近期，华东师范大学程亚教授研究组研发了一种在铌酸锂薄膜材料上实现超低损耗且快速可调光波导的微纳加工方法。他们首先利用飞秒激光刻蚀蒸镀在铌酸锂薄膜表面的一层铬膜，形成与光波导回路一样的铬掩膜图案。由于铬膜具有比铌酸锂高得多的硬度，因此在后续的化学机械抛光过程中，飞秒激光刻蚀形成的铬掩膜可以保护下方的铌酸锂材料，其余未被铬掩膜保护的区域中的铌酸锂薄膜则被去除。利用该方法得到的铌酸锂光波导具有非常光滑的表面，因此能够支持超低传输损耗。同时由于飞秒激光直写系统具有很大的运动行程，因此可以连续刻写出大尺寸的光子芯片。

利用该制备技术，该研究组实现了长度超越1米的铌酸锂光子延时线，突破了现有铌酸锂薄膜光波导制备中的长度限制。经测量，该新型波导的传输损耗仅为0.03 dB/cm，即光在该一米长的波导中传输后，其能量损失不到总输入光功率的50%. 由于铌酸锂晶体具有很高的电光系数，该研究组也通过将不同长度波导与一组电光开关在芯片上集成，实现了可快速切换延时量的可重构光子延时线。经过持续的技术与工艺提升，该波导制备技术已日趋成熟，可满足铌酸锂晶圆级全尺寸光子器件的流片需求。

以上工作已发表在CPL Express Letters栏目

Electro-Optically Switchable Optical True Delay Lines of Meter-Scale Lengths Fabricated on Lithium Niobate on Insulator Using Photolithography Assisted Chemo-Mechanical Etching

Jun-xia Zhou (周俊霞), Ren-hong Gao (高仁宏), Jintian Lin (林锦添), Min Wang (汪旻), Wei Chu (储蔚), Wen-bo Li (李文博), Di-feng Yin (尹狄峰), Li Deng (邓莉), Zhi-wei Fang (方致伟), Jian-hao Zhang (张健皓), Rong-bo Wu (伍荣波), and Ya Cheng (程亚)

Chin. Phys. Lett. 2020, 37 (8): 084201

应编辑部邀请，南京大学祝世宁教授为本文作了点评！

Shining Zhu (祝世宁)

Chin. Phys. Lett. 2020, 37 (8): 080102

Photonic integrated circuit (PIC) technology provides an enabling platform for emerging applications ranging from big data science and artificial intelligence to high sensitivity sensing and quantum information processing. An outstanding challenge in achieving the large-scale photonic integration is to realize low-loss optical waveguides with small radii and high tuning efficiencies on a single photonic chip. Traditionally, silicon-on-insulator (SOI) has been studied as a major material platform for PIC technology because of its mature processing technology and being compatible with CMOS process.

In merely two decades, however, lithium-niobate-on-insulator (LNOI) has proven itself to be a powerful competitor as a new material platform for PIC technology. Lithium niobate crystal, which has been described as “optical silicon”, has favorable low-loss limit, nonlinear, acousto-optic, and electro-optic coefficients. These properties, combined with the high-index-contrast ridge waveguides on LNOI wafer, have enabled low-loss tunable optical waveguides of small curvature radii. Such waveguides have been fabricated using lithographic techniques, following by dry etching, such as inductive coupled plasma (ICP), reactive ion etching (RIE), etc. Still, fabrication of large-scale low-loss LNOI waveguides remains to be an outstanding challenge due to the accumulation of the stitching errors in the traditional electron beam lithography (EBL) or ultraviolet (UV) lithography.

Recently, Zhou et al. overcome these challenges using a femtosecond laser lithographic patterning of a chromium (Cr) thin film coated on the top surface of LNOI to generate the waveguides mask, then followed by chemo-mechanical polish for selective etch of the thin film of lithium niobate. The technique termed photo-lithography assisted chemo-mechanical etching (PLACE) is proved to be particularly suited for fabrication of low-loss waveguide that is stitching-free, and therefore, large-scale PIC devices. Their work clearly demonstrates the ultra-smooth LN ridge waveguide surfaces produced by the PLACE technique can give rise to ultra-low propagation losses. They designed and fabricated reconfigurable optical true delay lines (OTDL) on the LNOI chip based on the electro-optic routing of the delay paths with several integrated Mach-Zehnder interferometer switches. The OTDL has a relative optical path length of 32.68 cm, corresponding to a relative time delay of 2.285 ns in theory. Experimentally, the output signal was measured to arrive at the photodetector with a time difference of ～ 2.2 ns using a femtosecond laser. The pulse propagation loss as low as ～ 0.03 dB/cm is confirmed. To examine the reliability of the fabrication approach for creating the LN ridge waveguides over large scales, the OTDLs with various lengths ranging from ～ 10 cm to ～ 100 cm are fabricated and the propagation losses in all the fabricated waveguides are measured on the level below 0.03 dB/cm.

The low-loss waveguides with highly miniaturized cross-sections are the basis of photonic integrated circuit technology. Besides OTDL, electro-optic switches and Mach-Zehnder interferometer, many other function components in PIC, such as optical micro-cavity, beam splitter, wavelength-division-multiplexing (WDM), frequency-division-multiplexing (FDM), and mode converter, all of these above consist of waveguides or waveguide circuits. Some active components, such as light sources (whether classical or quantum), optical modulators, optical memories, optical detectors, etc., are also highly dependent on high-performance optical waveguides or waveguide arrays. PLACE technique enables consistent low-loss waveguide fabrication over a large length at the meter level in a small on-chip footprint, and will have important impacts in the fields such as quantum photonic technology, microwave signal processing, optical gyroscopes, to name a few.

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