Stimulated Brillouin Scattering Interaction Research Articles

Fiber-Based Techniques to Suppress Stimulated Brillouin Scattering

Stimulated Brillouin scattering (SBS) is the major factor that limits the maximum optical fiber output power in narrow linewidth applications, which include important fields such as passive optical networks (PONs), high-power fiber amplifiers, and lasers. Great efforts have been dedicated to suppressing the SBS effect and increasing the maximum optical fiber output power. This paper focuses on key fiber-based techniques to suppress SBS. These techniques take advantages of the properties of optical fibers. We present how these properties (electric modes, acoustic modes, and material properties) could be utilized to suppress SBS. The fiber-based techniques are divided into transverse optical fiber design, longitudinal variant fiber design, and external perturbations (strain and temperature) on optical fibers. Transverse optical fiber design focuses on the mechanism electro-acoustic interaction. Large effective area fiber design and acoustic tailoring techniques have been discussed. Longitudinal variant fiber design considers the nonlinear SBS interaction along propagation distance, and various techniques related have been presented. External perturbations (strain and temperature) on optical fibers emphasize on how external static perturbations could modify the SBS effect.

Brillouin Interaction between Two Optical Modes Selectively Excited in Weakly Guiding Multimode Optical Fibers.

A multimode optical fiber supports excitation and propagation of a pure single optical mode, i.e., the field pattern that satisfies the boundary conditions and does not change along the fiber. When two counterpropagating pure optical modes are excited, they could interact through the stimulated Brillouin scattering (SBS) process. Here, we present a simple theoretical formalism describing SBS interaction between two individual optical modes selectively excited in an acoustically isotropic multimode optical fiber. Employing a weakly guiding step-index fiber approach, we have built an analytical expression for the spatial distribution of the sound field amplitude in the fiber core and explored the features of SBS gain spectra, describing the interaction between modes of different orders. In this way, we give a clear insight into the sound propagation effects accompanying SBS in multimode optical fibers, and demonstrate their specific contributions to the SBS gain spectrum.

Numerical simulation of noise in pulsed Brillouin scattering

We present a numerical method for modeling noise in stimulated Brillouin scattering (SBS). The model applies to dynamic cases such as optical pulses and accounts for both thermal noise and phase noise from the input lasers. Using this model, we compute the statistical properties of the optical and acoustic power in the pulsed spontaneous and stimulated Brillouin cases, and investigate the effects of gain and pulse width on noise levels. We find that thermal noise plays an important role in the statistical properties of the fields and that laser phase noise impacts the SBS interaction when the laser coherence time is close to the time scale of the optical pulses. This algorithm is applicable to arbitrary waveguide geometries and material properties and, thus, presents a versatile way of performing noise-based SBS numerical simulations, which are important in signal processing, sensing, microwave photonics, and opto-acoustic memory storage.

Optimization and visualization of phase modulation with filtered and amplified maximal-length sequence for SBS suppression in a short fiber system: a theoretical treatment.

We use a model to investigate both the temporal and spectral characteristics of a signal lightwave which has been spectrally broadened through phase modulation with a maximal-length sequence (MLS), which is a common type of pseudo-random bit sequence. The enhancement of the stimulated Brillouin scattering (SBS) threshold of the modulated lightwave in a fiber system is evaluated by numerically simulating the coupled three-wave SBS interaction equations. We find that SBS can build up on a nanosecond-level time scale in a short fiber, which can reduce the SBS suppressing capability of MLS modulation waveforms with GHz-level clock rate, if the sub-sequence ("run") lengths with the same symbol (zero or one) of the MLS extend over several nanoseconds. To ensure the SBS buildup is perturbed and thus suppressed also during these long sub-sequences, we introduce a low-pass filter to average the signal over several bits so that the modulation waveform changes gradually even during long runs and amplify the RF modulation waveforms to the level required for sufficient spectral broadening and carrier suppression of the optical signal. We find that the SBS suppression depends non-monotonically on the parameters of the filtered and amplified MLS waveform such as pattern length, modulation depth, and the ratio of low-pass filter cutoff frequency to clock rate for maximum SBS mitigation. We optimize the SBS suppression through numerical simulations and discuss it in terms of the temporal and spectral characteristics of the lightwave and modulation waveform using derived analytical expressions and numerical simulations. The simulations indicate that the normalized SBS threshold reaches a maximum for a RMS modulation depth of 0.56π and a ratio of filter cutoff frequency to clock rate of 0.54 and that MLS9 is superior to other investigated patterns.

Suppression of stimulated Brillouin scattering in high power fibers using nonlinear phase demodulation.

We demonstrate a modulation approach that relaxes the limitations imposed by stimulated Brillouin scattering (SBS) on amplification and propagation of narrow-linewidth light in fibers. By imposing synchronous amplitude and phase modulation on an input field, the optical spectrum after high-power fiber transmission is compressed using nonlinear self-phase modulation. This effectively reduces the SBS interaction length and increases the SBS threshold, enabling narrower linewidths. Using this technique, we demonstrate >2 × increase in SBS-limited spectral brightness from a kW-class amplifier. We show that SBS suppression becomes more effective for higher powers and longer fibers.

Brillouin instantaneous frequency measurement with an arbitrary response for potential real-time implementation.

We demonstrate an arbitrary-response Brillouin instantaneous frequency measurement (IFM) based on the principle of the frequency-to-power mapping in stimulated Brillouin scattering (SBS), which is named Brillouin IFM. The arbitrary response is realized by periodically sweeping the frequency of the local pump lightwave, and the response's amplitude depends on the sweeping slope. The Brillouin IFM is achieved by the control of the sweeping period lower than the double of the flight-of-time in the SBS interaction length. A Costas frequency coded pulse is experimentally measured to verify the feasibility of the proposed Brillouin IFM scheme. The bandwidth of the digital sampling in the scheme is reduced to 50MHz. The Brillouin IFM range without averaging reaches 6GHz at Ku band.

Sharp tunable and additional noise-free optical filter based on Brillouin losses

In this paper, we propose an additional noise-free, independent center frequency and bandwidth tunable optical filter based on stimulated Brillouin scattering (SBS) losses. By suppressing the out-of-band signal with two broadened symmetric SBS losses, tunable pass bandwidths from 500 MHz to 9.5 GHz and the independent center frequency tunability are demonstrated. Considering the limited SBS interaction in the center frequency range, a flat-top response with minimum 0.3 dB ripple is achieved. Assisted by the extra suppression from polarization pulling, a maximum selectivity of 20 dB and an ultrahigh 250 dB/GHz roll-off are reached. A gain-based SBS filter adds noise to the filtered signal. However, for our proposed filter setup, no additional noise is detected due to the transparency in the passband. Considering the wide independent bandwidth and center frequency tunability, flat-top response, and low-noise characteristic, our proposed filter can be perfectly used as a supplement of most commercialized conventional tunable optical single bandpass filters, whose minimum bandwidth is limited by 10 GHz.

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.

Demonstrating the saturation of stimulated Brillouin scattering by ion acoustic decay using fully kinetic simulations

The saturation of stimulated Brillouin scattering (SBS) by the decay to turbulence of the ion acoustic wave (IAW) that participates in the three-wave SBS interaction is demonstrated using a quasi-noiseless one-dimensional numerical solution to the Vlasov-Maxwell system of equations. This simulation technique permits careful examination of the decay process and its role in the complex evolution of SBS. The IAW decay process is shown to be an effective SBS saturation mechanism. In our example, the instantaneous plasma reflectivity saturates at ∼30% and drops to ∼0% as a direct consequence of IAW decay. A contrasting example where the reflectivity is controlled by dephasing due to the nonlinear frequency of the IAW is also discussed.

Stimulated Brillouin scattering in integrated photonic waveguides: Forces, scattering mechanisms, and coupled-mode analysis

Recent theoretical studies of Stimulated Brillouin Scattering (SBS) in nanoscale devices have led to an intense research effort dedicated to the demonstration and application of this nonlinearity in on-chip systems. The key feature of SBS in integrated photonic waveguides is that small, high-contrast waveguides are predicted to experience powerful optical forces on the waveguide boundaries, which are predicted to further boost the SBS gain that is already expected to grow dramatically in such structures because of the higher mode confinement alone. In all recent treatments, the effect of radiation pressure is included separately from the scattering action that the acoustic field exerts on the optical field. In contrast to this, we show here that the effects of radiation pressure and motion of the waveguide boundaries are inextricably linked. Central to this insight is a new formulation of the SBS interaction that unifies the treatment of light and sound, incorporating all relevant interaction mechanisms --- radiation pressure, waveguide boundary motion, electrostriction and photoelasticity --- from a rigorous thermodynamic perspective. Our approach also clarifies important points of ambiguity in the literature, such as the nature of edge-effects with regard to electrostriction, and of body-forces with respect to radiation pressure. This new perspective on Brillouin processes leads to physical insight with implications for the design and fabrication of SBS-based nanoscale devices.

Stability-improved slow light in polarization-maintaining fiber based on polarization-managed stimulated Brillouin scattering

With the idea of controlling the polarizations of the pump and Stokes beams in an optical fiber, a simple scheme is presented for enhancing the stability of the stimulated Brillouin scattering (SBS) interaction and thus that of the SBS-based slow light. For this purpose, a special slow-light element is constructed by fusing a polarization-maintaining circulator (its fast axis being blocked) to each terminal of the polarization-maintaining fiber (PMF). This configuration ensures that the pump and Stokes beams always have identical polarization in the whole fiber during the SBS interaction. An experimental setup is established accordingly. The SBS gain feature and the slow-light performance are studied. A tunable time delay with a slope of 0.82 ns dB−1 is demonstrated for a Gaussian pulse with a width of 88.9 ns. The experimental results with and without polarization management are compared. It is found that such a polarization-managed scheme can improve both the stability of the Brillouin gain and that of the time delay of the Stokes pulse. Moreover, for the same pump power, the Brillouin gain is also enhanced.

On the nature of optically controlled delay of a stokes pulse amplified by stimulated Brillouin scattering in optical fiber

In this work we show through an analytic solution of the set of two coupled dynamic equations for the slowly varying amplitudes of the Stokes optical field and material density in stimulated Brillouin scattering (SBS), its analysis and a set of experiments, that the SBS-induced delay of an output Stokes pulse which is amplified by the SBS interaction is due exclusively to the inertia of the acoustic wave excitation.

Single-End-Access Correlation-Domain Distributed Fiber-Optic Sensor Based on Stimulated Brillouin Scattering

We propose a single-end-access correlation-domain distributed fiber-optic sensor based on stimulated Brillouin scattering (SBS) in a polarization-maintaining optical fiber (PMF). Frequency-modulated pump and probe waves are launched into the same end of the PMF with orthogonal polarization states. Assisted by a polarization beam splitter and a polarization-maintaining isolator installed at the other end of the PMF, SBS interaction between the reflected probe wave and the forward pump wave is exclusively localized within the PMF. Unlike the previously-reported method based on placing a reflector at the far end of the fiber (Song , 2008), in which the measurement range is shortened to a half of the nominal value determined by the frequency modulation, as given in the work by Hotate and Hasegawa (2000), our new method maintains the range as the nominal value. Distributed sensing of temperature or strain is experimentally demonstrated with the measurement range/the spatial resolution of 200 m/16 cm, 56 m/5 cm, or 5.6 m/5 mm, respectively.

Investigation of the relationship between the prepulse energy and the delay time in the waveform preservation of a stimulated Brillouin scattering wave by prepulse injection

It is well known that a stimulated Brillouin scattering (SBS) wave undergoes deformation of its temporal waveform when the pulse is reflected from the SBS medium. A prepulse injection method is a promising technique that is used to preserve temporal pulse shape of a SBS wave by injecting the prepulse before the main pulse enters the SBS interaction region. In earlier research, it was reported that with a prepulse energy of 5 mJ and a delay time of 5 ns, the pulse shape is perfectly preserved. In this research, the prepulse energy required for waveform preservation according to the delay time was investigated experimentally, and the results are explained using a simple theoretical model.

Vector analysis of stimulated Brillouin scattering amplification in standard single-mode fibers

The polarization properties of stimulated Brillouin scattering (SBS) amplification or attenuation in standard single-mode fibers are examined through vectorial analysis, simulation and experiment. Vector propagation equations for the signal wave, incorporating SBS and birefringence, are derived and analyzed in both the Jones and Stokes spaces. The analysis shows that in the undepleted pump regime, the fiber may be regarded as a polarization-dependent gain (or loss) medium, having two orthogonal input SOPs, and corresponding two orthogonal output SOPs, for the signal, which, respectively, provide the signal with maximum and minimum SBS amplification (or attenuation). Under high Brillouin gain conditions and excluding zero-probability cases, the output SOP of arbitrarily polarized input signals, would tend to converge towards that of maximum SBS gain. In the case of high SBS attenuation the output SOP of an arbitrarily polarized signal would approach the output SOP corresponding to minimum attenuation. It is found that for a wide range of practical pump powers ( < or = 100mW) and for sufficiently long fibers with typical SBS and birefringence parameters, the signal aligned for maximum SBS interaction will enter/emerge from the fiber with its electric field closely tracing the same ellipse in space as that of the pump at the corresponding side of the fiber, albeit with the opposite sense of rotation. The analytic predictions are experimentally demonstrated for both Stokes (amplification) and anti-Stokes (attenuation) signals.

An accurate high-resolution technique for distributed sensing based on frequency-domain Brillouin scattering

A novel high-resolution temperature/strain profile reconstruction approach, based on frequency-domain stimulated Brillouin scattering (SBS) measurements in single-mode optical fibers, is proposed and experimentally demonstrated. The proposed technique is based on an accurate modeling of the SBS interaction in the frequency domain, where even the modulation of the acoustic wave involved in the scattering process is taken into account. The reconstruction algorithm determines the unknown profile by minimizing a cost function representing the error between the measured and the model data. Experimental results show that the proposed technique allows accurate temperature/strain profile reconstructions, even when dealing with modulation frequencies rising above the natural Brillouin gain linewidth. This permits us to achieve a spatial resolution below 1m.

Modeling of stimulated Brillouin scattering in optical fibers with arbitrary radial index profile

Stimulated Brillouin scattering (SBS) is an impairment seen in narrowband transmission over optical telecommunications fiber at high laser power. Accurate modeling is necessary to predict the strength of this phenomenon. One method of SBS control is the optimal design of the fiber radial index of refraction profile. Previous work has been limited to modeling optical fibers with one or more radial step changes in index. Here, the author derives and solves a general set of differential equations that allow the numerical solution of SBS spectral gain for an arbitrary radial index profile. Simulated and measured spectra are compared for several fibers with GeO/sub 2/-doped cores. It is found that radial material displacement plays a significant role in the SBS interaction in fibers with a complex radial index profile.

Measurement of the power distribution during stimulated brillouin scattering with focused gaussian beams

We show both theoretically and experimentally that, when stimulated Brillouin scattering (SBS) is produced using focused Gaussian beams, the spatial extent of the pump depletion region changes significantly with the input power. We also show that the location of the SBS interaction region depends on the ratio of the input power to the SBS threshold power. Only below and near SBS threshold is the interaction region largely located within several confocal parameters of the beam waist, as is often assumed in simplified treatments of the SBS process.

Measurement of the power distribution during stimulated Brillouin scattering with focused Gaussian beams

Abstract We show both theoretically and experimentally that, when stimulated Brillouin scattering (SBS) is produced using focused Gaussian beams, the spatial extent of the pump depletion region changes significantly with the input power. We also show that the location of the SBS interaction region depends on the ratio of the input power to the SBS threshold power. Only below and near SBS threshold is the interaction region largely located within several confocal parameters of the beam waist, as is often assumed in simplified treatments of the SBS process.

Effects of plasma long-wavelength hydrodynamical fluctuations on stimulated Brillouin scattering

Two important effects of long wavelength velocity and density perturbations on stimulated Brillouin scattering (SBS) have been identified: (i) nonlinear interaction of the SBS-produced ion acoustic wave with velocity perturbations provides detuning of the SBS interaction, formation of ion acoustic wave satellites and its additional damping; (ii) harmonic generation of long wavelength density perturbations enhances the spectral density of ion acoustic fluctuations over a wide range of wavevectors, including SBS resonant sound waves. The first effect reduces the magnitude of the SBS gain, and the second one enhances the nonthermal noise level for SBS. Long wavelength ion acoustic perturbations reduce SBS reflectivity more dramatically than short wavelength harmonics produced by the SBS driven nonlinear sound wave.