Saturation and depletion effect of Raman scattering in opticalfibers

The analytical solution of nonlinear power coupled equations involving three waves is given. The study reveals that the Raman gain saturation effect caused by the generation of the second Stokes has to be taken into consideration in light amplification by stimulated Raman scattering, though it can be ignored in spontaneous stimulated Raman scattering. It is also shown that it is advantageous to feed the fiber with a small low-order Stokes signal in amplifying the higher-order Stokes signal for easing the requirement of high pump power and long fiber length. A model to calculate higher-order Stokes radiation in low-loss optical fibers is presented     

Generation of optical phase-conjugate waves and compensation forpulse shape distortion in a single-mode fiber

The generation of optical phase-conjugate waves and the application of optical phase conjugation (OPC) to optical communication systems is described. The method of pulse shape distortion compensation by OPC is outlined including distortion due to both fiber dispersion and the optical Kerr effect. The generation of a forward-going phase-conjugate wave in a third-order nonlinear medium is discussed and that by a nondegenerate forward four-wave mixing in a zero-dispersion single-mode fiber (SMF) is investigated. Suppressing the stimulated Brillouin scattering (SBS) of a pump wave in the fiber prevents saturation of the generation efficiency of the phase-conjugate wave even when the pump power exceeds the SBS threshold. In transmission experiments through a 200-km standard SMF with a 16-Gb/s intensity-modulated signal and a 5-Gb/s continuous-phase FSK (CPFSK) modulated signal, it is shown the applicability of OPC is modulation independent and that OPC effectively compensates for both chromatic dispersion and the optical Kerr effect     

Limitations imposed by stimulated Raman scattering of 1550 nmhigh-energy pulse transmission

The limitations imposed by stimulated Raman scattering on 1550 nm high-energy pulse transmission in single-mode optical fiber are investigated. The generation of a Stokes pulse from an input pulse with peak power up to 60 W and duration on the order of 10 ns is experimentally studied and numerically simulated. This is the first demonstration of Stokes pulse generation in optical fiber from pump pulses within the 1550 nm telecommunications window. Experiment and theory show that there is an optimum input power for a given pulse duration and distance that maximizes the energy transmitted over long fibers     

Diffusion Brillouin stimulee dans un laser en anneau à fibre optique

A cw-pumped optical fiber ring oscillator leads to pulsed emission by stimulated Brillouin back-scattering (dbs for« diffusion Brillouin stimulee»). Numerical treatment of the three-wave coherent equations allows an interpretation of the process efficiency. Accumulation of stimulated acoustical phonons, which severely limits the coupled pump power and is at the origin of instabilities and broadening of the Brillouin pulses, has been avoided by coupling the pump wave with an acousto-optic deflector driven at the oscillator optical round trip frequency. Taking into account the optical Kerr effect, the numerical results are in good accordance with experiment.     

波长间隔可调谐多波长光纤光学参量振荡器

提出并实现了一种以高非线性色散位移光纤为增益介质,以光栅对形成谐振腔,简单线形结构的连续光抽运的波长间隔可调谐多波长光纤光学参量振荡器(MW-FOPO)。采用波长可调谐的窄线宽激光器作为抽运种子光源,以伪随机相位调制抽运光来抑制高非线性光纤中的受激布里渊(SBS)散射效应,结合高功率掺铒光纤放大器构成光纤光学参量振荡器的大功率抽运,通过四波混频(FWM)效应获得了室温下稳定的多波长激光输出。MW-FOPO的波长间隔可以通过调节抽运波长进行调谐。在1505~1615 nm光谱范围内,获得了17条消光比大于10 dB的多波长谱线。实验证明了MW-FOPO实现多波长激光光源的优异特性。     

Influence of the Stimulated Raman Scattering on the Four-Wave Mixing Process in Birefringent Fibers

The influence of the stimulated Raman scattering in the four-wave mixing process in high birefringent fibers is theoretically analyzed. We consider the dual pump configuration in both co-polarized and orthogonal polarization schemes and treat simultaneously the stimulated Raman scattering and the four-wave mixing processes. The obtained results are valid even when the shift between the pumps and signal lies inside the Raman band. Results show that when the phase-matching condition is achieved the generation of the idler wave is mostly dependent on the real part of the fiber nonlinear response function, whereas the amplification of the signal wave is mostly dependent on the imaginary part. Results also show that when the phase-matching condition is satisfied the optical power evolution of the signal and idler waves for small frequency detunings are mostly described by the anisotropic Raman response of the fiber, whereas for high frequency detunings the isotropic response dominates. Our analysis also shows that the stimulated Raman scattering can increase the efficiency of the four-wave mixing process, for certain frequency detunings.     

长腔被动锁模掺镱光纤激光器的方波脉冲产生

报道了一种基于非线性光纤环形镜(NOLM)、工作在耗散孤子共振(DSR)区的长腔被动锁模掺镱光纤激光器,该激光器谐振腔的总长度约为1502 m,可以输出重复频率为133.18 kHz的高能量方波脉冲,且输出脉冲的宽度和单脉冲能量均随泵浦功率的增大而呈线性增大。当泵浦功率增大到414.47 mW时,输出的方波脉冲具有最大宽度(761.6 ns),同时单脉冲能量达到了最大值(60.2 nJ)。通过改变NOLM中单模光纤的长度,进一步研究了谐振腔长度对输出方波脉冲特性的影响,结果表明:谐振腔越长,所得DSR方波脉冲越宽,脉冲峰值功率越低。     

基于干涉环结构的调Q光纤激光器

从光纤干涉环中受激瑞利散射(SRS)和光纤中受激布里渊散射(SBS)共同作用调Q的原理出发,研究了光纤干涉环耦合比及环长对调Q光纤激光器输出激光特性的影响。分别采用耦合比为50∶50,80∶20,90∶10的光纤干涉环在不同环长下搭建掺Er3 调Q光纤激光器进行实验。实验结果表明,干涉环的耦合比和环长是影响输出激光特性的关键因素,耦合率(~10%)的干涉环适宜产生脉冲激光,具有高耦合比干涉环的光纤激光器只有连续激光输出;环长根据干涉环耦合率(~10%)在2 m附近调整可以获得理想脉冲激光输出,环长过短时产生输出激光的弛豫振荡,反之产生输出激光脉冲的分裂。采用耦合比为90∶10,环长为2 m的光纤干涉环时,在37 mW的抽运功率下获得脉宽7.2 ns,重复频率212.4 kHz,输出功率5.4 mW的脉冲激光,脉冲波形较好,峰值功率有~30%的波动。     

Characterization of Brillouin fiber generator and amplifier for optimized working condition of distributed sensors

The Brillouin fiber amplifier (BFA) has been used in Brillouin optical time and frequency domain analyzer based sensors. For BFA based distributed sensor the optimized working condition is to have the highest Brillouin gain, i.e., highest SNR, which avoids high pump depletion induced gain saturation. We have found that the optimum working condition for distributed sensor system is associated with the stimulated Brillouin scattering (SBS) threshold for BFA, which can be experimentally determined with Stokes power inflexion and/or Stokes spectrum linewidth minimum methods. This threshold depends on both pump and probe power instead of just the pump power as in Brillouin fiber generator (BFG), as well as on sensing length as confirmed by our experimental results and theoretical simulation. This was achieved by introducing the concept of absorption coefficient of the sensing medium defined as the ratio of the total output power to the total input power. We find that the medium absorption is minimized when input Stokes power is an order of magnitude lower than BFA threshold. This minimum is a signature of the balance between maximum Stokes gain and pump depletion which is also the reason why Stokes spectrum linewidth goes through a minimum.     

多波长布里渊掺铒光纤激光器的结构优化研究

基于受激布里渊散射效应(SBS)和掺铒光纤(EDF)的线性增益机理研究了一种环形腔多波长布里渊掺铒光纤激光器(BEFL)。在该激光器中使用单模光纤作为布里渊增益介质,使用掺铒光纤来放大产生的斯托克斯(Stokes)信号,使该激光器在室温下产生稳定的多波长输出。通过对激光器结构中的环行器和耦合器的位置以及耦合器的接入方式的研究与分析发现:耦合器的接入方式、环行器与耦合器的位置均对激光器的输出有影响。在可调谐光源TLS(布里渊泵浦激光器)的功率大小为14dBm,980nm泵浦激光器的功率大小为23dBm,单模光纤长度为10km的情况下进行了实验测试,结果表明:耦合器的接入方式对BEFL的输出影响很明显,最后给出了详细的测试结果和相应解释。     

线性源啁啾对孤子脉冲压缩的影响

通过线性源啁啾对孤子脉冲压缩影响的研究,发现泵浦脉冲和孤子脉冲的源啁啾参量地脉冲压缩产生重要影响,且孤子脉冲压缩存在最佳光纤长度。在泵浦功率一定的条件下,选取负啁啾的泵浦脉冲正啁啾的孤子脉冲可以获得高压缩因子的光脉冲;同时选取负啁啾的泵浦脉冲和正啁啾的孤子脉冲易于产生更短的压缩光脉冲。     

S+C bands amplification in a distributed Er-doped Raman fiber

A novel hybrid optical amplifier covering entire S- and C-bands has been proposed. A silica fiber configured with an erbium (Er)-doped cladding and a germanium-doped core was used. Amplification was achieved by stimulated Raman scattering along with the Er ions' radiative transition in a fiber. A numerical simulation has been performed to analyze the amplification characteristics. Fiber parameters such as optical loss, Er concentration, fiber length, and pump power were taken into consideration to calculate the optimum pump power and fiber length for a flat gain characteristic over the entire S- and C-bands.     

高功率光纤激光器喇曼效应的数值分析

为了研究掺镱双包层高功率光纤激光器的结构参量对喇曼效应的影响,利用数值模拟方法求解有关抽运光、激光和斯托克斯波的稳态速率方程组,得到了激光和斯托克斯波在光纤轴向的分布特性,以及喇曼阈值抽运功率与光纤激光器结构参量的关系.结果表明,当光纤激光器采用大模面积、短腔长、小斯托克斯波辐射截面和长激光波长的结构参量时,能显著提高喇曼阈值抽运功率,降低喇曼效应.     

拉曼放大对孤子传输特性的影响

利用频率分辨光学门(SHG-FROG)技术研究了拉曼放大对光孤子传输特性的影响。研究表明,拉曼放大能够补偿光纤损耗,但不改变孤子脉冲的时域波形,对孤子脉冲的啁啾影响不大。当传输光纤长度小于拉曼放大有效光纤长度时,拉曼放大能够完全补偿光纤损耗;当传输光纤长度大于拉曼放大有效光纤长度时,拉曼放大能够部分补偿光纤损耗。拉曼放大对光纤损耗的补偿能力随着抽运功率的增加而增大。光孤子脉冲对拉曼放大抽运光偏振特性不敏感。     

Spontaneous and stimulated Raman scattering in long low loss fibers

This paper considers the problem of forward Raman scattering process in a single transverse mode fiber. Both pump wave depletion and spontaneous scattering are considered in the analysis. Analytic solutions of the governing differential equations are obtained. We examine the conditions under which a nondepleted pump approximation is valid. Expressions are derived for the maximum fiber loss and the minimum fiber length which allow significant pump to Stokes wave conversion. It is shown that for a given fiber length there is an optimal pumping power, or at a given pump power there is an optimal fiber length that yields maximum first-order Stokes power output. Good agreement with published experimental results in the threshold power prediction is obtained.     

基于双向二级喇曼抽运的偏振控制研究

为了发展具有实用价值的受激喇曼全光偏振控制方法,根据光纤中受激喇曼散射矢量理论和光纤随机双折射效应设计了基于双向二级喇曼抽运的全光偏振控制方案,构建了偏振吸引理论模型,采用严格数值迭代算法对该偏振控制方案进行了仿真分析,并完成了偏振控制方案最佳工作性能的优化设计。结果表明,当信号光波长为1550nm、功率为0.1mW、一级抽运光和二级抽运光的功率分别为1W和4W、其偏振态都为（0,1,0）、光纤长度为3km时,信号光偏振度可达0.85以上,且信号光强度波动低于35%。该方案有效降低了作用光波功率、减弱了信号光输出强度噪声,并提高了信号光输出偏振度。     

传输光纤对单频光纤放大器中受激布里渊散射的影响

在全光纤的单频光纤放大器系统中,泵浦耦合器带有的传输光纤将会对放大器的性能产生影响,为了研究其影响的规律,本文从含有SBS的速率方程出发,建立模型,讨论在含有传输光纤的情况下的泵浦方式,传输光纤长度,光纤结构对SBS的影响,所得结论为从实验上研究单频光纤放大器中SBS的变化规律提供理论依据。     

受激布里渊散射主被动混合调Q光纤激光器

基于瑞利散射和受激布里渊散射(RS-SBS)的被动调Q掺铒光纤(EDF)激光器的输出脉冲序列具有重复频率低、脉宽窄、功率高的特点,适合光时域反射(OTDR)系统对脉冲光源的基本要求,但是输出的脉冲序列不够稳定。提出在被动调Q激光腔中插入声光调制器(AOM)构成主被动混合调Q激光器。实验结果证明,这种混合调Q的方法既保持了声光调制器主动调Q激光器输出脉冲序列重复频率低而且稳定的特点,又发挥了瑞利散射和受激布里渊散射被动调Q机制动态速度快、输出脉冲宽度窄的优势。在120~200 mW的抽运功率条件下,得到的脉冲序列重复频率从30 Hz~90 kHz连续可调,脉冲宽度最小可达20 ns,峰值功率最高可达200 W,脉冲重复频率稳定度优于5%,脉冲幅度起伏不大于10%。脉冲峰值功率和脉冲宽度受抽运功率的影响不大,但随着调制频率增加,脉冲峰值功率降低而脉冲宽度加宽。     

A computer model of non-dispersion-limited stimulated Ramanscattering in optical fiber multiple-channel communications

The development of a computer model for non-dispersion-limited stimulated Raman scattering (SRS) in optical fiber multiple-channel communication systems is reported. The model takes into account the pump depletion due to SRS and the different attenuation coefficients at different wavelengths of operation. It can calculate the SRS effects in any fiber, provided the Raman gain profile for that fiber is known. Thus, the model is general in nature and can be used for a number of problems. It can be used to calculate the output power of all the channels, given the input of every channel and its operating wavelength. It can also be used, at least in principle, to determine the input power level if a desired level of output power is required. Moreover, it can be used to estimate the optical amplification of a signal due to SRS. This is possible because the governing equations for all of these processes are similar and therefore can be solved using the same approach     

Crosstalk due to optical fiber nonlinearities in WDM CATV lightwavesystems

Crosstalk in a two-wavelength 1550-nm standard fiber system at subcarrier frequencies 50-800 MHz is investigated. The dependence of the crosstalk on subcarrier frequency, wavelength spacing, and optical power is measured and analyzed. The observed crosstalk is attributed to three primary mechanisms: stimulated Raman scattering, cross-phase modulation, and the optical Kerr effect combined with polarization-dependent loss. At wavelength spacing greater than 9 nm, stimulated Raman scattering dominates. At wavelength spacing less than 5 nm, the primary contributor can be the optical Kerr effect with polarization dependent loss, except at higher modulation frequencies where cross-phase modulation also is significant. At even modest (by CATV standards) optical power, the crosstalk is as high as -40 to -45 dB