Echolocation click sounds from wild inshore finless porpoise (Neophocaena phocaenoides sunameri) with comparisons to the sonar of riverine N. p. asiaeorientalis

Acoustic signals from wild Neophocaena phocaenoides sunameri were recorded in the waters off Liao-dong-wan Bay located in Bohai Sea, China. Signal analysis shows that N. p. sunameri produced "typical" phocoenid clicks. The peak frequencies f(p), of clicks ranged from. 113 to 131 kHz with an average of 121 +/- 3.78 kHz (n=71). The 3 dB bandwidths Delta f ranged from 10.9 to 25.0 kHz with an average of 17.5 +/- 3.30 kHz. The signal durations At ranged from 56 to 109 mu s with an average 80 +/- 11.49 mu s. The number of cycles N, ranged from 7 to 13 with an average of 9 +/- 1.48. With increasing peak frequency there was a faint tendency of decrease in bandwidth, which implies a nonconstant value of f(p)/Delta f. On occasion there were some click trains with faint click energy presenting below 70 kHz, however, it was possibly introduced by interference effect from multiple pulses structures. The acoustic parameters of the clicks were compared between the investigated population and a riverine population of finless porpoise. (c) 2007 Acoustical Society of America.

The dynamic effect of a piled-foundation building on an incident vibration field from an underground railway tunnel

This paper presents a three-dimensional comprehensive model for the calculation of vibration in a building based on pile-foundation due to moving trains in a nearby underground tunnel. The model calculates the Power Spectral Density (PSD) of the building's responses due to trains moving on floating-slab tracks with random roughness. The tunnel and its surrounding soil are modelled as a cylindrical shell embedded in half-space using the well-known PiP model. The building and its piles are modelled as a 2D frame using the dynamic stiffness matrix. Coupling between the foundation and the ground is performed using the theory of joining subsystems in the frequency domain. The latter requires calculations of transfer functions of a half-space model. A convenient choice based on the thin-layer method is selected in this work for the calculations of responses in a half-space due to circular strip loadings. The coupling considers the influence of the building's dynamics on the incident wave field from the tunnel, but ignores any reflections of building's waves from the tunnel. The derivation made in the paper shows that the incident vibration field at the building's foundation gets modified by a term reflecting the coupling and the dynamics of the building and its foundation. The comparisons presented in the paper show that the dynamics of the building and its foundation significantly change the incident vibration field from the tunnel and they can lead to loss of accuracy of predictions if not considered in the calculation.

Lineshape Analysis of the Beat Signal Between Optical Carrier and Delayed Sidebands

This paper presents the lineshape analysis of the beat signal between the optical carrier and the shifted and delayed side-bands produced by sinusoidal amplitude modulation. It is shown that the beat signal has a typical lineshape with a very narrow delta-peak superposed on a quasi-Lorentzian profile. Theoretical explanation for the appearance of this peak has been given based on optical spectral structure constructed by a large number of optical wave trains. It is predicted that the delta-peak is originated from the beat between the wave trains in the carrier and those in the delayed sidebands when their average coherence length is longer than the delay line. Experiments carried out using different delay lines clearly show that the delta-peak is always located at the modulation frequency and decreases with the increasing delay line. Our analysis explicitly indicates that the linewidth is related to the observation time. It is also suggested that the disappearance of the delta-peak can be used as the criterion of coherence elimination.

Hyperfine spectral structure of semiconductor lasers

In this paper, we propose an interference technique that can provide a quantitative and ultrafine-resolution spectral analysis because the optical heterodyning is performed at nonzero frequency and interfering waves propagate in optical fiber. The spectrum of a laser consists of a large number of wave trains. Our study is focused on the features of wave trains. We demonstrate that wave trains emitting simultaneously have random frequency spacings, and the probability of occurrence of two or more joint wave trains with the same frequency is high. The estimated linewidth of the wave train is narrower than 1 mHz, corresponding to a wavelength range of 10(-23) m.

Passively Q-switched mode-locking of diode-pumped Nd : YVO4 laser with GaAs intracavity absorber grown at low temperature

Using a low temperature grown GaAs wafer as an intracavity saturable absorber, a temporal envelope duration of 11 ns of Q- switched and mode- locked ( QML) 1064 nm operation was achieved in a very simple compact plane- concave cavity Nd: YVO4 laser, it was so short that the pulses can be used as Q- switching pulses. The maximal average output power is 808 mW with the repetition rate of 25 kHz, and the corresponding peak power and energy of a single Q- switched pulse was 2.94 kW and 32.3 mu J, respectively. The mode- locked pulse trains inside the Q- switched pulse envelope had a repetition rate of 800 MHz.

Experiments of a diode-pumped Nd : GdVO4/LT-GaAs Q-switched and mode-locked laser

A diode-pumped passively Q-switched mode-locked (QML) Nd:GdVO4 laser with a low temperature GaAs (LT-GaAs) saturable absorber is presented. The maximal Q-switched mode-locked average output power was 798 mW with the Q-switched envelop having a repetition rate of 125 kHz. The mode-locked pulse trains inside the Q-switched pulse envelope had a repetition rate of similar to 750 MHz. The laser properties of the operational parameters on the pump power were also investigated experimentally.

Experimental investigation of intracavity absorber low temperature GaAs in diode-pumped Nd : GdVO4 laser

A passively Q-switched and mode-locked diode-pumped Nd:GdVO4 laser was demonstrated using a low-temperature-grown GaAs wafer (LT-GaAs) as an intracavity saturable absorber. The maximal Q-switched mode-locked average output power was 750 mW with the Q-switched envelop having a repetition rate of 167 kHz. The mode-locked pulse trains inside the Q-switched pulse envelope had a repetition rate of similar to 790 MHz.

MQW electroabsorption modulator-integrated DFB laser modules for high-speed transmission

Details of the design, fabrication and testing of a strained InGaAsP/InGaAsP multiple quantum well (MQW) electroabsorption modulator (EAM) monolithically integrated with a DFB laser by ultra-low-pressure selective area growth (SAG) are presented. The method greatly simplifies the integration process. A study of the controllability of band-gap energy by SAG has been performed. After being completely packaged in a seven-pin butterfly compact module, the device successfully performs 10 Gb s(-1) nonreturn to zero (NRZ) operation on uncompensated transmission span >53 km in a standard fibre with a 8.7 dB dynamic extinction ratio. A receiver sensitivity of -18.9 dBm at a bit error rate (BER) of 10(-10) is confirmed. 10 GHz short pulse trains with 15.3 ps pulsewidth have also been generated.

on string languages generated by spiking neural p systems

We continue the study of spiking neural P systems by considering these computing devices as binary string generators: the set of spike trains of halting computations of a given system constitutes the language generated by that system. Although the "direct" generative capacity of spiking neural P systems is rather restricted (some very simple languages cannot be generated in this framework), regular languages are inverse-morphic images of languages of finite spiking neural P systems, and recursively enumerable languages are projections of inverse-morphic images of languages generated by spiking neural P systems.

Efficient Simulation of Freak Waves in Random Oceanic Sea States

Numerical simulations of freak wave generation are studied in random oceanic sea states described by JONSWAP spectrum. The evolution of initial random wave trains is numerically carried out within the framework of the modified four-order nonlinear Schroedinger equation (mNLSE), and some involved influence factors are also discussed. Results show that if the sideband instability is satisfied, a random wave train may evolve into a freak wave train, and simultaneously the setting of the Phillips parameter and enhancement coefficient of JONSWAP spectrum and initial random phases is very important for the formation of freak waves. The way to increase the generation efficiency of freak waves though changing the involved parameters is also presented.

Environmentally stable ytterbium figure-of-eight fiber laser producing 177-fs pulses

Pulses of 177 fs and 1035 nm, with average power of 1.2 mW, have been generated directly from a passively mode-locked Yb-doped figure-of-eight fiber laser, with a nonlinear optical loop mirror for mode-locking and pairs of diffraction gratings for intracavity dispersion compensation. To our knowledge, these are the shortest pulses ever to come from a passively mode-locked Yb-doped figure-of-eight fiber laser. This represents a 5-fold reduction in pulse duration compared with that of previously reported passively mode-locked Yb-doped figure-of-eight fiber lasers. Stable pulse trains are produced at the fundamental repetition rate of the resonator, 24.0 MHz. (c) 2007 Elsevier B.V. All rights reserved.

Generation of 207-femtosecond pulses from a ytterbium figure-of-eight fibre laser

We report the generation of 207-fs pulses with 1.2mW average power at 1036 nm directly from a passively mode-locked Yb-doped fibre laser with a nonlinear optical loop mirror for mode-locking and pairs of diffraction gratings for intracavity dispersion compensation. These results imply a 4-fold reduction in pulse duration over previously reported figure-of-eight cavity passively mode-locked Yb-doped fibre lasers. Stable pulse trains are produced at the fundamental repetition rate of the resonator, 24.0MHz. On the other hand, this laser offers a cleaner spectrum and greater stability and is completely self-starting.

40-GHz wavelength tunable mode-locked SOA-based fiber laser with 40-nm tuning range

A 40-GHz wavelength tunable mode-locked fiber ring laser based oil cross-gain modulation in a semiconductor optical amplifier (SOA) is presented. Pulse trains with a pulse width of 10.5 ps at 40-GHz repetition frequency are obtained. The laser operates with almost 40-nm tuning range. The relationship between the key laser parameters and the output pulse characteristics is analyzed experimentally.

Comparative studies of semiconductor saturable absorber mirror mode-locking dynamics in pulsed diode-end-pumped picosecond Nd:GdVO4 and Nd:YAG lasers

Ultrashort pulses were generated in passively mode-locked Nd:YAG and Nd:GdVO4 lasers pumped by a pulsed laser diode with 10-Hz repetition rate. Stable mode-locked pulse trains were produced with the pulse width of 10 ps. The evolution of the mode-locked pulse was observed in the experiment and was discussed in detail. Comparing the pulse evolutions of Nd:YAG and Nd:GdVO4 lasers, we found that the buildup time of the steady-state mode-locking with semiconductor saturable absorber mirrors (SESAMs) was relevant to the upper-state lifetime and the emission cross-section of the gain medium.

Generation of optimized wavelength-tunable optical pulse from an uncooled gain-switched Fabry-Perot semiconductor laser using optical amplifier as external light injection

We present a novel system design that can generate the optimized wavelength-tunable optical pulse streams from an uncooled gain-switched Fabry-Perot semiconductor laser using an optical amplifier as external light source. The timing jitter of gain-switched laser has been reduced from about 3 ps to 600 fs and the pulse width has been optimized by using our system. The stability of the system was also experimentally investigated. Our results show that an uncooled gain-switched FP laser system can feasibly produce the stable optical pulse trains with pulse width of 18 ps at the repetition frequency of 5 GHz during 7 h continuous working. We respectively proved the system feasibility under 1 GHz, 2.5 GHz and 5 GHz operation. (c) 2008 Elsevier B.V. All rights reserved.