基于增益光纤长度优化的双波长运转掺铒光纤锁模激光器∗

石俊凯 纪荣祎 黎尧 刘娅 周维虎2)†

1)(中国科学院光电研究院激光测量技术研究室,北京 100094)

2)(中国科学院大学,北京 101407)

基于增益光纤长度优化的双波长运转掺铒光纤锁模激光器∗

石俊凯1)纪荣祎1)黎尧1)刘娅1)周维虎1)2)†

1)(中国科学院光电研究院激光测量技术研究室,北京 100094)

2)(中国科学院大学,北京 101407)

(2017年3月22日收到;2017年4月28日收到修改稿)

构建了可自启动的双波长运转掺铒光纤锁模激光器.通过优化增益光纤长度,利用掺铒光纤在1530 nm附近的再吸收效应调节激光器的增益谱,使激光器在1530 nm和1560 nm附近具有相同的增益强度.实验中采用31 cm掺铒光纤作为增益光纤,以透射式半导体可饱和吸收体作为锁模器件,实现了自启动双波长锁模运转.激光器锁模输出重复频率为58.01 MHz,信噪比为58.2 dB,最高输出功率为4.8mW.锁模输出的光谱在1532.4 nm和1552.3 nm处具有两个强度接近的谱峰,谱峰间距约为20 nm.该激光器无需手动调节即可实现双波长运转,更便于实际使用.

锁模激光器,双波长锁模运转,自启动锁模,自发辐射放大

1 引 言

近年来,多波长锁模激光器在光纤传感、光信号处理、精密光谱学、生物制药研究、微波/太赫兹光子学和波分复用光学光纤通信系统等领域的广泛应用[1,2],因此引起了人们极大的关注.多波长锁模运转已经在固体激光器,如Ti：sapphire激光器[3,4],Nd：CNGG激光器[5]和Yb：YAG陶瓷激光器[6]中成功实现.相比于固体激光器,光纤激光器具有结构简单、效率高、成本低等优点,成为获得多波长脉冲的更好选择.Schlager等[7]报道了基于双折射保偏光纤的双波长主动锁模光纤环形激光器.此后,人们在光纤激光器内引入色散光栅[8]、高非线性光纤[9]、偏置半导体光学放大器[10]或单采样光纤布拉格光栅[11],均实现了多波长主动锁模运转.多波长主动锁模光纤激光器具有高重频、窄线宽等优点,但是其结构复杂,需要引入腔外调解信号,不便于使用.多波长被动锁模光纤激光器具有脉冲短、峰值功率高、结构紧凑且无需腔外调节等优点而备受关注.

多波长被动锁模光纤激光器大多采用全光纤结构,以获得更高的环境稳定性.多波长被动锁模运转已经在基于非线性偏振旋转技术[12]或非线性光学环镜[13]锁模机制的光纤激光器中得以实现.近年来,真正的饱和吸收体(saturab le absorber, SA)如半导体可饱和吸收镜[14]、单壁碳纳米管[15]、石墨烯[16]和拓扑绝缘体[17]也已经被用于实现多波长被动锁模运转.目前已报道的多波长被动锁模光纤激光器需要在开机后将腔内的调节器件调整到合适的状态才能实现多波长运转,无法实现自启动,不便于使用.此外激光腔内引入的调制器件,如偏振控制器[14]或损耗调节器[1],增加了激光器结构的复杂度.

本文提出了一种在掺铒光纤(Er-doped fiber, EDF)激光器中实现双波长锁模运转的新方法.构建锁模激光器时选取合适长度的增益光纤,利用掺铒光纤的再吸收效应使激光器在1530 nm和1560 nm附近具有相同的增益强度.采用透射式半导体可饱和吸收体作为锁模器件,即可实现自启动双波长锁模运转.实验结果表明,当抽运功率高于锁模阈值时,激光器实现锁模运转,锁模输出的光谱在1532.4 nm和1552.3 nm处有两个谱峰.

2 实验装置

实验装置如图1所示.抽运源采用尾纤耦合输出的单模激光二极管(laser diode,LD),最大输出功率为600 mW,中心波长976 nm.抽运光通过波分复用器(wavelength division multiplexer, WDM)耦合进增益光纤,经过增益光纤后,残余抽运光(residual pum p,RP)由另一个WDM导出腔外.隔离器(isolator,Iso)为偏振无关隔离器,确保腔内激光的单向运转.增益光纤采用Nufern公司生产的SM-ESF-7/125型号光纤;腔内光纤器件的尾纤为Corning公司生产的EMF-28E型号光纤,总长度约为3.2 m.实验中采用光纤耦合的透射式半导体可饱和吸收体(Batop,德国)作为锁模器件,吸收率为58%,调制深度为35%,弛豫时间为2 ps.激光器以10：90光纤耦合器(optical coupler,OC)作为输出器件,将10%的脉冲能量导出腔外作为激光器输出.

图1 (网刊彩色)实验装置图 LD,激光二极管;W DM,波分复用器;EDF,掺铒光纤;Iso,隔离器;SA,饱和吸收体;OC,耦合输出;RP,残余抽运光Fig.1.(color on line)Experim ental setup:LD,laser d iode;W DM,wavelength d ivision m u ltip lexer;EDF, Er-doped fiber;Iso,isolator;SA,satu rab le absorber; OC,ou tpu t coup ler;RP,residual pum p.

实验中采用光谱仪(YOKOGAWA,AQ6370D)记录光谱,功率计(Thorlabs,PM 100D)测量激光功率,APE公司的自相关仪(Pu lse Check)测量脉冲的自相关曲线,锁模脉冲序列和一次谐波射频谱采用光电二极管(Newport,1801-FS)与数字示波器(KEYSIGHT,DSO9254A)和频谱分析仪(KEYSIGHT,N9010A)结合进行监测.

3 实验结果与分析

EDF辐射吸收谱如图2所示,增益谱谱峰和吸收谱谱峰在1530 nm附近重合.假设EDF长度不变,随着抽运功率降低,由于光纤的辐射再吸收效应,自发辐射放大(am plified spontaneous em ission,ASE)谱在1530 nm处的谱峰逐渐衰减,直至消失,ASE谱的谱峰从1530 nm转移到1560 nm[18].同理,假设抽运功率不变,随着EDF长度的增加,在辐射再吸收效应的作用下,ASE谱在1530 nm处的谱峰逐渐衰减,谱峰从1530 nm逐渐转移到1560 nm.在一定抽运功率下,选取合适长度的EDF,使ASE谱在两个波长处强度相同.以此作为增益光纤构建激光器,使激光器在两个波长处具有相同的增益强度,即可实现激光器的双波长运转.

图2 掺铒光纤的辐射与吸收截面谱[19]Fig.2.Em ission and absorp tion cross section spectra correspond ing to EDF[].

实验中分别选取长度为22,31,40和90 cm的EDF作为增益光纤构建激光器,分别测量低抽运功率下输出的ASE谱和最高抽运下输出的锁模光谱.图3(a)为不同增益光纤长度条件下输出的ASE谱.可以看出,激光器在1530 nm和1560 nm处有两个增益峰.当增益光纤长度为22 cm时,1530 nm处的增益强度大于1560 nm处.随着增益光纤长度的增加,在EDF辐射再吸收效应的作用下,1530 nm处的增益峰逐渐减弱.当EDF长度为31 cm时,两处增益峰的强度基本一致.当EDF长度增加到90 cm时,1530 nm处的增益峰接近湮灭.在这一过程中,同样在再吸收效应的作用下,1560 nm附近的增益峰向长波方向漂移.实验现象与分析结果一致.

图3 不同增益光纤长度条件下激光器输出的(a)ASE谱和(b)锁模输出光谱Fig.3.(a)ASE spectra and(b)m ode-locking outpu t spectra versus d iff erent length of EDF.

图3(b)为不同增益光纤长度条件下输出的锁模光谱.当EDF长度为22 cm时,对应的锁模光谱只有1530 nm处一个波峰,1560 nm处的激光频率在模式竞争的作用下几近湮灭.当EDF长度为31 cm时,由于1530 nm和1560 nm处的增益强度基本相同,因此激光器锁模时将两个波段同时锁定,且两个波段的锁模光谱强度也非常接近.随着EDF长度的继续增加,1530 nm处的锁模光谱逐渐衰减,直至完全湮灭.同时,锁模光谱在1560 nm附近的波峰在增益谱的影响下向长波方向漂移.

根据增益光纤长度对比实验结果,选取增益光纤长度为31 cm.激光器输出结果如图4和图5所示.图4为激光器输出的斜效率曲线,如图所示,随着抽运功率增加,激光器首先实现连续波(continuous wave,CW)运转.当抽运功率提升到390 mW时,激光器的CW输出功率为0.54 mW.激光的低转换效率是由短增益光纤提供的较低增益和SA的高非饱和损耗共同造成的.继续提升抽运功率至395 mW,激光器实现锁模运转,输出功率跃升至3.2 mW.在抽运功率为530 mW时,激光器获得最高输出功率4.8 mW.实验中发现,在激光器锁模运转状态下,将抽运功率降低至88 mW,激光器仍能维持锁模运转.这种现象被称为抽运滞后效应[20].

图4 激光器斜效率曲线Fig.4.Output power as a function of pum p power.

图5为最高抽运功率条件下激光器的输出特性.图5(a)为激光器输出光谱,插图为对数坐标下的光谱.光谱在1532.4 nm和1552.3 nm处有两个谱峰,半高宽分别为6.6 nm和5.2 nm. 1552.3 nm处谱峰与两谱峰之间的低谷处强度分别为1532.4 nm处强度的92%和13%.图5(b)为输出脉冲的自相关曲线,其半高全宽度(fullw idth half maximum,FWHM)为353.9 fs,假设脉冲为高斯型脉冲,对应的脉冲FWHM为250.3 fs.图5(c)和图5(d)分别为激光器输出的脉冲序列和一次谐波射频谱.激光器输出脉冲的重复频率为58.01MHz,对应的脉冲间距为17.24 ns.激光器锁模输出的信噪比为58.2 dB.以上测量结果表明,该激光器实现了可自启动的双波长锁模运转,且锁模状态运转稳定.

图5 最高抽运功率下激光器输出 (a)线性坐标和对数坐标(插图)下的光谱;(b)自相关曲线;(c)脉冲序列;(d)一次谐波射频谱Fig.5.Laser outpu t characteristics corresponding to m ax pum p power:(a)Spectra on linear scale and log scale(inset);(b)autocorrelation trace;(c)pu lse train;(d)RF spectrum.

4 结 论

构建了一种新型双波长运转掺铒光纤锁模激光器.根据不同长度增益光纤的对比实验结果,选取31 cm掺铒光纤作为增益光纤,激光器在1530 nm和1560 nm附近具有相同的增益系数,实现了自启动双波长锁模运转.激光器输出重复频率为58.01 MHz,信噪比为58.2 dB,最高输出功率为4.8 mW,锁模输出的光谱在1532.4 nm和1552.3 nm处具有两个谱峰,谱峰间距约为20 nm.该激光器无需手动调节即可实现双波长运转,使用时降低了对操作人员的要求,便于应用推广.

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(Received 22 March 2017;revised manuscript received 28 April 2017)

Dual-wavelength mode-locked Er-doped fiber laser based on optimizing gain fiber length∗

Shi Jun-Kai1)Ji Rong-Yi1)Li Yao1)Liu Ya1)Zhou Wei-Hu1)2)†

1)(Laboratory of Laser M easurem ent Technology,Academ y ofOpto-E lectronics,Chinese Academ y of Sciences,

Beijing 100094,China)
2)(University of Chinese Academ y of Sciences,Beijing 101407,China)

Recently,multi-wavelength pu lsed lasers have becom e a research hotspot due to their versatile app lications,such as precision spectroscopy,microwave/terahertz photonics,optical signal processing,and wavelength division mu ltip lexed op tical fiber communication system s.As a prom ising candidate,passively m ode-locked fiber laser has the advantages of ultrashort pulse,ultrahigh peak power,com pact structure and low-cost.In the existing mu lti-wavelength passively mode-locked fiber lasers,mu lti-wavelength mode-locked operation is achieved by ad justing the intracavity modu lators to a proper state after laser has worked.It is inconvenient for practical use,so,its app lication scope is restricted.In this paper,a new method to achieve dual-wavelength m ode-locked operation in an erbium-doped fiber laser is p roposed. For an erbium-doped fiber,the peaks of both absorption and em ission spectra overlap in the 1530 nm-region.So the em ission light in the 1530 nm-region w ill be re-absorbed by the erbium-doped fiber with low pum p power or long gain fiber.Utilizing the em ission re-absorption effect,the gain spectrum can be m odified by different lengths of gain fiber. In the experim ent,an all-fiber ring cavity is adopted and a transm ission-type sem iconductor saturable absorber is used as a modelocker.The cavity consists of～3.2-m-long single m ode fiber and an erbium-doped fiber.Gain fibers with different lengths are used in the cavity to reveal the dependence of em ission re-absorption on both gain spectrum and mode-locked output spectrum.According to the experimental resu lts,there are two hum ps in the am p lified spontaneous em ission spectrum located in the 1530 nm-region and 1560 nm-region,respectively.W ith the gain fiber length increasing, gain spectrum in the 1530 nm-region is suppressed,and gain intensity in the 1560 nm-region gradually surpasses that in the 1530 nm-region.Based on the experim ental results,self-starting dual-wavelength m ode-locked operation is achieved with a 31-cm-long gain fiber.The two spectral peaks with close intensity are located at 1532.4 nm and 1552.3 nm, respectively.The m aximum output power is 4.8 mW at a repetition rate of 58.01 MHz and a signal-to-noise ratio of 58.2 dB.This self-starting dual-wavelength m ode-locked erbium-doped fiber laser is convenient for practical use and can meet the requirements formany potential app lications.

mode-locked lasers,dual-wavelength mode-locking operation,self-starting mode-locking, am p lified spontaneous em ission

PACS:42.55.-f,42.55.Wd,42.55.Xi,42.60.Fc DO I:10.7498/aps.66.134203

∗国家自然科学基金(批准号:61475162)、中国科学院国际合作局对外合作重点项目(批准号:181811KYSB20160029)、中国科学院前沿科学重点研究项目(批准号:QYZDY-SSW-JSC 008)和国家重大科学仪器设备专项(批准号:2011YQ 120022, 2014YQ 090709)资助的课题.

†通信作者.E-m ail:zhouweihu@aoe.ac.cn

PACS:42.55.-f,42.55.Wd,42.55.Xi,42.60.Fc DO I:10.7498/aps.66.134203

*Pro ject supported by the National Natu ral Science Foundation of China(G rant No.61475162),the Key Pro ject of Bureau of International Co-operation,Chinese Academ y of Sciences(G rant No.181811KYSB 20160029),the K ey Research Pro ject of Bureau of Frontier Sciences and Education,Chinese Academ y of Sciences(G rant No.QYZDY-SSW-JSC008),and the National Key Scientifi c Instrum ents and Equipm ent Developm ent of China(G rant Nos.2011YQ 120022,2014YQ 090709).

†Corresponding author.E-m ail:zhouweihu@aoe.ac.cn