration to operate properly. In this work, we present the interferometer response characterization algorithm (IRCA), a robust three-step procedure designed to characterize the transmittance response of multi-aperture imaging spectrometers based on the interferometry of Fabry-Perot. Additionally, we propose a formulation of the image formation model for such devices suitable to estimate the parameters of interest by considering the model under various regimes of finesse. The proposed algorithm processes the image output obtained from a set of monochromatic light sources and refines the results using nonlinear regression after an ad-hoc initialization. Through experimental analysis conducted on four different prototypes from the Image SPectrometer On Chip (ImSPOC) family, we validate the performance of our approach for characterization. The associated source code for this paper is available from Zenodo (http://dx.doi.org/10.5281/zenodo.7978514).

Research on theoretical and experimental of Interference processing based on three-channel mutual injection phase-locked laser

With the advantage of obtaining multiple phase-locked laser outputs at the same time without complicated modulation devices, the multi-channel mutual injection technique provides a new idea to solve the problem of excessive use of beam splitting devices in traditional multi-beam interference processing systems. In this paper, we analyzed the phase-locking mechanism of the mutual injection technique, studied in detail the locking process of the phase difference of three lasers with time when their energy is injected into each other, and discussed the coherence characteristics of the three Gaussian beams after convergence. Accordingly, we built a three-beam interferometric processing system based on the three-channel mutual injection solid-state laser to process the surface of silicon material. The surface of the material was observed by SEM to exhibit a clear structure of regular shaped microporous arrays with a period of 3.0 µm distributed according to the regular pattern of positive hexagon, which was consistent with the interference pattern obtained from the simulation. The comprehensive theoretical and experimental results show that the laser obtained based on the three-channel mutual injection technique has good coherence characteristics and can be effectively applied in interference processing. This opens up a new idea for the development of interference processing equipment and has positive significance for the development of multi-beam interference processing.

Microfiber evanescent-field photothermal gas detection using acoustic-induced mode-dependent frequency shift

Abstract In this study, we experimentally showcase the microfiber evanescent-field photothermal gas detection by exploiting all-fiber MHz-level frequency shift scheme. Based on the acousto-optic interaction effect, the low-frequency shifts of 0.9 MHz and 1.83 MHz can be obtained through the cyclic conversion between the transverse core modes LP01 and LP11 in the few-mode fiber. Our proposed all-fiber frequency shifters show flexible MHz-level up(down) frequency shifts with superior sideband rejection ratio (over 40 dB) and low insertion loss (less than 1 dB). Furthermore, an all-fiber heterodyne interferometric detection system is implemented by leveraging the above low-frequency shifters, in which around 1-μm-diameter microfiber is investigated for photothermal gas detection. A pump-probe configuration is employed to obtain the photothermal effect induced by the gas absorption of the modulated evanescent field. By demodulating the phase of the beat signal output by the interferometer, an equivalent detection limit (1σ) of 32 ppm and a response time of 22 s are achieved for ammonia, as well as 0.24 % instability within 48 pump cycles. Given its compact all-fiber configuration and high sensitivity with fast response, the experimental results can pave the way for widespread applications like heterodyne detection, fiber optical sensors, and interplanetary coherent communications.

Dynamic damage characterization of slender masonry structures by radar interferometry: a case study

The work shows an experimental and numerical investigation on the dynamic identification of damage in slender masonry structures using the innovative remote sensing technique of ground-based radar interferometry. The case study concerns San Cataldo’s masonry lighthouse, which is located in Puglia (Italy). Here, we experimentally determine the natural frequencies of the lighthouse by carrying out Ambient Vibration Tests (AVT) using an interferometric radar system. Starting from the experimental results, we calibrate a numerical model of the structure and study the sensitivity of the first natural frequencies of the model to five possible damage scenarios involving different parts of the structure and different degrees of damage. Comparing numerical and experimental results, we show that the interferometric radar can be effectively applied to detect damage involving the lower part of the structure.

Compact generation of light beams carrying robust higher-order Poincaré polarization states

We propose a protocol for compact and efficient synthesis of a random vectorial source with a higher-order Poincaré (HOP) polarization state encoded into the spatial coherence structure. The procedure is based on the complex-random-mode representation of the cross-spectral density matrix and employs a single phase-only spatial light modulator (SLM) and a common path interferometric system for the mode construction. The SLM displays a set of elaborated multiplexed holograms, which both encode the HOP polarization state and determine the statistical properties of the source. We demonstrate that the beam from the synthesized source can be highly robust against obstructions in the propagation path in the sense that the encoded HOP polarization state is well reconstructed in the far field (focal plane) even when an obstacle is introduced to largely block the source. The results are useful for the transmission of polarization-encoded information in complex media.

Shortening Focal Length of 100‐mm Aperture Flat Lens Based on Improved Sagnac Interferometer and Bifacial Liquid Crystal

AbstractConventional lens technologies face great challenges in applications requiring large apertures and small f‐numbers, while a liquid crystal (LC) lens based on geometric phase is a potential solution. However, the challenge of manufacturing a large size LC flat lens with high efficiency and high phase gradient still remains. Here, an improved interferometric exposure system is proposed to acquire a large‐size vectorial optical field for photo‐alignment. Bifacial LC layers are applied to reduce the focal length while maintaining a large aperture, high diffraction efficiency, and light weight. LC layers are deposited on both sides of the substrate with the same vector system. Finally, a bifacial LC flat lens with a 10 cm diameter and 88.37% diffraction efficiency is fabricated by using optical elements smaller than 2 in., while the effective focal length is reduced from 91.5 cm to 46 cm comparing to the single side LC lens. This work demonstrates the great advantages of LC‐based flat lenses for flat optical systems with large aperture.

Cylindrical Multimode Waveguides as Focusing Interferometric Systems.

Delivery and focusing of radiation requires a variety of optical elements such as waveguides and mirrors or lenses. Heretofore, they were used separately, the former for radiation delivery, the latter for focusing. Here, we show that cylindrical multimode waveguides can both deliver and simultaneously focus radiation, without any external lenses or parabolic mirrors. We develop an analytical, ray-optical model to describe radiation propagation within and after the end of cylindrical multimode waveguides and demonstrate the focusing effect theoretically and experimentally at terahertz frequencies. In the focused spot, located at a distance of several millimeters to a few centimeters away from the waveguide end, typical for focal lengths in optical setups, we achieve a more than 8.4× higher intensity than the cross-sectional average intensity and compress the half-maximum spot area of the incident beam by a factor of >15. Our results represent the first practical realization of a focusing system consisting of only a single cylindrical multimode waveguide, that delivers radiation from one focused spot into another focused spot in free space, with focal distances that are much larger than both the radiation wavelength and the waveguide radius. The results enable design and optimization of cylindrical waveguide-containing systems and demonstrate a precise optical characterization method for cylindrical structures and objects.

Phase jump detection and correction based on the support vector machine

In general, interferometers are used to perform electron density measurements in magnetically confined plasma, where the electron density is dependent on the refractive index of the plasma. Measurements can be made through comparisons of the phase shift variation between the probe and reference laser beam. The plasma electron density should vary continuously during discharge; however, the fringe jump is a step-like change of the apparent electron density caused by a sudden jump of the measured phase shift. The appearance of fringe jump will degrade the interferometric measurements accuracy. This study attempted to solve the fringe jump problem on the polarimeter-interferometer (POINT) diagnostics system of the Experiment Advanced Superconducting Tokamak (EAST) by proposing a support vector machine model for electron density fringe jump detection and correction. The established model can efficiently classify the fringe jump data from the raw measurement data in a manner robust to noise and interference, and subsequently correct the jump. This model greatly improves the correction efficiency and precision of electron density data from the POINT system, and is expected to be embedded into the plasma control system to perform more accurate real-time electron density feedback control. Moreover, the algorithm is not limited to specific fusion devices or interferometer diagnostics, and is applicable to other interferometric measurement systems.

Dual Sagnac Interferometer Distributed Optical Fiber Localization Method Based on Hilbert–Huang Transform

In order to solve the problem that the dual Mach Zehnder (M-Z) interferometer system is easily affected by external environmental noise, a data signal-processing scheme based on Hilbert–Huang transform (HHT) is proposed to achieve high-precision location with distributed optical fibers. The polarization compensation module has a built-in dual Sagnac interferometer system which is used to stabilize the polarization state of the Sagnac interferometer. The eigenmode function is obtained by empirical mode decomposition of the received two optical signals, and then the Hilbert spectrum is obtained by superimposing the Hilbert transform, so that the high-similarity curve caused by the vibration signal can be clearly and intuitively extracted. The optical signal information can be calculated based on the cross-correlation and delay estimation algorithm to accurately obtain the vibration position information. The experimental results show that the positioning accuracy can reach ±11 m, with a sensing fiber length of 15 km and a sampling rate of 10 MHz. It is proven that the distributed optical fiber sensing technology based on the dual Sagnac interferometer system has high practical application value.

Inversion of Wind and Temperature from Low SNR FPI Interferograms

The temperature and wind in the middle and upper atmosphere can be obtained by recording the Doppler shift and broadening of the airglow emission, which is reflected by the interference ring from a ground-based Fabry–Perot interferometer (FPI) system. FPI observations are highly susceptible to weather and the external environment, which seriously affect the signal-to-noise ratio (SNR) of FPI interferograms. An SNR can significantly increase errors in determining the center of the interferogram, leading to inaccurate wind and temperature inversions. The calculation shows that the wind inversion from the interferogram decreases and the temperature increases for larger central errors. In this paper, we propose the maximum standard deviation method (MSDM) with high accuracy and robustness to determine the interference ring center. The performance of the MSDM is better achieved by using more than 100 1D interferogram bins to determine the center of interferograms. The robustness of the MSDM is investigated by computing numerous simulated interferograms with white Gaussian noise and Poisson noise, and compared with the two algorithms of binarization and peak fitting, which are usually used to invert wind and temperature from the interference ring of FPI. The results show that MSDM has higher accuracy and robustness than the other two algorithms. We also simulate the distortion interferogram when the FPI may be illuminated by inhomogeneous background light, which can introduce additional errors in wind and temperature, and the MSDM still performs better. Finally, we invert the wind and temperature from the real airglow interferogram by the Kelan (38.7°N, 111.6°E) FPI, which shows that both the wind and temperature inverted by MSDM better agree well with the FPI product than the other two algorithms. Therefore, the MSDM helps to improve the accuracy and stability to invert the wind and temperature.