• Structural Engineering and Mechanics
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• v.75 no.4
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• pp.435-444
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• 2020

Ultrasonic guided wave testing is a very promising non-destructive testing method for rails, which is of great significance for ensuring the safe operation of railways. On the basis of the semi-analytical finite element (SAFE) method, a analytical model of 59R2 grooved rail was proposed, which is commonly used in the ballastless track of modern tram. The dispersion curves of ultrasonic guided waves in free rail and supported rail were obtained. Sensitivity analysis was then undertaken to evaluate the effect of rail elastic modulus on the phase velocity and group velocity dispersion curves of ultrasonic guided waves. The optimal guided wave mode, optimal excitation point and excitation direction suitable for detecting rail integrity were identified by analyzing the frequency, number of modes, and mode shapes. A sinusoidal signal modulated by a Hanning window with a center frequency of 25 kHz was used as the excitation source, and the propagation characteristics of high-frequency ultrasonic guided waves in the rail were obtained. The results show that the rail pad has a relatively little influence on the dispersion curves of ultrasonic guided waves in the high frequency band, and has a relatively large influence on the dispersion curves of ultrasonic guided waves in the low frequency band below 4 kHz. The rail elastic modulus has significant influence on the phase velocity in the high frequency band, while the group velocity is greatly affected by the rail elastic modulus in the low frequency band.

• Korean Journal of Optics and Photonics
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• v.12 no.3
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• pp.158-164
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• 2001

We observed a velocity-selective saturated absorption spectrum when the pump beam intersects the probe beam at a finite angle with a saturated absorption spectroscopic configuration in the cesium vapor cell. And we also observed a dispersion-like signal by measuring the difference between two velocity-selective absorption spectra produced by two parallel probe beams intersected by one pump beam. The dispersion-like signal was changed with the crossing angle and the crossing position of the pump and probe beams and compared with the calculated result. The dispersion-like signal was used as a frequency discriminator, and the laser could be frequency-stabilized without any frequency modulation. As a result, the square root of Allan variance was $\sigma_y(\tau=1s)=7$\times10^{-12}$, for the sampling time of 1 s.of 1 s.

• Journal of the Korean Society for Nondestructive Testing
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• v.21 no.4
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• pp.391-397
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• 2001

The guided wave propagation in inhomogeneous multi-layered structures is experimentally explored based on theoretical dispersion curves. It turns out that proper selection of incident angel and frequency is critical for guided wave generation in multi-layered structures. Theoretical dispersion curves greatly depend on adhesive zone thickness, layer thickness and material properties. It was possible to determine the adhesive zone thickness of an inhomogeneous multi-layered structure by monitoring experimentally the change of dispersion curves.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.37 no.8
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• pp.45-51
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• 2000

The distortion of an electrical pulse, which has a rise/fall time due to the dispersion and the reflection, on tapered microstrip lines has investigated In time domain. The voltage and current transfer functions are shown for the tapered line. The dispersion distortion obtained by using these trans(or functions are represented for the nonideal square pulse along the triangular, Tchebycheff and exponential tapered lines, and analyzed the influence of the reflection and the frequency dispersion on the distorted voltage wave in the tapered lines. The observed overshoot in front of the distorted wane is caused due to the frequency dispersion and the sustained tail of that comes from the reflection in the tapered line.

• Proceedings of the Acoustical Society of Korea Conference
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• 1994.06a
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• pp.734-739
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• 1994

Elastic wave propagation in discrete random medium is studied to evaluate the effects of particle resonance on dispersion and attenuation of composite materials containing spherical inclusions. The frequency-dependent wave speed and attenuation coefficient can be obtained from proposed self-consistent method. It can be observed that the abrupt increase of effective wave speed and the concurrent peak of attenuation at low frequency is due to the lowest resonance of particles, whereas those in high frequency region are due to higher ones. The lowest resonance is mainly caused by the density mismatch and higher resonances by the stiffness mismatch between matrix and particles. The dispersion and attenuation of elastic waves in particulate composites are affected by the lowest resonance much than by higher ones.

• Proceedings of the IEEK Conference
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• 2000.11b
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• pp.309-312
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• 2000

In this paper, The Influence of chromatic fiber dispersion on the transmission distance of MMW(millimeter-wave) link is analyzed and discussed. It is shown that dispersion significantly limits the transmission distance in intensity modulated direct detection and heterodyne links operating in the above 20㎓ frequency region by inducing a carrier to noise penalty on the transmitted signal. We analyze and discuss the influence of dispersion induced CNR(Carrier to Noise Ratio) penalty for direct detection and heterodyne method from simulation.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.20 no.4
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• pp.348-351
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• 2007

Recently, Nanotechnology becomes a major issue in most part of industries. Nanotechnology is expected to develop various application products due to nano material mired composites is improved physical and electrical properties compared to conventional composites materials. Dielectric and insulation materials need to develop and improve like other field about nanotechnology. In this paper, we reported dielectric dispersion by size(no filler, $1.2{\mu}m$, 500 nm, 10 nm), frequencies(60, 120, 1 kHz), and temperatures($30{\sim}170^{\circ}C$). Dielectric constant of composites materials with filler shows higher than composites materials without filler and increased depending on rising temperatures in low frequency region. It was the effect that nano-filler and impurities in composites contributed to electrical conductivity. And dielectric properties depending on temperatures shows to change in low frequency region dramatically We analyzed interfacial polarization in low frequency region($10^{-2}$ Hz) and oriented polarization in high frequency region($10^{-5{\sim}6}$ Hz) on composites materials.

• The Journal of the Korea institute of electronic communication sciences
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• v.8 no.9
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• pp.1343-1349
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• 2013

In optical fiber communications, several newly-developed signal formats are used to obtain the best performance within limited spectral bandwidth. CSRZ (carrier-suppressed return-to-zero) format is one of the new signal formats, which has better spectral efficiency and better robustness to dispersion than RZ (return-to-zero) format. Thus it is widely used for demonstrating high-speed optical communication systems. In an earlier research, we proposed a clock-extraction method of CSRZ signal to monitor chromatic dispersion. However, the clock-frequency component extracted by the clock-extraction method can be affected by the bandwidth of a transmitter or a receiver. Therefore, in this paper, we investigate the effect of bandwidth on the chromatic dispersion monitoring of CSRZ signal based on clock-frequency component. As a result, we propose a couple of robust clock-extraction methods to monitor chromatic dispersion in CSRZ signal.

• JSTS:Journal of Semiconductor Technology and Science
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• v.4 no.1
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• pp.74-81
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• 2004

We propose a novel scheme to transmit high center frequency RF signals using electroabsorption devices (EADs) as frequency converters at the transmitter and the receiver. In this approach frequency heterodyning is employed for obtaining high center frequency. With the EAD as a detector/mixer at the receiver we demonstrated a smaller conversion loss than that of the conventional modulator/mixer. With EAD as a modulator/mixer at the transmitter and with two heterodyned lasers to generate an optical local oscillator (LO), we demonstrated a large reduction (${\sim}23dB$) in conversion loss, and the transmission is not limited by the optical saturation of the EAD. This transmission scheme has optical single-side-band transmission feature which greatly relieves the fiber dispersion effect.

• Proceedings of the Optical Society of Korea Conference
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• 2003.02a
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• pp.222-223
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• 2003

As laser pulse width becomes shortened from nanoseconds to femtoseconds, the effects caused by the dispersions of nonlinear optical mediums, such as group velocity mismatch and group velocity dispersion become considerably significant. The group velocity mismatch and group velocity dispersion are the major factors that lead to a decrease of frequency conversion efficiency and pulse spreading for picosecond and femtosecond pulses. (omitted)