Seed Laser Pulse Research Articles

Electron acceleration by amplified wakefield generated by two copropagating laser pulses in plasma

A detailed analytical theory is developed for studying the phenomenon of generation of high amplitude wakefield using a seed laser pulse and another coaxially propagating, trailing pulse. The two laser pulses are separated by a fixed distance between them and have the same polarization, frequency, length, intensity, and profile. The study shows that the maximum energy gained by the test electron significantly depends upon the distance between the two pulses. Phase space analysis shows that a test electron of significantly lower energy, injected behind the trailing laser pulse, will be trapped and accelerated to higher energy as compared to the single pulse case.

基于电光调制的激光光谱整形技术

A new spectrum shaping method, based on electro-optic modulation, to alleviate gain narrowing in chirped pulse amplification (CPA) system, is described and numerically simulated. Near-Fourier transform-limited seed laser pulse is chirped linearly through optical stretcher. Then the chirped laser pulse is coupled into integrated waveguide electro-optic modulator driven by an aperture-coupled-stripline (ACSL) electrical-waveform generator, and the pulse shape and amplitude are shaped in time domain. Because of the direct relationship between frequency interval and time interval of the linearly chirped pulse, the laser pulse spectrum is shaped correspondingly. Spectrum-shaping examples are modeled numerically to determine the spectral resolution of this technique. The phase error introduced in this method is also discussed.

High-peak-power femtosecond pulse compression with polarization-maintaining ytterbium-doped fiber amplification

We report the generation of 140 fs pulses with a peak power of up to 270 kW using a fiber pulse source based on a polarization-maintaining ytterbium-doped fiber amplifier and a semiconductor saturable absorber mirror mode-locked fiber laser seed. The seed laser pulses were amplified and chirped in the fiber amplifier and subsequently compressed in an external transmission grating pair. The use of a polarization-maintaining amplifier addresses nonlinear polarization-induced limitations to the obtainable compressed pulse duration and quality that can arise if isotropic fiber amplification is used. Numerical simulations of the system support the experimental measurements and also confirm the role of fiber dispersion in obtaining high-quality compressed pulses.

Design Method for Ultrabroadband Optical Parametric Chirped Pulse Amplification

Multiple amplification stages with different phase-matching angles were designed to yield stable and ultrabroadband amplification in optical parametric chirped pulse amplification by optimally controlling the idler laser pulses. Numerical results showed that the overall temporal duration of the amplified seed laser pulse and subsequently the spectral bandwidth can be amplified by using multiple amplification stages in comparison with those of the initial seed pulse laser, which is suitable to generate a high-energy pulse with ultrashort pulse duration in a simple manner.

Interplay of the chirps and chirped pulse compression in a high-gain seeded free-electron laser

In a seeded high-gain free-electron laser (FEL), where a coherent laser pulse interacts with an ultrarelativistic electron beam, the seed laser pulse can be frequency chirped, and the electron beam can be energy chirped. Besides these two chirps, the FEL interaction introduces an intrinsic frequency chirp in the FEL even if the above-mentioned two chirps are absent. We examine the interplay of these three chirps. The problem is formulated as an initial value problem and solved via a Green function approach. Besides the chirp evolution, we also give analytical expressions for the pulse duration and bandwidth of the FEL, which remains fully longitudinally coherent in the high-gain exponential growth regime. Because the chirps are normally introduced for a final compression of the FEL pulse, some conceptual issues are discussed. We show that to get a short pulse duration, an energy chirp in the electron beam is important.

Enhancement of Optical Parametric Chirped Pulse Amplification by Controlling Idler Laser Pulse

We present a novel method for optical parametric chirped pulse amplification to enhance the spectral bandwidth of amplified signal pulses by suppressing reconversion in a collinear degenerate geometry using a sequence of crystal pairs. The scheme consists of a conventional crystal pair used for achieving saturated amplification at the peak portion of the seed laser pulse, and a number of additional crystal pairs for optimizing the amplified seed laser pulse, especially in its spectral range and output stability. Numerical results showed that the proposed scheme can be used to produce a stable amplified seed laser pulse with a broader spectral bandwidth than the input spectrum under ordinary conditions.

Compact soft x-ray source using Thomson scattering

A compact soft x-ray source using Thomson scattering, enabled by the combination of a picosecond laser and an electron rf gun, was developed aiming at biological studies such as those using an x-ray microscope. The x-ray source included both a photoinjector system and a picosecond laser system with a tabletop size of 2×2m2. An infrared laser beam (λ0=1047nm) was obtained from an all-solid-state mode-locked Nd:YLF laser system and injected into the photocathode of an accelerator system. A 4.2MeV electron beam was generated from a laser-driven photocathode rf gun system. The residual laser beam was amplified up to about 4.2mJ/pulse using a flash-lamp-pumped laser amplifier. Upon collision of the electron beam with the amplified laser beam, 300eV soft x rays were generated by Thomson backscattering. The stable interaction between the two beams was achieved using the same seed laser pulse for irradiating the photocathode and the scattering process with laser photons.

Chirped pulse amplification of HGHG-FEL at DUV-FEL facility at BNL

The DUV-FEL facility has been in operation in the High Gain Harmonic Generation (HGHG) mode producing a 266-nm output from 177-MeV electrons for 1 year. In this paper, we present preliminary results of the Chirped Pulse Amplification (CPA) of the HGHG radiation. In the normal HGHG process, a 1-ps electron beam is seeded by a chirped 9 ps long, 800-nm Ti:Sapphire laser. The electron beam sees only a narrow fraction of the seed laser bandwidth. However, in the CPA case, the seed laser pulse length is reduced to 1 ps, and the electron beam sees the full bandwidth. We introduce an energy chirp on the electron beam to match the chirp of the seed pulse, enabling the resonant condition for the whole beam. We present measurements of the spectrum bandwidth for various chirp conditions.

Chirped pulse amplification of HGHG-FEL at DUV-FEL facility at BNL

The DUV-FEL facility has been in operation in the High Gain Harmonic Generation (HGHG) mode producing a 266-nm output from 177-MeV electrons for 1 year. In this paper, we present preliminary results of the Chirped Pulse Amplification (CPA) of the HGHG radiation. In the normal HGHG process, a 1-ps electron beam is seeded by a chirped 9 ps long, 800-nm Ti:Sapphire laser. The electron beam sees only a narrow fraction of the seed laser bandwidth. However, in the CPA case, the seed laser pulse length is reduced to 1 ps, and the electron beam sees the full bandwidth. We introduce an energy chirp on the electron beam to match the chirp of the seed pulse, enabling the resonant condition for the whole beam. We present measurements of the spectrum bandwidth for various chirp conditions.

Finite-duration seeding effects in powerful backward Raman amplifiers.

In the process of backward Raman amplification (BRA), the leading layers of the seed laser pulse can shadow the rear layers, thus weakening the effective seeding power and affecting parameters of output pulses in BRA. We study this effect numerically and also analytically by approximating the pumped pulse by the "pi-pulse" manifold (family) of self-similar solutions. We determine how the pumped pulse projection moves within the pi-pulse manifold, and describe quantitatively the effective seeding power evolution. Our results extend the quantitative theory of BRA to regimes where the effective seeding power varies substantially during the amplification. These results might be of broader interest, since the basic equations are general equations for resonant three-wave interactions.

Simulations of Raman laser amplification in ionizing plasmas

By using the amplifying laser pulse in a plasma-based backward Raman laser amplifier to generate the plasma by photoionization of a gas simultaneous with the amplification process, possible instabilities of the pumping laser pulse can be avoided. Particle-in-cell simulations are used to study this amplification mechanism, and earlier results using more elementary models of the Raman interaction are verified [D. S. Clark and N. J. Fisch, Phys. Plasmas 9, 2772 (2002)]. The effects (unique to amplification in ionizing plasmas and not included in previous simulations) of blueshifting of the pump and seed laser pulses as well as the generation of a wake are observed not significantly to impact the amplification process. As expected theoretically, the peak output intensity is found to be limited to I∼1017 W/cm2 by forward Raman scattering of the amplifying seed. The integrity of the ionization front of the seed pulse against the development of a possible transverse modulation instability is also demonstrated.

Scheme for time-resolved experiments based on the generation of femtosecond pulses by a sideband-seeded soft X-Ray SASE FEL

In this paper, we describe the extention of the soft X-ray SASE FEL facility for two-color time-resolved experiments. A new type of pump–probe technique is proposed which is based on the use of soft X-ray SASE FEL combined with an fs quantum laser. An ultrashort laser pulse is used for modulation of the energy and density of the electrons within a slice of the electron bunch at a frequency ω opt. The density modulation exiting the modulator (energy-modulation undulator and dispersion section) is about 10%. Following the modulator the beam enters an X-ray SASE FEL undulator, and is bunched at a frequency ω 0. This leads to an amplitude modulation of the beam density at the sidebands ω 0± ω opt. The sideband density modulation takes place at the part of the electron pulse defined by the duration of the seed laser pulse that is much shorter than the electron pulse. Following the SASE FEL undulator the beam and SASE radiation enter undulator section (radiator) which is resonant at the frequency ω 0− ω opt. Because the beam has a large component of bunching at the sideband, coherent emission is copiously produced within fs slice of the electron bunch. Separation of the sideband frequency from the central frequency by a time-compensated monochromator is used to distinguish the fs pulses from the sub-ps intense SASE pulses. Finally, the fs soft X-ray pulses are combined with optical pulses generated in the seed laser system. In the proposed scheme for pump–probe experiments, fs soft X-ray pulse is naturally synchronized with his fs optical pulse and cancel jitter.

Generation of high power femtosecond pulses by a sideband-seeded X-ray FEL

New proposal of a fs X-ray facility, which is described in this paper, is based on the use of X-ray SASE FEL combined with a fs quantum laser. An ultrashort laser pulse is used for modulation of the energy and density of the electrons within a slice of the electron bunch at a frequency ω opt. The density modulation exiting the modulator (energy-modulation undulator and dispersion section) is about 10%. Following the modulator the beam enters an X-ray SASE FEL undulator, and is bunched at a frequency ω 0. This leads to an amplitude modulation of the beam density at the sidebands ω 0± ω opt. The sideband density modulation takes place at the part of the electron pulse defined by the duration of the seed laser pulse that is much shorter than the electron pulse. Following the SASE FEL undulator the beam and SASE radiation enter undulator section (radiator) which is resonant at the frequency ω 0− ω opt. Because the beam has a large component of bunching at the sideband, coherent emission is copiously produced within fs slice of the electron bunch. Separation of the sideband frequency from the central frequency by a monochromator is used to distinguish the fs pulses from the sub-ps intense SASE pulses.

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 mu W 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 E-r(3+)-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 E-r(3+)-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.