Smoothing By Spectral Dispersion Research Articles

Optimization of smoothing by spectral dispersion with a sinusoidal phase modulation

In order to improve the propagation of a laser in a plasma, the intensity profile should be as uniform as possible. For this purpose, smoothing techniques are used. The most commonly used, smoothing by spectral dispersion (SSD), induces temporal fluctuations of the effective amplification length of Stimulated Brillouin scattering (SBS) or Stimulated Raman scattering (SRS). These fluctuations are random from shot to shot and may thus induce unpredictable levels of the efficient laser power used in laser-plasma experiments. Increasing the modulation frequency fm while maintaining the modulation bandwidth strongly reduces these fluctuations and even reduces the average SBS level. However, increasing fm has also an impact on laser propagation within the laser optical components. We show that, as long as chromatic dispersion is precompensated in the front-end, FM-to-AM conversion, a detrimental propagation effect, may remain sufficiently low and that anormal FM-to-AM conversion is not sensitive to the modulation frequency. Hence, there is no important bottleneck to an increase of the modulation frequency.

Dynamic control of the spatial frequency content of an intense laser via intra-beam energy transfer

We present theory and simulations demonstrating that bandwidth-inducing optical smoothing techniques such as smoothing by spectral dispersion (SSD) can initiate intra-beam energy transfer (IBET) within a single beam. IBET consists of energy transfer between the different frequency components of the beam and results in a simultaneous and correlated redistribution of the temporal and spatial frequencies of the laser's electric field. We identify a threshold in bandwidth for IBET to occur and validate the theory with simulations. The bandwidth and the color cycling of SSD can result in IBET in conditions relevant for high-energy density experiments at the National Ignition Facility and Omega Laser. We show that IBET could be used to manipulate a laser's spatial frequency content within a plasma. In particular, this technique could be used to increase a laser's effective f-number, with potential applications for self-guiding in plasmas.

Improving stimulated Brillouin scattering mitigation in weakly-damped plasmas: from spectral dispersion to spectral distribution

Optical smoothing techniques are widely used in large high-power laser facilities designed for inertial confinement fusion research, such as the Laser Mégajoule (LMJ) in France. Those techniques are of primordial interest because they reduce laser–plasma instabilities as, for example, the stimulated backward Brillouin scattering (SBS). The latter takes place in the hot plasmas produced inside a hohlraum by the ablation of high-Z or gold material where ion acoustic waves are weakly damped, easing SBS. However, despite the current optical smoothing implementation, like smoothing by spectral dispersion (SSD), SBS is not sufficiently mitigated and may also damage the laser optics by following back the laser path. Moreover, SSD limits the available energy and may produce unwanted amplitude modulations in the laser chain, resulting in an additional damage. Using spectral distribution with two colors (Duluc et al 2019 Phys. Rev. Appl. 12 054055) has proven to limit the latter damage, keeping the same efficiency for limiting SBS. In this study, we go a step forward and generalize this approach by exploring multi-color spectral distribution schemes and the role of dispersion by means of three-dimensional simulations. This study paves the road to new laser concepts that could tackle more efficiently the laser–plasma interaction issues leading to a better laser energy coupling.

Inhibition of stimulated Raman side-scattering with one-dimensional smoothing by spectral dispersion

Smoothing by spectral dispersion (SSD) is a beam smoothing technology aiming at improving irradiance uniformity in laser inertial confinement fusion, which has the potential to suppress many kinds of laser–plasma instabilities. Different effectivenesses of SSD on the suppression of instabilities were reported in previous works, suggesting SSD has different effects on different instabilities under various laser and plasma conditions. In this paper, inhibition of stimulated Raman side-scattering, deduced from the decrease in side-scattered light and hot electrons, in plastic plasmas at moderate laser intensity is observed in experiments with the application of one-dimensional SSD, the reason for which is deduced to be related to the suppression of filamentation. In contrast, two-plasmon decay and backward Raman scattering were not effectively suppressed by SSD in the experiments, the reason for which could be attributed to the limited modulation frequency and the directions of growth with respect to SSD induced rapid motion of laser spots.

Crossed beam energy transfer between optically smoothed laser beams in inhomogeneous plasmas.

Crossed beam energy transfer, CBET, in high-intensity laser–plasma interaction is investigated for the case of optically smoothed laser beams. In the two approaches to laser-driven inertial confinement fusion experiments, the direct-drive and the indirect-drive, CBET is of great importance because it governs the coupling of laser energy to the plasma. We use the two-dimensional wave-coupling code Harmony to simulate the transfer between two laser beams with speckle structure that overlap in a plasma with an inhomogeneous flow profile. We compare the CBET dynamics for laser beams with spatial incoherence and with spatio-temporal incoherence; in particular we apply the smoothing techniques using random phase plates (RPPs) and smoothing by spectral dispersion (SSD), respectively. It is found that for laser beams (wavelength λ0) with intensities (IL) above IL ∼ 2 × 1015 W cm−2(λ0/0.35 µm)−2(Te/keV), both the so-called plasma-induced smoothing as well as self-focusing in intense laser speckles induce temporal incoherence; the latter affects the CBET and the angular distribution of the light transmitted behind the zone of beam overlap. For RPP-smoothed incident beams, the resulting band width of the transmitted light can already be of the same order as the effective band width of the SSD available at major laser facilities. We examine the conditions when spatio-temporal smoothing techniques become efficient for CBET.This article is part of a discussion meeting issue ‘Prospects for high gain inertial fusion energy (part 1)’.

Impact of spatiotemporal smoothing on the two-plasmon–decay instability

Higher levels of hot electrons from the two-plasmon–decay instability are observed when smoothing by spectral dispersion (SSD) is turned off in directly driven inertial confinement fusion experiments at the Omega Laser Facility. This finding is explained using a hot-spot model based on speckle statistics and simulation results from the laser–plasma simulation environment. The model accurately reproduces the relative increase in hot-electron activity at two different drive intensities although it slightly overestimates the absolute number of hot electrons in all cases. Extrapolating from the current ≈360-GHz system while adhering to the logic of the hot-spot model suggests that a larger SSD bandwidth should significantly mitigate hot-electron generation, and legacy 1-THz OMEGA experiments appear to support this conclusion. These results demonstrate that it is essential to account for laser speckles and spatiotemporal smoothing to obtain quantitative agreement with experiments. A compilation of hot-electron data from the past two decades reveals several other important points: (1) many prior experiments are more easily understood using recent results from multibeam absolute instability theory and (2) experiments with ignition-scale conditions produce less hot electrons compared to OMEGA spherical experiments for a given vacuum overlapped intensity, which is a promising result for validating performance predictions based on hydrodynamic scaling relations.

Enhanced smoothing by spectral dispersion via insertion of complex coaxial vortex plate

An enhanced smoothing by spectral dispersion (SSD) scheme via the insertion of a complex coaxial vortex plate in the beamline is proposed. The complex coaxial vortex plate is comprised of an inner vortex plate that induces an azimuthal charge on the beam, and a surrounding vortex plate that induces an opposite charge on the beam. The surrounding vortex plate is supported by a base of same material that generates a time delay between the inner and the outer sub-beams. The time delay then brings about the frequency shift of the sub-beams with frequency modulation and spectral dispersion. As a result of the coherent superposition of the sub-beams, the focal spot exhibits a dominant envelope with transverse motion, and the inside speckles rotate rapidly in the picosecond regime. The simulation shows that the irradiation uniformity is slightly improved via the insertion of the complex coaxial vortex plate. More importantly, the laser-plasma interaction length is significantly shortened and then the reflectivity of stimulated Brillouin scattering is reduced by the use of the enhanced SSD scheme.

Combined implementation of smoothing technologies for improving illumination uniformity of laser quad in multi-directions

ABSTRACTAzimuthal smoothing (AS) that enables the focal spot of a laser quad to rotate in a period of a few picoseconds can improve the illumination uniformity. However, the smoothing performance of azimuthal smoothing is worse than that of smoothing by spectral dispersion (SSD) due to the low rotation speed in the center area of the focal spot. A combined implementation of the schemes including smoothing by spectral dispersion, radial smoothing (RS) and azimuthal smoothing is discussed to achieve better and faster illumination uniformity. The theoretical models of the combined smoothing scheme have been built up to analyse the smoothing performance. The key parameters affecting the smoothing performance of the combined smoothing scheme are analysed and optimized, including the topological charge and the wavelength difference. The simulation results show that the illumination uniformity of the laser quad can be significantly improved in multi-directions by the combined smoothing scheme.

Analysis on FM-to-AM conversion of SSD beam induced by etalon effect in a high-power laser system

FM-to-AM (frequency modulation-to-amplitude modulation) conversion caused by nonuniform spectral transmission of broadband beam is harmful to high-power laser facility. Smoothing by spectral dispersion (SSD) beam is a special broadband beam for its monochromatic feature at the given time and space on the near field. The traditional method which uses the optical spectral transfer function as filters cannot accurately describe its AM characteristics. This paper presents the theoretical analysis of the etalon effect for SSD beam. With a low-order approximation, the analytic model of the temporal shape of SSD beam is obtained for the first time, which gives the detailed AM characteristics at local and integral aspects, such as the variation of ripples width and amplitude in general situation. We also analyze the FM-to-AM conversion on the focal plane; in the focusing process, the lens simply acts as an integrator to smooth the AM of SSD beam. Because AM control is necessary for the near field to avoid optics damage and for the far field to ensure an optimal interaction of laser–target, our investigations could provide some important phenomena and rules for pulse shape control.

Performance of smoothing by spectral dispersion with consideration of the gain characteristic of Nd:glass amplifier

A key issue in developing a high-power laser driver, which can be used for inertial confinement fusion and laser produced plasma experiments, is to obtain uniform irradiation on the target surface, thus a number of spatial or temporal techniques have been proposed for laser beam smoothing. A scheme combining a lens array with the technique of smoothing by spectral dispersion (SSD) is being explored in the SG-II Laser Facility located in Shanghai Institute of Optics and Fine Mechanics. As the laser system involves a variety of optical elements, their influences have to be considered in the implementation of such a scheme. The Nd:glass amplifier is one of the most important parts of the system, and the phase-modulated laser beam will propagate through it along the long light path when SSD is employed. In this paper, the performance of uniform irradiation of the target pattern is studied based on two-dimensional simulations when the gain characteristic of the amplifier is taken into account. The major factors, such as the small signal gain profile of the amplifier, the amplification factor, the bandwidth of the phase-modulated laser beam and the difference between the central wavelength of the laser and the central wavelength of the amplifier gain curve, are analyzed in detail. The numerical results show that when the central wavelength of the incident beam is different from the central wavelength of the amplifier gain curve, intensity distribution of the target pattern will be affected to a degree depending on the amplification factor; while these two wavelengths are very close to or identical with each other, variation in the intensity distribution is trivial. The symmetry of the phase-modulated laser spectrum will be destroyed due to the gain characteristic of the amplifier, especially when the bandwidth is relatively wide. However, the slight asymmetry does not result in significant influence on the spatial power spectrum nor uniformity of the target pattern, even in the case where the central wavelength of the incident beam is different from that of the amplifier gain curve. The reasons would be 1) the gain curve of the amplifier is actually quite flat within the laser bandwidth, and 2) with the technique of SSD, all spectral components contribute to the target intensity distribution within an average time. The analysis indicates that the performance of uniform irradiation of the target pattern depends mainly on the bandwidth of the phase-modulated laser beam. A wider bandwidth can always generate better irradiation when it is within a certain range, say no more than 0.3 nm, but beyond this range, the nonuniformity tends to remain at a level about 0.250.3. Multistage Nd:glass amplifiers will be employed in the practical laser driver, and the case investigated in this paper involves only one stage for simplicity. The conclusion obtained in this paper is important for implementing the technique of SSD in the laser system.

Understanding the effects of laser imprint on plastic-target implosions on OMEGA

Understanding the effects of laser imprint on target performance is critical to the success of direct-drive inertial confinement fusion. Directly measuring the disruption caused by laser imprints to the imploding shell and hot-spot formation, in comparison with multidimensional radiation–hydrodynamic simulations, can provide a clear picture of how laser nonuniformities cause target performance to degrade. With the recently developed x-ray self-emission imaging technique and the state-of-the-art physics models recently implemented in the two-dimensional hydrocode DRACO, a systematic study of laser-imprint effects on warm target implosions on OMEGA has been performed using both experimental results and simulations. By varying the laser-picket intensity, the imploding shells were set at different adiabats (from α = 2 to α = 6). As the shell adiabats decreased, it was observed that (1) the measured shell thickness at the time the hot spot lit up became larger than the uniform one-dimensional (1-D) predictions; (2) the hot-spot core emitted earlier than the corresponding 1-D predictions; (3) the measured neutron yield first increased then decreased as the shell adiabat α was reduced; and (4) the hot-spot size reduced as α decreased for cases where SSD (smoothing by spectral dispersion) was on but became larger for low-α shots in cases where SSD was off. Most of these experimental observations are well reproduced by DRACO simulations with laser imprints including modes up to λmax = 200. These studies identify the importance of laser imprint as the major source of degrading target performance for OMEGA implosions of adiabat α ≤ 3. Mitigating laser imprints is required to improve low-α target performance.

Optical smoothing of laser imprinting in planar-target experiments on OMEGA EP using multi-FM 1-D smoothing by spectral dispersion

Direct-drive ignition on the National Ignition Facility (NIF) requires single-beam smoothing to minimize imprinting of laser nonuniformities that can negatively affect implosion performance. One-dimensional, multi-FM smoothing by spectral dispersion (SSD) has been proposed to provide the required smoothing [Marozas et al., Bull. Am. Phys. Soc. 55, 294 (2010)]. A prototype multi-FM SSD system has been integrated into the NIF-like beamline of the OMEGA EP Laser System. Experiments have been performed to verify the smoothing performance by measuring Rayleigh–Taylor growth rates in planar targets of laser-imprinted and preimposed surface modulations. Multi-FM 1-D SSD has been observed to reduce imprint levels by ∼50% compared to the nominal OMEGA EP SSD system. The experimental results are in agreement with 2-D DRACO simulations using realistic, time-dependent far-field spot-intensity calculations that emulate the effect of SSD.

Design and operation of the multiple-pulse driver line on the OMEGA laser

The multiple-pulse driver line (MPD) provides on-shot co-propagation of two separately generated pulse shapes in all 60 OMEGA beams at the Laboratory for Laser Energetics. Smoothing by spectral dispersion (SSD), which increases the laser bandwidth, can be applied to either one of the two pulse shapes, thereby enabling dynamic bandwidth reduction. The design of the MPD required careful consideration of beam combination as well as the minimum pulse separation for two pulses generated by two separate seed sources. A new combined-pulse-shape diagnostic needed to be designed and installed after the last SSD grating. The capability of MPD to reduce dynamic bandwidth has been used on a series of campaigns on OMEGA and the performance data are presented.

Research of beam conditioning technologies using continuous phase plate, Multi-FM smoothing by spectral dispersion and polarization smoothing

In the research of inertial confinement fusion, laser plasma interaction (LPI) is becoming a key problem that affects ignition. Here, multi-frequency modulation (Multi-FM) smoothing by spectral dispersion (SSD), continuous phase plate (CPP) and polarization smoothing (PS) were experimentally studied and implemented on the SG-III laser facility. After using these techniques, the far field distribution of SG-Ⅲ laser facility can be adjusted, controlled and repeated accurately. The output spectrums of the cascade phase modulators used for Multi-FM SSD were stable and the FM-to-AM effect can be restrained. Experiments on SG-III laser facility indicate that when the number of color cycles adopts 1, imposing SSD with 3.3 times diffraction limit (TDL) did not lead to pinhole closure in the spatial filters of preamplifier and main amplifiers with 30-TDL pinhole size. The nonuniformity of the focal spots using Multi-FM SSD, CPP and PS drops to 0.18, comparing to 0.26 with CPP+SSD, 0.57 with CPP+PS and 0.84 with only CPP and wedged lens. Polarization smoothing using flat birefringent plate in the convergent beam of final optics assembly (FOA) was studied. The PS plates were manufactured and equipped on SG-III laser facility for LPI research. Combined beam smoothing and polarization manipulation were also studied to solve the LPI problem. Results indicate that through adjusting dispersion directions of SSD beams in a quad, two dimensional SSD can be obtained. Using polarization control plate (PCP), polarization on the near field and far field can be manipulated, providing new method to solve LPI problem in indirect drive laser fusion.

Studies on design of 351 nm focal plane diagnostic system prototype and focusing characteristic of SGII-upgraded facility at half achievable energy performance.

In order to obtain the intensity distribution of a 351 nm focal spot and smoothing by spectral dispersion (SSD) focal plane profile of a SGII-upgraded facility, a type of off-axis imaging system with three spherical mirrors, suitable for a finite distance source point to be imaged near the diffraction limit has been designed. The quality factor of the image system is 1.6 times of the diffraction limit tested by a 1053 nm point source. Because of the absence of a 351 nm point source, we can use a Collins diffraction imaging integral with respect to λ=351 nm, corresponding to a quality factor that is 3.8 times the diffraction limit at 351 nm. The calibration results show that at least the range of ±10 mrad of view field angle and ±50 mm along the axial direction around the optimum object distance can be satisfied with near diffraction limited image that is consistent with the design value. Using this image system, the No. 2 beam of the SGII-upgraded facility has been tested. The test result of the focal spot of final optics assembly (FOA) at 351 nm indicates that about 80% of energy is encompassed in 14.1 times the diffraction limit, while the output energy of the No. 2 beam is 908 J at 1053 nm. According to convolution theorem, the true value of a 351 nm focal spot of FOA is about 12 times the diffraction limit because of the influence of the quality factor. Further experimental studies indicate that the RMS value along the smoothing direction is less than 15.98% in the SSD spot test experiment. Computer simulations show that the quality factor of the image system used in the experiment has almost no effect on the SSD focal spot test. The image system can remarkably distort the SSD focal spot distribution under the circumstance of the quality factor 15 times worse than the diffraction limit. The distorted image shows a steep slope in the contour of the SSD focal spot along the smoothing direction that otherwise has a relatively flat top region around the focal spot center.

Beam smoothing characteristics of multi-central frequency and multi-color smoothing by spectral dispersion.

A multi-central frequency and multi-color smoothing by spectral dispersion (SSD) scheme is presented for better beam smoothing than conventional SSD. One of the difficulties in conventional SSD is the narrow bandwidth limited by efficiency frequency conversion. The new scheme has overcome this problem by using multi-central frequency laser sources and a multi-color beam smoothing system. The multi-central frequency laser sources are incoherent combinations of several pulses in the time domain, each with different central frequencies and independent sinusoidal frequency-modulated components. Then, the laser beams from the multi-central frequency laser sources transmit in the multi-color smoothing system, spatially separated and independently spectrally dispersed by gratings array, independently achieve the third harmonic generation in KDP array with independent phase-matching for efficient conversion. Moreover, by adjusting the dispersion direction of the gratings to be perpendicular to each other, it is an effective way to achieve a 2D beam smoothing effect on the focal plane. Simulation results show that the novel beam smoothing scheme performs better than the conventional SSD.

A novel scheme of beam smoothing using multi-central frequency and multi-color smoothing by spectral dispersion

In typical techniques of smoothing by spectral dispersion (SSD), illumination uniformity cannot be further improved by increasing the pulse bandwidth due to the limitation of high-efficiency frequency tripling. Combined with the analysis of the schemes of four-color beam smoothing and multi-frequency modulator SSD, a novel scheme of beam smoothing using multi-central frequency and multi-color smoothing by spectral dispersion has been proposed, which not only can achieve high-efficiency frequency tripling, but also can obtain laser beams with nearly continuous spectrum and exhibit some specific advantages in far-field smoothing. Results show that the focal spot will be enlarged slightly but obviously further smoothed by the use of the new scheme. Compared to the typical SSD technique, the new scheme can decrease hot spots effectively and achieve the best irradiation in less time. Moreover, with independent combination of dispersion directions in each quadrant of grating array, a multi-dimensional smoothing on focal spot can be realized.

Propagation characteristics of small-bandwidth pulsed beams with smoothing by spectral dispersion in high power laser system

In this paper, we numerically study the propagation characteristics through spatial filter and free space of small-bandwidth pulsed beams with smoothing by spectral dispersion (SSD) in high power solid-state laser system. The numerical simulation with time-, space-and spectrum-resolution is achieved by using laser propagation and amplification software developed by Research Center of the Laser Fusion. The graphs of corresponding time-, space-and spectrum-distribution are presented, and the propagation characteristics between the SSD and non-SSD beam are compared. Besides, the effects of some SSD parameters on the diffraction propagation are further discussed. The results can help to choose the proper beam divergence of SSD which matches with the size of the spatial filter hole, to design the appropriate modulation frequency and the color circle number and so on. Moreover, it could guide the design of the light path, especially the design of free space propagation of small-diameter laser.

Investigation of stimulated rotational Raman scattering of the high-power broadband laser with applied angular dispersion

Smoothing by spectral dispersion (SSD) leads to considerable improvement on laser-irradiation uniformity in far field for fusion lasers. Phase modulation in time and spectral angular dispersion (SAD) across the beam introduced by SSD will affect the stimulated rotational Raman scattering (SRRS) gain in the near field. This paper focuses on the influence of SAD on SRRS gain under different laser conditions. Results show that the SAD will aggravate the generation of SRRS when the laser initial additional phase is constant. On the contrary, the SAD can reduce the SRRS gain if appropriate SSD parameters are adopted when the laser initial additional phase is variable. SSD has a certain application prospect in SRRS suppression.

Performance of target irradiation in a high-power laser with a continuous phase plate and spectral dispersion

We report on the performance of target irradiation at the SG-II high-power laser facility with a continuous phase plate (CPP) and the technique of smoothing by spectral dispersion (SSD). Simulative and experimental results are presented, where the irradiation uniformity and energy concentration of the target spots are analyzed. The results show that the designed CPP can focus the spot energy into the desired region and shape a profile with steep edge and flat top, but the actual performance of the fabricated CPP needs some improvements. It is also proved that the CPP is insensitive to the long-scale wavefront distortion in the incident beam. The one-dimensional SSD configuration evidently works in smoothing the fine-scale intensity modulation inside the target spot.