Relative intensity noise induced by four-wave mixing in the case of two-wave transmission in an optical fiber

Four-wave mixing (FWM) is a significant nonlinear effect in wavelength division multiplexing (WDM) fiber-optic systems. For two-wave transmission, it is easily found that the FWM noise power decreases with frequency spacing and increases with signal power. However, the variation of relative intensity noise (RIN) with frequency spacing and signal power is only 2 dB at most. The intensity fluctuations induced by the energy exchange between the FWM generated new waves and the original ones are trivial and the influence of FWM on RIN can be neglected. It is also found that the increase of RIN with signal power is mainly attributed to stimulated Brillouin scattering (SBS) rather than FWM.     

组合型光纤相位共轭器的数值模拟

 组合光纤是一种结合了小口径光纤SBS阈值低和大口径光纤损伤阈值高的特点而形成的新型光纤相位共轭器。通过建立数学模型，数值计算了组合光纤中SBS形成的过程，并对其特性参数进行了一定的理论分析。结果表明组合光纤不但可以大幅度降低SBS阈值、获得高反射率和扩大动态范围，而且通过优化其几何尺寸可以抑制Stokes波形的波动现象并显著压缩泵浦光脉冲。     

Enhanced four-wave mixing in a high-finesse highly nonlinear fiber Fabry-Perot resonator

We have achieved a four-wave mixing process in a high-finesse highly nonlinear fiber Fabry-Perot resonator, where the amplified signal and idler were enhanced in transmission by 6 dB and 10 dB respectively comparing with those in a single fiber. We used a 6 m long low-loss Fabry-Perot resonator with two high-reflectivity fiber Bragg gratings written directly into a highly nonlinear fiber provided by Sumitomo Electric Inc., where the minimised intracavity loss resulted in the finesse in excess of 100. The resonator length was locked for 30 min by means of a modified Pound-Drever-Hall technique. The maximum intracavity power was increased by 14.3 dB by increasing the stimulated Brillouin scattering threshold with periodic phase modulation by a pseudo-random bit sequence, with length matching that of the cavity.     

Stimulated Brillouin scattering of Q-switched laser pulses in large-core optical fibres

Q-switched Nd : glass laser pulses of 60 ns duration are transmitted through multimode fused-silica fibres of 0.4–1 mm core diam and lengths of up to 20 m. For laser radiation with narrow spectral width, stimulated Brillouin scattering (SBS) is observed for energies well below the threshold energy of fibre damage. The SBS threshold is shifted beyond the threshold of fibre damage through increasing the spectral width of the laser radiation. The SBS threshold energies of step-index and gradient-index fibres are measured for various fibre and laser parameters.     

基于受激布里渊散射的波长间隔可变多波长光纤激光器

提出了一个基于自激发受激布里渊散射的波长间隔可变多波长光纤激光器.利用单模光纤中自激发产生的非线性布里渊增益和掺铒光纤的线性增益组成混合增益光纤激光器,从而使光纤激光器在室温下产生稳定的多波长输出.改变双折射光纤环镜滤波器中保偏光纤的长度,可以实现波长间隔可变多波长激光产生,提高了多波长光纤激光器操作的灵活性和实用性.实验实现了波长间隔从0.8 nm至0.076 nm可变的多波长激光产生,波长数随波长间隔减小而增加,间隔为0.08 nm的激光波长数达86.     

Ge-codoped fibers for mitigating stimulated Brillouin scattering in high power fiber amplifiers

Rare-earth doped fibers for high power fiber amplifiers normally have a small refractive index difference between the core and inner cladding, but have a larger core diameter than conventional telecom fibers. We take advantage of this feature as well as the difference between the optical and acoustic waveguides and proposed several fiber designs to mitigate Stimulated Brillouin scattering (SBS) to produce higher output powers. The numerical modeling shows that an increase in SBS threshold by 3-6 dB can be achieved using Ge-codopant while maintaining diffraction limited beam quality. The key parameters that can affect the SBS performance are specified and optimal parameters for SBS suppression are obtained. The study also highlights the need to take into account the effects of minor refractive index profile variation in evaluating SBS performance.     

基于光子晶体光纤中受激布里渊散射的光载波抑制

设计了一种基于光子晶体光纤中受激布里渊散射的载波滤波器.该滤波器利用两个环形器和一段光子晶体光纤构成了一个环形腔,这种腔结构有效地降低了光纤中受激布里渊散射的阈值.理论分析了光子晶体光纤载波滤波器对提高光调制深度和射频增益的影响.利用25 m的高非线性光子晶体光纤作为受激布里渊增益介质,当入射载波功率为70 mW时,微波光子信号的射频增益为5.38 dB,实验结果与理论计算相吻合.     

Stimulated Brillouin scattering as the referencing mechanism of an optical fibre intensity sensor

An optical fibre intensity sensor referenced by stimulated Brillouin scattering is presented. The optical sensor uses Fresnel reflection signal at the sensor fibre end and employs an adequate relationship between Brillouin and Rayleigh scattering and Fresnel reflection to have a referenced optical fibre intensity sensor addressed in reflection.     

Stimulated Brillouin scattering（SBS） excited with a phase－modulated pump laser

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光纤放大器中的受激布里渊散射阈值

 从受激布里渊散射(SBS)耦合波理论出发,根据双包层光纤放大器中的受激布里渊散射阈值模型,理论仿真了信号带宽、纤芯直径、放大器增益对SBS阈值的影响,并从实验上研究了单频百纳秒脉冲信号在掺镱双包层光纤放大器中的受激布里渊散射现象。实验中输入脉冲信号重复频率1 Hz,脉宽200 ns,对不同的输入脉冲信号放大,前向放大脉冲在脉冲能量660 nJ、峰值功率3.3 W时出现畸变,产生后向SBS窄脉冲,达到了SBS阈值,实验计算的SBS阈值与理论分析结果基本一致。     

0.16 nm spaced multi-wavelength Brillouin fiber laser in a figure-of-eight configuration

A stable and compact multi-wavelength Brillouin fiber laser (BFL) operating at room temperature is experimentally demonstrated using a 100 m long photonic crystal fiber (PCF) in conjunction with a figure-of-eight configuration. At a Brillouin pump (BP) level of 15.3 dBm, 7 simultaneous lines with 20 GHz or 0.16 nm line spacing is achieved by removing the odd-order Stokes lines. The anti-Stokes lines are also generated via four wave mixing process in the laser cavity. Compared with the Erbium-based multi-wavelength laser, this BFL has advantages in term of channel spacing and flexibility in the choice of operating wavelength. The output spectrum of the proposed BFL can be tuned by 80 nm, dependent on the availability of an appropriate BP source. The multi-wavelength BFL shows a good stability with power fluctuations of less than 0.5 dB over more than 3 h.     

Stimulated Brillouin scattering phase conjugating properties of long multimode optical fibers

This paper describes and characterizes a detailed experiment that proves the phase conjugating properties of stimulated Brillouin scattering in multimode fibers (km of length), a problem still in debate in the literature, by studying the random aberrations correction abilities of this nonlinear interaction. Detailed qualitative and quantitative evaluation of aberration correction, by using the M² beam propagation ratio as a criterion to assess beam quality of the beams, proves the phase conjugation via SBS in long MM fibers. A new parameter to characterize the efficiency of the SBS phase conjugation process is proposed and conditions to obtain a complete healing of distortions (complete phase conjugation) are discussed.     

Numerical analysis of stimulated Brillouin scattering in high-power double-clad fiber lasers

A theoretical analysis of stimulated Brillouin scattering (SBS) in linear cavity Yb³⁺-doped double-clad fiber lasers is presented by solving the steady-state rate equations with the SBS. The effects of cavity length, fiber core diameter, input mirror reflectivity at Stokes wavelength, Yb³⁺ concentration and laser linewidth on the SBS are discussed. Numerical results show that the SBS threshold power can be improved by shortening the cavity length, using large mode area fiber, reducing the input mirror reflectivity at Stokes wavelength, lowering the Yb³⁺ concentration and broadening the laser linewidth, and the influence of the laser linewidth on the SBS threshold power is more noticeable than other system parameters.     

Stimulated Brillouin scattering of an electromagnetic wave by an acoustic-like mode in multi-ion species plasmas

Using an hydrodynamical model, analytical investigation of the stimulated Brillouin scattering of an electromagnetic wave by an acoustic-like mode has been done in a multi-ion species plasma. The acoustic-like mode is a new mode which propagates in a multi-ion plasma only. The non-linear dispersion relation and the growth rate of the excited modes are derived. The non-linearity arises through the motion of ions which is introduced through ponderomotive force. It is found that the growth rate can be controlled by several parameters like charge number, mass, density and temperature of the ions.     

Effect of four-wave mixing in WDM optical fibre systems

Wavelength division multiplexing (WDM) can both significantly enhance transmission capacity and provide more flexibility in optical network design. Through the use of erbium-doped fibre amplifiers (EDFAs), it is possible to build long-distance transparent optical transmission links without electrical regenerators. In such systems, fibre nonlinearities are likely to impose a transmission limit due to increased total interaction length. There are a number of optical nonlinear effects in optical fibres, such as stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), carrier-induced phase modulation and four-wave mixing (FWM). Out of these SRS and FWM are the dominant effects. In this paper, an algorithm has been suggested to study the effect of FWM in the total system noise considering the combined effect of SRS and FWM in the presence of amplified spontaneous noise (ASE). It has been found from the study that to maximize the signal-to-noise ratio of the transmitted signal in a WDM system FWM noise needs to be reduced as this is the dominant noise factor.     

Inherent spectral distortions in pump-probe Brillouin scattering spectroscopy

It is shown that the pump-probe technique in Brillouin scattering spectroscopy is accompanied by two specific distortions to a registered spectrum. The first appears due to an acoustic wave that unavoidably accompanies in a continuous medium the density wave driven by the pump and probe signals. The registered spectrum then splits to two spectral components, one corresponding to the propagation velocity of the driven density wave, and the other to the acoustic velocity. The other distortion is asymmetry of the spectrum which is shown to occur when the velocity of the driven density wave is greater than the acoustic velocity in the medium.     

大信号布里渊增强四波混频理论研究

 针对高功率激光相位共轭装置的应用需求,阐述了布里渊增强四波混频（BEFWM）在这一领域的应用潜力,并就该大信号强相互作用条件下BEFWM的作用机理及输出特征进行了系统的理论研究,揭示了BEFWM瞬态响应速度、共轭光波形保真度及稳定性、能量转换效率均随注入信号光的增强而提高的作用特点,为该技术在中高强度激光相位共轭系统中的实际应用提供了理论依据。     

Millimeter wave carrier generation based on Brillouin fiber laser with improved tuning capability

An all-optically generated millimeter wave carrier at 21.7 GHz, 43 GHz and 64.45 GHz are experimentally achieved. These frequencies are realized by generating two consecutively laser wavelengths and are detecting on the 70 GHz high-speed photo detector (HSPD). The initial mixing between the Brillouin pump and the second-order Stokes wavelengths is spaced by 0.178 nm. This spacing, which is doubled from an inherently generated Stokes shift, is accomplished through an isolated circulation of the first order Stokes wave in the double Brillouin Stokes shifter (DSBS) built with 25-km single mode fiber. The generated millimeter carrier is measured at 21.7 GHz, 43 GHz and 64.4 GHz achieved with BP power of 11 mW, 30 mW and 47 mW, respectively.     

光纤中的瞬态SBS过程的数值分析与探讨

 通过数值方法得到了光纤中的瞬态SBS过程中耦合波和声场的强度分布图，从而分析了光纤中瞬态SBS的发生过程以及光纤长度、泵浦脉宽等因素对SBS反射率及阈值的影响，据此提出了一种新的光纤相位共轭器。     

高非线性微结构光纤中基于受激布里渊散射的慢光延迟

全光连续可调的慢光技术在全光网络和光信息处理等领域具有重要的应用前景. 利用自行设计并拉制的高非线性微结构光纤, 实验研究了基于受激布里渊散射的可调谐慢光延迟. 采用单抽运光和单级延迟方案, 当抽运光功率为162.6 mW时, 在长度为120 m的高非线性微结构光纤中获得了最大76 ns的延迟量, 相当于0.76个脉冲宽度. 通过调节抽运光功率的大小, 可以实现对慢光延迟量的可调谐.该慢光延迟方案具有延迟量大、 全光可调谐及与现有光通信系统兼容等优势. 关键词： 慢光 微结构光纤 受激布里渊散射