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

: We report, for the first time, on the design and experimental demonstration of fiber-chip surface grating couplers based on subwavelength grating engineered nanostructure operating in the low fiber chromatic dispersion window (around 1.3 μ m wavelengths), which is of great interest for short-reach data communication applications. Our coupler designs meet the minimum feature size requirements of large-volume deep-ultraviolet stepper lithography processes. The fiber-chip couplers are implemented in a standard 220-nm-thick silicon-on-insulator (SOI) platform and are fabricated by using a single etch process. Several types of couplers are presented, specifically the uniform, the apodized, and the focusing designs. The measured peak coupling efficiency is − 2.5 dB (56%) near the central wavelength of 1.3 μ m. In addition, by utilizing the technique of the backside substrate metallization underneath the grating couplers, the coupling efficiency


Introduction
Silicon-on-insulator (SOI) is a compelling photonic platform for implementation of complex photonic functions and waveguide circuits on a compact optoelectronic chip [1,2]. Optical input and output couplers are essential building blocks in the library of silicon photonic components . Surface grating couplers are particularly interesting as they allow waferscale optical testing and yield comparatively large alignment tolerances for fiber attachment.
Propagation loss in the optical fiber is a key parameter determining the performance and reach of long-haul photonic links. Conversely, short-reach datacom interconnects exhibit substantially relaxed loss requirements. Hence, they can migrate from the telecommunications C-band (around 1.55 µm wavelengths), where the optical fiber exhibits very low loss, to the O-band (around 1.3 µm wavelengths) to beneficially exploit the zero-dispersion feature of standard single-mode optical fibers. This wavelength band is of special interest for highbandwidth applications in short-reach data communications such as Ethernet, cloud computing, or data storage, to name a few. However, silicon photonic components such as surface grating couplers, were developed for operation in the C-band (near 1.55 µm)  While single-etch step subwavelength gratings have proven to be a cost-effective solution for the implementation of high-efficiency couplers in the C-band [16,[29][30][31][32], no experimental or theoretical result has been reported on the use of subwavelength index engineered singleetch step grating couplers in the O-band. The main technical hurdle is the reduction in feature size required if a scaling approach is used to implement non-diffractive subwavelength grating nanostructures operating at 1.3 µm.
In this paper, we propose a comprehensive and general design procedure that enables the implementation of optimized subwavelength index engineered grating couplers with minimum feature sizes larger than 100 nm, compatible with large-volume fabrication processes like deep-ultraviolet stepper lithography. We also report, for the first time, on the design and experimental demonstration of 1.3 µm fiber-chip grating couplers with a subwavelength grating (SWG) engineered medium. The coupler is advantageously implemented in a standard 220 nm SOI photonic platform and fabricated using a single-etch step process.

Surface grating coupler design
The surface grating couplers are designed for an SOI wafer with a 220-nm-thick silicon waveguide layer and a 2-µm-thick SiO 2 burried oxide (BOX). Air is assumed as the superstrate medium. Silicon and silicon dioxide refractive indexes of n Si = 3.5047 and n SiO2 = 1.4484 at the wavelength of 1300 nm are used in the calculations. The design methodology is based on de-coupled two-dimensional (2-D) models in the vertical (y-z) and the horizontal (xz) planes [29]. For simulations, Fourier-Eigenmode Expansion Method (F-EEM) [33] and Finite Difference Time Domain (FDTD) calculations [34] are used. Coupler schematics is shown in Fig. 1 ((a), vertical cross-section; (b), in-plane view). Grating couplers are designed for a single etch fabrication process. Both the grating region and the interconnecting waveguides are defined in the same step of lithography patterning and etching, eliminating a second paterning step. 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. By controlling the SWG grating geometry, specifically the widths of the silicon segments (W Si,i ) and the air holes (W G,i ) within the SWG period (Λ SWG = W Si,i + W G,i ), a broad range of effective material refractive indexes can be achieved, providing the SWG period is sufficiently small to frustrate all but the zeroth diffraction order [35,38]. In this configuration, the SWG period must be shorther than the Bragg period [29,35], Λ SWG < Λ Bragg = λ/n eff , where λ is the free-space wavelength (here, λ = 1.3 µm) and n eff is the maximum effective index of the slab waveguide fundamental TE-polarized mode (here, n eff = 2.98). The range of effective material indexes which can be synthesized directly depends on the duty cycle (W Si /Λ SWG ) range determined by the minimum feature size which can be implemented with a given fabrication process. In Fig. 1(c), the SWG refractive index is shown as a function of air gap width for different SWG periods. The SWG refractive index (an equivalent homogeneous material index) is calculated using a 2-D multilayer slab model in the horizontal plane [29]. As can be observed in Fig. 1(c), a wide range of effective material indexes can be synthesized, assuming a minimum feature size >100 nm, compatible with 193 nm DUV stepper lithography.

Uniform grating couplers with high directionality
The grating directionality, throughout this work is defined as the ratio between the power diffracted in upwards direction and the sum of the powers diffracted in upwards and downwards directions (D = P up / (P up + P down )) [14,23,25]. 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. 2(a), the grating directionality is shown as a function of BOX thickness for two different coupling angles, 10° and 33°. It is observed that the grating directionality can readily be optimized for different BOX thicknesses by modifying the coupling angle. According to these calculations, the grating directionality is substantially enhanced for a standard 2-μm BOX thickness, yielding the peak value of 78% for a coupling angle of 33°, compared to 45% directionality for a coupling angle of 10°. The improved coupling performance is also observed in Fig. 2(b), which shows the grating directionality as a function of wavelength for different coupling angles. The geometrical parameters of the uniform grating coupler are: diffraction grating period Λ z = 800 nm, unetched silicon segment length L Si = 200 nm, SWG trench length L SWG = 600 nm, and number of periods NP = 30 (see Fig. 1(a) and (b)). The effective medium refractive index n SWG = 2.40 yields an optimal grating strength, with the field overlap of 78% between the diffracted field and the near-Gaussian mode of the optical fiber (the maximum theoretical overlap between the exponential radiated field profile and the Gaussian fiber mode is ~80% [7],). In the present work, an SMF-28 optical fiber is used, with the mode field diameter MFD = 9.2 μm, defined at 1/e 2 intensity and a wavelength of 1.3 μm. The effective medium index of 2.40 is synthesized for the SWG air gap width W G = 166 nm and SWG pitch Λ SWG = 400 nm (see Fig. 1(c)). The coupling efficiency of −2.4 dB (58%) is calculated, with back-reflections of 5%. As shown in Fig. 3, our F-EEM calculations are in excellent agreement with FDTD simulations.

Coupler apodization
Grating couplers are apodized by using SWG refractive index engineering. The variable grating strength along the mode propagation direction (z axis in Fig. 1(a)) is achieved by synthesizing different effective medium indexes in the subsequent SWG trenches. The apodization increases the field overlap with the optical fiber mode and it also reduces the effective index mismatch at the transition between the silicon access waveguide and the grating region, hence reducing back-reflections. Here we report on two types of apodization scheme, namely a two-step apodization and a continuous apodization. In both cases, the coupling angle is maintained constant (33°) for all grating segments. This is achieved by chirping the grating period in the apodized region to satisfy the phase matching condition: sinΘ = n B,j /n 0 + mλ/n 0 Λ z,j . Here, Θ is the coupling angle, n B,j is the effective index of the Bloch-Floquet mode in the j-th grating segment, n 0 is the refractive index of the superstrate medium (air), λ is the free-space wavelength, and Λ z,j is the period of the j-th grating segment. The design flow for the two-step apodized coupler can be summarized as follows: The structure is optimized by varying SWG refractive index, simulating different grating configurations with two grating sections, each with a specific SWG refractive index and number of periods (NP), as shown in Fig. 4(a). Accodingly, we determined that 6 periods in the first section and 24 periods in the second section are optimal, while maintaining the high grating directionality. The final design parameters of the nominal two-step apodized grating coupler used for fabrication are listed in Table 1. For this structure, the field overlap with the optical fiber mode is 91%. The overall coupling efficiency of −1.78 dB (66%) (see Fig. 3) is predicted and the back-reflections are reduced to 2.5%. The optimization procedure for the continuously apodized grating coupler was as follows: We carried out a set of 2D F-EEM calculations, varying the number of apodized grating periods, for a range of effective medium SWG indexes. In the first (apodized) section of the grating, the SWG index was varied linearly, while in the second section a constant SWG index was used. According to our simulation results shown in Fig. 4(b), the structure with 8 continuously apodized periods followed by a uniform grating section with 22 periods yields the highest coupling efficiency. The nominal design parameters of the continuously apodized grating coupler are specified in Table 2. The calculated field overlap is 93% and the overall coupling efficiency is −1.61 dB (69%) (see Fig. 3). The grating back-reflection is 1.4%.
We also carried out additional calculations for different minimum feature sizes (critical dimension, CD) of the SWG structure. In particular, we increased the nominal CD to 120 nm and 150 nm, for the continuously apodized coupler. These dimensions are typically accepted by the silicon photonics foundries using 193 nm DUV lithography [6,10,11,13]. As can be observed in Figs. 5(a) and (b), both the grating directionality and the field overlap remain nearly constant (75% and 91%, respectively) for the considered critical dimensions. As the result, the overall coupling efficiency is not substantially affected (<0.4 dB (<6%)) for this CD range (see Fig. 5(d)), while the grating reflectivity slightly increases with the CD (see Fig.  5(c)).

Grating coupler layouts
Design of the low-loss transition region between the interconnection waveguide (300 nm wide) and the grating coupler (13.1 μm wide) is essential to achieve a high coupling efficiency. This can be implemented as an adiabatic taper [7,22], but it requires comparatively long tapers, a few hundreds of micrometers in length. Alternatively, focusing grating couplers can be used, providing a high-efficiency short transition, with the length of a few tens of micrometers [2,17,39,40]. The grating trenches follow confocal ellipses in the chip plane [39], focusing the light onto a single-mode waveguide. The latter is connected with the focusing grating by either a short non-adiabatic taper or by a free-propagation slab waveguide region [40]. In this work, we experimentally evaluate performance of these three types of coupler geometries, namely i) the conventional (linear) grating coupler with adiabatic taper (500-μm-long), ii) the focusing grating coupler with non-adiabatic taper (40-μm), and iii) the focusing grating coupler with free-propagation region (17-μm).

Couplers fabrication and testing
The structures were fabricated on an SOI wafer with a 220-nm-thick Si layer a and 2-µm BOX. Electron beam lithography was utilized to define the pattern of the grating couplers and the interconnecting waveguides in the same (single) patterning step. The pattern was subsequently transferred onto the silicon layer by inductively coupled plasma reactive ion etching (ICP-RIE).
To perform optical testing, a pair of identical grating couplers was connected back-toback on the chip. The light was coupled into the chip by one grating and coupled out by the second grating. In this configuration, the polarization-maintaing optical fibre facets were angle-polished so that the facet are parallel to the chip surface. This way, the light is refracted at the interface between the optical fiber core and the superstrate medium (air) at the required coupling angle of 33°. The fiber-chip coupling efficiency was determined from the insertion loss measured for the grating coupler pair, subtracting the set-up system loss of interconnecting optical fibers.
The experimental results are presented in Figs. 6-8. These include the uniform, the twostep apodized, and the continuously apodized grating couplers in different layout configurations, specifically with a linear taper, a non-adiabatic taper, and a free-propagation region. The peak measured coupling efficiencies are −2.7 dB (54%) and −2.5 dB (56%), for the uniform and the apodized grating couplers, respectively. The impact of different coupler layout configurations on the fiber-chip coupling efficiency is observed to be negligible. However, compared to our theoretical predictions (FDTD), the experimental coupling efficiency is about 0.3 dB (~4%) smaller for a uniform grating coupler. For the apodized couplers, the coupling loss penalty is found to be 0.9 dB (~13%). Furthermore, as it can be observed in Figs. 6(a), 7(a), and 8(a), the measured spectral response for all fabricated structures is shifted by ~40 nm towards longer wavelengths. The observed coupling loss penalty and the spectral shift is attributed to the fabrication imperfections, specifically a fabrication bias of ~40 nm compared to the nominal design dimensions, as confirmed by SEM inspection and the FDTD calculations carried out for the biased structures (Fig. 9).   The measured spectral response exhibits a small ripple with a magnitude in the range of 0.2 -0.6 dB (3 -6%) for different grating geometries and layout configurations. The ripple is attributed to the back-reflections at the interface between the access taper waveguide and the grating region, and the reflection at the fiber facet. From the magnitude of the Fabry-Perot fringes, we estimate that the lower back-reflections (~3%) are achieved for curved grating couplers (uniform and apodized gratings, see Figs. 7(a) and 8(a)), while the reflections for the linearly-tapered grating couplers are about 6% (see Fig. 6(a)). The influence of the back-reflections can be effectively mitigated by established techniques such as curving and offsetting the grating lines [41].
In Fig. 9, we compare our experimental results with calculated spectral responses for different grating coupler designs with biased structural dimensions, as measured by SEM. A good agreement is observed between the measurement and simulation results. Fig. 9. Calculated and measured coupling effiency as a function of wavelength for the uniform, the two-step apodized, and the continuously apodized grating couplers. Here, the simulation results include the influence of 40 nm fabrication bias, as determined by the SEM inspection.
It is worth noting that albeit the measured coupling efficiency is 0.3 -0.9 dB (4 -13%) lower compared to the peak theoretical efficiency (because of the fabrication bias discussed above), these results still compare favorably with the previously reported 1.3 μm surface grating couplers, while the latter also demand multiple etch steps [26-28], custom-designed silicon wafers [26], or non-standard SOI thickness [27].

Grating coupler with a backside reflector
As we have shown, the coupling angle optimization and grating apodization using SWG effective medium engineering can be efficiently used to substantially increase the fiber-chip coupling efficiency. 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]. The second approach employs backside wafer metallization, which includes removing the silicon handle substrate by local etching, followed by metal mirror deposition step [14][15][16][17]. In the third approach, grating intrinsic directionality is increased (>95%) by blazing effect [23,25]. Here, we numerically investigate the second approach (backside metal processing) to potentially further increase the coupling efficiency of the coupler structures developed in this work. Specifically, we have carried out FDTD calculations of the nominal designs for the uniform, the two-step apodized, and the continuously apodized couplers presented in Section 2 with the metal mirror implemented underneath the grating, as schematically shown in the inset of Fig. 10. The FDTD simulation results are shown in Fig. 10 for all nominal grating coupler designs. The peak coupling efficiency of -1.71 dB (68%) is predicted for a uniform grating coupler, while the sub-decibel coupling performance is predicted for the apodized surface grating couplers. The two-step apodized design shows a calculated efficiency of −0.7 dB (85%), while the efficiency of −0.5 dB (89%) is predicted for the continuously apodized design.

Conclusion
We reported on the design and experimental characterizations of the surface grating couplers implemented in a standard 220 nm SOI substrate, for datacom wavelength region near 1.3 μm. By exploiting the concept of subwavelength grating refractive index engineering, both the coupler and the interconnecting waveguides were fabricated using a single etch process. The peak coupling efficiencies of −2.7 dB (54%) and −2.5 dB (56%) were measured for uniform and apodized surface grating couplers, respectively. Furthermore, by utilizing the metal reflector underneath the grating couplers, the peak fiber-chip coupling efficiency of up to −0.5 dB (89%) is predicted by FDTD calculations for our apodized design. These results open up promising prospects for the development of highly-efficient input/output optical coupling interfaces for silicon-based photonic integrated circuits for O-band optical communications.