We propose and experimentally demonstrate fiber-to-chip grating couplers with aligned silicon nitride (Si3N4) and silicon (Si) grating teeth for wide bandwidths and high coupling efficiencies without the use of bottom reflectors. The measured 1-dB bandwidth is a record 80 nm, and the measured peak coupling efficiency is −1.3 dB, which is competitive with the best Si-only grating couplers. The grating couplers are integrated in a Si3N4 on silicon-on-insulator (SOI) integrated optics platform with aligned waveguides in both the Si3N4 and Si, and we demonstrate a 1 × 4 tunable multiplexer/demultiplexer using the Si3N4-on-SOI dual-level grating couplers and thermally-tuned Si microring resonators.
© 2014 Optical Society of America
Grating couplers enable optical coupling between standard single-mode optical fiber and high index contrast waveguides, and they have gained widespread usage as on/off chip couplers for silicon-on-insulator (SOI) photonic integrated circuits [1–5]. Compared to edge couplers, grating couplers can be placed anywhere on the photonic chip and wafer-scale optical measurements can be performed without dicing. In addition, grating couplers easily couple to cleaved or polished standard single-mode fiber whereas edge couplers typically require extra processing steps to produce spot-size converters  or cantilever couplers .
Recently, several demonstrations of highly optimized SOI grating couplers have shown coupling efficiencies in the range 0.64 – 1.6 dB [8–17]. These efficiencies were obtained by apodizing the gratings to improve mode-matching to the fiber mode and increasing the directionality (i.e., fraction of radiated power directed toward the superstrate) using either reflectors under the gratings or optimized design of the silicon (Si) thickness and partial-etch depth. A disadvantage of SOI grating couplers is that their 1-dB bandwidths are 25 – 48 nm, which are relatively narrow compared to edge couplers. Large bandwidths can be obtained using silicon nitride (Si3N4) grating couplers, and a 1-dB bandwidth of 67 nm was demonstrated in . A limitation of Si3N4 grating couplers is that the directionality and coupling efficiency are low (only −4.2 dB in ) unless the Si3N4 thickness is large or reflectors are placed beneath the grating couplers , which complicates the fabrication due to either Si3N4 film stress or extra processing steps to define bottom reflectors. Through optical simulations, we have found that the Si3N4 thickness must be > 800 nm to simultaneously achieve directionalities above 80% and appropriate grating strenths; such thicknesses greatly exceed the waveguide thickness requirement for high optical confinement.
In this work, we propose and demonstrate Si3N4-on-SOI dual-level grating couplers that have high coupling efficiencies and large bandwidths (Section 2). The grating couplers use aligned Si3N4 and Si grating teeth, a moderate 400 nm Si3N4 thickness, and no bottom reflectors. Our demonstrated coupling efficiency to standard single-mode fiber is −1.3 dB (74%), and our demonstrated 1-dB bandwidth is 80 nm. In Fig. 1, our experimental result is plotted next to a summary of the best SOI and Si3N4 grating coupler demonstrations; the numbers next to the markers indicate the references. The 1-dB bandwidth demonstrated here is a record among high-efficiency grating couplers. The coupling efficiency is competitive with the best SOI grating couplers [8–17] and significantly larger than that of any reported Si3N4 grating coupler (with and without bottom reflectors) [3, 18]. In addition, the Si3N4 and Si layer thicknesses of our grating coupler are compatible with Si3N4-on-SOI photonic platforms, which support waveguides in both Si3N4 and Si to leverage the excellent passive waveguide properties of Si3N4 and the compatibility of Si waveguides with electro-optic modulators and detectors [19–22]. As an example of this platform compatibility, we demonstrate a 1 × 4 tunable multiplexer/demultiplexer using the Si3N4-on-SOI dual-level grating couplers and thermally-tuned Si microring resonators (Section 3).
2. Si3N4-on-SOI dual-level grating coupler
The Si3N4-on-SOI dual-level grating coupler is shown in Fig. 2(a) and consists of moderately-thick Si3N4 grating teeth above a set of thin, aligned, Si grating teeth. Owing to the proximity of the Si3N4 and Si teeth (≲ 200 nm), the grating behaves as a collection of composite Si3N4-Si grating teeth and not as a Si3N4 grating coupler with a Si back reflector. The combination of Si3N4 and Si breaks the vertical symmetry of the grating, and with proper design, we achieve constructively (destructively) interfering upwards (downwards) radiation from the different scattering interfaces (i.e., high directionality). Si grating couplers typically achieve this vertical asymmetry and high directionality through design of the Si thickness and partial-etch depth, but for Si3N4 grating couplers, the moderate refractive index contrast necessitates large thicknesses to simultaneously achieve high directionalities and appropriate grating strengths. Through optical simulations, we have found that partially-etched Si3N4 grating couplers require the Si3N4 thickness to be > 800 nm to radiate > 80% of the input optical power upwards over twice the fiber mode-field-diameter. Our composite Si3N4-Si grating tooth design circumvents this coupling efficiency limitation of Si3N4 grating couplers. In addition, since the Si3N4 is moderately-thick and the Si is relatively thin, the dual-level grating coupler’s period remains comparable to those of purely Si3N4 grating couplers, which allows large bandwidths due to fewer grating periods over the fiber mode diameter compared to Si grating couplers .
The grating coupler’s Si3N4 and Si thicknesses were chosen to be compatible with the Si3N4-on-SOI integrated optics platform in Fig. 2(b) and described in [21–23]. The grating coupler uses the fully-etched, 400 nm thick Si3N4 level and the partially-etched, 65 nm thick Si level. A planar, 135 nm thick layer of silica (SiO2) exists between the Si3N4 and Si grating teeth.
2.1. Device design
Our apodized dual-level grating coupler design is shown in Fig. 2(c); the design is targeted at the TE-polarization and coupling to standard single-mode fiber. The parameters of the first 11 grating periods are listed in the schematic; the last 5 periods are identical to the period on the far left. We chose a relatively large coupling angle of 21° to slightly enhance the bandwidth and coupling efficiency. In general, the bandwidth of a grating coupler increases weakly with the coupling angle . Also, for the 2 μm buried-oxide (BOX) thickness in the Si3N4-on-SOI platform, reflections from the substrate are in phase with the grating coupler’s upward radiation at a coupling angle of 21°. The efficiency of the grating coupler is optimized by: 1) improving the mode-matching to fiber via apodization, 2) choosing the offsets between the Si3N4 and Si teeth (L) to achieve a high directionality.
To obtain the apodized grating coupler parameters, we started with a uniform grating coupler with a period of 1.4 μm, wSi3N4 = 750 nm, wSi = 400 nm, and L = 250 nm, which we found to have a high directionality of 80%. Two extra Si teeth were included before the first Si3N4 tooth to provide a weak coupling strength at the beginning of the grating; the number of extra Si teeth was chosen to optimize the peak coupling efficiency of the uniform grating. The uniform grating coupler was apodized using two-dimensional finite-difference time-domain (2D-FDTD) simulations with the Si substrate included; fiber modes were launched toward the grating coupler and overlap integrals were calculated at the Si3N4 waveguide output. We performed exhaustive parameter sweeps on sets of two adjacent grating periods. Specifically, the period, wSi3N4, wSi, and L values were exhaustively swept for periods 1 and 2 and the parameters that maximized the peak coupling efficiency were applied to the grating; the process was repeated for periods 2 and 3, 3 and 4, etc. Figure 3(a) shows simulations of the coupling efficiency versus wavelength for the uniform and apodized grating couplers. The uniform design has a peak coupling efficiency of −1.8 dB and a 1-dB bandwidth of 114 nm, and the apodized design has a peak coupling efficiency of −1.0 dB and a 1-dB bandwidth of 82 nm. The 0.8 dB improvement in peak coupling efficiency via apodization is similar to Si grating coupler apodization results, however, a more complex apodization procedure using a genetic algorithm may yield improved performance . Also, the uniform design’s larger 1-dB bandwidth is due to the longer optical path lengths of the periods with the two extra Si teeth compared to the remaining periods; this difference in optical path lengths is removed in the apodized design.
The importance of L to the directionality, D, is evident from Fig. 3(b). D is defined as the fraction of radiated power from the grating directed toward the superstrate. In the figure, D is plotted against ΔL, the deviation in L from the apodized values, i.e., all the Si grating teeth are shifted by the same distance, ΔL, on the z-axis in Fig. 2(c). These simulation results were obtained using the 2D-FDTD method without the Si substrate; light was launched into the Si3N4 waveguide and scattered by the grating coupler. High directionalities (> 80%) are achieved when the Si teeth are pushed ahead of the Si3N4 teeth on the z-axis (ΔL ≈ 0). The directionality remains high for ΔL ≈ ±50 nm. For |ΔL| > 150 nm, the directionality degrades significantly, and this degradation is larger when ΔL < 0. ΔL = −50 nm is a more optimal point in terms of directionality, but ΔL = 0 nm was used for the final design because it provided a slightly higher peak coupling efficiency due to the substrate reflections and the mode-matching to fiber.
Alignment error between the Si3N4 and Si grating teeth will deteriorate the grating coupler performance. Alignment along the propagation axis (z-axis) of the grating (i.e., ΔL) is the most critical. CMOS fabrication processes allow alignment accuracy better than ±50 nm, and Fig. 4 shows the simulated spectra of the apodized grating coupler efficiency for ΔL = ±50 nm. The reduction in peak coupling efficiency is only 0.2 dB, and the 1-dB bandwidth grows to 94 nm for a +50 nm error and shrinks to 72 nm for a −50 nm error; the center wavelength is not significantly altered. Overall, the Si3N4-on-SOI grating coupler design can withstand ±50 nm of alignment error with only marginal performance degradations. Alignment errors perpendicular to the propagation axis (x-axis) of the grating are only relevant for focusing grating coupler designs, and for the focusing design in this work, misalignment on the x-axis has little effect on the performance. From 3D-FDTD simulations, alignment errors of ±100 nm on the x-axis reduce the peak coupling efficiency and 1-dB bandwidth by < 0.1 dB and < 1 nm, respectively.
The final step in the grating coupler design was curving the grating teeth to obtain a focusing grating coupler with a compact footprint. Following the design procedure in , we curved the grating teeth into confocal ellipses with a minimum grating order of 20. Si3N4 and Si grating teeth from the same period followed the same elliptical shape along their center-lines but had different tooth widths and the offset, L, applied to the Si tooth. The overall design is shown in the optical microscope image of the fabricated grating coupler [Fig. 5(a)] in the next subsection. From 3D-FDTD simulations, we found that this grating coupler design focused incident light into a spot size larger than the 900 nm single-mode Si3N4 waveguides, and to eliminate loss from this effect, we included a two-stage taper after the elliptical grating teeth. The Si3N4 is rapidly tapered down from the fiber mode width to a width of 4.3 μm over a length of 21.5 μm, and then, the Si3N4 is tapered down to a width of 900 nm using a 40 μm long taper.
Lastly, the fiber alignment sensitivity of our grating coupler is similar to that of Si grating couplers. This is expected since the grating’s radiation is well mode-matched to standard single-mode fiber and the alignment sensitivity is set by the profiles of the fiber mode and the grating’s radiation. We used FDTD simulations to verify that the 1-dB alignment sensitivity is ≈ 2 μm.
2.2. Experimental results
Grating couplers were fabricated in the Si3N4-on-SOI photonics platform in Fig. 2(b) at IME, A*STAR. The fabrication process is described in [21, 23]. An optical microscope image of a fabricated, apodized, focusing, Si3N4-on-SOI grating coupler is shown in Fig. 5(a). The grating coupler footprint is 27 μm × 87 μm, and the grating coupler connects to a 900 nm wide, single-mode, Si3N4 routing waveguide. Scanning electron microscope (SEM) images of the Si and Si3N4 grating teeth during fabrication are shown in Figs. 5(b) and 5(c).
The grating coupler measurements were performed on a calibration structure consisting of two nominally identical grating couplers connected by a U-shaped, 900 nm wide, 351 μm long, Si3N4 waveguide. The grating couplers were on a 250 μm pitch, and the short length of routing waveguide connecting the grating couplers was not normalized out of the measurement data. Light from a tunable laser was TE-polarized and coupled on/off the chip using a standard single-mode fiber array that was polished and tilted at 21°. Index matching fluid was applied to the chip to reduce reflections at the fiber-to-chip interface. The coupling efficiency versus wavelength of a single grating coupler was obtained by taking the square root of the raw transmission spectrum data of the two-grating-coupler structure on a linear-scale.
Figure 6 shows the measured coupling efficiency versus wavelength for the Si3N4-on-SOI grating coupler. The peak coupling efficiency was −1.3 dB (74%) at a wavelength of 1536 nm, and the 1-dB bandwidth was 80 nm. A 2D-FDTD simulation of the coupling efficiency versus wavelength is also shown in Fig. 6, and it closely agrees with the measurement data; the simulated peak coupling efficiency and 1-dB bandwidth are −1.0 dB and 82 nm, respectively. The ripples in the measured coupling efficiency versus wavelength plot were Fabry-Perot oscillations due to reflections from the grating couplers and the end of the fiber array. By assuming all the reflections were due to on-chip back-reflections from the grating couplers, we calculate the worst-case, on-chip reflectivity of a single grating coupler to be −16 dB over the 1-dB bandwidth . The reflectivity could be improved significantly by applying the design strategy in [24, 25], where the elliptical grating teeth are modified so that the on-chip reflections are directed away from the aperture of the focusing grating coupler.
3. Integration example: 1 × 4 tunable multiplexer/demultiplexer
To demonstrate integration of the Si3N4-on-SOI grating couplers on an integrated optics platform, we fabricated and measured the 1 × 4 tunable multiplexer/demultiplexer shown in Fig. 7(a) on the platform described in Fig. 2(b). The photonic integrated circuit (PIC) consists of four add-drop Si microrings coupled to a Si bus waveguide, which is connected to grating couplers at the input and output (“GCin” and “GCthru,” respectively). Each microring has an independent TiN thin film heater, and the drop port of each microring is connected to a grating coupler. The microrings are labeled as “Ring 1” to “Ring 4,” and the drop port grating couplers are labeled as “GC1” to “GC4.” The PIC uses the microring filters to demultiplex a multi-wavelength input at GCin into single-wavelength outputs at GC1 to GC4. The PIC is also capable of multiplexing inputs at GC1 to GC4 into an output at GCthru. Overall, the PIC uses all the levels in the platform, i.e., the Si3N4 and partially-etched Si for the grating couplers, the fully-etched Si for microrings and bus waveguides, and the TiN and contact metals for thin film heaters.
All the microrings in the PIC are nominally identical, and a schematic of the microring design is shown in Fig. 7(b). The Si waveguides are 500 nm wide and fully-etched. The microrings use 7.5 μm radius bends, and the through (“thru”) and drop port couplers are nominally identical and consist of 2.5 μm long straight coupling regions with 230 nm wide coupling gaps. The Si waveguides connect to grating couplers via adiabatic transitions from the Si to Si3N4 levels. Over a length of 15 μm, the Si narrows down from a width of 500 nm to a 180 nm wide blunt tip while the Si3N4 begins with a blunt 200 nm tip and widens to a 900 nm width.
The PIC was measured in the demultiplexer mode of operation. Light from a tunable laser was input into GCin and the transmission spectra at GCthru and GC1 to GC4 were measured. The input laser light was TE-polarized and coupled on/off the chip via a fiber array. We electrically probed the thermal tuners using a multi-contact wedge, and this prevented us from applying index matching fluid to the chip.
The fiber-to-fiber transmission measurement data is shown in Fig. 8. Figure 8(a) shows the thru port transmission spectra, over a wavelength range from 1510 to 1550 nm, before and after the microrings were thermally tuned, i.e., transmission from GCin to GCthru. Three of the four microrings were tuned, and the tuning powers for each microring were < 35 mW. The free-spectral range of the microrings was about 11.4 nm. Before thermal tuning, the microring resonances were unevenly spread due to fabrication variations in the waveguide dimensions. After thermal tuning, the resonances were evenly distributed with a channel spacing of about 2.5 nm and extinction ratios > 15 dB. The thru port spectrum had a peak transmission of about −5 dB, and we estimate the insertion loss can be broken down into about 1.5 – 2 dB per grating coupler and about 1 – 2 dB due to the Si3N4 to Si adiabatic transitions, microring directional couplers, and waveguide losses. Our loss estimate of the grating couplers is larger than the measurements in Section 2 for two reasons: 1) no index matching fluid was used, 2) the alignment accuracy of our measurement apparatus was worse since the fiber array and electrical probes simultaneously contacted the chip.
Figure 8(b) shows the drop port transmission spectra of the PIC with the microrings thermally tuned, i.e., transmission from GCin to GC1 – GC4. Over a wavelength range between 1510 and 1550 nm, the maximum transmission values of the drop port resonances ranged from −5.3 to −6.8 dB, and the 3-dB bandwidths and loaded quality factors of the resonances were about 0.5 nm and 3100, respectively. The variation in the maximum transmission values was due to the wavelength variation of the microring couplers, the Fabry-Perot oscillations from the grating coupler reflections, and the 80 nm 1-dB bandwidth per grating coupler. From Fig. 1, if the Si3N4-on-SOI grating couplers were replaced with Si-only grating couplers, the variation in the maximum transmission values of the drop port resonances would increase by about 1.5 dB or more over the 40 nm wavelength measurement range. The 40 nm wavelength range corresponds roughly to the 0.5-dB bandwidth for transmission through two Si3N4-on-SOI grating couplers and to the 2-dB or 3-dB bandwidth for transmission through two Si grating couplers.
In summary, we have proposed and demonstrated a high-efficiency, wide bandwidth grating coupler using aligned Si3N4 and Si teeth. The grating coupler uses the Si3N4 to achieve a wide bandwidth and the Si for a high directionality. The grating coupler can be integrated in Si3N4-on-SOI integrated optics platforms, and we demonstrated this by fabricating and measuring a thermally-tunable multiplexer/demultiplexer PIC that uses the grating couplers as well as independent waveguides in Si3N4 and Si. Our design approach of using moderate and high refractive index materials to produce high-performance grating couplers can be applied to other material systems such as Si3N4 on III–V semiconductors and aluminum nitride on SOI.
The financial support of the Natural Sciences and Engineering Research Council of Canada and the Canada Research Chairs program is gratefully acknowledged.
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