Frequency-modulated Continuous-wave Technique Research Articles

Two-wavelength digital holography using frequency-modulated continuous-wave technique for multiplexing in the time–frequency domain

Two-wavelength digital holography using frequency-modulated continuous-wave technique for multiplexing in the time–frequency domain

Machine Learning-Based Human Posture Identification from Point Cloud Data Acquisitioned by FMCW Millimetre-Wave Radar.

Human posture recognition technology is widely used in the fields of healthcare, human-computer interaction, and sports. The use of a Frequency-Modulated Continuous Wave (FMCW) millimetre-wave (MMW) radar sensor in measuring human posture characteristics data is of great significance because of its robust and strong recognition capabilities. This paper demonstrates how human posture characteristics data are measured, classified, and identified using FMCW techniques. First of all, the characteristics data of human posture is measured with the MMW radar sensors. Secondly, the point cloud data for human posture is generated, considering both the dynamic and static features of the reflected signal from the human body, which not only greatly reduces the environmental noise but also strengthens the reflection of the detected target. Lastly, six different machine learning models are applied for posture classification based on the generated point cloud data. To comparatively evaluate the proper model for point cloud data classification procedure-in addition to using the traditional index-the Kappa index was introduced to eliminate the effect due to the uncontrollable imbalance of the sampling data. These results support our conclusion that among the six machine learning algorithms implemented in this paper, the multi-layer perceptron (MLP) method is regarded as the most promising classifier.

Terahertz non-destructive testing of power generator bars with a dielectric waveguide antenna

AbstractTerahertz technologies for non-destructive testing (NDT) are continuing to find their way into the industrial sector in the context of very specific inspection tasks. Part of this development is the capability to adapt terahertz systems in such a way that they can meet the sometimes harsh challenges and requirements of real-world industrial scenarios. One such scenario is the inspection of components with limited available measurement space. In particular, we show here the terahertz NDT inspection of the mica insulation of generator bars of turbogenerators at power plants, where an early on-site detection of defects and cracks in the insulation can be crucial, but where only few centimeters of space between adjacent bars are available. To address this problem, we have developed a measurement system combining a 100 GHz all-electronic terahertz transceiver with a low-loss dielectric waveguide antenna with 90 degree tip. We achieve sub-wavelength image resolution by scanning the waveguide antenna's tip over the surface of the generator bars in a near-field measurement setup. Employing a frequency-modulated continuous wave technique, we obtain depth-resolved, volumetric terahertz images of the objects under test. We discuss here the implementation and performance of the implemented measurement system for terahertz NDT inspection. keywords: terahertz, non-destructive testing, dielectric waveguides, frequency-modulated continuous wave, millimeter waves, power generators

A Review of Sensing Technologies for Non-Destructive Evaluation of Structural Composite Materials

The growing demand and diversity in the application of industrial composites and the current inability of present non-destructive evaluation (NDE) methods to perform detailed inspection of these composites has motivated this comprehensive review of sensing technologies. NDE has the potential to be a versatile tool for maintaining composite structures deployed in hazardous and inaccessible areas, such as offshore wind farms and nuclear power plants. Therefore, the future composite solutions need to take into consideration the niche requirements of these high-value/critical applications. Composite materials are intrinsically complex due to their anisotropic and non-homogeneous characteristics. This presents a significant challenge for evaluation and the associated data analysis for NDEs. For example, the quality assurance, certification of composite structures, and early detection of the failure is complex due to the variability and tolerances involved in the composite manufacturing. Adapting existing NDE methods to detect and locate the defects at multiple length scales in the complex materials represents a significant challenge, resulting in a delayed and incorrect diagnosis of the structural health. This paper presents a comprehensive review of the NDE techniques, that includes a detailed discussion of their working principles, setup, advantages, limitations, and usage level for the structural composites. A comparison between these techniques is also presented, providing an insight into the future trends for composites’ prognostic and health management (PHM). Current research trends show the emergence of the non-contact-type NDE (including digital image correlation, infrared tomography, as well as disruptive frequency-modulated continuous wave techniques) for structural composites, and the reasons for their choice over the most popular contact-type (ultrasonic, acoustic, and piezoelectric testing) NDE methods is also discussed. The analysis of this new sensing modality for composites’ is presented within the context of the state-of-the-art and projected future requirements.

FMCW-ディジタルホログラフィによる直交直線偏光の参照光を用いた偏光分布の解析

Frequency-Modulated Continuous-Wave (FMCW) technique has been introduced into digital holography (DH) by using an injection-current-induced frequency modulation of a laser diode (LD) as a light source. Since the frequency of beat signals observed in time series holograms captured by a high speed digital camera depends on the optical path length difference between the reference and the object light waves, the introduction of FMCW technique into DH makes it possible to multiple record the object information in time-frequency domain. By the technique, a polarization state of the object wave transmitting through a PMMA sample was investigated using the proposed technique in which the object wave interfered with two orthogonally linearly polarized reference waves having the different optical path lengths.

2-D Self-Injection-Locked Doppler Radar for Locating Multiple People and Monitoring Their Vital Signs

This article presents the combination of a self-injection-locked (SIL) radar with frequency-modulated continuous-wave (FMCW) and switched phased-array (SPA) techniques to locate multiple people and detect their vital signs with high resolution and high sensitivity. The proposed system utilizes a subharmonic upconverter/downconverter with a 1.675-2.175-GHz chirp local oscillator (LO) signal to convert between a 2.45-GHz SIL IF signal and a 5.8-6.8-GHz FMCW RF signal. It transmits the FMCW RF signal and receives the echo signal using a transmit (Tx) antenna and a switched receive (Rx) antenna array, respectively, and processes the SIL IF signal to produce a range-azimuth map (RAM) that displays the positions of all people, their respiration rates (RRs), and their heart rates (HRs) in the monitoring region. The key advantage of combining the FMCW and SPA techniques is the provision of higher spatial resolution than obtained using each technique alone. This article also proposes a Doppler-weighted RAM to improve the identifiability of the images that are generated by two persons close to each other. Finally, the presented system was experimentally validated by measuring the locations and vital signs of up to five seated persons at a range of more than 1.5 m, of which two persons were seated almost shoulder by shoulder.

A High-Frequency Surface Wave Radar Simulation Using FMCW Technique for Ship Detection

Indonesia is an archipelagic country with a vast sea area. This vast sea area becomes a challenge in conducting regional surveillance to maintain maritime conditions. The use of buoys and satellites still has shortcomings in carrying out surveillance despite its excellent surveillance capabilities. A high-frequency radar technology with 3-30 MHz frequency and surface wave propagation are very suitable because it has a radar range that can cross the horizon or commonly refer to as Over the Horizon (OTH). The Frequency Modulated Continuous Wave (FMCW) technique on this radar obtains distance and velocity information by a continuously transmitted frequency modulation. The use of radar in Indonesia for marine surveillance is still infrequent. Therefore, it is relatively difficult to conduct testing and obtain data. In addition, the direct examination requires extended time, so a simulation program is needed. This paper discusses the design of a High-Frequency Surface Wave Radar (HFSWR) simulation program using FMCW modulation technique. The simulation program detected two objects based on time delays due to the distance and velocity of the object with a maximum range of 350 km. It displayed the results in an informative manner. The object detection was based on the results of the Fast Fourier Transform (FFT) from the mixed signals. The mixed signal is a combination of transmitted signal and reflected signal in which there are time delay components due to the object. The simulation program had been tested with input values of distance and velocity that vary, both for one object and two objects, in the radial direction. It generated output that was close to the input value with a level of accuracy of ± 2 km.

Fast and precise data acquisition for broadband microwave tomography systems

Microwave imaging (MWI) is a noninvasive diagnosis method, which has been investigated for a wide range of applications. MWI techniques include radar-based approaches as well as microwave tomography (MWT). One major challenge designing broadband MWI systems is the development of a data acquisition unit that allows for fast broadband scattering parameter measurements with a high measurement precision and a high dynamic range (DR), at reasonable cost. The cost-performance criteria cannot readily be achieved using commercial, continuous wave (CW) vector network analyzers (VNA) or pulse-based systems. Therefore, in this paper we propose a data acquisition unit, based on the well-known method of frequency modulated continuous wave (FMCW) network analysis. It offers fast scattering parameter measurements without compromising the measurement precision and the DR, and is particularly advantageous for MWI systems requiring a high number of frequency samples. A 2-port metadyne prototype electronics with low hardware complexity was developed, which allows very fast, precise, and accurate reflection and transmission measurements in the frequency range from 0.5 GHz to 5.5 GHz. To the best of the authors’ knowledge, a system with combined performance in terms of bandwidth, sweep time (1 ms), DR (80 dB) and maximum signal-to-noise-and-spurious ratio (65 dB) has not previously been reported. The design, the calibration, and the characterization of the prototype electronics are described in detail, and the measurement results are compared to those obtained with commercial high-end CW VNA. The advantages and limitations of the metadyne FMCW technique compared to the heterodyne CW technique are discussed. The applicability of the prototype electronics and the described calibration technique for microwave imaging has been demonstrated based on measurements using an 8-port MWT sensor and a switching matrix.

Focusing Bistatic FMCW SAR Signal by Range Migration Algorithm Based on Fresnel Approximation.

Frequency modulated continuous wave (FMCW) technique has recently been employed by synthetic aperture radar (SAR) to decrease the radar cost and volume. However, the operation range is limited by the direct energy leakage from the transmitting channel to receiving channel due to the operation principle of FMCW technique. Bistatic configuration is an efficient way to increase the isolation between the transmitter and receiver, which could significantly increase the radar standoff range. A bistatic FMCW SAR spectrum model is proposed by using the Fresnel approximation in this paper. This model is similar to that of a monostatic FMCW SAR spectrum, which allows the existing imaging algorithms to be used on bistatic image processing. Based on the new model and the characteristics of FMCW signal, a modified range migration algorithm (RMA) for FMCW SAR is proposed to focus the image, which requires less memory and computational load than the traditional RMA. Point-target simulation is used to verify the proposed spectral model and real data processing verified the effectiveness of the proposed RMA.

Characterization and validation of the frequency-modulated continuous-wave technique for assessment of photon migration in solid scattering media

The frequency-modulated continuous-wave (FM-CW) technique, based on the beat signal of a Mach–Zehnder interferometer employing a frequency-ramped light source, is studied for solid scattering media applications. The method is used to evaluate the mean time-of-flight (MTOF) of light traveling in scattering media, specifically polystyrene foams. We assume that each the time-of-flight (TOF) time corresponds to different light scattering paths resulting in a different phase shift. The phase shift variations produce a speckle pattern, which together with the frequency leakage induced by the discrete Fourier transform (DFT) cause “spikes” in the power spectrum of the beat signal, thus decreasing the accuracy of the measured MTOF values in solid scattering media. For comparison, time-of-flight spectroscopy (TOFS) is also employed to evaluate the MTOF for the same samples, while the geometrical difference between these two techniques is compensated for by using diffusion theory. The MTOFs measured by the FMCW and TOFS techniques agree well, which demonstrates a great potential to develop a robust FMCW setup for simplified MTOF assessment.

Optimization of the frequency-modulated continuous wave technique for referencing and multiplexing intensity-based fiber optic sensors

In this paper, we demonstrate the interrogation of fiber optic intensity sensors by using the combination of the frequency-modulated continuous wave concept with the spectral selective reflectivity of fiber Bragg gratings. Thus, we multiplex these kinds of sensors with this technique having simultaneously a referenced system. The basis of this dual functionality is described and results are presented for the case of interrogation of two multiplexed intensity sensors. Their evaluation permits to establish the conditions to address a sensor network of this type. Also, it is proposed a strategy to implement this sensing approach without the requirement of using optical fiber delay lines in the sensor heads.

Intensity-modulated linear-frequency-modulated continuous-wave lidar for distributed media: fundamentals of technique

We analyze the intensity-modulation frequency-modulated continuous-wave (FMCW) technique for lidar remote sensing in the context of its application to distributed media. The goal of the technique is the reproduction of the sounded-medium profile along the emission path. A conceptual analysis is carried out to show the problems the basic version of the method presents for this application. The principal point is the appearance of a bandpass filtering effect, which seems to hinder its use in this context. A modified version of the technique is proposed to overcome this problem. A number of computer simulations confirm the ability of the modified FMCW technique to sound distributed media.

Irregular sampling to reduce the limitations induced by the nonlinearities in wavelength scanning

A new numerical method for reducing the effects of the nonlinearity signal in the distance measurements by the frequency-modulated continuous wave technique is presented. It is shown that the phase shift of the interferogram produced by a light-scanning signal could be efficiently considered as polynomial. In the distance measurement method considered, the information is carried by the first-order polynomial phase signal. Our method eliminates the high orders of the polynomial pulse, which allows an improvement of the resolution. First results are reported in order to illustrate the new technique.

A PC-Controlled Nonincremental Distance Meter Based on a Comb-Spectrum Combined With a Frequency-Modulated Continuous Wave Interferometer

In this paper, a PC-controlled nonincremental interferometer that integrates frequency-modulated continuous wave (FMCW) and dispersive comb spectrum (DCS) techniques is presented. This system takes advantage of the wide measuring range of the FMCW technique and of the high resolution of the DCS technique. The system prototype presented in this paper performs measurements over a range of 30 mm with an accuracy of about 600 nm. The proposed combined interferometer is expected to also have applications in biomedical frameworks.

Atomic transition spectra markers for accurate frequency-modulated continuous-wave laser distance-meter

Atomic transition spectra are adopted to improve accuracy of a laser distance-meter based on frequency-modulated continuous-wave technique. A pair of lines in hyperfine structure of rubidium (Rb)–D2 atomic absorption spectrum are employed as stable frequency markers to calibrate the frequency sweep span of the measuring laser. To locate spectral peaks, the third derivative of the absorption signals taken by frequency scanning the laser is numerically gained using the sampled Lamb-dip profiles. The effectiveness and the utility of the present method are demonstrated by excellent results of block gauge length measurements. In spite of a sweep span (∼7 GHz) which is much narrower than that of a distance-meter system employing a wide-tunable external cavity laser diode, a relative error of 2.8×10−4 (σ=28 μm) at 100 mm is obtained.

Laser-diode phase-conjugate interferometer for distance measurement

A laser-diode (LD) phase-conjugate interferometer with two self-pumped (cat) phase-conjugate mirrors instead of two ordinary mirrors is constructed for distance measurement over extended ranges. The measurement is based on frequency-modulated continuous-wave techniques. A wide-tunable LD with an external cavity is used as a light source in a phase-conjugate wavefront-matched interferometer. The interferometer is insensitive to spatially nonuniform phase distortion, but can only detect the spatially uniform phase change introduced by the displacement of one cat mirror. The linear regression of the measured beat frequency as a function of the distance has been ascertained.

Improvement of the FMCW laser range-finder by an APD working as an optoelectronic mixer

A laser range-finder has been designed for distances from 1 to 20 m with diffusing targets. The measurement principle using a frequency modulation continuous wave technique is first described. Improvement for short distance measurements is then proposed with a new set-up using a delay line and an avalanche photodiode working as an optoelectronic mixer used to obtain a wide-band and a low-noise photoreception.

Multiplexing of optical fiber gas sensors with a frequency-modulated continuous-wave technique

We report on the use of a frequency-modulated continuous-wave technique for multiplexing optical fiber gas sensors. The sensor network is of a ladder topology and is interrogated by a tunable laser. The system performance in terms of detection sensitivity and cross talk between sensors was investigated and found to be limited by coherent mixing between signals from different channels. The system performance can be improved significantly by use of appropriate wavelength modulation-scanning coupled with low-pass filtering. Computer simulation shows that an array of 37 acetylene sensors with a detection accuracy of 2000 parts in 10(6) for each sensor may be realized. A two-sensor acetylene detection system was experimentally demonstrated that had a detection sensitivity of 165 parts in 10(6) for 2.5-cm gas cells (or a minimum detectable absorbance of 2.1 x 10(-4)) and a cross talk of -25 dB.

Reflectometric frequency-modulation continuous-wave distributed fiber-optic stress sensor with forward coupled beams

A distributed optical-fiber stress sensor whose principle of operation is based on the frequency-modulation continuous-wave technique is reported. The sensor consists of a length of birefringent fiber with a mirror attached to one end, a diode laser, and a p-i-n photodiode detector. The intensity and the location of an applied stress are determined simultaneously by detecting the amplitude and the frequency of the beat signal, which is produced by two forward-coupled mode beams. The system was found to have a reasonable spatial resolution of 0.85 m (rms error) in a sensing range of 100 m. The advantages and limitations of the sensor are also discussed.

Distributed temperature-change sensor based on Rayleigh backscattering in an optical fiber

A frequency-modulated continuous-wave technique is used to detect the presence of frequency shifts in the Rayleigh-backscattered light in a single-mode optical fiber as a result of a changing temperature. The system is able to detect a rate of temperature change of 0.014 K/s, when a 20-cm length of fiber is heated. The system is also able to demonstrate a spatial resolution of better than 15 cm.