Phase-sensitive Detection Method Research Articles

Phase sensitive detection for embedded sensors

The measurement of some physical properties requires detection of both amplitude and phase of an applied test signal. In portable sensors, this needs to be performed locally with integrated analog circuits or digital processing. Wireless sensor network allows the measurements over wide areas but raises a challenge of using efficient algorithm for phase sensitive detection (PSD) in nodes with constrained processing capacity. It is not straightforward to compare different PSD methods because their performance is closely related to the fine tuning of their parameters such as filter order and cut-off frequency. We propose a methodology to compare several PSD methods subjected to the same response time to assess its ability to provide accurate estimation in the presence of noise. We also determine the computational complexity of the investigated methods in terms of the number of operations required. We investigate both continuous and windowed operation. The methods were tested through simulations and verified using a dedicated embedded system. Our findings demonstrate that Goertzel algorithm rendered the best results, with smaller estimation error and less computation complexity.

Time-Domain Elastic Full Waveform Inversion With Frequency Normalization

Time-domain elastic full waveform inversion (FWI) uses seismic data to recover the high-resolution subsurface medium properties for structural imaging and lithologic identification. The approximate pulse generated by the explosion source in seismic exploration evolves into a limited bandwidth seismic wavelet through the propagation of the seismic wave in the underground medium, exhibiting strong energy near the dominant frequency and weak energy far away. Therefore, time-domain FWI using the band-limited seismic wavelet can only match the energy of the dominant frequencies present in a dataset to a large extent, resulting in insufficient low-wavenumber updates of model parameters because of the weak energy of low frequencies. To remove the effect of finite-frequency-band wavelet spectra from the time-domain FWI, we propose a time-domain elastic FWI without wavelet spectral limitation. In our method, a newly refined seismic wavelet with a normalized amplitude and accurate phase is used to propagate seismic waves in the time domain. Data residual measurement is performed based on the summation of single-frequency residuals between the normalized synthetic and observed frequencies. The adjoint-state method is used to approximate the gradients of the model parameters, and the decoupled wavefields obtained by the phase-sensitive detection method were involved in the gradient calculation. Overall, the proposed method helps FWI avoid falling into local minima by enhancing the low wavenumber reconstruction of the velocity models. The elastic FWI numerical tests and field data FWI application demonstrate that our approach can reliably recover high-precision inversion results.

Ground‐level DC electric field sensor for overhead HVDC/HVAC transmission lines in hybrid corridors

Measurement of electric fields is crucial for the evaluation of the electromagnetic environment in electric power transmission system. Due to limited transmission corridor resources, the hybrid transmission lines including high-voltage alternating current and high-voltage direct current (HVAC/HVDC) are more common nowadays. However, commonly used field-mill direct current electric field sensors (DC EFSs) can only be used beneath HVDC lines and the emergence of alternating current (AC) electric fields has adversely affected measurement results recorded by DC EFS. In this work, a DC EFS was designed that is capable of measuring ground-level DC electric fields generated by overhead HVAC/HVDC transmission lines in a hybrid corridor. A field-mill structure was used to modulate the DC and AC hybrid electric field and then a narrow band-pass filter was used to realise suppression of the AC electric field signal. Subsequently, the phase-sensitive detection method was used to realise amplitude demodulation and phase discrimination of the DC electric field. Sensor prototypes based on the foregoing method were experimentally tested in the China national high-voltage base. These DC EFSs could be mass-produced conveniently from current field-mill for hybrid corridors application.

Subsurface detection of opaque and solid material defect based on photoacoustic effect

This study develops a subsurface detection system based on a photoacoustic effect. The main parts of the system are a laser diode as an exciter, a standard microphone as an acoustic detector, and a software-based phase sensitive detection (PSD) method. Artificial defects of various depths were made inside an alumunium plate as the object which was painted with acrylic paint to cover the defects. The difference in the thermal diffusivity of alumunium and acrylic paint will cause different amplitudes of photoacoustic so they can be used to detect the subsurface defects. Photoacoustic signal amplitudes used in this study as measured by a microphone have an average amplitude of −57 dB with an SNR of 11.2. High noise level was found below 2.153 Hz, so that the photoacoustic signal amplitude cannot be detected. The accuracy of PSD method was tested by measuring amplitude of 400–600 Hz sinusoidal voltages from a signal generator. The range of the amplitudes was 0.73–6.27 μV with a resulting maximum error of 9%. The PSD method was succesfully implemented to detect the subsurface defects of a 300 μm thick alumunium plate at depths of 215 μm, 241 μm, and 254 μm with a resulting maximum error of 5.83%.

Frequency Response of Induced-Charge Electrophoretic Metallic Janus Particles.

The ability to manipulate and control active microparticles is essential for designing microrobots for applications. This paper describes the use of electric and magnetic fields to control the direction and speed of induced-charge electrophoresis (ICEP) driven metallic Janus microrobots. A direct current (DC) magnetic field applied in the direction perpendicular to the electric field maintains the linear movement of particles in a 2D plane. Phoretic force spectroscopy (PFS), a phase-sensitive detection method to detect the motions of phoretic particles, is used to characterize the frequency-dependent phoretic mobility and drag coefficient of the phoretic force. When the electric field is scanned over a frequency range of 1 kHz–1 MHz, the Janus particles exhibit an ICEP direction reversal at a crossover frequency at ~30 kH., Below this crossover frequency, the particle moves in a direction towards the dielectric side of the particle, and above this frequency, the particle moves towards the metallic side. The ICEP phoretic drag coefficient measured by PFS is found to be similar to that of the Stokes drag. Further investigation is required to study microscopic interpretations of the frequency at which ICEP mobility switched signs and the reason why the magnitudes of the forward and reversed modes of ICEP are so different.

Terahertz wire-grid circular polarizer tuned by lock-in detection method.

We report the design, construction, and testing of a broadband-tunable terahertz circular polarizer, which we developed for our magneto-optical measurements using the far-infrared/THz laser source in the range of 0.25-7.5 THz. We present a thorough analysis of the lock-in amplifier signal generated by using a rotating analyzer with regard to setting the desired state of polarization. The phase-sensitive detection method is applied to a combination of a wire-grid polarizer and a parallel translation mirror providing a tunable retardance. The proposed technique is appropriate not only to free-standing grids but also to those deposited on transparent substrates which may introduce additional non-linear effects. The method is preferred when the distance between the mirror and the grid cannot be exactly determined, but the relative displacements are measured. The device enables switching between left- and right-handed polarization states on a time scale of a few seconds. Practical use of the circular polarizer is demonstrated by directly probing the far infra-red magneto-optical properties of the two-dimensional electron gas on the 458 μm laser line.

An InGaAs/AlAsSb Avalanche Photodiode With a Small Temperature Coefficient of Breakdown

Dark current and avalanche gain M on AlAs0.56Sb0.44 (hereafter referred to as AlAsSb) separate absorption multiplication (SAM) avalanche photodiodes (APDs) were measured at temperatures ranging from 77 K to 300 K. To avoid possible ambiguity in breakdown voltage due to edge breakdown and tunneling current, a phase-sensitive detection method with a tightly focused light spot in the center of the device was employed to measure M accurately. An extrapolation of 1/M to zero was used to deduce the breakdown voltage, from which the temperature coefficient of breakdown voltage Cbd was derived. The value of Cbd 1/4 8 mV/K, obtained for AlAsSb SAM APDs, is much smaller than that for commercial Si and InGaAs/InP APDs, as well as other SAM APDs in the literature, demonstrating the potential of AlAsSb avalanche regions in improving the thermal stability of APDs.

Evaluation of phase sensitive detection method and Si avalanche photodiode for radiation thermometry

We report the evaluation of Si avalanche photodiodes (APDs) for use in radiation thermometry as an alternative to Si photodiodes. We compared their performance when operated under phase sensitive detection (PSD), where the signal is modulated, and direct detection (DD) methods. A Si APD was compared with a Si photodiode with reference black body temperatures of 275 to 600°C, in terms of the mean output voltage and signal-to-noise ratio (SNR), measured at different APD gain values. We found that using both PSD and DD methods, the high internal gain of the Si APD achieved a lower minimum detection temperature in order to satisfy a specific minimum output voltage of the detector-preamplifier combination employed. The use of PSD over DD for the Si APD allowed for improved performance of the thermometer, with a lower minimum measurable temperature, as well as improvement in the SNR. For instance we found that at 350°C, the Si APD biased at 150 V using PSD can provide ∼ 88 times enhancement in the system SNR over that of a Si photodiode using DD. A corresponding temperature error of ±0.05°C was achieved using the APD with PSD compared to an error of ±2.75°C measured using the Si photodiode with DD.

Study on the Signal Processing Methods of the Eddy Current Testing on the Steel Ball Surface Defects

In the detection of steel ball surface defects, the non-destructive eddy current testing has the advantage of reliability, high efficiency, non-contact and easy to automate, etc. Base on the establishment of the mathematical model of the ball surface unfolding, Correct processing of eddy current sensor output signal is the first conditions of accuracy. In the paper, base on the propose of unfolding mechanism of the ball's surface, with smooth, phase-sensitive detection, filtering and other methods to process the detected signal, so interference signals can be suppressed and to improve the signal to noise ratio; Then the signal can be further analyzed in time domain or frequency domain, the signal amplitude, frequency, phase and other characteristic quantities can be extracted, and the characteristic quantity of defect signal can be Calculated, So the surface quality of steel balls can be determined.

Measurement of the sheath capacitance of a planar probe

The sheath capacitance was measured on a planar probe dc-biased with respect to the plasma potential using the phase sensitive detection method in the region separated from the rf discharge plasmas by an immersed grid. It was observed that the sheath capacitance was negative when the collecting electrode of the probe was positioned downward toward the grid and biased near the plasma potential. This indicates that a double sheath had built up near the probe electrode. This tendency can be explained by the sheath capacitance, which is calculated using Poisson’s equation with a non-zero electrical field and an ion velocity condition at the sheath edge.

Low-Noise Chemical Detection with a Piezoresistive Microcantilever Array

Piezoresistive microcantilever systems are well suited for chemical sensing with low detection limits. A major hurdle in the realization of such systems is measurement noise associated with DC piezoresistance measurements. A piezoresistive microcantilever array has been fabricated and phase sensitive detection methods are used for realization of a low detection limit piezoresistive cantilever array. Reference cantilevers in the array can act as thermomechanical or chemical references to aid in lowering the detection limits.

Emission Characteristics of Cu in Dc Voltage Modulation Applied Glow Discharge Optical Emission Spectrometry

In order to obtain the depth profile of a thin film, we investigated the emission characteristics of a voltage modulation glow discharge to optimize the modulation parameters (modulation voltage, offset voltage, and modulation frequency). In this study, a phase-sensitive detection method with a lock-in amplifier to the modulation technique led to a higher sensitivity and a larger signal-to-noise ratio in the emission analysis compared to the normal dc amplification method. Upon increasing the maximum voltage, the emission intensity of the Cu atomic line (CuI 239.34 nm) increased linearly at a modulation voltage of 400 V and an offset voltage of 300 V. On the other hand, the emission intensity was gradually reduced when a modulation frequency increased. It is advantageous for surface analysis that the voltage modulation technique gives a lower sputtering rate rather than the conventional dc discharge.

A new method of phase sensitive detection in modulation spectroscopy applied to temperature induced folding and unfolding of RNase A

Modulation spectroscopy can be an adequate tool to separate a weak system response from a huge background absorption, provided the process under consideration enables a periodic external stimulation. Phase sensitive detection (PSD) is then used to demodulate the periodic system response. We introduce a new method of PSD that can be used in modulated excitation (ME)-FTIR spectroscopy without the need of a lock-in amplifier or spectrometer build-in hardware. An advantage of this method is that only standard procedures included in all FTIR instruments, such as the measurement of time-resolved spectra, are required. The principal mathematical formalism of the used PSD is described. As an example we show phase-resolved spectra of the amid I′ region obtained from temperature modulated FTIR spectroscopic experiments of RNase A. The advantage of the ME compared to a relaxation process is shown by the power of separation of overlapping absorption bands.

A Phase-Sensitive Detection Method Using Diffractive Optics for Polarization-Selective Femtosecond Raman Spectroscopy

A phase-sensitive detection method that uses two diffractive optics for femtosecond nonresonant Raman spectroscopy is demonstrated. One diffractive optic is used for generating the three input pulses for the third-order nonlinear experiment, and the second is used for recombining the signal with a passively phase-locked local oscillator derived from the probe pulse. This approach allows for phase-sensitive detection, direct phase calibration, control over all field polarizations, and removal of unwanted two-pulse signal contributions. Experiments on the intermolecular dynamics of CS2 and CH3CN demonstrate that the birefringent (in-quadrature) signal amplitude is significantly greater than the dichroic (in-phase) contribution. Polarization-selective measurements are used to project the isotropic birefringent response for CS2, which suppresses reorientational dynamics and allows interaction-induced effects to be observed.

AN ALTERNATIVE APPROACH TO POINCARÉ SECTIONING IN WEAKLY NONLINEAR SYSTEMS

A large array of dynamical systems can be broadly classified as weakly nonlinear, where a common feature is the presence of dual time-scale dynamics. Often it is desirable to isolate the dynamics at one time scale (typically the slower time scale), and this paper presents a generic phase-sensitive detection method that successfully isolates the slow-time dynamics, stripping off the less-relevant and often dominant fast-time dynamics. We shall also show for the first time, under certain phase considerations, the equivalence of the present method and the traditional Poincaré section of the original full response. The method is first presented in theory and then applied to data from a numerical simulation of the forced spherical pendulum followed by data from a similar spherical pendulum experiment. The method is successful provided certain conditions apply for phase stationarity.

Photoelectric properties ofp-CuInSe2 thin films obtained by pulsed laser vaporization

Using the method of phase-sensitive detection, we investigated the photoconductivity spectra of polycrystalline p-CuInSe2 films obtained in vacuum by the method of pulsed laser vaporization of initial single crystals. We show that, in contrast to single crystals, films are characterized by monopolar photoconductivity in the extrinsic and intrinsic regions of optical absorption and by a linear mechanism of recombination of nonequilibrium charge carriers at the intensities of illumination 0–100 mW/cm2 in the temperature range 80–300 K.

Measurement and analysis of complex modulated motions in a weakly nonlinear system

The two-timing dynamics of a forced, weakly nonlinear system are considered. Experimental results on a roll-forced spherical pendulum verify the occurrence of slow-time, subharmonically and chaotically modulated oscillations in a frequency band near resonance; the modulatory behavior follows the occurrence of a Hopf bifurcation in the slow-time coordinates. A phase-sensitive detection method for experimentally isolating the slow-time behavior is described. A Lagrangian model for the pendulum is developed, and slow-time equations are presented. A two-thirds power scalling law relating roll angle to the appearance of the modulations is derived from the slow-time equations and tested with experimental results.

Application of electroreflectance analysis for organic semiconductor thin films

Many organic semiconductors with conjugate bond structure possess photoconductivity. Conduction mechanism of organic materials exhibits ‘dualism’ since both intramolecular as well as inter-molecular aspects are involved in the excitation, absorption and transport of charge carriers. Modulation spectroscopy promises to be the most accurate method for analysis of organic photoconductors, especially of thin films. In this technique a periodic perturbation is applied to the material under study and the effect of the perturbation is separated from reflection or absorption while scanning through a given wavelength range by use of lock-in phase sensitive detection method. In electromodulation, particularly in electrolyte electromodulation, the applied field on the material produces changes in the dielectric function which corresponds to the change in reflectance. When the applied field is low the line-shape of spectrum is third-derivative like in comparison with the unmodulated reflectance spectrum. Using Aspnes three-point method the transitions corresponding to critical points can be determined. When the field is intermediate Franz-Keldysh oscillations, which are dc bias dependent, appear on the higher energy side of the transition energy from which the role of intra-molecular as well as intermolecular aspects in conduction mechanism can be understood and the carrier concentration could be determined. Though the electroreflectance method has been developed for inorganic semiconductors, it could be effectively applied for organic/molecular semiconductors as well if the constituent molecules are assumed to be the lattices. The study of organic photoconductors is very important since they are more and more promising especially in photocopying, photovoltaic and solar cells.

Photoelectric properties ofp-CuInSe2 crystals

Spectra of the photoelectromotive force of p-CuInSe2 single crystals obtained by directional crystallization of a stoichiometric-composition melt in an upright ampoule have been investigated by the method of phase-sensitive detection. A difference in the phase shifts between the exciting light and the arising photoelectromotive force in the extrinsic (impurity) and intrinsic regions of optical absorption is determined. The lifetime in the region of optical self-absorption is found to equal τ=15·10−6 at T=77 K.

Analysis of fluorescence lifetime and quenching of FITC-conjugated antibodies on cells by phase-sensitive flow cytometry.

Fluorescent antibodies are often used to measure the number of receptor sites on cells. The quantitative estimate of the number of receptor sites using this procedure assumes that the fluorescence intensity on a cell is proportional to the number of bound antibodies. Quenching may invalidate this assumption. For many fluorophores, intermolecular interactions and energy transfer between molecules in close proximity to one another results in self-quenching. This effect can occur in antibody probes with a high fluorochrome to protein (F/P) ratio. It can also occur due to close proximity antibodies relative to one another on a highly labeled cell surface. Since self-quenching is accompanied by a change in the fluorescence decay and a decrease in the fluorescence lifetime, it may be conveniently identified using fluorescence lifetime spectroscopy. In this paper we apply the phase-sensitive detection method to investigate the impact of self-quenching on fluorescence lifetimes by flow cytometry, using a model system consisting of FITC conjugated anti-mouse Thy1.2 antibodies bound to murine thymus cells. We show that in addition to the expected variation of lifetimes as a function of F/P ratio of the probes, the fluorescence lifetime diminishes also as a function of antibody labeling concentration on the cell surface. This is consistent with self-quenching effects expected at high densities of FITC molecules.