Amplification and compression of Ti:sapphire femtosecond pulses at high-repetition-rate

The experiment of the generation and amplification of femetosecond Ti:sapphire laser pulse at high repetition rate is reported. The laser pulses with minimum pulsewidth 15 fs, maximum spectrum width of 80 nm, average power of 200 mW are generated from a home-built self-mode-locked Ti:sapphire laser. As a seed pulse which is selected from the oscillator, the laser pulse is further amplified by using chirped-pulse-amplification technology in a Ti:sapphire amplifier from which a kind of pulses with single-pulse-energy of 100 uj, pulsewidth after compressing of 50 fs at 5 kHz repetition rate are produced. The system design and experimental results are discussed. Project supported by the National “Climbing Project” of China.     

Experimental studies of self-starting passive mode locking Er3+ -doped fiber ring soliton laser

The mechanism of femtosecond optical pulse generation in a self-starting Er³⁺-doped fiber ring soliton laser and experimental research results are discussed. Using the nonlinear polarization rotation effect of the fiber for sat-urable absorbers (and then self-amplitude modulation) which acts as the mode locking mechanism in an Er³⁺-doped fiber ring cavity laser, stable self-starting mode locking pulses have been generated. The shortest output pulse is 269 fs, with the central wavelength of 1,531 pm at the repetition rate of 21.37 MHz. The average output powen of the two terminators of the laser are 0.25 mW and 0.08 mW respectively. The threshold pump power which sustains the mode locking is 15 mW. Under high pump power, the laser works in a high order harmonic mode locking state. The mode locking pulse durations vs different cavity lengths are also studied. Project supported by Major Project of Chines: Academy of Sciences (No. KJ952-J1-705).     

A stochastic multiscale model for electricity generation capacity expansion

Long-term planning for electric power systems, or capacity expansion, has traditionally been modeled using simplified models or heuristics to approximate the short-term dynamics. However, current trends such as increasing penetration of intermittent renewable generation and increased demand response requires a coupling of both the long and short term dynamics. We present an efficient method for coupling multiple temporal scales using the framework of singular perturbation theory for the control of Markov processes in continuous time. We show that the uncertainties that exist in many energy planning problems, in particular load demand uncertainty and uncertainties in generation availability, can be captured with a multiscale model. We then use a dimensionality reduction technique, which is valid if the scale separation present in the model is large enough, to derive a computationally tractable model. We show that both wind data and electricity demand data do exhibit sufficient scale separation. A numerical example using real data and a finite difference approximation of the Hamilton–Jacobi–Bellman equation is used to illustrate the proposed method. We compare the results of our approximate model with those of the exact model. We also show that the proposed approximation outperforms a commonly used heuristic used in capacity expansion models.     

Rotor dynamics behavior of turbo-expander involving droplet impact

ABSTRACT

This paper dedicates on the rotor dynamics behaviour research on the turbo-expander rotor system involving droplet impact. A stochastic model based on Beta distribution and Bernoulli distribution of droplet generation is established and the formulations of droplet impact forces are deduced, which is applied on the rotor dynamics equations of the tilting pad bearing supported turbo-expander considering the temperature gradient for a further analysis. A time domain research is carried out and a conclusion that continuously droplet impact will perturb the steady vibration of the turbo-expander rotor system is obtained. Monte Carlo method is implemented for a statistics dynamics research and the results suggests that in the design of expander impellers, in order to decrease the uncertainty brought by droplet impact, the number of channels should be as few as possible, the droplet impact should be controlled to occur uniformly, and the collision, entrainment of the primary droplets and the stripping of the liquid film on the blade should be strictly restrained.     

Quantum equations of motion for multimode laser generation with a spatial dependence of the atom interaction with the field taken into account

We derive equations of motion for the electromagnetic field operators a_q′ ⁺ a_q″ for a three-level multimode laser with a spatial dependence of the interaction of atoms with the field of a standing wave in a cavity taken into account. We calculate and analyze the dynamics of means of photon numbers in the field modes and of the correlation function of field modes. We explore the effect of intermode correlations on the dynamics of establishing stationary laser generation. We find that taking the spatial dependence of the interaction of atoms with the field and the intermode correlation into account in investigating the means of photon numbers leads to revealing new properties of laser generation, such as saturation of the laser radiation intensity in a single-mode regime and generation of short light pulses of side below-threshold modes with the amplitudes depending on the initial state of the field in a cavity.     

Parametric generation of even polarization harmonics in a layer with cubic nonlinearity

The article consider the response of a cubically nonlinear medium to a small-period laser pulse when the optical frequency is close to the linear oscillator frequency of the medium. We demonstrate the possibility of parametric generation of even harmonics in response to a high-intensity femtosecond pulse when its amplitude exceeds some critical value. The shape of the generated pulses is shown to depend on the absolute phase of the input pulse. The formation of a supercontinuum is observed when the applied small-period pulse has a certain duration and a certain intensity. __________ Translated from Prikladnaya Matematika i Informatika, No. 22, pp. 31–49, 2005.     

Contact angle effects on microdroplet deformation using CFD

In this paper we use computational fluid dynamics (CFD) to study the effect of contact angle on droplet shape as it moves through a contraction. A new non-dimensional number is proposed in order to predict situations where the deformed droplet will form a slug in the contraction and thus have the opportunity to interact with the channel wall. It is proposed that droplet flow into a contraction is a useful method to ensure that a droplet will wet a channel surface without a trapped lubrication film, and thus help ensure that a slug will remain attached to the wall downstream of the contraction. We demonstrate that when a droplet is larger than a contraction, capillary and Reynolds numbers, and fluid properties may not be sufficient to fully describe the droplet dynamics through a contraction. We show that, with everything else constant, droplet shape and breakup can be controlled simply by changing the wetting properties of the channel wall. CFD simulations with contact angles ranging from 30° to 150° show that lower contact angles can induce droplet breakup while higher contact angles can form slugs with contact angle dependent shape.     

Control of chaos in laser plasma interaction

The chaotic dynamics originating from the equation governing the laser plasma interaction is studied. Our motivation is to show that it is possible to control this chaotic scenario either to a periodic state or to a totally steady state by adopting two different modes of control – one is the sinusoidal time variation of one parameter of the system and the other is the proportional pulse approach. Extensive use is made of Poincaré section, power spectrum analysis and phase space plot to prove the assertions. The observations can be of practical use in the simulation of plasma experiments.     

Experimental studies of high order soliton compression effect and gain characteristics in femtosecond laser pulses E3+r-doped fiber amplifier

Seed laser pulses with average power of 146 μW and pulse duration of 480 fs were amplified to 14.5 mW. The pulse duration was compressed to 260 fs using 6 m high concentration E3+r-doped fiber under forward pumping. The amplified signal pulse energy was 0.691 nJ (corresponding to a peak power of 2 657.7 W) and the repetition rate was 20.84 MHz. Spectrum breakup was observed simultaneously. The spectrum of pulses amplified by 3 m E3+r-doped fiber remains a single peak under different pump power. The amplified pulse duration was compressed abnormally with the increasing pump power using the backward pumping; that is, the amplified pulses were compressed with the increasing pump power under low pump power. When the pump power reached 38 mW, the shortest amplified pulse duration was 309 fs. With further increase in pump power, the amplified pulses began broadening, accompanied by a single peak spectrum under different pump power.     

Dynamics of electron in intense laser field

The induced magnetic field produced by a circular polarization laser pulse propagating in a cold plasma, and the dynamics of injected electron in the combination field of the laser field and the induced magnetic field are investigated. As a circular polarization laser propagates in a plasma, a quasistatic magnetic field in the direction of the wave propagation is rising. An evolution equation for the induced magnetic field is derived. Based on the derived equation, the properties of the induced magnetic field are discussed. The injected electron which satisfies the cyclotron resonance condition can be accelerated by the combination field. The energy equation for the injected electron is obtained. Finally, the classical dynamics of the injected electron in the combination field is analyzed.     

Experimental studies of high order soliton compression effect and gain characteristics in femtosecond laser pulses Er 3+-doped fiber amplifier

Seed laser pulses with average power of 146 μW and pulse duration of 480 fs were amplified to 14.5 mW. The pulse duration was compressed to 260 fs using 6 m high concentration E³⁺ _r -doped fiber under forward pumping. The amplified signal pulse energy was 0.691 nJ (corresponding to a peak power of 2 657.7 W) and the repetition rate was 20.84 MHz. Spectrum breakup was observed simultaneously. The spectrum of pulses amplified by 3 m E³⁺ _r -doped fiber remains a single peak under different pump power. The amplified pulse duration was compressed abnormally with the increasing pump power using the backward pumping; that is, the amplified pulses were compressed with the increasing pump power under low pump power. When the pump power reached 38 mW, the shortest amplified pulse duration was 309 fs. With further increase in pump power, the amplified pulses began broadening, accompanied by a single peak spectrum under different pump power.     

Experimental studies of high order soliton compression effect and gain characteristics in femtosecond laser pulses Er3+-doped fiber amplifier

Seed laser pulses with average power of 146 μW and pulse duration of 480 fs were amplified to 14.5 mW. The pulse duration was compressed to 260 fs using 6 m high concentration E³⁺ _r -doped fiber under forward pumping. The amplified signal pulse energy was 0.691 nJ (corresponding to a peak power of 2 657.7 W) and the repetition rate was 20.84 MHz. Spectrum breakup was observed simultaneously. The spectrum of pulses amplified by 3 m E³⁺ _r -doped fiber remains a single peak under different pump power. The amplified pulse duration was compressed abnormally with the increasing pump power using the backward pumping; that is, the amplified pulses were compressed with the increasing pump power under low pump power. When the pump power reached 38 mW, the shortest amplified pulse duration was 309 fs. With further increase in pump power, the amplified pulses began broadening, accompanied by a single peak spectrum under different pump power.     

The AC Stark Effect, Time-Dependent Born–Oppenheimer Approximation, and Franck–Condon Factors

We study the quantum mechanics of a simple molecular system that is subject to a laser pulse. We model the laser pulse by a classical oscillatory electric field, and we employ the Born–Oppenheimer approximation for the molecule. We compute transition amplitudes to leading order in the laser strength. These amplitudes contain Franck–Condon factors that we compute explicitly to leading order in the Born–Oppenheimer parameter. We also correct an erroneous calculation in the mathematical literature on the AC Stark effect for molecular systems. Communicated by Christian Gérard. Submitted: August 15, 2005; Accepted: October 13, 2005     

Changes in the mechanical properties of polycarbonate produced by a powerful light flux

The effects of a laser beam on PC are examined in relation to the power and duration of the light pulse. In uniaxial tension there is a sharp change in the relative elongation of the irradiated specimens (pulse length 10^–3 sec); the elastic modulus and the molecular weight remain unchanged. Irradiation of PC with a giant pulse does not affect its mechanical properties.Mekhanika Polimerov, Vol. 4, No. 2, pp. 288–292, 1968     

Sleep unconsciousness and breakdown of serial critical intermittency: New vistas on the global workspace

While several mental functions are characterized by parallel computation performed by moduli in the cortex, consciousness is sustained by a serial global integration: a single scene at a time takes place. Studies on complex systems show that macroscopic variables, integrating many components activities, undergo fluctuations with an intermittent serial structure when the system is in a state called “criticality”, characterized by avalanches with inverse-power-law (scale-free) distribution densities of sizes and inter-event times. Criticality has been established in human brain dynamics during wakefulness. Here we review how the critical hypothesis is able to explain many recent studies on brain complex dynamics. We focus, in particular, on the global, serial, intermittent behavior that can be assessed via high-density electroencephalograms, studying transitions between metastable states. Established as it is during wakefulness, it remained unsolved whether this global intermittent dynamics correlates with consciousness or with a non-task-driven default mode, also present in non-conscious states, like deep (NREM) sleep. Here we show that in NREM sleep seriality breaks down, and re-establishes during REM sleep (dreams), with unaltered spacial structure, in terms of complex branching of avalanches. We conjecture that this connectivity is exploited in NREM sleep by neural bistability, resetting and “parallelizing” portions of the cortex.     

The Initial Drift of a 2D Droplet at Zero Temperature

We consider the 2D stochastic Ising model evolving according to the Glauber dynamics at zero temperature. We compute the initial drift for droplets which are suitable approximations of smooth domains. A specific spatial average of the derivative at time 0 of the volume variation of a droplet close to a boundary point is equal to its curvature multiplied by a direction dependent coefficient. We compute the explicit value of this coefficient.      

Conservative finite-difference scheme for the problem of propagation of a femtosecond pulse in a nonlinear photonic crystal with nonreflecting boundary conditions

Conservative finite-difference schemes are constructed for the problems of self-action of a femtosecond laser pulse and of second-harmonic generation in a one-dimensional nonlinear photonic crystal with nonreflecting boundary conditions. The invariants of the governing equations are found taking into account these conditions. Nonreflecting conditions substantially improve the efficiency of conservative finite-difference schemes used in the modeling of complex nonlinear effects in photonic crystals, which require much smaller steps in space and time than those used in the case of linear propagation. The numerical experiments performed show that the boundary reflects no more than 0.01% of the transmitted energy, which corresponds to the truncation error in the boundary conditions. The amplitude of the reflected pulse is less than that of the pulse transmitted through the boundary by two (and more) orders of magnitude. The simulation is based on the approach proposed by the authors for the given class of problems.     

Propagation of high intensity light in semiconductors

For photons of energy below the bandgap energy, semiconductors exhibit high transparency and low irradiance light passes freely through the medium. However, for high irradiances of light, as from a laser, the transmission through the semiconductor becomes nonlinear. As the irradiance of the incident light increases, the transmission through the semiconductor decreases through nonlinear absorption as well as from the generation of free carriers during the laser pulse. The propagation of light through this irradiance dependent medium can be described by a set of coupled, inhomogeneous, partial differential equations. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)