Zheng Hong-Yan, Chen Bo-Liang, Lu Hsiao-Yen, Hsu Shih-Hsiang, Takabayashi Masanori
Department of Electronic Engineering, National Taiwan University of Science and Technology, No. 43, Sec. 4, Keelung Rd., Taipei 10607, Taiwan.
Department of Physics and Information Technology, Kyushu Institute of Technology, 680-4 Kawazu, Iizuka 820-8502, Japan.
Micromachines (Basel). 2022 Feb 26;13(3):373. doi: 10.3390/mi13030373.
A chip-based spectral-domain optical coherence tomography (SD-OCT) system consists of a broadband source, interferometer, and spectrometer. The optical power divider flatness in the interferometer's wavelength is crucial to higher signal-to-noise ratios. A Mach-Zehnder directional coupler (MZDC) structure could be utilized to smoothly maximize the splitting ratio of 50:50 on a silicon platform, with a sub-micrometer of decoupler optical path difference insensitive to the process variation up to 20 nanometers. However, the optical signal reflected from the reference and sample will go back to the same interferometer MZDC. The so-called bidirectional coupler MZDC will not illustrate a flat optical power response in the operating wavelength range but could still demonstrate at least 20 dB signal-to-noise ratio improvement in OCT after the echelle grating spectrum compensation is applied. For maintaining the axial resolution and sensitivity, the echelle grating is also insensitive to process shifts such as MZDC and could be further utilized to compensate a 3 dB bidirectional MZDC structure for a broad and flat 100 nm wavelength response in the interferometer-based on-chip SD-OCT.
基于芯片的谱域光学相干断层扫描（SD - OCT）系统由宽带光源、干涉仪和光谱仪组成。干涉仪中光功率分配器在波长范围内的平坦度对于获得更高的信噪比至关重要。马赫 - 曾德尔定向耦合器（MZDC）结构可用于在硅平台上平滑地实现50:50的分光比最大化，其解耦器光程差在亚微米级别，对高达20纳米的工艺变化不敏感。然而，从参考臂和样品反射回来的光信号会回到同一个干涉仪MZDC。所谓的双向耦合器MZDC在工作波长范围内不会呈现平坦的光功率响应，但在应用阶梯光栅光谱补偿后，在OCT中仍可实现至少20 dB的信噪比提升。为了保持轴向分辨率和灵敏度，阶梯光栅对诸如MZDC之类的工艺变化也不敏感，并且可进一步用于补偿基于芯片的干涉式SD - OCT中3 dB双向MZDC结构，以实现100 nm波长范围内宽且平坦的响应。
