Shape Of Transfer Function Research Articles

Flame transfer function shaping for robust thermoacoustic systems: Application to a kinematic flame model

This work presents a data-driven optimization technique for the optimization of the flame response within a low-order acoustic network model. The methodology exploits the Nyquist criterion to compute a measure of thermoacoustic robustness at targeted frequencies, which serves as an objective value for the optimized system. The method is demonstrated using a simple Rijke tube model coupled with a [Formula: see text]-equation solver to model flame dynamics. The approach is shown to efficiently increase the stability margin of the system by modifying the flame transfer function. The methodology is applied to two examples, based on which possible scenarios are discussed and the potential and limitations associated with the practical implementation of the method are analyzed.

Transfer function asymmetry in Fabry-Perot-based optical pressure sensors.

Optical resonators are some of the most promising optical devices for manufacturing high-performance pressure sensors for photoacoustic imaging. Among these, Fabry-Perot (FP)-based pressure sensors have been successfully used for a multitude of applications. However, critical performance aspects of FP-based pressure sensors have not been studied extensively, including the effects that system parameters such as beam diameter and cavity misalignment have on transfer function shape. Here, we discuss the possible origins of the transfer function asymmetry, ways to correctly estimate the FP pressure sensitivity under practical experimental conditions, as well as show the importance of proper assessments for real-world applications.

High quality factor phase gradient metasurfaces.

Dielectric microcavities with quality factors (Q-factors) in the thousands to billions markedly enhance light-matter interactions, with applications spanning high-efficiency on-chip lasing, frequency comb generation and modulation and sensitive molecular detection. However, as the dimensions of dielectric cavities are reduced to subwavelength scales, their resonant modes begin to scatter light into many spatial channels. Such enhanced scattering is a powerful tool for light manipulation, but also leads to high radiative loss rates and commensurately low Q-factors, generally of order ten. Here, we describe and experimentally demonstrate a strategy for the generation of high Q-factor resonances in subwavelength-thick phase gradient metasurfaces. By including subtle structural perturbations in individual metasurface elements, resonances are created that weakly couple free-space light into otherwise bound and spatially localized modes. Our metasurface can achieve Q-factors >2,500 while beam steering light to particular directions. High-Q beam splitters are also demonstrated. With high-Q metasurfaces, the optical transfer function, near-field intensity and resonant line shape can all be rationally designed, providing a foundation for efficient, free-space-reconfigurable and nonlinear nanophotonics.

RF-Photonic Filters via On-Chip Photonic–Phononic Emit–Receive Operations

The creation of high-performance narrowband filters is of great interest for many RF-signal processing applications. To this end, numerous schemes for electronic, MEMS-based, and microwave photonic filters have been demonstrated. Filtering schemes based on microwave photonic systems offer superior flexibility and tunability to traditional RF filters. However, these optical-based filters are typically limited to GHz-widths and often have large RF insertion losses, posing challenges for integration into high-fidelity radiofrequency circuits. In this article, we demonstrate a novel type of microwave filter that combines the attractive features of microwave photonic filters with high-Q phononic signal processing using a photonic-phononic emit-receive process. Through this process, a RF signal encoded on a guided optical wave is transduced onto a GHz-frequency acoustic wave, where it may be filtered through shaping of acoustic transfer functions before being re-encoded onto a spatially separate optical probe. This emit-receive functionality, realized in an integrated silicon waveguide, produces MHz-bandwidth band-pass filtering while supporting low RF insertion losses necessary for high dynamic range in a microwave photonic link. We also demonstrate record-high internal efficiency for emit-receive operations of this type, and show that the emit-receive operation is uniquely suitable for the creation of serial filter banks with minimal loss of fidelity. This photonic-phononic emitter-receiver represents a new method for low-distortion signal-processing in an integrated all-silicon device.

Contrast computation methods for interferometric measurement of sensor modulation transfer function

Accurate measurement of image-sensor frequency response over a wide range of spatial frequencies is very important for analyzing pixel array characteristics, such as modulation transfer function (MTF), crosstalk, and active pixel shape. Such analysis is especially significant in computational photography for the purposes of deconvolution, multi-image superresolution, and improved light-field capture. We use a lensless interferometric setup that produces high-quality fringes for measuring MTF over a wide range of frequencies (here, 37 to 434 line pairs per mm). We discuss the theoretical framework, involving Michelson and Fourier contrast measurement of the MTF, addressing phase alignment problems using a moiré pattern. We solidify the definition of Fourier contrast mathematically and compare it to Michelson contrast. Our interferometric measurement method shows high detail in the MTF, especially at high frequencies (above Nyquist frequency). We are able to estimate active pixel size and pixel pitch from measurements. We compare both simulation and experimental MTF results to a lens-free slanted-edge implementation using commercial software.

Wheelset curving guidance using H∞ control

ABSTRACTThis study shows how to design an active suspension system for guidance of a rail vehicle wheelset in curve. The main focus of the study is on designing the controller and afterwards studying its effect on the wheel wear behaviour. The controller is designed based on the closed-loop transfer function shaping method and control strategy. The study discusses designing of the controller for both nominal and uncertain plants and considers both stability and performance. The designed controllers in Simulink are then applied to the vehicle model in Simpack to study the wheel wear behaviour in curve. The vehicle type selected for this study is a two-axle rail vehicle. This is because this type of vehicle is known to have very poor curving performance and high wheel wear. On the other hand, the relative simpler structure of this type of vehicle compared to bogie vehicles make it a more economic choice. Hence, equipping this type of vehicle with the active wheelset steering is believed to show high enough benefit to cost ratio to remain attractive to rail vehicle manufacturers and operators.

A NON-PARAMETRIC APPROACH TO CONSTRAIN THE TRANSFER FUNCTION IN REVERBERATION MAPPING

ABSTRACT Broad emission lines of active galactic nuclei stem from a spatially extended region (broad-line region, BLR) that is composed of discrete clouds and photoionized by the central ionizing continuum. The temporal behaviors of these emission lines are blurred echoes of continuum variations (i.e., reverberation mapping, RM) and directly reflect the structures and kinematic information of BLRs through the so-called transfer function (also known as the velocity-delay map). Based on the previous works of Rybicki and Press and Zu et al., we develop an extended, non-parametric approach to determine the transfer function for RM data, in which the transfer function is expressed as a sum of a family of relatively displaced Gaussian response functions. Therefore, arbitrary shapes of transfer functions associated with complicated BLR geometry can be seamlessly included, enabling us to relax the presumption of a specified transfer function frequently adopted in previous studies and to let it be determined by observation data. We formulate our approach in a previously well-established framework that incorporates the statistical modeling of continuum variations as a damped random walk process and takes into account long-term secular variations which are irrelevant to RM signals. The application to RM data shows the fidelity of our approach.

Analysis of wavefront coding imaging with cubic phase mask decenter and tilt.

This paper has examined how the decenter and tilt of a cubic phase mask plate influence the imaging of a wavefront coding system. The calculated phase term of pupil function with mask decenter and tilt indicates that both decenter and tilt change the shape of the system modulation transfer function in a predictable way by changing the phase and defocus parameters. Simulation in an on-axis three-mirror Cassegrain system is presented to confirm the calculated formula result. Experimental results for mask decenter are also presented. The results demonstrate that the decenter of a phase mask has less effect on the point spread function in the z direction than the x and y directions.

Investigation of the coupling between pump amplitude noise and differential phase noise in an Er,Yb:glass two-polarization dual-frequency solid-state laser

We report experimental evidence of the coupling between the pump diode current and the output differential phase in a two-axis two-polarization dual-frequency solid-state laser. The effect has the same transfer function shape as that for a pump current to output power one, suggesting a pump AM/laser PM coupling in the laser medium. We report that the pump amplitude noise is negligible in the laser differential phase noise assuming a linear coupling. (C) 2016 Optical Society of America

A study on the seismic segmentation method of levee using the natural frequency of multiple layers

The Levees of Shinano River were subjected to many damages at Niigata Chuetsu Earthquake in 2004 as well as Tohoku-Pacific Ocean Earthquake in 2011. The damage of levees depends not only on the distance from the epicenter, but also on the soil profile of levee and its foundation. In this study, we classified the damage of levee into five types, and investigated the relationship the geological categorization and the seismic characteristics of the levee with the damage types. The natural frequency for multiple layers and the shape of transfer function were adopted as the seismic index, because the natural frequency can be express the seismic stiffness of levee. To obtain natural frequency of multiple layers, we carried out the linear elastic analysis based on multiple reflection theory using the results of standard penetration and laboratory soil tests. Those investigations of levee have already performed at each 1 km, for every main rivers managed by MLIT, therefore the new investigations are not required. However, the interval of 1 km is not enough for management length in Japan sea side region, because of its geological complexity compared with the Pacific Ocean side. To specify the representative levee length for one borehole logging, the geomorphologic data was considered. To estimate the damage rank, the influences of magnitude of ground motion should be considered. In this study, the multiple linear regression analysis, which the base acceleration, the natural frequency and the shape index of transfer function were adopted as dependent variables, and its explaining variable was damage rank, was performed to prediction of damage rank.

Formant measurement in children’s speech based on spectral filtering

Children’s speech presents a challenging problem for formant frequency measurement. In part, this is because high fundamental frequencies, typical of a children’s speech production, generate widely spaced harmonic components that may undersample the spectral shape of the vocal tract transfer function. In addition, there is often a weakening of upper harmonic energy and a noise component due to glottal turbulence. The purpose of this study was to develop a formant measurement technique based on cepstral analysis that does not require modification of the cepstrum itself or transformation back to the spectral domain. Instead, a narrow-band spectrum is low-pass filtered with a cutoff point (i.e., cutoff “quefrency” in the terminology of cepstral analysis) to preserve only the spectral envelope. To test the method, speech representative of a 2–3 year-old child was simulated with an airway modulation model of speech production. The model, which includes physiologically-scaled vocal folds and vocal tract, generates sound output analogous to a microphone signal. The vocal tract resonance frequencies can be calculated independently of the output signal and thus provide test cases that allow for assessing the accuracy of the formant tracking algorithm. When applied to the simulated child-like speech, the spectral filtering approach was shown to provide a clear spectrographic representation of formant change over the time course of the signal, and facilitates tracking formant frequencies for further analysis.

ROBOTIC REVERBERATION MAPPING OF ARP 151

We present the first results from the Las Cumbres Observatory Global Telescope (LCOGT) Network's Active Galactic Nuclei Key Project, a large program devoted to using the robotic resources of LCOGT to perform time domain studies of active galaxies. We monitored the Seyfert 1 galaxy Arp~151 (Mrk~40) for $\sim$200 days with robotic imagers and with the FLOYDS robotic spectrograph at Faulkes Telescope North. Arp~151 was highly variable during this campaign, with $V$-band light curve variations of $\sim$0.3 mag and H$\beta$ flux changing by a factor of $\sim$3. We measure robust time lags between the $V$-band continuum and the H$\alpha$, H$\beta$ and H$\gamma$ emission lines, with $\tau_\mathrm{cen} = 13.89^{+1.39}_{-1.41}$, 7.52$^{+1.43}_{-1.06}$ and 7.40$^{+1.50}_{-1.32}$ days, respectively. The lag for the \ion{He}{2} $\lambda4686$ emission line is unresolved. We measure a velocity-resolved lag for the H$\beta$ line, which is clearly asymmetric with higher lags on the blue wing of the line which decline to the red, possibly indicative of radial inflow, and is similar in morphology to past observations of the H$\beta$ transfer function shape. Assuming a virialization factor of $f$=5.5, we estimate a black hole mass of $M_\mathrm{BH}=6.2^{+1.4}_{-1.2}\times$10$^{6}$~$M_{\odot}$, also consistent with past measurements for this object. These results represent the first step to demonstrate the powerful robotic capabilities of LCOGT for long-term, AGN time domain campaigns that human intensive programs cannot easily accomplish. Arp 151 is now one of just a few AGN where the virial product is known to remain constant against substantial changes in H$\beta$ lag and luminosity.

Transmission performance of OSSB-RoF system using MZM electro-optical external modulator

Both the EAM and the MZM, the commonly used electro-optical external modulators in RoF systems, have pointed on their transfer function curve at which link performance is optimized. In discussing the difficulty of maintaining the optimum modulator bias point, we prefer MZM because unlike EAM, it has a predictable transfer function shape. In this paper, the simulative investigation is analyzed and discussed to study the impact of extinction ratio with low chirp of single- and dual-electrode Mach–Zehnder (SEMZM and DEMZM) external modulators over RF power degradation introduced in Radio-over-Fiber (RoF) systems due to fiber dispersion, which is one of the limiting factors to RoF link. Our simulative results show that longer transmission distance spanned between a receiver and transmitter with minimum RF power degradation can be achieved by using low extinction ratio and chirp of MZM modulators. Further, it is also observed that a dual-electrode MZM with low extinction ratio and chirp enables the transmission of RF signal not only with minimal dispersion but also with maximal second-order harmonic (HD2) reduction.

Imaging system fundamentals

Point-and-shoot, TV studio broadcast, and thermal infrared imaging cameras have significantly different applications. A parameter that applies to all imaging systems is Fλ/d, where F is the focal ratio, λ is the wavelength, and d is the detector size. Fλ/d uniquely defines the shape of the camera modulation transfer function. When Fλ/d<2, aliased signal corrupts the imagery. Mathematically, the worst case analysis assumes that the scene contains all spatial frequencies with equal amplitudes. This quantifies the potential for aliasing and is called the spurious response. Digital data cannot be seen; it resides in a computer. Cathode ray tubes, flat panel displays, and printers convert the data into an analog format and are called reconstruction filters. The human visual system is an additional reconstruction filter. Different displays and variable viewing distance affect the perceived image quality. Simulated imagery illustrates different Fλ/d ratios, displays, and sampling artifacts. Since the human visual system is primarily sensitive to intensity variations, aliasing (a spatial frequency phenomenon) is not considered bothersome in most situations.

New Transfer Functions for Correcting Turbulent Water Vapour Fluxes

We address the problem of the high-frequency correction of water vapour fluxes measured by eddy covariance with a closed-path infrared gas analyser (IRGA). Different transfer functions are compared and evaluated at a forested (Vielsalm, Belgium) and an agricultural (Lonzée, Belgium) site. Classical functions, usually applied to correct CO2 fluxes (Gaussian, Lorentzian), are found to be unsuited to water vapour cospectral corrections, being characterised by too sharp a decrease at high frequency. Two other functions characterised by a lower decreasing slope are found to better fit experimental transfer functions. They were calibrated and validated on experimental transfer functions and their dependency on air humidity is parameterised. On this basis, new correction coefficients are estimated. The coefficients are found to be larger than those based on the classical functions, even when the dependency of the latter on air humidity is taken into account. The difference amounts to 10% at the forested site and to 5% larger at the crop site. The study highlights the necessity of characterising the water transfer function shape and taking it into account in the correction factor at each site equipped with a closed path IRGA.

Bedrock Depth Mapping of the Coast South of İstanbul: Comparison of Analytical and Experimental Analyses

Local S-wave velocity-depth profiles and bedrock depth distribution are key factors in assessing seismic hazard and earthquake ground motion characteristics since they allow determination of the amplification potential of geological formations overlying bedrock. In this study, an empirical relationship between the thickness of Tertiary-Quaternary sediments (hereafter referred as cover) overlying Palaeozoic bedrock and their resonance frequencies was calculated for the İstanbul region and the bedrock depth distribution beneath the city was presented. The relationship was investigated by comparing transfer functions obtained from single station microtremor analyses and one-dimensional (1D) S-wave velocity profiles at sites where shallow velocity structure is known. Geotechnical data consisting of standard penetration test (SPT) blow counts and standard soil descriptions were evaluated from 15 boring sites and microtremor measurements were carried out. The bedrock depth of each site was determined by computing analytical transfer functions to fit the resonance frequency and the shape of experimental transfer functions. Based on those results, a relationship between the resonance frequency and the thickness of the cover was derived. Finally, the bedrock distribution beneath populated areas of İstanbul was obtained by applying the derived relationship to 86 strong-motion sites, where the resonance frequencies are known.

ICEARRAY: the first small-aperture, strong-motion array in Iceland

A small-aperture, strong-motion array, the ICEARRAY, has been deployed in South Iceland, a region with a history of destructive earthquakes, some exceeding magnitude 7. The array’s purpose is: (1) monitoring future significant events in the region, (2) quantifying spatial variability of strong-motion over short distances and (3) shedding light on earthquake source processes. The number of array stations and their arrangement were based on an optimisation of the shape of the corresponding array transfer function (ATF). The optimal ICEARRAY configuration comprises 14 stations, has an aperture of ~1.9 km and a minimum interelement distance of ~50 m and possesses a near-azimuthally independent ATF with a sharp main lobe, negligible sidelobes and a wavenumber range of 1.5–24 rad/km. Accordingly, the ICEARRAY has the intended capabilities of capturing seismic waves in the frequency range of 1–20 Hz, which is of main interest to earthquake engineering and engineering seismology applications.

Analysis of Scanning DMA Transfer Functions

A new approach to determine transfer functions of scanning differential mobility analyzers (DMAs) is introduced. An expression for non-diffusive particle trajectories in a cylindrical DMA operating in a scanning voltage mode is obtained analytically. Particle trajectory simulations are then used to determine the scanning DMA transfer functions. The results suggest that the shape of the scanning DMA transfer functions may vary with mobility for scan times smaller than a certain value, which depends on the DMA flowrates, and this provides one criteria limiting the fastest scan possible with the conventional DMAs. A simple approach to determine upscan scanning DMA transfer functions in near real-time with minimal number of simulations is described. For upscan operation, the choice of the scantime is seen to impose upper and lower limits on the particle mobility range classified by a scanning DMA. The predicted effect of voltage scanning on altering DMA transfer functions is validated with fast scan experiments.

Technical Note: A New Deconvolution Scheme for the Retrieval of True DMA Transfer Function from Tandem DMA Data

A new deconvolution scheme is proposed to retrieve the true transfer function of differential mobility analyzers (DMAs) from tandem DMA (TDMA) experimental data. Apart from the existing methods in which the transfer function is represented as either a triangular or a Gaussian shape, the true DMA transfer function in the new scheme is represented by a piecewise linear function. The conjugate gradient optimization method is used to find the best fit between the calculated and experimental TDMA data. It is shown that the scheme is insensitive to experimental error and able to differentiate the shapes of DMA transfer functions for different particle sizes. The scheme is further applied to obtain the true transfer function of Nano-DMA (Chen et al. 1998).

Dispersion penalty analysis for VSR —1 optical links

This paper presents and approach to calculate dispersion penalty for VSR-1 optical links. Based on parameters of a specific VSR-1 link, dispersion penalties are computed for various modal dispersion bandwidths respectively. The worst-case eye closure is expressed numerically by using the signal waveform at time 0, and the signal waveform is obtained in frequency domain through FFT algorithm. By this approach, the dispersion penalty is determined by the shape of transfer functions of the various components in the links. To simplify the derivation of multimode fiber link transfer function, a Gaussian form of normalized impulse response is used. This calculation approach can be used to estimate the worst-case dispersion penalty of VSR-1 links in the link budget analysis.