Stimulated Brillouin Scattering Suppression Research Articles

Modelling and characterization of time–frequency evolution dominated by stimulated Brillouin scattering in narrow-linewidth fiber oscillators

This paper mainly introduces several ways to suppress stimulated Brillouin scattering (SBS) in a narrow-linewidth fiber oscillator and explains how the methods take effect. The experiments are designed to investigate the factors influencing SBS generation in oscillators. Experimental results demonstrate that higher output power, a high reflector (HR) with a narrower linewidth as well as injection of feedback all could help reduce SBS. A theoretical model of SBS-dominated time–frequency evolution in the oscillator is also constructed. Simulations reveal that SBS initially oscillates in the cavity and propagates forward outside the oscillator if the cavity gain is low. However, as the cavity gain increases sufficiently, the SBS would be eliminated through the selection of the output coupler (OC) FBG. Additionally, incorporating external feedback with a wider linewidth and steadier temporal characteristic as a boundary condition of simulation would result in stable ns-scale temporal sequence of the oscillator laser at low power. In fact, not only the stable state of the oscillator with feedback is helpful for SBS suppression but the chaotic state of the oscillator at low power provides a compact and simple structure for multi-order SBS generation which could be useful in multi-parameter sensing, tunable microwave generation and supercontinuum producing.

Theoretical study of filtered and amplified PRBS phase modulation for SBS suppression in a 2.5 kW, 10 GHz monolithic fiber amplifier

We report a theoretical and experimental study on stimulated Brillouin scattering (SBS) suppression in a monolithic fiber amplifier with filtered and amplified pseudo-random binary sequence (PRBS) phase modulation. Theoretically, we use a time-dependent three-wave coupled nonlinear system considering both active fiber and passive fiber to describe the acoustic phonon, laser, and Stokes characteristics in a fiber amplifier. The SBS threshold power after filtered PRBS phase modulation is numerically evaluated to obtain the optimal parameters, and the time-averaged distributions of the counter-pump power, laser power, and Stokes power at different positions along the fiber length of the fiber system are simulated. Also, we established a four-stage fiber amplifier system to verify our theory. The configuration of the fiber amplifier system includes a filtered and amplified PRBS phase-modulated single-frequency fiber laser, a three-stage pre-amplifier, and a counter-pumping main stage, subsequently. 2.5 kW output power with an FWHM linewidth of 9.63 GHz is accomplished by a domestic ytterbium-doped double-clad fiber with core/cladding diameters of 20.2/400 µm. The reflectivity of the main stage is 0.049‰ at the maximum output power, which indicates the proposed architecture is under the SBS threshold. The experiments verify the accuracy of the theoretical model, which provides a reliable reference for evaluating the SBS suppression capability of the high-power narrow-linewidth fiber amplifier phase modulated by the filtered and amplified PRBS signal.

Spectral broadening scheme for high stimulated Brillouin scattering thresholds on multi-harmonic linear frequency modulation

This paper proposes a spectral-broadening scheme for suppressing the stimulated Brillouin scattering (SBS) in fibers using multi-harmonic linear frequency modulation (LFM). The frequency tuning range of the LFM lasers is further expanded by incorporating sinusoidal phase modulation (PM) to achieve more effective spectrum spreading. Furthermore, the ability of multi-harmonic-LFM light sources to suppress SBS is investigated using the effective Brillouin gain spectra (BGS). Meanwhile, the experimental results reveal that at a 3 dB bandwidth of ∼480 MHz, the SBS thresholds of multi-harmonic-LFM light sources were improved by 8.36 times compared to a single-frequency laser, and by appropriately 69% compared to the LFM light sources. Because the spectra of the multi-harmonic-LFM light sources and the multifrequency-sinusoidal-PM light sources have a near-rectangular shape, the capacity of the two schemes to suppress SBS is comparable, which verifies the advantages of the multi-harmonic-LFM light sources in suppressing SBS. Our study also investigated the effects of frequency intervals between each chirp harmonic and the chirp period of multi-harmonic-LFM light sources on the suppression of SBS, which provides guidance for optimizing multi-harmonic-LFM light sources.

Mitigating stimulated Brillouin scattering in multimode fibers with focused output via wavefront shaping.

The key challenge for high-power delivery through optical fibers is overcoming nonlinear optical effects. To keep a smooth output beam, most techniques for mitigating optical nonlinearities are restricted to single-mode fibers. Moving out of the single-mode paradigm, we show experimentally that wavefront-shaping of coherent input light to a highly multimode fiber can increase the power threshold for stimulated Brillouin scattering (SBS) by an order of magnitude, whilst simultaneously controlling the output beam profile. The SBS suppression results from an effective broadening of the Brillouin spectrum under multimode excitation, without broadening of transmitted light. Strongest suppression is achieved with selective mode excitation that gives the broadest Brillouin spectrum. Our method is efficient, robust, and applicable to continuous waves and pulses. This work points toward a promising route for mitigating detrimental nonlinear effects in optical fibers, enabling further power scaling of high-power fiber systems for applications to directed energy, remote sensing, and gravitational-wave detection.

Suppression of stimulated Brillouin and Raman scatterings using an alternating frequency laser and transverse magnetic fields

A novel scheme to suppress both stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) by combining an alternating frequency (AF) laser and a transverse magnetic field is proposed. The AF laser allows the laser frequency to change discretely and alternately over time. The suppression of SBS is significant as long as the AF difference is greater than the linear growth rate of SBS or the alternating time of the laser frequency is shorter than the linear growth time of SBS. However, the AF laser proves ineffective in suppressing SRS, which usually has a much higher linear growth rate than SBS. To remedy that, a transverse magnetic field is included to suppress the SRS instability. The electrons trapped in the electron plasma waves (EPWs) of SRS can be accelerated by the surfatron mechanism in a transverse magnetic field and eventually detrapped. While continuously extracting energy from EPWs, the EPWs are dissipated and the kinetic inflation of SRS is suppressed. The one-dimensional particle-in-cell simulation results show that both SBS and SRS can be effectively suppressed by combining the AF laser with a transverse magnetic field with tens of Tesla. The total reflectivity can be dramatically reduced by more than one order of magnitude. These results provide a potential reference for controlling SBS and SRS under the related parameters of inertial confinement fusion.

Nonlinear spectral compression in high-power narrow-linewidth polarization maintaining fiber amplifiers for SBS suppression.

In this paper, we propose a method for narrowing the spectrum in high-power narrow-linewidth polarization-maintaining (PM) fiber amplifiers and investigate its potential for suppressing the stimulated Brillouin scattering (SBS). In this method, in addition to common phase modulation to suppress SBS, precisely designed amplitude modulation is induced to generate self-phase modulation in a high-power PM fiber amplifier. In this co-modulation way, the spectrum can be gradually compressed along the fiber. Compared to phase modulation alone or fiber-Bragg-gratings (FBGs) based narrow-linewidth fiber oscillator schemes, in which the spectrum remains the same or broadens, this scheme can achieve a higher SBS threshold for the same output spectral linewidth. Experiments on a ∼ 3 kW peak power quasi-continuous wave (QCW) fiber amplifier show that the co-modulation scheme can compress the spectrum from 0.25 nm to 0.084 nm as output peak power increases from 13 W to 3.2 kW and enhances the SBS threshold by ∼1.7 times compared to traditional FBGs-based fiber oscillator schemes, and by ∼1.4 times compared to common phase modulation schemes. This co-modulation scheme has the potential for mitigating SBS in high-power fiber amplifiers.

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.

Suppression of stimulated Brillouin scattering by multicolor alternating-polarization bundle light in inertial confinement fusion

This study proposes a novel method to mitigate stimulated Brillouin scattering (SBS) using multicolor alternating-polarization bundle light. The bundle light combines multiwavelength, spike trains of uneven duration and delay for a single beam to multicolor alternating polarization for bundle beams. SBS suppression is verified using a three-dimensional large-scale laser plasma code. The numerical results show that the SBS reflectivity can be decreased by nearly two orders in low density plasma. The proposed method can extend the repetition time of a single beam from several picoseconds to tens of picoseconds. Moreover, it has potential applications in inertial confinement fusion research.

Spectral broadening scheme for suppressing SBS effects based on time-domain optimized chirp-like signals.

We propose a novel (to our knowledge) driving scheme to suppress the stimulated Brillouin scattering (SBS) effect in master oscillator power amplification (MOPA) systems based on an external high-order phase modulation. Since seed sources with the linear chirp can uniformly broaden the SBS gain spectrum with a high SBS threshold, a chirp-like signal was designed by applying further editing and processing to the piecewise parabolic signal. Compared with the traditional piecewise parabolic signal, the chirp-like signal has similar linear chirp characteristics and can reduce the driving power and sampling rate requirements, enabling more efficient spectral spreading. The SBS threshold model is constructed theoretically based on the three-wave coupling equation. The spectrum modulated by the chirp-like signal is compared with the flat-top and Gaussian spectra in terms of the SBS threshold and the bandwidth-distribution normalized threshold, and a considerable improvement is demonstrated. Meanwhile, the experimental validation is carried out in a watt-class amplifier based on the MOPA structure. At a 3 dB bandwidth of ∼10 GHz, the SBS threshold of the seed source modulated by the chirp-like signal is improved by 35% compared to the flat-top spectrum and 18% compared to the Gaussian spectrum, respectively, and the normalized threshold is also the highest among them. Our study shows that the SBS suppression effect is not only related to the power distribution of the spectrum but also can be improved by the time domain design, which provides a new idea for analyzing and improving the SBS threshold of narrow-linewidth fiber lasers.

Design a remote sensing of multi-BOTDR fiber optic sensors for fuel pipeline monitoring

The necessity for environmental protection has led to the development of different sensors for monitoring fuel pipelines to avoid explosion, leakage, and corrosion. Distributed optical fiber sensors-based Brillouin scattering have become progressively common because of their novel feature of simultaneously determining temperature, vibration, and strain. The stimulated Brillouin scattering (SBS) threshold power, sensing distance, single-mode fiber cable length, and sensing accuracy are serious parameters to the implementation of Brillouin optical time-domain reflectometry (BOTDR) sensor. In this study, remote sensing of multi-BOTDR fiber optic sensors for fuel pipeline monitoring is designed and presented. The results of simulations and theoretical explainations on suppressing fiber optic nonlinearity are analyzed. Results show that an SBS suppression via 16 dB is required to attain the maximum multi-remote sensing distance of up to 20 km for each network and a perfect frequency shift of 11 GHz. An open eye pattern corresponds to minimal signal distortion with a high Q-factor of 22 for the developed sensor. The highest sensing accuracy of 0.3318 %/K indicating power change (%) is observed from 290–300 K at 193.1 THz operating frequency.

Impact of finite bandwidth on PRBS-modulated optical spectra.

Applying a pseudo-random binary sequence (PRBS) to phase modulators is a recent development in the broadening of optical spectra of laser sources to defeat stimulated Brillouin scattering (SBS). The theoretical underpinning of this method relies on alternating the phase of the optical signal between its native value and an out-of-phase value (i.e., imposing a binary phase shift of 0 or π in a pseudo random manner) to prevent coherent buildup of the SBS acoustic grating. In real physical systems, realizing such a binary shift is impossible due to the finite response times of the electronics and electro-optic components. The influence of these effects is investigated in this work, specifically the finite bandwidth of the electronic PRBS generator and the frequency-dependent response of the phase modulator, on the resultant temporal waveforms of the PRBS signal and its RF optical spectra. It is found that the optimal SBS suppression in real systems is achieved when the phase modulator is driven at a voltage beyond V π, even though driving at V π has been deemed as ideal. Moreover, both the SBS suppression and spectral broadening are always weaker than what is predicted by analytical results since any degradation of the sharp edges of the PRBS signal result in loss of high-frequency content in the RF spectrum of the PRBS signal. Lastly, it is noticed that the typical ways of measuring spectral width are not relevant when applied to the complex PRBS RF optical spectrum. An 'equivalent spectral width' is defined to accurately quantify the correlation between spectral width an SBS suppression.

Suppression of Stimulated Brillouin Scattering in Laser Plasma under Conditions of Multifrequency Target Irradiation

Suppression of Stimulated Brillouin Scattering in Laser Plasma under Conditions of Multifrequency Target Irradiation

Optimization of filter frequency ratio in pseudo random modulated fiber amplifiers for maximally suppressing stimulated Brillouin scattering

A filtered pseudorandom binary sequence (PRBS) phase modulation scheme to broaden the linewidth of a fiber laser has previously been shown to simultaneously enhance stimulated Brillouin scattering (SBS) suppression and spectral brightness relative to a fiber laser under an unfiltered modulation scheme. We examined in detail the unwanted distortions to the broadened spectrum of an optical signal due to the low-pass filtering of the PRBS modulation signal, and show how one can navigate these deleterious effects for an optimal filtering strategy for SBS suppression. Through numerical simulations of the filtering effects on the optical spectrum, we determined the optimal ratios of the filter bandwidth to the PRBS modulation frequency to be 0.63, 0.56, 0.50, and 0.44 for bit numbers k = 7 , 8, 9, 10, respectively. A cursory experimental study for the k = 7 case verified the approach, which provides a more precise filtering-to-modulation frequency ratio for improved SBS suppression than the more general guideline of filtering at approximately half the modulation frequency reported in the literature. Applying this optimization strategy with a filtered 4.6 GHz PRBS scheme, we demonstrated a 1 kW, M 2 ∼ 1.27 , all-fiberized narrow linewidth amplifier.

Effective Brillouin Gain Spectra Broadening for SBS Suppression Based on Pseudo Random Bit Sequence Phase Modulation in Fiber System

We describe stimulated Brillouin scattering process in a fiber system with light field phase modulated by lowpass filtered and amplified pseudo random bit sequence (PRBS) using localized nonfluctuating source model. The effective Brillouin gain spectra and the ratio of normalized Stimulated Brillouin Scattering (SBS) threshold to RMS optical linewidth is used to characterize the SBS suppression capability of the PRBS phase modulation. The normalized effective Brillouin gain spectra for different modulation parameters are calculated to exhibit the Brillouin gain broadening process. Based on the theoretical model, we find that the SBS threshold reaches a maximum for the ratio of cutoff frequency to clock rate <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r_c</i> = 0.53 and RMS modulation depth <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k_rms</i> = 0.55 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</i> for PRBS9. An effective Brillouin gain spectrum measurement setup is established, and the theoretical predictions are verified by the experiment.

Enhancing the Efficacy of Noise Modulation for SBS Suppression in High Power, Narrow Linewidth Fiber Lasers by the Incorporation of Sinusoidal Modulation

Power scaling of continuous-wave, narrow-linewidth fiber lasers are limited by the onset of stimulated Brillouin scattering (SBS) which manifests at threshold as potentially damaging, peak power, backward propagating pulses. While several SBS mitigation techniques exist, optical linewidth broadening through external phase modulation using white noise is predominantly used for its simplicity. But the broadened line-shape is non-ideal and has a slow roll-off resulting in increased SBS seeding from reflections and hence limited power scaling. With better line-shape control, higher output powers can be achieved. In this work, we propose a simple and effective technique to improve the SBS suppression of these high power, narrow linewidth, noise broadened systems, by the addition of a sinusoidal modulation in cascade. The dual modulation results in an improved line-shape with a relatively flat topped-center and fast roll-off when compared to pure noise modulation at similar root-mean-square (RMS) linewidths. Using a multi-stage fully polarization maintaining, kW-class amplifier, we demonstrate >2.3X enhancement in SBS limited output power when compared to the pure noise modulated case at ∼7.3 GHz and >1.029 kW output power at ∼10.4 GHz RMS linewidth.

Suppression of the Stimulated Brillouin and Raman Scattering in Actively Q‐switched Fiber Lasers through Temporal Pulse Shaping

AbstractDespite the wide range of applications and versatility, actively Q‐switched fiber lasers (AQS‐FLs) suffer from the multi‐peak phenomenon or stepwise pulse generation under certain conditions. As such, the advantage of the stepwise tendency in AQS‐FL to modify the pulse shape and create a temporal profile that minimizes the power of stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) Stokes components is taken. Accordingly a multi‐objective genetic algorithm (GA) and a Monte‐Carlo method is developed to solve the improved model for SBS. Additionally, the suppression of SRS is predicted and examined successfully. The proposed method is deployed for the simulation of the 5 and 10 kW peak and 10 ns wide pulses on a 4 m long, single‐mode, ytterbium‐doped double clad fiber under the narrow linewidth of 0.05 nm with promising suppression of SBS and SRS effects. To the best of the authors' knowledge this is the first time that such a pulse‐shaping strategy is demonstrated for the SBS or SRS suppression. Furthermore reaching a 10 kW peak on single mode fibers was previously only possible via custom made fibers. Our novel method offers the possibility of optimizing and improving the utilization of different fiber types in AQS‐FL systems.

Stimulated Brillouin scattering mitigation using optimized phase modulation waveforms in high power narrow linewidth Yb-doped fiber amplifiers.

We demonstrate the mitigation of stimulated Brillouin scattering (SBS) in a double-clad single mode Yb-doped optical fiber amplifier through external phase modulation of narrow linewidth laser radiation using optimized periodic waveforms from an arbitrary waveform generator. Such optimized phase modulation waveforms are obtained through a multi-objective Pareto optimization based on a comprehensive model for SBS in high power narrow linewidth fiber amplifiers using Brillouin parameters determined from controlled measurements. The ability of our approach to mitigate SBS is tested experimentally as a function of RMS linewidth of the modulated optical radiation, and we measure an enhancement in SBS threshold with respect to optical linewidth of ∼ 10 GHz-1. Furthermore, we discuss the dependence of the SBS threshold enhancement on key parameters such as the amplifier length and the period of the optimized waveforms. Through simulations we find that waveforms of sufficiently long periods and optimized for a relatively long fiber (10 m) are effective for SBS suppression for shorter fibers as well. We also investigate the effect of increase in the bandwidth and amplitude of the modulation waveform on the SBS threshold enhancement observed at higher optical linewidth.

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.

Optical fibre with an offset core for SBS suppression

To raise the stimulated Brillouin scattering (SBS) threshold, we have fabricated an optical fibre having a core heavily doped with germanium oxide and displaced from the fibre axis. A method has been proposed for broadening the SBS gain spectrum of the fibre by winding it onto a small-diameter spool, and a broadening of the spectrum by 5.4 dB has been demonstrated, which indicates an increase in SBS threshold by 4.1 dB.

SBS Gain Suppression in a Passive Single-Mode Optical Fiber by the Multi-Mode Acoustic Waveguide Design

A novel method of stimulated Brillouin scattering (SBS) gain suppression in passive single-mode optical fibers is proposed and experimentally verified. The effect is achieved due to special propagation conditions created for a number of the core-bounded acoustic modes effectively involved in the SBS, such that its gain becomes approximately evenly distributed over a wide spectral range, thereby proportionally reducing its maximum. To verify and investigate this method in depth, single-mode large-mode-area (LMA) fibers with correspondingly optimized acoustic index profiles are fabricated, experimentally tested, and analyzed. Uneven complex doping of the core with two additives, P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> and F, forms a high-contrast gradient acoustic index profile while maintaining a quasi-uniform optical refractive index profile. With a contrast of P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> of more than 6 mol.%, we achieved 8 dB of SBS suppression over conventional uniformly doped LMA fibers. Moreover, it is shown that more than 10 dB of SBS gain suppression can be achieved if the optimal dopant distribution is realized.