Propagation Time Difference Research Articles

Temporal spreading generated by diffraction in the focusing of ultrashort light pulses with perfectly conducting spherical mirrors

We study femtosecond pulses at the focal plane of a perfectly conducting spherical mirror which is a dispersionless system, that is, it introduces no group velocity dispersion and no propagation time difference to the pulses after reflection. By using the scalar diffraction theory we will show that the neglected terms in the diffraction integral, when using the approximation of the bandwidth being smaller than the frequency of the carrier, have a significant influence on imaging if a laser pulse of a few femtoseconds is used in time-resolved imaging. The neglected terms introduce temporal spreading to extremely short pulses of a few optical cycles incident on the mirror, which avoids a fully compensated pulse, i.e., a one optical cycle pulse, at the focus of the mirror. The study in this paper also applies to refracting optical systems such as microscope objectives or lenses.

Gauss-Legendre quadrature method used to evaluate the spatio-temporal intensity of ultrashort pulses in the focal region of lenses

We analyze the spatio-temporal intensity of sub-20 femtosecond pulses with a carrier wavelength of 810 nm along the optical axis of low numerical aperture achromatic and apochromatic doublets designed in the IR region by using the scalar diffraction theory. The diffraction integral is solved by expanding the wave number around the carrier frequency of the pulse in a Taylor series up to third order, and then the integral over the frequencies is solved by using the Gauss-Legendre quadrature method. The numerical errors in this method are negligible by taking 96 nodes and the computational time is reduced by 95% compared to the integration method by rectangles. We will show that the third-order group velocity dispersion (GVD) is not negligible for 10 fs pulses at 810 nm propagating through the low numerical aperture doublets, and its effect is more important than the propagation time difference (PTD). This last effect, however, is also significant. For sub-20 femtosecond pulses, these two effects make the use of a pulse shaper necessary to correct for second and higher-order GVD terms and also the use of apochromatic optics to correct the PTD effect. The design of an apochromatic doublet is presented in this paper and the spatio-temporal intensity of the pulse at the focal region of this doublet is compared to that given by the achromatic doublet.

Distortion of ultrashort laser pulses in a spatial filter

By means of Collins diffraction integral, the propagation expression of an ultrashort laser pulse passing through a spatial filter is derived. The effects of the magnification of spatial filter on pulse broadening and distortion of pulse-front for the ultrashort optical pulse are analytically deduced and numerically simulated. It is found that pulse broadening and propagation time difference of a laser pulse getting through spatial filter is proportional to the magnification. As a conclusion, in an ultrashort pulse laser system with a large aperture, the effect of pulse broadening and distortion of pulse-front should be considered.

Exploiting the Path Propagation Time Differences in Multipath Transmission with FEC

We consider a transmission of a delay-sensitive data stream (e.g., a video call) from a single source to a single destination. The reliability of this transmission may suffer from bursty packet losses - the predominant type of failures in today's Internet. An effective and well studied solution to this problem is to protect the data by a Forward Error Correction (FEC) code and send the FEC packets over multiple paths. In this paper, we show that the loss rate of such a classic multipath FEC scheme can often be significantly reduced, while keeping the total transmission rate and delay unchanged. Our key observation is that the propagation times on the available paths often significantly differ, usually by 10-100ms. We propose to exploit these differences by appropriate packet scheduling that we call `Spread'. We evaluate our solution with a precise, analytical formulation and trace-driven simulations. Our studies show that Spread substantially outperforms the state-of-the-art solutions. It typically achieves two- to five-fold improvement (reduction) in the effective loss rate. Or conversely, keeping the same level of effective loss rate, Spread significantly decreases the observed delays and helps fighting the delay jitter.

Third-order dispersion effects generated by non-ideal achromatic doublets on sub-20 femtosecond pulses

Gaussian temporal envelope pulses with initial durations of 10 fs, 15 fs and 20 fs and a carrier wavelength of 810 nm were analyzed at the paraxial focal plane of non-ideal achromatic doublet lenses for well-collimated incoming pulses parallel to the optical axis. The wave vector is expanded up to third order, to investigate the effect of third-order group velocity dispersion on the pulse and the results are compared to those obtained when the wave number is expanded up to second order. The propagation time difference and the primary spherical aberration were included in the calculations using the thin lens approximation theory. Results are presented for a homogenous illumination beam.

PMD and DGD Performance Analysis in SMF due to Fiber Irregularities

A fundamental property of optical signal is polarization. Polarization refers to the electric field orientation of the light signal, which can vary significantly along the length of the fiber. Signal energy at a given wavelength occupies two orthogonal polarization modes .A varying birefringence along its length will cause each polarization mode to travel at significantly different velocity and polarization orientation is related with distance.The resulting difference in propagation time, DGD between two orthogonal polarizations will result in pulse splitting. PMD is related to the differential group delay, DGD caused by birefringence in optical fibers. Polarization mode dispersion, PMD is a phenomenon found in optical fiber and other wave guides .It is created by fiber irregularities and thus causes dispersion and distortion of the light pulse, thus increasing BER and limiting data rate. There are various types of irregularities in fiber, which arise due to an asymmetric fiber core or can be introduced through internal stresses during fiber manufacture, or through external stresses during cabling and installation. Optical fiber manufacturing processes are designed to yield fibers with a circular cross-section. Any deviation from this form will generally result in an elliptical core, which in turn will result in a refractive index difference between the X and Y-axes of the elliptical core. Even if the fiber core is manufactured with an ideal circular cross-section its refractive index can be asymmetric across its cross-section due to stresses built into the fiber during the manufacturing process or stress that is externally applied during deployment or operation. In the present paper, authors have analyzed PMD and DGD performance in the single mode fiber with different fiber irregularities.

10.1007/s11490-008-3007-5

One main function of short pulses is to concentrate energy in time and space [1]. The use of refractive lenses allows us to concentrate energy in a small volume of focusing around the focal point of the lens. When using refractive lenses, there are three effects that affect the concentration of energy around the focal point of the lens. These are the group velocity dispersion (GVD), the propagation time difference (PTD), and the aberrations of the lens. In this paper, we study lenses which are diffraction limited so that the monochromatic aberrations are negligible; the group velocity dispersion and the propagation time difference are the main effects affecting the spreading of the pulse at the focus. We will show that for 100-fs pulses the spatial spreading is larger than the temporal spreading of the pulse. It is already known that the effect of spatial spreading of the pulse due to PTD can be reduced by using achromatic optics. We use the theory proposed by A. Vaughan to analyze simple lenses and normal achromatic doublets, where normal means doublets that we can buy from catalogs. We then use the Vaughan theory to design achromatic doublets in phase and group, which produce no spatial spreading of the pulse, i.e., PTD = 0, when the doublet is designed for the carrier of the pulse. We compare these phase and group achromatic doublets with normal achromatic doublets. Finally, we show that apochromatic optics can give a much better correction of PTD than using normal achromatic doublets.

Aberration effects on femtosecond pulses generated by nonideal achromatic doublets

There are three main effects that affect the femtosecond pulse focusing process near the focal plane of a refractive lens: the group velocity dispersion (GVD), the propagation time difference (PTD), and the aberrations of the lens. In this paper we study in detail these effects generated by nonideal achromatic doublets based on a Fourier-optical analysis and Seidel aberration theory considering lens material, wavelength range, lens surface design, and temporally and spatially uniform and Gaussian intensity distributions. We show that the residual chromatic aberration in achromatic lenses, which has been neglected so far, has a considerable effect on the focusing of pulses shorter than 20 fs in the spectral range between the UV and IR, 300 to 1100 nm, and is particularly important in the blue and UV spectral range. We present a general fitted function for an estimation of the pulse stretching parameter, which depends only on the numerical aperture and focal length of the doublet as well as the wavelength of the carrier of the pulse.

J1602-2-7 地中レーダによる埋設管の管径および位置・方向の推定(機械の知能化:異分野からのアプローチ(2))

It has been difficult to extract the shape information of buried object such as radius of buried pipe from GPR image. But it is very important to get underground information as a previous knowledge before construction. In this paper, the authors proposed an estimation method of pipe radius, depth and direction of a buried pipe at a time from the propagation time differences between observation points, while scanning an antenna more than once and parallel. By scanning the antenna more than once, we can get multiple data sets containing inclination angle information of the buried pipe, and, by using a propagation time model of the reflected wave from the inclined buried pipe, we establish a method to estimate the pipe radius of buried pipe. The propagation time model consists of the shortest path between the antenna and the buried pipe. And we set a world coordinate system (i = 0) and the i-th moving coordinate system. We consider start point of the first scan as origin of the world coordinate system and another start point of the i-th scan as the origin of the z'-th moving coordinate system. Based on the antenna travelling distances are recorded to estimate the direction of the buried pipe, unknown parameters, i.e., the pipe radius, the depth and the direction of the buried pipe, are estimated by using a nonlinear leastsquares method.

Influence of load balancing on quality of real time data transmission

The routing algorithm with load balancing presented in [1] represents the modification of OSPF protocol, which enables the optimization to achieve higher network throughput. In the case of routing with load balancing, packets belonging to the same stream use different paths in the network. This paper analyzes the influence of the difference in packet propagation times on the quality of real-time data transmission. The proposed algorithm was implemented and the simulation network was formed to measure the jitter. .

Oesophageal bolus transit and clearance by secondary peristalsis in normal individuals

Secondary peristalsis is important for the clearance of retained refluxate or material from the oesophagus. Combined impedance and manometry can directly detect both oesophageal contraction and bolus transit. To apply combined impedance and manometry to characterize oesophageal bolus transit and clearance by secondary peristalsis in healthy individuals. Eleven healthy volunteers underwent combined impedance and manometry with a catheter containing seven impedance-measuring segments and eight water-perfusion pressure transducers. Saline and solid agar boluses of 5 ml were applied for primary peristalsis and secondary peristalsis was stimulated by rapid mid-oesophageal injections of saline. The rate for complete bolus clearance of secondary peristalsis with saline injections was less than that of primary peristalsis with saline swallows (69 vs. 95%, P=0.02). No statistical difference in bolus propagation time between primary and secondary peristalsis was observed (P=0.45). Bolus presence time of secondary peristalsis was significantly longer than that of primary peristalsis for all impedance-measuring segments (all P<0.05). Solid swallows differed from saline swallows with lower rate of complete bolus transit and longer bolus transit time. Our data show that the impedance technique can successfully characterize oesophageal bolus transit and clearance by secondary peristalsis. These findings suggest that secondary peristalsis may be less effective than primary peristalsis regarding oesophageal transit and clearance of a liquid bolus.

Achromatic doublets using group indices of refraction

One main function of short pulses is to concentrate energy in time and space [1]. The use of refractive lenses allows us to concentrate energy in a small volume of focusing around the focal point of the lens. When using refractive lenses, there are three effects that affect the concentration of energy around the focal point of the lens. These are the group velocity dispersion (GVD), the propagation time difference (PTD), and the aberrations of the lens. In this paper, we study lenses which are diffraction limited so that the monochromatic aberrations are negligible; the group velocity dispersion and the propagation time difference are the main effects affecting the spreading of the pulse at the focus. We will show that for 100-fs pulses the spatial spreading is larger than the temporal spreading of the pulse. It is already known that the effect of spatial spreading of the pulse due to PTD can be reduced by using achromatic optics. We use the theory proposed by A. Vaughan to analyze simple lenses and normal achromatic doublets, where normal means doublets that we can buy from catalogs. We then use the Vaughan theory to design achromatic doublets in phase and group, which produce no spatial spreading of the pulse, i.e., PTD = 0, when the doublet is designed for the carrier of the pulse. We compare these phase and group achromatic doublets with normal achromatic doublets. Finally, we show that apochromatic optics can give a much better correction of PTD than using normal achromatic doublets.

Multi-CFD Timing Estimators for PET Block Detectors

In a conventional PET system with block detectors, a timing estimator is created by generating the analog sum of the signals from the four photomultiplier tubes (PMT) in a module and discriminating the sum with a single constant fraction discriminator (CFD). The differences in the propagation time between the PMTs in the module can potentially degrade the timing resolution of the module. While this degradation is probably too small to affect performance in conventional PET imaging, it may impact the timing inaccuracy for time-of-flight PET systems (which have higher timing resolution requirements). Using a separate CFD for each PMT might allow for propagation time differences to be compensated through calibration and correction in software. In this paper we investigate and quantify the timing resolution achievable when the signal from each of the 4 PMTs is digitized by a separate CFD. Several methods are explored for both obtaining values for the propagation time differences between the PMTs and combining the four arrival times to form a single timing estimator. We find that the propagation time correction offsets are best derived through an exhaustive search, and that the "weighted average" method provides the best timing estimator. Using these methods, the timing resolution achieved with 4 CFDs (1052plusmn82 ps) is equivalent to the timing resolution with the conventional single CFD setup (1064plusmn216 ps)

Accurate interactive acoustic plate

The invention concerns a device for acquiring the position co-ordinates of a source of mechanical waves optionally generated by impacting the surface of a plate (PLQ) of finite dimensions comprising a set of acoustic sensors (PZT00 to PZT11) each formed by a pair of piezoelectric transducers (PZTa, PZTb) facing each other on either side of the plate, the device including processing means for determining the co-ordinates of the source by analysing the difference in propagation time of the acoustic waves generated by the source to the different sensors. Said device is characterised in that the processing means comprise combined with each sensor (PZT00 to PZT 11) a respective electronic circuit including means mounted in cascade for digitizing the amplified signal around a predetermined frequency, associated with means for limiting the digitization to a time window starting before the acoustic waves reach a sensor and ending when the acoustic waves have reached said sensor.

Effect of oral intake of choline-stabilized orthosilicic acid on skin, nails and hair in women with photodamaged skin

Chronic exposure of the skin to sunlight causes damage to the underlying connective tissue with a loss of elasticity and firmness. Silicon (Si) was suggested to have an important function in the formation and maintenance of connective tissue. Choline-stabilized orthosilicic acid ("ch-OSA") is a bioavailable form of silicon which was found to increase the hydroxyproline concentration in the dermis of animals. The effect of ch-OSA on skin, nails and hair was investigated in a randomized, double blind, placebo-controlled study. Fifty women with photodamaged facial skin were administered orally during 20 weeks, 10 mg Si/day in the form of ch-OSA pellets (n=25) or a placebo (n=25). Noninvasive methods were used to evaluate skin microrelief (forearm), hydration (forearm) and mechanical anisotropy (forehead). Volunteers evaluated on a virtual analog scale (VAS, "none=0, severe=3") brittleness of hair and nails. The serum Si concentration was significantly higher after a 20-week supplementation in subjects with ch-OSA compared to the placebo group. Skin roughness parameters increased in the placebo group (Rt:+8%; Rm: +11%; Rz: +6%) but decreased in the ch-OSA group (Rt: -16%; Rm: -19%; Rz: -8%). The change in roughness from baseline was significantly different between ch-OSA and placebo groups for Rt and Rm. The difference in longitudinal and lateral shear propagation time increased after 20 weeks in the placebo group but decreased in the ch-OSA group suggesting improvement in isotropy of the skin. VAS scores for nail and hair brittleness were significantly lower after 20 weeks in the ch-OSA group compared to baseline scores. Oral intake of ch-OSA during the 20 weeks results in a significant positive effect on skin surface and skin mechanical properties, and on brittleness of hair and nails.

Experimental evidence of 25-fs laser pulse distortion in singlet beam expanders

We report the measurement of spatiotemporal distortions of an ultrashort pulse in singlet beam expanders. With a simple second-order autocorrelator the temporal broadening of the pulse from 23 to 40 fs, due to propagation time difference (PTD), is determined. The delay due to PTD between different parts of the beam is also measured. This effect was theoretically studied for the first time by Bor [J. Mod. Opt. 35, 1907 (1988)]. These experimental results are in good agreement with the calculations of a dedicated three-dimensional ray-tracing program developed to simulate the spatial and temporal transformation of femtosecond pulses in optical systems.

Ultrasound Technique for Prediction of Statistical Characteristics of Grid-Generated Turbulence

Ultrasonic technique can be used to determine some characteristics of turbulent flows. In this work, measurements of the difference in propagation times of ultrasonic waves along two paths are combined with the Kolmogorov (2/3)-power law to study characteristics of grid-generated turbulence. The paper focuses on ultrasonic measurements of turbulent flow using the travel-time technique. The theoretical analysis is based on the well-known Kolmogorov theory and supported by experimental data. The present research is of interest from two points of view. First, it demonstrates a non-invasive method of investigation of turbulence, and second, it furthers understanding of the effect of turbulence on sound propagation.

Non-contact Sensor for the Measurement of Meniscus Velocity of Molten Metal Flow

An electromagnetic non-contact sensor was developed to measure the meniscus velocity cf molten metal flow. Wave motions were generated on the surface of a molten metal bath by applying a Lorentz force periodically. The meniscus velocity was obtained by detecting a difference between the propagation times of the waves to the upstream and downstream level sensors. This is because the propagation time difference depends on the meniscus velocity of molten metal flow. The effects of the frequency of the electric source, excitation frequency, and the existence of mold powder on the accuracy of the meniscus flow measurements were investigated based on cold model experiments using molten Wood's metal and silicone oil. The accuracy was satisfactory under the experimental conditions considered.

Cochlear and Neural Delays for Coincidence Detection in Owls

The auditory system uses delay lines and coincidence detection to measure the interaural time difference (ITD). Both axons and the cochlea could provide such delays. The stereausis theory assumes that differences in wave propagation time along the basilar membrane can provide the necessary delays, if the coincidence detectors receive input from fibers innervating different loci on the left and right basilar membranes. If this hypothesis were true, the left and right inputs to coincidence detectors should differ in their frequency tuning. The owl's nucleus laminaris contains coincidence detector neurons that receive input from the left and right cochlear nuclei. Monaural frequency-tuning curves of nucleus laminaris neurons showed small interaural differences. In addition, their preferred ITDs were not correlated with the interaural frequency mismatches. Instead, the preferred ITD of the neuron agrees with that predicted from the distribution of axonal delays. Thus, there is no need to invoke mechanisms other than neural delays to explain the detection of ITDs by the barn owl's laminaris neurons.

Influence of electrical source impedance on phase shift of sound wave in the ultrasonic velocity measurement

AbstractIn acoustic velocity measurement using ultrasonic waves, the composite resonator method with the sample and the transducer becomes a source of error. This phenomenon is considered as a phase shift at the boundary between the transducer and the sample and is analyzed by numerical simulation. From the result, it is found that the phase shift depends on the electrical source impedance and that the phase shift can be minimized by setting the electrical source impedance equal to the reactance of the transducer at resonance, depending on the acoustic condition of the backing of the transducer. The results of experiments with a varying electrical source impedance are found to be similar to those of simulations. Further, it becomes possible to measure the acoustic velocity of water with a discrepancy of less than 0.1% from the referenced value if the phase shift is eliminated by taking the propagation time difference in two samples with different lengths. © 2001 Scripta Technica, Electron Comm Jpn Pt 2, 84(4): 41–48, 2001