Po-Yen Lai

Parasitic Stimulated Amplification in High-Peak-Power and Diode-Seeded Nanosecond Fiber Amplifiers

The broadband parasitic amplification in a diode-seeded nanosecond ytterbium-doped fiber laser amplifier system is numerically and experimentally investigated. The amplification is originated from a weak and pulsed parasitic signal associated with the 1064-nm seed diode laser. Although the average power of the parasitic pulse is less than 5% of the total seed laser power, a significant transient spike is observed during the amplification. In agreement with the simulation, nonlinear effects caused by the transient spike limits the scaling of signal peak power in fiber preamplifiers. With the utilization of a narrow bandwidth filter to eliminate the parasitic pulse, the power and energy scalability of a multistage diode-seeded fiber amplifier laser system has been significantly improved. At 1064 nm, pulses with the peak power of 120 kW and energy of 1.2 mJ have been successfully generated in the multistage Yb3+-doped fiber amplifier with an energy gain of 63 dB and 56% conversion efficiency. In viewing of the parasitic pulses 8.8-nm bandwidth, it has the potential to become a novel seed source for high-peak-power fiber amplifiers.

High power broadband continuum source based on an all-PM-fiber master oscillator nonlinear power amplifier

In an all-polarization-maintaining-fiber master oscillator power amplifier system at 1064 nm under all normal dispersion, intense nanosecond emission was generated with spectral broadening from 980 to 1600 nm. In such a fiber nonlinear power amplifier, efficient power scaling is able to be free from significant depletion because both laser amplification and nonlinear conversion are simultaneously employed. As a result, output peak power up to 117 kW with a pulse energy of 1.2 mJ is generated with a maximum core intensity of 30 GW cm 2 . In addition, the conversion efficiency is 66% for a pulse duration of 6.1 ns at the moderate repetition of 20 kHz. The output level is close to the damage threshold for long-term operation. The onset and interplay of constituted fiber nonlinearities can be addressed, especially from single mode to a few modes, stage by stage. Furthermore, the seeding influence on the spectral broadening reveals its versatility for enabling many potential applications. For seeding by a highly controlled diode laser at the nanojoule level, a double-pass preamplifier significantly improves the energy extraction, resulting in a high input level for an efficient nonlinear power amplifier. Such a linearly polarized light source composed of an intense 1064 nm pump and a broad sideband seed is beneficial for efficiently driving broadband tunable optical parametric amplification.

Numerical thermalization in particle-in-cell simulations with Monte-Carlo collisions

Numerical thermalization in collisional one-dimensional (1D) electrostatic (ES) particle-in-cell (PIC) simulations was investigated. Two collision models, the pitch-angle scattering of electrons by the stationary ion background and large-angle collisions between the electrons and the neutral background, were included in the PIC simulation using Monte-Carlo methods. The numerical results show that the thermalization times in both models were considerably reduced by the additional Monte-Carlo collisions as demonstrated by comparisons with Turners previous simulation results based on a head-on collision model [M. M. Turner, Phys. Plasmas 13, 033506 (2006)]. However, the breakdown of Dawsons scaling law in the collisional 1D ES PIC simulation is more complicated than that was observed by Turner, and the revised scaling law of the numerical thermalization time with numerical parameters are derived on the basis of the simulation results obtained in this study.

Modeling extreme-ultraviolet emission from laser-produced plasma using particle-in-cell method

A one-dimensional (1D) collisional relativistic particle-in-cell (PIC) code with ionization processes has been developed to investigate the key semiconductor manufacturing device, i.e., the extreme ultraviolet (EUV) light source from laserproduced plasmas (LPP). Unlike hydrodynamic approach, the kinetic model describes laser heating, energy transport and ultrafast electron dynamics with least approximations. The two major numerical effects of PIC simulations, i.e., numerical self-heating and numerical thermalization, are also studied and mitigated in the collisional PIC model. The integrated numerical model is achieved by simulating the dense plasma using collisional PIC model and estimating EUV emission and mean opacities according to the respective weighted oscillator strengths of tin ions with charged states varying from 5+ to 13+.

Influences of amplified spontaneous emission on fiber laser amplifier chain

The time-dependent coupled rate equations based on the multi-channel treatment has been applied to study the influences of amplified spontaneous emission on fiber laser amplifier with the experimental verification and practical solutions for suppressing ASE.

A high-peak power nanosecond all-fiber MOPA system at high-repetition rate

By limiting the core diameter of 15 μm at maximum with NA=0.07±0.01 for the near-diffraction-limited output (V <3.6), we successfully generate the pulse with the peak power of 36 kW and the duration of 4.6 ns in FWHM at the repetition rate of 20 kHz. To the best of our knowledge, the signal pulse energy corresponding to 264 μJ is the highest to date in the diode-seeded 15-μm all-fiber MOPA system with the efficiency of 35%. The success in the energy/power scaling is attributed to the further raise of input energy for more extracted energy, the tradeoff between the Raman-limited signal energy and the amplifier slope efficiency for more signal energy ratio, and the proper adjustment of both pump wavelength and power for avoiding coat damage without forced cooling.

Study of discrete-particle effects in a one-dimensional plasma simulation with the Krook type collision model

The thermal relaxation time of a one-dimensional plasma has been demonstrated to scale with ND2 due to discrete particle effects by collisionless particle-in-cell (PIC) simulations, where ND is the particle number in a Debye length. The ND2 scaling is consistent with the theoretical analysis based on the Balescu-Lenard-Landau kinetic equation. However, it was found that the thermal relaxation time is anomalously shortened to scale with ND while externally introducing the Krook type collision model in the one-dimensional electrostatic PIC simulation. In order to understand the discrete particle effects enhanced by the Krook type collision model, the superposition principle of dressed test particles was applied to derive the modified Balescu-Lenard-Landau kinetic equation. The theoretical results are shown to be in good agreement with the simulation results when the collisional effects dominate the plasma system.

Numerical thermalization in one- and two-dimensional particle-in-cell simulations with Monte-Carlo collisions

Summary form only given. Numerical thermalization induced by discrete-particle effects was observed in particle-in-cell (PIC) simulations¹, which show that the thermal relaxation time scales with N_D² and N_D for one-dimensional (1D) and two-dimensional (2D) models², respectively. The parameter N_D denotes the number of particles in a Debye length (1D) or a Debye square (2D). However, it was found recently that the thermal relaxation time is anomalously shortened to scale with N_D while adding the Monte-Carlo collisions in a 1D PIC simulation.^3,4 In this work, we examine the numerical thermalization in two-dimensional ES PIC simulations with the consideration of electron-ion collision and electron-neutral collision.⁴ Our results show that the thermal relaxation time is less sensitive to the collision frequency as compared to the 1D cases, and the thermal relaxation time remains to scale with N_D as predicted by the theoretical analysis using the Balescu-Lenard-Landau kinetic theory⁵.

High-Intensity Nanosecond All-Fiber-Coiled Laser and Extreme Ultraviolet Generation

High-efficiency fiber-based extreme ultraviolet driver in the alignment-free configuration has been experimentally achieved with a maximum intensity of 6.4×1010 W/cm2 on target at a repetition rate of 20 kHz. The output EUV signal within 10~20 nm in wavelength was confirmed with a Si/Zr-coated x-ray photodiode by varying numbers of Be and Al foil filters. The measured spectral range is consistent with that obtained by the weighted oscillator strengths of Sn8+ to Sn+13 ions using an one-dimensional hydrodynamic code coupled with the ionization model of collisional-radiative equilibrium. The driver is based on a 1064-nm nanosecond coiled ytterbium all-fiber laser system in diode-seeded master oscillator power amplification. With an overall optical efficiency up to 56%, it can deliver a 1.16-mJ, 117-kW, 6.1-ns laser pulses with a FWHM linewidth of 10 nm and beam propagation factor of M2~1.55. The full advantages of using fiber laser for a movable LPP EUV metrology source are revealed.

Numerical thermalization of two-dimensional plasmas in the presence of binary collisions with the particle-in-cell method

Summary form only given. Numerical self-heating and thermalization are two unphysical artifacts in particle-in-cell (PIC) simulations due to the implicit assumptions used in the numerical method. Numerical self-heating in plasma simulations can typically be suppressed by using higher order finite-difference methods, field smoothing and particle weighting schemes. On the other hand, numerical thermalization is associated with the granularity of the particles that introduces relaxation in the velocity distribution functions (i.e. the particle velocity distribution approaches Maxwellian). In 2D collisionless plasma simulations, the rate of relaxation has been shown to be proportional to 1/ND, where ND is the number of particles per Debye length1. This scaling has been illustrated to be true in several previous works using electrostatic PIC codes. Here we will focus on the study of numerical thermalization for 2D collisional plasma using an in-house electromagnetic PIC (IPIC or Ihpc Particle-In-Cell) code. The binary collision model in IPIC is implemented based on the Fokker-Planck Collision operator to mimic Coulomb collisions in real plasmas2. Our numerical results confirm that without collisions, the 1/ND scaling remains intact as expected. However, in the presence of binary collisions, the relaxation rate is also scaled with 1/ND at small ND, but the scaling parameter varies distinctly as ND increases. The implications of this previously unidentified observation will be discussed.