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由于Er3+在1 550 nm波段的增益有一定的带宽,前文介绍的掺铒铌酸锂微腔激光器都是在多模状态下工作,存在虚信号、随机起伏和不稳定性等问题,阻碍了其实际应用。单模激光器具有单色性、高稳定性、输出波长可控等特点,在片上光通信等实际应用中具有巨大潜力,因而实现单模激光器的研究备受关注。目前实现单模激光的方式主要有以下四种[39]:(1)减小谐振腔的尺寸以扩大自由光谱范围(FSR),使得谐振腔在增益带内单纵模共振;(2)利用布拉格反射镜结构或分布反馈式谐振腔结构实现单模选模;(3)级联多个谐振腔通过游标效应实现单纵模共振;(4)利用空间选择性泵浦抑制高阶模或增加高阶模式的损耗。一般来说,减小谐振腔的尺寸会增加腔内光场的辐射损耗导致激光器的阈值增加,所以一般不采用该方案。下面介绍目前在Er-LNOI微腔单模激光器方面的研究进展。
中国科学院上海光学精密机械研究所的程亚、林锦添课题组,利用PLACE工艺制备了两直径分别为30.3 μm和23.6 μm耦合在一起的Er-LNOI微盘腔,两微盘满足泵浦波段977.7 nm和信号波段1 550.5 nm同时共振。由于游标效应有效地抑制了铒增益带宽内的模式数[40]。在977.7 nm激光泵浦下,首次观察到Er-LNOI单模激光,其阈值为200 μW,如图9所示。通过3×3光纤耦合器构成的迈克尔逊干涉仪测得激光器的线宽为348 kHz。
图 9 (a) 单模激光信号随着泵浦功率的增加而增加; (b) 泵浦激光光谱 (插图为泵浦光在耦合微盘腔内的光学显微镜照片); (c) 信号输出功率随泵浦功率的变化关系 (插图为出射激光时耦合微盘的显微镜照片)[40]
Figure 9. (a) Increasing single-mode lasing signal with increasing pump power; (b) Spectrum of pump laser (Inset: optical micrograph of coupled microdisks with pump laser); (c) Relationship between signal output power and pump power (Inset: optical micrograph of coupled microdisks when lasing)[40]
随后,笔者课题组设计了两半径分为85 μm和100 μm的微环构成的光学分子,基于游标效应,光学分子相邻的共振模式间隔波长增大到11 nm,并让一个共振波长位于Er3+最强增益波长(~1 532 nm)附近。利用电子束曝光、氩离子刻蚀等工艺制备了品质因子为2.97×105的Er-LNOI微环光学分子。在980 nm波段泵浦下,1 500~1 600 nm波段范围内实现了稳定的Er-LNOI单模激光,激光器的单模抑制比和阈值分别为~26.3 dB和~200 μW,如图10所示[41]。该工作提高了Er-LNOI单模激光器的集成度和可拓展性。
图 10 (a) 不同泵浦功率下Er-LNOI光学分子在1500~1 560 nm范围内实现单模出射;(b) 不同泵浦功率下的单模激光输出功率和模式线宽;(c) 900 μW泵浦功率下观察到的高边模抑制比信号(~26.3 dB)(插图为观测到的绿色上转换荧光)[41]
Figure 10. (a) Single mode emission of Er-LNOI optical molecules in the range of 1500-1560 nm under different pump powers; (b) Output power and mode linewidth of single-mode laser at different pump powers; (c) Observed high side-mode suppression ratio signal (~26.3 dB) at a pump power of 900 μW (Inset: the observed green up conversion fluorescence)[41]
同期,上海交通大学陈建平课题组设计了图11(a)中直径200 μm短微环腔和1.2 cm的长腔耦合系统[42],两腔的FSR分别为Fa~200 GH和Fb~10 GHz。基于游标效应,使得两个腔的共振模式在铒增益带内仅能够存在单个共振峰可以匹配。通过电子束曝光-氩离子刻蚀工艺制备的掺铒微环腔的Q值在1 531.1 nm处为5×104。在1 480 nm波段泵浦下,观察到了激光的单模输出,单模抑制比为31 dB。通过自外差探测法测得激光器的线宽约为~1.2 MHz。激光器的阈值和转换效率分别为13.54 mW和1.45×10−4。
图 11 (a)~(b) Er-LNOI双腔结构示意图和游标效应原理图[42]; (c) 制备的Er-LNOI单环腔SEM图; (d) 微环腔波导中支持的前四个模式的模场分布图[43]
Figure 11. (a)-(b) Schematic diagram of Er-LNOI dual cavity structure and vernier effect[42]; (c) SEM of prepared Er-LNOI single microring cavity; (d) Mode field distribution of the first four modes supported in microring cavity waveguide[43]
随后,该课题组在单个掺铒铌酸锂微环中也实现了单模激光[43]。图11(c)为制备的微环腔的SEM图,半径为105 μm,对应1 531.49 nm和1 484.45 nm的Q值分别为2.13×104和0.89×105。实现单模的机制为,通过合理设计微环环宽(2 μm),理论上支持TE00,TM00,TE10,TM10四个模式。图11(d)为通过COMSOL模拟得到的四个模式在微环波导中的电场分布图,可见,除TE00模式外,其余高阶模都与波导侧壁有较大的模场交叠。由于制备的微环波导的侧壁有一定的粗糙度,导致除TE00模式外,其余模式在微腔内传输的损耗较大,从而有效地抑制了高阶模式的增益,只让TE00模式获得较大增益,实现单模工作。实验上证实单模抑制比高至35.5 dB,激光器的线宽为1.27 MHz。单模激光器的阈值和转换效率分别为14.5 mW和1.2×10−4,最高输出功率为2.1 μW。
近期,中国科学院上海光学精密机械研究所的程亚、林锦添课题组,通过PLACE工艺制备了单个直径为29.8 μm的Er-LNOI微盘腔,在968 nm激光泵浦下,观察到了出射波长为1546 nm的单模激光[44]。其基本原理是通过调整锥形光的耦合位置,泵浦激发回音廊模式微腔中的多边形模式,利用泵浦波段和信号波段多边形模式具有高的模式交叠(Γ=0.75)和对应的FSR (11 nm)大的特点,有效抑制了传统回音廊模式信号的增益,从而实现了单模激光。图12(a)为不同泵浦功率下对应的单模激光输出功率光谱,通过外差法测得输出激光的线宽为98 Hz,是目前报导的线宽最窄Er-LNOI激光器。图12(b)为通过CCD捕获到的腔内出射四边形信号模式光学显微镜图像,图中分别为绿色上转换荧光(左)和泵浦光(右)的光学显微镜图像。该单模激光器的工作阈值和最高输出功率分别为25 μW和2 μW。表1总结了目前报导的LNOI微腔激光器的主要性能参数,可见,在过去的短短两年中,LNOI微纳激光器经历了从无到有,从盘到环,从多模到单模的快速发展过程。
图 12 (a) 不同泵浦功率下的Er-LNOI单模激光器的输出功率变化光谱; (b) 1546 nm波长方形激光模式的光学显微图像 (插图:550 nm波长上转换荧光(左)和泵浦光(右)的方形模式的光学显微镜图像)[44]
Figure 12. (a) Spectra of the output power of the Er-LNOI single mode laser at different pump powers; (b) Optical micrograph of the square lasing modes at 1 546 nm wavelength (Inset: the optical micrographs of the square modes of the up-conversion fluorescence around 550 nm wavelength (Left) and the pump light (Right))[44]
表 1 报导的LNOI微腔激光器性能参数对比
Table 1. Comparison of performance parameters of the reported LNOI microcavity lasers
Structure Pump wavelength/nm Threshold Conversion efficiency Maximum power Linewidth Model References Microdisk 974 2.99 mW 4.117×10−4% ~40 nW 0.12 nm Multimode [35] 1460 9.31 mW 3.15×10−3% ~500 nW 0.14 nm Multimode Microdisk 976 <400 μW 1.92×10−2% ~140 nW 0.024 nm Multimode [34] Microdisk 974 292 μW 6.5×10−5% ~0.4 nW ~0.01 nm Multimode [36] Microring 974 ~20 μW 6.61×10−5% ~0.1 nW ~0.01 nm Multimode [37] Microring ~980 ~3.5 mW 4.38×10−3% ~35 nW - Multimode [38] Coupling microrings 977.7 ~200 μW 7×10−3% ~50 nW 348 kHz Single mode [40] Coupling microrings 979.6 ~200 μW 4.4×10−3% ~40 nW ~0.005 nm Single mode [41] Coupling microrings 1484 13.54 mW 1.45×10−2% 0.31 μW 1.2 MHz Single mode [42] Microring 1484 14.5 mW 1.2×10−2% 2.1 μW 1.27 MHz Single mode [43] Microdisk 968 ~25 μW 1.3×10−2% 2 μW 98 Hz Single mode [44]
Research progresses of microcavity lasers based on lithium niobate on insulator (Invited)
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摘要: 绝缘体上铌酸锂薄膜凭借铌酸锂晶体优异的光学性能和薄膜器件的易加工和可集成特性,被视为理想的集成光学平台。除了波导、调制器等传输、控制器件方面的研究之外,最近铌酸锂薄膜激光器的研究也取得了显著的进展。文中将对最近迅速发展的铌酸锂薄膜微腔激光器的研究现状进行综述。首先,介绍铌酸锂晶体和铌酸锂薄膜稀土离子掺杂的主要技术方案,以及近期有关于稀土离子掺杂铌酸锂薄膜微纳光学器件制备方面的探索;其次,总结近年来掺铒铌酸锂薄膜微盘腔、微环腔激光器方面的研究进展;然后,阐述微腔激光器体系几种常见的实现单模激光器方法的工作机理,介绍研究者们利用“游标效应”调制模式损耗等方式实现掺铒铌酸锂薄膜单模激光器的研究进展;最后,基于目前报导的铌酸锂薄膜激光器研究成果,对目前研究存在的局限性以及未来的研究方向进行了探讨。Abstract: Lithium niobate on insulator (LNOI) was regarded as a competitive integrated optical platform due to the excellent optical performance of lithium niobate crystal and integration characteristics of thin-film devices. In addition to the research on transmission and control devices, such as waveguides and modulators, significant progress has been made in LNOI lasers recently. The research status of the rapidly developing LNOI microcavity laser was reviewed in this paper. Firstly, the main technical schemes of rare-earth ion doping of bulk lithium niobate and LNOI, as well as the recent exploration on the preparation of rare-earth ion doped LNOI micro-/nano- optical devices, were introduced; Secondly, the research progresses on Erbium-doped lithium niobate on insulator (Er-LNOI) microdisk and microring cavity lasers were summarized; Then, the working mechanism of several common methods to realize single-mode laser in microcavity laser system were described. The research progresses on Er-LNOI single-mode lasers utilizing "Vernier effect" and mode-loss modulation were introduced in the following; Finally, based on the reported research results of LNOI lasers, the limitations of the current research and the future research directions were discussed.
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Key words:
- lithium niobate on insulator /
- microcavity laser /
- integrated optics
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图 1 (a) 离子注入方式掺铒的结构和浓度分布[27]; (b) 离子注入方式掺镱的结构和浓度分布[28]; (c) Er-YSO键合掺杂结构示意图[30]; (d) 掺铥结构示意图[26]
Figure 1. (a) Structure and concentration distribution of erbium doped by ion implantation[27]; (b) Structure and concentration distribution of ytterbium doped by ion implantation[28]; (c) Schematic diagram of Er-YSO bonding doping structure[30]; (d) Schematic diagram of thulium doped structure[26]
图 4 (a)~(c) 974 nm泵浦下观察到的激光信号与线宽; (d) 激光功率和974 nm泵浦功率的变化关系 (插图:观察到的绿色上转换荧光图像); (e)~(g) 1460 nm泵浦下观察到的激光信号与线宽; (h) 激光功率和1460 nm泵浦功率的变化关系 (插图:观察到的绿色上转换荧光图像)[35]
Figure 4. (a)-(c) Observed laser signal and linewidth with the 974 nm pump; (d) Relationship between the emitted laser power and the 974 nm pump power (Inset: the observed green up-conversion fluorescence); (e)-(g) Observed laser signal and linewidth with the 1 460 nm pump; (h) Relationship between the emitted laser power and the 1 460 nm pump power (Inset: the observed green up-conversion fluorescence)[35]
图 7 (a)在 1 531.50~1 532.65 nm内的信号频谱,泵浦功率为 46.4 μW; (b) ~1 mW泵浦功率时观察到的多峰激光信号(插图:观察到的绿色上转换荧光图像); (c) 信号模式功率和 (d) 线宽随泵浦功率的变化关系[37]
Figure 7. (a) Collected signal spectrum in the range of 1 531.50-1 532.65 nm at 46.4 μW pump power; (b) Multi-peak lasing signal observed at a pump power of ~1 mW (Inset: the observed green up-conversion fluorescence); (c) Power and (d) linewidth of the signal mode under different pump powers[37]
图 8 (a) Er-LNOI“跑道形”微环腔随着输入泵浦功率的增加,观测到的光谱演化; (b) 激光输出功率与输入泵浦功率的关系; (c) 施加−300 V和+300 V电压时激光信号波长的漂移[38]
Figure 8. (a) Spectral evolution of the Er-LNOI racetrack microring resonator with increasing input pump powers; (b) Relationship between the emitted laser output power and the input pump power; (c) Laser signal wavelength by varying the electric voltage between −300 V and +300 V[38]
图 9 (a) 单模激光信号随着泵浦功率的增加而增加; (b) 泵浦激光光谱 (插图为泵浦光在耦合微盘腔内的光学显微镜照片); (c) 信号输出功率随泵浦功率的变化关系 (插图为出射激光时耦合微盘的显微镜照片)[40]
Figure 9. (a) Increasing single-mode lasing signal with increasing pump power; (b) Spectrum of pump laser (Inset: optical micrograph of coupled microdisks with pump laser); (c) Relationship between signal output power and pump power (Inset: optical micrograph of coupled microdisks when lasing)[40]
图 10 (a) 不同泵浦功率下Er-LNOI光学分子在1500~1 560 nm范围内实现单模出射;(b) 不同泵浦功率下的单模激光输出功率和模式线宽;(c) 900 μW泵浦功率下观察到的高边模抑制比信号(~26.3 dB)(插图为观测到的绿色上转换荧光)[41]
Figure 10. (a) Single mode emission of Er-LNOI optical molecules in the range of 1500-1560 nm under different pump powers; (b) Output power and mode linewidth of single-mode laser at different pump powers; (c) Observed high side-mode suppression ratio signal (~26.3 dB) at a pump power of 900 μW (Inset: the observed green up conversion fluorescence)[41]
图 11 (a)~(b) Er-LNOI双腔结构示意图和游标效应原理图[42]; (c) 制备的Er-LNOI单环腔SEM图; (d) 微环腔波导中支持的前四个模式的模场分布图[43]
Figure 11. (a)-(b) Schematic diagram of Er-LNOI dual cavity structure and vernier effect[42]; (c) SEM of prepared Er-LNOI single microring cavity; (d) Mode field distribution of the first four modes supported in microring cavity waveguide[43]
图 12 (a) 不同泵浦功率下的Er-LNOI单模激光器的输出功率变化光谱; (b) 1546 nm波长方形激光模式的光学显微图像 (插图:550 nm波长上转换荧光(左)和泵浦光(右)的方形模式的光学显微镜图像)[44]
Figure 12. (a) Spectra of the output power of the Er-LNOI single mode laser at different pump powers; (b) Optical micrograph of the square lasing modes at 1 546 nm wavelength (Inset: the optical micrographs of the square modes of the up-conversion fluorescence around 550 nm wavelength (Left) and the pump light (Right))[44]
表 1 报导的LNOI微腔激光器性能参数对比
Table 1. Comparison of performance parameters of the reported LNOI microcavity lasers
Structure Pump wavelength/nm Threshold Conversion efficiency Maximum power Linewidth Model References Microdisk 974 2.99 mW 4.117×10−4% ~40 nW 0.12 nm Multimode [35] 1460 9.31 mW 3.15×10−3% ~500 nW 0.14 nm Multimode Microdisk 976 <400 μW 1.92×10−2% ~140 nW 0.024 nm Multimode [34] Microdisk 974 292 μW 6.5×10−5% ~0.4 nW ~0.01 nm Multimode [36] Microring 974 ~20 μW 6.61×10−5% ~0.1 nW ~0.01 nm Multimode [37] Microring ~980 ~3.5 mW 4.38×10−3% ~35 nW - Multimode [38] Coupling microrings 977.7 ~200 μW 7×10−3% ~50 nW 348 kHz Single mode [40] Coupling microrings 979.6 ~200 μW 4.4×10−3% ~40 nW ~0.005 nm Single mode [41] Coupling microrings 1484 13.54 mW 1.45×10−2% 0.31 μW 1.2 MHz Single mode [42] Microring 1484 14.5 mW 1.2×10−2% 2.1 μW 1.27 MHz Single mode [43] Microdisk 968 ~25 μW 1.3×10−2% 2 μW 98 Hz Single mode [44] -
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