Subwavelength grating-assisted silicon nitride edge coupler with relaxed alignment tolerance

Abstract

We propose and realize a silicon nitride (SiN) edge coupler with enhanced alignment tolerance, achieved through the integration of a subwavelength grating (SWG) structure. The incident optical mode is guided from the SWG tri-tip to the main waveguide via two adiabatic transitions through intermediate bridge waveguides. The SWG tri-tip, composed of three laterally arranged SWG waveguides, provides a broader horizontal optical input region than a single-tip design, enhancing horizontal alignment tolerance. Moreover, careful engineering of the SWG period and fill factor lowers the effective index of the input waveguide compared to a solid waveguide, improving mode matching with optical fibers. The fabricated device demonstrates 1-dB alignment tolerances of ± 3 μm in the horizontal direction when coupled with a standard SMF-28 optical fiber, achieving a measured coupling efficiency of - 3.8 dB/facet at 1550 nm wavelength. This horizontal tolerance is notably wider than that of conventional single-tip inverse-tapered edge couplers, validating the effectiveness of the SWG tri-tip design in expanding the input acceptance region. These results highlight the effectiveness of the proposed design in relaxing alignment sensitivity while maintaining efficient coupling, making it suitable for practical packaging and integration in SiN photonic platforms.

Keywords: Alignment tolerance; Edge coupler; Silicon nitride; Subwavelength grating.

Fig. 1

Configuration of the proposed subwavelength grating (SWG)-assisted silicon nitride (SiN) edge coupler.

Fig. 2

(a) Cross-section of the SWG tri-tip. (b) Schematic of the proposed edge coupler.

Fig. 3

Schematic of (a) the SWG (left) and solid (right) taper waveguide. (b) Effective refractive index (n_eff) of the SWG and solid waveguide according to the waveguide width at a wavelength of 1550 nm. (c) Transmission according to the taper length of SWG and solid waveguide at a wavelength of 1550 nm.

Fig. 4

Schematic and cross-section mode profile of (a) the SWG tri-tip and (b) single-tip structure. (c) Calculated modal overlap in terms of the taper tip width at a wavelength of 1550 nm.

Fig. 5

(a) Schematic and electric field distribution of the adiabatic coupler 1 and (b) simulated adiabatic transition efficiency according to the coupling length at a wavelength of 1550 nm.

Fig. 6

(a) Schematic and electric field distribution of the adiabatic coupler 2. (b) Simulated adiabatic transition efficiency according to the coupling length at a wavelength of 1550 nm.

Fig. 7

(a) Electric field distribution of the proposed SWG-assisted SiN edge coupler at a wavelength of 1550 nm. (b) Guided mode profile of the SMF-28 fiber. (c) Cross-sectional electric field profiles at various x positions (x = 0, 30, 60, 70, 90, 110 μm).

Fig. 8

(a) Simulated coupling efficiency between the SMF-28 fiber and SWG-assisted SiN edge coupler as a function of the fill factor and grating period of SWG at a wavelength of 1550 nm. (b) Simulated coupling efficiency of the proposed edge coupler and conventional single-tip edge coupler with an SMF-28 fiber over a wavelength range of 1450–1650 nm, and (c) coupling efficiency as a function of horizontal misalignment at a wavelength of 1550 nm.

Fig. 9

Calculated coupling efficiency between the input fiber and SiN waveguide as a function of w_tip, t_SiN, and sidewall angle of the fabricated device at a wavelength of 1550 nm for the fundamental TE mode. (a) Coupling efficiency with t_SiN = 400 nm. (b) Coupling efficiency with w_tip = 200 nm.

Fig. 10

(a) Optical image of the fabricated device. (b) Optical microscope images of the fabricated SWG-assisted SiN edge coupler. Scanning electron microscope image of (c) cross-section of the SWG tri-tip and (d) SWG-assisted SiN edge coupler.

Fig. 11

Experimental setup for measuring the coupling efficiency between the SMF-28 fiber and SWG-assisted SiN edge coupler.

Fig. 12

(a) Calculated and measured coupling efficiency of the proposed edge coupler to an SMF-28 fiber. Alignment tolerances between the SMF-28 fiber and proposed edge coupler in (b) horizontal and (c) vertical directions at a wavelength of 1550 nm.

Fig. 13

(a) Schematic and (b) optical microscope image of the fabricated SWG-assisted edge coupler with an elongated tip. (c) Calculated coupling efficiency between the proposed SWG-assisted edge coupler and SMF-28 fiber based on the elongated tip length at a wavelength of 1550 nm.

Fig. 14

Calculated (black line) and measured (black dot) coupling efficiency according to the length of (a) SWG tri-tip, (b) adiabatic coupler 1, and (c) adiabatic coupler 2 at a wavelength of 1550 nm.