石俊凱 紀榮祎 黎堯 劉婭 周維虎2)?

1)(中國科學院光電研究院激光測量技術研究室,北京 100094)

2)(中國科學院大學,北京 101407)

基于增益光纖長度優(yōu)化的雙波長運轉摻鉺光纖鎖模激光器?

石俊凱1)紀榮祎1)黎堯1)劉婭1)周維虎1)2)?

1)(中國科學院光電研究院激光測量技術研究室,北京 100094)

2)(中國科學院大學,北京 101407)

(2017年3月22日收到;2017年4月28日收到修改稿)

構建了可自啟動的雙波長運轉摻鉺光纖鎖模激光器.通過優(yōu)化增益光纖長度,利用摻鉺光纖在1530 nm附近的再吸收效應調(diào)節(jié)激光器的增益譜,使激光器在1530 nm和1560 nm附近具有相同的增益強度.實驗中采用31 cm摻鉺光纖作為增益光纖,以透射式半導體可飽和吸收體作為鎖模器件,實現(xiàn)了自啟動雙波長鎖模運轉.激光器鎖模輸出重復頻率為58.01 MHz,信噪比為58.2 dB,最高輸出功率為4.8mW.鎖模輸出的光譜在1532.4 nm和1552.3 nm處具有兩個強度接近的譜峰,譜峰間距約為20 nm.該激光器無需手動調(diào)節(jié)即可實現(xiàn)雙波長運轉,更便于實際使用.

鎖模激光器,雙波長鎖模運轉,自啟動鎖模,自發(fā)輻射放大

1 引 言

近年來,多波長鎖模激光器在光纖傳感、光信號處理、精密光譜學、生物制藥研究、微波/太赫茲光子學和波分復用光學光纖通信系統(tǒng)等領域的廣泛應用[1,2],因此引起了人們極大的關注.多波長鎖模運轉已經(jīng)在固體激光器,如Ti：sapphire激光器[3,4],Nd：CNGG激光器[5]和Yb：YAG陶瓷激光器[6]中成功實現(xiàn).相比于固體激光器,光纖激光器具有結構簡單、效率高、成本低等優(yōu)點,成為獲得多波長脈沖的更好選擇.Schlager等[7]報道了基于雙折射保偏光纖的雙波長主動鎖模光纖環(huán)形激光器.此后,人們在光纖激光器內(nèi)引入色散光柵[8]、高非線性光纖[9]、偏置半導體光學放大器[10]或單采樣光纖布拉格光柵[11],均實現(xiàn)了多波長主動鎖模運轉.多波長主動鎖模光纖激光器具有高重頻、窄線寬等優(yōu)點,但是其結構復雜,需要引入腔外調(diào)解信號,不便于使用.多波長被動鎖模光纖激光器具有脈沖短、峰值功率高、結構緊湊且無需腔外調(diào)節(jié)等優(yōu)點而備受關注.

多波長被動鎖模光纖激光器大多采用全光纖結構,以獲得更高的環(huán)境穩(wěn)定性.多波長被動鎖模運轉已經(jīng)在基于非線性偏振旋轉技術[12]或非線性光學環(huán)鏡[13]鎖模機制的光纖激光器中得以實現(xiàn).近年來,真正的飽和吸收體(saturab le absorber, SA)如半導體可飽和吸收鏡[14]、單壁碳納米管[15]、石墨烯[16]和拓撲絕緣體[17]也已經(jīng)被用于實現(xiàn)多波長被動鎖模運轉.目前已報道的多波長被動鎖模光纖激光器需要在開機后將腔內(nèi)的調(diào)節(jié)器件調(diào)整到合適的狀態(tài)才能實現(xiàn)多波長運轉,無法實現(xiàn)自啟動,不便于使用.此外激光腔內(nèi)引入的調(diào)制器件,如偏振控制器[14]或損耗調(diào)節(jié)器[1],增加了激光器結構的復雜度.

本文提出了一種在摻鉺光纖(Er-doped fiber, EDF)激光器中實現(xiàn)雙波長鎖模運轉的新方法.構建鎖模激光器時選取合適長度的增益光纖,利用摻鉺光纖的再吸收效應使激光器在1530 nm和1560 nm附近具有相同的增益強度.采用透射式半導體可飽和吸收體作為鎖模器件,即可實現(xiàn)自啟動雙波長鎖模運轉.實驗結果表明,當抽運功率高于鎖模閾值時,激光器實現(xiàn)鎖模運轉,鎖模輸出的光譜在1532.4 nm和1552.3 nm處有兩個譜峰.

2 實驗裝置

實驗裝置如圖1所示.抽運源采用尾纖耦合輸出的單模激光二極管(laser diode,LD),最大輸出功率為600 mW,中心波長976 nm.抽運光通過波分復用器(wavelength division multiplexer, WDM)耦合進增益光纖,經(jīng)過增益光纖后,殘余抽運光(residual pum p,RP)由另一個WDM導出腔外.隔離器(isolator,Iso)為偏振無關隔離器,確保腔內(nèi)激光的單向運轉.增益光纖采用Nufern公司生產(chǎn)的SM-ESF-7/125型號光纖;腔內(nèi)光纖器件的尾纖為Corning公司生產(chǎn)的EMF-28E型號光纖,總長度約為3.2 m.實驗中采用光纖耦合的透射式半導體可飽和吸收體(Batop,德國)作為鎖模器件,吸收率為58%,調(diào)制深度為35%,弛豫時間為2 ps.激光器以10：90光纖耦合器(optical coupler,OC)作為輸出器件,將10%的脈沖能量導出腔外作為激光器輸出.

圖1 (網(wǎng)刊彩色)實驗裝置圖 LD,激光二極管;W DM,波分復用器;EDF,摻鉺光纖;Iso,隔離器;SA,飽和吸收體;OC,耦合輸出;RP,殘余抽運光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.

實驗中采用光譜儀(YOKOGAWA,AQ6370D)記錄光譜,功率計(Thorlabs,PM 100D)測量激光功率,APE公司的自相關儀(Pu lse Check)測量脈沖的自相關曲線,鎖模脈沖序列和一次諧波射頻譜采用光電二極管(Newport,1801-FS)與數(shù)字示波器(KEYSIGHT,DSO9254A)和頻譜分析儀(KEYSIGHT,N9010A)結合進行監(jiān)測.

3 實驗結果與分析

EDF輻射吸收譜如圖2所示,增益譜譜峰和吸收譜譜峰在1530 nm附近重合.假設EDF長度不變,隨著抽運功率降低,由于光纖的輻射再吸收效應,自發(fā)輻射放大(am plified spontaneous em ission,ASE)譜在1530 nm處的譜峰逐漸衰減,直至消失,ASE譜的譜峰從1530 nm轉移到1560 nm[18].同理,假設抽運功率不變,隨著EDF長度的增加,在輻射再吸收效應的作用下,ASE譜在1530 nm處的譜峰逐漸衰減,譜峰從1530 nm逐漸轉移到1560 nm.在一定抽運功率下,選取合適長度的EDF,使ASE譜在兩個波長處強度相同.以此作為增益光纖構建激光器,使激光器在兩個波長處具有相同的增益強度,即可實現(xiàn)激光器的雙波長運轉.

圖2 摻鉺光纖的輻射與吸收截面譜[19]Fig.2.Em ission and absorp tion cross section spectra correspond ing to EDF[].

實驗中分別選取長度為22,31,40和90 cm的EDF作為增益光纖構建激光器,分別測量低抽運功率下輸出的ASE譜和最高抽運下輸出的鎖模光譜.圖3(a)為不同增益光纖長度條件下輸出的ASE譜.可以看出,激光器在1530 nm和1560 nm處有兩個增益峰.當增益光纖長度為22 cm時,1530 nm處的增益強度大于1560 nm處.隨著增益光纖長度的增加,在EDF輻射再吸收效應的作用下,1530 nm處的增益峰逐漸減弱.當EDF長度為31 cm時,兩處增益峰的強度基本一致.當EDF長度增加到90 cm時,1530 nm處的增益峰接近湮滅.在這一過程中,同樣在再吸收效應的作用下,1560 nm附近的增益峰向長波方向漂移.實驗現(xiàn)象與分析結果一致.

圖3 不同增益光纖長度條件下激光器輸出的(a)ASE譜和(b)鎖模輸出光譜Fig.3.(a)ASE spectra and(b)m ode-locking outpu t spectra versus d iff erent length of EDF.

圖3(b)為不同增益光纖長度條件下輸出的鎖模光譜.當EDF長度為22 cm時,對應的鎖模光譜只有1530 nm處一個波峰,1560 nm處的激光頻率在模式競爭的作用下幾近湮滅.當EDF長度為31 cm時,由于1530 nm和1560 nm處的增益強度基本相同,因此激光器鎖模時將兩個波段同時鎖定,且兩個波段的鎖模光譜強度也非常接近.隨著EDF長度的繼續(xù)增加,1530 nm處的鎖模光譜逐漸衰減,直至完全湮滅.同時,鎖模光譜在1560 nm附近的波峰在增益譜的影響下向長波方向漂移.

根據(jù)增益光纖長度對比實驗結果,選取增益光纖長度為31 cm.激光器輸出結果如圖4和圖5所示.圖4為激光器輸出的斜效率曲線,如圖所示,隨著抽運功率增加,激光器首先實現(xiàn)連續(xù)波(continuous wave,CW)運轉.當抽運功率提升到390 mW時,激光器的CW輸出功率為0.54 mW.激光的低轉換效率是由短增益光纖提供的較低增益和SA的高非飽和損耗共同造成的.繼續(xù)提升抽運功率至395 mW,激光器實現(xiàn)鎖模運轉,輸出功率躍升至3.2 mW.在抽運功率為530 mW時,激光器獲得最高輸出功率4.8 mW.實驗中發(fā)現(xiàn),在激光器鎖模運轉狀態(tài)下,將抽運功率降低至88 mW,激光器仍能維持鎖模運轉.這種現(xiàn)象被稱為抽運滯后效應[20].

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

圖5為最高抽運功率條件下激光器的輸出特性.圖5(a)為激光器輸出光譜,插圖為對數(shù)坐標下的光譜.光譜在1532.4 nm和1552.3 nm處有兩個譜峰,半高寬分別為6.6 nm和5.2 nm. 1552.3 nm處譜峰與兩譜峰之間的低谷處強度分別為1532.4 nm處強度的92%和13%.圖5(b)為輸出脈沖的自相關曲線,其半高全寬度(fullw idth half maximum,FWHM)為353.9 fs,假設脈沖為高斯型脈沖,對應的脈沖FWHM為250.3 fs.圖5(c)和圖5(d)分別為激光器輸出的脈沖序列和一次諧波射頻譜.激光器輸出脈沖的重復頻率為58.01MHz,對應的脈沖間距為17.24 ns.激光器鎖模輸出的信噪比為58.2 dB.以上測量結果表明,該激光器實現(xiàn)了可自啟動的雙波長鎖模運轉,且鎖模狀態(tài)運轉穩(wěn)定.

圖5 最高抽運功率下激光器輸出 (a)線性坐標和對數(shù)坐標(插圖)下的光譜;(b)自相關曲線;(c)脈沖序列;(d)一次諧波射頻譜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 結 論

構建了一種新型雙波長運轉摻鉺光纖鎖模激光器.根據(jù)不同長度增益光纖的對比實驗結果,選取31 cm摻鉺光纖作為增益光纖,激光器在1530 nm和1560 nm附近具有相同的增益系數(shù),實現(xiàn)了自啟動雙波長鎖模運轉.激光器輸出重復頻率為58.01 MHz,信噪比為58.2 dB,最高輸出功率為4.8 mW,鎖模輸出的光譜在1532.4 nm和1552.3 nm處具有兩個譜峰,譜峰間距約為20 nm.該激光器無需手動調(diào)節(jié)即可實現(xiàn)雙波長運轉,使用時降低了對操作人員的要求,便于應用推廣.

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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

?國家自然科學基金(批準號:61475162)、中國科學院國際合作局對外合作重點項目(批準號:181811KYSB20160029)、中國科學院前沿科學重點研究項目(批準號:QYZDY-SSW-JSC 008)和國家重大科學儀器設備專項(批準號:2011YQ 120022, 2014YQ 090709)資助的課題.

?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