Stimulated Brillouin Scattering Threshold Research Articles

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.

Stimulated Brillouin Scattering Threshold in Presence of Modulation Instability for Optical Pulse in Long Optical Fiber

A theoretical and experimental study on the stimulated Brillouin scattering (SBS) threshold for optical pulse in the presence of modulation instability (MI) in long optical fiber is presented. The effects of MI on the SBS gain and threshold are analyzed based on the coupled-wave equation. An analytic expression is obtained to calculate the SBS threshold in the presence of MI. Numerical simulation is conducted to study the effects of the repetition rate and amplified spontaneous emission (ASE) noise on the SBS threshold for optical pulses. An experiment is conducted, and the results agree well with the theoretical analysis. The results clearly reveal how MI affects the Brillouin gain and SBS threshold of optical pulses in long optical fiber. The SBS threshold is affected by the ASE noise level and the repetition rate of the optical pulse. The study is helpful for the power evaluation of interferometric fiber sensing systems and optical power transmission systems.

Temporal characteristics of the light output from optical fibers with PRBS-modulated laser injection.

Pseudo-random binary sequence (PRBS) phase modulation is an effective method to promote the stimulated Brillouin scattering (SBS) threshold of high-power narrow linewidth fiber lasers. Most studies focus on the subjects of PRBS pattern selection, filter parameter optimization, threshold promotion, etc. In this paper, the temporal characteristics of the light output from optical fibers with PRBS modulated laser injection are investigated theoretically. We found that serious temporal fluctuations, i.e., high intensity pulses, existed in the output laser and backward Stokes light, even though the time-averaged backward Stokes power is only 1% of the input laser power. Based on the transient three wave coupling equations, through exploring the influence of clock rate and PRBS pattern, we demonstrate that this phenomenon arises from the long dwell times in PRBS patterns. Finally, modified PRBS patterns are proposed to significantly eliminate the intense pulses without decreasing the original SBS threshold.

Nonlinear Distortion by Stimulated Brillouin Scattering in Kramers-Kronig Receiver Based Optical Transmission.

Nonlinear distortion for single-sideband (SSB) signals will significantly reduce the performance of Kramers–Kronig (KK) receiver-based optical transmission. In this work, we present a proof-of-concept study of stimulated Brillouin scattering (SBS)-induced nonlinear distortion for 10 Gbaud and 28 Gbaud SSB QAM16 transmission over 80 km standard single mode fiber (SSMF) based on a KK receiver. Significantly reduced bit error rate (BER) has been experimentally observed due to the SBS and the threshold of SBS at about 7 dBm is detected for such an 80 km SSMF link. With left sideband (LSB) modulation of SSB, together with optical filtering, reduced SBS nonlinear distortion has been achieved with ~2 dB power tolerance improvement. The results reveal an important issue of SBS-induced nonlinear distortion, which would be of great significance for KK receiver-based optical transmission applications.

Equations describing the direct relation between laser phase and Stokes light for SBS pumped by phase-modulated laser in optical fibers

Phase modulation is one of the most important methods to increase the threshold of the stimulated Brillouin scattering (SBS) in optical fibers. The mainstream theoretical model used to predict the SBS threshold is the transient three-wave coupling model. However, it is a general model and does not establish a straight relation between the laser phase and the Stokes light. In this paper, undepleted pump approximation is applied to the transient three-wave coupling model. Equations describing the evolution of the Stokes light in both time domain and frequency domain are derived. They establish a direct relation between the laser phase and Stokes light. All terms in the equations have clear physical interpretations. The equations are validated numerically by comparing with the original coupling model as well. It is found that the results match well when the Stokes average reflectivity is not greater than 1% which is usually used to define the SBS threshold.

Influence of injected signal polarization on SBS, SRS, spectral broadening, and self-pulsing properties in high-power fiber amplifier

In this paper, we construct a narrow-linewidth amplifier based on polarization-maintaining (PM) fiber, which can be injected by a fiber oscillator seed or white noise signal phase-modulated seed. The stimulated Brillouin scattering (SBS), SRS, spectral broadening, self-pulsing properties of the fiber amplifier when its’ injected signal is 0° or 45° relative to the slow axes of the PM fiber are studied. It is indicated that when the injected signal is launched at 45° relative to the slow axes, the nonlinear effects are significantly suppressed compared with the case of the injected signal is launched parallel to the slow axis. The spectral broadening rate is reduced from 0.14 pm W−1 to 0.0747 pm W−1, the ratio of Raman to laser peak intensity is increased from 20 dB to 30 dB, the SBS threshold is increased by >24%, and the self-pulsing threshold is increased by 28.7%. It is worth noting that the nonlinear properties corresponding to the injected polarization state in our experiment are quite different from some earlier works.

694 W sub-GHz polarization-maintained tapered fiber amplifier based on spectral and pump wavelength optimization.

The comprehensive suppression of the stimulated Brillouin scattering (SBS) and transverse mode instability (TMI) is a critical issue for the power scaling of fiber laser with sub-GHz spectral linewidth. In this manuscript, a narrow linewidth and polarization-maintained (PM) fiber amplifier based on tapered Yb-doped fiber (T-YDF) is established, and the effects of spectral linewidth, spectral shape and pump wavelength on the SBS and/or TMI thresholds are investigated. Up to 694 W polarization-maintained fiber laser with just ∼790 MHz linewidth is obtained by combining the advantages of tapered Yb-doped fiber, near-rectangular spectral injection and 915 nm pump manner. This work could provide a well reference solution for the realization of high-power ultra-narrow linewidth fiber lasers.

Effect of the recurring random signal waveform on SBS and self-pulsing in a phase-modulated narrow-linewidth linearly polarized fiber amplifier

A high power, narrow linewidth, linearly polarized all-fiber amplifier with recurring random signal (RRS) or white noise signal (WNS) phase modulation is structured. The influence of the RRS waveforms on SBS and self-pulsing characteristics is experimentally studied. We find that choosing an appropriate RRS waveform can effectively improve the SBS and self-pulsing threshold. For that, the RRS phase modulation can provide larger than 1.6 times limited power compared to WNS phase modulation at the same linewidth. Meanwhile, we first demonstrate that the output power of a fiber amplifier based on RRS phase modulation is limited by the continuous wave (CW) SBS, while it is limited by the self-pulsing effect in the case of WNS phase modulation. The influence of random signal waveforms on SBS is analyzed theoretically. The results prove that the Stokes peak intensity is closely related to the fine structure of the modulated spectrum determined by the signal waveforms, and which cannot be characterized only from the spectral linewidth. The theoretical analysis is consistent with the experimental results. Finally, we obtain a 1.23 kW narrow linewidth linearly polarized laser output with a linewidth of ∼4 GHz based on RRS phase modulation.

Modeling the method for determining the stimulated brillouin scattering threshold in a single-mode optical fiber

Nonlinear optical effects in optical waveguides play an important role in the development of fiber and integrated optics systems for optical communication and information processing. On the one hand, nonlinear effects impose restrictions on the radiation power that can be transmitted through an optical fiber or light guide. In this paper, the problem of the occurrence of the phenomenon from the stimulated Brillouin scattering (SBS) effect is investigated using two optical sources of rays in a single-mode optical fiber at joint waves of 1310 nm and 1550 nm. Due to the fact that in all trunk fiber-optic lines, the intensity and energy of input signals is limited due to the influence of SBS, methods are currently being sought to reduce the influence of this phenomenon. It was found that the energy of the input beam in the combined propagation of the compound did not reach the value of the threshold of SBS in the values of 25 dBm and 27 dBm due to the discrepancy between the experimental results and the results of the model. The SBS effect was not observed when the threshold threshold of 15 dBm and 27 dBm – SBS was reached when combining a dual beam along a single optical fiber in one direction. As a result, by double integration, the value of the SBS threshold was raised, and the direction for future scientific research was determined. If the possibility of increasing the threshold of SBS is proved, then increasing the distance of amplifiers in the main networks, respectively, its economic effect increases. In addition, it can be noted that there are no scientific papers devoted to the study of the effects of optical nonlinear effects by combining and distributing these two compounds along a single optical fiber. This article discusses the issues of improving the capacity and determination of the threshold for stimulated Brillouin scattering. To increase the power threshold is invited to consider the dependence of the phase modulation frequency of the spectral width of the laser radiation. In addition, the fact that the SBS threshold did not reach the value at an energy of 27 DB in the case of the double-beam distribution can be proved by discrepancies in comparison with the results of experimental studies and the results of the model that determines the SBS effect

A Bidirectional RoF System for the Multi-Band Signal with Mitigation of the Nonlinear Effects

A bidirectional full duplex radio over fiber (RoF) system is proposed for multi-band signal generation and transmission. A multi-band signal is generated at the central station (CS), which includes 10 GHz baseband, 40, and 70 GHz millimeter-wave (mm-wave) by optical frequency multiplication technique of radio frequency (RF) local oscillator (LO) with frequency of 15 GHz. The simultaneous transmission of the 16-QAM OFDM-based multi-band downlink signal and 4-QAM uplink signal over a single fiber is demonstrated. The carrier for uplink transmission is extracted from the unmodulated downlink signal with tunable FBG which is modulated by OFDM-based 4-QAM uplink data obtained from the user end via an antenna. The frequency reuse from the downlink signal makes the base station (BS) free from the laser source. At the user end, the multi-band signals for multi-serve application are obtained by heterodyne beating of multiple bands. An increase in launch power induces fiber dispersion and non-linearities like self-phase modulation (SPM) and cross phase modulation (XPM), which depends on the nonlinear refractive index of the fiber. Different industrial standard fibers with common specifications and modulation format along with co-shared components are deployed, and system performance is analyzed. The stimulated Brillouin scattering (SBS) can be mitigated by optimizing the launch power within the SBS threshold. The nonlinear effects are minimized at the system level without the requirement of additional digital signal processor (DSP) . The theoretical and simulation analysis shows that the designed bidirectional RoF link exhibits an improved system performance for bidirectional transmission with negligible power penalty.

Investigation of Sbs Threshold in High-Power Narrow-Linewidth Fiber Amplifiers in Presence of Weak End Reflections

Investigation of Sbs Threshold in High-Power Narrow-Linewidth Fiber Amplifiers in Presence of Weak End Reflections

Enhanced Raman Distributed Temperature Sensor Using a High Raman Gain Fiber

We have designed and fabricated a single-mode Raman gain fiber (RGF) and use it to experimentally demonstrate a Raman distributed temperature sensor (RDTS) for high-precision temperature measurement with low power consumption. We have performed simulation to analyze the impact of fluorine-doped inner cladding on the control of the acoustic field and suppression of stimulated Brillouin scattering (SBS), showing that the SBS threshold can be improved by 37dB at 6wt% fluorine-doping concentration. Taking advantage of high Raman gain, a maximum signal-to-noise ratio (SNR) improvement of 6dB has been realized by using RGF under the same pump power as by using single-mode fiber (SMF), and the temperature uncertainty at the fiber end is enhanced from 0.9°C to 0.5°C. To achieve comparable sensing accuracy as that of SMF-based RDTS, our RGF-based RDTS only consumes half of the power, showing good energy efficiency. The magnitude of SNR and temperature uncertainty enhancement is limited by the relatively large fiber attenuation, but it can be further improved through optimization of the fiber design with high fluorine-doping concentration and graded index structure.

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.

A novel bidirectional RoF link with compensated SBS and RB for 16-QAM OFDM based mm-wave downlink and uplink vector signal generation and transmission on a single fiber

A novel bidirectional RoF link with compensated SBS and RB for 60 GHz OFDM-based 16-QAM downlink and uplink vector signal generation and co-propagation over a single fiber is proposed. At the central station (CS), 60 GHz optical mm-wave is generated and modulated in the optical single-sideband modulation (OSSB) technique by the 16-QAM OFDM downlink vector signal. At the base station (BS), the 60 GHz downlink signal is detected by a high-speed photodetector (PD). The 60 GHz electrical mm-wave signal is transmitted to the user end by an antenna. The uplink carrier is extracted from the unmodulated sideband of the mm-wave downlink vector signal. This reduces the complexity of BS and makes it free from the laser source. The 16-QAM OFDM-based mm-wave uplink signal is received from the user end through an antenna and down-converted to a 10 GHz uplink vector signal. The uplink signal modulates the extracted blank carrier in the OSSB technique. The uplink and downlink vector signals are co-propagated through the standard single bidirectional fiber. Although the carrier reuse over a single fiber reduces the fiber capacity by half, it generates non-linear backscattering such as Rayleigh Backscattering (RB) and Stimulated Brillouin Scattering (SBS). A theoretical analysis is made on the backscattered power and its effect on the fiber transmission. The compensation of SBS by maintaining the SBS threshold and mitigation of RB is carried out by appropriate modulation technique. In addition, downlink and uplink signal with different data rate is transmitted through the compensated bidirectional RoF link, and the transmission performance is analyzed. Based on the analysis, the theoretical model for simultaneous downlink and uplink signal transmission over a single fiber is proposed and verified by the software simulation. The compensated bidirectional RoF link shows up a better performance up to 75 km with negligible power penalty.

Limits of broadband fiber optic parametric devices due to stimulated Brillouin scattering

We experimentally find a practical stimulated Brillouin scattering (SBS) threshold for broadband high-performance fiber optical parametric devices relying on dispersion-stable GeO2-doped silica highly nonlinear fibers. We demonstrate that SBS limits the nonlinear phase shift in such fibers to ~0.3 rad per pump unless the SBS is mitigated in some way. We consequently derive corresponding limits on signal gain and conversion efficiency and find the required SBS mitigation factor for a range of fiber optic parametric devices’ applications. Finally, we examine the level of SBS mitigation using air gaps and fiber tapers for implementation in polarization-insensitive fiber optic parametric devices employing bidirectional loops. We observe that an air gap or fiber taper are not very efficient for SBS mitigation as they provided an increase in SBS threshold up to 0.7 dB attributed primarily to their excess loss.

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.

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.

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.

Suppression of stimulated Brillouin scattering in optical fibers by tilted fiber Bragg gratings.

Stimulated Brillouin scattering (SBS) has significant influence on optical fiber communication (OFC), optical fiber sensing (OFS), and narrow-linewidth fiber laser (NLFL) systems. How to effectively suppress it has always been a challenge. In this Letter, we propose and demonstrate a versatile solution, for the first time, to the best of our knowledge, by using tilted fiber Bragg gratings (TFBGs). A specially designed and fabricated TFBG can be used as an ultra-narrow spectral filter, precisely matching with the operation laser wavelength and the tiny frequency shift due to SBS. Experimental results show that the backward Stokes can be strongly rejected with a filtering ratio of >10dB; meanwhile, an obvious increasing of SBS threshold is observed with a maximum value of 1.7 times that without the TFBG, which enhances the effective transmission power by 33%. The operation stability of this method also is validated. This work opens new opportunities for SBS suppression in OFC, OFS, and high-power NLFL systems.