Backward Stimulated Brillouin Scattering Research Articles

Anti-resonant acoustic waveguides enabled tailorable Brillouin scattering on chip

Empowering independent control of optical and acoustic modes and enhancing the photon-phonon interaction, integrated photonics boosts the advancements of on-chip stimulated Brillouin scattering (SBS). However, achieving acoustic waveguides with low loss, tailorability, and easy fabrication remains a challenge. Here, inspired by the optical anti-resonance in hollow-core fibers and acoustic anti-resonance in cylindrical waveguides, we propose suspended anti-resonant acoustic waveguides (SARAWs) with superior confinement and high selectivity of acoustic modes, supporting both forward and backward SBS on chip. Furthermore, this structure streamlines the design and fabrication processes. Leveraging the advantages of SARAWs, we showcase a series of breakthroughs for SBS within a compact footprint on the silicon-on-insulator platform. For forward SBS, a centimeter-scale SARAW supports a large net gain exceeding 6.4 dB. For backward SBS, we observe an unprecedented Brillouin frequency shift of 27.6 GHz and a mechanical quality factor of up to 1960 in silicon waveguides. This paradigm of acoustic waveguide propels SBS into a new era, unlocking new opportunities in the fields of optomechanics, phononic circuits, and hybrid quantum systems.

Integrated photonic waveguides for on-chip SBS with OAM modes

Stimulated Brillouin Scattering (SBS) is a nonlinear optical process that can provide significant on-chip gain in various materials and platforms. The utilization of chalcogenide and silicon (Si) waveguides has facilitated the advancement of Brillouin lasers and diverse microwave photonic functionalities. The predominant approach has been centered on interactions between conventional guided modes, specifically TE/TM. This study introduces a novel approach that explores diverse guided-wave configurations capable of sustaining SBS with orbital angular momentum modes (OAM). These configurations include a cross-Si waveguide and a silica/chalcogenide waveguide. The simulations examine the forward and backward SBS utilizing OAM modes up to the 4th order in planar on-chip waveguides. A maximum on-chip SBS gain of up to ∼540W−1m−1 has been observed for pump and Stokes OAM mode numbers satisfying the topological charge conservation law. This OAM multiplexing approach can open new avenues for on-chip integrated nonlinear photonics for various applications, especially where pump and signal beam filtering or separation are critical.

Study on backward stimulated Brillouin scattering of chalcogenide inverted-ridge optical waveguide with air slot

The stimulated Brillouin scattering (SBS) effect has the advantages of narrow spectral line width, frequency stability, and sensitivity to gain direction, which is commonly used in the field of integrated photonic devices, such as lasers, slow light generation and microwave photonic filters. In practical applications, due to the low gain coefficient of SBS in traditional chalcogenide waveguides, there are high threshold of pumping power and long waveguide length. In this work, an inverted-ridge waveguide structure with air slot is designed by adopting As<sub>2</sub>S<sub>3</sub> and SiO<sub>2</sub>, which presents high backward stimulated Brillouin scattering (BSBS) gain coefficient. This chalcogenide inverted-ridge optical waveguide with air slot can better confine the optical field and acoustic field within the ridge region for improving the coupling efficiency between optical field and acoustic field. More significantly, adding an air slot into the ridge region of this chalcogenide waveguide will produce powerful radiation pressure at the boundary between the air slot and As<sub>2</sub>S<sub>3</sub>. Owing to the fact that the acoustic field is mainly distributed near the air slot in the ridge region, the coupling effect of the radiation pressure and acoustic field is significantly enhanced, leading to a significant increase in BSBS gain coefficient. In this work, the optical fundamental mode as optical mode due to the chalcogenide waveguide with submicron size structure and the six lowest order acoustic modes that meet the matching vector conditions as acoustic mode are calculated, and it is found that the fifth order acoustic mode achieves a maximum BSBS gain coefficient in the six acoustic modes. On this basis, by scanning the waveguide structural parameters of the air slot width, waveguide ridge width and height, and waveguide thickness, the BSBS gain coefficient is as high as 8.22×10<sup>4</sup> W<sup>–1</sup>·m<sup>–1</sup>, which is more than three times the currently reported chalcogenide waveguide with non-suspended structure. Additionally, the calculation results also indicate that this chalcogenide waveguide with a smaller effective mode field area has a higher BSBS gain coefficient in the same optical mode and acoustic mode, providing a new idea for further improving the BSBS gain coefficient in the design of waveguide structure. At the same time, the influence of optical loss on BSBS gain is also analyzed, and it is found that when the waveguide length exceeds the optimal value, the lost energy caused by the optical loss will be beyond the input energy of the pump optical wave, causing the power of the stokes optical wave to begin to decrease. However, the improvement of the power of pump optical wave not only increases the maximum power of the Stokes optical wave, but also raises the optimal value of the waveguide length. The results of simulation calculation show that when the input power of pump optical wave is about 20 mW, this chalcogenide waveguide with only 2 cm waveguide length has a BSBS gain of 100 dB, which has the advantages of low pumping power and short waveguide length in the currently reported on-chip integration of chalcogenide waveguides.

Experimental studies on the core-structure dependence of backward Brillouin gain in solid-core photonic crystal fibers.

Stimulated Brillouin scattering (SBS) in solid-core photonic crystal fibers (PCFs) differs significantly from that in standard optical fibers due to the tight confinement of both optical and acoustic fields in their µm-sized fiber cores, as resultantly evident in their Brillouin gain spectra. Despite many theoretical studies based on either simplified models or numerical simulations, the structural dependency of Brillouin gain spectra in small-core PCFs has not been characterized comprehensively using PCFs with elaborated parameter controls. In this work we report a comprehensive characterization on the core-structure dependences of backward SBS effects in solid-core PCFs that are drawn with systematically varied core-diameter, revealing several key trends of the fiber Brillouin spectrum in terms of its gain magnitude, Brillouin shift and multi-peak structure, which have not been reported in detail previously. Our work provides some practical guidance on PCF design for potential applications like Brillouin fiber lasers and Brillouin fiber sensing.

Time-dependent saturation and physics-based nonlinear model of cross-beam energy transfer

The nonlinear physics of cross-beam energy transfer (CBET) for multi-speckled laser beams is examined using large-scale particle-in-cell simulations for a range of laser and plasma conditions relevant to indirect-drive inertial confinement fusion (ICF) experiments. The time-dependent growth and saturation of CBET involve complex, nonlinear ion and electron dynamics, including ion trapping-induced enhancement and detuning, ion acoustic wave (IAW) nonlinearity, oblique forward stimulated Raman scattering (FSRS), and backward stimulated Brillouin scattering (BSBS) in a CBET-amplified seed beam. Ion-trapping-induced detuning of CBET is captured in the kinetic linear response by a new δf-Gaussian-mixture algorithm, enabling an accurate characterization of trapping-induced non-Maxwellian distributions. Ion trapping induces nonlinear processes, such as changes to the IAW dispersion and nonlinearities (e.g., bowing and self-focusing), which, together with pump depletion, FSRS, and BSBS, determine the time-dependent nature and level of CBET gain as the system approaches a steady state. Using VPIC simulations at intensities at and above the onset threshold for ion trapping and the insight from the time-dependent saturation analyses, we construct a nonlinear CBET model from local laser and plasma conditions that predicts the CBET gain and the energy deposition into the plasma. This model is intended to provide a more accurate, physics-based description of CBET saturation over a wide range of conditions encountered in ICF hohlraums compared with linear CBET gain models with ad hoc saturation clamps often used in laser ray-based methods in multi-physics codes.

Influence of a random phase plate on the growth of the backward stimulated Brillouin scatter.

We derive the analytical dispersion relation of a high-energy laser beam's backward stimulated Brillouin scattering (BSBS) in a hot plasma, that accounts both for the random phase plate (RPP) induced spatial shaping and its associated phase randomness. Indeed, phase plates are mandatory in large laser facilities where a precise control of the focal spot size is required. While the focal spot size is well controlled, such techniques produce small scale intensity variations that can trigger laser-plasma instabilities such as BSBS. Quantifying the resulting instability variability is shown to be crucial for understanding accurately the backscattering temporal and spatial growth as well as the asymptotic reflectivity. Our model, validated by means of a large number of three-dimensional paraxial simulations and experimental data, offers three quantitative predictions. The first one addresses the temporal exponential growth of the reflectivity by deriving and solving the BSBS RPP dispersion relation. A large statistical variability of the temporal growth rate is shown to be directly related to the phase plate randomness. Then, we predict the portion of the beam's sectionthat is absolutely unstable, thus helping to precisely assess the validity of the vastly used convective analysis. Finally, a simple analytical correction to the plane wave spatial gain is extracted from our theory giving a practical and effective asymptotic reflectivity prediction that includes the impact of phase plates smoothing techniques. Hence, our study sheds light on the long-time studied BSBS, deleterious to many high-energy experimental studies related to the physics of inertial confinement fusion.

Study on backward stimulated Brillouin scattering of chalcogenide inverted-ridge optical waveguide with air slot

The stimulated Brillouin scattering (SBS) effect has the advantage of narrow spectral line width, frequency stability, and sensitivity to gain direction, which is commonly used in the field of integrated photonic devices, such as lasers, slow light generation and microwave photonic filters. In practical applications, due to the low gain coefficient of SBS in traditional chalcogenide waveguides, there are high threshold of pumping power and long waveguide length. In this paper, a inverted-ridge waveguide structure with air slot is designed by adopting As<sub>2</sub>S<sub>3</sub> and SiO<sub>2</sub> materials, which presents high backward stimulated Brillouin scattering (BSBS) gain coefficient. This chalcogenide inverted-ridge optical waveguide with air slot can better confine the optical and acoustic fields within the ridge region for improving the coupling efficiency between optical and acoustic fields. More significantly, adding an air slot in the ridge region of this chalcogenide waveguide will produce powerful radiation pressure at the boundary between the air slot and As<sub>2</sub>S<sub>3</sub>. Owing to the fact that the acoustic field is mainly distributed near the air slot in the ridge region, the coupling effect of the radiation pressure and acoustic field is significantly enhanced, leading to a significant increase in BSBS gain coefficient. In this paper, the optical fundamental mode as optical mode due to the chalcogenide waveguide with submicron size structure and the six lowest order acoustic modes that meet the matching vector conditions as acoustic mode are calculated, and it is found that the fifth order acoustic mode achieves the maximum BSBS gain coefficient among the six acoustic modes. On this basis, by scanning the waveguide structural parameters of the air slot width, waveguide ridge width & height, and waveguide thickness, the BSBS gain coefficient is as high as 8.22×10<sup>4</sup> W<sup>-1</sup>·m<sup>-1</sup>, which is more than three times the currently reported chalcogenide waveguide with non-suspended structure. Additionally, the calculation results also indicate that this chalcogenide waveguide with a smaller effective mode field area has a higher BSBS gain coefficient in the same optical and acoustic mode, providing a new idea for further improving the BSBS gain coefficient in the design of waveguide structures. At the same time, the impact of optical loss on BSBS gain is also analyzed, and it is found that when the waveguide length exceeds the optimal value, the lost energy caused by the optical loss will be beyond the input energy of the pump optical wave, causing the power of the stokes optical wave to begin to decrease; However, the improvement of the power of pump optical wave not only increases the maximum power of the stokes optical wave, but also rises the optimal value of the waveguide length; The results of simulation calculation have shown that when the input power of pump optical wave is about 20 mW, this chalcogenide waveguide with the only 2 cm waveguide length has the BSBS gain of 100 dB, which has advantage of low pumping power and short waveguide length in the current reported on-chip integration of chalcogenide waveguides.

Guided-acoustic stimulated Brillouin scattering in silicon nitride photonic circuits.

Coherent optomechanical interaction known as stimulated Brillouin scattering (SBS) can enable ultrahigh resolution signal processing and narrow-linewidth lasers. SBS has recently been studied extensively in integrated waveguides; however, many implementations rely on complicated fabrication schemes. The absence of SBS in standard and mature fabrication platforms prevents its large-scale circuit integration. Notably, SBS in the emerging silicon nitride (Si3N4) photonic integration platform is currently out of reach because of the lack of acoustic guidance. Here, we demonstrate advanced control of backward SBS in multilayer Si3N4 waveguides. By optimizing the separation between two Si3N4 layers, we unlock acoustic waveguiding in this platform, potentially leading up to 15× higher Brillouin gain coefficient than previously possible in Si3N4 waveguides. We use the enhanced SBS gain to demonstrate a high-rejection microwave photonic notch filter. This demonstration opens a path to achieving Brillouin-based photonic circuits in a standard, low-loss Si3N4 platform.

Tailorable stimulated Brillouin scattering in a partially suspended aluminium nitride waveguide in the visible range.

Stimulated Brillouin scattering (SBS) has been widely applied in narrow line-width laser, microwave filters, optical gyroscopes, and other fields. However, most research is limited within near-infrared to mid-infrared range. This is due to the limited transparent window in most materials, such as silicon and germanium. Aluminium nitride (AlN) is a novel III-V material with a wide transparent window from 200 nm and an appropriate refractive index to confine the light. In this paper, we first validate the full-vectorial formalism to calculate SBS gain based on the measured results from a silicon platform. Compared to previous research, our model achieves higher accuracy in terms of frequency, Q factor, as well as Brillouin gain coefficient without modifying the waveguide width. It also reveals the importance of matching rotation matrix and crystalline coordinate system. Then, we investigate the SBS in a partially suspended AlN waveguide at 450 nm based on the validated method. It shows a wide tunability in frequency from 16 GHz to 32 GHz for forward SBS and a range from 42 GHz to 49 GHz for backward SBS. We numerically obtain the value of Brillouin gain of 1311 W-1m-1 when Q factor is dominated by anchor loss for forward SBS of transverse electric mode. We also find out that in the case for forward SBS of transverse-magnetic mode, anchor loss could be greatly suppressed when the node point of the selected acoustic mode matches with the position of pillar anchor. Our findings, to the best of our knowledge, pave a new way to obtain Brillouin-related applications in integrated photonic circuit within the visible range.

Influence of optical mode polarization state on the Brillouin gain spectrum in optical microfibers

During the stimulated Brillouin scattering (SBS) process, when the two H E 11 modes in the pump light and scattered light have different polarization states, between which is an angle θ , the Brillouin gain spectrum characteristics in a microfiber are studied in detail. The difference of θ directly changes the generated optical force distribution. When 0 ≤ θ ≤ π / 2 , for the T R 0 n acoustic mode in forward SBS and the L 0 n acoustic mode in backward SBS, the corresponding Brillouin gain values have a clear cosine-like functional relationship with θ . Considering the change of optical force, the T R 0 n and L 0 n acoustic modes with symmetrical displacement distributions of center circles are the key features to realize the change of gain with θ . The characteristics of gain value with θ may provide interesting theoretical enlightenment for Brillouin applications involving angles, such as Brillouin sensors and acousto-optic modulators.

The Investigation of Forward and Backward Brillouin Scattering in High-Q Chalcogenide Microspheres

Traditional optomechanical research is rarely studied in compound glass, especially chalcogenide glass. In this paper, the forward and backward Stimulated Brillouin Scattering (SBS) is demonstrated for the first time in a chalcogenide glass microsphere resonator. A high-purity chalcogenide glass microsphere with a high quality (<i>Q</i>) factor of 2.1 &#x00D7; 10⁷ is investigated using a 1550 nm tunable laser. In the experiment, the resulting mechanical vibration frequencies caused by forward and backward SBS are measured at 80 MHz and 7.8 GHz, respectively. The triply resonant Stimulated Brillouin scattering process greatly enhances the light&#x2013;acoustic interactions, enabling the threshold power to be 344 &#x03BC;W. The work demonstrated in the chalcogenide microresonator is important for the potential applications of chalcogenide glass, which has higher nonlinearity and low absorptions at mid-infrared band.

Stimulated Brillouin scattering enhanced by the stimulated Raman process near the quarter-critical density

Stimulated Raman scattering can occur near the quarter critical density in direct-drive fusions, and the frequency of Raman-scattered light is about half of the incident light frequency. The second harmonic of the Raman-scattered light can be produced due to the inhomogeneity density profile. It can serve as the seed of stimulated Brillouin scattering (SBS). When the second harmonic of stimulated Raman-scattered light propagates against the incident light, some components will match the frequency of backward SBS, and SBS is induced. Thus, SBS could be greatly enhanced.

Shock compression-induced enhancement of stimulated Raman scattering in heavy water

Stimulated Raman Scattering (SRS) of shock-compressed liquid heavy water was observed using a Nd:YAG laser in both the forward and backward directions of the laser beam. SRS spectra exhibited different characteristic features at weak and intense pump energies. As for the intense pump energy, the main SRS peak occurred at approximately 2400 cm −1 and subsequently shifted to a lower frequency. Interestingly, some new peaks were observed in both directions. Specifically, a lower frequency peak close to 2200 cm −1 and 2nd-order anti-Stokes peaks were observed in the backward direction, which further indicated that an ice-VII cubic structure was formed under shock compression conditions. In addition, the normalized intensity ratio of the two main peaks exhibited quite different energy-dependent behaviors. The results demonstrated that the shock-induced high pressure environment significantly enhanced SRS. The results could be attributed to the enhancement effect of Kerr self-focusing and the competitive Backward Stimulated Brillouin Scattering (BSBS) and Forward SRS (FSRS) processes.

Spectral structures of backward stimulated Brillouin scattering driven by a picosecond laser

Laser plasma interaction (LPI) is an important content in laser plasma related research, and it is one of the key issues related to the success or failure of inertial confinement fusion ignition, and has received extensive attention. In order to suppress the relevant LPI process as much as possible, the major laboratories around the world have developed a variety of beam smoothing methods through decades of research. However, the current understanding and suppression of LPI are still far from enough, and further in-depth studies are still needed. Generally, the research of LPI is based on nanosecond laser driving, and focuses mainly on the effects of the related LPI process caused by nanosecond lasers. However, the LPI processes, such as stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), etc., occur and develop on a time scale of picoseconds.The comprehensive effect can be studied only on a longer scale of nanosecond. For highly nonlinear LPI processes, the comprehensive effect may be difficult to reflect the real physical laws. The emergence of the picosecond laser has made it possible to study the LPI process in more detail and on a more appropriate time scale. The present research tries to gain an understanding of LPI from a more refined perspective. The experimental research of picosecond laser driving LPI is carried out on the Shenguang-Ⅱ upgrade and picosecond laser facilities. First, a nanosecond laser is used to irradiate a target to generate a large-scale plasma, and a few nanoseconds later, the picosecond laser is injected as an interaction beam to drive the LPI scattering such as SBS and SRS. The spectral signal of backscatter light is measured experimentally by using the method of diffuse reflector. From the research results it is found that the backward signals of the band near the laser wavelength contain, in addition to the true backward SBS component, a large number of interference signals introduced by picosecond laser and nanosecond laser. The interference signal introduced by nanosecond laser can be eliminated by using specific measures, but the interference signal introduced by picosecond laser cannot be eliminated experimentally, which will affect the estimation of the true share of the backward SBS. The comprehensive results show that under different experimental conditions, the backward scatter energy of SBS may be less than half that of the total recorded signals. This result is helpful in further understanding and re-recognizing previous relevant experimental data.

Pilot demonstration of correlation-domain distributed temperature sensing using forward Brillouin scattering

Forward Brillouin scattering (FBS) has been regarded as a key tool for distributed sensing of strain, temperature, acoustic impedance, etc. To date, FBS-based distributed sensing has been experimentally reported only by time-domain techniques. In this paper, we demonstrate correlation-domain distributed temperature sensing using cascaded FBS seeded by backward stimulated Brillouin scattering (SBS). By localizing the backward SBS using synthesized optical coherence function, cascaded FBS can also be localized. In the proof-of-concept experiment, we detect a 104 m long heated section in a 397 m long highly nonlinear fiber.

Broadband Brillouin Phase Shifter Utilizing RF Interference: Experimental Demonstration and Theoretical Analysis

Microwave photonic phase shifters based on stimulated Brillouin scattering (SBS) offer tunable and broadband, optically controllable phase shifts. However, achieving a 360° phase shift requires a large amount of SBS gain which often exceeds the available gain and power handling capability of an integrated waveguide. A Radio Frequency (RF) interference technique has recently been utilized in an integrated silicon platform, which uses forward Brillouin scattering in a suspended waveguide to compensate for the lack of available Brillouin gain in standard silicon on insulator platforms. This interference scheme amplifies the phase shift at the expense of link performance. Here, we demonstrate and analytically model a 360° ultra-broadband phase shifter using backward SBS in both fiber and on-chip by combining SBS and RF interference. The phase enhancement scheme greatly reduces the required Brillouin gain and thus the required optical power. Additionally, the backward architecture reduces filter requirements as the residual pump reflections are simpler to remove compared to the pump in the forward Brillouin scattering case, where the pump co-propagates with the signal. The model provides a deeper insight into the properties of the interferometric phase enhancement scheme and predicts the potential trade-offs of an optimized system, showing reduced link loss at higher levels of Brillouin gain. The model also predicts the sensitivity to variations of the interferometric components. Using this technique, we have demonstrated a broadband phase shift over an ultra-broad bandwidth of 0.1 - 65 GHz, limited only by the bandwidth of the available components. Also, we demonstrate a phase enhancement factor of 10 over a bandwidth of 18 GHz in an integrated chalcogenide waveguide.

Observation of Stimulated Brillouin Scattering in Silicon Nitride Integrated Waveguides.

Silicon nitride (Si_{3}N_{4}) has emerged as a promising material for integrated nonlinear photonics and has been used for broadband soliton microcombs and low-pulse-energy supercontinuum generation. Therefore, understanding all nonlinear optical properties of Si_{3}N_{4} is important. So far, only stimulated Brillouin scattering (SBS) has not yet been reported. Here we observe, for the first time, backward SBS in fully cladded Si_{3}N_{4} waveguides. The Brillouin gain spectrum exhibits an unusual multipeak structure resulting from hybridization with high-overtone bulk acoustic resonances of the silica cladding. The reported intrinsic Si_{3}N_{4} Brillouin gain at 25GHz is estimated as 4×10^{-13} m/W. Moreover, the magnitude of the Si_{3}N_{4} photoelastic constant is estimated as |p_{12}|=0.047±0.004, which is nearly 6 times smaller than for silica. Since SBS imposes an optical power limitation for waveguides, our results explain the capability of Si_{3}N_{4} to handle high optical power, central for integrated nonlinear photonics.

Enhanced stimulated Raman scattering by suppressing stimulated Brillouin scattering in liquid water

Stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) are two kinds of emblematic inelastic scattering processes resulting from the interaction of high-intensity laser with matter. Generally, competition between SBS and SRS is a common phenomenon in many substances. In liquid or high-pressure gas, if a single longitudinal mode laser is used as a pump source, both SBS and SRS can be excited, but the SBS will become very strong due to higher gain and optical phase conjugation. In comparison, the SRS gain is typically 2 orders of magnitude smaller than the SBS gain so that most of the pump laser energy is spent on the SBS and the SRS is greatly suppressed. To improve the output energy of SRS in liquid medium, a method of suppressing the SBS process by controlling temperature of medium is proposed. The SRS generation system using broadband pulse laser of 532 nm in wavelength as a pumping source is designed, the output energy of forward SRS (FSRS) and backward SBS (BSBS) in water with different temperatures are measured, and the physical mechanisms of the influences of water temperature, pumping linewidth and thermal defocusing on the output energy of SRS are analyzed. The experimental results indicate that by reducing the water temperature, the SBS process can be significantly suppressed, and the beam distortion caused by thermal defocusing effect can be reduced, thus effectively improving the output energy of SRS. Unlike the single longitudinal mode laser, when the pump source is handled in multiple longitudinal modes with a wide linewidth, the gain of FSRS is higher than that of the backward SRS (BSRS). Meanwhile, since the SBS gain coefficient is restricted by the linewidth of the pump laser, the FSRS process is dominant and both backward SBS and BSRS are significantly suppressed. It is necessary to state that none of the influence of backward SRS, self-focusing, optical breakdown and other non-linear effects on the output energy of SRS is considered in this paper, and only the effectiveness of reducing temperature to improve the energy output of forward SRS is verified from the perspective of temperature change. The results are of great significance for the multi-wavelength conversion of SRS in liquid medium.

Distributed forward Brillouin sensor based on local light phase recovery

The distributed fibre sensing technology based on backward stimulated Brillouin scattering (BSBS) is experiencing a rapid development. However, all reported implementations of distributed Brillouin fibre sensors until today are restricted to detecting physical parameters inside the fibre core. On the contrary, forward stimulated Brillouin scattering (FSBS), due to its resonating transverse acoustic waves, is being studied recently to facilitate innovative detections in the fibre surroundings, opening sensing domains that are impossible with BSBS. Nevertheless, due to the co-propagating behaviour of the pump and scattered lights, it is a challenge to position-resolve FSBS information along a fibre. Here we show a distributed FSBS analysis based on recovering the FSBS induced phase change of the propagating light waves. A spatial resolution of 15 m is achieved over a length of 730 m and the local acoustic impedances of water and ethanol in a 30 m-long uncoated fibre segment are measured, agreeing well with the standard values.

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

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