Stimulated Rotational Raman Scattering Research Articles

Optimization of stimulated rotational Raman scattering over vibrational scattering in a hydrogen-filled fiber.

We present the first, to the best of our knowledge, investigation of the gain competition between rotational and vibrational stimulated Raman scattering (SRS) in the transient regime for a hydrogen (H2)-filled antiresonant fiber (ARF) with the aim of generating multispectral emission composed of only rotational SRS. We show numerically and experimentally that purely rotational emission requires optimization of ARF length and spectral transmission, pump power and polarization, and H2 pressure. In this work, the H2-filled ARF is pumped by 40 kW, 7 ns pulses at λ = 1.06 µm to produce six discrete rotational lines from 1.1 to 1.7 µm with unique temporal profiles and pulse energies up to tens of microjoules.

Amplification of Supercontinuum Seed Pulses at ~1078–1355 nm by Cascade Rotational SRS in Compressed Hydrogen

Multiple higher-order Stokes of rotational stimulated Raman scattering (SRS) in compressed hydrogen covered the wavelength range from ~1.1 µm to ~1.4 µm using ~1.2 ps pump pulses from a Yb:YAG laser. In this study, the influence of polarization, energy, and the focusing conditions of the pump pulse, as well as gas pressure, on the conversion efficiency and modification of the spectral envelope of rotational and vibrational SRS was investigated. The use of a supercontinuum seed, as well as circular polarization of pump pulses at high hydrogen pressure, made it possible to significantly reduce the threshold for rotational SRS and suppress vibrational Stokes modes. The cascade excitation of rotational SRS, corresponding to a shift of 587 cm−1, ensured a spectrum spanning four Stokes orders withs a conversion efficiency of 52% and an output energy exceeding 3 mJ. The synthesized spectrum corresponded to ~14 fs transform-limited pulses.

Hydrogen Molecules Rotational Stimulated Raman Scattering in All-Fiber Cavity Based on Hollow-Core Photonic Crystal Fibers

Here, we report the rotational stimulated Raman scattering (SRS) of hydrogen molecules in an all-fiber cavity based on hollow-core photonic crystal fibers (HC-PCFs). The gas cavity consists of a 49 m long HC-PCF filled with 18 bar high-pressure hydrogen and two sections of fusion spliced solid-core fibers on both ends. When pumped by a homemade 1064 nm pulsed fiber amplifier, only rotational SRS occurs in the gas cavity due to the transmission spectral characteristics of the used HC-PCF, and 1135 nm Stokes wave is obtained (Raman frequency shift of 587 cm−1). By changing the pulse width and repetition frequency of the pump source, the output characteristics are explored. In addition, a theoretical model is established for comparison with the experimental results. This work is helpful for the application of gas Raman laser based on the HC-PCFs.

Pure rotational stimulated Raman scattering in H2-filled hollow-core photonic crystal fibers.

We conducted comprehensive theoretical research on rotational stimulated Raman scattering (SRS) of hydrogen molecules in hollow-core fibers. A reliable model for describing the steady-state rotational SRS of hydrogen was established and the influences of various factors was investigated. To verify the theoretical model, a single-pass fiber gas Raman laser (FGRL) based on hydrogen-filled hollow-core photonic crystal fibers pumped by a 1.5 µm nanosecond-pulsed fiber amplifier was constructed. Experimental results were congruent with simulation results. As the output powers and pulse shapes can be well calculated, the model can offer guidance for FGRL investigation, particularly for achieving high-efficiency and high-power FGRLs.

3 W tunable 1.65 µm fiber gas Raman laser in D2-filled hollow-core photonic crystal fibers

We demonstrate here a multi-watt tunable 1.65 µm fiber laser based on deuterium-filled hollow-core photonic crystal fibers (HC-PCFs) by stimulated Raman scattering (SRS). Owing to the fiber transmission properties of the HC-PCFs, pure rotational Stokes wave at 1.65 µm can be generated when pumped with a tunable 1.5 µm erbium-doped fiber amplifier (EDFA). When pumped at 1540 nm, with a 20-meter-long HC-PCF filled with 16 bar deuterium, a record average Stokes power of ∼2.91 W at 1645 nm is achieved, which is about 3.64 times higher than the similar experiment previously reported, and the corresponding power conversion efficiency is about 58%. Over the whole Stokes tunable range of 1645 to 1656 nm, the average output power is >2.5 W and the conversion efficiencies are >54%. A numerical model is also established to analyze the rotational SRS process in deuterium-filled HC-PCFs, the simulation results are in good agreement with experiments. This work opens a new opportunity for high-power tunable fiber lasers operating in the 1.7 μm band.

High-Efficiency, High Peak-Power, Narrow Linewidth 1.9 μm Fiber Gas Raman Amplifier

We reported here a high-efficiency, high peak-power, narrow linewidth 1.9 μ m fiber gas Raman amplifier based on stimulated Raman scattering (SRS) in an H2-filled antiresonance hollow-core fiber. A CW seed fiber laser near 1908 nm is amplified by the first vibrational Stokes wave of hydrogen molecules while pumping with a pulsed 1064 nm microchip laser. The seed laser significantly enhances the vibrational SRS and suppresses the rotational SRS, resulting in a record optical-to-optical conversion efficiency of ∼41% (quantum efficiency ∼73.5%) for such H2 vibrational Raman experiments in hollow-core fibers, with 1.4 m fiber and only 3 bar H2, which is more preferable in applications. The maximum average Stokes power of 74.2 mW and the peak power of ∼150 kW with the linewidth about 1 GHz (∼12 pm) are obtained. The linewidth characteristics of such fiber gas Raman laser sources are systematically studied for the first time by experimental methods. This paper provides an effective method to obtain efficient 1.9 μ m fiber sources of both high peak power and narrow linewidth.

Spectral and far-field broadening due to stimulated rotational Raman scattering driven by the Nike krypton fluoride laser.

Stimulated rotational Raman scattering (SRRS) in the ultraviolet region (λ=248 nm) has been observed at the Nike laser over extended propagation paths in air during high power operation. Although this phenomenon is not significant for standard operating configurations at Nike, broadening of the laser spectrum and far-field focal profiles has been observed once the intensity-path length product exceeds a threshold of approximately 1 TW/cm. This paper presents experimental results and a new theoretical evaluation of these effects. The observations suggest that significantly broader spectra can be achieved with modest degradation of the final focal distribution. These results point to a possible path for enhanced laser-target coupling with the reduction of laser-plasma instabilities due to broad laser bandwidth produced by the SRRS.

Stimulated rotational Raman scattering suppression by pulse stacking

Suppression of stimulated rotational Raman scattering (SRRS) is essential for the effective long-distance transmission of high-intensity laser pulses in air. In this work, a method for SRRS suppression is proposed based on the reconstruction of the initial laser pulse via subpulse stacking. The proposed method can suppress SRRS significantly and doubles the threshold distance when the initial laser pulse is stacked using 30 subpulses; this capability is attributed to the mutually perpendicular polarization directions of the adjacent subpulses. Additionally, the SRRS gain decreases even further if different phases are added to adjacent subpulses. This pulse stacking scheme also has potential for use in the suppression of other nonlinear effects, including transverse stimulated Raman scattering and transverse stimulated Brillouin scattering in large aperture optical components. There may be some difficulties in the implementation of such a scheme, and the energy losses that occur during the stacking process may even be greater than the losses due to SRRS, but we believe that the proposed method can serve as an inspiration for SRRS suppression scheme design.

Characteristics of 1.9 - μ m laser emission from hydrogen-filled hollow-core fiber by vibrational stimulated Raman scattering

We report here the characteristics of 1.9-μm laser emission from a gas-filled hollow-core fiber by stimulated Raman scattering (SRS). A 6.5-m hydrogen-filled ice-cream negative curvature hollow-core fiber is pumped with a high peak-power, narrow linewidth, linearly polarized subnanosecond pulsed 1064-nm microchip laser, generating a pulsed vibrational Stokes wave at 1908.5 nm. The maximum quantum efficiency of about 48% is obtained, which is mainly limited by the mode mismatch between the pump laser beam and the Stokes wave in the hollow-core fiber. The linewidths of the pump laser and the first-order vibrational Stokes wave are measured to be about 1 and 2 GHz, respectively, by a scanning Fabry–Perot interferometer. The pressure selection phenomenon of the vibrational anti-Stokes waves is also investigated. The pulse duration of the vibrational Stokes wave is recorded to be narrower than that of the pump laser. The polarization properties of the hollow-core fiber and the polarization dependence of the vibrational and the rotational SRS are also studied. The beam profile of the vibrational Stokes wave shows good quality.

The modified SRRS threshold criteria for high peak power laser pulses in long air-path transmission considering the near-field beam quality

This paper demonstrates that the stimulated rotational Raman scattering (SRRS) threshold for high peak power laser pulses propagating through a long air path can be influenced strongly by the near-field quality of the laser beams, and the relationship between the SRRS threshold and the near-field beam quality (i.e., spatial intensity modulation index and contrast ratio) can be evaluated quantitatively. By using our three-dimensional numerical model, which can describe the spatial–temporal evolution behaviors of SRRS and is verified by previously published SRRS experimental data, the criteria of the safe transmission distance for high peak power nanosecond laser pulses are obtained, and the modified SRRS threshold criterion formulas considering the near-field beam conditions are presented.

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.

Numerical investigation of the effects of smoothing by spectral dispersion on stimulated rotational Raman scattering

AbstractThe effect of smoothing by spectral dispersion (SSD) on stimulated rotational Raman scattering (SRRS) in air has been investigated both numerically and theoretically. The suppression effect of SSD on SRRS process is verified and it is demonstrated and proposed that the suppression effect is attributed to two aspects: the decreasing of the laser fluence modulation degree and the reducing of Stokes gain coefficient caused by the temporal and spatial variation of the phase of the incident laser pulse. The simulation results show that the SRRS threshold distance can be lengthened by choosing appropriate SSD parameters.

Numerical simulation of different aperture ICF high power ultraviolet laser propagating through a long air path

It is necessary for high power ultraviolet lasers to propagate through a long air path during experiments of inertial confinement fusion (ICF). When the product of gain efficiency and propagation distance is beyond the stimulated rotational Raman scattering (SRRS) threshold, the high power ultraviolet laser beam is subject to loss of energy and a decrease in beam quality, further resulting in the reduction of conversion efficiency of triple-harmonic, and even causing the optical components to be destroyed. This paper studies the four-dimension numerical modeling of SRRS effects, with emphasis on the variation of beam aperture and corresponding beam distributions in near field. The research showed that the variation of the pump beam aperture made little effect on SRRS transform efficiency ratio with the same incident energy and the same propagation distance, the transverse intensity distribution of beams became more even with an increase in the beam aperture. We have made the deduction that high power laser's SRRS effects through a long air path could be depressed by increasing the beam aperture on the condition of equal incident energy and propagation distance.

Stimulated rotational Raman scattering at multiwavelength under tea CO2 laser pumping with a multiple-pass cell

Stimulated rotational Raman scattering (SRRS) at multiwavelength pumped by TEA CO2 laser was demonstrated in this paper. Raman mediums were cooled by liquid-N2 and a multiple-pass cell (MPC) with 25 passes was designed and used. When the para-H2 was pumped by single-longitudinal-mode (SLM) circular polarized TEA CO2 laser on 10P(20), 9P(20), and 10R(20), 50 mJ 16.95 μm, 350 mJ 14.44 μm, and 536 mJ 16.9 μm radiations were obtained, corresponding to energy conversion efficiency of 1.2, 11.7, and 13.4%, respectively. When the ortho-D2 was pumped by CO2 laser on 10R(18), 108 mJ 12.57 μm Raman laser was obtained with energy conversion efficiency of 2.9%.

Experimental Observation of Near-Field Deterioration Induced by Stimulated Rotational Raman Scattering in Long Air Paths

We report the experimental investigation of a stimulated rotational Raman scattering effect in long air paths on SG-III TIL, with a 1053 nm, 20-cm-diameter, linearly polarized, 3 ns flat-topped laser pulse. An intense speckle pattern of near field with thickly dotted hot spots is observed at the end of propagation with an intensity-length product above 17TW/cm. The Stokes developing from the scattering of the laser beam by quantum fluctuations is characterized by a combination of high spatial frequency components. The observed speckle pattern with small-diameter hot spots results from the combination of the nonlinear Raman amplification and the linear diffraction propagation effect of the Stokes with a noise pattern arising from the spontaneous Raman scattering. A new promising suppression concept based on the special characteristic of the Stokes, called active and selective filtering of Stokes, is proposed.

强激光长程空气传输受激转动拉曼散射效应实验研究

强激光长程空气传输受激转动拉曼散射效应实验研究

转动能级拉曼抽运技术在NO2激光探测雷达中的应用

转动能级拉曼抽运技术在NO2激光探测雷达中的应用

Ultrahigh efficiency laser wavelength conversion in a gas-filled hollow core photonic crystal fiber by pure stimulated rotational Raman scattering in molecular hydrogen.

We report on the generation of pure rotational stimulated Raman scattering in a hydrogen gas hollow-core photonic crystal fiber. Using the special properties of this low-loss fiber, the normally dominant vibrational stimulated Raman scattering is suppressed, permitting pure conversion to the rotational Stokes frequency in a single-pass configuration pumped by a microchip laser. We report 92% quantum conversion efficiency (40 nJ pulses in 2.9 m fiber) and threshold energies (3 nJ in 35 m) more than 1 x 10(6) times lower than previously reported. The control of the output spectral components by varying only the pump polarization is also shown. The results point to a new generation of highly engineerable and compact laser sources.

Stimulated Raman scattering of intense laser pulses in air.

Stimulated rotational Raman scattering (SRRS) is known to be one of the processes limiting the propagation of high-power laser beams in the atmosphere. In this paper, SRRS, Kerr nonlinearity effects, and group velocity dispersion of short laser pulses and pulse trains are analyzed and simulated. Fully time-dependent, three-dimensional, nonlinear propagation equations describing the Raman interaction, optical Kerr nonlinearity due to bound electrons, and group velocity dispersion are presented and discussed. The effective time-dependent nonlinear refractive index containing both Kerr and Raman processes is derived. Linear stability analysis is used to obtain growth rates and phase matching conditions for the SRRS, modulational, and filamentation instabilities. Numerical solutions of the propagation equations in three dimensions show the detailed evolution of the Raman scattering instability for various pulse formats. The dependence of the growth rate of SRRS on pulse duration is examined and under certain conditions it is shown that short (approximately psec) laser pulses are stable to the SRRS instability. The interaction of pulses in a train through the Raman polarization field is also illustrated.

Rotational Raman scattering dependence on pump polarization

The influence of the ellipticity degree of the pump radiation polarization on threshold energies and energy conversion efficiencies of stimulated rotational Raman scattering (SRRS) are investigated in H2 with a XeCl excimer laser, by using different focusing geometries. For the first time, it is shown that the dependence of SRRS on pump radiation polarization is also affected by the focusing geometry, i.e. whether low- or high-angle focusing is applied. The experimentally determined threshold energies as a function of the pump radiation polarization are in accordance with theory mainly at high-angle focusing geometries. It is also shown for the first time that gain suppression effects on vibrational Raman scattering have to be considered in understanding the experimental results.