High‐efficiency single etch step apodized surface grating coupler using subwavelength structure

Abstract

Grating couplers are key elements enabling the coupling of light between planar waveguide circuits and optical fibers. In this work, it is demonstrated using simulations and experiments that a high coupling efficiency can be achieved for an arbitrary buried oxide thickness by judicious adjustment of the grating radiation angle. The coupler strength is engineered by subwavelength structure, allowing straightforward apodization and single etch step fabrication. The design has been implemented using Fourier-eigenmode expansion and finite difference time domain methods. The measured coupling loss of a continuously apodized grating is −2.16 dB with a 3 dB bandwidth of 64 nm, therefore opening promising prospects for low-cost and high-volume fabrication using 193 nm deep-ultraviolet lithography. It is also shown by simulations that a coupling loss as low as −0.42 dB is predicted for a modified coupler structure with bottom mirror.

Single-etch subwavelength engineered fiber-chip grating couplers for 13 µm datacom wavelength band

“…Grating directionality is typically limited by the interference effect due to the reflection at the interface between the BOX layer and the silicon substrate. We have demonstrated that a modification of the coupling angle is an effective technique to achieve the constructive interference condition [16,18]. In Fig.…”

“…This advantageous feature is afforded by utilization of the concept of subwavelength grating (SWG) refractive index engineering. The SWGs have successfully been used to locally engineer waveguide properties in integrated optical devices [16,18,19,25,[29][30][31][35][36][37]. Here, we implement an SWG structure in the direction perpendicular to the light propagation, as a periodic arrangement of high-index material (silicon) segments and rectangular holes etched in the waveguide core.…”

“…The main limiting factor which remains is the leakage loss to the silicon substrate through the BOX layer [3,7]. Presently, three main approaches have been used to circumvent this limitation [6,[14][15][16][17][18][19][20]. The first approach requires a specialized SOI platform with the dielectric (Bragg) reflector [18,20].…”

“…However, the SOI with 220-nm-thick silicon presently dominates in multi project wafer (MPW) shuttles, offered by the publicly accessible silicon photonic foundries [10,13]. To date, the state-of-the-art solutions to maximize the grating directionality exploit comparatively complex structures [14][15][16][17][18][19][20][21][22][23][24][25], including backside wafer processing and metal mirror deposition [14][15][16][17], or bonding dielectric mirrors [3,6,7,[18][19][20]. Yet another approach exploits the optimization of grating intrinsic directionality.…”

Grating-Assisted Fiber to Chip Coupling for SOI Photonic Circuits

“…The choice of fiber tilt angle of illumination θ, is highly design and application specific. It is possible to have very high efficiency non-uniform couplers with both shallow [18] as well as deep angles of incidence [19]. It may be pointed out that, for non-uniform designs, the modulation of individual grating strength results in a higher effective grating index than uniform couplers.…”