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以CFBG为色散补偿元件的掺Yb3+光纤色散管理锁模激光器的数值模型结构如图1所示。它由两段单模光纤(SMF1和SMF2)、一段增益光纤(YDF)、可饱和吸收体(SA)和一段CFBG组成。增益光纤是一段增益带宽为40 nm、中心波长为1030 nm的掺Yb3+光纤。SMF1、SMF2和YDF的二阶色散系数(β(2))为0.023 ps2/m。位于SMF2后的可饱和吸收体作为腔内的锁模元件,调制深度为34%,非饱和损耗为22%,恢复时间为700 fs,饱和通量为70 μJ/cm2。采用CFBG作为腔内色散管理元件,在模拟中,使用一个光耦合器表示CFBG的输出,一个滤波器模拟CFBG的光谱滤波效应,和一段用于模拟CFBG色散管理的色散补偿光纤(Dispersion Compesation Fiber, DCF)。光耦合器的耦合比为89%;色散补偿光纤在中心波长为1030 nm提供−1 ps2/m的群速度色散其长度为0.1 m;滤波器带宽为17.5 nm,其中心波长为1030 nm。
图 1 CFBG色散管理激光器模拟示意图,字母A-E依次代表SMF1、YDF、SMF2、SESAM、SMF2、YDF、SMF1、CFBG
Figure 1. Schematic illustration of the dispersion-managed fiber laser cavity, the letters A-E represents SMF1, YDF, SMF2, SESAM, and CFBG in turn. OC: output coupler, DCF: dispersion compensating fiber
通过非线性薛定谔方程模拟了脉冲在色散管理光纤锁模激光器中的演化过程[21]:
$$ \begin{split} \frac{{\partial A(z,\tau )}}{{\partial z}} + & \frac{i}{2}\left({\beta ^{(2)}} + ig\frac{1}{{{\Omega _g}}}\right)\frac{{{\partial ^2}A(z,\tau )}}{{\partial {\tau ^2}}} = \\ & \frac{g}{2}A(z,\tau ) + \frac{{{\beta ^{(3)}}}}{6}\frac{{{\partial ^3}A(z,\tau )}}{{\partial {\tau ^3}}} +\\ & i\gamma {\left| {A(z,\tau )} \right|^2}A(z,\tau ) \\ \end{split} $$ (1) 式中:A(z,τ)为脉冲包络的振幅;z为传播坐标;τ为时间延迟参数;β(2)和β(3)分别为光纤的二阶和三阶色散系数;γ为非线性系数;Ωg为YDF的增益带宽;YDF的增益系数g为:
$$ g = \frac{{{g_0}}}{{1 + \dfrac{E}{{{E_{sat}}}}}} $$ (2) 式中:g0为小信号增益;E为腔内能量;Esat为增益饱和能量。可饱和吸收体模型由以下方程建立:
$$ T = 1 - \Delta T/(1 + {P_{ave}}/{P_{sat}}) - {\alpha _{ns}} $$ (3) 式中:ΔT=A0−αns为SA的调制深度;Pave为脉冲的平均功率;Psat为饱和功率;αns为SA的非饱和损耗。
使用分步傅里叶算法对非线性薛定谔方程求解。如果两个相邻循环输出脉冲能量差值的绝对值小于10−9,则认为所得到的解是收敛的。如果在3000圈内不能得到收敛解,认为激光器是不稳定的。
Nonlinearity optimization of dispersion-managed mode-locked Yb-doped fiber lasers with near-zero net cavity dispersion
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摘要: 目前,飞秒激光脉冲因脉冲宽度窄和峰值功率高的特点被广泛运用在多种领域中。其中,色散管理光纤锁模激光器因其特有的腔内呼吸机制使输出的激光脉冲能量更高,光谱更宽、脉宽更窄。使用啁啾布拉格光纤光栅进行色散管理的光纤锁模激光器能够实现真正的全光纤结构,提升激光器的紧凑性和稳定性,因此基于啁啾布拉格光纤光栅进行色散管理的光纤锁模激光器具有更加实际的应用意义。采用数值模拟的方法,研究了基于啁啾布拉格光纤光栅进行色散管理的掺镱光纤锁模激光器中单模光纤在腔内的不同分布对脉冲动力学过程和输出脉冲参数的影响。系统分析了谐振腔内净色散值不同时,腔内单模光纤的分布对脉冲在腔内的动力学过程的影响。模拟结果表明,在腔内净色散值为负时,啁啾布拉格光纤光栅与增益光纤间的单模光纤越短,光纤激光器维持稳定单脉冲运行的最大泵浦强度更高且输出光谱更宽,从而能够获得脉宽更窄的去啁啾脉冲;腔内净色散值越接近零时,啁啾布拉格光纤光栅与增益光纤间的单模光纤长度对输出脉冲参数作用的影响越显著;腔内净色散值为正时,单模光纤在腔内的分布对输出脉冲影响逐渐减弱,优化单模光纤分布提升锁模激光器性能并不明显。最后,提出了一种通过改变单模光纤在腔内的分布来提高激光器输出性能的优化方法。Abstract: Femtosecond laser pulses are widely used in many fields due to their narrow pulse width and high peak power. The dispersion-managed fiber mode-locked laser has higher pulse energy, wider spectrum and narrower pulse due to its unique in-cavity breathing mechanism. The fiber mode-locked laser using chirped Bragg grating for dispersion management can realize the real all-fiber structure and improve the compactness and stability of the laser. Therefore, the fiber mode-locked laser using chirped Bragg grating for dispersion management has more practical significance. The effects of different distribution of single-mode fiber in ytterbium-doped fiber mode-locked laser based on chirped fiber Bragg grating on pulse dynamics and output pulse parameters are studied by numerical simulation. The influence of the distribution of single-mode fiber on the dynamic process of pulse in the cavity is analyzed when the net dispersion is different. The simulation results show that when the net cavity dispersion is negative, the shorter the single-mode fiber between the chirped fiber Bragg grating and the gain fiber, the higher the pumping threshold and the wider the output spectrum of the fiber laser can maintain the stable monopulsing operation, so that the narrow pulse width can be obtained. When the net cavity dispersion is close to zero, the effect of the length of single mode fiber between the chirped fiber Bragg grating and the gain fiber on the output pulse parameters is more significant. When the net cavity dispersion is positive, the influence of the single mode fiber distribution in the cavity on the output pulse gradually weakens, and the performance of the mode-locked laser is not significantly improved by optimizing the single-mode fiber distribution. Finally, an optimization method is proposed to improve the output performance of the laser by changing the distribution of single mode fiber in the cavity.
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Key words:
- fiber laser /
- mode-locked laser /
- ultrafast laser /
- chirped fiber Bragg gratings
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图 2 (a)最大输出光谱宽度、最大泵浦强度和B积分随SMF1长度的变化;插图:上:当SMF1长度为0.1 m时,去啁啾脉冲的自相关迹;下:当SMF长度为0.1 m时,稳定锁模输出的光谱;(b)~(c)脉冲和光谱宽度在腔内的演化过程;(d)脉冲光谱随循环圈数演化图;(e)~(f) 脉冲时域和频域分别随循环圈数演化图
Figure 2. (a) The output maximum spectral width, the maximum pump strength, and B-integral versus the length of SMF1. Inset: The auto correlation trace of dechirped pulse when the SMF1 length is 0.1 m; (b)-(c) The evolution of the spectral width and the pulse duration in the cavity; (d) Evolution of pulse spectrum with the number of round trips; (e)-(f)the spectral width and the pulse duration evolve with the number of round trips
图 3 (a)最大输出光谱宽度、最大泵浦强度和B积分随SMF1长度的变化;插图:左: 当SMF1长度为0.1 m时,去啁啾脉冲的自相关迹;右:当SMF长度为0.1 m时,稳定锁模输出的光谱;(b)~(c)脉冲和光谱宽度在腔内的演化过程;(d)脉冲光谱随循环圈数演化图;(e)~(f) 脉冲时域和频域分别随循环圈数演化图
Figure 3. (a) The output maximum spectral width, the maximum pump strength, and B-integral versus the length of SMF1. Inset: The auto correlation trace of dechirped pulse when the SMF1 length is 0.1 m; (b)-(c) The evolution of the spectral width and the pulse duration in the cavity; (d) Evolution of pulse spectrum with the number of round trips; (e)-(f) the spectral width and the pulse duration evolve with the number of round trips
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