⇦Chinese Physics Letters, 2020, Vol. 37, No. 8, Article code 080102Views & Comments Meter-Level Optical Delay Line on a Low-Loss Lithium Niobate Nanophotonics Chip Shining Zhu (祝世宁)1,2,3* Affiliations 1National Laboratory of Solid State Microstructures, School of Physics, College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, China 2Key Laboratory of Intelligent Optical Sensing and Manipulation (Ministry of Education), Nanjing University, Nanjing 210093, China 3Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China Received 16 July 2020; accepted 17 July 2020; published online 23 July 2020 *Corresponding author. Email: zhusn@nju.edu.cn Citation Text: Zhu S N 2020 Chin. Phys. Lett. 37 080102 Abstract DOI:10.1088/0256-307X/37/8/080102 PACS:01.10.-m, 42.82.-m, 42.79.-e © 2020 Chinese Physics Society Article Text 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.[1] 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.[2] Such waveguides have been fabricated using lithographic techniques,[3–6] 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.[7] 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 $\sim 2.2$ ns using a femtosecond laser. The pulse propagation loss as low as $\sim 0.03$ 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 $\sim 10$ cm to $\sim 100$ 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,[8] microwave signal processing,[9] optical gyroscopes,[10] to name a few. References Integrated Optical Devices in Lithium NiobateStatus and Potential of Lithium Niobate on Insulator (LNOI) for Photonic Integrated CircuitsSelf-starting bi-chromatic LiNbO 3 soliton microcombPeriodically poled thin-film lithium niobate microring resonators with a second-harmonic generation efficiency of 250,000%/WHigh-performance hybrid silicon and lithium niobate Mach–Zehnder modulators for 100 Gbit s−1 and beyondBroadband electro-optic frequency comb generation in a lithium niobate microring resonatorElectro-Optically Switchable Optical True Delay Lines of Meter-Scale Lengths Fabricated on Lithium Niobate on Insulator Using Photolithography Assisted Chemo-Mechanical EtchingOn-Chip Generation and Manipulation of Entangled Photons Based on Reconfigurable Lithium-Niobate Waveguide CircuitsA tutorial on microwave photonic filtersPhotonic technologies for angular velocity sensing
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