Stimulated Raman Scattering Threshold Research Articles

Super broadband light source: a promising master oscillator source for high brightness and high-power fiber amplifiers

Stimulated Raman scattering (SRS) and transverse mode instability (TMI) effects are crucial limitation factors for further power scaling of high-power fiber amplifiers with near-diffraction-limited beam quality. It is an important research direction to carry out laser system optimization from the perspective of seed construction. In this work, we experimentally investigated the impact of utilizing different seed lasers on the SRS and TMI characteristics in high power ytterbium-doped fiber amplifiers. Both the phase modulated single frequency fiber laser (PMSFL), fiber oscillator laser (FOL), and superfluorescent fiber source (SFS) as a typical type of temporally stable broadband light source (SBLS) are employed as seed lasers. The experimental results indicated that the SRS intensity in the fiber amplifier is influenced by the properties of the seed laser source. Notably, the SRS threshold of the SBLS could be comparable to that of a low-noise PMSFL source. There is a significant increase in TMI threshold when employing SBLS as a seed laser in high-power fiber amplifiers, which is 3.86 times that of the injected seed laser with a 3 dB narrow linewidth of 0.74 nm. Considering the combined threshold characteristics of SRS and TMI, it can be found that SBLS is a promising master oscillator source for high brightness and high-power fiber amplifiers. Our work could provide a good reference for the selection of a master oscillator source, and we believe that it is an important direction to go beyond the power limitation of high-power fiber lasers by constructing a light source with broader spectral linewidth and temporal stability.

Raman suppression in high-power fiber oscillators by femtosecond-written chirped and tilted fiber Bragg gratings

Realizing stimulated Raman scattering (SRS) suppression is a key topic for high-power fiber lasers. Here, we report an effective and simple strategy for SRS suppression using chirped and tilted fiber Bragg gratings (CTFBGs) in high-power fiber oscillators while maintaining the compactness and stability of the system. The CTFBG is inserted on the side of a cavity mirror FBG without cutting the gain fiber. To improve power handling capability, the CTFBG and cavity mirror FBGs are inscribed by femtosecond (fs) lasers. The optimal SRS suppression effect can be realized when the CTFBG is inserted into the resonant cavity and on the side of the output coupler FBG. The SRS threshold is increased by approximately 11% with an SRS suppression ratio of nearly 14 dB. Moreover, the output power of the fiber oscillator is improved to 3.5 kW, which is the maximum power achieved in fiber oscillators with SRS suppression using CTFBGs, to the best of our knowledge. The temperature of the air-cooled CTFBG is 50.2 °C, which has the potential to handle higher power. This work provides new insights for suppressing SRS in fiber oscillators, promoting the application of CTFBGs in high-power lasers.

Suppressing parametric instabilities in direct-drive inertial-confinement-fusion plasmas using broadband laser light

It has long been recognized that broadband laser light has the potential to control parametric instabilities in inertial-confinement-fusion (ICF) plasmas. Here, we use results from laser-plasma-interaction simulations to estimate the bandwidth requirements for mitigating the three predominant classes of instabilities in direct-drive ICF implosions: cross-beam energy transfer (CBET), two-plasmon decay (TPD), and stimulated Raman scattering (SRS). We find that for frequency-tripled, Nd:glass laser light, a bandwidth of 8.5 THz can significantly increase laser absorption by suppressing CBET, while ∼13 THz is needed to mitigate absolute TPD and SRS on an ignition-scale platform. None of the glass lasers used in contemporary ICF experiments, however, possess a bandwidth greater than 1 THz and reaching larger values requires the use of an auxiliary broadening technique such as optical parametric amplification or stimulated-rotational-Raman scattering. An arguably superior approach is the adoption of an argon-fluoride (ArF) laser as an ICF driver. Besides having a broad bandwidth of ∼10 THz, the ArF laser also possesses the shortest wavelength (193 nm) that can scale to the high energy/power required for ICF—a feature that helps to mitigate parametric instabilities even further. We show that these native properties of ArF laser light are sufficient to eliminate nearly all CBET scattering in a direct-drive target and also raise absolute TPD and SRS thresholds well above those for broadband glass lasers. The effective control of parametric instabilities with broad bandwidth is potentially a “game changer” in ICF because it would enable higher laser intensities and ablation pressures in future target designs.

Efficient frequency conversion and the crossing-pump effect of stimulated Raman scattering in an aqueous sodium sulfate solution.

The cascaded stimulated Raman scattering (SRS) of an aqueous sodium sulfate solution was investigated as well as the generation of the crossing-pump effect. With the introduction of dual sample cells, the first-order Stokes of the O-H stretching vibrational mode was able to act as the pump light to excite the Stokes of the S-O stretching vibrational mode, and a new Raman peak was obtained at 4423 cm-1. The dual sample cell device not only lowered the SRS threshold, but also enhanced the four-wave mixing (FWM) process. Compared to the input laser of 7 ns/pulse, the first-order Stokes of O-H was compressed to a pulse width of 413 ps after passing through the dual sample cells. The SRS of aqueous sodium sulfate solution covered an ultrabroad wavelength ranging from 441 nm to 720 nm (a Raman shift ranging from -3859 cm-1 to 4923 cm-1). The cone-shaped launch ring of the FWM process was also recorded. This work provides a reference for the establishment of laser frequency conversion devices using an aqueous sodium sulfate solution as the Raman medium.

Cascaded amplification via three-beam double stimulated Raman scattering in benzene

As stimulated Raman scattering (SRS) microscopy has emerged as a powerful tool for label-free imaging of biomolecules in biological systems, SRS has also received much attention in the field of nonlinear optics. Here, we enhanced the cascade SRS of benzene through an energy competition mechanism in the three-beam double SRS processes. A three-beam (pump, Stokes, and anti-Stokes beam) SRS scheme was formulated and demonstrated to simultaneously induce the parallel and cascaded SRS processes, which were related to the Raman active modes in the same molecule. Both two SRS processes generated high-order Stokes SRS through resonance enhancement of the double sample cell configuration, which effectively suppressed the CARS (Coherent anti-Stokes Raman Scattering) signal. It was shown in the experimental results that the cascaded SRS up to six orders from 562 to 780 nm were generated, and the SRS threshold of the multi-order Stokes components was lowered. The present work can potentially contribute to the development of super-resolution SRS microscopy.

Impact of amplified spontaneous emission noise on the SRS threshold of high-power fiber amplifiers

Impact of amplified spontaneous emission (ASE) noise on the stimulated Raman scattering (SRS) threshold of high-power fiber amplifiers is demonstrated numerically through a spectral evolution approach. The simulation results confirm that ASE noise in the Raman wavelength band could reduce the SRS threshold of high-power fiber amplifiers significantly. As for ASE noise originated the main amplifier, it becomes stronger and reduces the SRS threshold at shorter operation wavelength below 1052 nm. As for ASE noise originated from the seed laser, it reduces the SRS threshold at different operation wavelength under the condition that the Raman ratio is over –90 dB in the seed laser. The theoretical method and results in this work could provide a well reference to extend the operation wavelength of high-power fiber lasers.

Resonance enhanced cascaded stimulated Raman and Coherent Anti-Stokes Raman scattering of symmetric and antisymmetric O-D vibrations

Stimulated Raman scattering (SRS) of deuteroxide was investigated using a pulsed Nd:YAG laser with a wavelength of 532 nm. After the pump laser passed through the first sample cell (C1), the output light included the transmitted pump beam and the Stokes SRS lights, and the frequency difference between the two self-matched the O-D symmetric and antisymmetric stretching vibration modes. The 1st and the 2nd-order cascaded Stokes and the anti-Stokes SRS (Coherent Anti-Stokes Raman Spectroscopy-CARS) of both two modes were observed. The 2nd-order Stokes SRS and its CARS were firstly observed at a pump energy of 14.79 mJ. The pure Stokes SRS were always collinear with the pump beam along the axial direction. Some ring-like Stokes SRS and CARS, which originated from the four-wave mixing (FWM) processes, were also observed only in the forward direction along with different angles apart from the pump beam direction, respectively. This method not only reduced the SRS threshold but also increased the SRS intensity of the weak vibration mode, resulting in an energy conversion efficiency reaching up to 54%. The technique can be used in a tunable Raman laser.

High-Conversion Efficiency Pulse Compression by Stimulated Raman Scattering and Laser Induced Breakdown in Water

A novel temporal pulse compression technique, combining stimulated Raman scattering (SRS) and laser induced breakdown (LIB) principles was presented in this work, which employing an energy controllable dual-cell setup. As a novel type of plasma shutter, the 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> cell was used to transmit the high-power peak at the leading pulse edge and successfully block its low-power tail. The effect resulted in a 7 ns pulse duration from a Nd: YAG laser compressed into 330 ps, the pulse compression ratio was about 21×. The magnified optical power of the leading edge diminished the threshold of SRS and improved the beam quality of the output pulse. In addition, the output energy of Stokes was approximately 2 times than that of the 1 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">st</sup> cell process, and the 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> cell process increased the energy conversion efficiency from 4.2% to 15.8%. Thus, the 1 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">st</sup> -order Stokes SRS was greatly improved, and the 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> -order Stokes SRS was more easily generated. The method provided a simple but convenient way to generate a short pulse with a high energy conversion efficiency Raman laser.

Suppressing the enhancement of stimulated Raman scattering in inhomogeneous plasmas by tuning the modulation frequency of a broadband laser

The stimulated Raman scattering (SRS) instability can inhibit the performance of laser-driven inertial confinement fusion (ICF) implosions by scattering light into unwanted directions or by generating hot electrons that preheat the target fuel. In principle, ICF target designs can avoid parameter regimes conducive to large, linear SRS gains. In practice, kinetic inflation—the nonlinear enhancement of SRS due to electron trapping in the excited plasma wave—makes this difficult. Here, we show that laser bandwidth in the form of frequency modulation can either decrease or increase the inflationary SRS (iSRS) threshold in inhomogeneous plasmas depending on the maximum chirp of the laser pulse. The threshold, mapped out by a series of particle-in-cell simulations, exhibits a minimum when the frequency change within the pulse cancels the spatial detuning due to density inhomogeneities along the trajectory of the scattered light. By tuning the pump laser parameters away from this minimum, the iSRS threshold can be larger than at zero bandwidth, providing a path to mitigating kinetic inflation in ignition relevant plasmas.

Resonance enhancement stimulated Raman scattering of O–H stretching vibration in water molecule

Stimulated Raman scattering (SRS) of water molecules excited by 532 nm laser is greatly enhanced by the application of CW (continuous-wave) seeding laser with 650 nm. When the CW laser power is 1 mW, the normalized SRS intensity of O–H stretching vibration at 3405 cm−1 is increased by ten orders of magnitude. Two new lower frequency shoulder peaks are also observed after adding CW seeding laser, which shift to the low-wavenumber due to the pulse heating of water molecules in the laser focused volume. The CW seeding laser not only lowers the threshold of SRS, but also increases the intensity of SRS. The mechanism of enhancement is attributed to the frequency difference between pump laser (532 nm) and CW seeding laser (650 nm) matching the O–H stretching vibrational frequency of water molecule, which provides the possibility of using resonance methods to enhance weak Raman vibration modes.

Stimulated Raman Scattering in Ge Nanowires.

Investigating group-IV-based photonic components is a very active area of research with extensive interest in developing complementary metal-oxide-semiconductor (CMOS) compatible light sources. However, due to the indirect band gap of these materials, effective light-emitting diodes and lasers based on pure Ge or Si cannot be realized. In this context, there is considerable interest in developing group-IV based Raman lasers. Nevertheless, the low quantum yield of stimulated Raman scattering in Si and Ge requires large device footprints and high lasing thresholds. Consequently, the fabrication of integrated, energy-efficient Raman lasers is challenging. Here, we report the systematic investigation of stimulated Raman scattering (SRS) in Ge nanowires (NWs) and axial Al-Ge-Al NW heterostructures with Ge segments that come into contact with self-aligned Al leads with abrupt metal–semiconductor interfaces. Depending on their geometry, these quasi-one-dimensional (1D) heterostructures can reassemble into Ge nanowires, Ge nanodots, or Ge nanodiscs, which are monolithically integrated within monocrystalline Al (c-Al) mirrors that promote both optical confinement and effective heat dissipation. Optical mode resonances in these nanocavities support in SRS thresholds as low as 60 kW/cm2. Most notably, our findings provide a platform for elucidating the high potential of future monolithically integrated, nanoscale low-power group-IV-based Raman lasers.

Multibeam absolute stimulated Raman scattering and two-plasmon decay.

Multibeam absolute instability thresholds for stimulated Raman scattering (SRS) and two-plasmon decay (TPD) are calculated in three dimensions for conditions relevant to direct-drive inertial confinement fusion experiments on the OMEGA laser and at the National Ignition Facility (NIF). Although multibeam effects are found to be significant for both instabilities, SRS is found to have less efficient multibeam coupling than TPD. The results are consistent with the observation of a TPD-dominated regime on the OMEGA laser and a SRS-dominated regime on the NIF despite the single-beam SRS threshold being lower than the single-beam TPD threshold on both facilities. The minimum instability threshold for NIF plasma parameters occurs for SRS near quarter-critical densities with a shared electromagnetic wave propagating along the beam axis.

Stimulated Raman scattering mechanisms and scaling behavior in planar direct-drive experiments at the National Ignition Facility

Stimulated Raman scattering (SRS) has been explored comprehensively in planar-geometry experiments at the National Ignition Facility in conditions relevant to the corona of inertial confinement fusion ignition-scale direct-drive targets. These experiments at measured electron temperatures of 4 to 5 keV simulated density scale lengths Ln of 400 to 700 μm, and laser intensities at the quarter-critical density of up to 1.5 × 1015 W/cm2 have determined SRS thresholds and the scaling behavior of SRS for various beam geometries. Several SRS mechanisms, including saturated absolute SRS near the quarter-critical density and additional SRS, including near-backscatter or sidescatter at lower densities, have been identified. Correlation of time-dependent SRS at densities ∼0.15 to 0.21 of the critical density with hot-electron signatures as well as the magnitudes of these signatures across different experiments, is observed. Further modeling work is needed to definitively identify the density region in which hot electrons are generated and will guide SRS and hot-electron preheat mitigation strategies for direct-drive-ignition designs.

Influence of seed instability on the stimulated Raman scattering of high power narrow linewidth fiber amplifier

In this work, the effect of the temporal instability on stimulated Raman scattering (SRS) characteristics in a high power Yb-doped fiber amplifier is studied experimentally and theoretically. It is found that the SRS intensity is strongly influenced by the type of the seed source. When the power amplifier is seeded by a normal fiber oscillator laser, the SRS threshold is much lower than the value estimated by Smith threshold formula. The maximum output power in the experiment is 1910 W, which is limited by SRS effect. While the power amplifier is seeded by a phase modulated single frequency laser, the maximum output power is 2400 W without observation of any SRS. Such phenomenon can be explained by the different time-domain characteristics of the two different types of seed sources. On the one hand, the detailed measurements are performed experimentally. On the other hand, a numerical model is proposed. Both the experiment and the model reveal that strong intensity fluctuations exist in fiber amplifier seeded by a fiber oscillator laser. These intensity fluctuations lead to a much higher SRS output. With these considerations in mind, the model predictions are in good qualitative agreement with the experimental data.

Investigation of multispectral SF6 stimulated Raman scattering laser

In this work, SF6 as a Raman-active medium is investigated to generate a multispectral Raman laser by the combination of cascade stimulated Raman scattering (SRS) and four wave mixing. The Raman frequency comb from the 10th-order anti-Stokes to the 9th-order Stokes was generated, and its spectral range covered 377–846 nm. The photon conversion efficiency of 16.4% for the first Stokes was achieved, and the Raman gain coefficient at 1.5 MPa of SF6 under the 532 nm pump laser was calculated to be 0.83 cm/GW by the SRS threshold comparison with H2. Using helium as the carrier gas, the thermal effect of the SF6 Raman laser was improved dramatically under a repetition rate of 10 Hz.

Asymmetrical-cavity picosecond Raman laser at the water-air interface.

For the first time, to the best of our knowledge, we demonstrated a new type of Raman laser with asymmetrical cavity at the liquid-air interface. We observed an intriguing stimulated Raman scattering (SRS) threshold dependence when the pumping laser beam waist was transferred through the liquid-air interface, and we demonstrated a paradoxical 30-fold SRS threshold reduction in the vicinity of the water-air surface. The minimum SRS threshold was achieved when the pumping laser beam waist was located at the liquid surface. To explain the abnormal SRS threshold dependence, we suggested a new lasing mechanism. Finally, we demonstrated that threshold measurements at the liquid-air interface are a reliable method to quantitatively measure the interaction length in SRS experiments with a focused beam.

Anomalous Decrease in the Threshold of Stimulated Raman Scattering near the Surface of Liquid Nitrogen

An anomalously strong (up to 13 times) reduction of the threshold of the stimulated Raman scattering of 15-ps laser pulses in liquid nitrogen has been found when moving the caustic of a pump beam from the bulk through the free surface. A nonmonotonic N-shaped dependence of the stimulated Raman scattering threshold on the distance of the beam waist to the surface has been recorded for the first time with a minimum reached when the plane front of the beam caustic coincides with the surface. The physical mechanism of the detected phenomenon has been discussed.

Analytical study on the stimulated Raman scattering threshold in distributed-pumped fiber amplifiers.

In this paper, the stimulated Raman scattering (SRS) threshold of the distributed-pumped fiber amplifier consisting of multiple sub-amplifiers is analytically studied based on the rate-equation model. The SRS threshold formula is presented where the predictions agree well with the numerical results. With the help of the formula, the effect of the pumping scheme on the SRS suppression of the distributed-pumped fiber amplifier is systematically investigated. It is found that with a given number of pump injection ports, the SRS suppression can be optimized by making only some of sub-amplifiers counter-pumped with the others bi-directional-pumped. It is also suggested that the first sub-amplifier should be adopted to be counter-pumped for suppressing SRS if only one sub-amplifier can be single-directional-pumped. The variation of SRS threshold with the number of sub-amplifiers is also discussed in detail. We believe that these results can provide significant guidance on understanding and designing distributed-pumped fiber lasers and amplifiers.

Nanocavity-Enhanced Giant Stimulated Raman Scattering in Si Nanowires in the Visible Light Region.

Silicon photonics has been a very active area of research especially in the past two decades in order to meet the ever-increasing demand for more computational power and faster device speeds and their natural compatibility with complementary metal-oxide semiconductor. In order to develop Si as a useful photonics material, essential photonic components such as light sources, waveguides, wavelength convertors, modulators, and detectors need to be developed and integrated. However, due to the indirect electronic bandgap of Si, conventional light emission devices such as light-emitting diodes and lasers cannot be built. Therefore, there has been considerable interest in developing Si-based Raman lasers, which are nonlinear devices and require large stimulated Raman scattering (SRS) in an optical cavity. However, due to the low quantum yield of SRS in Si, Raman lasers have very large device footprints and high lasing threshold, making them unsuitable for faster, smaller, and energy-efficient devices. Here, we report strong SRS and extremely high Raman gain in Si nanowire optical cavities in the visible region with measured SRS threshold as low as 30 kW/cm2. At cavity mode resonance, light is confined into a low mode volume and high intensity electromagnetic mode inside the Si nanowire due to its high refractive index, which leads to strong SRS at low pump intensities. Electromagnetic calculations reveal greater than 6 orders of magnitude increase in Raman gain coefficient at 532 nm pump wavelength, compared to the gain value at 1.55 μm wavelength reported in literature, despite the 108 higher losses at 532 nm. Because of the high gain in such small structures, we believe that this is a significant first step in realizing a monolithically integrable nanoscale low-powered Si Raman laser.

Experimental study of SRS threshold dependence on the bandwidths of fiber Bragg gratings in co-pumped and counter-pumped fiber laser oscillator

With increasing output power, stimulated Raman scattering (SRS) becomes one of the main limitations for high power fiber laser oscillators. In this work, the SRS threshold dependence on the bandwidths of fiber Bragg gratings (FBGs) has been experimentally investigated in co-pumped and counter-pumped ytterbium-doped all-fiber laser oscillators, respectively. In both co-pumped and counter-pumped laser oscillators, FBGs with respective bandwidths of 0.24 nm, 0.5 nm, 1 nm and 2 nm are used as the output coupler. It is found that with the increasing bandwidth of the output-coupler FBG (OC-FBG), the SRS threshold of the laser oscillator also increases and when the bandwidth of the OC-FBG reaches over ∼1 nm, the SRS threshold increases slowly. Compared with co-pumping configuration, a counter-pumped laser oscillator holds a higher SRS threshold when the same OC-FBG is applied. By employing a counter-pumping configuration and increasing the bandwidth of the OC-FBG to 2 nm, the output laser power is further scaled to 3 kW-level with a slope efficiency of 70%. The M2 factor is measured to be ∼1.3 at the maximum output power.