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由于受增益介质上能级寿命的影响, 掺Er光纤光梳的梳齿线宽一般在百kHz量级. 为了实现光梳梳齿线宽的压窄, 一种有效的方法是在激光器中增加快速响应的电光晶体, 使光纤光梳的伺服锁定带宽提高到百kHz以上, 为光纤光梳的快速伺服锁定提供反馈机构. 这其中, 高品质的飞秒激光器是核心. 基于此, 本文主要研究了掺Er光纤飞秒激光器中电光晶体对激光器参数的影响. 通过计算电光晶体的折射率、色散、相位延迟等参数, 分析了电光晶体对激光器参数的影响, 并在实验上获得了电光晶体电压对激光器重复频率和载波包络偏移频率的影响, 进而通过电光晶体实现了对光纤光梳重复频率和载波包络偏移频率的锁定. 通过锁定光纤飞秒激光器与窄线宽激光器的拍频信号, 验证了电光晶体的引入使激光器的伺服锁定带宽提高到了236 kHz, 为窄线宽飞秒光学频率梳的建立提供了技术基础.
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关键词:
- 掺Er光纤激光器 /
- 掺Er光纤光学频率梳 /
- 电光晶体 /
- 伺服锁定
Narrow-linewidth femtosecond optical frequency comb plays an important role in the fields, such as optical clock comparison, time frequency transfer, ultrastable microwave generation, absolute distance measurement, high precision spectroscopy, etc. Due to the influence of the lifetime of the upper energy level in the gain medium, the linewidth of Er-fiber combs is generally on the order of several hundred kilohertz. In order to narrow the linewidth of comb teeth, an effective method is to insert a fast response electro-optic modulator (EOM) into the laser cavity, so that the servo bandwidth of fiber comb is extended to several hundred kilohertz, which provides a feedback mechanism for fast servo locking. Among them, a high quality femtosecond laser is the core. Based on this, the influence of the EOM on the parameters of Er-fiber femtosecond laser is studied in this paper. By calculating the refractive index, group velocity dispersion, and phase delay of the electro-optic crystal, the influence of the introduction of the EOM on the laser performance is analyzed. A LiNbO3 (LN) crystal with a length of 3 mm and x-cut is selected as the EOM and inserted into the laser cavity. The influence of the applied voltage of the EOM on the repetition rate and carrier envelope offset frequency of the laser are obtained experimentally. When the voltage on the LN crystal changes from -200 to 200 V, the adjustment of repetition rate is 60 Hz and the carrier envelope offset frequency is 25 MHz. Then the two parameters are phase locked through the EOM. Furthermore, by phase locking the beat note between the fiber comb and a narrow-linewidth continue wavelength laser at 1542 nm, it is verified that the introduction of the EOM can expand the servo bandwidth of the laser to more than 236 kHz, which provides a technical basis for establishing narrow linewidth femtosecond optical frequency combs. The following work will verify the performance of comb line, that is, when the comb is locked to a narrow-linewidth laser (such as 1542 nm), the performance of comb line at wavelength (such as 698, 729 nm, and so on) of distant place will be analyzed in detail.-
Keywords:
- Er-doped fiber femtosecond laser /
- Er-doped fiber optical frequency comb /
- electro-optic modulator /
- servo lock
[1] Fortier T, Baumann E 2019 Commun. Phys. 2 153
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Google Scholar
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图 3 腔内加入EOM的掺Er光纤光梳结构图. 其中, LD为激光二极管, WDM为980 nm/1550 nm波分复用器件, EDF为增益光纤, Col为光纤准直器, PBS为偏振分光片, ISO为隔离器, λ/4为四分之一波片, λ/2为二分之一波片, M为平面反射镜, PZT为压电陶瓷, EOM为电光晶体调制器, HNLF为高非线性光纤, F为透镜, PD为光电探测器, PPL为伺服锁定环路, PPLN为周期极化LN晶体, SW为微波线切换模块
Fig. 3. Schematic diagram of the Er-fiber comb with an intra-cavity EOM. LD, laser diode; WDM, 980 nm/1550 nm wavelength division multiplexing; EDF, Er-doped gain fiber; Col, fiber collimator; PBS, polarization beam splitter; ISO, isolator; λ/4, quarter wave plate; λ/2, half wave plate; M, plane mirror; PZT, piezoelectric transducer; EOM, electro-optic modulator; HNLF, highly nonlinear fiber; F, optical lens; PD, photoelectric diode; PPL, phase lock loop; PPLN, Periodically Poled Lithium Niobate; SW, signal switch module
图 4 激光器腔内有无EOM时的锁模光谱
Fig. 4. Spectra of the Er-fiber femtosecond laser with and without an intra-cavity EOM.
图 5 激光器中加入EOM锁模后的射频曲线, 其中插图为163 MHz处的频谱
Fig. 5. Radio frequency of the Er-fiber femtosecond laser with an intra-cavity EOM. The insert is the radio frequency at 163 MHz.
图 6 激光器载波包络偏移频率, 插图为扩谱后的倍频程光谱图
Fig. 6. Signal-to-noise ratio of carrier-envelop offset frequency in 100 kHz resolution bandwidth (RBW). The insert is octave spanning spectrum after HNLF.
图 7 EOM晶体电压对激光器参数的影响 (a) EOM晶体电压对激光器重复频率的影响; (b) EOM晶体电压对激光器载波包络偏移频率的影响
Fig. 7. Diagram showing the change in laser parameters at different voltage on EOM: (a) The change in repetition rate; (b) the change in carrier envelope offset frequency.
图 8 EOM晶体电压改变时, 激光器输出光谱变化
Fig. 8. Evolution of the spectra of the Er-fiber femtosecond laser with the changing of the voltage on EOM.
图 10 重复频率锁定后的频率变化 (a) 采用EOM锁定重复频率; (b) 采用PZT锁定重复频率
Fig. 10. Residual fluctuations of the repetition rate when it is phase-locked: (a) Phase-locked by EOM; (b) phase-locked by PZT.
图 11 采用EOM和PZT锁定重复频率后, 所获得的重复频率的相对Allan偏差曲线
Fig. 11. Calculated Allan deviations when the repetition rate was phase-locked by EOM and PZT respectively.
图 13 采用EOM锁定f0信号后的频率变化
Fig. 13. Residual fluctuations of the carrier envelope offset frequency when it is phase-locked by EOM.
图 14 飞秒激光器与1542 nm的单频激光的拍频信号 (a) 采用PZT锁定后的拍频信号, 其中分辨率带宽为100 kHz; (b) 采用EOM锁定后的拍频信号, 其中分辨率带宽为1 kHz
Fig. 14. Beat note between the Er-fiber comb and a 1542 nm laser: (a) Spectrum of the in-loop fb after phase-locking with PZT in 100 kHz RBW; (b) spectrum of the in-loop fb after phase-locking with EOM in 1 kHz RBW.
图 15 光梳与1542 nm激光拍频信号fb锁定后的频率变化
Fig. 15. Residual fluctuations of the beat note when the Er-fiber comb was phase-locked to a 1542 nm laser.
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[1] Fortier T, Baumann E 2019 Commun. Phys. 2 153
Google Scholar
[2] Liu T A, Shu R H, Peng J L 2008 Opt. Express 16 10728
Google Scholar
[3] Giorgetta F R, Swann W C, Sinclair L C, Baumann E, Coddington I, Newbury N R 2013 Nat. Photonics 7 434
Google Scholar
[4] Millo J, Boudot R, Lours M, Bourgeois P Y, Luiten A N, Coq Y. L, Kersalé Y, Santarelli G 2009 Opt. Lett. 34 3707
Google Scholar
[5] Minoshima K, Matsumoto H 2000 Appl. Opt. 39 5512
Google Scholar
[6] Picqué N, Hänsch T W 2019 Nat. Photonics 13 146
Google Scholar
[7] McCracken R A, Charsley J M, Reid D T 2017 Opt. Express 25 15058
Google Scholar
[8] Sinclair L C, Coddington I, Swann W C, Rieker G B, Hati A, Iwakuni K, Newbury N R 2014 Opt. Express 22 6996
Google Scholar
[9] Lezius M, Wilken T, Deutsch C, Giunta M, Mandel O, Thaller A, Schkolnik V, Schiemangk M, Dinkelaker A, Kohfeldt A, Wicht A, Krutzik M, Peters A, Hellmig O, Duncker H, Sengstock K, Windpassinger O, Lampmann K, Hülsing T, Hänsch T W, and Holzwarth R 2016 Optica 3 1381
Google Scholar
[10] Nakajima Y, Inaba H, Hosaka K, Minoshima K, Onae A, Yasuda M, Kohno T, Kawato S, Kobayashi T, Katsuyama T, Hong F L 2010 Opt. Express 18 1667
Google Scholar
[11] Schibli T R, Hartl I, Yost D C, Martin M J, Marcinkevicius A, Fermann M E, Ye J 2008 Nat. Photonics 2 355
Google Scholar
[12] Inaba H, Hosaka K, Yasuda M, Nakajima Y, Iwakuni K, Akamatsu D, Okubo S, Kohno T, Onae A, Hong F L 2013 Opt. Express 21 7891
Google Scholar
[13] Nicolodi D, Argence B, Zhang W, Targat R L, Santarelli G, Coq Y L 2014 Nat. Photonics 8 219
Google Scholar
[14] Hudson D D, Holman K W, Jones R J, Cundiff S T, Ye J 2005 Opt. Lett. 30 2948
Google Scholar
[15] Iwakuni K, Inaba H, Nakajima Y, Kobayashi T, Hosaka K, Onae A, Hong F L 2012 Opt. Express 20 13769
Google Scholar
[16] Torcheboeuf N, Buchs G, Kundermann S, Portuondo-Campa E, Bennès J, Lecomte S 2017 Opt. Express 25 2215
Google Scholar
[17] Schweyer S M, Eder B, Putzer P, Mayerbacher M, Lemke N, Schreiber K U, Hugentobler U, Kienberger R 2018 Opt. Express 26 23798
Google Scholar
[18] Ning K, Hou L, Fan S T, Yan L L, Zhang Y Y, Rao B J, Zhang X F, Zhang S G, Jiang H F 2020 Chin. Phys. Lett. 37 064202
Google Scholar
[19] Ma Y X, Meng F, Wang Y, Wang A M, Zhang Z G 2019 Chin. Opt. Lett. 17 041402
Google Scholar
[20] Wang H B, Han H N, Zhang Z Y, Shao X D, Zhu J F, Wei Z Y 2020 Chin. Phys. B 29 030601
Google Scholar
[21] 曹士英, 孟飞, 林百科, 方占军, 李天初 2012 61 134205
Google Scholar
Cao S Y, Meng F, Lin B K, Fang Z J, Li T C 2012 Acta Phys. Sin. 61 134205
Google Scholar
[22] Hisai Y, Akamatsu D, Kobayashi T, Okubo S, Inaba H, Hosaka K, Yasuda M, Hong F L 2019 Opt. Express 27 6404
Google Scholar
[23] Kim Y, Kim S, Kim Y J, Hussein H, Kim S W 2009 Opt. Express 17 11972
Google Scholar
[24] Hundertmark H, Wandt D, Fallnich C, Haverkamp N, Telle H R 2004 Opt. Express 12 770
Google Scholar
[25] Washburn B R, Fox R W, Newbury N R, Nicholson J W, Feder K, Westbrook P S, Jørgensen C G 2004 Opt. Express 12 4999
Google Scholar
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