Abstract
1 Introduction
Laser pulses with high peak power have made significant breakthroughs in the fields of high-field physics[
Laser pulses with durations of several hundreds of picoseconds, much shorter than nanosecond pulses and much longer than femtosecond pulses, have attracted the attention of fewer researchers. Furthermore, amplifying pulses with widths of several hundreds of picoseconds to generate pulses of high energy and power is difficult, and suffers from the limitations of both MOPA and CPA approaches. For MOPA, because of the damage threshold fluence of the laser system, shorter pulses require larger-aperture components, and thus a larger scale of the entire system, which means the output laser intensity is limited. Moreover, the output intensity must be modulated to avoid exceeding the B integral limit, to avoid self-focusing damage and the loss of beam quality. It must also be noted that the laser energy extraction efficiency is very low for short pulses. CPA and OPCPA are limited by the Fourier spectral widths in 100-ps laser-pulse amplification. A 200-ps laser pulse can only be expanded to 280 ps by a
Recently, 200-ps laser pulses have attracted interest in the field of ICF. Shock ignition[
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In this study, we report a novel frequency-matching scheme for SBS (stimulated Brillouin scattering) amplification to achieve high-power 200-ps laser pulses in existing ICF laser drivers that already achieve higher energies for longer pulses. However, it is difficult to efficiently produce strong amplification for a 200-ps pulse. We have adopted a method to realize high energies for such pulses, transferring energy from nanosecond pulses to 200-ps pulses through Brillouin power amplification (BPA). In this process, the high energy of a long-pulse-width pump is transferred to the 200-ps Stokes pulse. Distinguishing it from traditional SBS amplification, BPA Stokes pulses are downshifted and modulated in the fiber front end of the laser driver, rather than generated from the noise.
Experiments have been carried out in an SG-III prototype system. Our results show that the method can overcome the temporal instabilities of the Stokes beam, provide high gain, and thus make it possible to obtain high-efficiency 200-ps laser-pulse amplification in a high-power solid-state laser system.
2 Methods
The pulse train is amplified first by MOPA, and subsequently the energy of the nanosecond pulse is transferred to 200-ps pulses by BPA. This technique has the potential to significantly reduce the size and cost of laser systems while effectively amplifying the laser pulse to the desired intensity. To ensure that requirements for precision of synchronization are met, the Stokes pulse in our SBS amplifier undergoes injection seeding. The 200-ps laser pulse is generated at the front end. As SBS produces a frequency shift in the scattered wave spectra, it is necessary for the seed laser to be downshifted so that coupling between the laser beams is realized. Thus, the acoustic wave is driven by the coherence of the Stokes and pump light. Previous reports presented phase detuning leading to gain reduction in Brillouin amplification[
The shaping of 200-ps and 5-ns square pulses is achieved by driving an amplitude (Mach–Zehnder) modulator controlled by an arbitrary waveform generator (AWG). These two pulses are separated into two fibers by a splitter (Figure
In the experiments, the Brillouin amplification system is based on a noncollinear structure[
The noncollinear structure not only affects the frequency shift between the Stokes and pump beams, but also has an effect on the interaction volume of the two beams, which plays a more important role in energy extraction. Thus, we designed our noncollinear Brillouin amplification experiment with a
The laser beam is narrowed by a factor of
The medium selected for SBS amplification in this experiment is FC-43, a hydrocarbon fluid with stoichiometric formula
Frequency-mismatch conditions will result in a decrease in the gain. In this case, the frequency of the acoustic wave changes. The Brillouin gain decreases by half when the frequency shift is inaccurate by 500 MHz, and the accuracy of the frequency shift of the Stokes pulse requires strict modulation. According to the theoretical modulation, the accuracy of the frequency shift should be controlled to within 100 MHz.
In our case, the pump and Stokes pulses are coupled with pulse stacking, so the frequency shift between the Stokes and pump light should be modulated carefully. The Stokes pulse is a 200-ps Gaussian pulse and the pump pulse is a 5-ns eighth-order super-Gaussian pulse. The measured frequency shift is 1.55 GHz, which is suitable for amplification experiments.
The pre-shaped pulse should be considered carefully to compensate the spatial and temporal distortion caused by the amplification and transmission in the laser system. For convenient experimental analysis, as well as to reduce the risk of damaging the optical elements, the power for the 200-ps pulse is set to the same value as that of the 5-ns pulse.
In the experiments reported here, the waveform was measured by photodiodes (Ultrafast UPD-50-UP) and displayed on an oscilloscope (Tektronix DPO71604B). The energy was measured by an energy meter (Ophir PE100BF-DIF ROHS). As the Stokes and pump pulses are coupled by pulse stacking, the measured energy is the sum of the Stokes and the pump pulses. In order to obtain the exact energy of the Stokes and pump pulses in Brillouin amplification, we need to record the integrated area of the pulse. According to this, the Stokes or the pump energy can be calculated.
3 Experimental results
In order to improve the energy extraction efficiency, the position at which the pump encounters the Stokes seed needs to be strictly controlled. By modulating the delay time between the 200-ps and 5-ns pulses and regulating the re-entering optical path length, this position should be controlled at the entrance of the cell. Under this circumstance, as soon as the Stokes light enters into the cell, it can be amplified by the pump light.
The output Stokes intensity and waveform are plotted in Figure
According to our measurements, pulse compression occurs in SBS amplification (Figure
The temporal structure of the pulse for an input pump intensity of
Our work demonstrated the feasibility of this novel frequency-matching stimulated Brillouin scattering to achieve high-intensity 200-ps laser pulses in a large-scale solid-state laser system. In our experiments, via the active frequency-matching method, the acoustic wave is locked in the SBS interaction. In this way, energy can be effectively transferred from the pump pulse to the Stokes pulse.
4 Discussion
4.1 Theoretical analysis and nonlinear absorption
Our method depends on energy transmission between the pump light and the Stokes light through an acoustic field. Compared to direct MOPA in Nd:glass, this method has a higher extraction rate of the stored energy in the amplifier. However, if the 200-ps laser pulse is directly amplified in Nd:glass, small-scale self-focusing must be taken into consideration as the output intensity is limited by the B integral. For this SBS energy transmission method, because the medium used is liquid, if the chemical bonds in the medium remain stable, the entire system can be restored.
However, this experiment also exposes some technical problems. As the pump intensity varies from
The equations describing the pump and Stokes optical fields are derived from Maxwell’s equations. Moreover, the acoustic field in the medium is due to the Navier–Stokes equation. The slowly varying approximation is applied in solving the equations. As the bandwidth of the acoustic field is similar to its frequency, the second-order term in the acoustic field equation cannot be ignored. Thus, the wave equations can be described as[
Equation (
Considering both linear absorption and nonlinear absorption in the medium, the equations can be expressed as
Compared to the theoretical simulation results shown in Figure
Simulation results obtained by introducing nonlinear absorption into the theoretical analysis are shown in Figure
4.2 Phase-modulated light SBS amplification
In high-power solid-state laser systems, to avoid SBS optical damage in large components, the spectrum is always modulated into a small bandwidth by phase modulation. For high modulation frequencies,
When considering a phase-modulated laser in SBS amplification, the pulse should be modulated with a large frequency shift to ensure that each spectral line acts independently. Thus, the spectral line of the Stokes seed light should be frequency matched to that of the pump light, so SBS amplification can be used for phase-modulated lasers. This has been experimentally tested on a laser system[
The experimental pump spectrum is shown in Figure
The temporal structures of the pulse for an input pump intensity of
4.3 Noncollinear SBS amplification with large beams
For noncollinear SBS amplification, the interaction volume between the Stokes and the pump light decreases with an increase in the crossing angle. Generally, we would calculate the interaction length in noncollinear SBS amplification[
The variation of the equivalent interaction length with crossing angle can be calculated. For an 8th-order super-Gaussian pulse with a
In shock ignition, it is necessary to use a three-step pulse that can first achieve compression and then a shock pulse to achieve ignition. SBS technology does not have the capability of arbitrary waveform modulation. Therefore, the spatial domain composite technique can be used to separate the compression pulse from the ignition pulse, amplified in a different path, and coupled at the target for ignition.
5 Conclusions
In summary, we have described an amplification method and demonstrated its ability to obtain 7-
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