Stimulated Brillouin Scattering Process Research Articles

A Comprehensive Study on Phase Sensitive Amplification and Stimulated Brillouin Scattering in Nonlinear Fibers with Longitudinally Varying Dispersion

Fiber optic parametric and phase sensitive amplifiers (PSA) are interesting for modern day communication technologies due to their low noise and high gain amplification properties with a potential for all optical signal processing and wide band operation. PSAs are typically employed in either a single pump or dual pump configuration. In this article we explore the utilities of both configurations, however considering a fiber with a longitudinally varying dispersion profile. For the single pump case, PSA operation at large pump-signal detunings, that arise due to the longitudinal dispersion variation, were studied numerically, and recipes of using the system as a wide band wavelength selective filter were laid out. For the dual-pump case, emphasis was laid on achieving a larger signal gain, by reducing stimulated Brillouin scattering (SBS) that prevents large pump power transport through the nonlinear fiber. First, the effects of dispersion variation on the gain of a dual pump PSA were studied analytically and numerically in order to optimize the dispersion variation profile, neglecting SBS processes. Then we independently studied the SBS dynamics of the system numerically. A sinusoidally dispersion oscillating fiber (DOF) was found to be an optimal candidate with respect to its PSA and SBS performances. To establish this claim, we also experimentally compared the performance of an available DOF over a standard highly nonlinear fiber (HNLF) that has a constant dispersion profile and established its utility for designing a high gain PSA system, thanks to the SBS mitigation due to the longitudinal dispersion variation of the fiber.

Gain characteristics of stimulated Brillouin scattering in fused silica.

Stimulated Brillouin scattering (SBS) is a non-linear process which has the capacity to improve the beam quality and pulse characteristics of laser beams. In this paper, we theoretically and experimentally study the process of SBS in fused silica. In particular, we examine the energy reflection and pulse compression of input laser pulses as functions of focus position, pump energy and beam diameter. We utilized coupled wave equations and a distributed noise model to simulate the reflected energy and time waveform under different gain parameters. An experimental system is constructed and used to qualify the numerical simulations. The results reveal that the threshold for the SBS process and the energy reflectivity significantly change with laser focus position under the same pump and focusing parameters. Ultimately, the gain characteristics of the SBS material is the primary factor that influences the SBS output. This work presented here offers insight into the operation of short-length solid-state SBS lasers and serves as a basis for the design and optimization of such systems.

Cross beam energy transfer and backward stimulated Brillouin scattering in double-cone ignition experiment

<sec>In the research of direct-drive laser fusion, laser irradiation of a target pellet can stimulate various laser plasma instabilities, such as stimulated Brillouin scattering (SBS) and cross-beam energy transfer (CBET), which significantly reduce the energy coupling efficiency between the laser and target pellet as well as the laser irradiation uniformity, leading the implosion quality to degrade. In the double-cone ignition (DCI) scheme of laser fusion scheme, the diagnosis of SBS and CBET is important owing to the different target configurations and oblique incident laser irradiation from the traditional spherically symmetric direct-drive central ignition scheme. In this paper, a simple and reliable backscattering diagnostic system is developed and applied to the diagnosis of the time-resolved backscattering spectrum at wavelength near 351 nm in a DCI experiment on the Shenguang-II upgrade (SG-IIU) facility. We use the system to carry out an experimental study of the SBS process and CBET process in DCI.</sec><sec>The backscattering diagnostic system collects the backscattered light signal through the scattered light by reflector mirror via an optical fiber. The signal is dispersed by a spectrometer and then recorded by a streak camera. The signal contains both the laser reference signal from the frequency doubling crystal and the backscattered light. With the help of the reference signal, the diagnostic system can reliably give the energy fraction of backscattered light. The experimental results show that the energy fraction of backscattered light around 351 nm is not higher than 3%, which is significantly lower than the experimental result of the spherically symmetric irradiation direct-drive central ignition scheme.</sec><sec>By analyzing the correlation between the backscattered signal and the laser irradiation conditions and combining the results of a set of comparative experiments, we determine that the backscattered signal contains both CBET and SBS. There is a significant difference in the CBET fraction between the backscattered signal of the #5 laser and the backscattered signal of the #7 laser. By combining the polarisation state of the laser beams, we confirm that this phenomenon is related to the polarisation angle between the laser beams. This finding provides a reference for designing subsequent large-scale laser fusion devices.</sec>

Optimizing the spectrum of high power narrow linewidth fiber amplifier through the same complex degree of coherence

The spectra of narrow linewidth fiber amplifiers are closely related to both the stimulated Brillouin scattering (SBS) threshold in power scaling process and the combining efficiency in coherent beam combining (CBC) system. In this manuscript, the SBS thresholds of fiber amplifier with different spectral distributions (Gaussian, sinc2 and rectangular) have been compared under the same spectral complex degree of coherence (CDC), which could promise the same combining efficiency in CBC system. A SBS dynamic model is established to analyze the SBS process in fiber amplifier and a comparing experiment is also performed by measuring the SBS thresholds of different spectra that have the same CDC set to be 0.96. The FWHM linewidths of Gaussian, sinc2 and rectangular spectra are adjusted to be 1.1 GHz, 0.5 GHz and 1.06 GHz, respectively. The corresponding SBS thresholds are measured to be 108 W, 77 W, and 135 W. By contrast, the rectangular spectra could have most excellent capacity on improving SBS threshold in fiber amplifier under the same combining efficiency in CBC system. Overall, it could provide a feasible method on spectra designing in high power narrow linewidth fiber amplifiers used in CBC system.

Long-Range High-Spatial-Resolution Distributed Measurement by a Wideband Brillouin Amplification-Boosted BOCDA

Stimulated Brillouin scattering (SBS)-based distributed fiber optic sensing technique has found pervasive applications in measurement of real-world environmental variables. Its sensing capability, however, is affected by the tradeoff between sensing distance and spatial resolution. Herein, we propose and experimentally verify a wideband Brillouin amplification (WBA)-boosted BOCDA to achieve the high spatial resolution sensing in a longer distance. The WBA is generated by exploiting phase-uncorrelated SBS process between pump and probe waves. Through the WBA, the pump pulses at different scanning frequencies are amplified with the same ratio, which brings distortion-free enhancement of Brillouin signals. The signal-enhancement improves the sensing distance of high spatial resolution BOCDA. As a result, without any additional (Raman/Brillouin) amplification devices, a ∼0.46 MHz measurement certainty and a 6 cm spatial resolution are achieved over a ∼40 km sensing range. The proposed method offers a viable solution for the applications that need high-resolution measurement of remote targets.

High-precision Microwave Frequency Measurement Based on Stimulated Brillouin Scattering With Simple Configuration

We propose and implement a microwave frequency measurement (MFM) scheme based on stimulated Brillouin scattering (SBS) and an apodized fiber Bragg grating (AFBG), achieving high accuracy in a wide range of measurement with a highly simplified configuration. By sweeping a reference frequency during SBS process, a frequency-to-power mapping between the reference frequency and the monitored output optical power of the MFM system is established. Unknown frequencies can be identified through a specific numerical relationship among the unknown frequency, reference frequency, and Brillouin frequency shift (BFS). The transmission and reflection properties of the AFBG are exploited to offer pump light and probe light simultaneously for SBS process, thereby reducing complexity of the MFM system. It can be derived theoretically that all frequencies larger than one BFS can be measured except for 3/2 BFS. This measurement blind spot can be eliminated by utilizing two segments of single-mode fibers with different BFSs. A measurement error of less than ±1 MHz within 10.68 GHz to 20 GHz is achieved in a proof-of-concept experiment of the proposed MFM approach. This method has also been proved to be applied to dual microwave frequency measurements.

High-resolution wide-band optical frequency comb control using stimulated Brillouin scattering.

We introduce a technique to manipulate an optical frequency comb on a line-by-line basis using stimulated Brillouin scattering (SBS). The narrow-linewidth SBS process has been used to address individual lines in optical frequency combs, but previous demonstrations required a dedicated laser to modulate each comb tooth, prohibiting complete comb control. Here, we use a pair of frequency shifting fiber optic loops to generate both an optical frequency comb and a train of frequency-locked pulses that can be used to manipulate the comb via SBS. This approach enables control of the entire frequency comb using a single seed laser without active frequency locking. To demonstrate the versatility of this technique, we generate and manipulate a comb consisting of 50 lines with 200 MHz spacing. By using polarization pulling assisted SBS, we achieve a modulation depth of 30 dB. This represents a scalable approach to control large numbers of comb teeth with high resolution using standard fiber-optic components.

The effect of pump beam focusing characteristics on stimulated Brillouin scattering

Parameters including pump energy, beam waist radius, length of gain medium and beam divergence all affect the generation threshold and output pulse characteristics of lasers that utilize the stimulated Brillouin scattering (SBS) effect. In this paper, we investigate the effect that changes to the pump beam focusing parameters (namely beam divergence and beam waist) have on the output characteristics of the Stokes beam generated through the SBS process. This study is conducted both theoretically and experimentally using a single-cell SBS structure. The results highlight that pump beams with a small divergence angle at the focal point within the SBS cell are favorable for the generation of Stokes beams with short pulse lengths. Here, we demonstrate the generation of a Stokes beam with a pulse width of 854 ps using a pump pulse with a pulse width of 7.2 ns, which was focused to a spot with a divergence angle of 2.48 mrad.

Inhomogeneous acoustic grating model for stimulated Brillouin scattering

Stimulated Brillouin scattering (SBS) is well recognized as being useful in quantum information processing, optical fiber sensing, and LIDAR remote sensing. Here an inhomogeneous acoustic grating model based on coupled wave and dielectric grating theories is established to analyze the SBS process in water. The acoustic grating modulation process was theoretically analyzed based on the proposed model. The results show that variations of the Stokes peak intensity, frequency shift, and linewidth of SBS in water depend on the cosinusoidal refractive index modulation and periodic modulation of the acoustic grating. To verify the theoretical simulation results, an experimental system was designed to measure the SBS process in seawater with different temperatures and salinities. Theoretical results show coincidence with experimental values in the variation trend, and the numerical difference is about 10 MHz per degree Celsius or one thousandth of salinity. This work is essential to future applications of SBS in quantum communication, fiber sensing, and LIDAR technology.

Photonic microwave frequency measurement with improved resolution based on bandwidth-reduced stimulated Brillouin scattering

We propose and theoretically demonstrate a photonic-assisted frequency measurement system based on stimulated Brillouin scattering (SBS) with high accuracy, high resolution, and the capability of multiple frequencies estimation. The great performance is achieved by superimposing the gain spectrum with two loss spectrums for SBS gain bandwidth reduction. Meanwhile, the frequency information of multiple unknown signals is obtained by the bandwidth-reduced gain spectrum of the acoustic-optic interaction in the SBS process. Ultimately, the result show that the system is competent in identifying the frequency information among multiple unknown RF signals with high resolution of 14.5 MHz, high accuracy of 8 MHz, and the bandwidth of 20 GHz. Considering the comprehensive performance of this system, which is beneficial for applications in modern communication and radar applications.

Instantaneous response and suppression of SBS process in a short fiber system with binary sequence phase modulation.

We study the instantaneous response of stimulated Brillouin scattering (SBS) in a fiber system phase modulated by a binary sequence waveform. The buildup time constant of SBS kinetics is investigated analytically and experimentally. A series of binary sequences with adjustable dwell time and sequence period is constructed in order to examine both buildup and suppressing processes for SBS in 15 m short fiber. For a fiber system with SBS buildup time constant within several nanoseconds, the Stokes intensity can be effectively suppressed by implementing a binary sequence phase modulation with a dwell time close to or even less than the buildup time constant. Stokes intensity can be suppressed in several sequence periods, which exhibit a damped-oscillation-like trend.

Determination and Analysis of Line-Shape Induced Enhancement of Stimulated Brillouin Scattering in Noise Broadened, Narrow Linewidth, High Power Fiber Lasers

We investigate the origin of line-shape induced enhancement of stimulated Brillouin scattering (SBS) in narrow linewidth, noise broadened, high-power fiber lasers. A polarization-maintaining seed laser with continuously tunable linewidth (single frequency to >10 GHz), based on white noise modulation was developed for this study. With increasing linewidths, a substantial difference in SBS thresholds was observed depending on the end termination utilized. This observation can be explained by the line-broadened source, having significant power in the Stokes frequency at larger linewidths, seeding the SBS process. Here, SBS threshold for the system terminated with an anti-reflection coated delivery cable is compared with a simple angle cleaved end termination. The influence of end termination on SBS threshold becomes significant with increased linewidths, showing >20% difference in output power between the two cases at ~10 GHz linewidth. The experiments are complemented by simulations, analyzing relative contributions of Rayleigh scattering and fiber end-facet reflections to SBS. At larger linewidths, due to substantial overlap between laser line-shape and SBS Stokes, with low end-facet reflectivity, Rayleigh is the dominant mechanism, which gives way to end-facet reflections with increasing reflectivity. The Rayleigh contribution is negligible at smaller linewidths, and end-facet reflectivity has a weaker influence than with larger linewidths.

Effects of temperature and pressure on the threshold value of SBS LIDAR in seawater.

Effects of temperature and pressure on the threshold value of stimulated Brillouin scattering (SBS) in seawater were analyzed theoretically and experimentally. Theoretically, the change of threshold value of SBS versus the ocean depth was simulated based on the International Thermodynamic Equation of Seawater-2010 (TEOS-10) and the World Ocean Atlas 2013 (WOA13). Experimentally, an ocean temperature and pressure simulator (OTPS) was designed to measure the threshold value of SBS through simulating the changes of temperature and pressure of seawater in 0∼1000 meters. The theoretical and experimental results exhibit that the threshold value of SBS decreases with the increase of temperature at the same seawater pressure and increases with the increase of pressure at the same seawater temperature. The results imply that the SBS process is more likely to occur in upper seawater of lower-latitude areas. The theoretical and experimental results are helpful for remote sensing in ocean using the SBS LIDAR.

Seeding High Brightness Fiber Amplifiers With Multi-Phase Coded Signal Modulation for SBS Effect Management

We propose a new modulation approach based on multi-phase coded signal (MPCS) to increase stimulated Brillouin scattering (SBS) threshold in high power narrow-linewidth fiber amplifiers. To investigate the SBS suppressing capacity of the proposed modulation technique, the dynamics of SBS process with different spectra are simulated in active fibers with different dopants, core diameters, lengths and pumping lights based on three typical types of pumping sources (976 nm laser diodes, 915 nm laser diodes and 1018 nm fiber lasers). Before simulation, a spectral-correlated combining efficiency criterion based on coherent beam combining (CBC) system is introduced and is set to be identical in different spectra. As a result, compared with the typical white-noise-signal (WNS) and pseudo random bit sequence (PRBS) modulation techniques, the MPCS modulation yields a higher SBS threshold in 915 nm and 1018 nm pumped fiber amplifiers with relatively longer active fiber. Besides, in fiber amplifier constructed by general 20/ $400~\mu \text{m}$ active fiber and with 976 nm pumping strategy, MPCS modulation performs better than WNS and PRBS modulation as well. We believe that this modulation signal is expected to be applied in narrow-linewidth, large-mode-area (LMA) high-brightness fiber laser systems to balance the SBS effect and thermal mode instability (TMI) phenomenon.

Self-focusing of cosh-Gaussian laser beam and its effect on the excitation of ion-acoustic wave and stimulated Brillouin backscattering in collisionless plasma

An analytical and numerical study has been carried out for self-focusing of an intense cosh-Gaussian laser beam in collisionless plasma and its impact on the excitation of ion-acoustic wave and stimulated Brillouin backscattering process. The analytical model has been developed under Wentzel–Kramers–Brillouin and paraxial ray approximations. The nonlinearities of ponderomotive force on electron and the relativistic oscillation of the electron mass have been used in this study. The nonlinear differential equations have been set up for the beam width parameters of the main beam, ion-acoustic wave, backscattered wave and back reflectivity of stimulated Brillouin scattering (SBS). These equations have been solved numerically for different values of decentred parameter (b), relative plasma density (ωp0/ω0) and incident laser intensity (a). The results have been compared with only relativistic nonlinearity and Gaussian profile of laser beam. The focusing of laser beam, ion-acoustic wave and scattered wave are found to be strong under relativistic-ponderomotive regime compared to only relativistic regime. Further, it is observed that focusing/intensity of main laser beam, ion acoustic wave and SBS back reflectivity increases with increasing the values of b and ωp0/ω0. Itis also found that back reflectivity of SBS process gets suppressed with the increase in the value of a. This study may be useful in laser induced fusion scheme where back scattering of SBS plays very important role.

Slow-light based tunable delay and narrowband comb filtering at 2 μm.

Development of coherent sources, wideband thulium-doped fiber amplifiers, and fiber components has opened up the wavelength region around 2000nm for optical communications, sensing, and medical surgery. However, several key functionalities that are critical to enable these applications are not yet well developed and, therefore, need attention. Here, we present demonstration of two critical signal processing tasks viz: (1) tunable delay and (2) tunable narrowband filter, which are important for enabling optical communications using wavelengths around 2000nm. We exploit stimulated Brillouin scattering (SBS) in a 100m long SM1950 fiber to report the first demonstration of slow-light based tunable delay around 2000nm. For a 35ns input pulse, we tune the delay up to a maximum of 18.8ns, which corresponds to a relative delay of ∼0.5, by varying the pump power to achieve a maximum gain of 12.6dB. For the 1550nm wavelength regime, it has been shown that the Brillouin slow-light typically results in a delay of 1ns/dB, which is less than the 1.67ns/dB obtained at 2000nm. For the same gain, the large delay at 2μm compared to 1.55μm results from the narrow gain bandwidth (∼22 MHz) of the SBS process in this wavelength regime. Using the narrow gain bandwidth of SBS, we present a proof-of-concept experiment to demonstrate filtering of individual comb lines in a comb with frequency spacing <50 MHz. Demonstration of tunable narrowband filter and delay enables optical signal processing and line-by-line control of comb lines at 2μm.

Brillouin-induced self-heterodyne method for low jitter measurement of laser linewidth

In this paper, we study experimentally and analytically the stimulated Brillouin-induced self-heterodyne technique for linewidth measurement of lasers having narrow spectral widths. The output of a laser under test (LUT) is modulated by a RF signal to generate the sidebands. The carrier and the sidebands co-propagate in a single-mode fiber (SMF) along with the stimulated Brillouin scattering (SBS) signal generated by a counter-propagating pump. The SBS process amplifies one of the sidebands, which beats with the LUT carrier wave. The beat signal is detected using a photodiode and analyzed using an electrical spectrum analyzer to compute the laser linewidth. We analyze the SBS-induced self-heterodyne (SISH) scheme mathematically to obtain expression of the power spectral density of the photocurrent. Experimental results show 20 dB and 3 dB linewidths of 15 kHz and 5.5 kHz, respectively, which are in excellent agreement with the estimated theoretical values, thereby validating the analysis. We also perform the conventional delayed self-heterodyne measurement and obtain 20 dB linewidth of 15 kHz, which compares well with the SISH method. However, the SISH technique reduces jitter by 10 dB. We also study the dependence on linewidth measurement of the SISH scheme on fiber length. For 2 km SMF length, the 20 dB linewidth is 23 kHz, which is higher compared to the measurement obtained by using 25 km SMF.

Stimulated Brillouin scattering behaviors in multi-ion species plasmas in high-temperature and high-density region

Different IAW modes excited by stimulated Brillouin scattering (SBS) and different SBS behaviors in several plasmas composed of ion species relevant to inertial confinement fusion in the high-temperature and high-density region have been studied by Vlasov-Maxwell simulations. Firstly, the linear theory is taken to analyze the linear process of IAW and SBS in multi-ion species plasmas. Secondly, the nonlinear effects on SBS in CH and AuB plasmas have been studied in detail. Especially, particle trapping will lead to nonlinear Landau damping and nonlinear frequency shift (NFS) of IAW. Particle trapping will decrease IAW Landau damping and make SBS in CH plasmas transform from convective instability in the linear regime into absolute instability. The NFS from particle trapping and harmonic generation in AuB plasmas will lead to a positive shift of the IAW frequency and make the frequency of SBS scattering light experience an obvious negative shift. And, the nonlinear model can give a good explanation of the nonlinear phenomena. Lastly, the saturation mechanisms are analyzed. The main nonlinear saturation mechanisms of SBS include NFS from particle trapping and harmonic generation, energy dissipation by spectrum broadening, and pump depletion. Microscale kinetic simulations of SBS are interesting, difficult, and could have an impact on the current understanding of the instability, particularly in the nonlinear regime where wave-particle interactions contribute to the saturation of reflectivity.

Brillouin-Assisted Notch Filtering Based All-Optical Image Rejection Mixer

A novel approach for realizing a microwave photonic image rejection mixer is presented in this paper. It is based on using a widely tunable narrow-band optical notch filter implemented by stimulated Brillouin scattering (SBS) to remove the image signal. The image rejection mixer has a simple structure as it only requires a single integrated dual-polarization modulator for both frequency conversion and pump wave generation for the SBS process. The absence of electrical components enables the image rejection mixer to be operated over a wide frequency range with a high image rejection ratio. A technique is also proposed to suppress the noise generated by the SBS process in the image rejection mixer. Experimental results demonstrate a high and constant image rejection ratio of over 32 dB for the image rejection mixer operating over an RF signal frequency range of 11.8–19.6 GHz, and around −5 dB conversion efficiency.

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.