Phase noise measurement of high-power fiber amplifiers

We measure the phase fluctuation in a high-power fiber amplifier using a multi-dithering technique.Its fluctuation property is qualitatively analyzed by the power spectral density and integrated spectral density.Low frequency fluctuations caused by the environment are dominant in the phase fluctuations in an amplifier,whereas the high frequency components related to laser power affect the control bandwidth.The bandwidth requirement of the active phase-locking is calculated to be 300 Hz,670 Hz,1.6 kHz,and 3.9 kHz under the output power of 25,55,125,and 180 W,respectively.The approximately linear relationship between the control bandwidth and laser power needs to be further investigated.     

A 330-W single-frequency retrievable multi-tone monolithic fiber amplifier

A single-frequency retrievable phase modulated multi-tone fiber amplifier is presented in theory and demonstrated in experiment. A multi-tone seed laser generated by a sine wave phase modulated single-frequency laser is employed for stimulated Brillouin scattering suppression in an all-fiber amplifier. A demodulation signal which is π phase shifted with respect to the modulation signal is used to retrieve the single-frequency laser from the multi-tone laser. In experiment, we first optimize the all-fiber master-oscillator power-amplifier. With this amplifier, we demonstrate a single-frequency retrievable multi-tone laser with 330-W output when driven by the multi-tone seed, while the ultimate output power is only 130 W when driven by the single-frequency laser. Then, we carry out an experiment for retrieving the single-frequency laser from the amplified multi-tone laser. Results indicate that the single-frequency laser can be retrieved with a sideband suppression of more than 20 dB. Retrieving an even higher power single-frequency laser is possible if a high power demodulator is available.     

三角格点基底上磁性分形团簇形貌演化规律

在扩散限制凝聚模型基础上引入粒子的自旋自由度,将磁耦合系数扩展为随自旋间距离幂次变化的非常数项J/r^a,采用Monte Carlo方法研究在二维三角格点基底上具有幂次相互作用的磁性团簇形貌及其分形维数D_f的演化规律.模拟结果表明,对于较大的幂指数α值,即α≥5时,团簇形貌随耦合参数J的变化较小,其分形维数D_f在1.50~1.70之间;随着α值的减小,团簇形貌随参数J有一明显的演化过程,在模拟范围内,分形维数D_f在1.20~1.90之间.     

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.     

基于高功率激光输出端相位控制的光纤激光相干合成

利用压电陶瓷相位调制器在高功率光纤激光输出端进行相位控制,实现了两路瓦量级光纤激光相干合成.系统闭环时长曝光图样的条纹可见度为98%,峰值强度是开环情形下的1.87倍.     

Coherent Beam Combination of Three Fiber Amplifiers with Multi-dithering Technique

Coherent beam combination of three W-level fiber amplifiers with multi-dithering technique is demonstrated. The multi-dithering technique is used for phase control in two channels. In the experiment, two channels are modulated by sine wave with 70 kHz and 100 kHz respectively, and two regular commercial DSP lock-in amplifiers and an industrial computer are used for electric signal processing in the feedback loop. The fringe contrast is advanced from 12% to 81%, and 78% coherent combination efficiency is obtained when the feedback loop is closed.     

Synchronization and Coherent Combining of Two Pulsed Fiber Ring Lasers Based on Direct Phase Modulation

We demonstrate a scalable architecture for coherent combining of pulsed fiber ring lasers based on mutual injection and direct phase modulation. By direct phase modulation in the common arm of two ring lasers, synchronous pulsed lasers can be generated and coherent combining of the two synchronous lasers is obtained. Two pulsed fiber ring lasers are coherently combined with 0.55μJ pulse energy and 10 μs pulse duration at a repetition rate of 27.5 kHz. Experimental results show that the two fiber ring lasers are phase locked with an invariable phase difference of π and have good temporal synchronization and spatial coherence. The combining efficiency of the two pulsed fiber laser reaches 90% and the fringe contrast is larger than 40%. Neither active phase control nor polarization control is used in our experiment and this method can be extended to combine more beams and higher repetition rate scaling up to higher power.     

谱线和耦合系数对光纤激光器相互注入锁定的影响

相互注入锁定是相干合成技术中最具发展前景的方案之一. 在两个光纤激光器相互注入锁定的基础上,利用可调节光纤光栅改变其中一个激光器的中心谱线和利用不同的分束器改变两个激光器间的耦合系数,同时监测两个激光器的远场光斑和谱线,判断激光器谱线和耦合系数对相互注入锁定的影响. 通过理论和实验得到：提高耦合系数可以提高锁定的频率差;两个激光器的谱线越接近锁定的效果越好,当激光器的谱线超过一定范围时,两个激光器将不能被锁定. 关键词： 光纤激光器 相干合成 相位锁定     

腔内相位调制全光纤脉冲激光器Ⅱ: 理论

针对低频腔内直接相位调制时全光纤激光器的脉冲现象,提出了合理的理论解释。基于光纤激光器自脉冲和固体激光器空间耦合的理论,对脉冲光产生的机理进行了分析,解释脉冲光产生的物理机制。假定光纤激光器中存在两个正交的偏振态,且两个偏振态以固体激光器空间耦合的模型耦合; 在相位调制作用下,两个耦合的偏振态之间相位差变化,导致两个偏振态的耦合特性发生变化,从而产生脉冲光输出。建立相应的数学模型,进行定性的理论分析。数值模拟表明,理论分析与实验结果符合得很好。     

5 kW振荡放大一体化高可靠性光纤激光器

振荡放大一体化光纤激光器具有振荡器抗回光能力强和放大器效率高的双重优点,在工业领域有着很好的应用前景。国防科大首次实现了输出功率大于5 kW的振荡放大一体化光纤激光器,光光效率约为80%。重点研究了该激光器在高功率运行时的可靠性和控制逻辑。首先,类比普通放大器中“不开种子、只开放大”的情况:在振荡部分泵浦源关闭时,瞬间开启放大部分泵浦源;其次,类比普通放大器中“不关放大、只关种子”的情况:在放大部分泵浦源开启时,瞬间关闭振荡部分泵浦。实验结果表明,以上两种极端情况下激光器都可稳定正常工作。首次验证了该激光器在控制逻辑和泵浦源开关顺序方面无特殊要求。