Interferometric Signal Research Articles

Dynamical evaluation of interference fringe parameters by the Wiener adaptive filtering method

Application of the Wiener adaptive filtering method for estimation of interferometric signal parameters is presented and analyzed. It is shown that the fringe phase, as well as the envelope of low-coherence interference fringes, can be evaluated directly from the adaptive filter impulse response obtained under the criterion of the minimum error variance of a processed fringe signal and predefined reference signal. The method does not limit the number and value of the phase shifts and allows determination of the actual phase step. The accuracy of the method is evaluated and experimental results of dynamical fringe processing in low-coherence interferometry are presented.

Optical-Fiber Microsphere-Based Temperature Sensors with ZnO ALD Coating-Comparative Study.

This study presents the microsphere-based fiber-optic sensor with the ZnO Atomic Layer Deposition coating thickness of 100 nm and 200 nm for temperature measurements. Metrological properties of the sensor were investigated over the temperature range from 100 °C to 300 °C, with a 10 °C step. The interferometric signal was used to monitor the integrity of the microsphere and its attachment to the connecting fiber. For the sensor with a 100 nm coating, a spectrum shift of the reflected signal and the optical power of the reflected signal were used to measure temperature, while only the optical power of the reflected signal was used in the sensor with a 200 nm coating. The R2 coefficient of the discussed sensors indicates a linear fit of over 0.99 to the obtained data. The sensitivity of the sensors, investigated in this study, equals 103.5 nW/°C and 19 pm/°C or 11.4 nW/°C for ZnO thickness of 200 nm and 100 nm, respectively.

Effects of model incompleteness on the drift-scan calibration of radio telescopes

ABSTRACT Precision calibration poses challenges to experiments probing the redshifted 21-cm signal of neutral hydrogen from the Cosmic Dawn and Epoch of Reionization (z ∼ 30–6). In both interferometric and global signal experiments, systematic calibration is the leading source of error. Though many aspects of calibration have been studied, the overlap between the two types of instruments has received less attention. We investigate the sky based calibration of total power measurements with a HERA dish and an EDGES-style antenna to understand the role of autocorrelations in the calibration of an interferometer and the role of sky in calibrating a total power instrument. Using simulations we study various scenarios such as time variable gain, incomplete sky calibration model, and primary beam model. We find that temporal gain drifts, sky model incompleteness, and beam inaccuracies cause biases in the receiver gain amplitude and the receiver temperature estimates. In some cases, these biases mix spectral structure between beam and sky resulting in spectrally variable gain errors. Applying the calibration method to the HERA and EDGES data, we find good agreement with calibration via the more standard methods. Although instrumental gains are consistent with beam and sky errors similar in scale to those simulated, the receiver temperatures show significant deviations from expected values. While we show that it is possible to partially mitigate biases due to model inaccuracies by incorporating a time-dependent gain model in calibration, the resulting errors on calibration products are larger and more correlated. Completely addressing these biases will require more accurate sky and primary beam models.

Bond-selective interferometric scattering microscopy

Interferometric scattering (iSCAT) microscopy has been a very promising technology for highly sensitive label-free imaging of a broad spectrum of biological nanoparticles from proteins to viruses in a high-throughput manner. Although it can reveal the specimen’s size and shape information, the chemical composition is inaccessible in interferometric measurements. Infrared (IR) spectroscopic imaging provides chemical specificity based on inherent chemical bond vibrations of specimens but lacks the ability to image and resolve individual nanoparticles due to long IR wavelengths. Here, we describe a bond-selective iSCAT microscope where the mid-IR induced photothermal signal is detected by a visible beam in a wide-field common-path interferometry configuration. A thin film layered substrate is utilized to reduce the reflected light and provide a reference field for the interferometric detection of the weakly scattered field. A pulsed mid-IR laser is employed to modulate the interferometric signal. Subsequent demodulation via a virtual lock-in camera offers simultaneous chemical information about tens of micro- or nano-particles. The chemical contrast arises from a minute change in the particle’s scattered field in consequence of the vibrational absorption at the target molecule. We characterize the system with sub-wavelength polymer beads and highlight biological applications by chemically imaging several microorganisms including Staphylococcus aureus, Escherichia coli, and Candida albicans. A theoretical framework is established to extend bond-selective iSCAT microscopy to a broad range of biological micro- and nano-particles.

Retrieve Ice Velocities and Invert Spatial Rigidity of the Larsen C Ice Shelf Based on Sentinel-1 Interferometric Data

The Larsen C Ice Shelf (LCIS) is the largest ice shelf in the Antarctica Peninsula, and its state can be considered to be an indicator of local climate change. The goal of this paper is to invert the rigidity of the LCIS based on the interferometric synthetic aperture radar (InSAR) technique using Sentinel-1 images. A targeted processing chain is first used to obtain reliable interferometric phase measurements under the circumstance of rapid ice flow. Unfortunately, only the descending data are available, which disallows the corresponding 2-D velocity field to be directly obtained from such measurements. A new approach is thus proposed to estimate the interferometric phase-based 2-D velocity field with the assistance of speckle tracking offsets. This approach establishes an implicit relationship between range and azimuth displacements based on speckle tracking observations. By taking advantage of such a relationship, the equivalent interferometric signals in the azimuth direction are estimated, thereby recovering the interferometric phase-based 2-D ice velocity field of the LCIS. To further investigate the state of the LCIS, the recovered 2-D velocity field is utilized to invert the ice rigidity. The shallow-shelf approximation (SSA) is the core of the reverse model, which is closely dependent on boundary conditions, including kinematic and dynamic conditions. The experimental results demonstrate that the spatial distribution of the rigidity varies approximately from 70 MPa·s1/3 to 300 MPa·s1/3. This rigidity distribution can reproduce a similar ice flow pattern to the observations.

Nanometric Vibration Sensing Using Spectral Processing of Laser Self-Mixing Feedback Phase

In this paper, a method is proposed to recover displacements with nanometric precision based on spectral analysis of the feedback phase of self-mixing laser interferometric signal. This method is an enhancement of the previously developed method called Time-domain Frequency-domain Signal Processing (TFSP) which could only recover target vibration(s) with greater than lambda/8 amplitude (where lambda is the laser’s wavelength) so long as at least one of the vibration’s components has an amplitude greater than or equal to lambda/2. Using TFSP reconstruction as a first approximation of the main target displacement combined with parameter (optical feedback factor C and linewidth enhancement factor alpha) estimation, the proposed method can provide a better in-depth analysis of the self-mixing signal spectrum and thus achieve the detection of even lower amplitude vibrations. Studies presented in the paper indicate that the proposed method does not require accurate parameter estimation. Experimental results obtained using a laser diode based self-mixing sensor with lambda = 785 nm show that the adding this method on top of TFSP allows to recover target vibrations with amplitude down to approximately lambda/32 with an average amplitude root-mean-square error of 1.56 nm, which corresponds to a 4 fold improvement over TFSP.

Automated Quality Assessment of Interferometric Ring Laser Data.

In seismology, an increased effort to observe all 12 degrees of freedom of seismic ground motion by complementing translational ground motion observations with measurements of strain and rotational motions could be witnessed in recent decades, aiming at an enhanced probing and understanding of Earth and other planetary bodies. The evolution of optical instrumentation, in particular large-scale ring laser installations, such as G-ring and ROMY (ROtational Motion in seismologY), and their geoscientific application have contributed significantly to the emergence of this scientific field. The currently most advanced, large-scale ring laser array is ROMY, which is unprecedented in scale and design. As a heterolithic structure, ROMY’s ring laser components are subject to optical frequency drifts. Such Sagnac interferometers require new considerations and approaches concerning data acquisition, processing and quality assessment, compared to conventional, mechanical instrumentation. We present an automated approach to assess the data quality and the performance of a ring laser, based on characteristics of the interferometric Sagnac signal. The developed scheme is applied to ROMY data to detect compromised operation states and assign quality flags. When ROMY’s database becomes publicly accessible, this assessment will be employed to provide a quality control feature for data requests.

Measuring the primordial gravitational wave background in the presence of other stochastic signals

Standard methodologies for the extraction of the stochastic gravitational wave background (SGWB) from auto- or cross-correlation of interferometric signals often involve the use of a filter function.The standard optimal filter maximizes the signal-to-noise ratio (SNR) between the total SGWB and the noise.We derive expressions for the optimal filter and SNR in the presence of a target SGWB plus other unwanted components. We also generalize the methodology to the case of template-free reconstruction.The formalism allows to easily perform analyses and forecasts that marginalize over foreground signals, such as the typical ΩGW∝ f2/3 background arising from binary coalescence.We demonstrate the methodology with the LISA mission and discuss possible extensions and domains of application.

Optical Vernier sampling using a dual-comb-swept laser to solve distance aliasing

Optical interferometry using comb-swept lasers has the advantage of efficiently reducing the acquisition bandwidth for high-speed and long-range detection. However, in general, the use of a comb-swept laser involves a critical limitation in that the absolute distance cannot be measured, and, thus, multiple layers cannot be distinguished when measuring each position. This is because of the distance ambiguity induced by optical aliasing, in which there is periodic repetition of the frequency of an interferometric signal owing to discrete spectral sweeping, which does not occur in conventional optical interferometry that uses a continuous swept laser. In this paper, we introduce an optical Vernier sampling method using a dual-comb-swept laser to measure the absolute distances in a multi-layer target. For this, we designed a new type of dual-comb-swept laser to include two different free spectral ranges (FSRs) in separated wavelength bands to provide a stable lasing condition. Using a principle similar to that of a Vernier caliper for length measurement, the two different FSRs can be used to recover a higher frequency of an optical interferometric signal to measure longer distances from different layers in a target. Using the dual-comb-swept laser in optical interferometry, we solved the optical aliasing issue and measured the absolute distances of three layers separated over 83 mm using a point-scanning imaging setup and the simultaneous absolute distance of the top surfaces separated over 45 mm using a full-field imaging setup at 14 and 8 times lower acquisition bandwidth than a conventional continuous swept laser that is based on optical interferometry.

Multi-MHz MEMS-VCSEL swept-source optical coherence tomography for endoscopic structural and angiographic imaging with miniaturized brushless motor probes.

Swept source optical coherence tomography (SS-OCT) enables volumetric imaging of subsurface structure. However, applications requiring wide fields of view (FOV), rapid imaging, and higher resolutions have been challenging because multi-MHz axial scan (A-scan) rates are needed. We describe a microelectromechanical systems vertical cavity surface-emitting laser (MEMS-VCSEL) SS-OCT technology for A-scan rates of 2.4 and 3.0 MHz. Sweep to sweep calibration and resampling are performed using dual channel acquisition of the OCT signal and a Mach Zehnder interferometer signal, overcoming inherent optical clock limitations and enabling higher performance. We demonstrate ultrahigh speed structural SS-OCT and OCT angiography (OCTA) imaging of the swine gastrointestinal tract using a suite of miniaturized brushless motor probes, including a 3.2 mm diameter micromotor OCT catheter, a 12 mm diameter tethered OCT capsule, and a 12 mm diameter widefield OCTA probe. MEMS-VCSELs promise to enable ultrahigh speed SS-OCT with a scalable, low cost, and manufacturable technology, suitable for a diverse range of imaging applications.

Characterization of signals for a Divertor Tokamak Test facility interferometer/polarimeter system.

In magnetically confined fusion experiments, laser interferometer/polarimeter systems allow one to determine plasma density, give valuable information on the internal magnetic fields, and contribute to the evaluation of the plasma magnetic equilibrium and to the real-time estimation of the q profile to allow feedback configuration control. This work presents an analysis of the interferometric and polarimetric signals of a multi-chord far-infrared interferometer/polarimeter for the divertor tokamak test facility, the new tokamak device currently under construction in Italy. The polarimetric signals are calculated both with approximate formulas and by solving the equation describing the evolution of the laser beam polarization inside the plasma using the Mueller formalism. The latter method correctly accounts for crosstalk between Faraday rotation and the Cotton-Mouton effect. The impact of the plasma birefringence on the interferometric phase shift is also studied, and it is found that a perturbation of the interferometric phase shift is present also in the case of an initial fixed linear polarization of the probe laser beam.

High-bandwidth beam balance for vacuum-weight experiment and Newtonian noise subtraction

We report the experimental results of a prototype balance for the Archimedes experiment, devoted to measure the interaction between quantum vacuum energy and gravity. The prototype is a beam balance working at room temperature which shares with the final balance several mechanical and optical components. The balance sensitivity has been tested at the site of the Virgo gravitational wave detector in order to benefit from its quiet environment and control facilities. This allowed also the test of the coherence of the balance data with the Virgo interferometer signal and with the environmental data. In the low-frequency regime, the balance has shown a sensitivity of about 8times 10^{-12} {text {Nm}}/sqrt{text {Hz}}, which is among the best in the world, and it is very promising toward the final Archimedes measurement. In the high-frequency region, above a few Hz, relying on the behavior of the balance as a rotational sensor, the ground tilt has been measured in view of the next work devoted to Newtonian noise subtraction (NNS) in Virgo. The measured ground tilt reaches a minimum of about 8times 10^{-11} {text {rad}}/sqrt{{text {Hz}}} in the few Hz region and ranges from 10^{-10} to 10^{-9} {text {rad}}/sqrt{{text {Hz}}} in the 10–20 Hz region, where a very interesting coherence, at some frequencies, with the Virgo interferometer signal is shown.

Fibre-coupled, multiplexed methane detection using range-resolved interferometry

We describe the first use of range-resolved interferometric signal processing for measurement of spectral transmission. This was applied to gas sensing using tunable diode laser spectroscopy, allowing the simultaneous and independent measurement of methane concentrations in multiple gas cells. The system uses a single injection-current modulated diode laser and a single photodetector. For three gas cells, we show the ability of the system to measure methane at noise equivalent concentrations of less than 200 ppm for a 0.5 s measurement period and a potential noise equivalent concentration (1σ) of <20 ppm with 150 s averaging time. We further show that cross-talk between cells is below the experimental uncertainty for the system.

A coincidence null test for Poisson-distributed events

When transient events are observed with multiple sensors, it is often necessary to establish the significance of coincident events. We derive a universal null test for an arbitrary number of sensors motivated by the archetypal detection problem for independent Poisson-distributed events in gravitational-wave detectors such as LIGO and Virgo. In these detectors, transient events may be witnessed by myriad channels that record interferometric signals and the surrounding physical environment. We apply our null test to a broad set of simulated gravitational-wave events as well as to a real gravitational-wave detection to determine which auxiliary channels do and do not witness real gravitational waves, and therefore which are safe to use when constructing vetoes. We also describe how our approach can be used to study detector artifacts and their origin, as well as to quantify the statistical independence of candidate GW signals from noise artifacts observed in auxiliary channels.

Instantaneous frequency extraction of highly nonstationary optical interferometric signal using Reassigned Smoothed Pseudo Wigner Ville Distribution

Instantaneous frequency (IF) of optical interferometric signal often carries information on various physical parameters such as velocity, strain and displacement. We propose smoothed Pseudo Wigner-Ville distribution method for extracting IF of highly non-stationary polynomial frequency modulated (FM) fringe signal. Cohen class signal representation is followed and time and frequency resolutions are independently optimized using separable kernel. Reassignment technique is further applied for sharpening of the spectrogram with these optimized kernels that improves accuracy in IF significantly. Quadratic FM along with strong nonlinear IF based fringe signal is considered for showing the efficacy of proposed method. Results are compared with that of existing Polynomial Wigner Ville Distribution method and Continuous Wavelet Transform (CWT) based approach and found to be more accurate in proposed technique. Finally fringe signal generated by varying the length of one of the legs of Michelson interferometer has been used to validate the proposed method experimentally.

Assessment of parameters of interference fringes using a modified algorithm for adaptive Wiener filtering

We present a modified algorithm for adaptive Wiener filtering using two quadrature reference signals for a dynamic estimation of the phase and amplitude of interferometric signals. The advantages of the noise resistance of the modified algorithm with respect to the random deviations of the phase step and the observation noise effect are demonstrated in comparison with the original algorithm. We report the estimates of the error in determination of the phase shift step and signal phase in the interferometry of the controlled phase shift as well as the experimental results regarding the envelope of the partially coherent interferometric signals, which were obtained when determining the inner microstructure of the objects in optical coherence tomography.

Spectral-Phase Interferometry Detection of Ochratoxin A via Aptamer-Functionalized Graphene Coated Glass.

In this work, we report a novel method of label-free detection of small molecules based on direct observation of interferometric signal change in graphene-modified glasses. The interferometric sensor chips are fabricated via a conventional wet transfer method of CVD-grown graphene onto the glass coverslips, lowering the device cost and allowing for upscaling the sensor fabrication. For the first time, we report the use of graphene functionalized by the aptamer as the bioreceptor, in conjunction with Spectral-Phase Interferometry (SPI) for detection of ochratoxin A (OTA). In a direct assay with an OTA-specific aptamer, we demonstrated a quick and significant change of the optical signal in response to the maximum tolerable level of OTA concentration. The sensor regeneration is possible in urea solution. The developed platform enables a direct method of kinetic analysis of small molecules using a low-cost optical chip with a graphene-aptamer sensing layer.

A Fast Two-Dimensional Velocity Estimation Method for Multi-Channel UWB SAR

Moving target detection in ultra-wideband synthetic aperture radar (UWB SAR) has to accumulate target energy with long integration time to improve the probability of target detection. The traditional detection and parameters estimation algorithms usually rely on parameter searching, leading to a heavy computing burden. In this paper, a fast two-dimensional velocity estimation method of moving target for multi-channel UWB SAR is proposed. The method processes the multi-channel data separately to realize the target's coherent accumulation and one-dimensional velocity estimation. Then the multi-channel signals are processed jointly to realize the two-dimensional velocity estimation. Specifically, for each channel, the second-order keystone transform is used to correct the range curvature. The symmetric autocorrelation function in the range-frequency domain is used to accumulate the moving-target energy, achieving the target detection and target's azimuth signal extraction. Next, considering the long integration time, the azimuth signal is processed by the non-uniform cubic phase function (NUCPF) based on the fourth-order phase signal to estimate the relative velocity. The azimuth signal is transformed into the Doppler domain for multi-channel data, and the interferometric signal is calculated. The phase generated by the interferometric signal is related to the radial velocity of the target. Thus, the two-dimensional velocity of the target is obtained. Finally, the effectiveness of the algorithm is verified by simulation experiments.

SNR improvement of 8.2 dB in a self-mixing laser diode interferometer by using the difference signal at the output mirrors [Invited

At the mirrors of a laser diode self-mixing interferometer, the output beams carry anti-correlated (i.e., in phase opposition) interferometric signals, whereas the superposed noise fluctuations are (partially) correlated. Therefore, by using an instrumental output of the interferometer as the difference of the two, we double the amplitude of the self-mixing useful signal, while the superposed noise is reduced. To validate the idea, we first calculate the noise reduction by means of a second-quantization model, finding that in a laser diode the signal-to-noise ratio (SNR) can be improved by 8.2 dB, typically. Then, we also carry out an experimental measurement of SNR and find very good agreement with the theoretical result.

Spectral Interferometric Detection of NIR Optical Resonances of Rectangular Microcapillaries for Refractive Index Sensing

In this work, we propose a spectral interferometric technique for detecting the wavelength position of the resonances of low-cost optical resonators, consisting in rectangular glass microcapillaries. Moreover, we exploited these microdevices as core elements of a microoptofluidic sensing platform designed for the measurements of the refractive index (RI) of fluid samples, based on spectral phase shift interferometry. A detailed theoretical model was implemented to simulate the spectral shape of the interferometric signal in a wavelength range where cavity resonances of the capillary-based optical resonator occur. For experimental verification, the microdevice was placed at the end of the measurement arm of a Michelson interferometer and illuminated using a light source emitting broadband radiation in the near infrared region around 1.55 μm. The resonances of the empty capillary (i.e., filled with air) were successfully detected as steep phase jumps of the interferometric cosine signal recorded as a function of the wavelength, in good agreement with the theoretical predictions. Finally, further experimental measurements were carried out by filling the capillary channel with water-glucose dilutions with different concentrations to prove the suitability of the phase shift detection method to monitor RI variations with respect to a reference fluid.