Stimulated Brillouin Scattering Threshold Research Articles

Method for calculating SBS threshold in optical fibres

It is shown that the widely used method for calculating the SBS threshold in optical fibres yields erroneous results in a number of practically important cases. In particular, not all acoustic-antiwaveguide optical fibres demonstrate an increase in the SBS threshold. For the SBS threshold to increase, it is necessary to provide fast disruption of the acoustic wavefront due to refraction.

SBS threshold measurements and acoustic beam propagation modeling in guiding and anti-guiding single mode optical fibers

A 4.3 dB stimulated Brillouin scattering (SBS) threshold suppression is measured in a passive Al-doped acoustically anti-guiding single mode optical fiber relative to that of a Ge-doped acoustically guiding single mode optical fiber. Stimulated scattering is generated by the electrostrictive acoustic wave generated in the fiber core. This acoustic excitation has a decay length L(d) related to the sound absorption decay length L(abs) and the acoustic waveguide decay length L(wg) by: L(d)(-1)= L(abs)(-1)+ L(wg)(-1). The acoustic waveguide decay length L(wg) is associated with the diffraction, refraction and reflection of the acoustic wave in the elastically inhomogeneous optical fiber cores. The SBS gain is proportional to the net acoustic decay length L(d) and the relative SBS suppression is proportional to the ratio of the net decay lengths of the Al and Ge doped cores (L(Al)/ L(Ge)). An acoustic beam propagation model is used to calculate the evolution of the complex acoustic excitations in the optical cores and determine the acoustic wave decay lengths L(wg). Model predictions for the relative SBS suppression for the two fibers are in good agreement with experimental values obtained from Stokes power and optical heterodyne linewidth measurements.

Large delay tunable slow-light based on high-gain stimulated-Brillouin-scattering amplification in optical fibers

Tunable stimulated-Brillouin-scattering (SBS)-based slow-light in optical fibers has potential applications in optical buffering in the future all-optical router commutation systems. However, due to the low SBS threshold and relatively high realistic signal power, the gain in the usual SBS systems is limited at ∼30 dB. This paper presents a high-gain SBS scheme to realize large delay slow-light, which benefits from avoiding the depletion of the pump power in a short fiber as SBS media. The experiment demonstrates that, up to 50 dB non-saturated gain has been observed in the single-stage 591.8 m fiber SBS amplification. The slow-light delay can be obtained 52 ns, and the fractional delay can exceed 1.

On the pre-amplified linear cavity multi-wavelength Brillioun-erbium fiber laser with low SBS threshold highly nonlinear photonic crystal fiber

An efficient multi-wavelength Brillouin-erbium fiber laser utilizing low stimulated Brillouin scattering threshold highly nonlinear photonic crystal fiber (PCF) is presented. A Fabry-Perot pre-amplified Brillouin pump configuration was used which consists of 100 m PCF and 10 m erbium doped fiber (EDF). Up to 28 output channels with SNR of more than 20 dB could be generated with modest pump power.

Stimulated Brillouin scattering in single-frequency fiber amplifiers with delivery fibers

For an all-fiber-based single-frequency amplifier, a passive delivery fiber carrying the amplified signal out of the pump combiner may induce unwanted stimulated Brillouin scattering (SBS) and affect amplifier performance. To evaluate the impact, a rate-equation based model has been adopted to study SBS characteristics in the fiber amplifier with a delivery fiber. The model allows independent inputs of many critical parameters, such as the delivery fiber length, temperature distribution, core size, Brillouin gain coefficient, as well as the gain fiber length. The SBS thresholds of the amplifiers under various conditions are computed. The results indicate that the delivery fiber lengths, core diameter, and Brillouin gain coefficient are the most influential parameters which make great impacts on amplifier performance. In addition, the gain fiber length also plays a role on SBS thresholds, but its impact is modest and normally less than a few percent. For an amplifier with a 25 microm gain fiber, to achieve more than 400 W output, the delivery fiber should have a Brillouin gain coefficient <1 x 10(-11) m/W with a length of less than 2 m and a core diameter of greater than 35 microm.

Transient Stimulated Brillouin Scattering in Multimode As2S3 Glass Fiber

We studied backward stimulated Brillouin scattering (SBS) in a multimode As2S3 glass fiber using an 11.5 ns Nd:YAG laser operated at 1.064 µm. The threshold pump energy for SBS was 35 µJ for a 2.3-m-long chalcogenide fiber with a 360-µm-diameter core. The generated SBS was not steady-state but transient since the phonon lifetime of the glass is 11.5 ns. We estimated the SBS exponential gain to be M = 25.5 and the ratio of transient Brillouin-gain coefficient to steady-state Brillouin-gain coefficient to be gBtran/gB0 = 0.271 for 11.5 ns pump pulses from the measured SBS threshold power and the known value of steady-state gain gB0 (= 3.9 ×10-9 m/W) in a single-mode As2S3 fiber.

Investigation of optical limiting based on the combination of stimulated Brillouin scattering and carbon nanotube/HT-270 suspension

AbstractIn order to improve the optical limiting performance based on stimulated Brillouin scattering (SBS), a combination of SBS and carbon nanotube/HT-270 suspension for optical limiting is proposed in this paper. The dependence of output energy of optical limiting based on this hybrid approach on the pump energy is numerically simulated, and validated in Continuum's Nd: YAG seed-injected laser. The results indicate that the output energy based on this hybrid approach shows much better performance compared with that based on single SBS. In this approach, the SBS threshold can be controlled by adjusting the gain coefficient of medium and focal length of the lens, and the threshold of suspension can be controlled by altering the concentration and focal length of the lens, therefore, this hybrid approach to realize optical limiting has great potential in practical application.

High SBS-threshold single-mode single-frequency monolithic pulsed fiber laser in the C-band

We report a high SBS-threshold, single-frequency, single-mode, polarization maintaining (PM) monolithic pulsed fiber laser source in master oscillator and power amplifier (MOPA) configuration that can operate over the C-band. In order to achieve a narrow transform-limited linewidth for pulses longer than 100 ns, we use a single-frequency Q-switched fiber laser seed, which itself can be seamlessly tuned up to 1.24 micros. The Q-switched pulses are amplified in the power amplifier stage of MOPA using a high SBS threshold single-mode PM large core highly Er/Yb co-doped phosphate glass fiber (LC-EYPhF). This seed and amplifier combination represents the first monolithic, all-fiber implementation of a single-frequency pulsed laser with the highest pulse energy of 54 microJ and peak power of 332 W for 153-ns pulses at 1538 nm.

Improved output energy characteristic of optical limiting based on double stimulated Brillouin scattering

In order to improve the optical limiting performance based on stimulated Brillouin scattering (SBS), a method based on double SBS is proposed in this paper. The dependence of the output energy of optical limiting based on double SBS on the pump energy is numerically simulated, and experimentally validated with an Nd:YAG seed-injected laser. The results indicate that only the first SBS optical limiting works in the case of low pump energy. However, as the pump energy increases, the second SBS process can be activated if the transmitted power of the first SBS is still above the SBS threshold. Therefore, the output energy characteristic of optical limiting based on double SBS is much better than that based on single SBS. Owing to the sub-nanosecond response time and a high power threshold, the SBS optical limiting can provide protection in high power laser systems.

The Effect of Phase Conjugation Fidelity on Stimulated Brillouin Scattering Threshold

We present the effect of phase conjugation fidelity on stimulated Brillouin scattering (SBS) threshold in a multimode optical fiber. As fiber lengths decrease and phase conjugation fidelity increases, SBS threshold is observed to increase at a slower rate than that predicted by equations derived for single-mode fibers with an inverse dependence on fiber length. The deviation in threshold from standard models is shown to have a direct relationship to the phase conjugation fidelity.

Effects of Nonlinear Stimulated Brillouin Scattering on Performance Analysis of an Optical CDMA Transmission System

We conducted a computer simulation study to assess the effects of Stimulated Brillouin Scattering (SBS) on performance analysis of optical CDMA based on Enhanced Double Weight (EDW) code. It has been observed that SBS causes backward scattering of transmit signal and this in turn causes a reduction of received power and limitation improvement for the bit error rate. The maximum transmission distance is evaluated for optical CDMA based on BER equal to 10 -9 and transmitted power into fiber below SBS threshold power. Also in this paper, we demonstrate how the speed of data transfer, directly modulated technique and effective core area of optical fiber transmission influence the Brillouin threshold power in optical CDMA system.

High-Power $\hbox{Yb}^{{\bm 3}{\bm +}}$-Doped Phosphate Fiber Amplifier

We report on the development of novel high-power light sources utilizing a Yb3+-doped phosphate fiber as the gain element. This host presents several key benefits over silica, particularly much higher Yb2 O3 concentrations (up to 26 wt%), a 50% weaker stimulated Brillouin scattering (SBS) gain cross section, and the absence of observable photodarkening even at high population inversion. These properties result in a greatly increased SBS threshold compared to silica fibers, and therefore, potentially much higher output powers out of either a multimode large mode area or a single-mode fiber, which means in the latter case a higher beam quality. To quantify these predictions, we show through numerical simulations that double-clad phosphate fibers should produce as much as ~ 700 W of single-frequency output power in a step index, single-mode core. As a step in this direction, we report a short phosphate fiber amplifier doped with 12 wt% Yb2 O3 that emits 16 W of single-frequency single-mode output. We also describe a single-mode phosphate fiber laser with a maximum output power of 57 W. The laser slope efficiency is currently limited by the fairly high fiber loss ( ~ 3 dB/m). Measurements indicate that 77% of this loss originates from impurity absorption, and the rest from scattering.

Polarization dependence of stimulated Brillouin scattering in small-core photonic crystal fibers

Stimulated Brillouin scattering (SBS) generated from noise in small-core photonic crystal fibers (PCFs) exhibits a strong dependence on pump polarization. The polarization dependence of two small-core PCFs is investigated, yielding unexpected results. Both fibers exhibit birefringence resulting in a 90° polarization dependence for the SBS and a 3 dB difference in the SBS threshold between polarizations corresponding to minimum and maximum SBS, respectively. Surprisingly, the transmission of the smaller-core fiber also exhibits a 180° polarization dependence at lower powers due to polarization-dependent loss.

Spectrum evolutions of spontaneous and pump-depleted stimulated Brillouin scatterings in liquid media

A theoretical model for calculating spontaneous and stimulated Brillouin scattering(SBS) spectra is described. An empirical formula for the Stokes output spectral linewidth, a function of spontaneous Brillouin linewidth and the exponential gain coefficient, is obtained by the calculated data fitting. The formula holds true for two cases involving pump undepletion and depletion. The lineshape change from spontaneous to highly pump-depleted SBS spectra is also investigated. The result shows that for the pump power below the SBS threshold, the Stokes output spectral lineshape evolves from Lorentzian to approximately Gaussian as the pump power increases. For the pump power near or beyond the threshold, the SBS spectrum is in the form of a steady Gaussian profile, and the spectral linewidth comes to a certain value about 7 times narrower than the spontaneous one. The theoretical results are experimentally demonstrated by using several common liquid media.

Stimulated Brillouin Scattering in Ultrahigh-Speed In-Phase RZ and CS-RZ OTDM Transmission

We report that stimulated Brillouin scattering (SBS) plays an important role in ultrahigh-speed bitwise phase-controlled optical time-division multiplexing transmissions such as in-phase and carrier-suppressed return-to-zero (RZ). We show that the optical power can easily exceed the SBS threshold for such high-speed coherent RZ signals. The SBS-induced performance limitation in a 160-Gb/s-520 km transmission is described in detail.

Direct Measurements of an Increased Threshold for Stimulated Brillouin Scattering with Polarization Smoothing in Ignition Hohlraum Plasmas

We demonstrate a significant reduction of stimulated Brillouin scattering by polarization smoothing in large-scale high-temperature hohlraum plasma conditions where filamentation is measured to be negligible. The stimulated Brillouin scattering experimental threshold (defined as the intensity at which 5% of the incident light is backscattered) is measured to increase by a factor of 1.7+/-0.2 when polarization smoothing is applied. An analytical model relevant to inertial confinement fusion plasma conditions shows that the measured reduction in backscatter with polarization smoothing results from the random spatial variation in polarization of the laser beam, not from the reduction in beam contrast.

A theoretical treatment of two approaches to SBS mitigation with two-tone amplification

A technique that employs two seed signals for the purpose of mitigating stimulated Brillouin scattering (SBS) effects in narrow-linewidth Yb-doped fiber amplifiers is investigated theoretically by constructing a self-consistent model that incorporates the laser gain, SBS, and four-wave mixing (FWM). The model reduces to solving a two-point boundary problem consisting of an 8x8 system of coupled nonlinear differential equations. Optimal operating conditions are determined by examining the interplay between the wavelength separation and power ratio of the two seeds. Two variants of this 'two-tone' amplification are considered. In one case the wavelength separation is precisely twice the Brillouin shift, while the other case considers a greater wavelength separation. For the former case, a two-fold increase in total output power over a broad range of seed power ratios centered about a ratio of approximately 2 is obtained, but with fairly large FWM. For the latter case, this model predicts an approximately 100% increase in output power (at SBS threshold with no signs of FWM) for a 'two-tone' amplifier with seed signals at 1064nm and 1068nm, compared to a conventional fiber amplifier with a single 1068nm seed. More significantly for this case, it is found that at a wavelength separation greater than 10nm, it is possible to appreciably enhance the power output of one of the laser frequencies.

Investigation on the output energy characteristic of optical limiting based on stimulated Brillouin scattering

Stimulated Brillouin scattering (SBS) featuring sub-nanosecond response time takes place at a high power threshold, which enables its application at a high power density. When the intensity of input light excesses the SBS threshold, strong SBS process takes place through SBS medium, leading to a quick energy transfer from pump to the Stokes and thereby an optical limiting characteristic in the output energy. In this paper, the correlation between SBS output energy and input power density is numerically simulated and validated in the Nd: YAG Q-switch laser system. The results indicate that not only the output energy exhibits an optical limiting characteristic, but also the clamped value of output energy can be controlled by changing the medium or the focal length.

Threshold reduction of stimulated Brillouin scattering by Stokes seeds via acousto-optic effect

We present a method of threshold reduction of stimulated Brillouin scattering by importing Stokes seeds, which are generated from the light passing through the Brilloun medium via an acousto-optic medium. To realize the scheme, the acousto-optic medium and the Brillouin medium should be of the same material, and the frequency of the transducer must be identical with the Brillouin shift. Essentially, the transducer and the acousto-optic medium constitute a special acousto-optic shifter. Calculations suggest that it is easy to reduce the SBS threshold by at least one magnitude and enhance the reflectivity remarkably by this scheme.

Effects of different Raman pumping schemes on stimulated Brillouin scattering in a linear cavity

The effects of backward, forward, and bidirectional Raman pumping schemes on stimulated Brillouin scattering (SBS) is investigated in this study. By using a linear cavity, we utilize residual Brillouin pump (BP) and Raman pump (RP) power after each transmission through a 25 km single-mode fiber (SMF) used as a gain medium. The SBS threshold power is reduced in the forward, backward, and bidirectional Raman pumping schemes by 2.5, 1.75, and 2.75 dB, respectively when the 1480 nm RP power is fixed at 150 mW and the BP wavelength is 1580 nm. Surprisingly, it is revealed that the SBS threshold reduction depends strongly and solely on Raman gain and it is independent of the Raman pumping schemes. In addition, the effect of Raman amplification on SBS is more effective at the SBS threshold, especially in the bidirectional and forward schemes.