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基于光栅对和光谱滤波器的可调谐飞秒掺镱光纤激光器装置如图1所示。整个激光器为线性腔,所使用的光纤元件均为保偏器件。在此次装置中,976 nm泵浦光通过976/1030 nm波分复用器耦合进激光腔,1.0 m的掺镱光纤 (Nufern,PM-YDF-HI-HP,250 dB/m@975 nm)被用来提供增益。商用的SESAM (semiconductor saturable absorber mirror, BATOP GmbH-SAM-1030-55-500 fs)不仅可以用来实现激光器的锁模,同时作为腔镜置于激光腔的一端。起偏器用来确保激光在腔内的以固定的偏振态运行。为了顺利地使光栅对(FINISAR,T-1000-1040-3225-94,1000 lines/mm)为激光腔提供负色散,使用一个中心波长为1030 nm,最大工作距离200 mm的准直器输出激光,激光经由光栅对后穿过一个可调谐狭缝(Thorlabs,VA100/M)到达高反镜(1030 nm,99%反射率)再原路返回至准直器。为了使腔内保持足够的功率密度,使用输出比例为10%的保偏光纤耦合器作为输出端。各光纤型无源器件及掺镱光纤提供正色散,PM-YDF群速度色散(GVD)约为24 ps2/km。所有光纤无源元件均由PM 980制成,在1030 nm处的GVD约为24 ps2/km。实验装置中,光纤长度共5 m,正色散量约为+0.24 ps2,提供负色散的器件为光栅对,根据公式:
$$ GDD=\frac{{d}^{2}\phi }{d{\omega }^{2}}=\frac{-{\lambda }^{3}G}{2\pi {c}^{2}{d}^{2}{{\rm{cos}}}^{3}(\gamma -\theta )} $$ (1) 式中:
$ GDD $ 为光栅对的二阶色散;$ \gamma $ 为入射角;$ (\gamma -\theta ) $ 为衍射角;$ G $ 为光栅间距;λ为中心波长;$ d $ 为光栅常数。通过改变光栅间距,从而改变腔内净色散量。图 1 基于腔内光栅对和光谱滤波器的可调谐飞秒掺镱光纤激光器装置
Figure 1. Setup of tunable femtosecond Yb-doped fiber laser device based on grating pair in cavity and spectral filter
实验中研究并证明了输出耦合器的位置对输出情况的影响,为了获得更好的实验结果,输出耦合器被放置于图中增益光纤后端。当输出耦合器远离色散管理部分时,输出的光谱较宽,相对应的脉宽较窄[16]。
为了保证实验的顺利进行,首先对准直器的效率进行了测试,空间部分总光程为0.33 m,空间光往返一次的耦合效率为52%(光栅单次透过效率为93%,准直器耦合效率为70%)。实验中,可调谐狭缝等同于光谱滤波器,狭缝间距限制光谱调谐的范围,此次实验中,考虑到SESAM的工作带宽为30 nm,因此设计调谐带宽为40 nm,计算了应选取的狭缝宽度,狭缝带宽光路示意图如图2所示,计算公式如下:
$$ {\rm{sin}}\gamma + {\rm{sin}}{\theta _x} = {\rm{m×}}{\lambda _x}/{{d}} $$ (2) $$ {\omega _{1 - 2}} = G×\left( {\tan {\theta _2} - \tan {\theta _1}} \right)×\cos \gamma $$ (3) 式中:λx为激光波长;γ为激光入射角;θx为激光衍射角;G为光栅对间距;光栅常数d=0.001 mm;ω为所求狭缝带宽。将数值代入后可得到狭缝带宽为2.02 mm。
输出光通过光谱分析仪(YOKOGAWA AQ6373B,测量分辨率为0.02 nm)、示波器(RIGOL DS2302A, 300 MHz)和自相关仪(FEMTOCHROME FR-103XL)来测量其光谱、脉冲序列以及脉冲宽度。
1 015-1 046 nm tunable femtosecond Yb-doped fiber laser
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摘要: 近年来,超快掺镱锁模光纤激光器由于其转换效率高、操作方便、免维护、尺寸紧凑等优点,被广泛应用于工业加工、医疗外科、多光子成像等领域。在激光器中补偿群速度色散是获得皮秒甚至飞秒脉冲的有效方法。通过利用光栅对与光谱滤波器,实现了对激光波长、腔内色散和光谱宽度的灵活调节。该激光器能输出稳定的锁模脉冲,对应的基本重复频率为19.41 MHz。在+ 0.0127 ps2色散时中心波长1015~1037 nm可调;在+0.007 ps2色散时中心波长1015~1045 nm可调以及在−0.0127 ps2色散时中心波长1020~1046 nm可调。同时,当净腔色散从反常色散到近零色散变化时,光谱带宽可从1.40 nm调到19.38 nm,对应的压缩后脉冲宽度可从1.03 ps调至175.9 fs。该方案具备连续调整激光器状态的能力,有望用于高功率大能量飞秒激光前端,可以满足对激光器有多种谱宽及波长的应用需求。Abstract: In recent years, ultra-fast ytterbium doped mode-locked fiber lasers have been widely used in industrial processing, medical surgery, multiphoton imaging and other fields due to their high conversion efficiency, convenient operation, free maintenance and compact size. Compensation of group velocity dispersion in lasers was an effective method to obtain picosecond or femtosecond pulses. A grating pair and a spectral filter were used to adjust the dispersion and spectral width of laser wavelength in cavity flexibly, the laser can output stable mode-locked pulses with the corresponding fundamental repetition frequency of 19.41 MHz. The central wavelength can be adjusted from 1015 nm to 1037 nm when the net cavity dispersion was +0.0127 ps2. In the case of +0.007 ps2 dispersion, the central wavelength can be adjusted from 1015 nm to 1045 nm and when the net cavity dispersion was −0.0127 ps2, the central wavelength can be adjusted from 1020 nm to 1046 nm. Meanwhile, when the net cavity dispersion changed from abnormal to nearly zero dispersion, the spectral bandwidth can be adjusted from 1.40 nm to 19.38 nm, and the corresponding pulse width after compression can be adjusted from 1.03 ps to 175.9 fs. The proposed and demonstrated scheme is capable of continuously adjusting the state of the laser, and is expected to be used in the development of a femtosecond laser front-end with high power and high energy, which can meet the application requirements of lasers with a alterable of spectrum widths and wavelengths.
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Key words:
- femtosecond /
- ytterbium doped fiber laser /
- grating pair /
- laser tuning
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图 3 (a) 激光器在近零色散工作,光谱可从1015.97 nm调谐到1045.67 nm;(b) 激光器在正色散下工作,光谱可从1015.74 nm调谐到1037.72 nm;(c) 激光器在负色散下工作,光谱可从1020 nm调谐到1046 nm
Figure 3. (a) Laser can be tuned from 1015.97 nm to 1045.67 nm when the net cavity dispersion near zero; (b) Laser works in positive dispersion and the spectrum can be tuned from 1015.74 nm to 1037.72 nm; (c) Laser operates under negative dispersion and has a spectrum tuned from 1020 nm to 1046 nm
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