# High-Tolerance Grating Couplers for Vertical Backside Coupling
###### Abstract  
To facilitate convenient packaging of photonic integrated circuits on a fiber tip, a silicon grating coupler designed for vertical backside coupling has been developed. In order to comply with foundry capabilities and streamline the fabrication processes, the grating coupler features a minimum feature size larger than 200 nm and a single-etched structure on silicon. By inverse design-based optimization, the vertical backside grating coupler achieves a coupling efficiency of nearly 40% (\(-\)3.97 dB), while showcasing high fabrication and misalignment tolerance. g +
Footnote †: journal: Applied sciences  
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Footnote †: journal: Applied sciences  
0000-0002-491X/0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0  
configuration based on multicore fibers provides more flexibility in the design of the circuit and potentially a higher contrast in the readout. One of the key components in PICs is the GC, which must enable efficient fiber-to-chip coupling. Typically, a GC comprises a high-index slab featuring partially or fully etched periodic corrugations, inducing light scattering out of the slab at the desired angle [11]. In comparison with conventional oblique incidence GCs, vertical GCs have recently received significant attention, with a design approach being proposed [11; 12; 13; 14; 15]. These couplers enable efficient coupling of light from vertically aligned fibers or fiber arrays into waveguide chips from the front side [16]. However, the performance of vertical GCs is influenced by the Bragg condition [13], which governs the coupling efficiency in diffraction-based systems. In the case of symmetric vertical GCs, this condition leads to the coupled light being split between two counter-propagating directions. As a result, a significant portion of the coupled light is directed in the undesired direction within the photonic integrated circuit (PIC), with only a small fraction of the light successfully coupling into the target output waveguide. Consequently, vertical GCs typically exhibit lower coupling efficiency (CE) compared to conventional oblique incidence GCs. To enhance CE, various methods such as dual-layer GC [11], tilted-angle GC [12], binary blazed grating [15; 17], apodized GC [13], the use of an extra metallic mirror [9], the use of dielectric metasurfaces [18; 19], and more exotic couplers [20] have been demonstrated. Adding a supplementary metallic mirror and dual layer GC require additional fabrication processes, and the tilted angle GC is achieved by collapsing the grating structure during the fabrication process to achieve a specific angle, which requires sophisticated fabrication capabilities. Integration of metasurfaces on top of GCs will improve their CE by deflecting at an angle the incoming light from a fiber towards an underneath GC, therefore preventing bidirectional transmission within it. Such dielectric metasurfaces, along with binary blazed GCs consisting of the combination of multiple sub gratings within one period, face two major issues. To meet the first-order Bragg condition, it is often necessary to integrate very small features (120 nm or less), which requires high-resolution lithography techniques (EBL), and thus is not compatible with most commercial silicon photonics foundries. Additionally, all mentioned GCs are optimized for an air-to-chip coupling or are not suited to fiber-tip sensing applications, where light must be coupled from silica to the chip (backside coupling) and the chip must be transferred on the fiber's tip (monitoring the surrounding medium, typically air). Therefore, GCs with a larger minimum feature size (over 200 nm) and a simple (single etch and single lithographic  
Figure 1: A 3D example sketch of a mechanically transferred PIC on the facet of a multicore fiber. Vertical backside GCs are used for -in and -out coupling of the light. step), adapted structure for sensing would not only enhance fabrication tolerance but also reduce complexity of fabrication and thus satisfy mass production standards.