基于45°倾斜光纤光栅的光纤激光频率梳(特邀)创刊六十周年特邀
1 引言
作为连接微波频率和光学频率的桥梁,光学频率梳的发明使得光学频率的精确测量取得了革命性的突破。如今光学频率梳作为光学领域的重要工具,已经从最初精密光学计量、光学原子钟、高分辨率光谱学、基本物理常数的精确测定等基础研究领域,逐步拓展至高带宽光通信、精密测距、超稳微波产生等高新技术领域,并在地外行星探测、高精度时频传递、量子光学等前沿领域显示出重要的应用潜力[1-6]。作为光学频率梳的核心器件,飞秒锁模激光器决定了光频梳系统的性能。在发展初期通常依赖于钛宝石固体锁模激光器来构建光学频率梳,随着光纤技术的成熟和发展,以及飞秒光纤激光器小型化、抗干扰、成本以及维护方面的优势,光纤激光频率梳在近年来逐渐成为主流[3]。光学频率梳的梳齿方程为
起偏器作为NPR技术的关键器件,在实现光纤化的过程中得到了众多关注。其中45°倾斜光纤光栅(45°-TFG)特殊的倾斜结构打破了光纤的圆对称性,可以实现偏振相关的模式耦合,是最具有代表性的光纤型偏振相关器件之一。2000年45°-TFG首次被证明可以用作一种光纤偏振计[11],2005年英国阿斯顿大学的Zhou等[12]在掺锗光纤上刻写了在1550 nm处具有33 dB偏振消光比的45°-TFG,工作范围可达100 nm,使45°-TFG用于锁模光纤激光器成为可能。与其他商用光纤型起偏器相比,45°-TFG具有高偏振相关损耗(PDL)、低插入损耗、宽带响应、灵活的波长设计性以及简单的制备方式等优势,且可以在诸多类型特种光纤内进行刻写。自2010年Mou等[13]利用45°-TFG首次实现NPR锁模光纤激光器以来,45°-TFG已经在不同波段的光纤激光器中作为关键功能器件实现了不同工作模式的锁模脉冲输出[14-22]。然而,尽管已经有大量基于45°-TFG锁模光纤激光器的研究,但仍集中在飞秒激光种子源的研制阶段,并未将其用于实际激光应用系统中。因此,为了进一步探索基于45°-TFG锁模光纤激光器的应用潜力,本文利用基于45°-TFG的展宽脉冲锁模光纤激光器作为种子源,自主搭建了一套光纤激光频率梳系统,实现了重复频率
2 实验装置
基于45°-TFG的光纤激光频率梳的实验结构示意图如
图 1. 基于45°-TFG光纤激光频率梳系统结构示意图。(a)基于45°-TFG的锁模光纤激光振荡器;(b)脉冲放大光路;(c)倍频程超连续谱产生及 自参考拍频探测光路;(d)重复频率 的锁定电路;(e)载波包络偏移频率 的锁定电路
Fig. 1. Schematic diagram of a 45°-TFG fiber laser frequency comb system. (a) Mode-locked fiber laser oscillator based on 45°-TFG; (b) pulse amplification; (c) octave supercontinuum spectrum generation and interferometer; (d) stabilization circuit of ; (e) stabilization circuit of
如
图 2. 45°-TFG的特性表征。(a)45°-TFG 的最大和最小传输损耗;(b)45°-TFG 的 PDL 和插入损耗
Fig. 2. Characterization of 45°-TFG. (a) Maximum and minimum transmission loss of 45°-TFG; (b) PDL and insertion loss of 45°-TFG
如
在实现
3 基于45°-TFG锁模光纤振荡器的输出特性
当LD1的泵浦功率达到20 mW时,激光器出现连续波信号,继续提高泵浦功率至90 mW以上,仔细调节腔内偏振,可以获得稳定的锁模脉冲。如
图 3. 泵浦功率228 mW时的展宽脉冲锁模输出特性。(a)输出光谱(橘色:以dBm为纵坐标单位的对数光谱。紫色:纵坐标单位归一化后的线性光谱);(b)直接输出脉冲和压缩后(插图)脉冲的自相关曲线,以及相应的高斯拟合曲线;(c)输出脉冲序列;(d)射频频谱
Fig. 3. Performances of a stretched-pulse mode-locked laser at 228 mW pump power. (a) Optical spectrum with logarithmic scale (violate) and normalized linear scale (orange); (b) AC traces of the compressed and direct output (inset) pulses, and corresponding Gaussian fits; (c) output pulse train; (d) RF spectra
在实现锁模后,通过对腔内偏振态的仔细优化,振荡器能在较大泵浦功率范围内维持稳定的单脉冲展宽脉冲锁模,
图 4. 振荡器输出可靠性及稳定性测量。(a)光谱随泵浦功率的变化;(b)输出功率和单脉冲能量随泵浦功率的变化;(c)12 h内光谱演变;(d)12 h内输出功率演变
Fig. 4. Output reliability and stability measurements of the oscillator. (a) Evolution of optical spectrum with pump power; (b) variation of output power and single pulse energy with pump power; (c) evolution of optical spectrum within 12 h; (d) evolution of output power within 12 h
4 重复频率与载波包络偏移频率的锁定
以上对基于45°-TFG锁模光纤振荡器输出特性的测量表明,无论从锁模脉冲输出特性,还是锁模输出的稳定性以及锁模动态范围,该振荡器已经可以作为后续构建光纤激光频率梳系统稳定可靠的种子源。
图 5. 脉冲放大特性。(a)放大器前的脉冲自相关曲线;(b)放大器输出功率随泵浦功率的变化;(c)压缩后脉冲光谱和(d)自相关曲线
Fig. 5. Characteristics of pulse amplification. (a) AC trace before amplifier; (b) variation of the output power of the amplifier with the pump power; (c) spectrum and (d) corresponding AC trace of the compressed pulse
将获得的高峰值功率脉冲输入至HNLF,分别使用近红外波段的光谱仪(AQ6370B,Yokogawa,日本)和中红外波段的光谱仪(AQ6375,Yokogawa,日本)测量在HNLF末端的光谱,如
图 6. 光谱。(a)经 HNLF 后的倍频程超连续谱;RBW分别为(b)100 kHz和(c)3 kHz时探测的 信号射频频谱
Fig. 6. Spectra. (a) Full octave-spanning supercontinuum spectrum at the output of the HNLF; RF spectra of the signal with RBW of (b) 100 kHz and (c) 3 kHz
在实现
图 7. 信号的锁定。(a) 信号随PZT控制电压的变化;(b) 信号锁定后的频率抖动;(c) 信号锁定后的Allan偏差曲线(归一化至 信号)
Fig. 7. Stabilization of signal. (a) signal variation with PZT control voltage; (b) frequency fluctuation after signal stabilization;(c) Allan deviation curve after signal stabilization (normalized to signal)
与重复频率
图 8. 信号的锁定。(a) 信号随振荡器泵浦电流的变化;(b) 信号锁定经256分频后频谱;(c) 信号锁定后的经256分频后的频率抖动;(d) 信号锁定后的Allan偏差曲线(归一化至光频192 THz)
Fig. 8. Stabilization of signal. (a) signal variation with pump current of oscillator; (b) RF spectrum of after stabilization; (c) frequency fluctuation after signal stabilization; (d) Allan deviation curve after signal stabilization (normalized to 192 THz)
图 9. 信号锁定前后基于45°-TFG 锁模光纤激光器的强度噪声分布
Fig. 9. Intensity noise distribution of 45°-TFG-based mode-locked fiber laser before and after stabilization
5 结论
本文采用45°-TFG作为NPR锁模技术中的光纤型起偏器,自主搭建了一台基于45°-TFG的光纤激光频率梳系统。通过优化基于45°-TFG的锁模光纤激光振荡器,获得了3 dB带宽为60.4 nm、脉冲宽度约为68 fs的超短脉冲输出,且具有良好的锁模可靠性及稳定性。经过脉冲放大以及高非线性光纤扩谱,获得了超过倍频程的超连续谱,并搭建了
[1] Picqué N, Hänsch T W. Frequency comb spectroscopy[J]. Nature Photonics, 2019, 13(3): 146-157.
[2] Fortier T, Baumann E. 20 years of developments in optical frequency comb technology and applications[J]. Communications Physics, 2019, 2: 153.
[3] Droste S, Ycas G, Washburn B R, et al. Optical frequency comb generation based on erbium fiber lasers[J]. Nanophotonics, 2016, 5(2): 196-213.
[4] Kim J, Song Y J. Ultralow-noise mode-locked fiber lasers and frequency combs: principles, status, and applications[J]. Advances in Optics and Photonics, 2016, 8(3): 465-540.
[5] 卢振旭, 李培丽, 王浩然. 基于光学频率梳的可选频毫米波生成多业务分层WDM-ROF系统[J]. 中国激光, 2023, 50(10): 1006002.
[6] 刘桐君, 叶慧琪, 唐靓, 等. 天文光谱高精度波长定标技术研究进展(特邀)[J]. 光子学报, 2023, 52(5): 0552203.
[7] Tamura K, Haus H A, Ippen E P. Self-starting additive pulse mode-locked erbium fibre ring laser[J]. Electronics Letters, 1992, 28(24): 2226-2228.
[8] Li X, Zou W W, Yang G, et al. Direct generation of 148 nm and 44.6 fs pulses in an erbium-doped fiber laser[J]. IEEE Photonics Technology Letters, 2015, 27(1): 93-96.
[9] Zhou L, Liu Y, Xie G H, et al. Generation of stretched pulses from an all-polarization-maintaining Er-doped mode-locked fiber laser using nonlinear polarization evolution[J]. Applied Physics Express, 2019, 12(5): 052017.
[10] 夏传青, 武腾飞, 赵春播, 等. 光纤飞秒光学频率梳载波包络偏移频率锁定的实验研究[J]. 激光与光电子学进展, 2016, 53(12): 123201.
[11] Westbrook P S, Strasser T A, Erdogan T. In-line polarimeter using blazed fiber gratings[J]. IEEE Photonics Technology Letters, 2000, 12(10): 1352-1354.
[12] Zhou K M, Simpson G, Chen X F, et al. High extinction ratio in-fiber polarizers based on 45° tilted fiber Bragg gratings[J]. Optics Letters, 2005, 30(11): 1285-1287.
[13] Mou C B, Wang H, Bale B G, et al. All-fiber passively mode-locked femtosecond laser using a 45°-tilted fiber grating polarization element[J]. Optics Express, 2010, 18(18): 18906-18911.
[14] Zhang Z X, Yan Z J, Zhou K M, et al. All-fiber 250 MHz fundamental repetition rate pulsed laser with tilted fiber grating polarizer[J]. Laser Physics Letters, 2015, 12(4): 045102.
[15] Liu X L, Wang H S, Wang Y S, et al. Single-polarization, dual-wavelength mode-locked Yb-doped fiber laser by a 45°-tilted fiber grating[J]. Laser Physics Letters, 2015, 12(6): 065102.
[16] 林彦吕, 黄梓楠, 黄千千, 等. 基于Lyot滤波器的脉冲态可切换掺镱光纤激光器[J]. 中国激光, 2021, 48(19): 1901004.
[18] Zou M, Ran Y L, Hu J, et al. Multiwavelength mode-locked fiber laser based on an all fiber Lyot filter[J]. IEEE Photonics Technology Letters, 2020, 32(22): 1419-1422.
[20] Bharathan G, Hudson D D, Woodward R I, et al. In-fiber polarizer based on a 45-degree tilted fluoride fiber Bragg grating for mid-infrared fiber laser technology[J]. OSA Continuum, 2018, 1(1): 56.
[21] Huang Q Q, Zou C H, Mou C B, et al. 23 MHz widely wavelength-tunable L-band dissipative soliton from an all-fiber Er-doped laser[J]. Optics Express, 2019, 27(14): 20028-20036.
[22] Li W X, Huang Z N, Xiao X P, et al. 0.017 nm, 143 ps passively mode-locked fiber laser based on nonlinear polarization rotation[J]. Optics Letters, 2023, 48(10): 2676-2679.
[23] Wang T X, Yan Z J, Huang Q Q, et al. Mode-locked erbium-doped fiber lasers using 45° tilted fiber grating[J]. IEEE Journal of Selected Topics in Quantum Electronics, 2018, 24(3): 1101506.
Article Outline
黄梓楠, 黄千千, 田昊晨, 闫志君, 邹萌, 孙敬华, 顾澄琳, 王开, 徐子硕, 李卫淅, 戴礼龙, 梁新栋, 牟成博. 基于45°倾斜光纤光栅的光纤激光频率梳(特邀)[J]. 激光与光电子学进展, 2024, 61(1): 0106005. Zinan Huang, Qianqian Huang, Haochen Tian, Zhijun Yan, Meng Zou, Jinghua Sun, Chenglin Gu, Kai Wang, Zishuo Xu, Weixi Li, Lilong Dai, Xindong Liang, Chengbo Mou. Fiber Laser Frequency Comb Based on a 45° Tilted Fiber Grating (Invited)[J]. Laser & Optoelectronics Progress, 2024, 61(1): 0106005.