Ultra-low loss SiN edge coupler interfacing with a single-mode fiber

Abstract

In this work, an ultra-low loss silicon nitride (SiN) edge coupler was designed and fabricated to interface with a single-mode fiber (SMF). Unlike other works that focus on the core structure, this work focuses on the cladding structure. First, it is demonstrated that the cladding structure ultimately determines the size and shape of the mode when the taper tip width is small enough. Then, the thickness of the up-cladding is optimized to provide enough space for mode expansion in the vertical direction. Air trenches are added to confine the mode laterally. In addition, the refractive index (RI) of the up-cladding layer is slightly increased to prevent light from leaking into the Si substrate. This edge coupler is then fabricated on the SiN platform at Chongqing United Microelectronics Center. For the TE mode at 1630 nm, a coupling loss of 0.67 dB/facet was obtained. At 1550 nm, 0.85 dB/facet and 1.09 dB/facet were measured for the TE and TM modes, respectively, which means that the polarization-dependent loss is 0.24 dB. Although the design method and the structure are based on a pure SiN platform, they are applicable to a silicon-on-insulator platform as well.

Broadband Polarization-Independent Edge Couplers With High Efficiency Based on SiN-Si Dual-Stage Structure

“…Silicon nitride (SiN), a common CMOS-compatible material with low index contrast to SiO2 cladding (ΔnSiN-SiO2 ≈ 0.5) is a promising candidate [26][27][28]. Recently, SiN edge couplers have been extensively studied [27,[29][30][31][32][33][34][35][36][37][38]. Siwei Sun et al proposed an edge coupler design based on a cross-shaped arrangement of SiN waveguides surrounded by SiO2 cladding on a standard 220 nm-thick silicon SOI platform and it can couple the light from a standard SMF-28 fiber at 1550 nm to silicon wire waveguide with an overall coupling loss lower than 0.44 dB [33].…”

Graphene‐Based Silicon Photonic Devices for Optical Interconnects

“…The ecosystem developed for the silicon photonics industry, including process-design kits (PDKs), packaging, and assembly, is fully compatible with SiN. The moderate index contrast and relatively low TO coefficient (≈13% of silicon) [58] of SiN enables the development of passive silicon photonic devices with superior performance, such as ultra-low loss waveguides, [61][62][63] high coupling efficiency edge couplers, [64][65][66] ultra-high quality factor micro-ring resonators, [67][68][69][70][71][72][73][74] fabricationtolerant and temperature-insensitive (de)multiplexers, [75][76][77][78][79] and high-performance optical phased arrays. [80][81][82][83] Moreover, the negligible TPA of SiN makes it an ideal platform for handling high optical power, thereby paving the way for on-chip nonlinear optics.…”