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Frontiers of Optoelectronics

Front. Optoelectron.    2019, Vol. 12 Issue (3) : 276-285
Design and fabrication of compact Ge-on-SOI coupling structure
Jianfeng GAO, Junqiang SUN(), Heng ZHOU, Jialin JIANG, Yang ZHOU
Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China
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In this paper, we have proposed and demonstrated a simple approach to fabricate vertical integrated structure for coupling between active germanium (Ge) waveguide and silicon-on-insulator (SOI) waveguide. The active Ge waveguide is sputtered after etching the underlying passive silicon (Si) waveguide. This method scuttles away from the difficulty involved in the waveguide fabrication by avoiding the etching process for the Ge waveguide, and thereby the waveguide quality is improved. The influences of the coupling structural parameters on the coupling loss are analyzed and discussed. The optimizing parameters are obtained for the fabrication. The minimal coupling loss is experimentally measured about 2.37 dB, and variation tendency of coupling loss against the structural parameters is consistent with the theoretical result. The proposed approach offers an effective path for vertical coupling between Ge and SOI optical components.

Keywords taper coupler      integrated optics device      guided waves      silicon-on-insulator (SOI) waveguide      germanium (Ge) waveguide      active Ge device      Ge-on-SOI coupling structure     
Corresponding Authors: Junqiang SUN   
Just Accepted Date: 12 September 2018   Online First Date: 31 October 2018    Issue Date: 16 September 2019
 Cite this article:   
Jianfeng GAO,Junqiang SUN,Heng ZHOU, et al. Design and fabrication of compact Ge-on-SOI coupling structure[J]. Front. Optoelectron., 2019, 12(3): 276-285.
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Jianfeng GAO
Junqiang SUN
Jialin JIANG
Fig.1  (a) Schematic diagram of the Ge active waveguide and laterally tapered coupler; (b) two routes of optical transmission within the coupling structure
Fig.2  Fundamental mode scattering as a function of the active waveguide width in the coupling structure under different active waveguide heights tGe, with the sidewall inclination of 90°
Fig.3  Fundamental mode scattering as a function of the active waveguide width in the coupling structure under different sidewall inclinationsof the active waveguide, with the active waveguide height of 200 nm
Fig.4  Schematic diagram of the designed tapered active waveguide. Ws and We are the starting width and ending width of the middle section
Fig.5  Coupling efficiency as a function of central width of the middle section in the taper when the width difference WeWs is kept at 200 nm
Fig.6  Coupling efficiency as a function of width difference of the middle section in the taper with the central width of 260 nm
Fig.7  For different central widths Wc, the normalized profile of the electric field distribution at the middle of Ge and SOI layers. (a) Wc = 160 nm; (b) Wc = 260 nm; (c) Wc = 360 nm. The width range WeWs = 200 nm
Fig.8  For different width differences WeWs, the normalized profile of the electric field distribution at the middle of Ge and SOI layers. (a) WeWs = 100 nm; (b) WeWs = 200 nm; (c) WeWs = 300 nm. The central width Wc = 260 nm
Fig.9  Schematic diagram of the fabrication process of Ge-on-SOI coupling structure. (a) Making metal mark; (b) coating ZEP520A; (c) electron beam lithography; (d) ICP etching; (e) removing the photoresist ZEP520A; (f) coating PMMA; (g) electron beam lithography; (h) sputtering Ge material; (i) removing the photoresist PMMA
Fig.10  SEM image of the fabricated Ge-on-SOI coupling structure
Fig.11  Schematic diagram of the coupling alignment between optical fibers and photonic crystal gratings
width range of the middle section 60−260 nm 160−360 nm 260−460 nm
absorption loss 4.62 dB 4.70 dB 4.65 dB
scattering loss 3.18 dB 3.10 dB 3.02 dB
Tab.1  Absorption loss and the scattering loss for different taper shapes
width range of the middle section 60−260 nm 160−360 nm 260−460 nm
coupling loss 2.49 dB 2.37 dB 2.67 dB
total loss 10.29 dB 10.17 dB 10.34 dB
Tab.2  Coupling loss and the total loss for the different taper shapes
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