Pump Pulse Shape Research Articles

Spectral gain control using shaped pump pulses in a counter-pumped cascaded fiber Raman amplifier

We investigate the Raman gain spectra produced from pulse-pumping a highly nonlinear fiber with shaped optical pulses delivered from a Yb-doped fiber MOPA pump source. Cascaded Raman wavelength shifting up to seven Stokes orders is demonstrated and the counter-propagating gain is measured across all seven Stokes orders. Step-shaped optical pulses with varying instantaneous powers are then used to pump the highly nonlinear fiber, generating a controllable gain spectrum across multiple Stokes orders. Furthermore, we extend this work by using multiple pump wavelengths along with step-shaped pulses to increase the bandwidth of the Raman gain spectrum.

Design, performance, and limitations of fibers for cladding-pumped Raman lasers

In this paper, we present basic design rules for double-clad fibers that enable efficient 1st-Stokes operation of cladding-pumped fiber Raman devices. Limiting factors that we treat include unwanted 2nd-Stokes generation, material damage in the core, pump pulse dispersion, inner-cladding NA, background loss, pulse shape and pump noise. With a well-designed fiber, we calculate that a pump-to-signal brightness enhancement of over 2000 is possible, with certain pump parameters. On experimental work, we report a 100 W cladding-pumped fiber Raman laser at 1120 nm. Furthermore, a double-clad Raman fiber with large-mode area, ∼40 μm core diameter, pumped by a Q-switched Nd:YAG laser is used to generate Stokes pulses with 1 mJ energy.

Testing ultrafast two-photon spectral amplitudes via optical fibres

We test two-dimensional TPSA of biphoton light emitted via ultrafast spontaneous parametric down-conversion (SPDC) using the effect of group-velocity dispersion in optical fibres. Further, we apply this technique to demonstrate the engineering of biphoton spectral properties by acting on the pump pulse shape.

Improving the efficiency of an optical parametric oscillator by tailoring the pump pulse shape

The conversion efficiency of an optical parametric oscillator is reduced by energy consumption during build-up of signal and idler intensities and due to back-conversion effects. By tailoring the pump pulse temporal shape, we are able to improve the conversion efficiency by minimizing build-up time and back-conversion. Simulations predict a significant improvement in 1064 nm to 4000 nm idler conversion by using a double-rectangular temporal shape rather than using a simple Gaussian pulse. Experimental results qualitatively verify the effect resulting in a 20% improvement of a rectangular pulse over a Gaussian pulse.

Q-switch rise time effect in flash-pumped solid state lasers

The effect of the rise time of the Pockels cell on the output laser pulse is investigated. The pump system in a flash pumped Nd:YAG laser with unstable resonator is numerically analyzed and simulated. The time dependent flash discharge current is calculated and the optical time-dependent variation of the flash pulse is deduced. The numerical simulation of the whole laser gives the solution of the system of four level Nd:YAG differential rate equations. The pump power is considered variable and dependent on the flash pump pulse shape and power. The Q-switch rise time is included in the rate equations as a variable time dependant loss parameter. The laser pulse shape and output characteristics are calculated. Numerical and some experimental results are presented and discussed.

10.1007/s11449-008-3011-8

Echo responses of a three-level medium formed by the λ-scheme of quantum transitions under exposure to optical pulses consisting of an arbitrary number of oscillations have been studied theoretically. The cases of pulses consisting of few optical oscillations (for which the concept of an envelope cannot be used) and combinations of such pulses and quasi-monochromatic resonance signals have been considered. The approach used can be reduced to the renormalization of dipole moments of allowed quantum transitions through their multiplication by coefficients depending on the shape and duration of pump pulses and having absolute values in the range from zero (for nonresonance pulses) to unity (for resonance quasi-monochromatic pulses and broadband pulses consisting of few oscillations, whose spectrum covers the quantum transitions). A general equation has been proposed for the pulse area. In the limit of a large number of oscillations, it transforms into the well-known definition of the area of a quasi-monochromatic signal. The characteristics of primary and longlived photon echoes have been analyzed in detail. It has been shown that, when a medium is exposed to only pulses consisting of a few oscillations, three echo responses of both types can be principally generated at each frequency of the λ-scheme. Introduction of quasi-monochromatic pulses in pump pulse series decreases the number of echoes, and their qualitative character has a non-commutative property with respect to pulse permutation in a series. The extension of the proposed approach to more complex schemes of quantum transitions with the large number of quantum levels faces no principal difficulties.

Analytical investigation on transient thermal effects in pulse end-pumped short-length fiber laser

The transient heat conduction and thermal effects in pulse end-pumped fiber laser are modeled and analytically solved. For the arbitrary temporal shape of pump pulse, a three-dimensional (3D) temperature expression is derived via an integral transform method, and the thermal stress field is deduced through solving the Navier displacement equations. The results show that pulse shape has an important influence on the peak thermal stress and transient phase shift induced by heating of the fiber. Reasonable design for pulse duration and period can reduce thermal effects and optimize the performance of high-power fiber laser.

Control of excited-state population and vibrational coherence with shaped-resonant and near-resonant excitation

We investigated numerically and experimentally the enhancement of vibrational coherence and population transfer in solution using tailored pulses. The general control mechanism is based on the precise control of the absorption after excitation with multipulses. Transient absorption was used as an experimental method to quantify the enhancement after the excitation with a shaped pump pulse in the low-pulse energy regime. A density-matrix approach was used to investigate the effect of the shaped excitation of a four-level system (two ground-state vibrational levels + two excited-state vibrational levels). Density-matrix simulation results suggest similar wavelength dependence as observed in the transient absorption experiment. The results show that enhancement of population transfer and vibrational coherence depends crucially on the overlap between the excitation pulse spectrum and the molecular absorption band.

Photon echo under excitation of a medium by pulses consisting of an arbitrary number of oscillations

Echo responses of a three-level medium formed by the λ-scheme of quantum transitions under exposure to optical pulses consisting of an arbitrary number of oscillations have been studied theoretically. The cases of pulses consisting of few optical oscillations (for which the concept of an envelope cannot be used) and combinations of such pulses and quasi-monochromatic resonance signals have been considered. The approach used can be reduced to the renormalization of dipole moments of allowed quantum transitions through their multiplication by coefficients depending on the shape and duration of pump pulses and having absolute values in the range from zero (for nonresonance pulses) to unity (for resonance quasi-monochromatic pulses and broadband pulses consisting of few oscillations, whose spectrum covers the quantum transitions). A general equation has been proposed for the pulse area. In the limit of a large number of oscillations, it transforms into the well-known definition of the area of a quasi-monochromatic signal. The characteristics of primary and longlived photon echoes have been analyzed in detail. It has been shown that, when a medium is exposed to only pulses consisting of a few oscillations, three echo responses of both types can be principally generated at each frequency of the λ-scheme. Introduction of quasi-monochromatic pulses in pump pulse series decreases the number of echoes, and their qualitative character has a non-commutative property with respect to pulse permutation in a series. The extension of the proposed approach to more complex schemes of quantum transitions with the large number of quantum levels faces no principal difficulties.

Control of wave packets in Li2 by shaping the pump and probe pulses for a state-selected pump-probe analysis of the ionization continuum

Wave packet signals in Li(2) prepared by shaped pump pulses are also detected with state-selected shaped probe pulses in the ionization continuum. The results show that the final states are discrete Rydberg states instead of continuum states. Final autoionizing states in the continuum are observed and characterized. By selecting specific resonant rovibrational electronic transitions from the superposition states prepared in the wave packets to the final autoionizing states with the pulse shaping system, the modulation depths of the wave packet signals are increased by as much as 5.20+/-0.03 times. Control of the wave packets is also realized by shaping the probe pulses to select specific resonant transitions between the states in the wave packets and the highly excited Rydberg states. The detected amplitude ratio of one specific vibrational quantum beat to one specific rotational quantum beat can be decreased by ten times.

Shaping of picosecond pulses for pumping optical parametric amplification

The use of temporally shaped pump pulses for optical parametric chirped-pulse amplification can increase the efficiency and suppress possible spectral distortions in this process. With this goal in mind a novel method for shaping narrowband picosecond pulses has been developed. The method is based on the pulse stacking principle, where replicas of the incoming pulse are created in a specially designed four-beam interferometer. The replicas are recombined with appropriate relative delays. The interferometer design allows for a unique flexibility in varying the pulse shape, since all relevant degrees of freedom, such as relative intensities and delays between the pulse replicas are independently adjustable. Here we describe the design of our device in detail and report on the experimental demonstration of its pulse-shaping capabilities.

Singlet versus triplet dynamics of β-carotene studied by quantum control spectroscopy

Biomolecules very often present complex energy deactivation networks with overlapping electronic absorption bands, making their study a difficult task. This can be especially true in transient absorption spectroscopy when signals from bleach, excited state absorption and stimulated emission contribute to the signal. However, quantum control spectroscopy can be used to discriminate specific electronic states of interest by applying specifically designed laser pulses. Recently, we have shown the control of energy flow in bacterial light-harvesting using shaped pump pulses in the visible and the selective population of pathways in carotenoids using an additional depletion pulse in the transient absorption technique. Here, we apply a closed-loop optimization approach to β-carotene using a spatial light modulator to decipher the energy flow network after a multiphoton excitation with a shaped ultrashort pulse in the near-IR. After excitation, two overlapping bands were detected and identified as the S 1 state and the first triplet state T 1. Using the transient absorption signal at a specific probe delay as feedback, the triplet signal could be optimized over the singlet contribution.

Analysis of ultrashort-pulse second-harmonic generation in both phase- and group-velocity-matched structures

Second-harmonic generation (SHG) of ultrashort pulse in both phase- and group-velocity mismatch structures is studied theoretically in temporal and frequency domains taking account of the effects of group-velocity dispersion (GVD) and high-order phase mismatch. Under the plane-wave and pump undepletion approximation, a set of simple analytical expressions is derived in frequency domain for Gaussian pulse. Furthermore, a formula describing the SH pulse is obtained under pump depletion condition. The obtained expressions are valid in large range of interaction length and have features such as explicit form, high accuracy, and less computing time. Meanwhile, the influences of intensity, pulse duration, chirp coefficient and shape of pump pulses, as well as the linear loss of material, on the pulsewidth preservation length are investigated by numerical method. In the case of both phase- and group-velocity mismatch structures, the conversion efficiency larger than 45% could be obtained without pulse distortion for weak input ultrashort pulse.

Optimal control theory for a target state distributed in time: optimizing the probe-pulse signal of a pump-probe-scheme.

Optimal control theory (OCT) is formulated for the case of a two-color pump-probe experiment. The approach allows to calculate the pump-pulse shape in such a way that the probe-pulse absorption signal is maximized. Since the latter quantity is given by the time-averaged expectation value of a time dependent operator (the probe-pulse field-strength times the dipole operator) a version of OCT has to be used where the target state is distributed in time. The method is applied to a molecular three-level system with the pump-pulse driving the transition from the electronic ground state into the first-excited electronic state and the probe-pulse connecting the first-excited state with a higher lying electronic state. Depending on the probe-pulse duration, the vibrational wave packet becomes localized or at least highly concentrated in the Franck-Condon window for the transition into the higher-excited state. The dependence on the probe-pulse duration and on the delay time between the optimized pump-pulse and the probe-pulse is discussed in detail. The whole study demonstrates the feasibility of laser pulse induced temporal wave packet localization and the use of spectroscopic quantities as target states in experiments on femtosecond laser pulse control.

Influence of pump pulse structure on a transient collisionally pumpedNi-likeAgx-ray laser

Results of numerical simulations on a $\mathrm{Ni}$-like silver x-ray laser pumped by a single picosecond laser pulse are presented. Since the mechanisms responsible for the significant reduction in the pump energy are not well understood, the results of theoretical simulations with emphasis on the plasma kinetics and dynamics in a $\mathrm{Ni}$-like $\mathrm{Ag}$ x-ray laser are presented and referred to the experimental data. Special attention has been paid to the influence of the pump pulse shape and length on the gain and its duration. It was found that a low-level pulse pedestal being an integral part of the leading edge of the pump pulse is very beneficial to the pump energy reduction. The thermal cooling process has been identified as the mechanism strongly contributing to gain termination if a low-energy single-profile laser pulse with the width of a few picoseconds is used in the pump process.

Mid infrared pulse shaping by optical parametric amplification and its application to optical free induction decay measurement

We produce microjoule energy shaped mid infrared (MIR) pulses in an optical parametric amplification (OPA) process by imposing the phase and amplitude profile of an arbitrarily shaped pump pulse onto the idler pulse. Using phase locked pulses created using this technique, we measure for the first time, complex optical free induction decay (OFID) of the vibrational coherence of a C-H stretching mode.

Generation of single dispersion precompensated 1-fs pulses by shaped-pulse optimized high-order stimulated Raman scattering.

We propose and theoretically analyze a new approach for generating and shaping 1-fs pulses. It combines the ideas of strong-field molecular optics and optimal control to manipulate light generation in a pump-probe Raman regime. Flexible phase control over the generated spectrum of about 3 eV width is achieved by controlling the input pulses and maximizing the coherence of medium excitation by adiabatically aligning molecules in the medium with a specially shaped pump pulse. The generated pulse is optimized for an output window, precompensating for its dispersion to all orders.

Linewidths in energy-resolved two-photon photoemission spectroscopy

Energy-resolved two-photon photoemission is modeled in the framework of optical Bloch equations for a three-level system. For pulsed excitation, analytical expressions for the photocurrent have been derived which elucidate numerical solutions, The information that can be extracted from linewidth analysis is demonstrated to depend substantially on the shape of pump and probe pulses. The analysis is applied to image-potential states leading to overall good agreement.

Femtosecond spin dynamics probed by linear and nonlinear magneto-optics

A theoretical study is performed on ultrafast spin dynamics as probed by linear and nonlinear (magneto-)optics, such as second harmonic generation (SHG). Both intrinsic spin and charge dynamics occur on a time scale of 10 fs. A clear delay of spin dynamics with respect to charge dynamics is observed. Such delay is clear not only for the linear response, but also for the nonlinear response such as χ (2). We find that the different pump pulse shape could result in a different dynamical process. And for a larger probe frequency, the oscillatory behavior of the spin dynamics is clearer, the period is shorter, and the decay time is longer. All of these effects show that one can actively tune the dynamics by properly choosing the experimental conditions. Our study provides some insights into ultrafast spin dynamics.

Time-domain pulse property of stimulated Raman scattering in a multimode optical fiber

The time-domain pulse property of quasi-steady stimulated Raman scattering (SRS) in a multimode optical fiber, pumped by an A-O Q-switched Nd: YAG laser, is studied both experimentally and theoretically. Good agreement has been achieved between theory and experiment. Discussion shows that the variation of the Stokes pulse width (FWHM)t s depends on the shape of input pump pulse.