劉歡 曹士英于洋 林百科 方占軍

1)(清華大學(xué)精密儀器系激光與光子技術(shù)研究室,北京 100084)

2)(中國計量科學(xué)研究院時間頻率計量研究所,北京 100029)

級聯(lián)摻Y(jié)b增益光纖提高拍頻信號信噪比的實驗研究?

劉歡1)曹士英2)?于洋1)林百科2)方占軍2)

1)(清華大學(xué)精密儀器系激光與光子技術(shù)研究室,北京 100084)

2)(中國計量科學(xué)研究院時間頻率計量研究所,北京 100029)

(2016年7月2日收到;2016年8月31日收到修改稿)

飛秒光學(xué)頻率梳的出現(xiàn)使對未知激光的絕對頻率測量成為可能,極大地簡化了激光絕對頻率的量值溯源和比對工作.為了保證測量數(shù)值的準(zhǔn)確性,飛秒光學(xué)頻率梳與未知激光的拍頻信號fb的信噪比要求大于30 dB.針對碘穩(wěn)頻532 nm激光絕對頻率測量的特定需求,以532 nm激光的基頻光1064 nm激光的絕對頻率測量為著眼點,本文采用303 MHz重復(fù)頻率的摻Er光纖光學(xué)頻率梳,首先通過激光放大和光譜展寬技術(shù)使光譜覆蓋到1μm波段,然后采用級聯(lián)摻Y(jié)b增益光纖技術(shù),將擴譜后1μm波段的激光功率進行放大,提高了摻Er光纖光學(xué)頻率梳擴譜后1μm波長附近的激光強度.采用碘穩(wěn)頻532 nm激光的基頻光作為待測光源與飛秒光學(xué)頻率梳進行拍頻.實驗表明,與未經(jīng)過光譜增強的激光相比,光譜增強后的激光與1064 nm激光拍頻信號的信噪比提高了5 dB,保持在35 dB附近.該技術(shù)有效地緩解了采用摻Er光纖光梳測量1064 nm激光絕對頻率時對直接擴譜所獲得的1μm波長激光的強度要求.

摻Er光纖飛秒激光器,光譜增強,拍頻信號,光纖光學(xué)頻率梳

1 引 言

在飛秒光學(xué)頻率梳誕生之前,諧波光頻鏈?zhǔn)菧y量光學(xué)頻率的惟一工具.諧波光頻鏈成本高、難度大、使用和維護復(fù)雜.世界上只有美國、德國、法國等少數(shù)幾個國家擁有這種測量裝置.飛秒光學(xué)頻率梳的出現(xiàn),極大地簡化了光學(xué)絕對頻率測量工作的復(fù)雜性.一臺飛秒光學(xué)頻率梳就可以實現(xiàn)光學(xué)絕對頻率測量,把長度單位“米”直接溯源到時間單位“秒”,并可以通過協(xié)調(diào)世界時(UTC)實現(xiàn)實時的國際比對和等效互認(rèn).飛秒光學(xué)頻率梳的波長測量范圍廣,可以測量從可見光到近紅外區(qū)域內(nèi)的所有激光波長,并進行量值傳遞.飛秒光學(xué)頻率梳測量精度高,可達10-14量級,取決于外參考源.

采用飛秒光學(xué)頻率梳對未知激光的絕對頻率測量,主要依靠計數(shù)器采集飛秒光學(xué)頻率梳與未知激光的拍頻信號fb的頻率值,然后通過N×fr±f0±fb獲取絕對頻率,其中fr是飛秒光學(xué)頻率梳的重復(fù)頻率,f0是載波包絡(luò)位相偏移頻率,N是數(shù)值很大的整數(shù),可以通過波長計粗測或者通過微小改變重復(fù)頻率值來獲取[1].通常為了保證計數(shù)的準(zhǔn)確性,fb的信噪比要求至少大于30 dB,否則會造成計數(shù)器觸發(fā)不正常導(dǎo)致數(shù)據(jù)采集不準(zhǔn).此外,為了實現(xiàn)長時間絕對頻率測量,高信噪比的fb信號有助于對激光絕對頻率漂移的長時間監(jiān)測.為了提高拍頻信號的信噪比,光學(xué)系統(tǒng)中可以引入緩沖激光技術(shù)[2],電學(xué)系統(tǒng)中可以引入跟蹤濾波放大技術(shù)[3].

光纖飛秒光學(xué)頻率梳,簡稱“光纖光梳”,由于體積小巧、成本低廉、穩(wěn)定性高、易于操作等諸多優(yōu)點而受到人們的廣泛青睞.光纖光梳與外激光的拍頻信號fb的信噪比更容易保持恒定.摻Er光纖光梳的中心波長在1.5μm附近.盡管通過高非線性光纖擴譜可以實現(xiàn)1000-2200 nm的寬帶光譜輸出[4-6],但該光譜處于紅外波段,很難滿足實際的激光波長測量需求.通過采用放大-倍頻-擴譜技術(shù)[7]或單點倍頻技術(shù)[8],可以實現(xiàn)激光輸出波長向可見光波長的擴展,并可以保證與未知激光30 dB信噪比拍頻信號的獲取,滿足可見光波長激光頻率的測量要求.但放大-倍頻-擴譜中采用的光子晶體光纖仍然存在長時間光譜漂移造成拍頻信號信噪比下降的風(fēng)險.

在復(fù)現(xiàn)國際單位制(SI)單位“米”的幾個推薦的波長標(biāo)準(zhǔn)中,碘穩(wěn)頻633 nm He-Ne激光器[9]和碘穩(wěn)頻532 nm Nd：YAG激光器[10,11]應(yīng)用最為廣泛.因此,人們更感興趣的是碘穩(wěn)頻532,633 nm等國際推薦值的激光頻率的測量和激光絕對頻率的長期監(jiān)測.與碘穩(wěn)頻633 nm He-Ne激光器相比,碘穩(wěn)頻532 nm Nd：YAG激光器具有穩(wěn)定度高、功率高、無調(diào)制、光纖輸出等諸多優(yōu)點[12],因此對碘穩(wěn)頻532 nm Nd：YAG激光的頻率監(jiān)測更為重要.

目前飛秒光纖激光器的研究主要集中于1,1.5,2μm等幾個波段.其中以摻Y(jié)b光纖為增益介質(zhì)的1μm激光器的研究最為顯著,在高能量、窄脈沖、高重復(fù)頻率、低時域抖動、新型色散補償及脈沖整形技術(shù)等方向都取得了飛速的發(fā)展[13-17].而以摻Er光纖光梳為光源的級聯(lián)光纖光譜增強技術(shù)主要集中于摻Y(jié)b增益光纖的1μm和摻Tm：Ho增益光纖的2μm波長附近.提取摻Er光纖飛秒激光器通過高非線性光纖擴譜后的1μm波長激光,通過級聯(lián)摻Y(jié)b增益光纖的放大可以獲得1040 nm處1.2 nJ的激光輸出[18],進一步的功率放大可以獲得大于5 W的平均功率輸出[19],可以作為一種新的1μm飛秒激光產(chǎn)生方法.后續(xù)的倍頻程擴譜和拍頻信號探測可以達到摻Y(jié)b光纖光梳的技術(shù)指標(biāo)[18].該方法使基于摻Er光纖飛秒激光器的高功率1μm光學(xué)頻率梳成為可能.提取摻Er光纖飛秒激光器通過高非線性光纖擴譜后的2μm波長激光,可以級聯(lián)摻Tm：Ho增益光纖,提高2μm附近光譜強度,一方面可以有效改善摻Er光纖飛秒激光器中f0信號的信噪比[20],另一方面也可以實現(xiàn)基于摻Er光纖飛秒激光器的高功率人眼安全的2μm激光輸出[21],為大氣中氣體成分的遠程監(jiān)測、相干多普勒雷達提供有效光源[22].

本文基于級聯(lián)光纖的光譜增強技術(shù),針對碘穩(wěn)頻532 nm激光絕對頻率測量的特定需求,以532 nm激光的基頻光1064 nm激光的絕對頻率測量為著眼點,采用303 MHz重復(fù)頻率的摻Er光纖光梳,通過級聯(lián)摻Y(jié)b增益光纖技術(shù)有效地提高了光纖光梳擴譜后1μm波長附近激光的強度,使摻Er光纖光梳與碘穩(wěn)頻532 nm激光的基頻光1064 nm激光拍頻信號的信噪比得到了改善.該技術(shù)有效地緩解了摻Er光纖光梳直接擴譜時對1μm波長的強度要求.

2 實驗裝置與實驗結(jié)果

2.1 1μm光源的產(chǎn)生

系統(tǒng)中采用的光源為高重復(fù)頻率摻Er光纖飛秒激光器,如圖1中A部分所示,其詳細結(jié)構(gòu)和參數(shù)配置參考文獻[23].抽運光LD1最大輸出功率為700 mW.激光器重復(fù)頻率為303 MHz.連續(xù)光狀態(tài)下輸出的平均功率為130 mW,鎖模后的平均功率為80 mW.鎖模后激光器工作在展寬脈沖鎖模狀態(tài),其輸出的光譜曲線如圖2所示.經(jīng)腔外壓縮可獲得小于60 fs的激光脈沖.采用壓電陶瓷(PZT)拉伸光纖的方式控制激光器腔長,PZT對激光器重復(fù)頻率的調(diào)節(jié)范圍約為1.5 kHz.整個激光器置于一塊尺寸為21 cm×15 cm×12 cm的合金鋁材料的光學(xué)平板上,采用聚四氟乙烯材料進行四周及頂蓋密封.激光器底板的溫度控制在23°C.

激光器輸出的飛秒激光經(jīng)一分三分束器后,其中的兩路分別進入f0和fr探測單元,用于探測f0信號和fr信號,第三路光進入測量單元,通過局部光譜增強后進入拍頻模塊D.

經(jīng)一分三分束器分束后一路功率約為25 mW的激光首先進入兩級放大器,如圖1中B部分.兩級放大器均采用后向抽運方式,抽運源在最大抽運電流1.5 A時輸出功率為800 mW.兩級放大器采用LIEKKI公司的Er80-4/125型號的光纖,它在1530 nm處的吸收系數(shù)為80 dB/m.一級放大器增益光纖的長度為50 cm,二級放大器增益光纖的長度為85 cm.種子光經(jīng)過一級放大后功率可以提升至111 mW,經(jīng)過二級放大后功率可提升至356 mW.

圖1 (網(wǎng)刊彩色)級聯(lián)摻Y(jié)b增益光纖提高拍頻信號信噪比的實驗系統(tǒng)結(jié)構(gòu)圖 其中：LD1-LD4為980 nm單模抽運源,WDM為波分復(fù)用器,Col1-Col8為光纖準(zhǔn)直器,PBS為偏振分光鏡,ISO為空間光隔離器,Isolator為光纖光隔離器,PZT為壓電陶瓷,HNLF為高非線性光纖,λ/4為四分之一波片,λ/2為二分之一波片,M1-M6為平面反射鏡,G為衍射光柵,PD為InGaAs光電探測器Fig.1.(color online)Experimental setup for improving the SNR of the beat note by cascading an Yb-doped fiber amplifier in an Er-fiber comb.LD1-LD4,980 nm single-mode laser diodes;WDM,wavelength division multiplexer;Col1-Col8,fiber collimators;PBS,polarization beam splitter;ISO,space isolator;PZT,piezoelectric transducer;HNLF,highly nonlinear fiber;λ/4,quarter wave plate;λ/2,half wave plate;M1-M6,plane mirrors;G,diffraction grating;PD,InGaAs photoelectric detector.

圖2 摻Er光纖飛秒激光器輸出的光譜曲線Fig.2.Output spectrum from the Er-doped fiber femtosecond laser in log scale.

采用國產(chǎn)高非線性光纖作為光譜展寬器件,其有效非線性系數(shù)為10.6 W-1·km-1,在1550 nm處的色散為-0.137 ps/(nm·km),在1550 m處的色散斜率為0.0158 ps/(nm2·km).系統(tǒng)中采用的高非線性光纖長度為40 cm.高非線性光纖由于纖芯較細,因此與普通單模光纖熔接時損耗較大.為了減小熔接損耗,我們通過優(yōu)化放電次數(shù)和放電時間,可將單個熔點損耗降低至1 dB.放大器輸出激光經(jīng)波片組合后進入高非線性光纖,經(jīng)高非線性光纖擴譜后功率為185 mW.通過優(yōu)化波片組合,可以實現(xiàn)不同范圍的波長展寬,光譜短波可以覆蓋至1μm波段,如圖3中紅色曲線所示.在兩級放大系統(tǒng)中,當(dāng)一二級抽運源均未打開時,高非線性光纖后的輸出功率為9 mW,光譜如圖3中黑色曲線所示,當(dāng)一級抽運打開,二級抽運關(guān)閉時,高非線性光纖后的輸出功率為77.5 mW,光譜如圖3中藍色曲線所示,當(dāng)二級抽運打開,一級抽運關(guān)閉時,高非線性光纖后的輸出功率為107.7 mW,光譜如圖3綠色所示.在兩級放大系統(tǒng)中二級放大器不僅效率高,而且對擴譜的影響大.

圖3 (網(wǎng)刊彩色)高非線性光纖獲得的光譜展寬Fig.3. (color online)Broadened spectra after the HNLF under different pump conditions of the amplifi er.Red curve,the two pump sources were running simultaneously;blue curve,only the first-stage pump source was turned on;green curve,only the secondstage pump source was turned on;black curve,the two pump sources were both closed.

當(dāng)其中一級放大器抽運電流固定在1.5 A(對應(yīng)抽運功率800 mW)時,經(jīng)過高非線性光纖獲得的900-1200 nm范圍內(nèi)的光譜展寬隨著另外一級抽運功率的變化趨勢如圖4所示.從圖4中可以看出,當(dāng)二級抽運功率固定時,經(jīng)過高非線性光纖獲得的光譜展寬隨著一級抽運功率的變化存在著最佳值0.9 A(對應(yīng)抽運功率473 mW),此后光譜展寬量開始減小.而當(dāng)一級抽運功率固定時,經(jīng)過高非線性光纖獲得的光譜展寬隨著二級抽運功率的增加而增加.在最大抽運功率800 mW的條件下,所獲得的最佳光譜展寬如圖3中紅色曲線所示.

圖4 (網(wǎng)刊彩色)高非線性光纖獲得的光譜展寬隨抽運功率的演變 (a)二級放大器抽運電流固定在1.5 A;(b)一級放大器抽運電流固定在1.5 AFig.4.(color online)Supercontinuum bandwidth versus pump current：(a)The pump current of the secondstage amplifier was set at 1.5 A;(b)the pump current of the first-stage amplifier was set at 1.5 A.

2.2 1μm光譜的增強

由于1μm激光附近強度較低,因此與碘穩(wěn)頻532 nm Nd：YAG激光器輸出的1064 nm基頻光拍頻信號fb的信噪比最高只有30 dB,處于計數(shù)器采數(shù)閾值附近.為了進一步提高測量的準(zhǔn)確性以及長時間測量的可靠性,我們通過采用級聯(lián)摻Y(jié)b增益光纖放大技術(shù)來增強高非線性光纖擴譜后1064 nm激光強度.摻Y(jié)b增益光纖在974 nm處的吸收系數(shù)為2750 dB/m(SCF-YB550-4/125-19,Coractive),光纖長度為15 cm,采用后向抽運方式,抽運功率最高為780 mW.擴譜后的激光通過摻Y(jié)b增益光纖后,在無抽運功率的情況下,透射功率為27 mW.隨著抽運功率的增加,經(jīng)摻Y(jié)b增益光纖放大后的激光功率也逐漸增加,如圖5所示.在最大抽運功率780 mW的激勵下,放大后輸出功率為376 mW.

圖6給出了高非線性光纖展寬后光譜經(jīng)過摻Y(jié)b光纖放大器光譜增強前后對比圖.其中紅色曲線為摻Y(jié)b光纖放大器抽運源關(guān)閉時的輸出光譜,黑色曲線為摻Y(jié)b光纖放大器抽運源抽運功率780 mW時的輸出光譜.從圖6(a)-(c)中可以看出,當(dāng)摻Er光纖放大器的一級和二級抽運源均關(guān)閉或者只有一個關(guān)閉時,經(jīng)過高非線性光纖后的光譜由于無法覆蓋到1μm波段,因此即使打開摻Y(jié)b光纖放大器的抽運源,1100 nm以下部分僅僅為摻Y(jié)b光纖的熒光光譜.由于1100 nm以上的光譜成分在摻Y(jié)b光纖中沒有增益,因此黑色曲線在1100 nm以上部分與紅色曲線完全重合.

圖5 光譜增強輸出功率隨抽運功率的變化Fig.5.The output power of the Yb-doped fiber amplifier versus the pump power.

當(dāng)摻Er光纖放大器的一級和二級抽運源均打開時,經(jīng)過高非線性光纖后的光譜逐漸擴展到1100 nm波段以下并達到摻Y(jié)b光纖的增益譜線范圍,此時高非線性光纖擴譜后1030 nm附近的光譜成分在摻Y(jié)b光纖中產(chǎn)生增益放大,圖6中1030 nm附近的尖峰消失,轉(zhuǎn)化為1030 nm激光輸出.從圖6(d)中可以看出,摻Y(jié)b光纖放大器抽運功率為780 mW時輸出光譜在950-1150 nm之間的光譜強度明顯高于摻Y(jié)b光纖放大器抽運源關(guān)閉時輸出的光譜強度.圖7給出光譜增強前后輸出光譜的對比圖.采用中心波長1064 nm,帶寬10 nm的窄帶濾波器進行光譜濾波,濾波后的輸出功率為43 mW.

圖6 (網(wǎng)刊彩色)高非線性光纖展寬后光譜經(jīng)過摻Y(jié)b光纖放大器光譜增強前后對比圖 (a)摻Er光纖放大器中兩級放大器抽運源均關(guān)閉;(b)摻Er光纖放大器中一級放大器抽運源關(guān)閉,二級放大器抽運電流固定在1.5 A;(c)摻Er光纖放大器中二級放大器抽運源關(guān)閉,一級放大器抽運電流固定在1.5 A;(d)摻Er光纖放大器中兩級放大器抽運電流均固定在1.5 AFig.6.(color online)Comparisons of optical spectra after cascading an Yb-doped fiber amplifier without spectral enhancement(red curve,the pump source of the Yb-doped fiber amplifier was closed)and with spectral enhancement(black curve,the pump power of the Yb-doped fiber amplifier was 780 mW)：(a)The two pump sources of the two-stage Er-doped fiber amplifier were both closed;(b)the pump source of thefirst-stage amplifier was closed and the pump current of the second-stage amplifier was set at 1.5 A;(c)the pump source of the second-stage amplifier was closed and the pump current of the first-stage amplifier was set at 1.5 A;(d)the pump currents of the two pump sources were both set at 1.5 A.

圖7 (網(wǎng)刊彩色)光譜增強前后輸出光譜的對比圖Fig.7.(color online)Comparison of spectra before and after spectral enhancement.Red curve,output spectrum after the HNLF;black curve,output spectrum after the Yb-doped fiber amplifier without spectral enhancement;and blue curve,output spectrum after the Yb-doped fiber amplifier with spectral enhancement.

2.3 拍頻信噪比的改善

經(jīng)過高非線性光纖光譜展寬后的激光與經(jīng)過摻Y(jié)b光纖光譜增強后的激光分別通過光纖傳輸至拍頻模塊,圖1中D部分.碘穩(wěn)頻532 nm激光器輸出的1064 nm基頻光最高可達2 W.將其中一小部分激光耦合進入光纖傳遞至拍頻模塊.光纖后耦合輸出激光功率保持在10 mW,與光譜展寬后的激光進行拍頻.兩束光分別經(jīng)過準(zhǔn)直器轉(zhuǎn)換成空間光路,再經(jīng)過波片和PBS合束.利用1200/mm的光柵G(Thorlabs,GR13-1210)將光譜成分在空間分開,選取1064 nm待測激光的頻率成分通過反射鏡耦合進入InGaAs光電探測器PD(APD110C/M,Thorlabs)中,探測拍頻信號fb.經(jīng)過仔細調(diào)節(jié)兩路光的空間重合以及偏振狀態(tài),在頻譜儀300 kHz帶寬分辨率的條件下獲得35 dB的拍頻信號,如圖8所示.其中圖8(a)為1μm光譜增強前,即經(jīng)過高非線性光纖光譜展寬后的激光與1064 nm激光的拍頻信號,圖8(b)為1μm光譜增強后,即經(jīng)過高非線性光纖光譜展寬后再經(jīng)過摻Y(jié)b光纖放大后的激光與1064 nm激光的拍頻信號.從圖中可以看出,拍頻信號信噪比提高了5 dB.

圖8 摻Er光纖光學(xué)頻率梳與小型化碘穩(wěn)頻532 nm Nd：YAG激光器的拍頻信號,其中RBW為300 kHz(a)1μm光譜增強前與1064 nm激光的拍頻信號;(b)1μm光譜增強后與1064 nm激光的拍頻信號Fig.8.The beat note between the comb light and an iodine-stabilized 532 nm laser at 300 kHz RBW：(a)The beat note between a 1μm laser without spectral enhancement and a 1064 nm laser;(b)the beat note between a 1μm laser with spectral enhancement and a 1064 nm laser.

經(jīng)過摻Y(jié)b光纖光譜增強后,雖然1064 nm光譜能量得到了較大的提高,但由于放大過程中噪聲的存在,使得目前信噪比只提高了5 dB.此外,摻Y(jié)b增益光纖的增益光譜的峰值處于1030 nm,而待測激光的波長1064 nm處于增益較低的位置,也是導(dǎo)致1064 nm能量提高較小的原因.但由于摻Er光纖光梳系統(tǒng)穩(wěn)定度高,特別是全光纖的放大擴譜結(jié)構(gòu),使得擴譜后光譜非常穩(wěn)定,與外激光拍頻信號信噪比保持恒定,可以滿足數(shù)天以上長時間的連續(xù)監(jiān)測.

3 結(jié) 論

針對碘穩(wěn)頻532 nm激光絕對頻率測量的特定需求,以532 nm激光的基頻光1064 nm的絕對頻率測量為著眼點,采用303 MHz重復(fù)頻率的摻Er光纖光梳,通過級聯(lián)摻Y(jié)b增益光纖技術(shù)有效地提高了光纖光梳擴譜后1μm波長附近激光的強度.實驗表明,與未經(jīng)過光譜增強的激光相比,光譜增強后的激光與1064 nm拍頻信號的信噪比提高了5 dB,保持在35 dB附近,可以滿足數(shù)天以上長時間的連續(xù)監(jiān)測.該技術(shù)有效地緩解了采用摻Er光纖光梳測量1064 nm激光絕對頻率時對直接擴譜所獲得的1μm波長激光的強度要求.

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PACS:42.55.Wd,42.65.Re,06.30.Ft DOI:10.7498/aps.66.024206

Experimental study on increasing signal-to-noise ratio of a beat note by cascading an Yb-doped fiber in an Er-fiber comb?

Liu Huan1)Cao Shi-Ying2)?Yu Yang1)Lin Bai-Ke2)Fang Zhan-Jun2)

1)(Center for Photonics and Electronics,Department of Precision Instrument,Tsinghua University,Beijing 100084,China)
2)(Division of Time and Frequency Metrology,National Institute of Metrology,Beijing 100029,China)

2 July 2016;revised manuscript

31 August 2016)

The harmonic optical frequency chain is the only tool for measuring optical frequency till the advent of a femtosecond optical frequency comb(FOFC).However,its disadvantages are obvious,such as high cost,difficult construction,complex usage,and complicated maintenance.The emergence of femtosecond optical frequency combs(FOFCs)makes it possible to measure the absolute frequency of a laser,which greatly simplifies the quantity traceability of the absolute frequency value and comparison,and allows the length unit “m” to be directly traced back to the time unit “s”.The beat note(fb)between an FOFC and a test laser is one of the most important data in measuring absolute frequency of the test laser.In order to ensure the accuracy and reliability of the measurement,the signal-to-noise ratio(SNR)offbshould be above 30 dB at 300 kHz resolution bandwidth.Among the wavelength standards recommended to replicate“meter” (SI),iodine-stabilized 633 nm lasers and iodine-stabilized 532 nm lasers have been widely used.Compared with iodine-stabilized 633 nm lasers,iodine-stabilized 532 nm lasers have the advantages of high stability,high output power,no modulation and fiber coupled output.Therefore,it is of great importance to measure and monitor the absolute frequency of an iodine-stabilized 532 nm laser.Aiming at the specific requirements for absolute frequency measurement of an iodine-stabilized 532 nm laser,the absolute frequency measurement of its fundamental 1064 nm laser has been studied.In this paper,a high-repetition-rate Er-doped femtosecond fiber laser is adopted as an optical source in the system.The repetition rate of the fiber laser is 303 MHz,the output power in the continuous-wave state is 130 mW and the average output power in the mode-locking state is 80 mW.The highest SNR offbbetween the comb light and a 1064 nm laser generated by an iodine-stabilized 532 nm laser is only 30 dB due to the low intensity at 1μm wavelength in the supercontinuum,which just reaches the SNR threshold meeting the counter’s working condition.In order to improve the accuracy and reliability of absolute frequency measurement,the technique of cascading an Yb-doped fiber amplifier after spectral broadening is adopted to enhance the spectral intensity at 1μm wavelength.The experimental results indicate that the SNR offbbetween a 1μm laser after spectral enhancement and a 1064 nm laser is increased by 5 dB and kept at 35 dB for several days,meeting requirements for long-term continuous monitoring.This method can effectively reduce the intensity requirements at 1μm wavelength when the spectrum is directly broadened in the Er-FOFC.

Er-doped fiber femtosecond laser,spectral enhancement,beat note,fiber optical frequency comb

:42.55.Wd,42.65.Re,06.30.Ft

10.7498/aps.66.024206

?清華大學(xué)自主科研計劃——青年教師自主選題基礎(chǔ)研究(批準(zhǔn)號：20131089299)和質(zhì)檢公益性行業(yè)科研專項(批準(zhǔn)號：201310007)資助的課題.

?通信作者.E-mail：caoshiying@nim.ac.cn

*Project supported by Tsinghua University Initiative Scientific Research Program,China(Grant No.20131089299)and the Special Scientific Research Foundation of General Administration of Quality Supervision,Inspection and Quarantine of China(Grant No.201310007).

?Corresponding author.E-mail：caoshiying@nim.ac.cn