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双波长单纵模光纤激光器的原理如图 1所示。980nm抽运光源发出的光经过3dB耦合器C1后被分为两路,一路光经过波分复用器(wavelength division multiplexer, WDM)WDM1和掺铒光纤(erbium-doped fiber, EDF)EDF1,激发出1550nm波段的荧光, 此荧光经过环形器(circulator, CIR)CIR1到达光纤布喇格光栅(fiber Bragg grating, FBG)FBG1。满足FBG1布喇格波长的光被其反射回来,再次经过CIR1、3dB耦合器C2、C3和C4、1 ∶9耦合器C、环形器CIR3、光纤光栅FBG3、光纤隔离器(isolator, ISO)以及WDM1,又通过EDF1,光强被放大,此光经过CIR1到达FBG1,再次被FBG1反射,重复以上路径,当增益大于损耗时,从1 ∶9耦合器C的一端输出波长为FBG1的布喇格波长的激光,输入光谱仪检测。另一路光经过波分复用器WDM2和掺铒光纤EDF2,激发出1550nm波段的荧光, 此荧光经过环形器CIR2到达光纤光栅FBG2。满足FBG2布喇格波长的光被其反射回来,再次经过CIR2、3dB耦合器C2、C3和C4、1 ∶9耦合器C以及CIR3,到达FBG3。由于FBG3的布喇格波长与FBG2的布喇格波长相同,光到达FBG3后即被其反射回来,再次经过CIR3、WDM2和EDF2,光强被放大,此光经过CIR2到达FBG2,再次被FBG2反射,重复以上路径,当增益大于损耗时,从1 ∶9耦合器C的一端输出波长为FBG2的布喇格波长的激光,输入光谱仪检测。
3dB耦合器C3和C4的作用是构成复合子谐振腔,使每个激光谐振腔都成为复合激光谐振腔,从而使得每个激光谐振腔都输出单纵模激光。3dB耦合器C3和C4构成3个子腔,分别是C3构成的腔长为L1的子腔、C4构成的腔长为L2的子腔,以及由C3和C4组合构成的腔长为L1+L2的子腔。
每个子腔的纵模间隔分别为:
$ \left\{ {\begin{array}{*{20}{l}} {\Delta {\mathit{v}_1} = \frac{c}{{n{L_1}}}}\\ {\Delta {\mathit{v}_2} = \frac{\mathit{c}}{{\mathit{n}{\mathit{L}_2}}}}\\ {\Delta {\mathit{v}_3} = \frac{c}{{\mathit{n}({\mathit{L}_1} + {\mathit{L}_2})}}} \end{array}} \right. $
(1) 式中,c为光速,n为光纤的折射率,L1,L2和L1+L2分别为3个子腔的腔长。
根据游标效应,子腔串联加入激光谐振腔后,每个激光谐振腔的纵模间隔扩大为主腔纵模间隔与Δν1, Δν2和Δν3的最小公倍数。只要使每个激光谐振腔的纵模间隔不小于光纤光栅的反射谱宽,即可使每个激光谐振腔发出单纵模激光。
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三波长单纵模光纤激光器是在双波长单纵模光纤激光器的基础上再增加一个激光谐振腔, 其工作原理如图 2所示。分别以FBG1, FBG2, FBG3为波长选择元件构成3个激光环行谐振腔。每个激光环行谐振腔的路径与图 1中的激光环行谐振腔的路径相似。
与图 1所示系统相同,图 2中的3dB耦合器C1和C2的作用是构成复合子谐振腔,与每个激光环行谐振腔共同作用,使每个激光环行谐振腔都输出单纵模激光。
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如图 1所示的双波长单纵模光纤激光器中,FBG1的布喇格中心波长为1543.214nm,3dB带宽为0.043nm,反射率为50%;FBG2的布喇格中心波长为1554.552nm,3dB带宽为0.042nm,反射率为50%;FBG3的布喇格中心波长为1554.550nm,3dB带宽为0.6nm,反射率为99.9%以上;EDF1和EDF2的长度约为1m。由FBG1反射的光经过FBG3时透射,在对应的激光谐振腔传输,当增益大于损耗时,从1 ∶9耦合器C的一端输出波长为λ1=1543.212nm的激光;由FBG2反射的光经过FBG3时被反射,在对应的激光谐振腔传输,当增益大于损耗时,从1 ∶9耦合器C的同一端输出波长为λ1=1554.552nm的激光。输出双波长激光由光谱仪检测,如图 3所示。
由于两个波长的激光分别由两个激光谐振腔形成,每个激光谐振腔都有各自的掺铒光纤作为增益介质,所以两个波长之间没有模竞争,都有稳定的功率。为了测试每个波长的功率稳定性,将两个波长的光分开,如图 4所示。FBG4的布喇格波长与FBG1的布喇格波长相同,两个波长的光分别透过FBG4,及被FBG4反射。
分别用光功率计探测两个波长的功率,测试结果如图 5所示。两个波长的功率变化范围分别不超过±0.22μW和±0.20μW,两个波长的功率都很稳定。
利用光谱仪分别对两个波长的波长稳定性进行测试,测试结果如图 6所示。在4h内,两个波长值的最大变化量不大于0.01nm,波长稳定性达10-6。
设计利用外差光纤马赫-曾德尔干涉实验来验证每个波长的激光是否是单纵模。实验系统如图 7所示,以研制的双波长光纤激光器作为光源,声光调制器的调制频率为1.2MHz,将双波长光纤激光器的两个波长先后输入光纤马赫-曾德尔干涉仪。
当在双波长光纤激光器的谐振腔中没有嵌入由3dB耦合器构成的复合子腔时,探测器探测到的外差干涉信号如图 8所示。外差干涉信号的幅值不断地变化,说明激光器同时输出多个纵模激光,探测到的外差干涉信号是每一个纵模的外差干涉信号的叠加。
然后,在激光谐振腔中嵌入两个3dB耦合器,两个3dB耦合器构成3个激光谐振子腔。3个谐振子腔分别是由两个3dB耦合器构成的腔长为L1=40mm和L2=45mm的两个子腔,以及这两个子腔共同构成腔长为L3=85mm的第3个子腔。此时,激光谐振腔的纵模间隔是这3个子腔的纵模间隔与激光主谐振腔的纵模间隔的最小公倍数。
3个子腔的纵模间隔分别为:
$ \left\{ {\begin{array}{*{20}{l}} {\Delta {\mathit{v}_1} = \frac{c}{{n{L_1}}} = 5.\;172 \times {{10}^8}{\rm{Hz}}}\\ {\Delta {\mathit{v}_2} = \frac{\mathit{c}}{{\mathit{n}{\mathit{L}_2}}} = 4.\;598 \times {{10}^8}{\rm{Hz}}}\\ {\Delta {\mathit{v}_3} = \frac{c}{{\mathit{n}({\mathit{L}_1} + {\mathit{L}_2})}} = 2.\;434 \times {{10}^8}{\rm{Hz}}} \end{array}} \right. $
(2) 激光主谐振腔的长度为1.8m,主谐振腔的纵模间隔为Δν=c/(nL)=1.149×108Hz。激光主谐振腔的纵模间隔与3个子腔的纵模间隔的最小公倍数为6.651×109Hz,嵌入两个3dB耦合器构成的子腔以后,激光谐振腔的纵模间隔扩大。两个激光谐振腔的波长选择元件FBG1和FBG2的布喇格波长的3dB带宽约为0.043nm,对应的频率宽为5.369×109Hz。激光纵模间隔大于波长选择元件的反射谱带宽,所以,激光谐振腔中只能一个纵模起振并形成激光。
将嵌入两个3dB耦合器构成的子腔的光纤激光器作为光源,重复以上实验,探测器探测到的外差干涉信号如图 9所示。外差干涉信号的幅值恒定不变,说明这是一个激光纵模产生的外差干涉信号。由此可知,此双波长光纤激光器实现了单纵模输出。
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在双波长单纵模光纤激光器的基础上再增加一个激光环行谐振腔,构成原理如图 2所示的三波长单纵模光纤激光器。在第3个激光谐振腔中,波长选择元件FBG3的布喇格中心波长为1548.051nm,3dB带宽为0.041nm,反射率均为55%,FBG4的布喇格中心波长与FBG3的布喇格中心波长相同,3dB带宽超过0.6nm,反射率99.9%以上。当980nm抽运光源输出功率约400mW时,从1 ∶9耦合器C的一个端口输出三波长激光,由光谱仪检测,如图 10所示。
同样地,利用检测双波长单纵模光纤激光器各个波长的功率稳定性的方法,测试三波长单纵模激光器的每个波长的功率稳定性。测试结果如图 11所示,三波长激光的功率变化范围分别不超过0.20μW, 0.17μW, 0.22μW,输出功率非常稳定。
同样地,利用检测双波长单纵模光纤激光器各个波长稳定性的方法,测试三波长单纵模激光器的每个波长的稳定性。测量结果如图 12所示。测试期间3个波长的最大漂移量均小于0.01nm,每个波长的稳定性均达10-6。
同样地,利用与双波长单纵模光纤激光器相同的方法,验证了此三波长光纤激光器每个波长均是单纵模激光。
可调谐单纵模多波长光纤激光器的研究
Research on tunable single-longitudinal multi-wavelength fiber laser
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摘要: 为了实现对台阶高度和绝对距离等物理量的高精度干涉测量,采用在一个光纤激光器中构建多个激光谐振腔的方法,构建了能同时发出多波长激光的光纤激光器。每个激光谐振腔都利用掺铒光纤作为增益介质,利用光纤光栅作为波长选择元件,改变光纤光栅的布喇格波长,即可改变对应谐振腔的激光波长。各个激光谐振腔独立但部分重叠,在重叠区域利用光纤耦合器构成复合子腔,使每个激光谐振腔都是复合激光谐振腔,从而使每个激光谐振腔都发出单纵模激光。结果表明,该光纤激光器能同时发出功率和频率都稳定的多波长激光,且每个波长都是单纵模激光;在4h内,每个波长的波长稳定性优于0.01nm。该设计对可调谐单纵模多波长光纤激光器的研究是有帮助的。Abstract: In order to achieve high-precision interferometric measurement of physical quantities such as step height and absolute distance, etc, an optical fiber laser was constructed which includes multiple fiber laser resonators and can emit several wavelengths simultaneously. The erbium-doped fibers were used as gain material in each laser resonator, and the fiber Bragg gratings were employed as wavelength-choosing elements. By changing the Bragg wavelength of a fiber Bragg grating, the wavelength emitted from the corresponding resonator can be changed. Each of the cavities was independent and all of the cavities were overlapped each other. In the overlapped part, optical fiber couplers were used to configure sub-cavities and thus each resonator can emit single longitudinal mode laser. The experimental results show that each of the resonators can emit single longitudinal mode laser with stable power and stable wavelength. And the stability of each wavelength can be less than 0.01 nm within 4h. This design is helpful to the research of tunable single longitudinal mode multi wavelength fiber laser.
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Key words:
- lasers /
- multi-wavelength /
- resonant cavity /
- single longitudinal mode
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