Phase-sensitive Detection Technique Research Articles

A Device for Measuring the Electrical Conductivity of Liquids Using Phase Sensitive Detection Technique

A Device for Measuring the Electrical Conductivity of Liquids Using Phase Sensitive Detection Technique

Data reduction for spatially resolved reflectance anisotropy spectrometer.

We show that in spatially resolved reflectance anisotropy (RA) spectrometers, off-axis optical rays introduce a spurious signal component that cannot be addressed by optical alignment. Such a component is associated with the difference between the reflectivities s and p of the sample and depends, in a complex manner, on the incidence position of the incident light on the surface of the sample. We report a data-reduction procedure to easily identify and remove spurious RA signals associated with the off-axis optical rays, based on the singular value decomposition analysis of spatially resolved RA spectra. We validated this approach by developing a spatially resolved RA spectrometer based on an 8 × 8 multi-anode photomultiplier (PMT). The PMT allowed the use of phase-sensitive detection techniques to enhance the signal-to-noise ratio, which is essential for the evaluation of the proposed data reduction procedure.

Coherent Fourier scatterometry nanoparticle detection enhanced by synthetic optical holography.

We demonstrate that the sensitivity of nanoparticle detection on surfaces can be substantially improved by implementing synthetic optical holography (SOH) in coherent Fourier scatterometry (CFS), resulting in a phase-sensitive confocal differential detection technique that operates at very low power level (P = 0.016 mW). The improvement in sensitivity is due to two reasons: first, the boost in the signal at the detector due to the added reference beam; and second, the reduction of background noise caused by the electronics. With this new system, we are able to detect a 60-nm polystyrene latex (PSL) particle at a wavelength of 633 nm (∼λ/10) on a silicon wafer with an improvement in the signal-to-noise ratio (SNR) of approximately 4 dB.

A digital feedback system for advanced ion manipulation techniques in Penning traps.

The possibility of applying active feedback to a single ion in a Penning trap using a fully digital system is demonstrated. Previously realized feedback systems rely on analogcircuits that are susceptible to environmental fluctuations and long term drifts, as well as being limited to the specific task they were designed for. The presented system is implemented using a field-programmable gate array (FPGA)-based platform (STEMlab), offering greater flexibility, higher temporal stability, and the possibility for highly dynamic variation of feedback parameters. The system's capabilities were demonstrated by applying feedback to the ion detection system primarily consisting of a resonant circuit. This allowed shifts in its resonance frequency of up to several kHz and free modification of its quality factor within two orders of magnitude, which reduces the temperature of a single ion by a factor of 6. Furthermore, a phase-sensitive detection technique for the axial ion oscillation was implemented, which reduces the current measurement time by two orders of magnitude, while simultaneously eliminating model-related systematic uncertainties. The use of FPGA technology allowed the implementation of a fully-featured data acquisition system, making it possible to realize feedback techniques that require constant monitoring of the ion signal. This was successfully used to implement a single-ion self-excited oscillator.

Anisotropy magnetoresistance differential probe for characterization of sub-millimeter surface defects on galvanized steel plate

Defects such as cracks can cause dangerous damage to the metal structure and may lead to structural collapses. Cracks can exist in various shapes and sizes where they can start to develop from small scale lower than 1 mm and spread to contribute to the complete fracture of components. Hence, early discovery and monitoring of any cracks in their early stage are crucial to prevent any sudden fatal accidents in the future. This work presents the study and detailed analysis of an ECT probe’s development based on AMR sensors to identify sub-millimeter surface cracks in galvanized steel plates. The probe consists of an excitation coil that induces an eddy current in sample plates and two AMR sensors that detect the differential eddy current-induced magnetic response. A phase-sensitive detection technique with a lock-in amplifier is used to evaluate the magnetic field distribution detected by the AMR sensors. The measured magnetic responses are classified to the depth, width, length, and complex shapes of artificial slits, and the probe is used to perform line scans and 2-D map scans above the slits’ positions. The probe was able to characterize slits with a depth and width as low as 210 and 50 µm, respectively, by using an excitation current of 4 mA at 1 kHz. The slit orientations that were perpendicular to the differential direction of the AMR sensors were clearly visualized, with their estimated lengths showed a good correlation with the physical slit lengths. In the future, the developed system can be expected to help towards the development of a more sophisticated crack detection system where real-time inspections can be realized and applied in various fields.

A Low-Frequency Eddy Current Probe Based on Miniature Fluxgate Array for Defect Evaluation in Steel Components

Detecting defects in high-permeability steel components can be challenging when using the eddy current technique. This is due to the strong magnetization signal that is simultaneously generated along with the eddy current signal. To minimize the regions of the induced eddy current and its detection, an eddy current probe based on an array of miniature fluxgate sensors (DRV425, Texas Instrument, USA) and axial inductors was proposed and fabricated in the study. The fluxgate sensors were arranged in two layers, and the sensors were sandwiched between two layers of inductors. The output signals from the fluxgate array were sampled to obtain a differential signal of the eddy current intensity and direction. A phase-sensitive detection technique was implemented to isolate the strong magnetization signal from the detected eddy current signal and utilized to characterize artificial slits with varying depths. The developed probe successfully characterized both the vertically and horizontally oriented slits with depths from 2 to 10 mm on a 12 mm thick mild steel plate. A better sensitivity was notable in the evaluation of the vertical slits where the vertical slits would increase the eddy current intensity in contrast to the horizontal slits around the slits. It was shown that the eddy current’s distribution map obtained from the developed probe could be used to reveal the physical dimension of the slit.

Extraction of Flux Leakage and Eddy Current Signals Induced by Submillimeter Backside Slits on Carbon Steel Plate Using a Low-Field AMR Differential Magnetic Probe

Magnetic Flux Leakage (MFL) and Eddy Current Testing (ECT) are commonly employed as the non-destructive evaluation (NDE) techniques used to detect defects within the steel. The MFL technique is advantageous in terms of deep defects detection, while the ECT technique excels in providing dense information regarding defects. In this work, artificial MFL and eddy current (EC) signals in ferromagnetic materials are studied, and an experimental magnetic probe that utilizes both techniques is developed for signal verification. The separation between MFL and EC signals is achieved by utilizing the phase-sensitive detection technique, implementing a dynamic referencing method as opposed to the conventional static phase referencing. A finite element method (FEM) based simulation is employed to study and verify the MFL and EC signals measured by the proposed magnetic probe. The proposed magnetic probe features highly sensitive anisotropic magneto-resistive sensors capable of measuring the MFL and EC signals induced by artificial slits of varying depths engraved onto a 2-mm carbon steel plate. Finite element simulations indicate different flux leakage patterns and eddy current signals detected in the vicinity of the back-side slits. A good agreement is observed between the simulated and the measured MFL and EC signals for the optimized frequency range of 110-210 Hz with the corresponding Lissajous curve for the detection of submillimeter back-side slits. The study has shown that the combination of MFL and EC signals can be successfully captured by an appropriate magnetic probe for an enhanced detection performance of back-side defects in ferromagnetic materials.

Spin Seebeck effect detection by harmonic analysis

We present an alternative measurement technique for the longitudinal spin Seebeck effect (SSE), based on harmonic analysis of the signal measured in the Y3Fe5O12/Pt bilayer. In our experimental setup, we use a harmonic magnetic field and a static thermal gradient in the longitudinal configuration to induce a square-wave SSE signal. By using a phase-sensitive detection technique (lock-in), we can reduce the impact of electrical noise, which improves sensitivity and, through harmonic analysis of the measured signal, decouples in real time the SSE signal from other thermoelectric effects.

Unexpected magnetic phase in the weakly ordered spin- 12 chain cuprate Sr2CuO3

We present the magnetic phase diagram of a spin-1/2 chain antiferromagnet Sr$_2$CuO$_3$ studied by ultrasound phase-sensitive detection technique. We observe an enhanced effect of external magnetic field on the ordering temperature of the system, which is in the extreme proximity to the quantum critical point. Inside the N\'eel ordered phase, we detect an additional field-induced continuous phase transition, which is unexpected for a collinear Heisenberg antiferromagnet. This transition is accompanied by softening of magnetic excitation mode observed by electron-spin resonance, which can be associated with a longitudinal (amplitude) mode of the order parameter in a weakly-coupled system of spin-1/2 chains. These results suggest transition from a transverse collinear antiferromagnet to an amplitude-modulated spin density wave phase induced by magnetic field.

Exploring hyperfine levels of non-Rydberg excited states in a Ξ system using Autler-Townes splitting.

In this experimental work we report our findings about a cascade (Ξ) transition 5S1/2→5P3/2→5D3/2 of both 85,87Rb atoms under different laser detuning combinations. The relative power levels of two individual lasers are adjusted under a counter-propagating configuration so that the system exhibits Autler-Townes splitting (ATS). However, the ATS, which is otherwise difficult to detect in a room-temperature alkali vapor cell offering large Doppler background, is well resolved here by using a combination of modulation transfer and phase-sensitive detection techniques. The results show that the AT components clearly indicate the hyperfine structure of 5D3/2 level for 87Rb isotope. For 85Rb, the resolution of ATS is limited by the relatively closer proximity of 5D3/2 hyperfine components. The results are also verified through blue fluorescence detection by monitoring the 5D3/2→6P3/2→5S1/2 non-degenerate decay arm. The technique is easy to implement and is able to reveal the hyperfine structure of the excited levels. However, the technique is not a good choice when an excited level with dense hyperfine structure is targeted.

Development a low-cost carbon monoxide sensor using homemade CW-DFB QCL and board-level electronics

A mid-infrared sensor was demonstrated for the detection of carbon monoxide (CO) at trace level. In order to reduce cost, a homemade continuous-wave mode distributed feedback quantum cascade laser (CW-DFB QCL), a mini gas cell with 1.6-m optical length, and some self-development electronic modules were adopted as excitation source, absorption pool, and signal controlling and processing tool, respectively. Wavelength modulation spectroscopy (WMS) and phase sensitive detection (PSD) techniques as well as wavelet filtering software algorithm were used to reduce the influence of light source fluctuation and system noise and to improve measurement precision and sensitivity. Under the selected P(11) absorption line located at 2099.083 cm−1, a limit of detection (LoD) of 26 parts per billion by volume (ppbv) at atmospheric pressure was achieved with a 1-s acquisition time. Allan deviation was used to characterize the long-term performance of the CO sensor, and a measurement precision of ∼3.4 ppbv was observed with an optimal integration time of ∼114 s. As a field measurement, a continuous monitoring on indoor CO concentration for a period of 24 h was conducted, which verified the reliable and robust operation of the developed sensor.

Exchange biased spin valve-based gating flux sensor

A hybrid type of magnetic sensor, combining the advantages of both fluxgate sensors and giant magnetoresistance sensors (GMR), was developed in this work by applying the fluxgate principle into the GMR sensors. The well-known measuring principle of the fluxgate sensor is based on the offset of the hysteresis curve and the second harmonic characteristic using the phase sensitive detection technique. The GMR spin valve was used as the soft magnetic core and the pickup coil of the fluxgate. Based on the harmonic analysis using Fourier series, the second harmonic component of the hybrid GMR-fluxgate sensor output was found to be dominated. The sensitivity of the device was maximized by turning the amplitude and the phase of the sinusoidal excitation current. The obtained sensitivity was 1.52 mV/mT and the field noise was found to be 5.7 nT/√Hz@1Hz. The non-hysteresis of the transfer V-B curve was achieved, and the nonlinearity was about 2.5% within the operation range of ±0.2 mT. The proposed sensor is an appropriate design for low field measurements and geomagnetic applications.

A novel CMOS transducer for giant magnetoresistance sensors.

In this work, an ASIC (application specific integrated circuits) transducer circuit for field modulated giant magnetoresistance (GMR) sensors was designed and fabricated using a 0.18-μm CMOS process. The transducer circuits consist of a frequency divider, a digital phase shifter, an instrument amplifier, and an analog mixer. These comprise a mix of analog and digital circuit techniques. The compact chip size of 1.5 mm × 1.5 mm for both analog and digital parts was achieved using the TSMC18 1P6M (1-polysilicon 6-metal) process design kit, and the characteristics of the system were simulated using an HSpice simulator. The output of the transducer circuit is the result of the first harmonic detection, which resolves the modulated field using a phase sensitive detection (PSD) technique and is proportional to the measured magnetic field. When the dual-bridge GMR sensor is driven by the transducer circuit with a current of 10 mA at 10 kHz, the observed sensitivity of the field sensor is 10.2 mV/V/Oe and the nonlinearity error was 3% in the linear range of ±1 Oe. The performance of the system was also verified by rotating the sensor system horizontally in earth's magnetic field and recording the sinusoidal output with respect to the azimuth angle, which exhibits an error of less than ±0.04 Oe. These results prove that the ASIC transducer is suitable for driving the AC field modulated GMR sensors applied to geomagnetic measurement.

Status and outlook of CHIP-TRAP: The Central Michigan University high precision Penning trap

At Central Michigan University we are developing a high-precision Penning trap mass spectrometer (CHIP-TRAP) that will focus on measurements with long-lived radioactive isotopes. CHIP-TRAP will consist of a pair of hyperbolic precision-measurement Penning traps, and a cylindrical capture/filter trap in a 12T magnetic field. Ions will be produced by external ion sources, including a laser ablation source, and transported to the capture trap at low energies enabling ions of a given m/q ratio to be selected via their time-of-flight. In the capture trap, contaminant ions will be removed with a mass-selective rf dipole excitation and the ion of interest will be transported to the measurement traps. A phase-sensitive image charge detection technique will be used for simultaneous cyclotron frequency measurements on single ions in the two precision traps, resulting in a reduction in statistical uncertainty due to magnetic field fluctuations.

Methode of phase correction of displacement measurement using Fabry-Perot interferometer in calculable capacitor

The calculable capacitor is a classical and fundamental experimental apparatus in precision electromagnetic measurements. It is the alternating current (AC) impedance primary standard, and an important tool in measuring the fine structure constant. The calculable capacitor provides a way to directly link the capacitance unit to the mechanical unit of length. In the calculable capacitor, the displacement measurement of the guard electrode is an essential part, because the average value of the cross capacitances is directly proportional to the linear displacement of the moving guard electrode. In order to measure the displacement with a high accuracy of 10-9 or lower, a Fabry-Perot interferometer, whose cavity length is traceable to a stabilized laser by the phase sensitive detection technique, is employed. Considering that the Fabry-Perot interferometer is irradiated by the Gaussian laser beam, the effect of the phase shift of the Gaussian field, relative to the plane wave, should be carefully considered in the displacement measurement. The amplitude of the Gaussian laser beam disperses out of the region where it can be assumed to be plane-wave propagation, so its wavefronts bend and their spacing is different from that of the plane wave. As a result, the corresponding distance of an interference fringe from the coherent Gaussian laser beams is not strictly equal to /2, and it means that the displacement correction based on the phase shift of the Gaussian laser beam in the Fabry-Perot interferometer is inevitable. Therefore, the measured result should add or subtract the correction value to obtain the actual displacement of the interferometer. In order to determine the Gouy phase correction, an interferometer model based on the calculable capacitor is studied analytically and numerically. Using the free space propagation and lens transformation of the Gaussian beam field, the complex amplitude of the partial beam transmitted through the interferometer is obtained, and its phase versus the longitude propagation distance is analyzed. The amplitude and phase of the total transmitted beam, which is the coherent superposition of all the partial beams, are presented. Since the Fabry-Perot interferometer in the calculable capacitor is actively locked to a stabilized laser at two different cavity lengths, the phase of the transmitted beam at each cavity length is calculated individually. The phase difference between the two transmitted beams versus the longitude propagation distance is also analyzed numerically. The simulation result demonstrates that the minimum value of the displacement correction can be obtained by actively detecting the laser light at a distance of 560 mm from output mirror, when the Fabry-Perot interferometer moves from the cavity length of 111.3 mm to 316.3 mm, and it means that a displacement correction value of 0.7 nm, with a relative value of |L|/|L| = 3.410-9, should be added to the measured displacement of the guard electrode.

Terahertz magnetic circular dichroism induced by exchange resonance in CoCr₂O₄ single crystal.

Terahertz (THz) time domain spectroscopy (THz-TDS) of a CoCr₂O₄ single crystal has been performed under magnetic fields up to 8 Tesla. The magnetic field dependences of inter-sublattice exchange resonance at different temperatures have been investigated. Benefiting from the phase and polarization sensitive detection technique in THz-TDS, the circular absorption dichroism and Faraday ellipticity in the THz frequency region are observed and are found to be tunable by the external magnetic field. The complex indices of refraction are obtained under different magnetic field, which present distinct rotatory dispersions arising from the exchange magnetic resonance.

Compact sound-speed sensor for quartz enhanced photoacoustic spectroscopy based applications.

A compact sound-speed sensor based on a phase difference method was developed. The sensor employs a U-shaped stainless steel tube with two holes located on its front and back ends, which serves as a sound wave guide. The phase difference between the two holes was measured using two mini-microphones by means of a phase-sensitive detection technique. This method offers the advantage of eliminating the influence of signal fluctuations. The frequency of a sound source offered by a loudspeaker can be scanned between 1 kHz and 50 kHz. The slope of the phase difference as a function of frequency was obtained by scanning the frequency of the sound source. The speed of sound was retrieved from the rate of change of the phase difference. The performance of the sensor was evaluated over a wide range of speeds of sound from 260 m/s to 1010 m/s in different gas mixtures. The measured speed of sound was found to be in good agreement with the theoretical value for the sound-speed sensor.

The Application of Phase-Sensitive Detection Technology in the Lateral Logging

In the course of logging, the instrument needs to measure the formation parameters, at the same time we need to solve the problem that the various signals with different frequencies will be produced. The technology with the use of phase sensitive detection filter can reduce mutual interference between the various signals, which can achieve the purpose of improving the measurement accuracy. This paper analyzed the work principal of phase sensitive detection technique in lateral logging applications, the data shows that this method can effectively eliminate or reduce the impact phase, thereby improving the measurement accuracy of lateral logging instrument.

Chemical Selectivity in Diffraction by Statistical Analysis of in situ XRPD Data

X-ray diffraction methods in general allow only a limited chemical selectivity. Structural information on a subset of atoms can be obtained by a modulation enhanced diffraction (MED) experiment, using a periodic stimulus supplied in situ on a crystal, while diffraction data are collected several times within a stimulus period. The data are then treated by statistical methods such as phase sensitive detection (PSD) and Principal component analysis (PCA) techniques. The application of PSD to diffraction has been proposed as a tool to extract crystallographic information on a subset of atoms [1], thus allowing to introduce selectivity in diffraction. Simulated and experimental PSD-MED powder data were produced by using a TS-1 zeolite as spectator, in which Xe, acting as active species, is adsorbed and desorbed in a periodically modulated mode. By first demodulating these data, MED allowed to obtain the powder diffraction pattern of the active subset, i.e. to obtain selectively the crystallographic information on Xe, by solving the crystal structure of the active species out of the zeolite framework. The "real world" experiments indicated that the PSD-MED approach has some limitations related to its theoretical assumptions. PCA is widely used in spectroscopic analyses and was recently applied to XRPD data by some of us [2]. PCA was exploited to evaluate the in situ XRPD data quality, to speed up the data analysis and data pre-treatment required by PSD and improve the extraction of the substructure information from MED data. It resulted that the first two components obtained by PCA are related to the 1- and 2-omega patterns from PSD. The two approaches (PCA and PSD) are finally compared from the viewpoint of their capacity of gathering information on the Xe substructure inside the zeolite channels and used in a synergic way to obtain the optimal data analysis efficiency.

Doppler Optical Coherence Tomography

Optical Coherence Tomography (OCT) has revolutionized ophthalmology. Since its introduction in the early 1990s it has continuously improved in terms of speed, resolution and sensitivity. The technique has also seen a variety of extensions aiming to assess functional aspects of the tissue in addition to morphology. One of these approaches is Doppler OCT (DOCT), which aims to visualize and quantify blood flow. Such extensions were already implemented in time domain systems, but have gained importance with the introduction of Fourier domain OCT. Nowadays phase-sensitive detection techniques are most widely used to extract blood velocity and blood flow from tissues. A common problem with the technique is that the Doppler angle is not known and several approaches have been realized to obtain absolute velocity and flow data from the retina. Additional studies are required to elucidate which of these techniques is most promising. In the recent years, however, several groups have shown that data can be obtained with high validity and reproducibility. In addition, several groups have published values for total retinal blood flow. Another promising application relates to non-invasive angiography. As compared to standard techniques such as fluorescein and indocyanine-green angiography the technique offers two major advantages: no dye is required and depth resolution is required is provided. As such Doppler OCT has the potential to improve our abilities to diagnose and monitor ocular vascular diseases.