Experimental investigation on the competition between wideband stimulated Brillouin scattering and forward stimulated Raman scattering in water

The utilization of a simple focused optical cell to bring to light the competition between wideband stimulated Brillouin scattering (WSBS) and forward stimulated Raman scattering (FSRS) is investigated experimentally. A pulsed, wide bandwidth second-harmonic Nd:YAG laser is used as the pump source. We found that, the competition between WSBS and FSRS is an alternate process, which one dominated depends on the linewidth and energy of the pump laser, focal length, and optical breakdown.     

Theoretical investigation on the threshold value of stimulated Brillouin scattering in terms of laser intensity

The threshold value of stimulated Brillouin scattering (SBS) was investigated theoretically basing on a new concept of real gain length (RGL). It shows that for ideal steady-state SBS or ideal transient SBS, the threshold values are both constants and the later is five orders of magnitude larger than the former. Under general conditions, whether steady-state SBS or transient SBS will occur is determined by RGL. The SBS occurred in a long material may not be steady-state SBS.     

Amplification effect on stimulated Brillouin scattering in the S-band forward G652 fiber Raman amplifier

The amplification effects on forward and backward stimulated Brillouin scattering (SBS) lines in the forward pumped S-band distributed G652 fiber Raman amplifier (FRA) have been studied. There is a pump threshold power of Stokes backward stimulated Brillouin scattering (B-SBS) line in the forward pumped G652 FRA, it is about 1 mW. The Stokes B-SBS lines are amplified by FRA and fiber Brillouin Brillouin gain. In experimental work, the saturation gain of the first order Stokes backward SBS line is about 58 dB and the saturation gain of 25-km G652 forward FRA is about 25 dB, so the gain of FBA is about 33 dB. The forward stimulated Brillouin scattering (F-SBS) is generated and amplified in S-band G652 FRA. The stimulated threshold powers of the forward first order Stokes SBS (SB1-), second order Stokes SBS (SB2-), and third order SBS (SB3-) in the forward pumped FRA are 2.3, 1.6, and 1.6 mW,respectively. In experimental work, the saturation gains of SB1-, SB2-, and SB3- are about 38, 62, and 60 dB, respectively. The saturation Raman gain of 25-km G652 forward FRA is about 8.8 dB, so the Brillouin gains of SB1-, SB2-, and SB3- are about 29.2, 53.2, and 51.2 dB, respectively. The forward and backward cascaded SBS lines have been observed.     

水中受激布里渊散射阈值的研究

在对水中受激布里渊散射阈值的研究中,针对传统定义上的不足,提出一种新的定义和新的测量方法.测量了不同能量的光束在不同长度的水介质中传输,在宽线宽和窄线宽情况下水的衰减系数.通过水的衰减系数的变化来确定不同情况下受激布里渊散射的阈值.试验发现,水中的受激布里渊阈值在理想稳态和理想瞬态下是一个常数,只与泵浦激光和介质属性有关,和介质长度无关;在中间情况下,受激布里渊阈值是否与介质长度有关,还要根据具体情况来分析.     

后向受激布里渊散射诱导的光学材料破坏机理研究

 区别于传统的受激布里渊散射(SBS)发生器和放大器,提出了一种新型的SBS模型：自供种子光模型 (self Stokes seeding,SSS)。数值求解了SBS耦合波方程组,得到了SBS诱导应力的时空分布。基于SSS建立了高功率激光辐照下光学材料破坏阈值的计算模型,研究了SBS破坏阈值与激光脉宽以及作用区长度的关系。研究发现,SBS作为一种破坏机制,表现为前表面破坏,且破坏阈值与激光脉宽以及作用区长度均成反比。     

Investigation on the suppression of stimulated Brillouin scattering in single-frequency Raman fiber amplifiers

High power operation of single-frequency Raman fiber amplifiers (SF-RFAs) is usually limited by the onset of stimulated Brillouin scattering (SBS). In this paper, we present our theoretical investigation on the suppression of SBS in SF-RFAs, based on the intensity equations combining SBS and stimulated Raman scattering (SRS). The effects of increasing the mode area of fiber and utilizing temperature and strain gradients along the fiber on the suppression of SBS are discussed. The simulation results suggest that it is not an advisable method to suppress SBS in SF-RFAs by increasing the mode area. Besides, although tensile strain and temperature gradients can suppress SBS in RFA singly, the conditions are too rigorous to realize. Based on the numerical simulation results, a feasible scheme for the suppression of SBS in RFA is proposed using the temperature gradients together with tensile strain gradients along the fiber length, resulting in an increase of ～3 times of amplifiers output power.     

Study on forward stimulated Brillouin scattering in a backward pumped fiber Raman amplifier

Strong multi-order forward stimulated Brillouin scattering (SBS) has been observed in the backward pumped S-band distributed fiber Raman amplifier (FRA) with tunable narrow signal source (less than 100 MHz) when the pump power of FRA reached the SBS threshold. This does not obey the theory that only weak backward SBS lines exist according to the conservation of energy and momentum and the wave vector selected rule. This is because the sound waveguide characteristic weakens the wave vector rule, and the forward transmitted sound waveguide Brillouin scattering lines are generated and amplified in FRA.When the pump power is further increased, 11 orders of SBS lines and comb-like profile are observed. For the excited line, the frequency is 197.2296 THz and the power is 0 dBm. The even order SBS lines are stronger than odd order SBS lines, the power of the 2nd and 4th order SBS lines is 1.75 dBm, which is 16 dB higher than that of the 1st and 3rd order SBS lines. The odd order SBS lines are named BrillouinRayleigh scattering lines.     

Method for measuring the threshold value of stimulated Brillouin scattering in water

A new method for accurate measurement of the threshold value of stimulated Brillouin scattering (SBS) in water in terms of pump laser intensity is investigated. The threshold value of SBS is determined by the point of the deviation of the value of the attenuation coefficients of wide- and narrow-linewidth lasers rather than the intensity of the backscattered SBS signal in the material.     

Polarization switching of stimulated Raman scattering in optical fibers by dual-frequency pumping

Experiments are presented showing that under dual-frequency, orthogonal-polarization pumping, stimulated Raman scattering in optical birefringent fibers can be suppressed in one of the two fiber axes. For relatively small-frequency spacing and group-velocity mismatch between the pumps, Stokes light is suppressed along the polarization direction of the highest-frequency pump, owing to the orthogonal component of the Raman gain.     

Experimental measurement of bulk viscosity of water based on stimulated Brillouin scattering

This paper presents a new method for measuring water bulk viscosity directly by using stimulated Brillouin scattering (SBS). The relationships among shear viscosity, bulk viscosity, thermal capacity, and the line-width were deduced theoretically. The line-width and the bulk viscosity in water with different temperatures were investigated experimentally, and the error of experimental result was analyzed. The results obtained demonstrate that the proposed way is an accurate method for measuring bulk viscosity of water.     

Theoretical study of stimulated Raman scattering in long tapered fiber amplifier

A model that is based on the propagation equation and coupled mode theory is introduced in order to describe stimulated Raman scattering(SRS) effects in long tapered fiber amplifiers.Based on the presented model,fiber amplifiers with uniform and long tapered fibers are theoretically and numerically simulated.It can be drawn from the results of our simulations that the long tapered fiber has the advantage in suppressing SRS when applied in fiber laser amplifiers.Our results can provide guidance in the designing of system configuration in long tapered-fiber-based fiber laser systems.     

On the material response in stimulated Brillouin scattering

We re-examine behavior of the material response in stimulated Brillouin scattering and from this revise its theoretical formalism. We show that the response out of exact Brillouin resonance splits into two processes: one is the induction of a moving density grating, propagation characteristics of which follow these of the beat pattern of pump and Stokes fields, and the other is the induction of an acoustic wave, frequency of which coincides with that of the beat pattern but which propagates at the acoustic velocity in the medium. The resulting equations for the response amplitudes are solved analytically in several representative cases.     

Effect of water temperature on pulse duration and energy of stimulated Brillouin scattering

The water temperature has a strong effect on the kinematic viscosity, which is inversely proportional to the phonon lifetime and the gain coefficient. The higher the temperature is, the smaller the kinematic viscosity is, and the larger the phonon lifetime is. At a low pump power and a short focal length, we can derive a single-peak stimulated Brillouin scattering (SBS) pulse. The duration of this single-peak SBS pulse depends mainly on the phonon lifetime of water. With the increase of the water temperature, the duration of such a single-peak SBS pulse will become longer, and the SBS energy will become higher for the gain coefficient, which is related to the phonon lifetime. Therefore, varying the medium temperature can lead to the changes of SBS pulse duration and SBS energy.     

Bursting behaviours in cascaded stimulated Brillouin scattering

Stimulated Brillouin scattering is studied by numerically solving the Vlasov-Maxwell system. A cascade of stimulated Brillouin scattering can occur when a linearly polarized laser pulse propagates in a plasma. It is found that a stimulated Brillouin scattering cascade can reduce the scattering and increase the transmission of light, as well as introduce a bursting behaviour in the evolution of the laser-plasma interaction. The bursting time in the reflectivity is found to be less than half the ion acoustic period. The ion temperature can affect the stimulated Brillouin scattering cascade, which can repeat several ×at low ion temperatures and can be completely eliminated at high ion temperatures. For stimulated Brillouin scattering saturation, higher-harmonic generation and wave-wave interaction of the excited ion acoustic waves can restrict the amplitude of the latter. In addition, stimulated Brillouin scattering cascade can restrict the amplitude of the scattered light.