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基于NPR锁模的掺镱光纤激光器原理图如图1所示,腔内的增益介质是0.65 m长的掺镱光纤(YDF,Nufern SM-YSF-HI),其在975 nm波长处纤芯吸收率为250 dB/m。波长为980 nm的台式泵浦激光器,通过980/1 060 nm波分复用器(WDM)对掺镱光纤进行泵浦。两个偏振控制器(PC)与一个偏振相关隔离器(PD-ISO)组合构成锁模器件,实现非线性偏振旋转技术锁模。其中偏振控制器可调节腔内循环光的偏振状态,偏振相关隔离器用来确保光的单向传播方向。为了增加腔的克尔非线性和调节正色散值,腔内还接入了一段100 m长的1 060 nm单模光纤(HI1060)。整个环形腔的长度为115 m。最后谐振腔通过一个20/80的光纤耦合器(OC)将激光输出到腔外用于探测。示波器(Agilent, MSO6104A)、光谱分析仪(OSA, Anritsu MS9740A)以及射频谱仪(UNI-T UTS2020)分别用来记录输出激光的脉冲序列、光谱和射频谱。
图 1 (a) NPR锁模掺镱光纤激光器示意图, (b) NPR诱导的梳状透射谱仿真图
Figure 1. (a) Schematic of Yb-doped fiber laser mode locked by NPR, (b) comb transmission spectra induced by NPR
腔内NPR结构等效为一个Lyot滤波器,其透射函数由下式来表述[12]:
$$\begin{aligned} {\left| T \right|^2} =\;& {\sin ^2}\left( {{\alpha _1}} \right){\sin ^2}\left( {{\alpha _2}} \right) + {\cos ^2}\left( {{\alpha _1}} \right){\cos ^2}\left( {{\alpha _2}} \right) +\\ & 0.5\sin \left( {2{\alpha _1}} \right)\sin \left( {2{\alpha _2}} \right)\cos \left( {\Delta {\varphi _{\rm{L}}} + \Delta {\varphi _{{\rm{NL}}}}} \right) \end{aligned}$$ 式中:α1和α2分别为起偏器、检偏器与光纤快轴之间的夹角;ΔφL= φ0+2πL(ny-nx) / λ为线性相移,ΔφNL= 2πn2PLcos(2α1) /(λAeff)为非线性相移,φ0为光进入PC的初相位,ny和nx为光纤快轴和慢轴的折射率,L为腔长,λ为工作波长,n2为非线性折射率系数,P为瞬时光功率,λAeff为有效纤芯截面。上述公式的透射谱依赖于波长变化,其通道的间隔与双折射大小有关,因此通过调节PC,可以改变α1和α2的角度,PC挤压光纤会引起双折射大小的变化,使得腔内NPR诱导的梳状滤波透射谱的峰值位置和间隔随之发生相应的变化,从而在光纤激光器中实现可切换以及可调谐锁模[13-14]。
Wavelength switchable and tunable dissipative soliton mode-locking Yb-doped fiber laser
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摘要: 采用非线性偏振旋转(NPR)锁模技术,在全正色散掺镱光纤激光器中研究了波长可调谐可切换耗散孤子锁模现象。由于NPR所诱导的腔内梳状滤波效应,光纤激光器在中心波长1 042.8~1 050.2 nm以及1 040.9~1 048.1 nm处实现了波长可切换运作,可切换波长间隔分别为7.4 nm和7.2 nm,光谱宽度约为5.5 nm和2.7 nm。同时在1 042.77~1 045.33 nm之间观察到波长可调谐运作,调谐范围2.7 nm。另外在光纤激光器中还获得了稳定的双波长锁模和二阶谐波锁模。该实验的研究有利于加深人们对掺镱光纤激光器中锁模动力学行为的理解,并为多功能激光光源的设计提供了借鉴。Abstract: Wavelength switchable and tunable dissipative soliton mode-locking Yb-doped fiber laser based on nonlinear polarization rotation was studied. Due to the in-cavity comb filtering effect induced by NPR, the central wavelength of laser spectrum could switch between 1 042.8-1 050.2 nm, and 1 040.9-1 048.1 nm. The switchable wavelength intervals were 7.4 nm and 7.2 nm, respectively, and the spectral widths were about 5.5 nm and 2.7 nm. At the same time, the wavelength tunable operation was observed from 1 042.77 nm to 1 045.33 nm with the tuning range of 2.7 nm. In addition, stable dual wavelength mode locking and second harmonic mode locking were obtained in the fiber laser. The research of this experiment is helpful to deepen people's understanding of mode-locking dynamics in Yb-doped fiber laser and provide reference for the design of multi-function laser light source.
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