Stimulated Brillouin Scattering Power Research Articles

Gain performance analysis of optical parametric amplifiers employing different modulation techniques

In this paper, experimental verification of optical fiber-based parametric amplification has been studied over different modulation formats. Based on principle of parametric amplification, which is essentially a nonlinear phenomenon, these fiber amplifiers offer discrete amplification along with the greater use of bandwidth without any major compromise on usable gain and vice versa. We have demonstrated the gain profile of fiber optical parametric amplifier using stimulated Brillouin scattering (SBS) control mechanism which is based on SBS power pump, which in turn determines the gain of lumped amplifier. Different modulation formats like amplitude modulation, frequency modulation and phase modulation were investigated, both with and without the parametric amplification over given length of fiber. During performance evaluation, it was observed that FM- and PM-modulated data responded with a considerable increase in gain with activation of SBS pump, while the similar gain improvement was found to be missing in AM-modulated data.

Stimulated Brillouin Review: Invented 50 Years Ago and Applied Today

Stimulated Brillouin scattering (SBS) is embedded today in a variety of optical systems, such as advanced high-power lasers, sensors, microwave signal processors, scientific instrumentation, and optomechanical systems. Reduction in SBS power requirements involves use of optical fibers, integrated optics, micro-optic devices, and now nano-optics, often in high Q cavities. It has taken fifty years from its earliest invention by conceptual discovery until today for SBS to become a practical and useful technology in a variety of applications. Some of these applications are explained and it is shown how they are tied to particular attributes of SBS: phase conjugation, frequency shifts, low noise, narrow linewidth, frequency combs, optical and microwave signal processing, etc.

Laser propagation in a subcritical foam: Ion and electron heating

We develop a model for laser propagation and heating in a subcritical foam (homogeneous electron density as a fraction of critical ne,0/nc<1). Our model describes the partition of energy between ions and electrons in an expanding foam element irradiated by a laser, and we solve for the plasma conditions generated by burning down the foam microstructure. We find that a substantial fraction of laser energy goes into breaking down and homogenizing the foam microstructure, which slows down the laser heat front. We also find that the ion temperature in a plasma generated by burning down a foam can exceed the electron temperature. This is because laser energy is deposited into the expansion kinetic energy of ions as the foam microstructure burns down explosively. The higher ion temperature increases ion-acoustic wave damping which reduces stimulated Brillouin scattering (SBS). We test our model against data from an experiment that shot a subcritical foam with a high intensity laser pulse on the Janus laser facility at LLNL. We find that by modeling the effect of the foam microstructure, we can explain both the experimentally measured laser propagation velocity and the backscattered SBS power.

Stimulated Brillouin Scattering in an AlGaN Photonics Platform Operating in the Visible Spectral Range

We present Stimulated Brillouin Scattering (SBS) process in AlGaN integrated photonic waveguides. The wide bandgap of this III-Nitride material platform allows operating at visible wavelengths enabling large Stokes shifts. For this study, we employ a multiphysics approach that includes electric-photoelastic, magnetic-photoelastic, material interface displacement effects, and for optimal waveguide dimensions to find the Brillouin-active acoustic modes involved in the SBS process. The SBS power gain and the Stokes frequency shift are investigated for both backward and forward scattering processes, and it is shown that stokes shift larger than 50 GHz with high gain are achievable. Moreover, a parametric analysis is presented in order to demonstrate the possibility of realizing Brillouin lasers operating at blue wavelengths.

Investigations of Backscattering Effects in Optical Fibers and Their Influences on the Link Monitoring

In this paper, first, we employed the coupled-mode theory to investigate the backscattering power characteristics of Rayleigh backscattering (RB) and stimulated Brillouin scattering (SBS) both synchronously and precisely. We discovered how the RB and SBS power changed with pump powers and fiber lengths. The scattering power curves with different pump powers could be divided into three regions. The first was called “the linear region of RB,” in which the pump power was mainly converted into RB and the RB power grew linearly with pump powers. The second region was called “the stimulation region of Brillouin scattering,” in which the power was mainly converted into SBS. In “the gain-saturated region,” when the pump power was much higher than the SBS threshold, both the RB and SBS power were gain-saturated. We also obtained the power curves with different fiber lengths. The power curves were helpful to set up proper launch powers in practical engineering applications, such as optical time-dome reflectometry systems and distributed fiber sensors. Then, we verified our theoretical results by experiments. Finally, according to the measurements and numerical solutions of the coupled-mode equations, we discussed the application of backscattering and gave advice for pump power selections in different fiber link monitoring systems.

Analysis of the modification of four-wave mixing phase matching by stimulated Brillouin scattering

Controlling the phase-matching condition of four-wave mixing (FWM) using stimulated Brillouin scattering (SBS) has been experimentally demonstrated, but the full extent of the capabilities of this method has remained unknown. Here we theoretically and numerically investigate these capabilities, considering the SBS and the FWM separately. We show that, with modest optical power, exponential gain is achievable for normal dispersion. The method is limited by SBS-induced power variations and by the need to limit the SBS power levels.

Long-range wind monitoring in real time with optimized coherent lidar

Two important enabling technologies for pulsed coherent detection wind lidar are the laser and real-time signal processing. In particular, fiber laser is limited in peak power by nonlinear effects, such as stimulated Brillouin scattering (SBS). We report on various technologies that have been developed to mitigate SBS and increase peak power in 1.5-μm fiber lasers, such as special large mode area fiber designs or strain management. Range-resolved wind profiles up to a record range of 16 km within 0.1-s averaging time have been obtained thanks to those high-peak power fiber lasers. At long range, the lidar signal gets much weaker than the noise and special care is required to extract the Doppler peak from the spectral noise. To optimize real-time processing for weak carrier-to-noise ratio signal, we have studied various Doppler mean frequency estimators (MFE) and the influence of data accumulation on outliers occurrence. Five real-time MFEs (maximum, centroid, matched filter, maximum likelihood, and polynomial fit) have been compared in terms of error and processing time using lidar experimental data. MFE errors and data accumulation limits are established using a spectral method.

Brillouin lasing in whispering gallery micro-resonators

Thresholds of stimulated Brillouin scattering (SBS) in solid-state whispering gallery mode (WGM) microresonators are analyzed. It is shown that the SBS interaction is substantially different here from that known in the bulk case and in the case of water droplet resonators. The reason is the absence of pure longitudinal acoustic WGMs owing to strong coupling of the longitudinal (l) and transverse (t) acoustic displacements at the surface of the resonator. As a result, a considerable increase of the SBS thresholds takes place, and the lowest thresholds correspond to the hybrid tl-modes with very large radial indices. Nevertheless, the thresholds lie in the μW range of the pump power. Dependence of the SBS power thresholds on the modal numbers and the possibility of self-tuning to the SBS resonance are analyzed.

Pseudo-random binary sequence phase modulation for narrow linewidth, kilowatt, monolithic fiber amplifiers.

We report on pseudo random binary sequence (PRBS) phase modulation for narrow-linewidth, kilowatt-class, monolithic (all-fiber) amplifiers. Stimulated Brillouin scattering (SBS) threshold enhancement factors for different patterns of PRBS modulated fiber amplifiers were experimentally analyzed and agreed well with the theoretical predictions. We also examined seeding of the SBS process by phase modulated signals when the effective linewidth is on the same order as the Brillouin shift frequency. Here ~30% variations in SBS power thresholds were observed from small tunings of the modulation frequency. In addition, a 3 GHz PRBS modulated, 1.17 kW fiber amplifier was demonstrated. Near diffraction-limited beam quality was achieved (M2 = 1.2) with an optical pump efficiency of 83%. Overall, the improved SBS suppression and narrow linewidth achieved through PRBS modulation can have a significant impact on the beam combining of kilowatt class fiber lasers.

Nonsinusoidal phase modulations for high-power laser performance control: stimulated Brillouin scattering and FM-to-AM conversion

High-power lasers, such as the Laser MegaJoule (LMJ), have to be phase modulated to avoid stimulated Brillouin scattering (SBS) that may strongly damage optics at the end of the laser chain. Current spectral broadening on LMJ is performed with a sinusoidal phase modulation. This pure sinusoidal phase modulation leads to inhomogeneous spectral power densities (SPD). Thus, for a same SBS power threshold, the sinusoidal phase-modulated spectrum has to be larger than the equivalent ideal SPD with isoenergetic peaks. We present in this paper a technique to generate energy-balanced Dirac peaks spectra thanks to nonsinusoidal phase modulations. Thus, we can build a narrower spectrum with a nonsinusoidal phase modulation that has the same SBS threshold as a sinusoidal phase modulation, and we show that FM-to-AM conversion can be strongly reduced, which is of great interest for LMJ laser performance, with reductions up to 40%.

Dependence of stimulated Brillouin scattering in multimode fibers on beam quality, pulse duration, and coherence length

We experimentally studied the threshold power for stimulated Brillouin scattering (SBS) in multimode optical fibers as a function of input beam product (a measure of the beam quality), using a Q-switched Nd:YAG laser producing short pulses with a coherence length of at least 8 m. Numerical simulations of the dependence of this threshold on pulse duration were performed to extrapolate the experimental data to the continuous-wave (CW) case. The CW SBS threshold was found to increase with increased beam product (decreased beam quality), with a threshold value of 1 kW, for example, for a beam product of 2.5 mm mrad at the input of a 100-μm core-diameter, 10-m-long fiber. When we compared our measured results with published data for SBS in single-mode fibers recalculated to our core diameter, we found that our CW SBS power threshold values were two to four times higher. Using further numerical simulations, we extrapolated the SBS threshold for the long coherence length to the case of a short coherence length (of the order of centimeters) as in a high-power CW Nd:YAG laser for material processing.

Statistical properties of stimulated Brillouin scattering in single-mode optical fibers above threshold

We performed numerical simulations to obtain statistical and spectral characteristics of stimulated Brillouin scattering (SBS) initiated by Gaussian noise in single-mode optical fibers. Recently published experimental spectra of SBS power [e.g., Phys. Rev. Lett. 85, 1879 (2000)] are explained completely by a one-dimensional SBS model. We give a clear physical insight into the problem and, for what is to our knowledge the first time, reveal how the probability function of Stokes power, the power-correlation function, and the SBS spectra evolve as key parameters of the model vary, leading to a modification of Stokes field statistics.

Effect of dynamic stimulated Brillouin scattering on millimeter-wave fiber radio communication systems

The effect of dynamic stimulated Brillouin scattering (SBS) on millimeter-wave fiber radio communication systems has been studied and compared with results by steady-state analysis (SSA). More information is provided by the dynamic SBS analysis as the probability density functions (pdf's) of both SBS power and received signal can be calculated approximately. The worst-case power by SSA overestimates the effect of SBS greatly while the average power by SSA underestimates it. The dynamic SBS analysis, therefore, should be used for estimating the effect of SBS on millimeter-wave fiber radio communication systems, especially for low bit rate and short length ones.

Stimulated brillouin scattering threshold representing bit‐error ratio degradation in single‐mode optical fiber transmission

AbstractAchieving maximum repeater spacing in optical fiber transmission systems is compromised by the input power limit due to stimulated Brillouin scattering (SBS). In digital transmission, SBS causes bit‐error rate degradation. The calculation method for the SBS equation under steady state in a single‐mode optical fiber has been established here; boundary conditions and actual environment are both considered. A new SBS threshold has been proposed as the input optical power at which the backscattered SBS power equals the Rayleigh backscattering power at the fiber input end. Based on a detailed comparison with experimental results, it has been confirmed that the proposed threshold expresses accurately the bit‐error rate degradation due to the SBS; it is approximately 2.5 dB lower than the conventional threshold. Use of the proposed threshold makes accurate system design possible.