Frequency Modulation Bandwidth Research Articles

Research on the OFDR strain measurement method based on similarity features of dual-segment RSS.

Optical frequency domain reflectometry (OFDR) is a research hotspot in fiber optic sensing technology. This technology can be used for strain, vibration and temperature sensing and has great application prospects in fields such as deformation analysis of aerospace components and bridge monitoring. This article analyzes the reasons for strain demodulation errors under large strains. In response to the problem of reduced similarity between the reference state signal and the measured state signal, a strain measurement method based on the similarity feature of a double-segment Rayleigh scattering spectrum is proposed. Local segments at both ends of the reference state signal are used as new fingerprint spectra, and the offset of the measured state signal similarity spectrum is synchronously searched after extension. At the same time, by revealing the mechanism of strain edge demodulation errors, a strain edge optimization method based on automatic adjustment of the sliding window center position is proposed. A comparison experiment was conducted with traditional methods to verify the effectiveness of the above method. Finally, a sensing unit length of 32.6 mm was achieved with a frequency modulation bandwidth of 5 nm, and the measurement range was from ± 2000 µɛ to ± 2500 µɛ. The measurable spectral offset was increased from 48% to 60%, with a maximum standard deviation of 1.9 µɛ.

Absorption-diffusion integrated stacked metamaterials by multi-compound strategy for broadband electromagnetic attenuation

The meta-structural design of flat absorbers demonstrates significant potential for improving microwave attenuation performance. However, the single mechanism of amplitude attenuation through resonance-enhanced effect necessitates considerable material thickness, restricting its application in electromagnetic (EM) defense. Utilizing the synergistic effect of multiple mechanisms is expected to break through the performance limits of microwave stealth metamaterials at the sub-wavelength scale. Herein, absorption-diffusion integration stacked metamaterials (ADISM) were designed using a multi-compound strategy, which comprises bionic porous spherical carbonyl iron powder (SCIP) coatings and stacked vortex metasurfaces (VMs). Such a metamaterial integrates EM scattering modulation with microwave absorption, enabling the creation of a broadband, high-efficiency, low-reflection design, providing an effective absorption bandwidth (EAB) reaching 10.42 GHz (7.58–18 GHz) at a thickness of 2.48 mm. Manipulating the orbital angular momentum (OAM) to effect wavefront transformation, the dissipation of EM wave energy is achieved. The absorption peak frequency modulation and absorption bandwidth broadening are achieved by the intrinsic loss capability of the porous coating, and corresponding mechanisms are demonstrated by simulation models. In short, the synergistic effects of microwave absorption and scattering modulation significantly enhance the return loss capability and facilitate broadband microwave attenuation. This research offers new insights into the multifunctional integration of EM stealth metamaterials.

Performance Analysis of Six Electro-Optical Crystals in a High-Bandwidth Traveling Wave Mach-Zehnder Light Modulator

In this study, a traveling wave Mach-Zehnder intensity modulator (TW-MZM) was designed and optimized for six different electro-optical (EO) crystals: lithium niobate (LNB), potassium niobate (KNB), lithium titanate (LTO), beta barium borate (BBO), cadmium telluride (CdTe), and indium phosphide (InP). The performance of each EO crystal, including optical and radio frequency (RF) loss, applied voltage, and modulation bandwidth, was estimated and compared. The results suggest that, in theory, KNB, LTO, BBO, and CdTe have the potential to outperform LNB. However, it should be noted that the loss associated with KNB and LTO is comparable to that of LNB. The findings demonstrated that BBO and CdTe exhibit a modulation bandwidth exceeding 100 GHz and demonstrate the lowest loss among the considered crystals based on the assumed geometry.

Extreme Ultra‐Wideband Optoelectronic Frequency‐Modulated Continuous‐Wave Terahertz Radar

AbstractA novel photonic terahertz measurement system based on a frequency‐modulated continuous‐wave (FMCW) radar approach is presented. In previous works, fast frequency modulation has been demonstrated in connection with a continuous wave terahertz spectroscopy setup based on the photomixing principle. In this paper, a terahertz radar based on both a photomixing transmitter and a photomixing receiver, in contrast to the rigid spectroscopy approach, is reported. Hereby, frequency modulation bandwidths of more than 1.65 THz in radar operation is achieved. This corresponds to an order of magnitude more than what is previously achieved by terahertz radar systems. At the same time, measurement rates can be achieved that are comparable with radar systems based on Monolithic Microwave Integrated Circuits (MMICs) according to the current state of the art. Within the scope of the work, two operating modes are realized, one with a measurement rate of about 560 Hz at 600 GHz modulation bandwidth and one with 200 Hz at 1.65 THz modulation bandwidth, which can be set within the spectrum from 50 GHz to about 4.5 THz. The possibility to adjust the operating range of the radar without necessary hardware adaptations is another unique feature of the presented system, which allows the operator to choose a suitable frequency band that corresponds best to a certain measurement scheme via software settings. Besides the potential for multi‐layer thickness inspections, the capabilities of this technique for terahertz imaging applications are presented.

Highly-time-resolved FMCW LiDAR with synchronously-nonlinearity-corrected acquisition for dynamic locomotion.

Highly-time-resolved and precise tracking of position, velocity, and acceleration is urgently required when highly dynamic legged robots are walking, trotting, and jumping. Frequency-modulated continuous-wave (FMCW) laser ranging is able to provide precise measurement in short distance. However, FMCW light detection and ranging (LiDAR) suffers from a low acquisition rate and poor linearity of laser frequency modulation in wide bandwidth. A sub-millisecond-scale acquisition rate and nonlinearity correction in the wide frequency modulation bandwidth have not been reported in previous studies. This study presents the synchronous nonlinearity correction for a highly-time-resolved FMCW LiDAR. The acquisition rate of 20 kHz is obtained by synchronizing the measurement signal and the modulation signal of laser injection current with a symmetrical triangular waveform. The linearization of laser frequency modulation is conducted by resampling of 1000 intervals interpolated in every up-sweep and down-sweep of 25 µs, while measurement signal is stretched or compressed in every period of 50 µs. The acquisition rate is demonstrated to be equal to the repetition frequency of laser injection current for the first time to the best of authors' knowledge. This LiDAR is successfully used to track the foot trajectory of a jumping single-leg robot. The high velocity up to 7.15 m/s and high acceleration of 365 m/s2 are measured during the up-jumping phase, while heavy shock takes place with high acceleration of 302 m/s2 as the foot end strikes the ground. The measured foot acceleration of over 300 m/s2, which is more than 30 times gravity acceleration, is reported on a jumping single-leg robot for the first time.

Narrowband and wideband frequency modulation spectral characterization and bandwidth calculation

Frequency modulation (FM) is a class of analog modulation techniques that rely on varying the instantaneous frequency of a radio-frequency (RF) carrier in accordance with the baseband signal. The amplitude of the FM signal is constant, while its instantaneous frequency is time variant; therefore, the zero crossing rate (ZCR) of the carrier differs at a rate proportional to the Nyquist rate of the baseband signal. Unlike amplitude modulation (AM), FM is a nonlinear modulation technique; therefore, the calculation of the spectrum of an FM signal is expected to be a tedious mathematical problem.

SIGNAL FORMATION AND PROCESSING FEATURES FROM AUTODYNE RADAR WITH A WIDE FREQUENCY MODULATION BAND. (РART 1)

Subject and Purpose. In Part 1 of the paper, a mathematical model of an autodyne self-oscillator with frequency tuning by varactor capacitance varying is built and thoroughly analyzed for the features of signal formation in autodyne radar with a wide frequency-modulation bandwidth and a nonlinearity in the modulation characteristic. The aim of the study is to appreciate the action that the nonlinearity of the oscillator modulation characteristic exerts on the spectral characteristics of signals from frequency-modulation autodyne radar. Methods and Methodology. The research method is a mathematical analysis of the operation of an autodyne oscillator with electronic frequency tuning. To examine formation processes of emitted autodyne signals, the spectral, frequency and amplitude characteristics of signals from frequency-modulation autodyne radar are constructed with the use of numerical modeling techniques. Results. Numerical modeling of autodyne response signal spectra has been performed for various distances to the reflecting object and different modulating voltages across the varactor. It has been shown that a nonlinear dependence of the generator frequency on the varactor voltage makes for the broadening of the autodyne response signal spectrum. It has been found that as the object distance increases, the frequency of the autodyne response signal moves towards the higher frequencies, while the nonlinearity makes the spectrum broaden. The obtained calculation results refer to an 8-mm Gunn diode autodyne. Conclusion. The performed research of the spectral characteristics and into the features of signal formation in autodyne transceiver devices with a wide frequency tuning has shown that in order to achieve high resolution figures from autodyne radar, certain methods are needed to be developed for adjusting the laws of frequency modulation and for the processing of response signals from reflecting objects. Such a method and ways to solve these problems will be presented in Part 2 of the work.

Ultra-wideband signal generation and fusion algorithm for high-resolution terahertz FMCW radar imaging.

The terahertz frequency modulated continuous wave (THz FMCW) imaging has proved to be a novel nondestructive testing (NDT) technology for non-metal materials, and the large bandwidth is usually required to meet high range resolution demands in many applications such as multilayer sample under test (SUT). However, broadband THz hardware is difficult to design. In this paper, an ultra-wideband THz FMCW generation method is proposed, which provides frequency modulation bandwidths of up to 386 GHz by time-division multiplexing. Furthermore, an ultra-wideband signal fusion algorithm (USFA) is also proposed and significantly improves the range resolution to 0.46 mm in air. Results from the artificially constructed multilayer structure demonstrate the superiority and effectiveness of our method quantitatively.

Aircraft Target Classification Method for Conventional Narrowband Radar Based on Micro-Doppler Effect

For a conventional narrowband radar system, its insufficient bandwidth usually leads to the lack of detectable information of the target, and it is difficult for the radar to classify the target types, such as rotor helicopter, propeller aircraft, and jet aircraft. To address the classification problem of three different types of aircraft target, a joint multifeature classification method based on the micro-Doppler effect in the echo caused by the target micromotion is proposed in this paper. Through the characteristics analysis of the target simulation echoes obtained from the target scattering point model, four features with obvious distinguishability are extracted from the time domain and frequency domain, respectively, that is, flicker interval, fractal dimension, modulation bandwidth, and second central moment. Then, a support vector machine model will be applied to the classification of the three different types of aircraft. Compared with the conventional method, the proposed method has better classification performance and can significantly improve the classification probability of aircraft target. The simulations are carried out to validate the effectiveness of the proposed method.

A high-precision range extraction method using an FM nonlinear kernel function for DFB-array-based FMCW lidar

Frequency modulated continuous wave (FMCW) lidar is a high-precision absolute distance measurement technology in which the resolution is proportionate to the frequency modulation (FM) bandwidth of the light source. The distributed feedback (DFB) array exhibits advantages that meet FMCW lidar, such as a wide FM range, narrow linewidth, high FM speed, and low cost. However, the existing range extraction method for DFB array-based FMCW lidar has problems such as signal splicing error, residual FM nonlinearity, high sample rate requirement, and dispersion mismatch. Here, we propose and demonstrate an FM nonlinear kernel function-based range extraction method, which improves the measurement precision of DFB array-based FMCW lidar. The distance to a target located at 5 m is experimentally measured using this method, and a precision of 1.9 μm is obtained, which is much better than the existing range extraction method under the same conditions.

Hybrid integrated low-noise linear chirp frequency-modulated continuous-wave laser source based on self-injection to an external cavity

A hybrid integrated low-noise linear chirp frequency-modulated continuous-wave (FMCW) laser source with a wide frequency bandwidth is demonstrated. By employing two-dimensional thermal tuning, the laser source shows frequency modulation bandwidth of 10.3 GHz at 100 Hz chirped frequency and 5.6 GHz at 1 kHz chirped frequency. The intrinsic linewidth of 49.9 Hz with 42 GHz continuous frequency tuning bandwidth is measured under static operation. Furthermore, by pre-distortion linearization of the laser source, it can distinguish 3 m length difference at 45 km distance in the fiber length measurement experiment, demonstrating its application potential in ultra-long fiber sensing and FMCW light detection and ranging.

Automatic identification of epileptic seizure signal using optimized added kernel support vector machine (OAKSVM)

In this work, empirical mode decomposition (EMD)-based optimized added kernel least square support vector machine (OAKLSSVM) hybridized model is proposed for automatic identification of epileptic electroencephalogram (EEG) signals where the kernel parameters are being optimized using water cycle algorithm (WCA). The proposed model with EMD decomposition and WCA optimization together is known as EMD-OAKLSSVM-WCA. Here, two kernel functions i.e., radial basis function and wavelet kernel functions are deployed together to form the added kernel framework. From EMD, intrinsic mode functions (IMFs) are obtained where Hilbert transform (HT) is used to obtain analytic form of IMFs. For classifying seizure and non-seizure EEG signals, the frequency modulation bandwidth and amplitude modulation bandwidth parameters are obtained from the analytical IMFs and are used as features for the OAKLSSVM model. The experimental results validate the efficiency of the proposed model which provides better classification accuracy (99.33%) as compared to different promising classifiers and some state-of-the-art models.

Modeling of detection techniques for FMCW lidar using OptiSystem

In this paper, we investigated the frequency modulation continuous wave (FMCW) lidar rang measurement principle, then designed a simulated distance measurement scheme and building the model. Using the OptiSystem system modeling simulation, the relationship between frequency modulation (FM) bandwidth, FM time and rang measurement accuracy is investigated, and the effect of different sweep parameters on the accuracy of the final rang measurement results is discussed in the time and frequency domains. The simulation tests the delay of the target at 30, 60 and 90 meters respectively, and the distance is derived by beating the frequency of the optical echo signal with the local oscillation optical signal and calculating the beat frequency.

Laser frequency scanning interferometry based on estimating signal parameters via rotational invariance technique

The laser frequency scanning interferometry, as a non-contact method, has non-ranging blind zone and achieves multi-target testing in a single measurement. The beat frequency of target can be extracted by Fourier transform, and then the distance can be solved. However, due to the limitation of laser frequency modulation bandwidth, the resolution of target obtained by Fourier transform is limited to the inherent resolution. In order to solve this problem, in this paper we propose to use the estimating signal parameter via rotational invariance technique (ESPRIT) to perform spectrum analysis on the measured signal. In the experiment, the resampling method is adopted to correct the non-linearity of the measured signal beat frequency, and then the ESPRIT algorithm is used to obtain the target distance. The results show that the Fourier transform algorithm cannot distinguish the target signal from the frequencies of adjacent target, but the ESPRIT algorithm can do. The thickness of the measured target is 2.08 mm. This provides ideas for measuring, such as damage point in the proximity of the fiber, height of thin step, or small hole.

Epilepsy seizure detection using kurtosis based VMD’s parameters selection and bandwidth features

This paper presents an automated seizure detection method based on variational mode decomposition (VMD). In VMD, the number of decomposed modes K and the penalty coefficient α are selected empirically based on experience and observation which impacts its adaptability. To overcome this difficulty, we have proposed a novel method based on kurtosis to select K and α automatically for VMD decomposition of EEG signals. Primarily, the five sets of EEG data obtained from Bonn University database are decomposed into bandlimited intrinsic mode functions (BIMFs) using VMD with parameters K and α selected using the proposed kurtosis method, then amplitude modulation bandwidth (AMBω), frequency modulation bandwidth (FMBω) and spectral features of VMD decomposed BIMFs are evaluated. The significant features are obtained using Kruskal-Wallis test and fed to four different classifiers including Decision Tree (DT), K-Nearest Neighbor (KNN), Support Vector Machine (SVM) and Random Forest (RF) for five clinically relevant classification cases. Experimental results show that our presented work deals efficiently with all the five classification cases and accuracies greater than or equal to 98.7% have been achieved using RF classifier. Finally, in comparison with related works, our proposed seizure detection scheme performs better with higher accuracies in all the five cases.

40 W, 780 nm laser system with compensated dual beam splitters for atom interferometry.

We demonstrate a narrow-linewidth 780 nm laser system with up to 40W power and a frequency modulation bandwidth of 230 MHz. Efficient overlap on nonlinear optical elements combines two pairs of phase-locked frequency components into a single beam. Serrodyne modulation with a high-quality sawtooth waveform is used to perform frequency shifts with >96.5% efficiency over tens of megahertz. This system enables next-generation atom interferometry by delivering simultaneous, Stark-shift-compensated dual beam splitters while minimizing spontaneous emission.

Multilayer Thickness Measurements below the Rayleigh Limit Using FMCW Millimeter and Terahertz Waves

We present thickness measurements with millimeter and terahertz waves using frequency-modulated continuous-wave (FMCW) sensors. In contrast to terahertz time-domain spectroscopy (TDS), our FMCW systems provide a higher penetration depth and measurement rates of several kilohertz at frequency modulation bandwidths of up to 175 GHz. In order to resolve thicknesses below the Rayleigh resolution limit given by the modulation bandwidth, we employed a model-based signal processing technique. Within this contribution, we analyzed the influence of multiple reflections adapting a modified transfer matrix method. Based on a brute force optimization, we processed the models and compared them with the measured signal in parallel on a graphics processing unit, which allows fast calculations in less than 1 s. TDS measurements were used for the validation of our results on industrial samples. Finally, we present results obtained with reduced frequency modulation bandwidths, opening the window to future miniaturization based on monolithic microwave integrated circuit (MMIC) radar units.

Epileptic Seizure Detection Based on Bandwidth Features of EEG Signals

Detection of seizure from EEG signals may help neurologist analyze the condition of epileptic patients. Variance in types of epileptic seizures provides challenge in recognizing the pattern of seizure signals from normal ones. In this paper, we discuss the classification of seizure and non-seizure conditions in epilepsy based on bandwidth features of EEG signals. These features were extracted using Empirical Mode Decomposition (EMD). This method decomposes signal into eight Intrinsic Mode Functions (IMFs) and its residue. The IMFs is then analyzed by Hilbert transform to obtain amplitude modulation bandwidth (BAM) and frequency modulation bandwidth (BFM). These features are fed as input to Support Vector Machine (SVM). We propose to combine the first four IMFs to extract bandwidth features and we implement this method on two data sets, public data set that contains signals from both extracranial and intracranial EEG, and data set OF Rumah Sakit Universitas Airlangga (RSUA) that contains signals from extracranial EEG only. The results showed that in general, the values of accuracy and specificity using combined features of first four IMFs outperformed those using features of single IMF, for all SVM kernel functions. The best accuracy and specificity on public data set were 97.3% and 98.25% respectively using RBF kernel, while the best sensitivity was gained using polynomial kernel based on only IMF1 bandwidth features by 99.0%. The experiments on RSUA data showed that the best accuracy was 96.5% and the best specificity was 99.38% when using Mexican Hat kernel based on combined features of first four IMFs, while the best sensitivity was 94% when using polynomial kernel based on only IMF1 bandwidth features.

Small and Large Signal Analysis of Photonic Crystal Fano Laser

We analyze the small- and large-signal response of a photonic crystal Fano laser. Conventional current modulation, as well as modulation of the laser via the mirror, is investigated using a numerical approach as well as linear small-signal analysis. The results show that the amplitude modulation bandwidth of one of the laser ports (the through-port) and the frequency modulation bandwidth of another port (the cross-port) extend to the THz region. Large-signal simulations of the laser response to a 500 Gb/s pseudo-random bit sequence modulation of the nanocavity are in good agreement with the predictions of the small-signal analysis. Finally, it is investigated how the design of the Fano mirror, in particular, the quality-factor ( Q ) of the nanocavity, affects the modulation properties.

Modes, stability, and small-signal response of photonic crystal Fano lasers.

Photonic crystal Fano lasers have recently been realised experimentally, showing useful properties such as pinned single-mode lasing and passive pulse generation. Here the fundamental properties of the modes of the Fano laser are analysed, showing how the laser functionality depends sensitively on the system configuration. Furthermore the laser stability is investigated and linked to the small-signal response, which shows additional dynamics that cannot be explained with a conventional rate equation model, including a damping of relaxation oscillations and a frequency modulation bandwidth that is only limited by the nanocavity response.