Interferometric Fringe Research Articles

Interferometric Fringe Visibility Null as a Function of Spatial Frequency: A Probe of Stellar Atmospheres

We introduce an observational tool based on visibility nulls in optical spectro-interferometry fringe data to probe the structure of stellar atmospheres. In a preliminary demonstration, we use both Navy Precision Optical Interferometer (NPOI) data and stellar atmosphere models to show that this tool can be used, for example, to investigate limb darkening. Using bootstrapping with either multiple linked baselines or multiple wavelengths in optical and infrared spectro-interferometric observations of stars makes it possible to measure the spatial frequency [Formula: see text] at which the real part of the fringe visibility [Formula: see text] vanishes. That spatial frequency is determined by [Formula: see text], where [Formula: see text] is the projected baseline length, and [Formula: see text] is the wavelength at which the null is observed. Since [Formula: see text] changes with the Earth’s rotation, [Formula: see text] also changes. If [Formula: see text] is constant with wavelength, [Formula: see text] varies in direct proportion to [Formula: see text]. Any departure from that proportionality indicates that the brightness distribution across the stellar disk varies with wavelength via variations in limb darkening, in the angular size of the disk, or both. In this paper, we introduce the use of variations of [Formula: see text] with [Formula: see text] as a means of probing the structure of stellar atmospheres. Using the equivalent uniform disk diameter [Formula: see text], given by [Formula: see text], as a convenient and intuitive parameterization of [Formula: see text], we demonstrate this concept by using model atmospheres to calculate the brightness distribution for [Formula: see text] Ophiuchi and to predict [Formula: see text], and then comparing the predictions to coherently averaged data from observations taken with the NPOI.

Angular alignment and fidelity of neutron phase-gratings for improved interferometer fringe visibility

The recent development of phase-grating moiré neutron interferometry promises a wide range of impactful experiments from dark-field imaging of material microstructure to precise measurements of fundamental constants. However, the contrast of 3% obtained using this moiré interferometer was well below the theoretical prediction of 30% using ideal gratings. It is suspected that non-ideal aspects of the phase-gratings was a leading contributor to this deficiency and that phase-gratings needed to be quantitatively assessed and optimized. Here we characterize neutron diffraction from phase-gratings using Bragg diffraction crystals to determine the optimal phase-grating orientations. We show well-defined diffraction peaks and explore perturbations to the diffraction peaks and the effects on interferometer contrast as a function of grating alignment. This technique promises to improve the contrast of the grating interferometers by providing in-situ aides to grating alignment.

Analysis of a high-stability Stern–Gerlach spatial fringe interferometer

The discovery of the Stern–Gerlach (SG) effect almost a century ago was followed by suggestions to use the effect as a basis for matter-wave interferometry. However, the coherence of splitting particles with spin by a magnetic gradient to a distance exceeding the position uncertainty in each of the arms was not demonstrated until recently, where spatial interference fringes were observed in a proof-of-principle experiment. Here we present and analyze the performance of an improved high-stability SG spatial fringe interferometer based on two spatially separate wave packets with a maximal distance that is more than an order of magnitude larger than their minimal widths. The improved performance is enabled by accurate magnetic field gradient pulses, originating from a novel atom chip configuration, which ensures high stability of the interferometer operation. We analyze the achieved stability using several models, discuss sources of noise, and detail interferometer optimization procedures. We also present a simple analytical phase-space description of the interferometer sequence that demonstrates quantitatively the complete separation of the superposed wave packets.

Contribution study of monogenic wavelets transform to reduce speckle noise in digital speckle pattern interferometry

Wavelets shrinkage is the most illustrative of wavelets transform for speckle noise reduction. We aim to study the performance of a monogenic wavelet transform to reduce the speckle noise in digital speckle pattern interferometric fringes. The proposed method is implemented on simulated and experimental speckle fringe patterns and its performance is appraised on the basis of peak signal-to-noise ratio (PSNR) and quality index (Q). The ability to reduce the speckle noise by the proposed method is compared with other classical speckle denoising methods. The obtained results corroborate the effectiveness of the proposed method for speckle noise reduction in speckle fringes in terms of PSNR and Q. It is also observed that the method provides better qualitative and quantitative results. Furthermore, the proposed method preserves the edge information of the speckle fringes, a feature that is quantified by the edge preservation index.

Fringe Disappearance in Self-Mixing Interferometry Laser Sensors: Model and Application to the Absolute Distance Measurement Scheme

The fringes disappearance phenomenon in the self-mixing interferometry occurs whenever the external round-trip phase at free-running state is modulated by either external modulation such as external cavity length changes or internal modulation when the laser injection current is modulated with a high back-scattered light power. This phenomenon has been observed by many authors for harmonic motion or vibration application and more recently in the case of the absolute distance measurement scheme when the laser injection current is modulated in the triangle waveform. However, there are no clear explanations on the fringes disappearance phenomenon and its dependence on the self-mixing regimes. In this paper, a novel approach that depicts the mechanism of interferometric fringes disappearance is proposed that highlight with a new perspective the impact of the coupling strength between the laser and the external cavity on the number of missing fringes. The proposed modeling is validated by a set of measurements using the optical feedback interferometry absolute distance measurement scheme.

Phase-stepping algorithms for synchronous demodulation of nonlinear phase-shifted fringes.

In optical metrology synchronous phase-stepping algorithms (PSAs) estimate the measured phase of temporal linear-carrier fringes with respect to a linear-reference. Linear-carrier fringes are normally obtained using closed-loop, feedback, optical phase-stepped devices. On the other hand, open-loop phase-stepping devices usually give fringe patterns with nonlinear phase steps. The Fourier spectrum of linear-carrier fringes is composed of Dirac deltas only. In contrast, nonlinear phase-shifted fringes are wideband, spread-spectrum signals. It is well known that using linear-phase reference PSA to demodulate nonlinear phase stepped fringes, one obtains a spurious-piston. The problem with this spurious-piston is that it may wrongly be interpreted as a real thickness in any absolute phase measurement. Here we mathematically find the origin of this spurious-piston and design nonlinear phase-stepping PSAs to cope with nonlinear phase-stepping interferometric fringes. We give a general theory to tailor nonlinear phase-stepping PSAs to synchronously demodulate nonlinear phase-stepped wideband fringes.

Field Reconstruction of Holograms for Interactive Free Space True Three Dimensional Display

Nowadays, the 3D display technology has attracted great academic and industrial attention due to its rapid development and applications in providing more realistic, natural, and extra depth images far superior than the traditional 2D display. But traditional 3D techniques suffer from many drawbacks and hard to meet the requirement of commercialization. In the present study, the field reconstruction method of holograms is proposed for the first time to realize the display of true 3D image in free space which is more practical and better than traditional holographic technique in image quality. An optical switch array (OSA) and a projection lens are used to generate a series of 2D image slice, which can form a virtual 3D object, in free space. Then the light from the virtual object and reference light interference with each other forming an interferometric fringe on the screen and thus the 3D image of the virtual object is reconstructed in free space, timely. Meanwhile, the OSA can work as an image generator and sensor simultaneously which allows user to interact with the reconstructed 3D scenery without extra equipment.

Optical Bio-sensing with an Asymmetric Hybrid Plasmonic Mach-Zehnder Interferometer

A silicon (Si) suspended metal strip loaded hybrid plasmonic waveguide is designed and optimized to obtain a high waveguide sensitivity for bio-sensing applications. The waveguide forms a horizontal slot which confines a high ∼59% power with the optimized design parameters. Any liquid refractometric changes can be detected by this waveguide and its quasi-TM waveguide sensitivity shows a very high value of >1. Finally, an asymmetric Mach-Zehnder interferometer (AMZI) is designed to detect a small refractometric change in terms of interferometric fringe shift. Our proposed design exhibits a high refractometric sensitivity of > 400 nm/RIU. The AMZI is then considered to use for an accurate detection of haemoglobin (Hb) concentration of human whole blood. This ex vivo detection process exhibits a promising device sensitivity of -781.05 pm/g/dl with a detection limit of 1.3 mg/dl which makes this device useful for a precise detection of anaemic status of a patient.

Phase unwrapping of interferometric fringes based on a mutual information quality map and phase recovery strategy

Phase unwrapping in regions of abnormal fringes caused by noise, aliasing, fracturing, and the presence of mass discontinuous phases of unwrapped results remains an unresolved issue. We present an approach that combines an iterated discontinuous phase recovery strategy and a quality-guided algorithm based on mutual information quality maps. First, the quality-guided unwrapping algorithm based on mutual information is applied, and the results of the discontinuous phase region are excluded. Second, the remainder of the continuous phase region is rewrapped and unwrapped iteratively until there are no discontinuities in the result. Finally, the discontinuous region is recovered by a continuity operation. The method is validated through both simulations and experiments, thereby demonstrating that the proposed algorithm is robust in the presence of abnormal fringes and that the discontinuous phase can be reduced significantly.

Automatic Detection of Multi-Modality in Self-Mixing Interferometer

Laser feedback-based self-mixing interferometry (SMI) has been demonstrated for diverse metric sensing applications. Typically, SMI sensors are based on such laser diodes (LDs) which provide mono-modal emission resulting in SMI signals in which each interferometric fringe occurs due to change in optical path length of $\lambda $ /2, where $\lambda $ is emission wavelength. However, in case multiple laser modes undergo SMI, then each mode contributes its own set of fringes. As LDs can emit multiple modes under variable operating conditions, so, non-detection of multiple SMI modes can cause drastic increase in measurement error due to wrong interpretation of fringes. Previously, the detection of multiple laser modes undergoing SMI was achieved by adding spectroscopic instruments to the SMI set-up. This, however, compromises the inherent simplicity of SMI sensing. In this paper, an automatic SMI-based multi-modality detection method is proposed which is able to detect if multiple modes of deployed LD are undergoing SMI and are contributing additional fringes within the SMI signal under variable sensing conditions. Such detection enables correct interpretation of SMI fringe count and can be used to signal occurrence of mode-hopping or secondary mode excitation. The method uses an artificial neural network, able to automatically identify uni-, bi-, or tri-modal SMI signals. Two different LDs (emitting at 637 and 650 nm) were used to acquire 131 experimental uni-, bi-, and tri-modal SMI signals for variable operating conditions and target vibration amplitude. The proposed system has achieved modality detection accuracy of 98.57% on 70 unseen experimental SMI signals.

Interference probe ptychography for computational amplitude and phase microscopy.

We have developed an approach to Fresnel domain ptychography in which the illumination consists of an interference pattern. This pattern is conveniently created by overlapping two coherent beams at an angle. Only the phase and orientation of the interferometric fringe pattern needs to be scanned to reconstruct a high-fidelity object image, which alleviates the requirements for accurate sample positioning and system stability. As such, the resulting imaging systems can be constructed in an extremely simple and robust way. Object images are reconstructed from recorded Fresnel diffraction data using a modified ptychographical iterative engine. We demonstrate the capabilities of this imaging system by recording images of various biological samples, demonstrating quantitative phase contrast as well as a spatial resolution better than 2.2 μm.

Towards the realization of an optical pressure standard

A novel pressure standard based on optical interferometry has been realized. The paper presents a preliminary achievement of the system, in which the standard pressure between 1 kPa and 120 kPa is obtained through the measurement of the refractive index of a gas. The response of the optical system has been initially evaluated measuring its sensitivity for nitrogen and air, in terms of the ratio between a pressure variation and the number of detected interferometric fringes. Afterwards, the interferometer has been geometrical characterized in order to perform an absolute measurement of the refractive index of the gas, allowing an optical measurement of the gas pressure. The following step has been to compare the results of a series of measurements obtained with the optical system with a reference pressure standard at 120 kPa. Finally, the standard has been compared with two capacitance diaphragm gauges in the range between 1 kPa and 120 kPa.The device has a relative standard uncertainty equal to 170 ppm at atmospheric pressure.

A balloon-borne very long baseline interferometry experiment in the stratosphere: Systems design and developments

The balloon-borne very long baseline interferometry (VLBI) experiment is a technical feasibility study for performing radio interferometry in the stratosphere. The flight model has been developed. A balloon-borne VLBI station will be launched to establish interferometric fringes with ground-based VLBI stations distributed over the Japanese islands at an observing frequency of approximately 20 GHz as the first step. This paper describes the system design and development of a series of observing instruments and bus systems. In addition to the advantages of avoiding the atmospheric effects of absorption and fluctuation in high frequency radio observation, the mobility of a station can improve the sampling coverage (“uv-coverage”) by increasing the number of baselines by the number of ground-based counterparts for each observation day. This benefit cannot be obtained with conventional arrays that solely comprise ground-based stations. The balloon-borne VLBI can contribute to a future progress of research fields such as black holes by direct imaging.

Integration of Range Split Spectrum Interferometry and conventional InSAR to monitor large gradient surface displacements

Incorrect unwrapping of dense interferometric fringes caused by large gradient displacements make it difficult to measure mining subsidence using conventional Interferometric Synthetic Aperture Radar (InSAR). This paper presents a Range Split Spectrum Interferometry assisted Phase Unwrapping (R-SSIaPU) method for the first time. The R-SSIaPU method takes advantage of (i) the capability of Range Split Spectrum Interferometry of measuring surface displacements with large spatial gradients, and (ii) the capability of conventional InSAR of being sensitive to surface displacements with limited spatial gradients. Both simulated and real experiments show that the R-SSIaPU method can monitor large gradient mining-induced surface movements with high precision. In the case of the Tangjiahui mine, the R-SSIaPU method agreed with GPS with differences of approximately 4.2 cm, whilst conventional InSAR deviated from GPS with differences of nearly 1 m. The R-SSIaPU method makes phase unwrapping less challenge, especially in the cases with large surface displacements. In addition to mining subsidence, it is believed that the R-SSIaPU method can be used to monitor surface displacements caused by landslides, earthquakes, volcanic eruptions, and glacier movements.

Three-dimensional measurement of small-object surfaces on the basis of fiber optic interferometric fringe projection

We used a system that exploits the Mach–Zehnder interferometer structure and Young’s double-pinhole interference principle to measure the three-dimensional topographies of small objects at high precision. Next, we performed phase profilometry to measure small objects and achieve a height measurement within a 10 × 10-cm area. The accuracy of the measurement system improved by 44.1%, and the measurement time was reduced by 63.2%.

Speckle noise reduction in digital speckle pattern interferometric fringes by nonlocal means and its related adaptive kernel-based methods.

Digital speckle pattern interferometry (DSPI) is widely used in many scientific and industrial applications. Besides its several advantages, one of the basic problems encountered in DSPI is the undesired speckle noise existing in the fringe pattern. In this paper, we demonstrate the performance of nonlocal means (NLM) and its related adaptive kernel-based filtering methods for speckle noise reduction in DSPI fringes. The NLM filter and its related kernel-based filters such as NLM-average, NLM-local polynomial regression, and NLM-shape adaptive patches are implemented first on simulated DSPI fringes, and their performances are quantified on the basis of peak signal-to-noise ratio (PSNR), mean square error (MSE), and quality index (Q). Further, their effectiveness and abilities in reducing speckle noise are compared with other speckle denoising methods. These filtering methods are then employed on experimental DSPI fringes. The obtained results reveal that these filtering methods have the ability to improve the PSNR and Q of the DSPI fringes and provide better visual and quantitative results. It is also observed that the proposed filtering methods preserve the edge information of the DSPI fringes, which is evaluated on the basis of the edge preservation index of the resultant filtered images.

Application of Machine Learning to Classification of Volcanic Deformation in Routinely Generated InSAR Data

AbstractRecent improvements in the frequency, type, and availability of satellite images mean it is now feasible to routinely study volcanoes in remote and inaccessible regions, including those with no ground‐based monitoring. In particular, Interferometric Synthetic Aperture Radar data can detect surface deformation, which has a strong statistical link to eruption. However, the data set produced by the recently launched Sentinel‐1 satellite is too large to be manually analyzed on a global basis. In this study, we systematically process >30,000 short‐term interferograms at over 900 volcanoes and apply machine learning algorithms to automatically detect volcanic ground deformation. We use a convolutional neutral network to classify interferometric fringes in wrapped interferograms with no atmospheric corrections. We employ a transfer learning strategy and test a range of pretrained networks, finding that AlexNet is best suited to this task. The positive results are checked by an expert and fed back for model updating. Following training with a combination of both positive and negative examples, this method reduced the number of interferograms to ∼100 which required further inspection, of which at least 39 are considered true positives. We demonstrate that machine learning can efficiently detect large, rapid deformation signals in wrapped interferograms, but further development is required to detect slow or small deformation patterns which do not generate multiple fringes in short duration interferograms. This study is the first to use machine learning approaches for detecting volcanic deformation in large data sets and demonstrates the potential of such techniques for developing alert systems based on satellite imagery.

Dual-Wavelength DC Compensation Technique for the Demodulation of EFPI Fiber Sensors

A dual-wavelength DC compensation technique for the demodulation of extrinsic Fabry-Perot interferometric (EFPI) fiber sensors is developed. The technique can overcome two problems in the measurement: the cavity length of the EFPI and wavelengths of light sources must be matched, and the DC component of the interferometric fringe must be eliminated. The proposed demodulation technique is verified by demodulating EFPI sensors with different cavity lengths and different phase modulations. Experimental results show that the demodulation technique can interrogate an EFPI sensor with any cavity length and any phase modulation. Besides, the technique has properties of high frequency, large dynamic range, and high sensitivity.

Multicenter Study of Intense Pulsed Light Therapy for Patients With Refractory Meibomian Gland Dysfunction

Purpose:To evaluate the efficacy of intense pulsed light (IPL) therapy combined with meibomian gland expression (MGX) for refractory meibomian gland dysfunction (MGD) in a prospective study conducted at 3 sites in Japan.Methods:Patients with refractory obstructive MGD were enrolled and underwent 4 to 8 IPL-MGX treatment sessions at 3-week intervals. Clinical assessment included the Standard Patient Evaluation of Eye Dryness questionnaire; noninvasive breakup time of the tear film and interferometric fringe pattern as determined by tear interferometry; lid margin abnormalities, fluorescein breakup time of the tear film, corneal and conjunctival fluorescein staining (CFS), and meibum grade as evaluated with a slit-lamp microscope; meibomian gland morphology (meiboscore); and tear production as measured by the Schirmer test without anesthesia.Results:Sixty-two eyes of 31 patients (17 women, 14 men; mean age ± SD, 47.6 ± 16.8 years) were enrolled. The Standard Patient Evaluation of Eye Dryness score (P < 0.001), noninvasive breakup time (P < 0.001), and interferometric fringe pattern (P < 0.001) were significantly improved after therapy, with 74% of eyes showing a change in the interferometric fringe pattern from 1 characteristic of lipid deficiency to the normal condition. Meibum grade, lid margin abnormality scores, fluorescein breakup time, and CFS were also significantly improved (P < 0.001, P < 0.001, P < 0.001, and P = 0.002, respectively) after treatment, whereas the meiboscore and Schirmer test value remained unchanged.Conclusions:IPL-MGX ameliorated symptoms and improved the condition of the tear film in patients with refractory MGD and is therefore a promising treatment option for this disorder.

Fiber-optic micro vibration sensors fabricated by a femtosecond laser

Fiber-optic micro vibration sensors fabricated by a femtosecond laser are proposed and experimentally demonstrated. The proposed sensor is an extrinsic Fabry–Perot interferometer (EFPI), which is the three-section structure sandwiched by a single mode fiber (SMF), a hollow core fiber (HCF) and a coreless silica fiber (CF). A femtosecond laser is employed to fabricate the cantilever beam. The ablation process does not affect reflectivity of the mirrors of the EFPI, resulting in the high visibility of the interferometric fringe. The vibration can be interrogated by using the quadrature passive demodulation. The experimental results show that the sensitivity of 20.678 mV/g@500 Hz is achieved in the acceleration range of 0–10 g. The frequency response of the ANSYS simulation and the experimental results are compared with the frequency range from 100 Hz to 3000 Hz, and the resonant frequency is 1920 Hz.