Stimulated Brillouin Scattering Reflectivity Research Articles

Reducing the total stimulated brillouin scattering of two-color lasers through two-ion decay

Abstract A mechanism coupling the stimulated Brillouin scattering (SBS) of two-color lasers with wavelengths of 527 and 351 nm via the two-ion decay 
instability is proposed. When the SBS reflectivities of both lasers exceed 10%, the ion-acoustic wave excited by the 527 nm laser seeds the decay process of the ion-acoustic wave excited by the 351 nm laser, thereby promoting the decay of the latter into the former. This results in a significant reduction in the SBS reflectivity of the 351 nm laser, while the SBS of the 527 nm laser exhibits minimal variation, consequently reducing the total SBS reflectivity. The total SBS reflectivity initially decreases and then increases as the intensity fraction α of the 527 nm laser rises. When α ∽(20%-30%), the laser energy reflections from both lasers become approximately equal, achieving the minimum total reflectivity. Through this mechanism, the incidence of the two-color lasers can achieve lower reflected energy compared to monochromatic lasers with the same total intensity. These results demonstrate the significant potential of replacing a small fraction of high-frequency light with low-frequency light in enhancing the laser-target coupling efficiency for inertial fusion energy.

Stability enhancement with nonlinear gain modulation in high-power SBS-PCM

Stimulated Brillouin scattering (SBS) is an effective method for compensating wavefront aberrations in high-energy lasers due to its phase conjugation property. However, SBS phase conjugate mirrors (SBS-PCMs) under high-power pumping often suffer from significant spatial aberrations and decreased energy reflectivity, indicating instability in the nonlinear gain. Here, a nonlinear gain modulation method is proposed to realize the stable output. Experiments show that the energy reflectivity of SBS decreased due to the thermal effect in high-repetition-rate pumped SBS-PCMs. The nonlinear gain modulation was accomplished with simultaneous adaptation of radial and axial focusing parameters resulting in a higher SBS gain under short-focus conditions. The feasibility of the method was experimentally confirmed by obtaining SBS energy reflectivity stabilized at 60%, while the root mean squares of steady-state Stokes energy and pulse time delay enhanced to 1.97% and 1.81%, respectively, in an HT-230 medium. This scheme ensures stable and efficient SBS output under high-power pumping, which is of great significance for expanding the application of SBS-PCMs in high-repetition-rate laser systems.

Coupled modes analysis of stimulated Brillouin scattering in the regimes of nonlinear ion acoustic waves

The nonlinear saturation of stimulated Brillouin scattering (SBS) in long scale length plasmas is studied in detail through coupled mode equations. Our model incorporates harmonic and subharmonic generation of ion acoustic waves (IAWs), as well as nonlinear Landau damping and the nonlinear frequency shift of IAWs induced by particle trapping. Numerical simulations are carried out across various IAW wavenumbers (k a λ De ) and electron-ion temperature ratios (Z i T e /T i ) within different SBS instability regimes. The results demonstrate that our model can distinguish the importance of each effect contributing to the nonlinear behavior in SBS under different plasma conditions. Furthermore, we examine the scaling of SBS reflectivity with laser intensity under conditions relevant to inertial confinement fusion.

Revisit of electron temperature effect on stimulated Brillouin scattering in homogenous plasma

Stimulated Brillouin scattering (SBS) has complex dependence on the plasma electron temperature via the Landau damping, particle trapping, and consequent nonlinear frequency shift. It is found from our numerical simulation that the SBS reflectivity in its saturation stage tends to increase with the plasma electron temperature within a certain range, although the linear growth rate of SBS normally reduces with the increasing electron temperature. This is because the phase velocity of an ion acoustic wave (IAW) increases with the electron temperature, which tends to reduce the Landau damping of the IAWs and hence reduce ion trapping. In the nonlinear saturation stage, ion trapping will modify the ion distribution function and induce a negative frequency shift in the IAW. This nonlinear frequency shift will break the three-wave coupling, thereby causing saturation of the SBS. With further increase in the electron temperature, however, electron trapping will dominate over ion trapping, which induces a positive frequency shift in the IAW and can lead to the SBS saturation as well. As a result, the SBS reflectivity first increases and then decreases with increase in the electron temperature. At around the peak of the SBS reflectivity, the positive frequency shift of IAW induced by electron trapping roughly offsets the negative frequency shift induced by ion trapping.

Effects of frequency-modulated pump on stimulated Brillouin scattering in inhomogeneous plasmas

The effects of a frequency-modulated pump on stimulated Brillouin scattering (SBS) in a flowing plasma are investigated by theoretical analysis, three-wave simulations, and kinetic simulations. The resonance point of SBS oscillates in a certain spatial region with time when frequency modulations are applied. There exists a certain frequency modulation that causes the velocity of resonant points to be similar to the group velocity of the seed laser, which increases the SBS reflectivity. The SBS can also be suppressed by frequency modulation with larger bandwidth. In the kinetic simulations, the effects of the frequency-modulated pump on the reflectivity agree with our theoretical predictions. Multi-location autoresonance is also observed in the narrow-bandwidth frequency modulation case, which can also increase the SBS reflectivity. Our work provides a method for selecting the laser bandwidth to inhibit SBS in inhomogeneous plasmas.

Suppression of stimulated Brillouin scattering by multicolor alternating-polarization bundle light in inertial confinement fusion

This study proposes a novel method to mitigate stimulated Brillouin scattering (SBS) using multicolor alternating-polarization bundle light. The bundle light combines multiwavelength, spike trains of uneven duration and delay for a single beam to multicolor alternating polarization for bundle beams. SBS suppression is verified using a three-dimensional large-scale laser plasma code. The numerical results show that the SBS reflectivity can be decreased by nearly two orders in low density plasma. The proposed method can extend the repetition time of a single beam from several picoseconds to tens of picoseconds. Moreover, it has potential applications in inertial confinement fusion research.

Laser transport and backscatter in low-density SiO2 and Ta2O5 foams

Experiments using a single 527 nm wavelength beam interacting with sub- and supercritical density SiO2 and Ta2O5 foams examined laser propagation and backscatter from laser–plasma instabilities such as Stimulated Brillouin Scattering (SBS). Two densities of each material were examined, and multiple diagnostics were used to characterize the propagation and backscatter. For 5 mg/cc SiO2 (ne/nc = 0.375), the laser propagation distance was well approximated by treating the foam as a gas. However, for the 2 mg/cc SiO2 foam (ne/nc = 0.15), the same model over-predicts the propagation distance by ∼40%. Existing analytical theories on propagation through subcritical foams were able to account for this difference. The laser heat wave propagated ∼1/2 as far in Ta2O5 than SiO2 foams with similar electron density. We showed that this difference is due to the increased radiation losses in the higher Z foam. The fraction of backscattered light scales linearly with incident laser intensity for the range of intensities examined. Ta2O5 foams had significantly lower levels of backscatter (1–3%) than the SiO2 (4–8%), which is consistent with estimates of large Landau damping due to the presence of the oxygen atoms. The measured fraction of SBS backscattered laser energy for a 2 mg/cc SiO2 foam shot was ∼4 times lower than predicted by simulations assuming a gas-like foam. We found that we needed to assume increased ion heating such that Ti/Te ∼ 1.2–1.5 in the plasma to agree with the measured SBS reflectivity. Analytical models of laser-heated foams predict preferential heating of the ions as has been observed in previous experiments.

Interaction of parametric instabilities from 3ω and 2ω lasers in large-scale inhomogeneous plasmas

The mechanism of energy transfer from 3ω pump light to stimulated Brillouin scattering (SBS) generated by 2ω pump light is proposed. The backscattering light of stimulated Raman scattering (SRS) generated by 3ω pump light can work as a seed of the SBS of 2ω pump light, which results in the increase of the latter. Adding 2ω pump light into 3ω pump light will decrease the reflectivity of SBS generated from 3ω pump light. The total reflectivity will first decrease and then increase with an increase of the ratio of 2ω light intensity to the intensity of 2ω and 3ω lights , and will be controlled in a lower level when this ratio f is about 10%−20%. These results give a method to control the total reflectivity of SBS and SRS in inertial confinement fusion (ICF) by adding 2ω light into 3ω light.

Growth rate and gain of stimulated Brillouin scattering considering nonlinear Landau damping due to particle trapping

Growth rate and gain of stimulated Brillouin scattering (SBS) considering the reduced Landau damping due to particle trapping has been proposed to predict the growth and average level of SBS reflectivity. Due to particle trapping, the reduced Landau damping has been taken used of to calculate the gain of SBS, which will make the simulation data of SBS average reflectivity be consistent to the Tang model better. Other nonlinear effects such as harmonic generation and pump depletion have been also considered in the later evolution of SBS. This work may solve the pending questions in laser-plasma interaction and have wide applications in parametric instabilities.

Burst behavior due to the quasimode excited by stimulated Brillouin scattering in high-intensity laser–plasma interactions

The strong-coupling mode, called the “quasimode”, is excited by stimulated Brillouin scattering (SBS) in high-intensity laser–plasma interactions. Also SBS of the quasimode competes with SBS of the fast mode (or slow mode) in multi-ion species plasmas, thus leading to a low-frequency burst behavior of SBS reflectivity. Competition between the quasimode and the ion-acoustic wave (IAW) is an important saturation mechanism of SBS in high-intensity laser–plasma interactions. These results give a clear explanation of the low-frequency periodic burst behavior of SBS and should be considered as a saturation mechanism of SBS in high-intensity laser–plasma interactions.

The interplay between the kinetic nonlinear frequency shift and the flowing gradient in stimulated Brillouin scattering

The effect of the kinetic nonlinear frequency shift (KNFS) on backward stimulated Brillouin scattering (SBS) in homogeneous plasmas and inhomogeneous flowing plasmas is investigated by three-wave coupled-mode equations. When the positive contribution to the KNFS from electrons as well as the negative contribution from ions is included, the net KNFS can become positive at a large electron-ion temperature ratio . In homogeneous plasmas, KNFS can greatly reduce the SBS reflectivity at low or large but has a weak effect on SBS at where the positive frequency shifts from electrons almost cancels out the negative shifts from ions. In inhomogeneous plasmas, the net negative frequency shift can enhance the backward SBS reflectivity for the negative gradient of the plasma flowing, and can suppress the reflectivity for the positive case. On the contrary, the net positive frequency can suppress the reflectivity for the negative case of the flowing gradient and enhance the reflectivity for the positive case. This indicates that the SBS in inhomogeneous flowing plasmas can be controlled by changing the sign of the nonlinear frequency shift.

Anti-Stokes scattering and Stokes scattering of stimulated Brillouin scattering cascade in high-intensity laser–plasma interaction

Anti-Stokes scattering and Stokes scattering in stimulated Brillouin scattering (SBS) cascades have been researched using the Vlasov–Maxwell simulation. In high-intensity laser–plasma interactions, stimulated anti-Stokes Brillouin scattering (SABS) will occur after second stage SBS rescattering. The mechanism of SABS has been put forward to explain this phenomenon. In the early phase of SBS evolution, only first stage SBS appears and total SBS reflectivity comes from first stage SBS. However, when high-stage SBS and SABS occur, SBS reflectivity will display burst behavior and the total reflectivity comes from the SBS cascade and SABS superimposition. The SABS will compete with the SBS rescattering to determine the total SBS reflectivity. Thus, SBS rescattering including SABS is an important saturation mechanism of SBS and should be taken into account in high-intensity laser–plasma interaction.

Stimulated Brillouin backscattering of hollow Gaussian laser beam in collisionless plasma under relativistic–ponderomotive regime

AbstractStimulated Brillouin backscattering of an intense hollow Gaussian laser beam (HGLB) from collisionless plasma has been investigated under relativistic–ponderomotive regime. The main feature of considered hollow Gaussian laser beam is having the same power at different beam orders with null intensity at the center. Backscattered radiation is generated due to nonlinear interaction between main beam (pump beam) with pre-excited ion acoustic wave (IAW). Modified coupled equations has been set up for the beam width parameters of the main beam, ion-acoustic wave, back-scattered wave, and back reflectivity of stimulated Brillouin scattering (SBS) with the help of the Wentzel–Kramers–Brillouin approximation, fluid equations and paraxial theory approach. These coupled equations are solved analytically and numerically to study the laser intensity in the plasma, the variation of amplitude of the excited IAW and back reflectivity of SBS. The back reflectivity of SBS is found to be highly sensitive to the order of the HGLB, intensity of main laser beam, and plasma density for typical laser and plasma parameters. The focusing of main laser beam (hollow Gaussian) and IAW significantly affected the back reflectivity of SBS. The results show that the self-focusing and back reflectivity is enhanced for higher order modes of HGLB.

Stimulated Brillouin scattering reduction induced by self-focusing for a single laser speckle interacting with an expanding plasma

The origin of the low level of stimulated Brillouin scattering (SBS) observed in laser-plasma experiments carried out with a single laser speckle is investigated by means of three-dimensional simulations and modeling in the limit when the laser beam power P is well above the critical power for ponderomotive self-focusing We find that the order of magnitude of the time averaged reflectivities, together with the temporal and spatial SBS localization observed in our simulations, are correctly reproduced by our modeling. It is observed that, after a short transient stage, SBS reaches a significant level only (i) as long as the incident laser pulse is increasing in amplitude and (ii) in a single self-focused speckle located in the low-density front part of the plasma. In order to describe self-focusing in an inhomogeneous expanding plasma, we have derived a new Lagrangian density describing this process. Using then a variational approach, our model reproduces the position and the peak intensity of the self-focusing hot spot in the front part of the plasma density profile as well as the local density depletion in this hot spot. The knowledge of these parameters then makes it possible to estimate the spatial amplification of SBS as a function of the laser beam power and consequently to explain the experimentally observed SBS reflectivity, considerably reduced with respect to standard theory in the regime of large laser beam power.

Interaction of 0.53 μm laser pulse with millimeter-scale plasmas generated with gas-bag target

Reported are the interactions of 0.53 μm laser pulse with millimeter-scale plasmas that are produced with gas-bag target irradiated with 0.35 μm laser pulses. The generated plasmas are characterized with collective Thomson scattering and space- and time-resolved X-ray images. The density of the plasmas is about 0.1 n c corresponding to the 0.53 μm laser beam, and the temperature is about 0.64 keV with a filling gas of C5 H12 . The interaction laser beam can output 1.0 kJ energy within the duration of 1 ns at the wavelength of 0.53 μm, leading to a nominal intensity less than 1.5 × 1015 W/cm2 . The reflectivity of stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) is measured with a full aperture backscatter system. The experimental results show than the reflectivity of SBS is less than 5% and that of SRS is less than 1%. It is a positive result with respect to using 0.53 μm lasers as drivers for laser fusion.

Stimulated Brillouin Scattering of Gaussian Laser Beam in Relativistic Plasma

This paper presents an investigation of stimulated Brillouin scattering (SBS) of Gaussian laser beam in relativistic plasma. The pump beam interacts with a pre-excited ion-acoustic wave thereby generating a back-scattered wave. In the high intensity laser beam, electron oscillatory velocity becomes comparable to the velocity of light, which modifies the refractive index of plasma due to increase in the electron mass. This modification of refractive index affects the incident laser beam, ion-acoustic wave and back scattered beam. We have set up non-linear differential equations for the beam width parameters of the main beam, ion-acoustic wave, back-scattered wave and derived expression of SBS-reflectivity by taking full non-linear part of the dielectric constant of relativistic plasma with the help of moment theory approach. It is observed from the analysis that focusing of waves greatly affects the SBS reflectivity.

Study of stimulated Brillouin scattering in extended paraxial region

AbstractThis communication presents a comparison of two cases to study the process of laser beam propagation and Stimulated Brillouin Scattering (SBS) in laser plasma interaction. These two cases are, with imposing the restriction of paraxial approximation on the laser beam and, relaxing this restriction up to a certain extent. In this work, the study done by Sharma et al. (2009) using paraxial approximation is extended by taking contribution of the off-axial rays in the laser beam profile. The spitted profile of the laser beam is obtained due to uneven focusing of the off-axial rays and its effect on localization of the ion acoustic wave (IAW) has been studied. Including the off-axial part of the laser beam semi-analytical solution of the nonlinear coupled IAW equation has been found and this is examined that off-axial part affects background densities. The nonlinear coupling between the laser beam and IAW is severely affected by modified profile of the laser beam. Further, it is investigated that the SBS is also influenced by this coupling process. Therefore, the reflectivity of SBS has been compared with and without contribution of off-axial rays. A notable change is found in the magnitude of SBS reflectivity in modified-paraxial case in comparison to the paraxial case.

Stimulated Brillouin scattering with high reflectivity and fidelity in liquid-core optical fibers

The characteristics of stimulated Brillouin scattering (SBS) in a liquid-core optical fiber (LCOF) contained CS2 with 10 ns pulses at 532 nm wavelength are investigated experimentally. The threshold, reflectivity, pulse-shape (PS) fidelity, and phase-conjugation (PC) fidelity of LCOFs with different core diameters and lengths are measured. Threshold energy is as low as 2.5 μJ, and the SBS reflectivity, the PS fidelity, and PC fidelity exceed simultaneously 90 % when the input energy is over 20 times threshold.

Phase fluctuation of self-phase-controlled stimulated Brillouin scattering waves via K8 glass

As stimulated Brillouin scattering (SBS) materials, solids have many advantages such as compactness, innoxiousness, and ease of handling. The SBS threshold and reflectivity of K8 glass, a solid material, are investigated. A SBS phase control experiment using K8 glass to achieve coherent beam combination is performed with a self-phase-control technique below the optical damage threshold. The experimental results show that the short-term phase has been stabilized well with λ/33.8 fluctuation during 256 shots (25.6s), and that the long-term phase change with non-convective K8 glass is much slower than that with convective HT-70 liquid.

Performance of large aperture tapered fiber phase conjugate mirror with high pulse energy and 1-kHz repetition rate

A large aperture fused silica tapered fiber phase conjugate mirror is presented with a maximum 70% stimulated Brillouin scattering (SBS) reflectivity, which is obtained with 1 kHz repetition rate, 15 ns pulse width and 38 mJ input pulse energy. To the best of our knowledge, this is the highest SBS reflectivity ever reported by using optical fiber as a phase conjugate mirror for such high pulse repetition rate (1 kHz) and several tens of millijoule (mJ) input pulse energy. The influences of fiber end surface quality and pump pulse widths on SBS reflectivity are investigated experimentally. The results show that finer fiber end surface quality and longer input pulse widths are preferred for obtaining higher SBS reflectivity with higher input pulse energy. Double passing amplification experiments are also performed. 52 mJ pulse energy is achieved at 1 kHz repetition rate, with a reflected SBS pulse width of 1.5 ns and a M(2) factor of 2.3. The corresponding peak power reaches 34.6 MW. Obvious beam quality improvement is observed.