Efficiency Enhanced Grating Coupler for Perfectly Vertical Fiber-to-Chip Coupling

Abstract

In this work, a bidirectional grating coupler for perfectly vertical coupling is proposed. The coupling efficiency is enhanced using a silicon nitride (Si3N4) layer above a uniform grating. In the presence of Si3N4 layer, the back-reflected optical power into the fiber is diminished and coupling into the waveguide is increased. Genetic algorithm (GA) is used to optimize the grating and Si3N4 layer simultaneously. The optimal design obtained from GA shows that the average in-plane coupling efficiency is enhanced from about 57.5% (−2.5 dB) to 68.5% (−1.65 dB), meanwhile the average back-reflection in the C band is reduced from 17.6% (−7.5 dB) to 7.4% (−11.3 dB). With the help of a backside metal mirror, the average coupling efficiency and peak coupling efficiency are further increased to 87% (−0.6 dB) and 89.4% (−0.49 dB). The minimum feature size of the designed device is 266 nm, which makes our design easy to fabricate through 193 nm deep-UV lithography and lowers the fabrication cost. In addition, the coupler proposed here shows a wide-band character with a 1-dB bandwidth of 64 nm and 3-dB bandwidth of 96 nm. Such a grating coupler design can provide an efficient and cost-effective solution for vertical fiber-to-chip optical coupling of a Wavelength Division Multiplexing (WDM) application.

Broadband High-Efficiency Grating Couplers for Perfectly Vertical Fiber-to-Chip Coupling Enhanced by Fabry-Perot-like Cavity

“…Considering chip-to-fiber coupling, the second-order diffracted mode will cause a strong reflection back into the waveguide. However, thanks to the symmetry of the bi-directional GC, with two in-plane ports, the reflected wave in one in-plane port will be diminished by the destructive interference, with the wave transmitted from the other in-plane port, as discussed in our previous work [ 29 ].…”

Desing of grating couplers for submicron optical waveguides

“…TE polarization was chosen for these simulations since most SOI devices are designed to work in this mode [8]. In this work, the physical parameters are optimized by taking measurements of the electric field below the first grating (P1), in the middle of the two gratings (P2) and above the second grating (P3) shown in Fig.…”

“…Another important achievement proposes a bidirectional grating for vertical coupling. Such an improvement was achieved by using a Si 3 N 4 layer on the grating, which decreases the reflected power to the fiber and increases coupling to the guide [8]. An interesting strategy, which has allowed the improvement in the efficiency of grating couplers, has been the use of grating period apodization and fill factor (FF), together with the optimization of the etch depth of the grating coupler and presented the best performance for SOI structures without the use of any self-reflector [9,10].…”

A 60 μm-Long Fiber-to-Chip Edge Coupler Assisted by Subwavelength Grating Structure with Ultralow Loss and Large Bandwidth

“…Owing to the flexibility in use of the grating coupler, it has been employed as the main input and output couplers for wafer-level device testing at present with advantages of compact size and being able to choose arbitrary design position on the fabrication layout. However, the grating coupler also 2 of 12 has obvious drawbacks, e.g., narrow working bandwidth, low coupling efficiency (or large coupling loss), incident angle dependence, and polarization dependence, all of which are derived from the intrinsic phase matching requirement of the grating coupler [13,14,17]. By contrast, the edge coupler works as a mode evolution method where the injected light from the optical fiber is gradually coupled to the silicon waveguide through inverse tapers [15,16,18].…”