Coherent Raman amplification.
Coherent Raman amplification dynamics, considering pump depletion and coupling saturation and stimulated Raman scattering by polarization waves
POLARIZATION CHARACTERISTICS↗
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Hot Raman amplification
A parameter regime is identified for Raman amplification at high temperatures, where deleterious laser–plasma instabilities that limit current amplification experiments are avoided, yet sufficient gain for an effective power amplifier is attained. Furthermore, calculations and kinetic simulations show that even at high temperatures, an amplifier is able to satisfy the criteria set forth to present a proof-of-principle system scalable to high powers, where energy transfer efficiencies are ≥30%, intensity gains are ≥10, and output intensities are ≥100× the pump intensity.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Optical phase conjugation in backward Raman amplification
Compression of an intense laser pulse using backward Raman amplification (BRA) in plasma, followed by vacuum focusing to a small spot size, can produce unprecedented ultrarelativistic laser intensities. The plasma density inhomogeneity during BRA, however, causes laser phase and amplitude distortions, limiting the pulse focusability. To solve the issue of distortion, we investigate the use of optical phase conjugation as the seed pulse for BRA. We show that the phase conjugated laser pulses can retain focusability in the nonlinear pump depletion regime of BRA, but not so easily in the linear amplification regime. This somewhat counterintuitive result is because the nonlinear pump depletion regime features a shorter amplification distance, and hence less phase distortion due to wave–wave interaction, than the linear amplification regime.
Jia, Qing↗
Focusability in the multi-pump Raman amplification of short laser pulses
Spatially combining multiple strong laser beams is a promising concept for achieving ultrahigh laser intensities. Proof-of-principle experiments have been conducted at the National Ignition Facility to report a combination of up to 20 pulses with high energy conversion efficiency. However, the combination process might damage the seed focusability due to mismatch of the seed and pump wavefronts. Here, we investigate the effect of the finite pump beam size on the focusability of the seed pulse. We propose an approach to retain and even improve the seed focusability by specifically arranging multiple pump beams. The results are demonstrated by the numerical solution of coupled nonlinear Schrödinger equations. Our findings also apply to seed amplification by filamented pump pulses.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Stimulated Raman amplification, oscillation, and linewidth in barium nitrate
Measurements of Raman gain in a Ba(NO3)2 crystal are reported at 532 nm using a Raman oscillator/amplifier arrangement for differential absorption lidar measurements of ozone. The experimentally determined gain coefficient will be compared with theoretical results. The effect of single and multi-longitudinal mode pumping upon the amplification process will be discussed. Measurement of the Raman linewidth for 1st 2nd and 3d stokes shifts arc presented.
McCray, Christopher J.↗
A robust plasma-based laser amplifier via stimulated Brillouin scattering
Brillouin amplification in plasma is more resilient to fluctuations in the laser and plasma parameters than Raman amplification, making it an attractive alternative to Raman amplification. In this work, we focus on high plasma densities, n 0 > n cr /4, where stimulated Raman scattering is not possible and laser beam filamentation is the dominant competing process. Through analytic theory and multi-dimensional particle-in-cell simulations, we identify a parameter regime for which Brillouin amplification can be efficient while maintaining filamentation of the probe at a controlled level. We demonstrate pump-to-probe compression ratios of up to 72 and peak amplified probe fluences over 1 kJ cm -2 with ≃50% efficiency. High pulse quality is maintained through control of parasitic filamentation, enabling operation at large beam diameters. Provided the pump and probe pulse diameters can be increased to 1 mm, our results suggest that Brillouin amplification can be used to produce sub-picosecond pulses of petawatt power.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
New criteria for efficient Raman and Brillouin amplification of laser beams in plasma
Raman or Brillouin amplification of a laser beam in plasma has long been seen as a way to reach multi-PW powers in compact laser systems. However, no significant plasma-based Raman amplification of a laser pulse beyond 0.1 TW has been achieved in nearly 20 years, and only one report of Brillouin amplification beyond 1 TW. In this paper, we reveal novel non-linear criteria for the initial seed pulse that will finally open the door to efficient Raman and Brillouin amplification to petawatt powers and Joule-level energies. We show that the triple product of the coupling constant \(\Gamma \) , seed pulse duration \(\tau \) and seed pulse amplitude a for the Raman seed pulse (or \(a^{2/3}\) for Brillouin) must exceed a specific minimum threshold for efficient amplification. We also analyze the plasma-based Raman and Brillouin amplification experiments to date, and show that the seed pulses used in nearly all experiments are well below our new threshold, which explains the poor efficiency obtained in them. Finally, we analyze a recent Brillouin amplification experiment that used increased seed pulse power to obtain Joule-level amplification, and find excellent agreement with our theory.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Ultra-Long Distance BOTDA Sensor System Employing Hybrid Amplification and Advanced Noise Reduction Techniques
Brillouin Optical Time Domain Analysis (BOTDA) sensor systems play a pivotal role in distributed sensing, which enables precise measurements of strain and temperature across extensive fiber lengths. This research offers a comprehensive strategy to extend the sensing range of BOTDA systems beyond tens of kilometers while maintaining high spatial resolutions. Such enhanced sensing is realized through the integration of distributed Raman amplification, inline amplification using erbium-doped fiber amplifiers (EDFA), and advanced noise reduction techniques. Optimizing Raman pump and probe wave profiles ensures the robustness of Brillouin scattering signals, effectively countering attenuation-induced losses. Additionally, enhancing weak Brillouin signals by strategically placing EDFAs at optimal fiber locations keeps sensor sensitivity high. Leveraging inherent redundancy in measured data as a function of frequency and fiber distance, the non-local means (NLM) filter removes noise while preserving essential physical information. In summary, this research has shown a holistic exploration of extending BOTDA’s distance sensing capabilities up to 150 km with spatial resolutions of 8 meters.
Bhatta, Hari↗
Ultra-Long Distance BOTDA Sensor System Employing Hybrid Amplification and Advanced Noise Reduction Techniques
Brillouin Optical Time Domain Analysis (BOTDA) sensor system play a pivotal role in distributed sensing, which enables precise measurements of strain and temperature across extensive fiber lengths. Nonetheless, challenges emerge as distances grow due to signal attenuation and noise interference resulting in measurement errors. This research offers a comprehensive strategy to extend the sensing range of BOTDA systems beyond 10’s of kilometers while maintaining high spatial resolutions. Such enhanced sensing is realized through the integration of distributed Raman amplification, inline amplification using erbium-doped fiber amplifiers (EDFA), and advanced noise reduction techniques. Leveraging inherent redundancy in measured data as a function of frequency and fiber distance, the non-local means (NLM) filter removes noise while preserving essential physical information. This approach proves particularly advantageous in BOTDA systems, where accurate measurement of Brillouin scattering signals is paramount for long-range sensing, while concurrently safeguarding high spatial resolutions. In summary, this research has shown a holistic exploration of extending BOTDA's distance sensing capabilities up to 150 km with spatial resolutions of 8 meters.
Bhatta, Hari↗
Seed source for plasma compression in the long wavelength infrared
Two color laser pulses are used to form an air plasma and generate broadband infrared radiation suitable as a seed for backward Raman amplification of CO 2 laser pulses. Broadband radiation in the atmospheric window from 8-14 microns is observed. The infrared radiation is characterized using a long wavelength grating spectrometer specially designed to accept an ionizing laser filament at its input plane. The LWIR yield is greatly enhanced by chirping the drive pulse. Unidirectional pulse propagation simulations suggest that this is due in part to the dependence of the nonlinear refractive index on the pulse
43 PARTICLE ACCELERATORS↗
Large-mode-area optical fibers and optical fiber amplifiers in the eye-safe regime
The present technology provides large mode area optical fibers engineered to have normal dispersion around 1600 nm, enabling high power Raman amplification at eye safer wavelengths. The fibers can have a main core and one or more side cores disposed relative to the main core so that modes of the main core and the one or more side cores hybridize into supermodes with modified dispersion.
Pax, Paul H.↗
Tunable Laser-Plasma Amplifier (Final Report)
Exploration at the laser intensity frontier has always offered new avenues for physics and reaching beyond this frontier is a grand challenge. Present-day petawatt-class lasers provide focused intensities on target of 10 22 W/cm 2 , corresponding to electric fields of 200 TV/m, while laser-plasma amplification opens a route for focused intensities well above 10 23 W/cm 2 . Intensities in this range provide the ability to test quantum electrodynamics in the unexplored low-energy, strong-field regime where signatures for new physics may arise. The behavior of matter under such extraordinary conditions is a rich and fascinating subject, not only in its own right in fundamental plasma physics, but for the many potential applications that promise to enrich the natural sciences in the future, including compact electron-beam, ion-beam particle accelerators, and ultra-bright X-ray sources. Although laser systems are now under construction internationally to access intensities of 10 23 W/cm 2 , the current technologies used appear to be fundamentally limited to these intensities. The realization of intensities beyond 10 23 W/cm 2 using parametric amplification in plasmas promises a breakthrough in high-energy density physics. Parametric amplification using Raman scattering in a plasma could provide the enabling technology for the generation of ultra-high-power laser pulses, but a more complete understanding of the nonlinear optics of plasmas is required. There is a significant gap in well-diagnosed laser-plasma instability studies of nonlinear plasma-wave phenomena, which are critical to understand for future laser-plasma devices. To achieve an efficient laser-plasma amplifier, plasma waves must be driven to large amplitude where significant energy can be rapidly transferred from the pump to the seed over the pulse duration of the seed. Simulations suggest that this nonlinear pump depletion regime can be achieved in the “pi-pulse” amplification regime. While simulations show this optimal regime with efficient amplification, it has remained elusive in experiments and there is a growing consensus within the community that thermal effects and pump beam limitations prevent laser-plasma amplifiers from progressing through the linear regime into the nonlinear pump depletion regime. Previous experiments have been significantly limited by the laser power available at the necessary wavelengths for the seed laser; therefore, the amplification is required to start in the linear regime where it is sensitive to many deleterious effects. The enabling technology (currently unique to plasma-wave amplification in the world) at the University of Rochester is the ability to provide a seed pulse with sufficient power (4 mJ/100 fs seed) to immediately drive nonlinear plasma waves into the pi-pulse regime and to tune its wavelength to optimize the efficiency of energy transfer. This in combination with the state-of-the-art OMEGA heater beams providing multiple kilojoules in a nanosecond to sufficiently heat the plasma make this system distinct from previous studies. These heater beams will provide, for the first time in Raman amplification studies, a homogeneous electron temperature high enough to prevent pump beam propagation issues that have plagued previous experiments. These systems will provide a platform for driving electron-plasma waves into the nonlinear regime where pump depletion and pulse shortening are predicted to lead to high amplification efficiencies (>30%). The Team has made significant progress through prior support from DOE Fusion Energy Sciences [DOE Office of Science Award Number DE-SC0016253 (2016-2022)]. This includes twenty-two peer-reviewed manuscripts, one patent, ten contributed talks presented at international conferences, and research that was highlighted as invited talks at fifteen international conferences. The broader impacts of this research are evident in the support of early career scientists, two Ph.D. theses, four current graduate students, a Masters Project, two undergraduate researchers, and an underrepresented minority student hired through the California Alliance for Minority Participation who now works as a Research Engineer in the group. This research met all of the funded research objectives and the highlights from primary Raman amplification thrust of this work are discussed below and form the foundation for the proposed research.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Boosting the performance of Brillouin amplification at sub-quarter-critical densities via reduction of parasitic Raman scattering
Brillouin amplification of laser pulses in plasma has been shown to be a promising approach to produce picosecond pulses of petawatt power. A key challenge is preservation of the quality of the amplified pulse, which requires control of parasitic instabilities that accompany the amplification process. At high plasma densities (> cr /4), ponderomotive filamentation has been identified as the biggest threat to the integrity of the amplifying pulse. It has therefore been proposed to perform Brillouin scattering at densities below n cr /4 to reduce the influence of filamentation. However, parasitic Raman scattering can become a problem at such densities, contrary to densities above n cr /4 where it is forbidden. In this paper, we investigate the influence of parasitic Raman scattering on Brillouin amplification at densities below n cr /4. We expose the specific problems posed by both Raman backward and forward scattering, and how both types of scattering can be mitigated, leading to an increased performance of the Brillouin amplification process.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
MoS 2 Nanoplatelets on Hybrid Core-Shell (HyCoS) AuPd NPs for Hybrid SERS Platform for Detection of R6G
In this work, a novel hybrid SERS platform incorporating hybrid core-shell (HyCoS) AuPd nanoparticles (NPs) and MoS 2 nanoplatelets has been successfully demonstrated for strong surface-enhanced Raman spectroscopy (SERS) enhancement of Rhodamine 6G (R6G). A significantly improved SERS signal of R6G is observed on the hybrid SERS platform by adapting both electromagnetic mechanism (EM) and chemical mechanism (CM) in a single platform. The EM enhancement originates from the unique plasmonic HyCoS AuPd NP template fabricated by the modified droplet epitaxy, which exhibits strong plasmon excitation of hotspots at the nanogaps of metallic NPs and abundant generation of electric fields by localized surface plasmon resonance (LSPR). Superior LSPR results from the coupling of distinctive AuPd core-shell NP and high-density background Au NPs. The CM enhancement is associated with the charge transfer from the MoS 2 nanoplatelets to the R6G. The direct contact via mixing approach with optimal mixing ratio can effectively facilitate the charges transfer to the HOMO and LUMO of R6G, leading to the orders of Raman signal amplification. The enhancement factor (EF) for the proposed hybrid platform reaches ~10 10 for R6G on the hybrid SERS platform.
36 MATERIALS SCIENCE↗
Phase noise limited frequency shift impulsive Raman spectroscopy
We introduce a method to enable optical amplification of a coherent Raman spectroscopy signal, which we call radio frequency (RF) Doppler Raman spectroscopy. In this article, we consider the perturbation of a probe pulse in a sample due to an excited Raman vibrational coherence as a generalized Doppler shift, which connects a time-varying optical path length (the product of the propagation length and refractive index, OPL = n ℓ) with an optical frequency shift. Amplification of a Raman signal outside of the focused interaction is enabled by converting the Doppler frequency shift experienced by a laser probe pulse into a periodic timing jitter. This transit time perturbation is detected through the phase of a RF electronic signal measured at a harmonic of the probe pulse train with a method adapted from precision metrology techniques used to measure laser pulse train timing jitter. Measurement of a timing jitter allows access to much lower noise floors than other coherent Raman techniques, and by exploiting the new capability to scale the signal of a coherent Raman spectroscopic signal, this method opens the potential to detect very weak Raman signals that are currently not observable due to limits of illumination intensity imposed by laser damage to the specimen and noise.
47 OTHER INSTRUMENTATION↗
Progress of research on water vapor lidar
Research is summarized on applications of stimulated Raman scattering (SRS) of laser light into near infrared wavelengths suitable for atmospheric monitoring. Issues addressed are conversion efficiency, spectral purity, optimization of operating conditions, and amplification techniques. A Raman cell was developed and built for the laboratory program, and is now available to NASA-Langley, either as a design or as a completed cell for laboratory or flight applications. The Raman cell has been approved for flight in NASA's DC-8 aircraft. The self-seeding SRS technique developed here is suggested as an essential improvement for tunable near-IR DIAL applications at wavelengths of order 1 micrometer or greater.
Wilkerson, Thomas D.↗
Laser–plasma amplification of an ultrabroadband laser pulse to 0.3 TW
Producing on-target laser intensities much greater than 10 23 W cm −2 with current laser technologies is a roadblock to accessing new regimes of physics such as strong-field quantum electrodynamics. Laser–plasma amplifiers show promise to realize these intensities by augmenting the final amplifier and compressor in traditional chirped-pulse-amplification architectures with a plasma-based amplification and compression stage that operates at a much higher damage threshold. Here we demonstrate amplification of an ultrabroadband (>60 nm) pulse in a laser–plasma Raman amplifier. We directly amplified seed intensities up to 3.7 × 10 15 W cm −2 and measured efficiencies up to 8.7%. Single-shot SPIDER measurements show a factor-of-2 reduction in the amplified pulse duration with final powers up to 0.3 TW, a 10× improvement over previous results. Final pulse durations of 64 fs are measured. Energy transfers greater than 220 mJ from the picosecond pump into the seed result in a 30× energy amplification of a 7.6 mJ seed. These results set the stage for a compact plasma afterburner based on Raman amplification that could extend the scientific capability of existing petawatt-class laser facilities to enable experiments at the intensity frontier.
Shaw, J. L. [Univ. of Rochester, NY (United States↗
A self-seeded SRS system for the generation of 1.54-micron eye-safe radiation
A light source is described for the efficient generation of 1.54-micron for eye-safe aerosol lidar operation. The system is based upon an Nd:YAG laser at 1.06 micron which is then Raman-shifted in methane to produce light at the first Stokes wavelength of 1.54 micron. First Stokes light generated in the backward direction was retroreflected back into the Raman cell for amplification in the tail of the 10 ns pump beam. The energy conversion efficiency and the spatial beam quality of the amplified first Stokes were found to be adversely affected when operated at higher repetition rates due to a thermal gradient produced in the generation region. A Stokes energy of 25 mJ was obtained for a pumping energy of 140 mJ at a repetition rate of 10 Hz. The beam divergence of the amplified Stokes radiation was measured to be less than 1 mrad. The optimized results demonstrate the applicability of this radiation for eye-safe lidar measurements.
Chu, Z.↗