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图1为单、双波长可切换单频光纤激光器的实验装置示意图。采用480 mW的980 nm单模尾纤输出半导体激光器作为泵浦源,泵浦光由980/1060 nm波分复用器(Wavelength Division Multiplexer, WDM)耦合进入谐振腔内泵浦长度为1.5 m的高掺杂镱离子增益光纤(Ytterbium-doped Fiber, YDF, LIEKKI Yb1200-4/125);增益光纤与中心波长为1064 nm的环形器(Circulator, CIR)1端口连接,环形器2端口与长度为0.9 m保偏掺镱光纤(Polarization Maintaining Ytterbium Doped Fiber, PM-YDF, Coractive PM-Yb 401-4/125)相连接,在其另一端连接一带宽约为2 nm的高反光纤布拉格光栅(Fiber Bragg Grating, FBG,反射率大于99%)。偏振控制器(Polarization Controller, PC)、0.9 m保偏掺镱光纤和带宽为2 nm、反射率大于99%的FBG组成高精度滤波器,高精度滤波器对谐振腔内模式个数进行抑制,实现对谐振腔内单纵模的选取。环形器3端口与1×2的耦合器(Optical Coupler, OC,分光比为10∶90)一端相连接,耦合器另一端的90%端口与WDM的一端连接形成单频光纤激光器环形腔结构,所产生的单频激光由耦合器的10%端口输出,整个谐振腔长约为9.5 m,对应纵模频率间隔约为21.7 MHz。
环形器的2端到1端、3端到2端有大于45 dB的隔离度作用,确保光在谐振腔内的传输过程保持单向传输。同时,也能进一步阻止经过1.5 m高掺杂镱离子增益光纤的980 nm残余泵浦光进入PM-YDF,使得PM-YDF不被泵浦。FBG反射回来的光再次进入PM-YDF与入射光形成驻波干涉效应;驻波处波腹与波节的光强呈周期性分布从而引起折射率周期性变化,最后在未泵浦的PM-YDF中形成了动态光栅。该动态光栅具有极窄的反射带宽,同时具有中心波长自适应的特性,可以有效地抑制跳模现象。
整个滤波器中PM-YDF形成动态光栅的反射率的半高全宽(Full Width at Half Maxima, FWHM)的表达式为[21]:
$$ \Delta \lambda =\frac{C}{\lambda }\kappa \sqrt{{\left(\frac{\Delta n}{2n}\right)}^{2}+{\left(\frac{\lambda }{2n{L}_{g}}\right)}^{2}} $$ (1) 式中:
$ n、\lambda 、\Delta n、{L}_{g} $ 分别为光纤折射率、入射光波长、折射率差和PM-YDF的长度;其中$ \kappa = 2\Delta n/(\lambda n) $ 为动态光栅的耦合系数。图2为反射带宽与$ {L_g} $ 和$ \Delta n $ 的变化趋势,$ \Delta n $ 与输入进PM-YDF的光功率或泵浦光强度有关。 反射带宽随PM-YDF的加长和$ \Delta n $ 的减小而变小。图中曲线上标记的是实验中选取的PM-YDF的长度$ {L_g} $ 和折射率$ \Delta n $ 变化。在该实验中,结合理论与实验测试结果,最终选取PM-YDF的长度为0.9 m、对应的折射率差$ \Delta n $ 为1.39×10−7。图 2 动态光栅的反射带宽随PM-YDF长度和折射率差的变化
Figure 2. Variation of reflection bandwidth of dynamic grating with PM-YDF length and refractive index difference
另外,PM-YDF的双折射效应与FBG共同作用下可以产生梳状谱(周期性透射光谱),如图3(a)所示。通过旋转PC改变光路中的总相移量,不仅梳状光谱的透射系数会随之变化,而且梳状谱的透射曲线整体也会发生相应的波长平移。实验中,当梳状谱透射曲线的透射峰处于FBG反射带宽内时,则在该透射峰对应的波长处会产生单一波长激光振荡输出。通过调谐PC使得梳状谱曲线的透射峰在FBG反射带宽内发生移动,则可在FBG反射带宽范围内实现单波长调谐输出。当梳状谱曲线透过率最小位置(即透过率曲线波谷位置)处于FBG反射带宽中心位置附近时,该光纤激光器则有可能在如图3(b)中A、B两点处实现双波长激光振荡输出。
为了对理论分析结果进行验证,搭建如图4所示的检测装置对梳状光谱的特性进行验证,此检测光路采用980 nm半导体激光器作为泵浦源,经过980/1060 nm WDM与增益光纤连接,经过掺镱光纤中镱粒子增益放大后产生自发辐射光,经过CIR1端进入2端后的PM-YDF光进入光纤全反镜,返回2端后从CIR3端射出,输出端连接光谱仪进行测量。
在泵浦功率为250 mW时,对检测装置输出端的光信号进行采集,图5为采集的光谱信号。光谱周期为3.8 nm,与理论分析结果近乎吻合。在对PC进行旋转时,由于PC会对光纤进行一定的挤压,导致挤压点与其余段的单模光纤的应力分布不同,从而改变接触点的折射率分布。因此,在对PC旋转挤压操作时,使整个输出光谱如图5产生一定量的相移。图5中黑色的几字型曲线为FBG实测的反射带宽,如图5(a)所示,当梳状谱透射峰落在FBG反射带宽内,此时输出激光处于单波长输出状态;继续转动PC,使梳状谱波谷位置移动至FBG反射带宽中心附近时,如图5(b)所示,则此时输出的激光为双波长状态。
Wavelength switchable and tunable single-frequency narrow linewidth ytterbium doped fiber laser (Invited)
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摘要: 基于光纤环形激光器,设计出由三端口环形器、偏振控制器、未泵浦保偏掺镱光纤和光纤布拉格光栅组成的滤波器件作为高精度滤波器对谐振腔内的模式个数进行抑制,通过调谐偏振控制器,在保偏掺镱光纤内形成的梳状光谱和动态光栅,实现了窄线宽、单、双波长可切换单频掺镱光纤激光器。单波长运行时,在1064.37 nm处测得激光器输出线宽346 Hz,光信噪比大于50 dB,30 min内该激光器波长及功率的不稳定性均在0.01 nm和0.2 dB范围内。通过调节偏振控制器,单波长和双波长可以实现互相切换,双波长分别位于1064.156 nm和1065.236 nm。该技术为超窄线宽激光器的双波长输出提供了新的途径。Abstract: Based on fiber ring lasers, we designed a single-wavelength and dual-wavelength switchable single-frequency ytterbium-doped fiber laser. A high-finesse filter was composed of a three-port circulator, an unpumped ytterbium-doped fiber and a fiber Bragg grating, which was used to suppress the number of modes in the resonator. By tuning polarization controller, comb spectra and dynamic gratings were formed within Polarization Maintaining Ytterbium Doped Fiber(PM-YDF) and realized the output of a single-frequency fiber laser with narrow linewidth. The output linewidth of the laser was 346 Hz at 1064.37 nm, and the optical signal-to-noise ratio was greater than 50 dB. The instability of wavelength and power was within 0.01 nm and 0.2 dB in 30 min. By adjusting the polarization controller, the single and dual wavelengths could be switched to each other, and the dual wavelengths were located at 1064.156 nm and 1065.236 nm, respectively. This technology provides a new way for dual wavelength output of ultra-narrow linewidth lasers.
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Key words:
- fiber laser /
- narrow linewidth /
- switchable /
- double wavelength
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