Impulse Response Functions for Characterizing Pulse-to-Pulse Junction Temperature Behavior in Laser Diode Pulse Trains

<div class="htmlview paragraph">This paper summarizes a recent analytical study of a pulsed Gallium Arsenide diode laser. The study was conducted to investigate the maximum junction temperature for a pulsed diode and to explore the possibility of junction temperature reduction by varying the diode pulse duration and changing the diode mounting configuration. Due to large local heat generation, the junction's temperature quickly rises during the pulse, and large temperature gradients are created in several directions. For a given diode column and optical power level, the effects of diode length and pulse duration on junction temperatures were examined. Also, mounting configurations were investigated to the extent of varying top and bottom mounting plate thicknesses. Variation of the diode length and pulse duration provided the most potential for the reduction of junction temperatures, while minimum copper mounting plate thicknesses are required for the prevention of sharp increases in junction temperatures.</div>

Fluorescence lifetime imaging is independent of signal intensity and is thus efficient and robust. Additionally, lifetime can be used to differentiate fluorophores and sense fluorophore micro-environment change. A time-resolved optical system is usually used to measure fluorescent decay kinetics, and then one fits the decay to get lifetime. Since the system impulse response function (IRF) is finite, it impacts lifetime fitting. Deconvolution of the IRF can diminish its impact. In thick tissues, light diffusion due to scattering is also convolved with the fluorescence decay. One can recover the decay using an inversion algorithm. However, processing data in this way is computationally intensive and therefore not practical for real time imaging. We present here results of our studies on the IRF impact to fluorescence lifetime fitting in a turbid medium over a wide range of parameters, using a unique time-domain imaging system. Fluorophores were submerged inside a turbid medium that models tissue. Analytical analysis and computation show that when the lifetime is 1.5 times larger than the FWHM of system IRF, reasonable fluorescence lifetimes can be obtained by fitting the decay tail without taking into account IRF. For small source-fluorophore-detector separation, the effect of optical diffusion on the lifetime fitting is also negligible. This gives a guidance of system precision limit for fluorescence lifetime imaging by fast tail fitting. Experimental data using a fs laser with a streak camera and a pulsed diode laser with PMT-TCSPC for ICG, Cy5.5, and ATTO 680 support the theoretical results.

We show that daily sunspot areas can be used in a simple, single parameter model to reconstruct daily variations in several other solar parameters, including solar spectral irradiance and total magnetic flux. The model assumes that changes in any given parameter can be treated mathematically as the response of the system to the emergence of a sunspot. Using cotemporal observational data, we compute the finite impulse response (FIR) function that describes that response in detail, and show that the response function has been approximately stationary over the time period for which data exist. For each parameter, the impulse response function describes the physical evolution of that part of a solar active region that is the source of the measured variability. We show that the impulse response functions are relatively narrow functions, no more than 3 years wide overall. Each exhibits a pre-active, active, and post-active region component; the active region component dominates the variability of most of the parameters studied.

A basic tenet of linear invariant systems is that they are sufficiently described by either the impulse response function or the frequency transfer function. This implies that we can always obtain one from the other. However, when the transfer function contains uncanceled poles, the impulse function cannot be obtained by the standard inverse Fourier transform method. Specifically, when the input consists of a uniform train of pulses and the output sequence has a finite duration, the transfer function contains multiple poles on the unit cycle. We show how the impulse function can be obtained from the frequency transfer function for such marginally stable systems. We discuss three interesting discrete Fourier transform pairs that are used in demonstrating the equivalence of the impulse response and transfer functions for such systems. The Fourier transform pairs can be used to yield various trigonometric sums involving sin⁡πk/Nsin⁡πLk/N, where k is the integer summing variable and N is a multiple of integer L.

An approach is presented to compute the time-dependent force acting on a piston in a rigid infinite planar baffle as a result of the specified velocity of the piston. The approach to computing the force is applicable to both sinusoidal and nonsinusoidal velocity pulses and is valid for all piston shapes. The approach, which is based on a Green's-function solution to the time-dependent boundary value problem, utilizes a transformation of coordinates to simplify the evaluation of the double surface integrals. An impulse response function is defined such that the time-dependent force can be obtained by differentiating the convolution of the impulse response and piston velocity time functions. A closed-form expression for the impulse response of a circular piston is derived and discussed. Numerical results for the impulse response and the forces on large square pistons resulting from sinusoidal piston velocities are then presented and discussed to compare the transient and steady-state behavior of the forces. Finally, an approach is presented to compute the radiation impedance as a function of normalized frequency from the impulse response data, and the approach is used to obtain the normalized radiation resistance and reactance for square pistons.

A multitude of environmental and hydrologic processes embody both the elements of chance and the descriptive laws of physics. A finer process description at one scale is lost through the processes of integration in time and space and through averaging. This justifies simplification in representation of the processes. It is hypothesized that if an environmental process is described by a linear or linearized governing equation, then the solution of this equation for a unit impulse (or Dirac delta) function can be interpreted as a probability density function for describing the probabilistic properties of the process. This hypothesis is tantamount to mapping from the unit impulse response (UIR) function, h(t), to the probability density function (PDF), f(x), where h is the UIR as a function of time or space variable denoted by t and f is the PDF as a function of the random variable of the process. For example, the impulse response of a diffusion equation for pollutant transport described by the space-time variation of concentration can be used as a probability distribution for pollutant concentration in a medium, such as a river, a lake, conduit storm water, soil, or a saturated geologic formation. Likewise, the impulse response of a linearized diffusion model of channel flow can be interpreted as a probability distribution for frequency analysis of extreme values (such as floods, droughts, hurricanes, earthquakes, and so on). Similarly, the impulse response of a linear reservoir can be used as an exponential probability distribution model. The impulse response of a cascade of equal linear reservoirs is the gamma distribution, which has a number of applications in environmental and water resources data analysis. In this vein, a number of impulse responses of physically based equations that apply to environmental and hydrologic processes and data are discussed and illustrated by using field or laboratory data.

Joint confidence sets for structural impulse responses

Objective To evaluate the efficacy and safety of intense pulsed light and diode laser for axillary hair removal. Methods Clinical trials on 61 persons using intense pulsed light and diode laser to depilate axillary hairs were conducted. 36 persons were treated by IPL and 25 persons by diode laser.Treatments were carried out in three times at 8-week intervals, and a final assessment was made 3 months following the third theatment. Results Both IPL and diode laser reduced the hair count substantially! the IPL group effective rates were 80. 6 % and the diode laser group, 76. 0 %. They had no statistical significance was (P>0. 05)). Conclusions Intense pulsed light and diode laser are effiective and safe for hair removal. Key words: Hair removal; Intense pulsed light; Didoe laser

Impulse response functions (IRFs) are useful for characterizing systems’ dynamic behavior and gaining insight into their underlying processes, based on sensor data streams of their inputs and outputs. However, current IRF estimation methods typically require restrictive assumptions that are rarely met in practice, including that the underlying system is homogeneous, linear, and stationary, and that any noise is well behaved. Here, I present data-driven, model-independent, nonparametric IRF estimation methods that relax these assumptions, and thus expand the applicability of IRFs in real-world systems. These methods can accurately and efficiently deconvolve IRFs from signals that are substantially contaminated by autoregressive moving average (ARMA) noise or nonstationary ARIMA noise. They can also simultaneously deconvolve and demix the impulse responses of individual components of heterogeneous systems, based on their combined output (without needing to know the outputs of the individual components). This deconvolution–demixing approach can be extended to characterize nonstationary coupling between inputs and outputs, even if the system’s impulse response changes so rapidly that different impulse responses overlap one another. These techniques can also be extended to estimate IRFs for nonlinear systems in which different input intensities yield impulse responses with different shapes and amplitudes, which are then overprinted on one another in the output. I further show how one can efficiently quantify multiscale impulse responses using piecewise linear IRFs defined at unevenly spaced lags. All of these methods are implemented in an R script that can efficiently estimate IRFs over hundreds of lags, from noisy time series of thousands or even millions of time steps.

In view of the increasing attention to the time responses of complex fluids described by power-laws in association with the need to capture inertia effects that manifest in high-frequency microrheology, we compute the five basic time-response functions of in-series or in-parallel connections of two elementary fractional derivative elements known as the Scott-Blair (springpot) element. The order of fractional differentiation in each Scott-Blair element is allowed to exceed unity reaching values up to 2 and at this limit-case the Scott-Blair element becomes an inerter--a mechanical analogue of the electric capacitor that its output force is proportional only to the relative acceleration of its end-nodes. With this generalization, inertia effects may be captured beyond the traditional viscoelastic behavior. In addition to the relaxation moduli and the creep compliances, we compute closed form expressions of the memory functions, impulse fluidities (impulse response functions) and impulse strain-rate response functions of the generalized fractional derivative Maxwell fluid, the generalized fractional derivative Kelvin-Voigt element and their special cases that have been implemented in the literature. Central to these calculations is the fractional derivative of the Dirac delta function which makes possible the extraction of singularities embedded in the fractional derivatives of the two-parameter Mittag-Leffler function that emerges invariably in the time-response functions of fractional derivative rheological modes.

1. The dynamic properties of unit responses to amplitude-modulated tones were studied using modulation with pseudorandom noise and described by cross-covariance and integrated cross-covariance functions between the discharge rate and the modulation. Under the experimental conditions used, these two functions are valid approximations of the system's impulse and step response functions respectively. 2. On the basis of their impulse response functions units could be classified into two groups, Type I with a low adaptation and Type II with a large degree of adaptation as well as a damped oscillation in their impulse response functions. 3. The response pattern of the Type II units is most likely the result of a negative feed-back striving to keep the discharge rate at a nearly constant level. 4. The cross-covariance functions are shown to remain unchanged during long duration recordings from the same unit.

A procedure based on the theory of generalized functions is applied to the calculation of impulse response (Green's) functions of a structural network with rigid body motion. The system considered may be viewed as representation of certain large space structures and consists of torsional members with no shear and bending capacity, and Timoshenko beams with no torsional capacity. All members are rigidly connected at the network joints. Equations yielding impulse response functions are derived, and results of numerical calculations based on these equations are presented. The results confirm the practical feasibility of using the procedure for the calculation of impulse response functions for relatively complex structural networks.

Human motor control is often explained in terms of engineering 'servo' theory. Recently, continuous, optimal control using internal models has emerged as a leading paradigm for voluntary movement. However, these engineering paradigms are designed for high band-width, inflexible, consistent systems whereas human control is low bandwidth and flexible using noisy sensors and actuators. By contrast, engineering intermittent control was designed for bandwidth-limited applications. Our general interest is whether intermittent rather than continuous control is generic to human motor control. Currently, it would be assumed that continuous control is the superior and physiologically natural choice for controlling unstable loads, for example as required for maintaining human balance. Using visuo-manual tracking of an unstable load, we show that control using gentle, intermittent taps is entirely natural and effective. The gentle tapping method resulted in slightly superior position control and velocity minimisation, a reduced feedback time delay, greater robustness to changing actuator gain and equal or greater linearity with respect to the external disturbance. Control was possible with a median contact rate of 0.8±0.3 s(-1). However, when optimising position or velocity regulation, a modal contact rate of 2 s(-1) was observed. This modal rate was consistent with insignificant disturbance-joystick coherence beyond 1-2 Hz in both tapping and continuous contact methods. For this load, these results demonstrate a motor control process of serial ballistic trajectories limited to an optimum rate of 2 s(-1). Consistent with theoretical reasoning, our results suggest that intermittent open loop action is a natural consequence of human physiology.

Modal properties of a structure can be identified by experimental modal analysis (EMA). Discrete frequency response functions (FRFs) and impulse response functions (IRFs) between response and excitation series are bases for EMA. In the calculation of a discrete FRF, the discrete Fourier transform (DFT) is applied to both response and excitation series, and a transformed series in the DFT is virtually extended to have an infinite length and be periodic with a period equal to the length of the series; the resulting periodicity can be physically incorrect in some cases, which depends on an excitation technique used. An efficient and accurate methodology for calculating discrete FRFs and IRFs is proposed here, by which fewer spectral lines are needed and accuracies of resulting FRFs and IRFs can be maintained. The relationship between an IRF from the proposed methodology and that from the least-squares (LS) method is shown. A coherence function extended from a new type of coherence functions is used to evaluate qualities of FRFs and IRFs from the proposed methodology in the frequency domain. The extended coherence function can yield meaningful values even with response and excitation series of one sampling period. Based on the extended coherence function, a fitting index is used to evaluate overall qualities of the FRFs and IRFs. The proposed methodology was numerically and experimentally applied to a two degrees-of-freedom (2DOF) mass–spring–damper system and an aluminum plate to estimate their FRFs and IRFs, respectively. In the numerical example, FRFs and IRFs from the proposed methodology agree well with theoretical ones. In the experimental example, an FRF and its associated IRF from the proposed methodology with a random impact series agreed well with benchmark ones from a single impact test.

4 ps pulses with low temporal jitter (0.6 ps) and reduced pedestal (> 25 dB extinction) have been generated from a 1548 nm gain switched distributed feedback laser diode (DFB) pulse source at 2.5 GHz using continuous wave (CW) light injection for jitter suppression and an electroabsorption modulator (EAM) for pedestal suppression.