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Abstract
A compact, high-efficiency grating coupler is demonstrated for interfacing a silicon waveguide and a perfectly-vertical fiber at O-band. The grating lies on a tilted silicon membrane for minimizing the reflections. Circular grating lines are adopted to shorten the overall device length to about 60μm. 57% peak coupling efficiency and >28nm 1-dB coupling bandwidth are obtained experimentally. Back reflections of 1% to the silicon waveguide and the single mode fiber are theoretically estimated. The processing flow to realize the proposed structure is discussed in detail. The fabrication control over the tilted angle of the silicon membrane is investigated. The approach by applying an oxide cladding to improve the stability of the membrane is also introduced. The present grating coupler is compatible to common fabrication processes for silicon photonic chips.
© 2017 Optical Society of America
1. Introduction
Grating couplers have been widely used for interfacing photonic integrated circuits based on silicon-on-insulator (SOI) platform with other discrete optical components, such as optical fibers [1,2], lasers [3–6], and photo-detectors [7]. Conventionally, these grating coupler structures utilize a second-order design, where light is coupled out from the SOI waveguide vertically. However, in order to avoid a large amount of first-order back reflection, the direction of the light output is normally slightly off the perfectly vertical direction, i.e., oblique with respect to the chip substrate. This brings some difficulties in, e.g., testing and packaging, especially when integrating with lasers [4, 5]. Grating couplers on SOI working for the perfectly vertical direction have been pursued for many years [8–10]. Recently, a high-performance perfectly-vertical grating coupler has been demonstrated [10]. The design is based on a tilted silicon membrane structure, which facilitates a low back-refection.
In this paper, a perfectly-vertical grating coupler on silicon working at O-band is demonstrated. O-band devices become more important for short range optical interconnect, such as in data-centers, due to more robust laser sources as compared to C-band ones. Conventional obliquely-incident grating couplers at O-band have been introduced [11, 12]. Here, structural parameters of a perfectly-vertical grating coupler are optimized. Circular grating lines are also employed, which forms a focusing grating design and decreases significantly the footprint of the whole structure by avoiding long tapers. 57% peak coupling efficiency to a single-mode fiber is obtained experimentally. Back reflections of 1% to the SOI waveguide and the single mode fiber are theoretically estimated. The present grating coupler is also compatible with common post-fabrication processes for SOI photonic chips.
2. Design
The structure of the present grating coupler is sketched in Fig. 1(a). The grating is located on a tilted silicon membrane waveguide, which exhibits a certain angle with respect to the horizontal plane (the substrate surface). Effectively, the light from a single-mode fiber is obliquely coupled to the silicon waveguide, provided that the fiber is perfectly vertical to the substrate. Here, the grating teeth is designed to be extruded from the waveguide layer with a height h of 200nm. The waveguide layer has a thickness t of 140nm, which is within the single mode region for O-band light and also rigid enough to keep the tilted membrane from breaking. The rest of the structural parameters are grating period p = 454nm, grating groove width g = 0.68p = 309nm, and the thickness of the buried oxide b = 2μm. The tilted angle θ of the membrane is 11°, which is concluded from some test fabrication runs (see Sec. 3). The simulated performance of the above optimized structure in two dimensions (2D) is shown in Fig. 1(b). We used a commercial finite-difference time-domain solver to perform the simulations here [13], as well as those in three dimensions (3D) below. One can find that over 84% of the input light from the SOI waveguide is diffracted upward towards the fiber by the grating. This is due to a careful tuning of the height of the grating teeth, which enables a constructive interference for the scattered light towards the fiber while a destructive interference for that towards the substrate [1]. The upward diffracted light field has ~81% overlap with the mode of a standard SMF-28 fiber at 1.31μm wavelength, which gives a fiber-coupled efficiency of 68.7% at peak. This field overlap can be improved by using a non-uniform grating design [1], which would further increase the coupling efficiency. The 1-dB coupling bandwidth is 33nm. The back reflections are both about 1% when light is incident from both the waveguide and the fiber, due to the fact that the direction of the diffracted light here is eventually oblique to the grating surface and the first-order reflection is avoided [10].
Fig. 1 (a) Schematic structure of the present grating coupler. (b) Simulated performance, including the coupling efficiency between the fundamental transverse-electric mode in the SOI waveguide and the fiber, the upward diffracted power when light is incident from the waveguide, and the reflections to the input modes when light is incident from the waveguide (w-w ref.) or the fiber (f-f ref.), of an optimized grating couple with t = 140nm, b = 2μm, p = 454nm, g = 0.68p, h = 200nm, and θ = 11°.
In order to further decrease the footprint of the present grating coupler, a focusing grating design is then investigated. The 3D structure is sketched in Fig. 2. Here, the grating lines are designed as concentric arcs with a radius r to the leading edge of the first grating line of 25μm. The period p and groove width w of the grating are the same as those in 2D discussed above. The width of the tilted membrane is 10μm, which is matched with the beam size of the fiber. This circular grating can focus the light within the SOI waveguide, forming a spot of about 2.8μm wide at a distance of 20μm to the first grating line, as shown in Fig. 3(a). An SOI rib waveguide of a matching width is therefore put at this position to collect the focused light, and a short taper of 30μm is also included to further decrease the width of the waveguide to 600nm within the single-mode region. The simulated performance in 3D of this configuration is also shown in Fig. 3. One can find that the peak coupling efficiency is 66.2% which is slightly lower than the 2D result. The 1-dB bandwidth and the reflections are all similar to those in the 2D case. With the help of the circular grating lines, the total length of the device is decrease to about 60μm from over 200μm in the original non-focusing design [10].
Fig. 3 Simulated performance of the present grating coupler in 3D. (a) Time snap-shot of the Ex field propagation in the structure at 1.31μm wavelength. (b) Coupling efficiency and reflection spectra. The model is shown in Fig. 2.
3. Fabrication and characterization
The fabrication process of the proposed grating coupler is sketched in Fig. 4. We start with an SOI wafer of 340nm top silicon and 2μm buried oxide layers. First, the grating lines are defined and shallowly dry-etched for 200nm (height of the grating teeth). Then, the waveguide structure is defined and the rest of the top silicon layer, i.e., 140nm, is fully dry-etched. The third lithography step is subsequently followed to define a photoresist opening on top of the grating area. The whole chip is then dipped into a hydro-fluoric (HF) solution to wet-etch the silicon oxide layer around and beneath the grating structure. It is crucial to have the edge of the photoresist opening sit on the silicon layer, since it is more stable during the wet-etching. A photoresist-oxide interface, on the other hand, can easily peer off in HF solution, which would unprotect the rest of the circuit other than the grating area. Finally, the photoresist is removed and the whole chip is dried on a hot plate. The suspended grating structure collapses and its end sticks to the exposed silicon substrate. This results in a tilted membrane structure.
Prior to the fabrication of the real grating coupler structure, some test runs were conducted in order to find out the tilted angle θ of an 140nm thick and 10μm wide silicon membrane fixed at one end. We read the length of the suspended part of the collapsed membrane, and use a simple trigonometric relation, taking into account the 2μm thickness of the buried oxide layer, to extract the tilted angle θ. The statistics of θ on a fabricated sample is shown in Fig. 5(a). The experimental data is also fitted using normal distribution, which exhibits a mean of 11.3° (11° was used in the theoretical analyses above) and a standard deviation σ of 0.183°. In order to show how the fabrication uncertainty in the tilted angle affects the performance of the proposed grating coupler, we further investigate the coupling efficiency at different θ while keeping other parameters the same as those in Fig. 1(b). As shown in Fig. 5(b), the coupling efficiency spectra red-shifts when θ increases. The shift rate of the peak coupling wavelength is about 6nm/o. According to the statistical data in Fig. 5(a), a 3-σ boundary would lead to a variation of 3 × σ × 6nm/o = 3.3nm. Such a spectra shift is much smaller, by an order of magnitude, than the 1-dB coupling bandwidth of the grating coupler (33nm as discussed above). This indicates that the proposed processing flow can well support the fabrication of the present grating coupler without spoiling the yield. The performance of the present grating coupler responds to variations of other structural parameters are also illustrated in Fig. 5(b). The peak coupling wavelength shifts at 2.1nm/nm with respect to the changes in the period p. Using a silicon oxide as cladding, instead of air, the peak coupling wavelength red-shifts for about 50nm to 1.36μm, and the peak coupling efficiency decreases slightly.
Fig. 5 (a) Statistics of the tilted angle θ of 140nm thick and 10μm wide silicon membranes fabricated on a test sample. Probability density function (PDF) of a fit using normal distribution is also included. (b) Coupling efficiency spectra for the present coupler in 2D with different structural parameters. The rest of the parameters are the same as those in Fig. 2
The designed grating structure was fully fabricated using conventional electron-beam lithography, photo-lithography, dry-etching, and wet-etching processes. Figure 6 shows some pictures of a finished sample. Two gratings with the same parameters are connected with a 0.9mm-long straight waveguide. The fabricated structures were then characterized using two cleaved single-mode fibers, a broadband super-luminescent light-emitting diode, and an optical spectrum analyzer. The measured spectrum responses were further normalized using that from a direct fiber-to-fiber configuration, and the results are shown in Fig. 7(a). One can find that the peak coupling efficiency reaches 57%, slightly less than what is predicted in simulation. However, the peak coupling wavelength moves significantly to 1.286μm. This is probably due to a not-well-controlled etching for defining the grating teeth. The etching depth was slightly more than the nominal 200nm. This could be solved by using an in situ end-point detector for the dry etching process. Nevertheless, as suggested in Fig. 5(b), this wavelength shift can be compensated by, e.g., increasing the grating period. For a grating with p = 474nm, the peak coupling wavelength is at 1.327μm. The relative wavelength shift is matched well with the simulation prediction (2.1nm/nm). The measured 1-dB coupling bandwidth is about 28nm‒39nm, which is also matched with the simulation.
Fig. 6 (a) Optical microscope picture of the waveguide structure for testing the present grating coupler. (b) Zoom-in view of the grating coupler area. The gratings are slightly out of focus due to tilting. (c) Scan-electron microscope pictures of the grating.
Fig. 7 (a) Measured coupling efficiency spectra for fabricated grating couplers with two different grating periods. (b) Measured coupling efficiency spectra before and after applying the SiO2 cladding. The inset shows a picture of the oxide-covered grating. Here, p = 454nm.
Since the definition of silicon patterns lies in the front-end of the whole processing flow for an SOI photonic chip, the thin suspended silicon membrane in the proposed grating coupler could be vulnerable to the harsh conditions in the process steps which would follow, such as, passivation layer deposition, lithography, metallization, annealing, and packaging. In order to protect the present membrane structure, the aforementioned sample was further covered with a thin spin-on-glass layer and then a thick oxide layer through plasma enhanced chemical vapor deposition. The total thickness of the oxide cladding is about 1μm. The measured responses before and after the oxide cladding are shown in Fig. 7(b). The peaking coupling wavelength red-shifts for about 34nm, which is smaller than the simulation result, i.e., 50nm. This is mostly likely due to incomplete filling of the air region beneath the suspended membrane by oxide. Nevertheless, one can find that the coupling efficiency and bandwidth were not degraded much by the deposition process. The oxide cladding serves not only a passivation layer for the rest of the processing, but also greatly increases the stability of the present structure. We tried several iterations of photoresist spin-coating, acetone cleaning, and annealing at 400°C. No breaking of the membranes was observed. This proves that the present grating coupler structure is compatible with the conventional fabrication processes of SOI photonic chips.
4. Conclusion
A high-efficiency grating coupler on silicon working for a perfectly vertical fiber has been demonstrated based on a tilted silicon membrane. Circular grating lines are adopted, which forms a focusing grating design. Simulations in 3D have shown that the designed grating coupler exhibits a peaking coupling efficiency of 66.2% and a 1-dB coupling bandwidth of 33nm. The back reflections to the SOI waveguide and the fiber are both small, about 1%. The fabrication processing flow for the present structure has been introduced. The tilted angle of the silicon membrane is well-controlled during the fabrication. Experimentally, a peaking coupling efficiency of 57% and a 1-dB coupling bandwidth of 28nm-39nm have been achieved. The present structure has been made more stable by applying a silicon oxide cladding without compromising the performance. Due to the process compatibility, the proposed grating coupler can be readily adopted in the fabrication flow for SOI photonic chips.
Funding
National Natural Science Foundation of China (NSFC) (61675069); Guangzhou Science and Technology Program (201707010444).
References and links
1. G. Roelkens, D. Van Thourhout, and R. Baets, “High efficiency Silicon-on-Insulator grating coupler based on a poly-Silicon overlay,” Opt. Express 14(24), 11622–11630 (2006). [CrossRef] [PubMed]
2. L. Liu, M. Pu, K. Yvind, and J. M. Hvam, “High-Efficiency, Large-Bandwidth Silicon-on-Insulator Grating Coupler based on a Fully-Etched Photonic Crystal Structure,” Appl. Phys. Lett. 96(5), 051126 (2010). [CrossRef] [PubMed]
3. K. S. Kaur, A. Z. Subramanian, P. Cardile, R. Verplancke, J. Van Kerrebrouck, S. Spiga, R. Meyer, J. Bauwelinck, R. Baets, and G. Van Steenberge, “Flip-chip assembly of VCSELs to silicon grating couplers via laser fabricated SU8 prisms,” Opt. Express 23(22), 28264–28270 (2015). [CrossRef] [PubMed]
4. Y. Wang, S. S. Djordjecvic, J. Yao, J. E. Cunningham, X. Zheng, A. V. Krishnamoorthy, M. Muller, M.-C. Amann, R. Bojko, N. A. F. Jaeger, and L. Chrostowski, “Vertical-cavity surface-emitting laser flip-chip bonding to silicon photonics chip,” in IEEE Optical Interconnects Conference (2015), pp. 122–123.
5. H. Lu, J. S. Lee, Y. Zhao, C. Scarcella, P. Cardile, A. Daly, M. Ortsiefer, L. Carroll, and P. O’Brien, “Flip-chip integration of tilted VCSELs onto a silicon photonic integrated circuit,” Opt. Express 24(15), 16258–16266 (2016). [CrossRef] [PubMed]
6. Y. Yang, G. Djogo, M. Haque, P. R. Herman, and J. K. S. Poon, “Integration of an O-band VCSEL on silicon photonics with polarization maintenance and waveguide coupling,” Opt. Express 25(5), 5758–5771 (2017). [CrossRef] [PubMed]
7. H. Li, Y. Liu, M. Zhang, W. Zhou, C. Zhang, E. Li, C. Miao, and C. Tang, “Highly efficient polarization-independent grating coupler used in silica-based hybrid photodetector integration,” Opt. Eng. 53(5), 057105 (2014). [CrossRef]
8. X. Chen, C. Li, and H. K. Tsang, “Fabrication-tolerant waveguide chirped grating coupler for coupling to a perfectly vertical optical fiber,” IEEE Photonics Technol. Lett. 20(23), 1914–1916 (2008). [CrossRef]
9. M. Dai, L. Ma, Y. Xu, M. Lu, X. Liu, and Y. Chen, “Highly efficient and perfectly vertical chip-to-fiber dual-layer grating coupler,” Opt. Express 23(2), 1691–1698 (2015). [CrossRef] [PubMed]
10. L. Liu, J. Zhang, C. Zhang, S. Wang, C. Jin, Y. Chen, K. Chen, T. Xiang, and Y. Shi, “Silicon waveguide grating coupler for perfectly vertical fiber based on a tilted membrane structure,” Opt. Lett. 41(4), 820–823 (2016). [CrossRef] [PubMed]
11. G. Roelkens, D. Van Thourhout, and R. Baets, “Silicon-on-insulator ultra-compact duplexer based on a diffractive grating structure,” Opt. Express 15(16), 10091–10096 (2007). [CrossRef] [PubMed]
12. M. Streshinsky, R. Shi, A. Novack, R. T. P. Cher, A. E.-J. Lim, P. G.-Q. Lo, T. Baehr-Jones, and M. Hochberg, “A compact bi-wavelength polarization splitting grating coupler fabricated in a 220 nm SOI platform,” Opt. Express 21(25), 31019–31028 (2013). [CrossRef] [PubMed]
13. F. D. T. D. Solutions, Lumerical Solutions Inc., www.lumerical.com.
