Optimizing the performance of TOAD by changing the wavelength and power of control pulse

The performance of terahertz optical asymmetric demultiplexer (TOAD) has been studied by modelling the semiconductor optical amplifier (SOA) in which the intraband effects had been taken into account. Numerical results are coincident with the experiment results. We interpret why there are three peaks in the switching window, which has never been reported before. In addition, we put forward the definition of the flatness of the switching window of TOAD for the first time By analysing the different phase of clockwise and counter clockwise signal pulse changed by SOA, appropriate peak power of control pulse and wavelength of signal and control pulse have been calculated in order to obtain large output power and flat switching window of TOAD.     

Study of characteristics of ultrasound pulse generated by a laser pulse

I.IntroductionThegenerationofacousticpulsebylaserirradiationofametalsurfacewasfirstsuggestcdbyWhitein1963[1l.SincethatdateLaserU1trasoundtechniquchasbeendcvclopedrapidly.Becausethistechniquehasanumberoftechnicalfcatures,suchasnon-contact,highbandwidth,highhme-spacia1resolution,quantitativeteshng,generationoflongitudinal,shcarandRay1cighwaves(simu1taniously),andsoon,ithasbccnwidelyapp1icdtomcasurementsofmatcrialproperties,detectionofdefects,andcalibrationoftransd.ccrsl'-'o].Inordertodcve1opth…     

Effect of the ratio of transition dipole moments on few-cycle pulse propagation

Propagation of a few-cycle laser pulses in a dense V-type three-level atomic medium is investigated based on full-wave Maxwell-Bloch equations by taking the near dipole-dipole (NDD) interaction into account. We find that the ratio,γ,of the transition dipole moments has strong influence on the time evolution and split of the pulse:whenγ≤1,the NDD interaction delays propagation and split of the pulse,and this phenomenon is more obvious when the value ofγis smaller;whenγ=2~(1/2),the NDD interaction accelerates propagation and split of the pulse.     

Modeling the dynamics of one laser pulse surface nanofoaming of biopolymers

Self standing films of biopolymers like gelatine, collagen, and chitosan irradiated with single nanosecond or femtosecond laser pulse easily yield on their surface, a nanofoam layer, formed by a cavitation and bubble growth mechanism. The laser foams have interesting properties that challenge the molecular features of the natural extracellular matrix and which make them good candidates for fabrication of artificial matrix (having nanoscopic fibers, large availability of cell adhesion sites, permeability to fluids due to the open cell structure). As part of the mechanistic study, the dynamics of the process has been measured in the nanosecond timescale by recording the optical transmission of the films at 632.8 nm during and after the foaming laser pulse. A rapid drop 100→0% taking place within the first 100 ns supports the cavitation mechanism as described by the previous negative pressure wave model. As modeled a strong pressure rise (∼several thousands of bar) first takes place in the absorption volume due to pressure confinement and finite sound velocity, and then upon relaxation after some delay equal to the pressure transit time gives rise to a rarefaction wave (negative pressure) in which nucleation and bubble growth are very fast.     

On the error of the time–pulse method of measuring air consumption in mines

The derivation of a formula for the time during which a sound signal propagates between two given points A and B in a stationary gas flow is considered. It is shown that the gas flow changes the signal reception time by a quantity proportional to the consumption, regardless of the detailed velocity profile. The difference between the reception time of signals from point B to the point A and vice versa is proportional to air consumption with high accuracy. It is shown that the relative error of the obtained formula does not exceed the squared maximum Mach number in the gas flow. This allows measurement of the consumption of gas moving in a mine with an arbitrary stationary subsonic velocity field.     

Development of the pulse NMR method utilizing the short-lived β-emitter 12 B

Measurements of the spin-spin relaxation time T ₂ of the short-lived $\upbeta $ -emitter $^{12}B (I^{\uppi }$ = 1^?+?, T _1/2 = 20 ms) in Si have been performed. By applying the spin echo method to $\upbeta $ -NMR measurements, the intrinsic T ₂ for ¹²B in Si was successfully determined with the effect of the static magnetic field inhomogeneity removed. The temperature dependence of T ₂ for ¹²B in Si shows that T ₂ becomes longer with increasing temperature, which seems to reflect the effect of motional narrowing.     

Generation of the cascaded fifth-order nonlinear phase shifts with femtosecond pulse

Due to group velocity mismatch, the generation of the cascaded fifth-order nonlinear phase shifts with femtosecond pulse will inevitably cause the severe pulse distortion. By theoretical study and numerical simulations, we show that undistorted femtosecond pulse which is impressed the cascaded fifth-order nonlinear phase shifts can be obtained at large values of the phase mismatching △κL.     

Effect of magnetic pulse treatment on the magnetic characteristics of yttrium–iron garnets

The effect weak (10–100 kA m^–1) low-frequency (10–20 Hz) pulsed magnetic fields have on the surface structure and magnetic characteristics of yttrium–iron garnet Y₃Fe₅O₁₂ is studied by means of electron and Mössbauer spectroscopy. A mechanism is proposed for the variation of saturation magnetization in Y₃Fe₅O₁₂ after magnetic pulse treatment. The mechanism is associated with the change in the spin state of iron ions localized in the tetrahedral sublattice.     

Precise measurement of the micron-scale spot of ultrashort laser pulse based on film scanning

A novel and precise micron-scale nanosecond laser spot measurement based on film-scanning method is presented.The method can be used to measure the spot size,beam profile,and intensity distribution of the pulse.The central spot radius of the pulsed Bessel beams with pulse width of 25 ns is measured to be 94.86±5μm in our experiment through the analysis of the digital image by film scanning,and the result is consistent with the theoretical value of 92.33μm.Compared with charge-coupled device/complementary metal oxide semiconductor(CCD/CMOS)laser beam profilers,the film-scanning method shows higher measurement accuracy,single shot ultrashort pulse measurable,larger measurable size,and wider measur- able wavelength range.     

Measurement of the third order nonlinear optical coefficients of GaP and chirp parameter of laser pulse

The spectral resolved femtosecond pump-probe experiment is applied to measure the third order nonlinear refractive index coefficient, two-photon absorption coefficient of GaP crystal, and the chirp parameter of the input laser pulses. The results show that nonlinear refractive index coefficient is 2 × 10 -18 m2/W, two-photon absorption coefficient is 2×10-11 m/W at wavelength of 783.5 nm, and the chirp parameter of laser pulse is 1×1025 s-2. Furthermore, the mechanism of nonlinear refraction due to two-photon absorption in GaP crystal and experimental results are discussed.     

Theoretical and experimental research on the pulse duration of excimer laser output

The effects of discharge stability and cavity buid-up time of pulse laser on thepulse duration of excimer laser are investigated theoretically.The characteristics of the ex-cimer laser pulse duration can be explained by these theoretical analysis.     

Effect of diffraction on the ultrasonic velocity measuredby the pulse interference method in VHF range

1 IntroduCtionIt has been demonstrated that, the frequenCy dependences of ultraSOnic velocity and at-tenuatiOn in VHF range are very useful in the ultrasenic characterizations of silicate glasses,especmp the siedcant vefocity djspersions dsited in tfor frequncy rang and the power despendences of freqUeny of attenuation coefficients can be erpected to be used as the alternativeacoustic paramters in the ultrasonic characterizations of materiaIs[1].In order to accurately measure the frequellc…     

On the propagation of a light pulse in the “transparency window” of a medium with population inversion

The propagation of a light pulse through a layer of a substance with population inversion and, correspondingly, with an anomalous dispersion in the transparency range is considered. It is shown that, when the wave packet approaches the layer of the substance with a strong anomalous dispersion, two additional wave packets arise at the rear boundary of the layer. One of these packets has a negative energy and moves in the layer from its rear to the front boundary, while the other packet moves away from the rear boundary of the layer into a vacuum. As the initial wave packet comes into contact with the front boundary of the layer of the substance, it meets the packet of the negative energy. As a result, the two packets annihilate one another, after which only one packet remains, which moves away from the layer.     

Investigation of the magnetization vector hodographs corresponding to the 90° pulse magnetization of ferrite-garnet films with easy-plane anisotropy

The magnetization vector hodographs corresponding to the regime of 90° pulse magnetization of ferrite-garnet films with easy-plane anisotropy have been analyzed. It is found that when the magnetizing pulse amplitude H _m exceeds some threshold value H*_m (≈15 Oe), magnetization occurs through the mechanism of uniform magnetization vector rotation. The weak dependence of the intensity of nonlinear oscillations (accompanying the magnetization process) on the rise time of the magnetizing pulse, observed in the fields H _m > H*_m, is explained by the presence of biaxial anisotropy in the plane of real films.     

Phase evolution of the reemitted field in the semiconductor quantum wells under the femtosecond pulse train intersubband excitation

Property of the phase of the reemitted field in the semiconductor quantum wells (QWs) excited by femtosecond pulse train is investigated. It is shown that the phase evolution of the reemitted field is controlled by the relative phase between the successive pulses of the incident train. For all the odd pulses excitation, the reemitted field is from out-of-phase to in-phase, then again to out-of-phase with the incident pulses, whereas for all the even pulses excitation, the situation is the opposite, i.e., it is from in-phase to out-of-phase, then again to in-phase with the incident pulses.     

Optimisation of the pulse duration of a discharge-pumped XeF(B→X) excimer laser

In an x-ray preionised XeF(BX) discharge-excited laser driven by a magnetic-spiker sustainer circuit with a magnetic pulse compressor the influence of the various parameters on the optical pulse duration was experimentally investigated. Laser pulses at = 351 nm with a duration of 212 ns (FWHM) have been achieved in a NF₃/Xe/Ne mixture by using very low (0.25 mbar) NF₃ partial pressures in a total gas pressure of 2.5 bar and with a high-reflecting output coupling mirror.     

Simultaneous compression of the pulsewidth and pulse train of a passively mode-locked Nd:YAG laser

Simultaneous compression of the pulsewidth and the pulse train of a passivelymode-locked Nd:YAG laser has been achieved by using a plano-convex unstable resonatorwith a nonlinear Sagnac ring interferometer.A single pulse energy of >30mJ and apulsewidth of ≤10ps have been obtained.Using this system,the LAGEOS and ETALONsatellites laser ranging have been performed successfully.     

Synchronization of long/short light pulse

Based on rough synchronization of long/short light pulse,fine synchronizationof long/short light pulse was achieved with a photoconductive switch.     

Propagation of a light pulse inside matter in a context of the Abraham–Minkowski dilemma

It is shown that a light pulse propagating in an optical medium exerts the optical pressure on the medium in the regions where leading and trailing edges are propagating. This effect is derived from analysis of unambiguous thought experiments which results contradict one other. It is shown that a magnitude of the pressure is equal to (n − 1/n)W₀, where n and W₀ is the refractive index of the medium and the momentum flux density of the same pulse in free space, respectively. The Abraham form of the momentum of light is redundant if the optical pressure is taken into account. In this case the dilemma disappears because one of the rival alternatives disappears.     

Frequency shift and pulse compression of an ultrashort pulse in a large-amplitude plasma wave

By calculating the momenta of a coupled set of nonlinear equations of laserpulse-plasma wave interaction in the weak relativistic approximation,the conditions for fre-quency up-shift have been found.That the energy change of the pulse due to frequency shiftis compensated by the change of plasma wave energy results in photon number conservation.Some factors that affect the frequency up-shift are studied.It is also found that the laser pulsecan be compressed when it is located near the valley of plasma density perturbation and ifsome threshold value of the plasma wave field is exceeded.