Ultra-Wideband Electromagnetic Radiation from GaAs Photoconductive Switches

The experiment results of ultrawide band electromagnetic radiation with DC biased GaAs photoconductive semiconductor switch combining double ridge horn antenna triggered by high repeat frequency femto-second laser pulse are reported.The GaAs switches are insulated by solid multi-layer transparent dielectrics and the distance of two electrodes is 3mm.The electrode material of the switch is ohmic contact through alloy technics with definite proportion of Au/Ge/Ni.This switch and double ridge horn antenna are integrated and the receive antenna is connected with the test instrument.From receiving antenna,ultra fast electrical pulse of 200ps rise time and 500ps pulse width is obtained,the repetition rate of the pulse is about 82MHz and the frequency spectrum is in the range of 4.7MHz～14GHz.The radiation characteristic of the ultrafast electrical pulse is analyzed.

Pulsed digital micro-holography of femto-second order by double-wavelength recording

Double-wavelength recording used in a pulsed digital micro-holographic system to record ultra-fast processing of the order of femto-second is reported for the first time, where a BBO crystal is used to generate harmonic wave of the incident laser wave, and both of the basic and the frequency doubled waves are time-delayed and introduced to a Michelson＇s interferometer to record two sub-holograms with different spatial frequencies on a single frame of a CCD. In the experiment, an ultrafast dynamic process of air ionization induced by a single femto-second laser pulse is recorded with holography by this system, and both of intensity and phase difference images digitally reconstructed are obtained through Fourier transformation and digital faltering, which show clearly the dynamic process of formation and propagation of the plasma, with a time resolution of the order of femto-second.

Generation of low jitter and discrete tunable dual-wavelength optical pulses at arbitrary repetition rates

A novel and simple method to generate low timing jitter and discrete tunable dual-wavelength optical pulses at arbitrary repetition rates is demonstrated in this paper.Two multiple quantum wells distributed feedback laser diodes,were used as the external seeding sources to inject the external photons into a gain-switched Fabry-Perot laser diode.The output wavelengths can be tuned discretely to coincide with any two lasing modes in the gain spectra range of the Fabry-Perot Laser diode,and the output side mode suppression ratio was better than 25 dB.Moreover,the timing jitter of optical pulses was reduced from 1.89 ps to 0.83 ps.It was empirically found that the lowest timing jitter operation occurred when the injected light wavelength is 0.2-0.3 nm shorter than the locked mode of the Fabry-Perot laser diode.To our knowledge,this is the first report of using two DFB laser diodes as a seeding source to reduce pulses jitter and select lasing dual-wavelength simultaneously.

Theoretical study on pulse-shaping of Stokes pulse with steep leading edge by two Brillouin amplifiers

A novel configuration of two Brillouin amplifiers, which contains a main amplifier combined with a reshaping amplifier, is suggested to control pulse shape of Stokes pulses with steep leading edge. Dependences of pulse shapes on several param- eters are numerically simulated. By changing the distance between the two amplifiers, the leading edge of amplified pulses can be finely adjusted. Smooth and symmetrical pulses or pulses with slow leading-edge are achieved. Theoretical re- searches prove that this system is fit for shaping pulses with steep leading edge, especially, for controlling leading edge of pulses. The results provide useful and necessary theoretical basis and guidance for the future experimental research.

Third-Harmonic Generated in EH32 Mode of a Gas-Filled WavegUide by fs Laser Pulses

In this paper it is reported for the first time that the third harmonic generated in EH32 mode of a gas-filled waveguide by fs pulses has higher generation efficiency. The new finding contrasts with the experiment in It. G. Durfee Ⅲ, S. Backus, M.M. Murnane, and H.C. Kapteyn, Opt. Left. 22 （1997） 1565]. Some possible factors, which produce the contradiction, are discussed briefly.

High-Dimensional Nonlinear Envelope Equations and Nonlinear Localized Excitations in Photonic Crystals

We investigate the nonlinear localized structures of optical pulses propagating in a one-dimensional photonic crystal with a quadratic nonlinearity. Using a method of multiple scales we show that the nonlinear evolution of a wave packet, formed by the superposition of short-wavelength excitations, and long-wavelength mean fields, generated by the self-interaction of the wave packet, are governed by a set of coupled high-dimenslonal nonlinear envelope equations, which can be reduced to Davey-Stewartson equations and thus support dromionlike high-dimensional nonlinear excitations in the system.

A Review on Fiber Lasers

After a half century of development, fiber laser has evolved from a concept to a great family penetrating into various fields of applications. This paper reviews the history and current development of fiber lasers, with topics covering both continuous wave and short pulse fiber lasers. Important issues such as the major rare earth dopants, fiber laser brightness, polarization effects, clad pumping technology, beam combination, mode locking and pulse shaping are discussed in this paper.

Performance of engineered cementitious composite containing stone slurry powder waste

Most of the materials used in engineered cementitious composite are fine in size to achieve ductile nature.Stone slurry powder(SSP)is an inert material obtained from stone industries as by-product which may cause hazardous impact on environment.In this research work,partial replacement of silica sand(SS)and fine sand(FS)by SSP with different contents(25%and 50%each)for making engineered cementitious composite has been explored.The performance was evaluated on the basis of strength,tensile strain,mid span deflection capacity,ultra-sonic pulse velocity and microstructure.Mechanical strength was found to be increased at 25%SSP in both replacements;whereas,strength decreased slightly at 50%replacement.Tensile strain,mid span deflection and quality of concrete were enhanced with increase in SSP content.Using SSP formed denser cementitious composite can help to save the natural resources and contribute in making green cementitious composite.

Microwave-Controlled Light-Pulse Propagation in a Driven A-Type Atomic System with Two-Folded Levels

The temporal and spatial dynamics of one weak probe laser pulse, propagating through a A-type atomic medium with two-folded levels under the resonant excitation of one microwave driving field and one strong control field, is investigated in this paper. By numerically solving coupled Bloch-Maxwell equations, it is found that, in the absence of the microwave driving field, the atomic medium is transparent to the probe pulse at line center, which propagates over sufficiently long distances. By contrast, when the microwave driving field is applied, the probe pulse at line center can be rapidly absorbed on propagation. This substantial reduction of probe transmittance caused by the microwave driving field may lead to potential applications in designing a new kind of optical switching.

Femtosecond Stimulated Raman Line Shapes:Dependence on Resonance Conditions of Pump and Probe Pulses

Resonance enhancement has been increasingly employed in the emergent felntosecond stimu- lated Raman spectroscopy （FSRS） to selectively monitor molecular structure and dynamics with improved spectral and temporal resolutions and signal-to-noise ratios. Such joint eflforts by the technique- and application-oriented scientists and engineers have laid the foundation for exploiting the tunable FSRS methodology to investigate a great variety of photosensitive systems and elucidate the underlying functional mechanisms on molecular time scales. Dur- ing spectral analysis, peak line shapes remain a major concern with an intricate dependence on resonance conditions. Here, we present a comprehensive study of line shapes by tuning the Rarnan pump wavelength from red to blue side of the ground-state absorption band of the fluorescent dye rhodarnine 6G in solution. Distinct line shape patterns in Stokes and anti-Stokes FSRS as well as from the low to high-frequency modes highlight the competition between multiple third-order and higher-order nonlinear pathways, governed by difl^rent res- onance conditions achieved by Raman pump and probe pulses. In particular, the resonance condition of probe wavelength is revealed to play an important role in generating circular line shape changes through oppositely phased dispersion via hot luminescence （HL） pathways. Meanwhile, on-resonance conditions of the Rarnan pump could promote excited-state vibrational modes which are broadened and red-shifted from the coincident ground-state vibrational modes, posing challenges for spectral analysis. Certain strategies in tuning the Raman pump and probe to characteristic regions across an electronic transition band are discussed to improve the FSRS usability and versatility as a powerful structural dynamics toolset to advance chemical, physical, materials, and biological sciences.

Synthesis of Two-Color Laser Pulses for the Harmonic Cutoff Extension

Increasing simultaneously both the cutoff energy and efficiency is a big challenge to all applications of high-order harmonic generation(HHG).For this purpose,the shaping of the waveform of driving pulse is an alternative approach.Here,we show that the harmonic cutoff can be extended by about two times without reducing harmonic yield after considering macroscopic propagation effects,by adopting a practical way to synthesize two-color fields with fixed energy.Our results,combined with the experimental techniques,show the great potential of HHG as a tabletop light source.

Propagation of Solitary Pulses in Optical Fibers with Both Self-Steepening and Quintic Nonlinear Effects

Pulse dynamics and stability in optical fibers in the presence of both self-steepening and quintic nonlinear effects are analyzed. Propagating profiles of the quintic derivative nonlinear Schr¨odinger model are isolated via two invariants of motion. The resulting canonical equation admits exact periodic propagating patterns in terms of the Jacobi elliptic functions, and solitary pulses are recovered in the long wave limit, i.e. degenerate cases of periodic profiles where each pulse is widely separated from the adjacent ones. Two families of such exact wave profiles are identified. The first one has a precise constraint concerning the magnitude of self-steepening and quintic nonlinear effects, while the second one permits more freedom. The reduction to the well established temporal soliton in an optical fiber waveguide in the absence of self-steepening and quintic nonlinearity is demonstrated explicitly. Numerical simulations are performed to identify regimes of parameter values where robust propagation patterns exist.

A microwave photonic filter based on multi-wavelength fiber laser and infinite impulse response

A microwave photonic filter(MPF) based on multi-wavelength fiber laser and infinite impulse response(IIR) is proposed. The filter uses a multi-wavelength fiber laser as the light source, two sections of polarization maintaining fiber(PMF) and three polarization controllers(PCs) as the laser frequency selection device. By adjusting the PC to change the effective length of the PMF, the laser can obtain three wavelength spacings, which are 0.44 nm, 0.78 nm and 1.08 nm, respectively. And the corresponding free spectral ranges(FSRs) are 8.46 GHz, 4.66 GHz and 3.44 GHz, respectively. Thus changing the wavelength spacing of the laser can make the FSR variable. An IIR filter is introduced based on a finite impulse response(FIR) filter. Then the 3-d B bandwidth of the MPF is reduced, and the main side-lobe suppression ratio(MSSR) is increased. By adjusting the gain of the radio frequency(RF) signal amplifier, the frequency response of the filter can be enhanced.

Integrated Optical Waveguide Sensor for Lighting Impulse Electric Field Measurement

A Lithium niobate （LiNbO3） based integrated optical E-field sensor with an optical waveguide Mach-Zehnder interferometer （MZI） and a tapered antenna has been designed and fabricated for the measurement of the pulsed electric field. The minimum detectable E-field of the sensor was 10kV/m. The sensor showed a good linear characteristic while the input E-fields varied from 10kV/m to 370kV/m. Furthermore, the maximum detectable E-field of the sensor, which could be calculated from the sensor input/output characteristic, was approximately equal to 1000kV/m. All these results suggest that such sensor can be used for the measurement of the lighting impulse electric field.

Optical pulse shaping based on a double-phase-shifted fiber Bragg grating

Based on the transfer matrix method,a detailed theoretical and numerical study on double-phase-shifted fiber Bragg grating(FBG)is investigated.Temporal responses of the double-phase-shifted FBG to optical pulse are analyzed and the influence of the two phase-shifts’position on the reflected output pulse is evaluated.Results demonstrate that very different temporal pulse waveforms can be achieved by adjusting the length ratio(α=L2/L1).Specifically,a transform-limited Gaussian input optical pulse can be shaped into flat-top square pulse(α=1.81)or two identical optical pulse sequences(α=1.93).

Surface morphology of refractive-index waveguide gratings fabricated in polymer films

The characteristic modifications are reported on the surface of polymeric waveguide film in the process of vol- ume-grating fabrication. The light from a mode-locked 76 MHz femtosecond laser with pulse duration of 200 fs and wavelength of 800 nm is focused normal to the surface of the sample. The surface morphology modifications are as- cribed to a fact that surface swelling occurs during the process. Periodic micro-structure is inscribed with increasing incident power. The laser-induced swelling threshold on the grating, which is higher than that of two-photon initiated photo-polymerization （TPIP） （8 mW）, is verified to be about 20 mW. It is feasible to enhance the surface smoothness of integrated optics devices for further encapsulation. The variation of modulation depth is studied for different values of incident power and scan spacing. Ablation accompanied with surface swelling appears when the power is higher. By ootimizing the laser carvinR oararneters, hizhly efficient grating devices can be fabricated.

Non-invasive human extremely weak pulse wave measurement based on a high-precision laser self-mixing interferometer

In this paper, a technique based on laser self-mixing effect for acquiring pulse wave profile is proposed and demonstrated. For the characteristics of extremely weak vibration in human pulse waves, a method of multiple reflections with external cavity is designed to improve the measurement accuracy to λ/4. Based on the fringe counting algorithm, one period of pulse wave is successfully reconstructed. The measurement results show that the root mean square error(RMSE) of the reconstructed pulse wave is 0.912 μm at sampling rate of 8 k Hz. Besides, the correlation coefficients and heart beats of this method and the traditional photoplethysmography(PPG) measurement are analyzed.

Multi-wavelength erenkov radiations in a microring resonator in combination with two gratings

In this paper,the erenkov radiation of light pulse in a microring and gratings is simulated and investigated.The system design consists of a two-defect grating incorporating a microring,connected with a uniform grating.In simulation,the continuous wave(CW) light pulse with wavelength centered at 1.55 μm is input into the microring device via the two-defect grating.The resonant outputs from the two-defect grating propagate through the microring and uniform grating,where the time delays of those two input pulses with different wavelengths through the system are distinguished by the output uniform grating.From the obtained resonant output pulses,we find that the red-shifted and blue-shifted erenkov pulses are observed.In applications,such a proposed system can be used to form two different optical delay pulses,in which the change in erenkov radiation of them,i.e.,time delay within a microring device system,can be useful for erenkov radiation imaging and sensing applications.