基于超快激光光刻的有源铌酸锂光子集成 下载: 919次特邀综述
The development of photonic integration technology provides an effective approach to constructing communication, sensing, computing, and information processing devices with high performance, low cost, scalability, and reliability. Among various material platforms, lithium niobate (LN) has long been considered one of the most suitable materials for realizing photonic integrated circuits (PICs). It possesses superior optical properties, including a wide transparent window (0.35-5 μm), large nonlinear/electro-optic coefficients, and strong acousto-optic effects. Significant progress has been made in the fabrication process of thin film LN (TFLN) wafers, which has laid a material foundation for manufacturing photonic devices with high refractive index contrast and strong light field confinement. To date, researchers have achieved a wide range of photonic integrated functional bricks on TFLN, such as modulators, optical frequency converters, splitters, quantum light sources, and delay lines. These devices have demonstrated notable photonic characteristics, including low transmission loss, high-speed controllability, efficient optical frequency conversion, and low energy consumption. However, due to the lack of optical gain characteristics in LN crystals themselves, it is challenging to directly fabricate essential components for on-chip integration, such as micro-lasers and optical amplifiers, on TFLN wafers.
One approach to achieving optical gain on TFLN is by doping gain media within the TFLN film. Rare-earth ion-doped (REI-doped) TFLN has been employed to realize on-chip micro-lasers and optical amplifiers at different wavelengths, such as around 1550 nm and 1030 nm. The specific working wavelength is determined by the intrinsic optical spectra of the rare-earth-doped ions. Although active integration of TFLN photonic devices is still in its early stages, the exceptional optical properties of LN crystals, combined with low-loss photonic chip fabrication techniques and innovative device designs, will endow on-chip TFLN photonic devices with unparalleled scalability and exceptional functionality.
In recent years, the combination of commercial TFLN wafers and low-loss LN photonic device nanostructuring technology has resulted in a series of high-performance photonic device applications. In less than a decade, several important manufacturing techniques for LN photonic chips have been developed internationally, enabling the realization of practical high-quality photonic chip prototypes. These techniques include focused ion beam fabrication of high-performance LN nanostructures, as well as the use of electron beam lithography or ultraviolet photolithography combined with ion etching to produce high-quality LN photonic chips. Additionally, the photolithography-assisted chemo-mechanical etching technology (known as PLACE) has emerged as a promising micro/nanofabrication technique.
Thanks to the rapid advancements in high-repetition-rate and highly stable femtosecond lasers and large-stroke high-precision high-speed motion stages, the PLACE technique for fabricating active photonic devices on REI-doped TFLN has demonstrated both high processing efficiency while maintaining its inherent high-precision processing quality. Many corresponding advances have been achieved, which is important and necessary to summarize the existing research to guide the future development of this field more rationally.
The fabrication process of the PLACE technique is summarized (Fig. 1). The home-built ultra-high-speed high-resolution femtosecond laser lithography fabrication system is reported (Fig. 2). The demonstration of integrated active LN photonic devices such as on-chip tunable micro-lasers and waveguide amplifiers on REI-doped TFLN using the PLACE technique are comprehensively reviewed. Specifically, an erbium ion-doped LN waveguide amplifier with a maximum internal net gain exceeding 20 dB is achieved (Fig. 18), and an electro-optically tunable single-frequency laser in a high-Q LN microdisk is demonstrated with an ultra-narrow linewidth of 454.7 Hz (Fig. 5). We also achieve an electrically driven microring laser by monolithically integrating a diode laser (Fig. 10) and an erbium ion-doped TFLN microring laser (Fig. 3). A novel hybrid integration scheme of passive and active LN microdevices is performed using a continuous lithographic processing approach (Fig. 19 and 20). Lastly, we summarize the afore mentioned results and give an outlook on this vibrant and promising field of research.
The utilization of PLACE technology for the fabrication of TFLN active photonic devices holds great importance. This cutting-edge technique, with its high processing efficiency, intrinsic high precision, and wafer-scale integration capability, enables the production of cost-effective and high-performance photonic devices. This breakthrough has the potential to revolutionize the field of photonic science and technology, promoting sustainable development across various scientific disciplines and applications such as high-speed communication, artificial intelligence, and precision measurement.
1 引言
光子集成技术的发展为构建具有高性能、低成本、可扩展和高可靠性的通信、传感、计算与信息处理设备提供了有效的技术途径[1]。而铌酸锂(LN)材料一直被认为是最适合实现光子集成芯片的材料平台之一。它具有优越的光学特性,包括较宽的透明窗口(0.35∼5 μm)、较大的非线性/电光系数和较强的声光效应[2]。近年来,铌酸锂薄膜(TFLN)晶圆制造工艺取得了变革性进展,为制造具有高折射率比、强光场约束的光子器件奠定了材料基础,尤其是商业化TFLN晶圆与低损耗铌酸锂光子器件纳米加工技术的结合,产生了一系列的高性能光子器件应用[2-8]。在过去的不到十年中,国际上发展了多个重要的铌酸锂光子芯片制造技术路线,能够实现具有实用意义的高品质光子芯片原型器件,包括聚焦离子束制备高性能铌酸锂纳米光子结构[9]、利用电子束曝光[10]或紫外光刻[11]技术结合离子刻蚀制备高品质铌酸锂光子芯片,以及超快激光辅助的化学机械抛光制备高品质、大尺寸铌酸锂光子芯片[12]等。其中,利用现有的较为成熟的电子束曝光工艺与离子刻蚀技术,一旦找准工艺条件,可以快速复制该技术路线,进行小批量光子原型器件的研发,因此被多个国内外研究团队所采纳。而超快激光光刻辅助化学机械刻蚀技术(PLACE)作为一种新兴微纳加工技术,得益于飞秒激光微纳加工技术及高重复频率高稳定性激光器、大行程高精度高速运动平台等相关设备系统的快速发展,飞秒激光光刻技术在保持其内禀高精度加工品质的同时,也已具有较高的加工效率且未来提升空间巨大。目前经过近五年的发展,该技术已实现传输损耗小于3 dB·m-1、总长度大于1.1 m的单模脊形波导,是一项具有晶圆级集成能力的光子器件制造技术[13]。
迄今为止,研究人员在TFLN上已经实现了一大批光子集成功能单元,比如调制器[14-19]、光频转换器[20-23]、分束器[24-31]、量子光源[32-34]、延时线[13]等,并研究演示了它们所具备的低传输损耗、可高速调控、高效光频转换以及低能耗等光子学特性[4-6,8,21-22,35-36],因此,TFLN是实现大规模光子集成的理想材料平台[37]。然而,由于铌酸锂晶体本身不具备光学增益特性,像片上集成的微激光器与光放大模块这类光子集成中不可缺少的核心部件很难直接在TFLN晶圆上制备出来。相比铌酸锂,III-V族或II-VI族等直接带隙半导体材料更加适合片上的光产生,通过转印、倒装焊接、混合集成、异质集成等方式将半导体异质材料结合到TFLN平台上可以获得光学增益[38],实现多种电驱动的片上通信波段光源与光放大器件[39-44]。例如:将商业化的InP增益芯片与含有可调级联微环腔、电光调制器等结构的无源铌酸锂光子回路混合集成[45]演示实现了宽带可调的O波段铌酸锂/III-V激光器,并在此基础上实现了O波段铌酸锂/III-V发射器[46];将分布式反馈(DFB)激光器倒装焊接到制备有高速可调马赫-曾德尔调制器阵列的铌酸锂薄膜硅衬底上实现的C波段高功率激光发射器[39];利用直接晶圆键合实现低损耗氮化硅波导回路与铌酸锂薄膜的异质集成,结合铌酸锂的较大的Pockels系数,构建了本征线宽仅3 kHz、频率调制速率达12 PHz·s-1的超快可调自注入锁模激光,并基于该器件展示了分辨率可达15 cm的调频连续波(FMCW)激光雷达测距应用[42];通过在铌酸锂薄膜光子回路上转印单晶硅中间层用于刻蚀硅波导,以及转印预制III-V族半导体光放大器等操作,已实现铌酸锂上III-V族单模激光器与光放大器,在1537 nm处最大增益可达11.8 dB[43]。这些光源器件由于结合了具有工艺较为成熟的商业化半导体外部光源或增益器件和低损耗的铌酸锂薄膜光子芯片,往往可实现较高的输出功率、较窄的线宽和直接电泵,可用于高速光收发、大数据中心、激光雷达等光通信与光传感应用。然而,由于器件性能高度依赖高精度的对准键合工艺、高效的光耦合接口,大大增加了制备工艺的复杂程度与器件成本,因此这类器件仍处于研发阶段,制备效率仍面临着巨大挑战。
另一种更加直接地在铌酸锂薄膜上实现光学增益的方式是在铌酸锂薄膜中掺杂增益介质,并且已经在稀土离子掺杂铌酸锂薄膜(REI-doped TFLN)上,实现了不同波段(如~1550 nm和1030 nm波长附近)的片上微激光器与光放大器[38,47-68],具体的工作波段由所掺杂的稀土离子的本征光谱决定。截至目前,利用电子束光刻结合氩离子刻蚀技术在稀土离子掺杂铌酸锂薄膜上制备的微腔,实现的负载品质因子(Q)可达到106量级[63],而利用PLACE技术,则可实现品质因子Q达107量级的微腔[49]。铌酸锂的Pockels效应和χ(2)效应也已经被应用于高速波长转换与调谐[4,35]。通过结合片上铌酸锂微腔激光器的窄线宽与快速频率调谐两个优势,可以对相干通信、激光雷达、精密测量、高精度传感等一系列重要应用产生深远影响。与此同时,片上光放大功能也已经在稀土离子掺杂的铌酸锂薄膜脊形波导上得到演示,并获得了高达23 dB的增益系数[50]。通常,在稀土离子掺杂的铌酸锂薄膜材料平台上,有源光子元件是由分立的外部半导体激光二极管泵浦的。近期,利用商业化泵浦激光芯片与掺铒铌酸锂微环腔芯片对接耦合,制备出了混合集成的电驱动激光器[52]。此外,有源与无源波导阵列的低损耗同片集成[69]也得到了演示。
尽管铌酸锂薄膜光子器件的有源集成仍处于起步阶段,铌酸锂晶体的极佳光学特性,配合低损耗的光子芯片制备技术与新颖的器件设计,将为铌酸锂薄膜片上光子器件赋予无与伦比的可扩展性和优异功能。本文将聚焦由PLACE技术实现的有源铌酸锂薄膜集成光子学的最新发展。首先回顾了有望成为大规模高品质铌酸锂光子器件可行制备途径的PLACE技术。其次介绍了使用PLACE技术构建的高品质因子铌酸锂微腔和片上微激光器,并介绍了基于铌酸锂微激光器的几种前沿应用。然后介绍了PLACE技术在制备低损耗波导和光放大器方面的最新成果。最后,总结了上述结果,并对这一充满活力和前景的研究领域进行了展望。
2 基于飞秒激光的铌酸锂光子芯片制备技术
超快激光PLACE技术[12,70]目前在制备超低损耗的铌酸锂单晶薄膜器件方面得到了有效的验证,已经成功演示了本征品质因子Q值高达1.23×108的铌酸锂微盘腔和传输损耗极低(0.34 dB·m-1)的微环波导腔[71-72]。该技术可以直接应用在掺杂稀土离子的铌酸锂薄膜上制备有源光子器件。如
图 1. 基于飞秒激光的铌酸锂光子芯片制备技术[12,70,72-73]。(a)REI-doped TFLN晶圆的横截面结构示意图[下图:由数码相机拍摄的掺铒TFLN(Er3+:TFLN)晶圆和未掺杂TFLN晶圆的图片];(b)稀土离子掺杂薄膜铌酸锂光子器件的超快激光PLACE制造工艺示意图;(c)利用PLACE技术制造的超高Q值光学微盘的俯视图显微照片;(d)通过光学振铃效应测量获得的图(c)中微盘的透射光谱,测得微盘的本征Q因子为1.23×108
Fig. 1. Fabrication technique of TFLN photonic integrated chip based on femtosecond laser micromachining[12, 70, 72-73]. (a) Schematic of the cross-section structure of REI-doped TFLN wafer [bottom inset: the picture of an Er3+-doped TFLN (Er3+: TFLN) wafer and an undoped TFLN wafer taken by a digital camera]; (b) schematic of the PLACE fabrication process of the REI-doped TFLN photonic device; (c) overview of the optical micrograph of an ultra high-Q LN microdisk fabricated by the PLACE technique; (d) transmission spectrum of the microdisk in Fig. (c) obtained by ring-down measurement, the intrinsic Q-factor of the microdisk is measured to be 1.23×108
得益于飞秒激光直写技术的独特优势,该方法制备的光子集成芯片(PIC)器件的尺寸仅受到薄膜晶圆大小或激光加工平台运动行程的限制,而这两个制约因素可以很轻松地根据特定的应用放大提升。因此,这项技术可以实现大尺寸的PIC器件制备,几乎没有物理限制。此外,光子结构经过化学机械抛光,留下了光学级的光滑表面,粗糙度低于1 nm[74],使得器件的传输损耗可以逼近衬底材料的吸收极限。这些独特优势对于发展大规模集成乃至未来的超大规模集成光子芯片而言至关重要。已报道的PLACE技术[12,70,73-75]采用传统高精度三维运动平台实现掩模刻写,要实现更高加工效率,通常需要平台具备更大的速度、加速度及更好的稳定性,对运动台的性能要求更高,然而,受限于运动台的加减速问题,这种方式会很大程度牺牲加工效率,而我们自主搭建的新一代飞秒光刻系统[76]采用高速旋转的多面镜实现水平方向的扫描,具备更大的扫描速度的同时也不存在加减速问题,同时结合自主开发的控制系统实现高速扫描转镜、飞秒激光器及运动平台的同步精确控制保证掩模刻写的精度,同时自主开发了实时追焦系统,保证刻写的均匀性。总体上,相比之前的PLACE技术,在保证相同制备器件性能的同时,制备效率可提升1~2个数量级。
图 2. 自主搭建的超高速高分辨率飞秒激光光刻系统。(a)飞秒激光光刻系统的晶圆加工平台;(b)飞秒激光实时直写铬掩模图案过程的显微镜监控图像;(c)经过飞秒激光光刻直写的4英寸晶圆,晶圆表面带有11×68个MZI光调制器阵列图案的铬掩模;(d)CMP过程后图(c)中晶圆上其中一个MZI的显微镜图像
Fig. 2. Home-built ultra high-speed high-resolution femtosecond laser lithography fabrication system. (a) Wafer stage of the fs laser lithography fabrication system; (b) microscope image of the real-time femtosecond laser writing process for Cr patterning; (c) a 4 inch wafer with a Cr mask of a 11 × 68 array of MZI optical modulators; (d) microscope images of one of the MZI on the wafer in Fig. (c) after the CMP process
3 基于高品质因子微腔的片上激光器
3.1 低阈值掺铒铌酸锂薄膜微盘腔激光器
利用如
图 3. 通过制备高品质掺铒铌酸锂薄膜微盘腔实现C波段片上波长可调微激光器[47]。(a)光学显微镜下,掺铒铌酸锂薄膜微盘腔与光纤锥耦合时的俯视图;(b)透射光谱的洛伦兹拟合结果表明,在1563.86 nm波长下测量的微盘的品质因子为1.8×106;(c)泵浦光功率为0.92 mW时的掺铒微激光器光谱(上方左边插图为1560 nm附近的放大光谱,显示为多模激光光谱;上方右边插图为微盘发出的绿色上转换荧光);(d)在976 nm处所有发射线的总强度与吸收泵浦功率的关系(实验数据用圆圈表示,曲线为线性拟合,激光阈值小于400 μW)
Fig. 3. Demonstration of the C-band on-chip wavelength tunable microlaser by constructing high Q Er3+∶TFLN microdisk[47]. (a) Optical micrograph of an Er3+∶TFLN microdisk; (b) Lorentzian fitting of the transmission curve indicates the Q-factor of the microdisk reaches 1.8×106; (c) spectrum of the Er3+-doped laser when the pump power is around 0.92 mW (upper left inset is enlarged spectrum around 1560 nm, shows the multimode lasing spectrum; upper right inset is micrograph of the strong green upconversion fluorescence of the microdisk); (d) dependence of the intensity of all emission lines on the absorbed pump power at 976 nm (experimental data are marked by circles, and the linear fitting in solid line indicates a lasing threshold lower than 400 μW)
3.2 高品质微腔多边形模式与单模超窄线宽掺铒微盘激光器
在高品质的铌酸锂微盘腔中,本征频率很接近的固有回音壁模式(WGM)在受到耦合光纤锥等的弱微扰诱导下,发生重组,相干合成了多边形模式与星形模式主导的新模式[77]。通过调节激发波长或者改变锥形光纤与微腔的耦合位置,可以在单个谐振腔中观察到各种多边形与星形模式。最近发展的一种有效且可控的模式修剪和聚类理论可以很好地解释和再现这些现象[78]。
WGM谐振腔中多边形模式的产生源自腔模普遍且规律的准简并现象。考虑WGM谐振腔中的准稳态解,其本征值(
式中:P和Q分别代表径向量子数与方位量子数的变化量;
图 4. 高品质回音壁微腔在弱微扰下的模式修剪与聚类[78]。(a)不同波长下本征模式的弦角的余弦值[黑点和彩色虚线分别代表准稳态解和由式(1)给出的近似结果,右侧图像为其在轻微扰动下的本征模式绘图];实验观测得到的模式图样(b)三角形模式、(c)双局域三角形模式(第一激发态)、(d)星形模式、(e)四边形模式、(f)五边形模式、(g)六边形模式、(h)七边形模式、(i)八边形模式
Fig. 4. Modes trimming and clustering in a weakly perturbed high-Q whispering gallery microresonator[78]. (a) Cosine of the chord angle of the eigenmodes at different wavelengths [black dots and colored dotted line represent the quasi-steady-state solution and the approximate result given by Eq. (1), respectively]; observed mode patterns of (b) triangle mode, (c) triangle mode with first excited state, (d) star mode, (e) square mode, (f) pentagon mode, (g) hexagonal mode, (h) heptagon mode, and (i) octagon mode
实验上,在Z切Er3+∶TFLN晶片上制备的直径为83.44 μm微盘中,利用双光子荧光激发,实现了模态的可视化。当锥形光纤与微盘中心之间的距离调谐到38.5 μm时,如
波长快速可调谐的单频超窄线宽片上微激光器在相干通信、光学检测和其他光子学应用中具有重要的应用价值。根据Schawlow-Townes理论,激光器的基础线宽与无掺杂的谐振腔负载品质因子Q值的二次方成反比,故获得超窄线宽激光需要高品质因子的谐振腔[79-81]。然而,制备具有超高光学品质因子的激光微腔,并在单个微腔的光学增益谱宽内只支持单模输出,仍然是一个挑战。如前介绍可知,由于WGM微腔中产生多边形模式的条件严格,在一个自由光谱范围内只能形成一个特定的多边形模式。因此,相比于传统的回音壁模式,多边形模式在光学增益带宽内分布较为稀疏,可以抑制回音壁模式泵浦时容易激发的高阶横模,避免多模激射。
最近,这种独特的基于诱导微腔中多边形模式构建单频超窄线宽激光的机制被提出并得到实验证明,在单个掺铒铌酸锂薄膜微盘腔内,实现了泵浦波长和激光波长处的高品质多边形模式的同时激发[49]。这些多边形模式是由锥形光纤与圆对称的微盘腔耦合形成,由于光纤锥引入的微扰很弱,回音壁模式固有的高品质因子得以维持,这是形成高品质多边形模式的关键[77-78]。如
图 5. 单模超窄线宽掺铒微盘激光器表征[49]。(a)上转换荧光和泵浦光的光谱图[插图为550 nm波长处的四边形模式上转换荧光(左)和泵浦光(右)的光学显微图];(b)激光输出功率随泵浦功率增加的变化图;(c)不同泵浦功率下测得的微激光器输出功率(左上插图为在1546 nm波长处的四边形激射模式的光学显微图,右上插图为理论计算得到的四边形激射模式的强度分布图);(d)测试线宽的实验装置示意图;(e)测得的拍频信号的光谱图;(f)高频范围测得频率噪声为196.5 Hz2·Hz-1,表明激光的基本线宽为436.6 Hz;(g)激光线宽随激光输出功率增加呈线性增长
Fig. 5. Characterization of single-mode laser with ultra-narrow linewidth in Er3+-doped microdisk[49]. (a) Spectrum of the up-conversion fluorescence and the pump light [upper insets are optical micrographs of the square modes of the upconversion fluorescence near the 550 nm wavelength (left) and the pump light (right)]; (b) laser output power at increasing pump power; (c) spectra of the microlaser output powers at different pump power levels (upper left inset is optical micrograph of the lasing mode with a square pattern at 1546 nm wavelength, upper right inset is theoretically calculated intensity distribution of the square lasing mode); (d) illustration schematic of the experimental setup for the linewidth measurement; (e) spectrum of the detected beating signal; (f) frequency noise is measured to be 196.5 Hz2·Hz-1 in the high-frequency range, indicating that the fundamental linewidth is 436.6 Hz; (g) linewidth glows linearly with the output power of the microlaser
微激光器在许多光子学应用中,通常需要快速的波长调谐功能。如
图 6. 超窄线宽激光器的波长调谐特性[49]。(a)不同泵浦功率下测得的激射波长;(b)激射波长随着泵浦功率变化图;(c)不同直流偏压下测得的激射波长;(d)激射波长随着直流偏压变化图
Fig. 6. Wavelength tuning performance of the ultra-narrow linewidth mcirolaser[49]. (a) Lasing wavelength at different pump powers; (b) lasing wavelength drifts with pump power; (c) lasing wavelength at different direct current (DC) biases; (d) lasing wavelength drifts with DC bias
3.3 在有源铌酸锂微盘腔上实现电光可调的低相位噪声微波合成器
单个弱微扰的掺铒铌酸锂薄膜微盘谐振腔,除了能合成单个多边形模式,也可以形成近简并的多边形模式,进而获得双波长微激光器[55]。这对近简并的激光多边形模式,仍然由锥形光纤与圆对称微盘耦合引起近简并的回音壁模式重组而产生,它们具有很小的波长间距。由于这些模式同样具有很高的品质因子,通过拍频即可合成可调谐的低相位噪声微波信号。
为了产生近简并多边形模式,在Z切向的掺铒铌酸锂薄膜晶圆上制备了直径为106.33 μm的微盘谐振腔,并使用2 μm直径的锥形光纤耦合到距离微腔中心47.0 μm的微腔边缘上方。如
图 7. 双波长微激光器[55]。(a)微激光器的光谱图双峰精细结构,双波长激光的间隔为8 pm(左上插图为微激光器的激光光谱图,右上插图为泵浦光附近光谱图);(b)上转换荧光(上)、泵浦光(左下)和输出激光(右下)多边形模式的光学显微图;(c)微激光器的输出功率与泵浦功率的依赖关系图;(d)波长间隔为70 pm的双波长微激光器的光谱图[插图为上转换荧光(左)和激射模式(右)的多边形光学显微图]
Fig. 7. Dual-wavelength microlasers[55]. (a) Spectrum of the microlasers with a fine structure of two peaks, showing a wavelength interval of 8 pm [insets are transmission spectrum of the microlaser near the lasing wavelength (left) and the pumping wavelength (right)]; (b) optical micrographs of polygon modes of the up-conversion fluorescence (top), pump laser (bottom left), and output laser (bottom right); (c) output power of the microlaser as a function of the pump power; (d) spectrum of the microlasers with a large wavelength interval of 70 pm [insets are optical micrographs of polygon-shaped patterns of the up-conversion fluorescence (left) and lasing modes (right)]
为了产生微波信号,将两个微激光器的输出功率经掺铒光纤放大器放大到0.047 mW,然后用快速光电探测器通过信号拍频来合成微波信号。
图 8. 微波信号合成[55]。(a)测得的微波信号(插图为探针施加在微盘腔中心的电极上的光学显微示意图);(b)微波载波信号的相位噪声图;(c)微波信号的电光调制图;(d)波长间隔为70 pm的双波长激光合成的微波信号
Fig. 8. Microwave signal synthesis[55]. (a) Detected microwave signal (inset is optical micrograph of the probe contracted with the electrode on the center of the microdisk); (b) phase noise of the microwave signal carrier; (c) electro-optic tuning of the microwave signal; (d) microwave signal synthesized by the dual-wavelength laser of a wavelength interval of 70 pm
通过对双波长激光稳定的物理机制分析可知,两个近简并的激光多边形模式,具有很接近的空间模场分布,但相位相差了π。它们与泵浦光的多边形模式具有很接近的模场空间重叠度,但沿着微腔边缘呈现准周期性变化的模场空间重叠度的分布仍然相差π,因此两个近简并激光模式避免了增益竞争,从而获得稳定的双波长激光输出,进而形成稳定的微波源。
3.4 单片集成的大功率窄线宽掺铒微盘激光器
上述已经介绍了基于单个圆对称的掺铒铌酸锂的悬空微盘腔与锥形光纤耦合,形成弱微扰的有源微腔,进而产生窄线宽单频/双频激光。这些微激光器,在面向光子学应用时,仍然存在一些不足。首先,悬空微盘与锥形光纤耦合,不是一体化、便携式集成。其次,单频掺铒铌酸锂微腔激光器的输出功率需要工作在较小腔长的微腔来实现平衡单频运转,因此所获取的光学增益长度有限,限制了激光输出功率的提升(为数微瓦)。为解决这些问题,利用PLACE技术制备毫米尺寸的圆对称非悬空微盘,并在微腔侧面集成脊形光波导,借助该光波导引入模式微扰,可以获得窄线宽、大输出功率的单片集成微激光器[82]。
采用X切700 nm厚、掺铒摩尔分数为0.5%的掺铒铌酸锂薄膜晶圆作为样品。圆对称微盘腔的直径选为409 μm和1 mm两种尺寸,侧面耦合的多模脊形光波导距离微腔边缘4 μm。铌酸锂薄膜的刻蚀深度为300 nm,如
图 9. 单片集成的大功率窄线宽微盘激光器[82]。(a)片上集成微结构的光学显微图;(b)直径409 μm微腔的单频激光光谱;(c)空间分布为六边形的微腔上转换荧光的光学显微图;(d)激光输出功率随泵浦光功率演化曲线;(e)直径1 mm的微腔在1551.68 nm附近的单频激光光谱(插图为微腔上转换荧光的空间分布,由于多边形的边数更多,接近圆形);(f)激光拍频信号(黑点)的洛伦兹拟合结果显示,激光线宽为0.11 MHz
Fig. 9. Monolithically integrated high-power narrow-bandwidth microdisk laser[82]. (a) Optical micrograph of the fabricated integrated microdisk laser; (b) spectrum of the integrated microdisk laser with 409 μm diameter, exhibiting a single-frequency lasing at 1552.82 nm; (c) green upconversion fluorescence of the integrated microdisk laser, showing a hexagon pattern; (d) laser output power versus pump power dropped to the cavity; (e) spectrum of the integrated microdisk laser with 1 mm diameter, confirming a single-frequency lasing at 1551.68 nm wavelength (inset is micrograph of the upconversion fluorescence); (f) Lorentz fitting of the detected beating signal (black dots) featuring a laser linewidth of 0.11 MHz
3.5 激光二极管泵浦的紧凑型混合铌酸锂微环激光器
PLACE技术还可用于制备集成泵浦激光二极管的铌酸锂薄膜环腔激光器[52]。
图 10. 激光二极管泵浦的紧凑型混合铌酸锂微环激光器[52]。(a)器件结构示意图,由CoS封装的半导体激光器和高Q Er3+∶TFLN微环激光器组成;(b)紧凑型混合铌酸锂微环激光器的实物照片;(c)CoS封装的半导体激光器和 Er3+∶TFLN 微环之间界面的特写光学显微照片;(d)微环激光器发射光谱随泵浦功率增加的变化;(e)微环激光器的片上激光功率与片上泵浦功率的函数关系;(f)微环激光器的片上激光功率与驱动电功率的函数关系
Fig. 10. Laser diode-pumped compact hybrid lithium niobate microring laser[52]. (a) Illustration of the microlaser device, which consists of a commercial CoS laser diode and an Er3+∶TFLN microring; (b) overview picture of the microlaser device taken by a digital camera; (c) top-view optical micrograph of the interface between the CoS laser diode and the array of Er3+∶TFLN microrings; (d) spectra of the microring laser with increasing pump power; (e) on-chip laser power drifts with input pump power; (f) on-chip laser power drifts with driving electric power
3.6 基于Sagnac环形反射器的单模激光器
到目前为止,所有稀土离子掺杂的TFLN激光器都是使用高品质因子的WGM谐振腔进行演示的,包括微盘谐振腔和微环谐振腔。TFLN WGM谐振腔往往需要精密复杂的耦合技术,如棱镜耦合、锥形光纤和耦合波导,实现高功率、高效率激光输出。与WGM激光器相比,Fabry-Pérot(FP)谐振腔激光器可以简单地通过延长谐振腔中的增益长度,产生高功率激光输出,其重要组成部分是端面反射器。常用的DFB半导体激光器选用的端面反射器是分布式布拉格反射器(DBR)与高反射率镀膜[83-89],具有较高的集成度,但对于铌酸锂这种高折射率材料而言,DBR要求具备极高的加工精度,在芯片端面制备多层结构高反膜的镀膜工艺也十分复杂,因此基于FP腔的REI-TFLN激光器难以直接借鉴DFB半导体激光器的构型。利用PLACE技术在铌酸锂薄膜上构建高性能光耦合器与低损耗波导的优势,可以制备出由两个Sagnac环反射器(SLR)对接构成的FP腔单模激光器。每个Sagnac环反射器由一个2×2 3 dB光耦合器组成,其两个输出端口连接形成一个环波导。SLR中的耦合器在两个输出之间施加
图 11. 基于Sagnac环形反射器的Er3+∶TFLN片上单模激光器[90]。(a)Er3+∶TFLN FP 谐振腔显微图像(右下角的箭头表示铌酸锂晶轴 X、Y 和 Z),以及利用两个980 nm半导体激光器泵浦的Er3+∶TFLN FP 激光器的绿色上转换荧光;(b)激光器局部的光学显微镜图像(上图为3 dB定向耦合器,左下图为耦合区域,右下图为Sagnac环形反射器);(c)1544 nm附近放大激光光谱,红色实线为激光谱峰的洛伦兹拟合曲线(插图为近红外相机拍摄的Er3+∶TFLN FP腔激光器输出端口光斑的伪彩色成像);(d)微环激光器的片上激光功率与泵浦光功率的函数关系
Fig. 11. Er3+∶TFLN single-mode laser based on Sagnac loop reflectors[90]. (a) Optical microscope image of an Er3+∶TFLN FP resonator (the arrows in bottom right corner illustrate the LN crystallographic axes X, Y, and Z), and the green upconversion fluorescence of the Er3+∶TFLN FP resonator pumped by the 980 nm LD; (b) zoomed-in optical microscope image of a 3 dB directional coupler (the upper inset), the coupling region (the bottom-left inset), and a Sagnac loop reflector (the bottom-right inset); (c) enlarged spectrum around wavelength 1544 nm, the lasing peak is fitted with a Lorentzian line shape (red) (inset is the infrared image of the output port of the Er3+∶TFLN FP resonator); (d) on-chip laser power of Er3+∶TFLN FP resonator laser drifts with absorbed pump power
3.7 片上自注入锁模波长可调窄线宽激光器
随着光芯片加工工艺的逐渐成熟,基于磷化铟和硅异质集成的商业化电泵浦DFB激光芯片已广泛应用于光纤通信网络与数据中心的构建。但由于工艺不完美等,激光芯片自身的腔精细度往往不高,此类激光器的激光频率不可避免地存在涨落,仅靠激光芯片自身难以实现集成窄线宽激光器的构建[92]。自注入锁定是一种利用外部光腔的谐振光反馈使电泵浦激光二极管产生窄线宽激光的有效方案,该方案简单、紧凑且成本低廉,目前已经在硅基和碳化硅平台上实现了工作在中红外波段和可见光波段的片上自注入锁定窄线宽激光器[93-94],而受材料透射光谱限制[95],980 nm波段的窄线宽片上激光器则鲜有报道。
利用高品质因子的铌酸锂(LN)微环谐振腔可以在较宽的波段范围内实现电泵浦DFB激光二极管激光模式的自注入锁定。
图 12. 片上自注入锁模波长可调窄线宽激光器[96]。(a)由商用CoS激光器二极管和高Q LN微环激光器组成的窄线宽自注入锁定激光器示意图;(b)由相机拍摄的自注入锁定激光器的照片;(c)激光在975.36 nm处LN微环激光的透射光谱,微腔Q值在975.36 nm处为6.91×105;(d)自由运行的DFB激光和经过LN微环腔自注入锁定的DFB激光线宽比较;(e)980 nm窄线宽自注入锁定激光发射的光谱;(f)窄线宽激光波长对外加电功率的依赖性
Fig. 12. On-chip wavelength-tunable narrow-linewidth laser diode based on self-injection locking[96]. (a) Illustration scheme of the narrow linewidth self-injection-locked laser, which is composed of a commercial CoS laser diode and a high-Q LN microring laser; (b) close-up optical micrograph of the interface between the CoS laser diode and LN microring; (c) Lorentz fitting (red curve) reveals a Q-factor of 6.91×105 at the wavelength of 975.36 nm; (d) comparison of the laser linewidth for the free-running DFB case and the case where the DFB is self-injection-locked to a LN microring cavity; (e) spectrum of the 980 nm narrow linewidth self-injected locking laser emission; (f) dependence of the narrow linewidth laser wavelength on the applied electrical power
3.8 掺镱铌酸锂薄膜微环腔激光器
三价镱离子(Yb3+)以其宽广的增益光谱宽度、低量子缺陷和无自我淬灭的特性,成为晶体或玻璃基底中完美的活性离子,为激光器和放大器提供了不可或缺的增益,具有优异的光学和机械性能[97-99]。对于工作在1 μm左右波长的激光振荡器或放大器来说,Yb3+是一个很好的选择[51,57,65-66]。通过PLACE技术在掺镱铌酸锂薄膜(Yb3+∶TFLN)上制备的微环激光器,可在1025 nm附近实现多模激射[100]。加工的微环半径为200 μm,周长大概是1 mm,耦合区长度约为80 μm(弱耦合)。
图 13. 在980 nm波长的泵浦下,Yb3+∶TFLN微环的激光特性[100]。(a)在不同的泵浦功率下激光模式的演变;(b)Yb3+∶TFLN微环的激光功率与泵浦功率关系(通过线性拟合可得,阈值和斜率效率分别为10 mW和1.77×10-3%);(c)在1025.62 nm处的激光光谱,其线宽为0.035 nm;(d)在泵浦功率为20 mW的情况下,有14个纵模的激光发射
Fig. 13. Lasing characterization of the Yb3+∶TFLN microring when pumped by a 980 nm wavelength laser[100]. (a) Evolution of the lasing modes at different pump powers; (b) Yb3+∶TFLN microring lasing power versus the increasing pump powers (threshold and the slope efficiency deduced from the linear fitting are 10 mW and 1.77×10-3%, respectively); (c) spectrum around the laser emission at 1025.62 nm and featuring a linewidth of 0.035 nm; (d) lasing spectrum features 14 longitudinal modes when the pump power is 20 mW
表 1. 利用超快激光光刻技术制备的片上激光器关键参数
Table 1. Typical parameters for the on-chip microlasers fabricated by PLACE technique
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4 基于低损耗光波导的片上光放大器
4.1 片上可调低损耗光波导延时线
图 14. 米级长度铌酸锂波导OTDL[13]。(a)输出端连接两个不同长度的波导的片上分束器示意图(上输出端波导为光学实时延时线);(b)长度为109.26 cm的OTDL的实物照片;(c)在图(b)中OTDL弯曲部分的SEM图像;(d)器件中分束器部分的光学显微镜图像;(e)输出端口2处模式的近场分布(插图为脊形波导的横截面SEM图像);(f)随着OTDL长度的增加,传输损耗的变化(插图为输出端口1和2处TE模式的近场分布)
Fig. 14. Meter-scale length LN waveguide OTDL[13]. (a) Schematic design of a beam splitter connected with two waveguides of different lengths (upper waveguide serves as the OTDL); (b) digital camera picture of an OTDL with a total length of 109.26 cm; (c) SEM image of the bend section of the OTDL in Fig. (b); (d) optical micrograph of the fabricated beam splitter in the OTDL device; (e) near-field distributions of the modes at the output port 2 (inset is cross-sectional SEM image of the ridge waveguide); (f) propagation loss as a function of the length of the OTDL (inset is near-field distributions of the TE modes at the output ports 1 and 2)
此外,通过在螺旋波导延时线附近直接集成微电极阵列,还可以实现延时的连续可调[101]。
图 15. 220 fs范围内延时连续电光可调的铌酸锂波导ODL[101]。(a)包含一条可调ODL臂的TFLN不等臂干涉仪显微镜俯视图;(b)ODL传输损耗与波导长度的关系,其中损耗数据来自三个两臂的长度差分别为10 cm、20 cm和30 cm的不等臂干涉仪的透射谱,损耗与波导长度呈线性关系;(c)图(a)中不等臂干涉仪的波长(空心方块数据,蓝色线性拟合)与延时(空心圆形数据,红色线性拟合)随直流电压的线性变化图(测得波长调节效率约为2.42 pm/V,对应的延时调节效率为3.18 fs/V)
Fig. 15. Electro-optically tunable ODL with a continuous tuning range of 220 fs in TFLN[101]. (a) Micrograph of a TFLN unbalanced MZI with a tunable ODL arm; (b) measured losses derived from the measured transmission spectra of unbalanced MZIs without microelectrodes in the arm-length differences of 10 cm, 20 cm, and 30 cm, showing a linear dependence on the length; (c) linear wavelength shift (marked by black squares with blue linear fitting curve) and time delay (marked by black circles with red linear fitting curve) of the device in Fig. (a) as a function of the DC voltage (measured tuning efficiency is 2.42 pm/V, indicating a time tuning efficiency of 3.18 fs/V)
4.2 基于掺铒铌酸锂薄膜的光波导放大器
4.1节中的低损耗波导制备技术可直接应用于片上铌酸锂光波导放大器的构建,如
图 16. 单片集成掺铒铌酸锂波导放大器[48]。(a)芯片示意图;(b)片上光放大器直波导光学显微镜图像;(c)片上光放大器弯曲波导部分光学显微镜图像;(d)增益测量实验装置示意图
Fig. 16. The monolithic Er3+-doped waveguide amplifier[48]. (a) Schematic of the device; (b) optical micrograph of the straight waveguide of the on-chip amplifier; (c) optical micrograph of the curved waveguide of the on-chip amplifier; (d) schematic of the experimental setup of the gain measurement
4.3 掺铒铌酸锂薄膜包层光波导放大器
4.2节中演示的基于TFLN的EDWA使用了没有包层的裸露TFLN波导,这可能会导致放大器在长期运行时因外部扰动而发生性能波动和不稳定的情况。因此,对器件最直接的保护措施是在TFLN波导的顶部沉积一层包层。此外,波导中的光学模式可以通过包层进行控制,便于进一步优化提升放大器性能。同样在600 nm厚的Z切掺Er3+铌酸锂薄膜上可以制备如
图 17. 带介质包层的Er3+∶TFLN波导特征[53]。(a)顶部沉积Ta2O5包层的Z切 Er3+∶TFLN铌酸锂波导横截面示意图;(b)空气包层LNOI波导的俯视显微镜图;(c)Ta2O5包层LNOI波导横截面的SEM图;(d)空气包层和(e)Ta2O5包层波导在泵浦和信号波长下的TE基模模式分布仿真计算图(在每个图表中都标注了功率限制系数Г)
Fig. 17. Cladded Er3+∶TFLN waveguide configuration[53]. (a) Cross-sectional schematic of the waveguide fabricated on the Z-cut Er3+∶TFLN wafer with a cladding layer of Ta2O5; (b) top-view microscope image of the air-clad TFLN waveguide; (c) SEM image of the Ta2O5-clad TFLN waveguide cross-section; simulated mode distribution of the fundamental TE modes for the (d) air-clad and (e) Ta2O5-clad waveguides at the pump and signal wavelengths, respectively (power confinement factor Г is labelled in each panel)
为了表征包层TFLN波导放大器的增益性能,采用了如
图 18. 掺铒铌酸锂波导放大器光学增益测量[53]。(a)实验装置示意图(PC为偏振控制器,OSA为光谱分析仪,中间为受激励Er3+∶TFLN波导芯片的实物照片);(b)空气包层(蓝色方块)和Ta2O5包层(红色圆圈)放大器内部净增益随入射泵浦光强变化图
Fig. 18. Optical gain measurement of the Er3+∶TFLN waveguide amplifiers[53]. (a) Experimental setup for optical gain measurement (PC is polarization controller, OSA is optical spectrum analyzer, the digital camera photograph of the excited Er3+∶TFLN waveguide chip is shown in the center); (b) internal net gain measured from the air-clad (blue squares) and Ta2O5-clad (red circles) amplifiers varies with the incident pump light intensity
两种波导测量的小信号内部净增益如
5 单片集成有源/无源铌酸锂光子器件
PLACE技术仅仅通过单次连续的掩模图案光刻,即可实现无源和有源TFLN光子元件的直接集成[69],从而绕过繁琐的对准和接合工艺。
图 19. 利用拼合芯片实现有源/无源铌酸锂光子器件稳定低损耗光学互连的制备工艺流程图[69]。(a)制备无掺杂和稀土离子掺杂TFLN晶片样品;(b)将无掺杂和稀土离子掺杂TFLN晶片样品倒装到抛光后的平板玻璃上;(c)利用特质夹具将无掺杂和稀土离子掺杂TFLN晶片无缝拼合,并将紫外固化胶滴在两片晶片底部,实现晶片键合;(d)利用激光焊接,将两片拼合的TFLN晶片与一块石英晶片焊接键合;(e)完成无源/有源TFLN芯片的拼接;(f)利用PLACE技术在拼接晶片上制备光子结构
Fig. 19. Schematic of the fabrication process for robust low-loss optical interconnection of passive and active LN photonics using stitch-chips[69]. (a) Prepare non-doped and REI-doped TFLN samples; (b) non-doped and REI-doped TFLN samples are flip-chip on a polished plate glass; (c) non-doped and REI-doped TFLN samples are stitched seamlessly using customized fixtures and the UV glue is applied on the bottom of the stitched chips to bond the two samples; (d) subsequent laser welding is operated in the boundary of the two TFLN samples and quartz substrate to achieve durable bonding; (e) completed stitch-chip of passive and active TFLN; (f) photonic structures fabricated using the PLACE technique on the stitch-chips
图 20. 有源无源拼接集成的四通道阵列光波导放大器[69]。(a)器件设计图;(b)实物图;(c)1550 nm波长信号光在四通道掺铒光波导内的模式分布(插图)与强度分布图;(d)四通道阵列光波导放大器受976 nm波长泵浦时照片;器件中各个放大器通道在信号光波长分别为(e)1550 nm和(f)1530 nm时的增益特性曲线
Fig. 20. Four-channel waveguide amplifiers fabricated on the monolithically integrated active/passive TFLN chip[69]. (a) Illustration of the device design; (b) digital picture of the four-channel waveguide amplifiers; (c) mode (insets) and intensity distribution of the 1550 nm wavelength signal in the four-channel Er3+-doped waveguides; (d) photo of the four-channel waveguide amplifier array when pumped by a 976 nm diode laser; gain characterization of the four Er3+-doped LN waveguides array for the signal wavelengths of (e) 1550 nm and (f) 1530 nm
6 结束语
铌酸锂薄膜集成光子学在近十年中取得了飞速发展,得益于高品质铌酸锂单晶薄膜的商业化和微纳加工技术的日趋成熟,大量性能优异的铌酸锂集成光子器件不断涌现。通过在铌酸锂薄膜中掺入稀土激活离子,各种有源铌酸锂光子器件在近年来被成功演示。本文介绍了利用超快激光PLACE技术在掺杂有源发光稀土离子的铌酸锂薄膜衬底上实现片上激光与光放大的最新进展,包括超高Q值微腔、超低损耗米级长度波导、片上电光可调单模极窄线宽激光器、高增益波导光放大器、单片集成有源/无源光子回路等等高性能的光子器件,所制备的有源铌酸锂光子器件在光通信、传感、计算与处理等多个领域都有丰富的应用前景与需求。未来基于PLACE技术,结合器件设计与仿真、基底掺杂品质优化、多体系异质集成,有望在大幅提升所制备器件性能的同时有效控制器件成本,为有源铌酸锂光子器件走向大规模制造与广泛应用奠定坚实的基础。
[1] ColdrenL A, CorzineS W, MašanovićM L. Diode lasers and photonic integrated circuits: coldren/diode lasers 2E[M]. Hoboken: John Wiley & Sons, Inc., 2012.
[2] Weis R S, Gaylord T K. Lithium niobate: summary of physical properties and crystal structure[J]. Applied Physics A, 1985, 37(4): 191-203.
[3] Boes A, Corcoran B, Chang L, et al. Status and potential of lithium niobate on insulator (LNOI) for photonic integrated circuits[J]. Laser & Photonics Reviews, 2018, 12(4): 1700256.
[6] Zhu D, Shao L B, Yu M J, et al. Integrated photonics on thin-film lithium niobate[J]. Advances in Optics and Photonics, 2021, 13(2): 242-352.
[7] Jia Y C, Wang L, Chen F. Ion-cut lithium niobate on insulator technology: recent advances and perspectives[J]. Applied Physics Reviews, 2021, 8(1): 011307.
[8] Kong Y F, Bo F, Wang W W, et al. Recent progress in lithium niobate: optical damage, defect simulation, and on-chip devices[J]. Advanced Materials, 2020, 32(3): 1806452.
[9] Lin J, Xu Y, Fang Z, et al. Fabrication of high-Q lithium niobate microresonators using femtosecond laser micromachining[J]. Scientific Reports, 2015: 5 8072.
[10] Wang C, Burek M J, Lin Z, et al. Integrated high quality factor lithium niobate microdisk resonators[J]. Optics Express, 2014, 22(25): 30924-30933.
[11] Wang J, Bo F, Wan S, et al. High-Q lithium niobate microdisk resonators on a chip for efficient electro-optic modulation[J]. Optics Express, 2015, 23(18): 23072-23078.
[12] Wu R B, Zhang J H, Yao N, et al. Lithium niobate micro-disk resonators of quality factors above 107[J]. Optics Letters, 2018, 43(17): 4116-4119.
[13] Zhou J X, Gao R H, Lin J T, et al. Electro-optically switchable optical true delay lines of meter-scale lengths fabricated on lithium niobate on insulator using photolithography assisted chemo-mechanical etching[J]. Chinese Physics Letters, 2020, 37(8): 084201.
[14] Xu M Y, He M B, Zhang H G, et al. High-performance coherent optical modulators based on thin-film lithium niobate platform[J]. Nature Communications, 2020, 11: 3911.
[15] Xu M Y, Zhu Y T, Pittalà F, et al. Dual-polarization thin-film lithium niobate in-phase quadrature modulators for terabit-per-second transmission[J]. Optica, 2022, 9(1): 61-62.
[16] Kharel P, Reimer C, Luke K, et al. Breaking voltage–bandwidth limits in integrated lithium niobate modulators using micro-structured electrodes[J]. Optica, 2021, 8(3): 357-363.
[17] Wang C, Zhang M, Chen X, et al. Integrated lithium niobate electro-optic modulators operating at CMOS-compatible voltages[J]. Nature, 2018, 562(7725): 101-104.
[18] Rao A, Patil A, Rabiei P, et al. High-performance and linear thin-film lithium niobate Mach–Zehnder modulators on silicon up to 50 GHz[J]. Optics Letters, 2016, 41(24): 5700-5703.
[19] 刘海锋, 郭宏杰, 谭满清, 等. 铌酸锂薄膜调制器的研究进展[J]. 中国光学, 2022, 15(1): 1-13.
[21] Zheng Y L, Chen X F. Nonlinear wave mixing in lithium niobate thin film[J]. Advances in Physics: X, 2021, 6(1): 1889402.
[22] Xie R R, Li G Q, Chen F, et al. Microresonators in lithium niobate thin films[J]. Advanced Optical Materials, 2021, 9(19): 2100539.
[23] 刘时杰, 郑远林, 陈险峰. 铌酸锂薄膜上的非线性频率变换[J]. 光学学报, 2021, 41(8): 0823013.
[24] Wu R B, Lin J T, Wang M, et al. Fabrication of a multifunctional photonic integrated chip on lithium niobate on insulator using femtosecond laser-assisted chemomechanical polish[J]. Optics Letters, 2019, 44(19): 4698-4701.
[25] Hu Y W, Yu M J, Zhu D, et al. On-chip electro-optic frequency shifters and beam splitters[J]. Nature, 2021, 599(7886): 587-593.
[26] Xu Q, Liu J M, Zhang D L, et al. Ultra-compact lithium niobate power splitters designed by an intelligent algorithm[J]. Optics & Laser Technology, 2023, 160: 109057.
[27] Zhang L, Zhang L, Fu X, et al. Polarization-independent symmetrical directional coupler utilizing orientation-engineered method on the x-cut lithium-niobate-on-insulator[J]. Optics Communications, 2021, 479: 126365.
[28] Chen G Y, Da Ng J, Lin H L, et al. Design and fabrication of high-performance multimode interferometer in lithium niobate thin film[J]. Optics Express, 2021, 29(10): 15689-15698.
[29] KeW, LinZ J, CaiX L. A compact Y-branch power splitter with an arbitrary power splitting ratio based on thin-film lithium niobate platform[C]//Asia Communications and Photonics Conference, October 24-27, 2021, Shanghai. Washington, D.C.: Optica Publishing Group, 2021: W4C.6.
[30] Shen Y, Ruan Z L, Chen K X, et al. Broadband polarization splitter-rotator on a thin-film lithium niobate with conversion-enhanced adiabatic tapers[J]. Optics Express, 2023, 31(2): 1354-1366.
[31] Wu Y N, Sun X R, Li H, et al. Lithium niobate thin film polarization beam splitter based on asymmetric directional coupling[J]. Journal of Lightwave Technology, 2022, 40(24): 7843-7847.
[32] Aghaeimeibodi S, Desiatov B, Kim J H, et al. Integration of quantum dots with lithium niobate photonics[J]. Applied Physics Letters, 2018, 113(22): 221102.
[33] Xia K W, Sardi F, Sauerzapf C, et al. Tunable microcavities coupled to rare-earth quantum emitters[J]. Optica, 2022, 9(4): 445-450.
[34] 成然, 黄帅, 徐强, 等. 铌酸锂量子器件研究进展[J]. 激光技术, 2022, 46(6): 722-728.
[35] Qi Y F, Li Y. Integrated lithium niobate photonics[J]. Nanophotonics, 2020, 9(6): 1287-1320.
[36] 季晓伟, 崔建民, 冯立辉, 等. 基于LNOI的环形谐振腔压力传感器[J]. 激光与光电子学进展, 2022, 59(3): 0323001.
[37] 乔玲玲, 汪旻, 伍荣波, 等. 超低损耗铌酸锂光子学[J]. 光学学报, 2021, 41(8): 0823012.
[39] Shams-Ansari A, Renaud D, Cheng R, et al. Electrically pumped laser transmitter integrated on thin-film lithium niobate[J]. Optica, 2022, 9(4): 408-411.
[40] Zhang X, Liu X Y, Ma R, et al. Heterogeneously integrated III-V-on-lithium niobate broadband light sources and photodetectors[J]. Optics Letters, 2022, 47(17): 4564-4567.
[42] Snigirev V, Riedhauser A, Lihachev G, et al. Ultrafast tunable lasers using lithium niobate integrated photonics[J]. Nature, 2023, 615(7952): 411-417.
[43] Op de Beeck C, Mayor F M, Cuyvers S, et al. III/V-on-lithium niobate amplifiers and lasers[J]. Optica, 2021, 8(10): 1288-1289.
[44] Li M X, Chang L, Wu L, et al. Integrated pockels laser[J]. Nature Communications, 2022, 13: 5344.
[45] Han Y, Zhang X, Huang F J, et al. Electrically pumped widely tunable O-band hybrid lithium niobite/III-V laser[J]. Optics Letters, 2021, 46(21): 5413-5416.
[46] Han Y, Zhang X, Ma R, et al. Widely tunable O-band lithium niobite/III-V transmitter[J]. Optics Express, 2022, 30(20): 35478-35485.
[47] Wang Z, Fang Z W, Liu Z X, et al. An on-chip tunable micro-disk laser fabricated on Er3+ doped lithium niobate on insulator[J]. Optics Letters, 2021, 46(2): 380-383.
[48] Zhou J X, Liang Y T, Liu Z X, et al. On-chip integrated waveguide amplifiers on erbium-doped thin-film lithium niobate on insulator[J]. Laser & Photonics Reviews, 2021, 15(8): 2100030.
[50] Liang Y T, Zhou J X, Liu Z X, et al. A high-gain cladded waveguide amplifier on erbium doped thin-film lithium niobate fabricated using photolithography assisted chemo-mechanical etching[J]. Nanophotonics, 2022, 11(5): 1033-1040.
[51] Zhou Y, Wang Z, Fang Z, et al. On-chip microdisk laser on Yb3+-doped thin-film lithium niobate[J]. Optics Letters, 2021, 46(22): 5651-5654.
[52] Zhou J X, University E C N, Huang T, et al. Laser diode-pumped compact hybrid lithium niobate microring laser[J]. Optics Letters, 2022, 47(21): 5599-5601.
[53] Liang Y T, Zhou J X, Wu R B, et al. Monolithic single-frequency microring laser on an erbium-doped thin film lithium niobate fabricated by a photolithography assisted chemo-mechanical etching[J]. Optics Continuum, 2022, 1(5): 1193-1201.
[54] Gao R H, Guan J L, Yao N, et al. On-chip ultra-narrow-linewidth single-mode microlaser on lithium niobate on insulator[J]. Optics Letters, 2021, 46(13): 3131-3134.
[55] Gao R H, Fu B T, Yao N, et al. Electro-optically tunable low phase-noise microwave synthesizer in an active lithium niobate microdisk[J]. Laser & Photonics Reviews, 2023, 17(5): 2200903.
[57] Zhang Z H, Fang Z W, Zhou J X, et al. On-chip integrated Yb3+-doped waveguide amplifiers on thin film lithium niobate[J]. Micromachines, 2022, 13(6): 865.
[58] Chen Z X, Xu Q, Zhang K, et al. Efficient erbium-doped thin-film lithium niobate waveguide amplifiers[J]. Optics Letters, 2021, 46(5): 1161-1164.
[59] Liu Y A, Yan X S, Wu J W, et al. On-chip erbium-doped lithium niobate microcavity laser[J]. Science China Physics, Mechanics & Astronomy, 2021, 64(3): 234262.
[60] Li T Y, Wu K, Cai M L, et al. A single-frequency single-resonator laser on erbium-doped lithium niobate on insulator[J]. APL Photonics, 2021, 6(10): 101301.
[61] Xiao Z Y, Wu K, Cai M L, et al. Single-frequency integrated laser on erbium-doped lithium niobate on insulator[J]. Optics Letters, 2021, 46(17): 4128-4131.
[62] Cai M L, Wu K, Xiang J M, et al. Erbium-doped lithium niobate thin film waveguide amplifier with 16 dB internal net gain[J]. IEEE Journal of Selected Topics in Quantum Electronics, 2022, 28(3): 8200608.
[63] Luo Q, Hao Z Z, Yang C, et al. Microdisk lasers on an erbium-doped lithium-niobite chip[J]. Science China Physics, Mechanics & Astronomy, 2021, 64(3): 234263.
[64] Zhang R, Yang C, Hao Z Z, et al. Integrated lithium niobate single-mode lasers by the Vernier effect[J]. Science China Physics, Mechanics & Astronomy, 2021, 64(9): 294216.
[65] Luo Q A, Yang C, Hao Z Z, et al. Integrated ytterbium-doped lithium niobate microring lasers[J]. Optics Letters, 2022, 47(6): 1427-1430.
[66] Luo Q A, Yang C, Hao Z Z, et al. On-chip ytterbium-doped lithium niobate microdisk lasers with high conversion efficiency[J]. Optics Letters, 2022, 47(4): 854-857.
[67] Xu Q, Chen F, Chen Z X, et al. Er3+-doped lithium niobate thin film: a material platform for ultracompact, highly efficient active microphotonic devices[J]. Advanced Photonics Research, 2021, 2(12): 2100081.
[68] Minet Y, Herr S J, Breunig I, et al. Electro-optically tunable single-frequency lasing from neodymium-doped lithium niobate microresonators[J]. Optics Express, 2022, 30(16): 28335-28344.
[69] Zhou Y, Zhu Y R, Fang Z W, et al. Monolithically integrated active passive waveguide array fabricated on thin film lithium niobate using a single continuous photolithography process[J]. Laser & Photonics Reviews, 2023, 17(4): 2200686.
[70] Wu R B, Wang M, Xu J, et al. Long low-loss-litium niobate on insulator waveguides with sub-nanometer surface roughness[J]. Nanomaterials, 2018, 8(11): 910.
[72] Gao R H, Zhang H S, Bo F, et al. Broadband highly efficient nonlinear optical processes in on-chip integrated lithium niobate microdisk resonators of Q-factor above 108[J]. New Journal of Physics, 2021, 23(12): 123027.
[73] Wang M, Wu R B, Lin J T, et al. Chemo-mechanical polish lithography: a pathway to low loss large-scale photonic integration on lithium niobate on insulator[J]. Quantum Engineering, 2019, 1(1): e9.
[74] Zhang J H, Fang Z W, Lin J T, et al. Fabrication of crystalline microresonators of high quality factors with a controllable wedge angle on lithium niobate on insulator[J]. Nanomaterials, 2019, 9(9): 1218.
[75] Wu R B, Gao L, Liang Y T, et al. High-production-rate fabrication of low-loss lithium niobate electro-optic modulators using photolithography assisted chemo-mechanical etching (PLACE)[J]. Micromachines, 2022, 13(3): 378.
[76] Chen J M, Liu Z X, Song L B, et al. Ultra-high-speed high-resolution laser lithography for lithium niobate integrated photonics[J]. Proceedings of SPIE, 2023, 12411: 1241109.
[77] Fang Z W, Haque S, Farajollahi S, et al. Polygon coherent modes in a weakly perturbed whispering gallery microresonator for efficient second harmonic, optomechanical, and frequency comb generations[J]. Physical Review Letters, 2020, 125(17): 173901.
[79] Schawlow A L, Townes C H. Infrared and optical masers[J]. Physical Review, 1958, 112(6): 1940-1949.
[80] Henry C. Theory of the linewidth of semiconductor lasers[J]. IEEE Journal of Quantum Electronics, 1982, 18(2): 259-264.
[81] Goldberg P, Milonni P W, Sundaram B. Theory of the fundamental laser linewidth[J]. Physical Review A, 1991, 44(3): 1969-1985.
[83] Abdelsalam K, Ordouie E, Vazimali M G, et al. Tunable dual-channel ultra-narrowband Bragg grating filter on thin-film lithium niobate[J]. Optics Letters, 2021, 46(11): 2730-2733.
[85] Baghban M A, Schollhammer J, Errando-Herranz C, et al. Bragg gratings in thin-film LiNbO3 waveguides[J]. Optics Express, 2017, 25(26): 32323-32332.
[86] Escalé M R, Pohl D, Sergeyev A, et al. Extreme electro-optic tuning of Bragg mirrors integrated in lithium niobate nanowaveguides[J]. Optics Letters, 2018, 43(7): 1515-1518.
[87] Ulbrich N, Scarpa G, Sigl A, et al. High-temperature (T≥470 K) pulsed operation of 5.5 µm quantum cascade lasers with high-reflection coating[J]. Electronics Letters, 2001, 37(22): 1341-1342.
[88] Qin C Y, Jia K P, Li Q Y, et al. Electrically controllable laser frequency combs in graphene-fibre microresonators[J]. Light: Science & Applications, 2020, 9: 185.
[89] Jia K P, Wang X H, Kwon D, et al. Photonic flywheel in a monolithic fiber resonator[J]. Physical Review Letters, 2020, 125(14): 143902.
[90] Yu S P, Fang Z W, Laboratory H N, et al. On-chip single-mode thin-film lithium niobate Fabry–Perot resonator laser based on Sagnac loop reflectors[J]. Optics Letters, 2023, 48(10): 2660-2663.
[91] Liu X M, Yan X S, Liu Y A, et al. Tunable single-mode laser on thin film lithium niobate[J]. Optics Letters, 2021, 46(21): 5505-5508.
[92] Kondratiev N M, Lobanov V E, Shitikov A E, et al. Recent advances in laser self-injection locking to high-Q microresonators[J]. Frontiers of Physics, 2023, 18(2): 21305.
[93] Shim E, Gil-Molina A, Westreich O, et al. Tunable single-mode chip-scale mid-infrared laser[J]. Communications Physics, 2021, 4: 268.
[94] Corato-Zanarella M, Gil-Molina A, Ji X C, et al. Widely tunable and narrow-linewidth chip-scale lasers from near-ultraviolet to near-infrared wavelengths[J]. Nature Photonics, 2023, 17(2): 157-164.
[95] Ling J W, Staffa J, Wang H M, et al. Self-injection locked frequency conversion laser[J]. Laser & Photonics Reviews, 2023, 17(5): 2200663.
[96] Huang T, Ma Y, Fang Z W, et al. Wavelength-tunable narrow-linewidth laser diode based on self-injection locking with a high-Q lithium niobate microring resonator[J]. Nanomaterials, 2023, 13(5): 948.
[97] Hönninger C, Paschotta R, Graf M, et al. Ultrafast ytterbium-doped bulk lasers and laser amplifiers[J]. Applied Physics B, 1999, 69(1): 3-17.
[98] Krupke W F. Ytterbium solid-state lasers. The first decade[J]. IEEE Journal of Selected Topics in Quantum Electronics, 2000, 6(6): 1287-1296.
[99] Paschotta R, Nilsson J, Tropper A C, et al. Ytterbium-doped fiber amplifiers[J]. IEEE Journal of Quantum Electronics, 1997, 33(7): 1049-1056.
[100] Ma Y, Zhou J X, Fang Z W, et al. Monolithic Yb3+-doped thin film lithium niobate microring laser fabricated by photolithography-assisted chemo-mechanical etching technology[J]. Journal of the Optical Society of America B, 2023, 40(5): D1-D4.
[101] Song L B, Chen J M, Wu R, et al. Electro-optically tunable optical delay line with a continuous tuning range of ∼220 fs in thin-film lithium niobate[J]. Optics Letters, 2023, 48(9): 2261-2264.
Article Outline
汪旻, 乔玲玲, 方致伟, 林锦添, 伍荣波, 陈锦明, 刘招祥, 张海粟, 程亚. 基于超快激光光刻的有源铌酸锂光子集成[J]. 光学学报, 2023, 43(16): 1623014. Min Wang, Lingling Qiao, Zhiwei Fang, Jintian Lin, Rongbo Wu, Jinming Chen, Zhaoxiang Liu, Haisu Zhang, Ya Cheng. Active Lithium Niobate Photonic Integration Based on Ultrafast Laser Lithography[J]. Acta Optica Sinica, 2023, 43(16): 1623014.