-
铌酸锂薄膜波导采用典型的LNOI脊型结构,如图1所示,由硅基底、二氧化硅层(SiO2)和铌酸锂(LN)薄膜脊波导构成,通常LN薄膜典型厚度为600 µm,脊高度0.25 µm,其满足单模条件的模式面积比较小,约1 µm。典型单模保偏光纤的模场直径一般大于6 µm,导致光纤与波导之间存在显著的模式失配,导致较大的耦合损耗。
为了实现铌酸锂薄膜波导与光纤的高效光耦合,采用在铌酸锂薄膜波导端面集成模斑转换器实现模式和能量的传递与转换。文中采用模斑转换器由SiO2、SiON锥形结构以及双层LN锥形结构组成,其结构如图2所示,其中双层LN锥形结构、SiON锥形结构用于光耦合与绝热模式转换,其中SiO2波导(Ⅰ)将用于定义与光纤耦合端面上的光模式,模式变换区包括SiON锥形、双层铌酸锂锥形结构,分为三个部分:SiON锥形线性变换区域(Ⅱ)、LN平板锥形区(Ⅲ)、以及LN脊型和平板构成的锥形区(Ⅳ),最后为LN脊波导区(Ⅴ)。
-
三维有限光束差分传输算法可用于计算波导的本征模式及光束在波导中的传输情况[18-24]。在光纤和波导中,电磁波中的电场(磁场)满足Helmholtz方程:
$$ \frac{{{\partial ^2}\phi }}{{{\partial ^2}x}} + \frac{{{\partial ^2}\phi }}{{{\partial ^2}y}} + \frac{{{\partial ^2}\phi }}{{{\partial ^2}z}} + k{(x,y,z)^2}\phi = 0 $$ (1) 考虑到光轴主要沿Z轴传输,作近轴和慢变化近似,则
$$ \phi (x,y,z) = u(x,y,z){{\rm{e}}^{i\bar kz}} $$ (2) 式中:$ {{u}}(x,y,z) $为光波导中的传输场;$ \bar k = {k_0}\bar n $,表示$ \phi (x,y,z) $的平均相位变化,为参考波数;$ \bar n $为参考折射率,将公式(2)代入公式(1),得到:
$$ \frac{{{\partial ^2}u}}{{\partial {z^2}}} + 2i\bar k\frac{{\partial u}}{{\partial z}} + \frac{{{\partial ^2}u}}{{\partial {x^2}}} + \frac{{{\partial ^2}u}}{{\partial {y^2}}} + ({k^2} - {\bar k^2})u = 0 $$ (3) 作慢变化近似,即忽略$ \dfrac{{{\partial ^2}u}}{{\partial {z^2}}} $,则
$$ \frac{{\partial u}}{{\partial z}} = \frac{i}{{2\bar k}}\Bigg[\Bigg(\frac{{{\partial ^2}u}}{{\partial {x^2}}} + \frac{{{\partial ^2}u}}{{\partial {y^2}}} + ({k^2} - {\bar k^2})\Bigg]u $$ (4) 对公式(4)实行差分,得到:
$$ \frac{{u_i^{n + 1} - u_i^n}}{{\Delta z}} = \frac{i}{{2\bar k}}\left[ {\frac{{{\delta ^2}}}{{\Delta {x^2}}} + {k^2}({x_i},{z_{n + 1/2}}) - {{\bar k}^2}} \right]\frac{{u_i^{n + 1} + u_i^n}}{2} $$ (5) 式中:$ u_{\text{i}}^{\text{n}} $为第n个纵向平面上第i个格点处的电磁场分布;$ \Delta z $和$ \Delta {{x}} $为纵向平面之间的间隔和横向格点之间的间隔;$ {\delta ^2} $为标准二阶差分因子,$ {\delta ^2}{u_i} = ({u_{i + 1}} + {u_{i - 1}} - 2{u_i}) $,$ {z_{n + 1/2}} = {z_n} + \Delta z/2 $,根据公式(5)可求出波导传输方向任意位置处的电磁场分布。
对于波导与光纤之间的模式耦合效率可表示为:
$$ \eta = {\eta _{\text{1}}}{\eta _{\text{2}}}{\eta _{\text{3}}} $$ (6) 式中:耦合效率$ {\eta }_{1} $为耦合端面上氧化硅波导与单模光纤的耦合效率,由氧化硅波导与光纤中的电场重叠积分以及模式有效折射率所决定,公式表示为:
$$ {\eta _{\text{1}}} = \frac{{{{\left| {\displaystyle\iint {U_{fb}^ * \cdot {U_{WG}}{\rm{d}}x{\rm{d}}y}} \right|}^2}}}{{\displaystyle\iint {{{\left| {{U_{fb}}} \right|}^2}{\rm{d}}x{\rm{d}}y \cdot \displaystyle\iint {{{\left| {{U_{WG}}} \right|}^2}{\rm{d}}x{\rm{d}}y}}}} $$ (7) 耦合效率$ {\eta }_{2} $和$ {\eta }_{3} $分别表示从SiO2波导耦合端面传输到SiON波导模式转换效率和从SiON波导端面经双层LN锥形结构传输到LN脊型波导的模式转换效率。为了提高这俩部分耦合效率需要采用绝热模式传输,要求波导壁的局部扩展必须慢于最低阶模的衍射,即锥形波导的宽度变化尽可能缓慢,因此设计时需要在波导长度与模式转换效率之间进行权衡,在保持较高的模式转换效率的前提下尽可能缩短波导长度。
-
摘要: 铌酸锂薄膜光子集成技术在高速光电子领域不断凸显,被广泛用于各种片上功能实现,如电光调制、光频梳、滤波器、非线性光学频率转换器、非线性量子光源、激光器等。在铌酸锂薄膜光子集成技术发展过程中,目前面临的一个重要的技术瓶颈就是铌酸锂薄膜纳米波导与单模光纤的高效耦合。针对这一问题,设计了一种基于SiO2、SiON锥形结构以及双层铌酸锂锥形结构的模斑转换器,实现铌酸锂薄膜纳米波导与单模光纤之间模式和能量的高效传递与转换。采用三维有限差分光束传播法对器件结构进行了模拟仿真,并优化了结构参数,可实现与铌酸锂薄膜波导与单模光纤的高效耦合,耦合效率在82.2%~89.0%之间,同时,得到了±1.8 µm光纤耦合对准容差,可为下一步制备出高效耦合的铌酸锂薄膜光子器件提供参考。Abstract:
Objective The photonic integration technology based on lithium niobate thin films has become increasingly prominent in the field of high-speed optoelectronics, and is widely used for various on-chip functions, such as electro-optical modulation, optical frequency comb, filter, nonlinear optical frequency converter, nonlinear quantum light source, laser etc. In the development of lithium niobate film photonic integration technology, there is an important technical bottleneck which is the effective coupling of lithium niobate film nanowaveguides and single-mode fibers, which is also the key to hinder the practical application of lithium niobate thin film photonic devices. On-chip mode size converter is widely used in mode field transformation to realize waveguide mode field transformation. Although the existing researches have improved the coupling efficiency by using bilayer tapered waveguides or composite structures, they are all coupled with tapered fiber or thin diameter fiber, which still cannot achieve effective coupling with single-mode fiber. To solve this problem, a mode size converter based on SiO2 waveguide, SiON tapered waveguide and bilayer LN tapered waveguide is designed to achieve efficient mode and energy transfer and conversion between lithium niobate film nanowaveguide and single-mode fiber. Methods The structure of the mode size convertor composed of SiO2 waveguide, SiON tapered waveguide and bilayer LN tapered waveguide is simulated by using the three-dimensional finite difference beam propagation method, and the structural parameters of each section are sequentially optimized through optical pattern matching design and adiabatic mode transmission design, and the optical coupling efficiency and adiabatic mode conversion efficiency of each section are simulated. Results and discussions The research results show that when the refractive index difference between the core layer and cladding layer of the SiO2 waveguide is 0.75% and the size of SiO2 waveguide is 6 μm×6 μm, the coupling efficiency between SiO2 waveguide and single-mode fiber is about 93% (Fig.6). When the mode field size of the wide end of SiON tapered waveguide is 2.5 μm×2.5 μm-3.5 μm×3.5 μm, the refractive index of the corresponding core layer is 1.48-1.51, the length of the SiON tapered waveguide (L1) is greater than 250 μm and the width of the tapered tip W3 is 0.1-0.3 μm, the optical mode is gradually converted from the SiO2 waveguide to the SiON waveguide, and the conversion efficiency of the SiON tapered waveguide is 93%-97.2% (Fig.8). The bilayer LN tapered waveguide includes the LN tapered planar waveguide and the LN tapered ridge waveguide. In the LN tapered planar waveguide, when the tapering length (L2) changes in the range of 200-300 μm, the width of the tapered tip W4 changes within 0.1-0.15 μm, and the width of the wide end (W5) changes in the range of 0.8-1.4 μm, the optical mode profile in LN tapered planar waveguide increases with the increase of the inverse taper width of LN tapered planar waveguide, while that in SiON layer decreases, and the conversion efficiency of the LN tapered planar waveguide is 96%-98.5% (Fig.9). In the LN tapered ridge waveguide, when the length of LN ridge tapered waveguide (L3) varies from 40 to 100 μm, and the width of the tapered tip of LN ridge tapered waveguide W6 varies from 0.1 μm to 0.3 μm, the optical mode is gradually converted into LN ridge waveguide optical mode, and conversion efficiency of the LN tapered ridge waveguide exceeds 99% (Fig.10). Through the above design, effective coupling with lithium niobate film waveguide and single-mode fiber can be realized, and the coupling efficiency is 82.2%-89.0% (Fig.11). At the same time, ± 1.8 μm fiber coupling alignment tolerance is obtained (Fig.12). Conclusions The proposed mode size converter based on SiO2 waveguide, SiON tapered waveguide and bilayer LN tapered waveguide provides a new method for the coupling and integration of lithium niobate thin film photonic devices, which can provide a reference for the next step of preparing highly efficient coupling lithium niobate thin film photonic devices, and is beneficial to further realize the integrated application of lithium niobate devices. -
-
[1] 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. doi: 10.1038/s41586-018-0551-y [2] Morton P A, Khurgin J B, Morton M J. All-optical linearized Mach-Zehnder modulator [J]. Optics Express, 2001, 29(23): 37302-37313. [3] 孙时豪, 蔡鑫伦. 高性能硅和铌酸锂异质集成薄膜电光调制器(特邀)[J]. 红外激光与工程, 2021, 50(7): 20211047. doi: 10.3788/IRLA20211047 Sun Shihao, Cai Xinlun. High-performance thin-film electro-optical modulator based on heterogeneous silicon and lithium niobate platform (Invited) [J]. Infrared and Laser Engineering, 2021, 50(7): 20211047. (in Chinese) doi: 10.3788/IRLA20211047 [4] Gao R, Yao N, Guan J, et al. Lithium niobate microring with ultra-high Q factor above 108 [J]. Chinese Optics Letters, 2022, 20(1): 011902. doi: 10.3788/COL202220.011902 [5] Gao R, Zhang H, 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 J Phys, 2021, 23(12): 123027. doi: 10.1088/1367-2630/ac3d52 [6] Zheng Y, Chen X. Nonlinear wave mixing in lithium niobate thin film [J]. Advances in Physics: X, 2021, 6(1): 1889402. doi: 10.1080/23746149.2021.1889402 [7] Lin J, Yao N, Hao Z, et al. Broadband quasi-phase-matched harmonic generation in an on-chip monocrystalline lithium niobate microdisk resonator [J]. Phys Rev Lett, 2019, 122(17): 173903. doi: 10.1103/PhysRevLett.122.173903 [8] Xu B Y, Chen L K, Lin J T, et al. Spectrally multiplexed and bright entangled photon pairs in a lithium niobate microresonator [J]. Sci China-Phys Mech Astron, 2022, 65(9): 294262. doi: 10.1007/s11433-022-1926-0 [9] Xue G T, Niu Y F, Liu X, et al. Ultrabright multiplexed energy-time-entangled photon generation from lithium niobate on insulator chip [J]. Phys Rev Appl, 2021, 15(6): 064059. doi: 10.1103/PhysRevApplied.15.064059 [10] Lin J, Farajollahi S, Fanget Z, et al. Electro-optic tuning of a single-frequency ultranarrow linewidth microdisk laser [J]. Adv Photon, 2022, 4(3): 036001. [11] Zhang P, Huang H, Jiang Y, et al. High-speed electro-optic modulator based on silicon nitride loaded lithium niobate on an insulator platform [J]. Optics Letters, 2021, 46(23): 5986-5989. doi: 10.1364/OL.446222 [12] Wang C, Zhang M, Yu M J, et al. Monolithic lithium niobate photonic circuits for Kerr frequency comb generation and modulation [J]. Nature Communications, 2019, 10(1): 978. doi: 10.1038/s41467-019-08969-6 [13] Zhang M, Buscaino B, Wang C, et al. Broadband electro-optic frequency comb generation in a lithium niobate microring resonator [J]. Nature, 2019, 568(7752): 373-377. doi: 10.1038/s41586-019-1008-7 [14] Pohl D, Escalé M R, Madiet M, et al. An integrated broadband spectrometer on thin-film lithium niobate [J]. Nat Photonics, 2020, 14(1): 24-29. doi: 10.1038/s41566-019-0529-9 [15] He L, Zhang M, Shams-Ansari A, et al. Low-loss fiber-to-chip interface for lithium niobate photonic integrated circuits [J]. Optics Letters, 2019, 44(9): 2314-2317. doi: 10.1364/OL.44.002314 [16] Hu C, Pan A, Li T, et al. High-efficient and polarization independent edge coupler for thin-film lithium niobite waveguide devices[EB/OL]. (2020-09-07)[2022-12-27]. https://arxiv.org/abs/2009.02855. [17] Pan Y, Heyun T, Zhang J, et al. Low-loss edge-coupling thin-film lithium niobate modulator with an efficient phase shifter [J]. Optics Letters, 2021, 46(6): 1478-1481. doi: 10.1364/OL.418996 [18] Press W H, Flannery B P, Teukolsky S A, et al. Numerical Recipes: The Art of Scientific Computing[M]. 3rd ed. New York: Cambridge University Press, 1986: 156-163. [19] Rsoft Design Group. Beam PR, OP7.0 user guide[Z]. Ossining: Rsoft Design Group Inc, 2006. [20] Hadley G R. Transparent boundary condition for beam propagation method [J]. Optics Letters, 1991, 16(9): 624-626. doi: 10.1364/OL.16.000624 [21] Hadley G R. Transparent boundary condition for the beam propagation method [J]. IEEE Journal of Quantum Electronics, 1992, 28(1): 363-370. doi: 10.1109/3.119536 [22] Vassalo C, Collino F. Highly efficient absorbing boundary condition for the beam propagation method [J]. Journal of Lightwave Technologyvol, 1996, 14(6): 1570-1577. doi: 10.1109/50.511688 [23] Huang W P, Xu C L, Lui W, et al. The perfectly matched layer (PML) boundary condition for the beam propagation method [J]. IEEE Photonics Technology Letters, 1996, 8(5): 649-651. doi: 10.1109/68.491568 [24] Chiou Y P, Chang H C. Complementary operators method as the absorbing boundary condition for the beam propagation method [J]. IEEE Photonics Technology Letters, 1998, 10(7): 976-978. doi: 10.1109/68.681289