窄线宽纳秒脉冲光纤拉曼放大器的理论模型和数值分析

窄线宽纳秒脉冲光纤拉曼放大器在非线性频率变换、遥感探测和量子信息等领域有广泛的应用前景.综合考虑受激拉曼散射(stimulated Raman scattering,SRS)、受激布里渊散射(stimulated Brillouin scattering,SBS)、自相位调制(self-phase modulation)和交叉相位调制(cross-phase modulation)等非线性效应,建立了窄线宽纳秒脉冲光纤拉曼放大器的非线性动力学模型.仿真分析了放大器中脉冲激光的时频演化特性,对比研究了抽运脉冲宽度、光纤长度和信号光功率等因素对放大器性能的影响.研究发现,上述因素会影响放大器的SRS阈值、SBS阈值、输出激光线宽、激光转换效率等.例如,当脉冲宽度为800 ns时,SBS随着抽运功率的增加而发生,限制了激光功率的提升;减短抽运脉宽可以抑制SBS,但是输出激光的线宽易于展宽到数百MHz以上;增加光纤长度可以获得更低的SRS阈值和更高的转换效率,但是SBS效应和光谱展宽程度也随之增强.系统搭建中需要平衡各非线性效应,选择合适的系统参数.研究内容可以为窄线宽纳秒脉冲光纤拉曼放大器的设计搭建提供参考.

Theoretical and numerical study on narrow-linewidth nanosecond pulsed Raman fiber amplifier

Narrow-linewidth nanosecond pulsed Raman fiber amplifiers possess many applications such as in nonlinear frequency generation, remote sensing and quantum information. By considering nonlinear effects such as stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), self-phase modulation (SPM) and cross-phase modulation (XPM), we build a nonlinear dynamical model of narrow-linewidth nanosecond pulsed Raman fiber amplifier. A numerical simulation model is also built and the simulation is carried out based on the parallelizable bidirectional finite difference time-domain method. The pulse evolution processes in time and spectral domain are simulated. The influences of pump pulse width, fiber length and signal laser power are studied in detail. It is found that SRS peak power threshold is not influenced by pump pulse width, however, pump pulse width will affect SBS threshold and output linewidth. When the pump pulse width is 800 ns, tens of MHz narrow linewidth can be obtained, but the SBS occurs as the increasing of pump energy, which limits the power scaling of the narrow-linewidth laser pulses. When the pump pulse width is 80 ns, the SBS is effectively suppressed and the peak power can be further increased, but the linewidth of output laser is easily broadened to hundreds of MHz. The simulation results also show that lower SRS threshold and higher efficiency can be obtained by using longer passive fiber, however, if shorter passive fiber is used, SPM and XPM can be weakened and narrower linewidth can be obtained. We build an experimental setup to study the influence of fiber length. In our experiment, a polarization-maintained passive fiber with a core diameter of 10 μm and core numerical aperture of 0.08 is used as the Raman gain fiber. The signal laser is a 1120 nm single frequency continuous wave fiber laser with an average power of 20 mW, and the pump laser is a 1064 nm pulsed laser with a pulse width of~40 ns and repetition rate of 500 kHz. When the fiber lengths are 100 m and 80 m, the efficiencies of the pulsed Raman amplifier are, respectively, 51.5% and 38.2% at a pump power of 6.8 W. It can also be found that increasing signal power can increase the efficiency of the amplifier, but it will reduce the SBS threshold at the same time. Therefore, in order to balance the different nonlinear effects in the arrow-linewidth nanosecond pulsed Raman fiber amplifier, we should take laser power, linewidth and efficiency into consideration, and choose the suitable system parameters such as pump pulse width, fiber length and signal power. These analyses can serve as design guidelines for narrow-linewidth nanosecond pulsed fiber Raman amplifiers.