Broadband Continuous Spectrum Research Articles

Single-hydrophone-based passive localization for vertically moving targets in shallow water

In this article, we consider underwater passive localization for vertically moving targets in shallow water. A method that utilizes parameters derived from interference structures is introduced, which enables the implementation of both range and depth estimations for targets with a single hydrophone. Underwater moving targets radiate broadband continuous spectrum signals by forming multipath signals through interface reflections, resulting in stable interference structures in the time-frequency domain of the received signals. Ray theory is utilized to derive the trajectory equations of the interference fringes. Considering the periodic characteristics of interference structures, a specialized homomorphic filter is designed to effectively restore the peaks of the fringes to reduce localization errors. Furthermore, with a priori information on the target's initial depth, the Hough Transform is used to extract the target's location information within interference fringes formed by direct signal and surface-reflected signal. Addressing the issue of multi-peaks in parameter domain, parameter information across multiple fringes is fused. The effectiveness and robustness of the proposed method have been demonstrated through numerical simulations, lake trial, and error assessments, even when underwater environments information is not precisely known.

Mass dependency of high-wavenumber turbulence in a linear partially magnetized plasma

We investigated the mass dependency of the high-wavenumber turbulence, which occurs at scales smaller than the ion effective Larmor radius, in a partially magnetized plasma column. In this system, two different types of fluctuations were observed: one exhibiting a coherent discrete spectrum, while the other displaying a broadband continuous spectrum. The phase velocities of both types showed a similar mass dependency, approximately matching the ion sound velocity or electron diamagnetic drift velocity. Additionally, we found that the discrete spectrum has a peak interval frequency comparable to the ion cyclotron frequency, which is consistent with ion cyclotron ranges of fluctuations, including ion Bernstein waves.

Waveguide invariant estimation based on correlation coefficient of tonal acoustic intensity interference fluctuation

The waveguide invariant is usually not extracted from the striation pattern of broadband continuous spectrum in the low frequency analysis and recording spectrum at low signal-to-noise ratio. In this paper, the correlation coefficient of multi-tonal acoustic intensity interference fluctuation is utilized to estimate waveguide invariant. Since the proportional relationship between the reference intensity interference fluctuation and the scaled intensity interference fluctuation is dependent on waveguide invariant, the waveguide invariant estimation can be done. Firstly, the tonal intensity interference fluctuation has been resampled in the new time domain under the radial and non-radial motion models respectively. Then, the similarity degree between the resampling reference tonal intensity interference fluctuation and the scaling tonal intensity interference fluctuation can be described by correlation coefficient, and the peak of correlation coefficient is corresponding to the true value. Finally, the numerical results and the experimental results prove the proposed method's effectiveness.

Novel computer-generated hologram encoding method based on partially temporal coherent light.

Partially temporal coherent light (PTCL) has been applied to holographic reconstruction to reduce speckle noise in display systems, while the encoding methods of computer-generated hologram (CGH), based on PTCL, have not been reported. We propose a novel method to encoding CGH, in which a PTCL with a broadband continuous spectrum is used to illuminate the object image. The continuous spectrum is discretized into different wavelengths and a weight value associated with PTCL power spectrum is assigned to each wavelength. The diffractive transmission is based on Fresnel diffraction theory. The phase distribution of the encoded CGH is obtained using the sum of multiplying the different CGH phase distributions of corresponding discrete wavelengths by the corresponding weight values. The modulation results without iteration are performed to verify the feasibility of the proposed method and the iterative algorithm is introduced to improve the quality of the modulation. The reconstructed images from the proposed encoding method exhibit high quality as compared with that obtained from the encoding method based on ideal temporal coherent light. Numerical simulations and optical experiments are good consistent with each other. The proposed method can provide a reference for various wave-front modulations.

Analysis of fluctuating force acting on two cylinders in different arrangements through Lattice Boltzmann Method

The force components on two circular cylinders in tandem, side-by-side and staggered arrangements and in a uniform incoming flow are investigated systematically with Reynolds number being taken Re=200. The numerical simulations are undertaken using the lattice Boltzmann method and the boundary condition is imposed though the immersed boundary method. At an intermediate centre-to-centre spacing which is based on the ratio of distance between cylinder centres and the cylinder diameter, the fluid force is periodic and dominated virtually by a single frequency component in the tandem arrangement and is random with a continuous frequency spectrum in the side-by-side arrangement. Here random means that the components of the force are not in the form of discrete frequencies, but are in the form of continuous spectrum. Detailed analysis has been undertaken to show this evolution process with the alignment angle α, which is defined as the angle between the incoming flow direction and the line linking two cylinders. As α increases, the frequency component of the force on each individual cylinder changes from a single value to multiple ones, then to a large number of discrete ones and eventually to a broadband continuous spectrum. The results have important implications to vortex induced vibration and fatigue analysis of the cylinder, which has important applications, such as risers in offshore engineering.

Tapered Yb3+-doped photonic crystal fiber for blue-enhanced supercontinuum generation

The absorption spectrum of the Yb3+-doped photonic crystal fiber covers from 770 nm to 980 nm. A less flatness broadband continuous spectrum and a short peak are generated in the uniform Yb3+-doped photonic crystal fiber. Supercontinuum from 388 nm to 1590 nm is received in the tapered Yb3+-doped photonic crystal fiber, which covers almost two octaves, generating an ultra flat (7 dB) continuous spectrum coming from 926 nm to 1554 nm in the long waveband, and receiving a 62 nm width, high and violet peak from 388 nm to 450 nm in the purple waveband by femtosecond pulse. The up conversion efficiency of pump power in tapered Yb3+-doped photonic crystal fiber is up to 59.4%, while the uniform is only 6.3%.

Numerical and Experimental Studies on the Chaotic Dynamics of Driven Gunn Oscillator

Dynamics of a microwave (X band) Gunn oscillator in presence of a periodic forcing signal has been critically studied to explore the existence of chaos in the behavior of the driven Gunn oscillator (DGO). A broad band continuous spectrum is experimentally observed at the output of the DGO just at the verge of the synchronization band for certain strength and frequency of the forcing signal. For characterisation of the obtained broad band signal, time domain analysis of the frequency down converted DGO output is done and different chaos quantifiers like Maximum Lyapunov exponent and Correlation dimension are evaluated to confirm the chaotic mode of oscillations. Numerical simulation of the system equations also indicate that the dynamics of the DGO enters into a chaotic domain at these conditions.

All-fiber spectrometer based on speckle pattern reconstruction

A standard multimode optical fiber can be used as a general purpose spectrometer after calibrating the wavelength dependent speckle patterns produced by interference between the guided modes of the fiber. A transmission matrix was used to store the calibration data and a robust algorithm was developed to reconstruct an arbitrary input spectrum in the presence of experimental noise. We demonstrate that a 20 meter long fiber can resolve two laser lines separated by only 8 pm. At the other extreme, we show that a 2 centimeter long fiber can measure a broadband continuous spectrum generated from a supercontinuum source. We investigate the effect of the fiber geometry on the spectral resolution and bandwidth, and also discuss the additional limitation on the bandwidth imposed by speckle contrast reduction when measuring dense spectra. Finally, we demonstrate a method to reduce the spectrum reconstruction error and increase the bandwidth by separately imaging the speckle patterns of orthogonal polarizations. The multimode fiber spectrometer is compact, lightweight, low cost, and provides high resolution with low loss.

Spectrum optimization-based chaotification using time-delay feedback control

In this paper, a spectrum optimization method is developed for chaotification in conjunction with an application in line spectrum reconfiguration. A key performance index (the objective function) based on Fourier spectrum is specially devised with the idea of suppressing spectrum spikes and broadening frequency band. Minimization of the index empowered by a genetic algorithm enables to locate favorable parameters of the time-delay feedback controller, by which a line spectrum of harmonic vibration can be transformed into a broad-band continuous spectrum of chaotic motion. Numerical simulations are carried out to verify the feasibility of the method and to demonstrate its effectiveness of chaotifying a 2-DOFs linear mechanical system.

SOME NUMERICAL AND EXPERIMENTAL OBSERVATIONS ON THE GROWTH OF OSCILLATIONS IN AN X-BAND GUNN OSCILLATOR

The dynamics of the onset of oscillations in a wave guide cavity based Gunn Oscillator (GO) has been critically examined through numerical simulations and experimental studies. The transition of the GO from a non-oscillatory to an oscillatory state and the same in the reverse direction occurs at different critical values of the dc bias voltage applied to the GO. In presence of a weak RF field in GO cavity, oscillations with broad band continuous spectrum and multiple discrete line spectrum are observed at the GO output for different values of dc bias below the above mentioned critical values. Analysing the numerically obtained time series data, chaos quantifiers have been obtained to prove the occurrence of the chaotic oscillations in the GO. Experimental results and observations of numerical simulation show good qualitative agreement.

Investigations on acousto-optically Q-switched frequency-shifted feedback laser with Nd:YVO_4 crystal

An acousto-optically Q-switched frequency-shifted feedback (FSF) diode-pumped laser with a Nd:YVO(4) crystal is demonstrated. The laser emits more than 1W average power with multi-ten ns duration at up to 50 kHz repetition rate, and peak power higher than 1 kW is achieved at frequency of 20 kHz. This kind of pulsed laser has special value for some applications with outstanding features of broadband continuous spectrum and chirping frequency in the oscillation spectrum. Simulation results of the FSF laser through rate equations in Q-switched regime are also presented.

Chaotic behavior of hot electron plasma in Ni-compensated Ge

Abstract Spontaneous conductivity oscillations are discovered and investigated in Ni-compensated Ge under electron heating at high dc electric field. These oscillations develop from periodic to chaotic state with the applied electric field increases and are characterized by broadband continuous spectra in the range of 4 – 12 kHz. The model discussed includes the hot electron trapping on the repulsive defect centres.

The elements of an acoustic phonetic theory.

This article describes the essential properties of an acoustic theory of phonetics and demonstrates the dependence of such a theory on physiological phonetics. The article defines the acoustical characteristics of speech and summarizes the relationships of these characteristics to physiological speech parameters and to descriptive phonetics. The acoustical characteristics of speech are defined as the speech wave types and the acoustical speech parameters. The basic speech wave types specified are quiescent, burst, quasi-random, quasi-periodic, double-periodic, and irregular-periodic. The acoustical speech parameters are separated into acoustic phonetic parameters and acoustic prosodic parameters. The basic acoustic phonetic parameters are specified as gap, voice bar, broad-band continuous spectrum, the frequencies of the vowel and consonant formants, the amplitudes of the vowel and consonant formants, and the frequencies of the consonant antiresonances. The acoustic prosodic parameters are defined as acoustic phonetic duration, average fundamental voice frequency, and average speech power. The relationships among the speech wave types and the acoustical speech parameters are shown by means of a table. The essential relationships among the speech wave types, the acoustical speech parameters, and the physiological speech parameters are also shown in a set of tables. The manner in which the speech wave types and the acoustic phonetic parameter values may be employed to identify the phone types of descriptive phonetics is briefly discussed and illustrated.