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基于NPR的可饱和吸收效应与激光器内的偏振状态和泵浦功率水平有关[24]。因此,在340 mW的泵浦功率条件下,通过调节激光腔内PC的旋转角度,激光器实现了单脉冲状态运行。如图2(a)所示,锁模脉冲光谱在中心波长两侧具有典型的Kelly边带[25],为孤子锁模的典型特征。锁模光谱的中心波长为1733 nm,3 dB带宽为6.3 nm,对应于501 fs的变换极限脉冲(双曲正割型)。由于NPR锁模光纤激光器中的Lyot滤波效应,通过调节PC激光器输出的中心波长可以在小范围内调谐。图2(c)是测量的脉冲序列,脉冲重复频率为19.56 MHz对应于51.1 ns的脉冲间隔。激光器的输出功率为1.4 mW,通过计算可知获得的单脉冲能量约为72 pJ。这里脉冲输出的能量受到孤子面积定理的限制,进一步升高泵浦功率会导致脉冲分裂,并形成多脉冲。图2(b)是脉冲的射频谱图,频率峰值位于19.56 MHz,这与激光器的基本重复频率一致。此外,射频谱信号表明激光脉冲输出的信噪比约为55 dB,证明了激光器工作在稳定单脉冲状态。实验中,由于输出脉冲功率较低,因此无法测量出可以表征脉冲宽度的自相关信号。此外,由于掺铥光纤在长波具有很高的发射截面[16],这导致激光器会在长波产生宽的ASE光。如图3所示,光纤带通滤波器的反射端口输出光谱包括长波ASE光谱,以及未被吸收的1560 nm泵浦光。因此,该结果证明了该光纤带通滤波器有效地过滤了这些光谱成分,从而保证了激光器在1.7 μm波段的稳定运行。
为了进一步研究脉冲在激光器内的传输特性,笔者数值模拟了脉冲在激光器内的演化过程。基于标准的对称分步傅里叶算法,采用非线性薛定谔方程对脉冲演化进行模拟:
$$ \frac{\partial A}{\partial z}=-i\frac{{\beta }_{2}}{2}\frac{{\partial }^{2}A}{\partial {t}^{2}}+\frac{g}{2}A+i\gamma {\left|A\right|}^{2}A $$ (1) 式中:A为脉冲的慢变包络振幅;i为虚数单位;变量z和t分别代表了传输距离和时间;β2、g和γ分别代表了群速度色散(GVD)、掺铥光纤增益系数和光纤克尔非线性系数。掺铥光纤的增益系数为:
$$ g=\frac{{g}_{0}}{\left( { 1+\dfrac{{E}_{P}}{{E}_{s}} } \right)} $$ (2) 式中:g0为小信号增益;
${E}_{P}=\displaystyle \int {\left|A\right|}^{2}{\rm d}t$ 是脉冲能量;Es为掺铥光纤的饱和能量,并且可以代表泵浦功率强度。另外,在模拟中掺铥光纤的增益带宽被设置为200 nm。NPR锁模过程可以用强度传递函数来表示:$$ T=1-\frac{{q}_{0}}{1+\dfrac{{\left|A\left(t\right)\right|}^{2}}{{P}_{0}}} $$ (3) 式中:q0代表了调制深度;
$ {\left|A\left(t\right)\right|}^{2} $ 和P0分别为瞬时强度和饱和功率。此外,考虑到激光器的实际参数和光纤位置排布,数值模拟使用了以下参数:g0=3 dBm−1,γ=0.003 W−1 m−1,增益光纤长度和色散分别为LTDF=1 m,β2=−0.025 ps2 m−1,单模光纤长度和色散为LSMF=9.5 m,β2=−0.045 ps2 m−1。调制深度和饱和功率分别为q0=0.5和P0=100 W,耦合输出比例为Rout=80%。数值模拟中超高斯型带通滤波器的中心波长和3 dB带宽分别设置为1734 nm和25 nm。光脉冲在激光器腔中经过多次循环后,最终得到了稳定的输出。图 2 激光器输出特性。(a) 锁模光谱;(b) 射频谱;(c) 脉冲序列
Figure 2. Laser output character. (a) Mode-locked spectrum; (b) Radio frequency spectrum; (c) Pulse train
图 3 光纤带通滤波器反射端口输出的泵浦光和ASE光谱
Figure 3. Pump light and ASE spectra from the reflection port of the fiber-based band pass filter
图4(a)为数值模拟的激光器孤子脉冲输出光谱,其展示出反常色散锁模孤子光纤激光器的典型光谱特征,即光谱中带有明显的Kelly边带。锁模孤子光谱的中心波长为1733.4 nm,3 dB带宽为6.35 nm,对应的输出脉冲宽度为497 fs,如图4(b)所示。通过与图2的实验结果相比较,可以看出数值模拟时域、频域结果与实验相吻合。注意到实验测量孤子脉冲光谱的Kelly边带具有不对称性,因此笔者模拟了脉冲在腔内不同位置的光谱演化来进一步分析边带不对称性,如图4(c)所示。可以看出:由于滤波器对孤子脉冲的整形作用,锁模光谱在经过滤波器时长波部分的边带被滤掉,进而导致了锁模光谱上不对称的Kelly边带[26]。图4(d)展示了锁模光谱的3 dB带宽在腔内演化过程,可以看到锁模孤子光谱带宽具有呼吸行为,这是由于激光腔内不同色散光纤及单元器件对孤子脉宽影响不同而导致的。
Research on a 1.7 μm all-fiber mode-locked Tm-doped fiber laser
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摘要: 1.7 μm超短脉冲光纤激光器在生物成像和材料加工等领域具有重要的应用前景,受到了科学家们的极大关注。基于非线性偏振旋转锁模技术,实验搭建了全光纤结构的1.7 μm锁模脉冲掺铥光纤激光器。通过在激光器内加入光纤滤波器抑制掺铥光纤中的长波激光发射,同时采用纤芯泵浦的方式有效获得了1.7 μm波段的增益。激光器输出脉冲的光谱中心波长为1733 nm,3 dB带宽为6.3 nm。锁模脉冲的重复频率为19.56 MHz,平均功率为1.4 mW。同时,数值模拟了脉冲在激光器的腔内演化。文中提出的1.7 μm全光纤锁模激光器有利于进一步提高1.7 μm激光源的稳定性和集成度,在生物成像等领域具有重要的应用价值。
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关键词:
- 超快光纤激光器 /
- 1.7 μm激光光源 /
- 掺铥光纤 /
- 被动锁模 /
- 超短脉冲
Abstract: The 1.7 μm ultrashort pulse fiber laser has received great attention for its promising applications in various fields, such as bioimaging and materials processing. We experimentally built a 1.7 μm all-fiber structure mode-locked Tm-doped fiber laser based on the nonlinear polarization rotation technique. The optical gain at the 1.7 μm waveband is effectively obtained by using a core-pumping scheme, and the ASE at long wavelengths is suppressed by a fiber-based bandpass filter in the cavity. The proposed fiber laser delivers an ultrashort pulse with a central wavelength of 1733 nm and a 3 dB bandwidth of 6.3 nm. The mode-locked pulse has a repetition frequency of 19.56 MHz and an average power of 1.4 mW. In addition, the evolution of the pulse inside the laser cavity is numerically simulated. The proposed 1.7 μm all-fiber mode-locked laser is beneficial to further improve the stability and integration of the 1.7 μm laser source, which could find important applications in fields such as bioimaging.-
Key words:
- ultrafast fiber laser /
- 1.7 μm laser source /
- Tm-doped fiber /
- passive mode-locking /
- ultrashort pulse
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