Mid-infrared (mid-IR) (2–20 µm) silicon photonics has attracted much attention in the past few years [1–6]. Since the mid-IR spectral range covers three atmospheric transparency windows [7,8] and rich characteristic absorption spectra resulting from fundamental vibrational transitions of molecules [9,10], it has enormous applications in free-space optical communications, light detection and ranging (lidar), and molecular analysis. As a CMOS-compatible platform to develop mid-IR applications on a chip, silicon photonics has exhibited several advantages. First, in the long wavelength region, linear optical losses introduced by the waveguide sidewall roughness could be greatly eliminated due to the reduced Rayleigh scattering [11]. Second, nonlinear optical losses, namely, the two-photon absorption (TPA) loss, could be significantly eliminated at wavelengths beyond 2.2 µm [12,13]. Moreover, it is feasible to develop high-volume, high-reproducibility, and low-cost mid-IR silicon photonic integrated circuits (PICs) based on the multi-project wafer (MPW) foundry services [14]. Consequently, mid-IR silicon photonics has been widely explored for nonlinear optics [15,16], gas sensing [17,18], optical interconnects [19,20], and communications [21,22].

To couple mid-IR light to silicon waveguides, grating coupler techniques are essential approaches. Compared with the edge-coupling methods [23], the grating coupler technique offers the merits of no need for mid-IR lens fibers, small device footprints, and flexible light coupling locations on a chip. To date, many efforts have been made in developing mid-IR grating couplers, namely shallowly etched grating couplers [24], subwavelength grating (SWG) couplers [25,26], and ultra-thin SWG couplers [27], providing various strategies to achieve mid-IR light coupling with high efficiencies, low reflections, and wide bandwidths. It is worthwhile to note that due to the intrinsic fragile nature of chalcogenide or ZBLAN materials, it is challenging to develop chalcogenide-fiber or ZBLAN-fiber-based polarization controllers, introducing the difficulty of using the grating couplers in long-wavelength regions. As a result, it is significant to explore polarization-insensitive grating coupling techniques in mid-IR wavelengths. However, such study is seldom reported to the best of our knowledge.

In this paper, we demonstrate an ultra-thin mid-IR polarization-insensitive grating coupler based on an MPW service followed by a silica cladding and buried oxide (BOX) removal process. We first demonstrate the grating coupler with a maximum coupling efficiency of −11.5 dB and a polarization-dependent loss of 2.12 dB at a center wavelength of ∼2200 nm by using Ge-doped silica-core optical fibers coupling. Then, we demonstrate that the proposed grating coupler has a maximum coupling efficiency of −12 dB at a wavelength of 2714 nm by using ZBLAN fibers coupling. Our study opens an avenue to the development of silicon PICs for on-chip mid-IR applications.

A schematic of the device is shown in Fig. 1(a). The device was designed based on a silicon-on-insulator (SOI) wafer. A two-dimensional (2D) polarization-splitting grating coupler separates arbitrarily polarized light from an optical fiber into TE₀-mode polarized light propagating in two directions on the chip. The fiber is tilted along the bisection line of the 2D grating coupler. Here, θ is the angle between the fiber axis and the axis perpendicular to the chip plane, known as the coupling angle. Figure 1(b) shows the top view of the grating coupler, where the period and fill factor parameters are indicated as $\mathrm{\Lambda}$ and FF. First, 1D focusing grating lines were obtained based on the following equation [28,29]:

 

Fig. 1. Schematic of the ultra-thin mid-IR polarization-insensitive grating coupler and SWG-cladding waveguides. (a) 3D view of the device. The insets show the simulated electric-field distributions of the TE₀ mode in the waveguide and grating coupler. (b) Top view of the grating coupler. (c) Cross section view of the waveguide.

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First, the focal point of the 1D focusing grating was set at the center of the bottom line of the taper. Then, the 2D focusing grating coupler was formed by the superposition of two orthogonal focusing grating couplers. Here, an ultra-thin grating coupler design was used to reduce the backreflection of the grating coupler [27]. In addition, we employed a suspended SWG-cladding waveguide, as shown in Fig. 1(c). The BOX underneath the devices was designed to be removed to reduce the silica absorption of the mid-IR light.

We designed the ultra-thin mid-IR polarization-insensitive grating coupler by using the 3D finite-difference time-domain (FDTD) simulation tool. First, we studied the grating directionality and backreflection as a function of the top-layer silicon thickness, as shown in Fig. 2(a). The simulation shows that the directionality increases from 51.3% to 74.6% as the top-layer silicon thickness changes from 220 nm (h₁) to 70 nm (h₂); meanwhile, the backreflection reduces from −12.0 dB to −29.2 dB. Therefore, we chose 70 nm as the thickness of the top silicon layer for achieving a high directionality and low backreflection. A thickness of 70 nm would be compatible with the device fabrication processes of standard silicon photonics foundries. Then, we optimized the coupling efficiency by changing the fill factor (FF). As shown in Fig. 2(b), the simulation shows that the grating coupler with FF = 0.6 has a peak coupling efficiency of −6.6 dB with a minimum low backreflection of −23.0 dB with an incident angle of 14°. Appropriately increasing the top-layer silicon thickness could further improve the coupling efficiency. It is worthwhile to note that, due to the low RI of the ultra-thin silicon membrane, the designed grating coupler exhibits a wide coupling spectral bandwidth and large tolerance to the light incident angle, as shown in Fig. 2(c). When θ is set as 14°, the grating coupler has the best coupling efficiency at the center wavelength of ∼2400 nm. Finally, we simulated the polarization-dependent coupling efficiency of the ultra-thin mid-IR polarization-insensitive grating coupler and a TE₀-mode ordinary SWG coupler, as shown in Fig. 2(d). In this design, a period of 2280 nm and FF of 0.5 have been set in the grating direction, while a period of 600 nm and FF of 0.5 have been set in the SWG structure direction. The simulation result shows that the polarization-dependent loss of the ultra-thin mid-IR polarization-insensitive grating coupler is 0.5 dB when the polarization of the incident light is swept through 90°, which is much lower than that of the ordinary SWG coupler of (22.9 dB). Here, the best coupling efficiency occurs with 45° linearly polarized light due to the focusing grating design.

 

Fig. 2. Design of the ultra-thin mid-IR polarization-insensitive grating coupler based on the 3D FDTD simulator. (a) Directionality and backreflection of the grating coupler with different top-layer silicon thicknesses. (b) Coupling efficiency and backreflection of the grating coupler with different fill factors. (c) Coupling efficiency of the grating coupler with different incident angles. (d) Dependence of the peak coupling efficiency on the polarization state of the incident light.

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We fabricated the devices on an SOI wafer based on an MPW foundry service. A 220-nm-thick top silicon layer was first etched to leave a 70-nm-thick top silicon layer. Then, the ultra-thin mid-IR polarization-insensitive grating couplers and suspended SWG-cladding waveguides were fabricated on the 70-nm-thick top silicon layer. Finally, the silica cladding and BOX were removed by immersing the silicon chip in hydrogen fluoride (HF) acid solution. A scanning electron microscopy (SEM) image of the fabricated device is shown in Fig. 3(a). A zoom-in view of the ultra-thin mid-IR polarization-insensitive grating couplers is shown in Fig. 3(b). There is a clear trace around the edge of the grating coupler resulting from the HF acid wet etching process.

 

Fig. 3. SEM images of the ultra-thin mid-IR polarization-insensitive grating coupler and SWG-cladding waveguides. (a) Top view of the whole device. (b) Zoom-in view of the grating coupler.

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In the experiment, we measured the fabricated ultra-thin mid-IR polarization-insensitive grating coupler and ordinary SWG coupler, as shown in Fig. 4. To characterize the fabricated devices, we used a continuous wave single-frequency tunable mid-IR laser (IPG CLT-2250/500-1-SF) and a power meter (Thorlabs S148C) controlled by using a Labview program. We used Ge-doped-silica-core optical fibers with a mode field diameter of ∼13 µm to couple light into/out of the suspended SWG-cladding waveguide in the wavelength range of 2.05–2.35 µm. Meanwhile, a fiber polarization controller was used to control the polarization state of the input light. The laser is incident from the fiber at three different incident angles, namely, ∼12°, ∼14°, and ∼16°. The experiment result illustrates that the ultra-thin mid-IR polarization-insensitive grating coupler exhibits a peak coupling efficiency of −11.5 dB at the center wavelength of ∼2200 nm with a 1-dB bandwidth of ∼148 nm, as shown in Fig. 4(a). Compared with the simulation result in Fig. 2(c), the coupling efficiency of the grating coupler is moderated while the center wavelength has a blueshift. The measurement results should be mainly attributed to the variation of the top-layer silicon thicknesses due to the device fabrication errors. The experimental results agree well with the simulation of the 50-nm-thick polarization-insensitive grating coupler, as shown in Fig. 4(a). Then, we measured polarization-dependent losses induced by the ultra-thin mid-IR polarization-insensitive grating coupler and the ordinary SWG coupler. We adjusted the polarization state of the light coupled out of the optical fiber by rotating the polarization controller. Here, the polarization states were estimated by calibrating the variation of the coupling efficiency of the ordinary SWG coupler. The experimental results show that the ultra-thin mid-IR polarization-insensitive grating coupler exhibits a polarization-dependent loss of 2.1 dB, which is much better than that of the ordinary SWG coupler (9.6 dB). The measurement has a similar trend to the simulation results in Fig. 2(d). We also measured the straight suspended SWG-cladding waveguides with different lengths to estimate the optical loss. The experiment result illustrates that the suspended SWG-cladding waveguide exhibits an optical loss of 25.5 dB/cm, which could be caused by the surface roughness of the ultra-thin waveguide induced by the silicon etching processing.

 

Fig. 4. Performance of the grating coupler at the center wavelength of ∼2200 nm. (a) Simulation and experimental results of the coupling efficiency of the grating coupler with different incident angles. (b) Dependence of the peak coupling efficiency on the polarization state of the incident light.

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Finally, based on the same principle, we fabricated and characterized the ultra-thin mid-IR polarization-insensitive grating coupler at 2700-nm wavelength to verify the feasibility of the designed grating coupler in the long wavelength region. Here we used a mid-IR Er³⁺-doped ZBLAN fiber laser (Thorlabs LFL2700) with a center wavelength of 2714 nm and single-mode ZrF₄ fibers (Thorlabs P3-23Z-FC-2) for mid-IR light coupling. The simulation and measurement results of the coupling efficiency of the grating coupler with five different periods, namely, 2590 nm, 2710 nm, 2830 nm, 2950 nm, and 3070 nm, are shown in Fig. 5(a). Similar to the grating coupler results in the wavelength range of 2.05–2.35 µm, the measured coupling efficiency of the grating coupler is reduced compared with the simulation. In our experiment, a maximum coupling efficiency of –12.0 dB was measured. Also, we measured the coupling efficiency of the grating coupler with the period of 2830 nm under different angles of the incident light from ∼11° to ∼16°, as shown in Fig. 5(b). The experiment results show that the light coupling angle of ∼14° provides the maximum coupling efficiency.

 

Fig. 5. Performance of the ultra-thin mid-IR polarization-insensitive grating coupler at the wavelength of 2714 nm. (a) Simulation and measurement results of the coupling efficiency of the grating coupler with different periods. (b) Coupling efficiency of the grating coupler with different angles of the incident light.

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In conclusion, we reported the mid-IR polarization-insensitive grating coupler based on the MPW foundry service. We experimentally demonstrated the maximum coupling efficiency of −11.5 dB with the polarization-dependence of 2.1 dB at the center wavelength of ∼2200 nm. Moreover, based on the same method, we designed and demonstrated the ultra-thin mid-IR polarization-insensitive grating coupler with a maximum coupling efficiency of −12.0 dB at the center wavelength of 2714 nm. Our study paves the way toward developing silicon PICs for mid-IR applications.