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Abstract
We demonstrate a lateral, planar fiber-to-waveguide coupling strategy for photonic integrated circuits with diffraction grating couplers using angle-polished silica waveguide blocks fabricated with well-established planar lightwave circuit technologies. Compared to the conventional lateral coupling scheme with angle-polished fibers, the demonstrated scheme can significantly decrease the diverging distance between the reflective angle-polished facet and the grating couplers, and thereby maintains the overall coupling efficiency and alignment tolerances of the vertical coupling approach. The proposed method shows a small penalty in coupling efficiency (< 0.1 dB), and in-plane (out-of-plane) alignment tolerance for 1 dB excess loss is approximately 5 µm (9 µm).
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Silicon-based photonic integrated circuits (PICs) have a number of promising applications in optical communications [1], sensing [2], and computing [3]. High refractive index contrast between the silicon waveguide core (∼3.45) and the silica cladding (∼1.45) at near-infrared wavelengths results in highly-confined light propagation in the condensed core area on the order of 0.1 µm2 [4], which permits ultra-compact guided optical components while allowing more design flexibilities in photonic circuit layouts.
Although the sub-micrometer-scale silicon photonic waveguides significantly increase the PIC integration density, the size mismatch between the integrated silicon waveguides and silica-based optical fibers makes efficient and reliable optical coupling challenging. Diffraction grating-based coupling (also referred to as grating coupling or surface coupling) has been extensively studied not only to mitigate the mode size difference between the optical fibers and the integrated waveguides but also to support scalable optical interfaces from the surface-normal direction [5–10]. The grating coupling scheme has a couple of advantages in optical interfacing and packaging perspectives. It improves the design flexibility of the PIC layouts, allows wafer-scale device characterization without requiring post-processing such as chip dicing and polishing, and relaxes mechanical alignment tolerances whose typical range for 1 dB excess losses is ±2 µm [11]. This alignment margin is much larger than a typical 1-dB alignment tolerance of the conventional end-fire coupling scheme, which is on the order of ±0.5 µm [7]. The alignment tolerance of the end-fire coupling can be improved close to that of the grating coupling with additional waveguide taper structures made of low refractive index materials, such as SiN or SiON [12,13]. Despite these advantages, the diffraction grating-based coupling is inherently an out-of-plane approach, so a vertically tall packaging profile is required rather than a flat planar shape that is usually preferred for compact form factors. Such non-planar structures tend to occupy more volume and generally have issues of mechanical durability [14].
To take advantage of the out-of-plane grating coupling scheme while satisfying the planar packaging requirements, researchers have investigated the planar coupling method with grating-coupled PICs using ∼40 degree angle-polished fibers [15–17]. In this approach, the angle-polished standard single-mode fibers (SMFs) are placed on the PICs in parallel to its surface, and the polished fiber facet converts the light propagation direction from horizontal (surface-parallel) to vertical (out-of-plane) or vice versa through light reflection at the fiber’s angled end facet. However, the reflected light diverges quickly while passing through the fiber cladding region (approximately corresponding to the fiber radius of ∼60 µm), which may cause an additional insertion loss compared to conventional vertical fiber coupling with shorter fiber-to-grating distances (∼10 µm). For instance, the overall fiber-to-chip coupling loss becomes larger when employing the lateral coupling scheme with angle-polished fibers (3.25 dB) [16] than the typical case of vertical fiber coupling (1.5 dB) [18]. To deal with this drawback, it is possible to polish two surfaces of an optical fiber end [19]. However, the additional polishing process increases the manufacturing complexity and might be subject to chipping during the polishing fabrication.
In this paper, we suggest a simple planar packaging scheme using an intermediate silica waveguide block (SWB) with an angle-polished facet, which can solve the aforementioned problems by taking advantages of mature planar lightwave circuit (PLC) manufacturing technologies and thereby significantly decreasing the distance between the silica waveguides and grating couplers as well as the waveguide pitch. The optical fiber is first coupled to the silica waveguide in the intermediate SWB, whose mode shape is similar to the optical fiber mode, allowing highly efficient optical coupling between the fiber and SWB. The silica waveguide core can be placed close to the flat silica block surface (∼10 µm), and the incident angle to the grating coupler can be accurately defined by the facet polishing angle. Another important advantage of our approach is that one grating coupler design can accommodate both out-of-plane and surface-parallel fiber-to-PIC coupling schemes, namely a conventional vertically-placed angle-polished optical fiber and a horizontally-placed SWB, by optimizing the silica waveguide core dimension to the grating coupler.
2. Design of the intermediate silica waveguide block
2.1 Principle of operation
As described in Fig. 1, the angle-polished SWB is inserted between an array of single-mode optical fibers and a silicon-based PIC. For high-efficiency coupling between the optical fiber and SWB, the silica waveguide core dimension was designed to be 6 µm × 6 µm. The refractive indices for the silica core and cladding regions are 1.455 and 1.444, respectively. The optical coupling between a diffraction grating-based coupler in the PIC and a silica waveguide parallel to the PIC surface is realized by changing the direction of the light at the reflective polished facet of the SWB, from the surface-parallel direction to the desired coupling angle for the grating coupler, as schematically illustrated in the inset of Fig. 1(a). For a grating coupler with a radiation angle of θoffset in the air, light refraction at the SWB’s bottom surface should be considered to obtain the optimal polishing angle β. Here, depending on the grating coupler design and its orientation, θoffset may have a negative value as illustrated in the inset of Fig. 1(a).
Fig. 1. (a) Schematic of light coupling between single-mode optical fibers and a grating-coupled PIC using a 16-channel angle-polished silica waveguide block. Inset: Magnified view (not to scale) of a coupling between a silica waveguide block and a grating coupler. (b) Photograph of a 16-channel assembly of single-mode optical fibers, a V-groove fiber holder, and an angle-polished silica waveguide block. (c) Scanning electron micrograph (top view) of a grating coupler used in characterization.
A simple ray optics can be applied so as to determine the polishing angle of SWB (β = (90°-sin−1(nsilica−1sin θoffset))/2, where nsilica represents the refractive index of the SWB) to achieve the optimal incident angle to the grating coupler, θoffset. After determining the polishing angle, the total internal reflection (TIR) condition on the angle-polished facet should be considered to judge a necessity of an additional high-reflection (HR) coating on the polished end facet. The TIR condition is satisfied when the incident angle at the polished surface (90°-β) is greater than the critical angle (43.42°), and this corresponds to θoffset>-4.6°. When θoffset<-4.6°, TIR does not occur at the silica-air interface, and the additional HR coating on the angle-polished end facet is necessary. For the HR coating, we deposited a 50 nm-thick gold layer on the angle-polished facet.
2.2 Simulation results
A commercial finite-difference time-domain (FDTD) Maxwell’s equation solver (FDTD Solutions, Lumerical) was employed to simulate the overall coupling efficiencies as well as the translational alignment tolerances of the SWB with respect to the grating coupler. All simulation results were obtained with TE polarization, a minimum mesh size of 30×25×10 nm3, a total simulation volume of 50×30×35 µm3, and a minimum time step of 0.03 fs. Two-dimensional FDTD analysis was first employed for iterative design stages, while three-dimensional approach was used for verification and further analysis. A simple grating coupler, whose design is quite conventional as shown in Fig. 1(c), was employed in both simulations and measurements. The design parameters for the grating coupler, which operates at 1550 nm wavelength, were 10 µm for the width, 2 µm for the thickness of the buried oxide, 220 nm for the thickness of the silicon device layer, 620 nm for the grating period, 0.5 for the fill factor, and 80 nm for the grating shallow etching depth. The radiation angle of the grating coupler is θoffset∼10°. The maximum coupling efficiency with a conventional angled physical contact (APC) fiber is -3.57 dB.
The simulation results of the alignment tolerances are illustrated in Fig. 2. The coupling efficiency was calculated by obtaining the relative optical power in the silicon waveguide coupled in from the SWB using the mode overlap analysis. In each figure, the horizontal axis indicates the relative translational offsets from the optimum position where the highest achievable coupling efficiency between the SWB and the grating coupler is obtained. The vertical axis, denoted as power penalty, represents the additional decrease of the coupling efficiency when compared to the highest coupling efficiency. When the best coupling efficiency is achieved, the end of the angle-polished waveguide core in the SWB is slightly apart from the center of the silicon grating coupler in the x-direction (∼0.7 µm), while its y-position is exactly matched in the middle of the grating coupler due to the symmetry in the y-direction. The vertical gap between the SWB and the PIC is about 10 µm in the z-direction, and this is similar to the conventional vertical coupling case with an 8° angle-polished fiber.
Fig. 2. Simulated alignment tolerance along the (a) x-direction, (b) y-direction, and (c) z-direction.
The 1-dB alignment tolerance is calculated to be over 4.5 µm in the x-direction (parallel to the waveguide optical axis) and 3.5 µm in the y-direction (normal to the optical axis). Figure 2(c) shows the simulation results when the working distance of the SWB (the distance between the SWB’s bottom surface and the PIC’s top surface) is varied. The simulation results show some fluctuations due to the Fabry-Pérot interference between two partially reflective interfaces, the SWB’s bottom surface and the grating coupler. For 1-dB excess optical losses, the SWB’s alignment tolerance for the vertical direction is as large as 8 µm. In terms of SWB manufacturing tolerance and optimum coupling wavelength accuracy, according to our FDTD simulations, a polishing-angle change of 0.5° corresponds to a center wavelength shift within 10 nm (∼20 nm per degree) as it alters the incident angle to the grating coupler. The peak coupling efficiency change for the 0.5° polishing angle variation remains within only 0.5 dB.
3. Fabrication and characterization
The angle-polished SWBs suggested in this work were manufactured based on standard PLC fabrication processes, as schematically illustrated in Fig. 3. Although we used Ge-doped SiO2 to form the core of the silica waveguide as this process makes it easier to control the core refractive index [20], other methods can also be employed to fabricate the fiber-matched silica waveguides in the SWB. The core height and width can be modified to optimize the optical coupling performance for various grating coupler designs by changing the deposition thickness of the core layer and the photoresist patterning. The single-mode optical fibers mounted on V-groove array were attached to the flat polished facet of SWB using optical adhesive. The fabricated SWB assembly showed a low fiber-to-SWB coupling loss of <0.1 dB.
Fig. 3. Fabrication process of the angle-polished silica waveguide block. Additional gold coating might be required depending on the polishing angle and the desired incident angle to the grating coupler (θoffset<-4.6°).
The angle-polished SWB can significantly decrease the light-diverging distance from the waveguide core to the grating coupler. In our case, the distance from the waveguide core to the bottom surface of the silica block is ∼10 µm, which is similar to the conventional ∼8° angle-polished APC optical fiber case. Therefore, a same grating coupler design can be used with modest coupling efficiencies for both conventional angle-polished optical fiber coupling and surface-parallel silica block coupling.
The number of waveguide channels in the SWB is scalable, and the channel density can be improved by modifying the pitch of the silica waveguide array. For conventional fiber array-based coupling, the fiber cores are typically separated by at least 127 µm or more often 250 µm, mainly because of the fiber diameter itself (125 µm). However, when utilizing the intermediate SWB, the waveguide pitch at the grating coupler side (Pgc) can be significantly reduced while maintaining the fiber array pitch (Pfiber) equal to the standard fiber array pitch. It implies that this approach can be a promising candidate for large-scale packaging and multiple-port device characterization. For characterization of alignment tolerances, a single-channel SWB and a 16-channel SWB were fabricated and attached to fiber/V-groove assemblies. An example of a 16-channel SWB aligned on a silicon-based PIC is shown in Fig. 4(a). To experimentally measure the alignment tolerances, the 16-channel device and the 1-channel device were used as illustrated in Fig. 4(b) and 4(c), respectively. Note that the grating couplers’ orientations are different for two cases, which results in different θoffset polarity (positive θoffset for 16-channel case shown in Fig. 4(b) and negative θoffset for 1-channel case shown in Fig. 4(c)). Our measurement results, which will be explained later, demonstrate that efficient optical coupling can be achieved regardless of θoffset when the polishing angle β is optimized for θoffset.
Fig. 4. (a) Photograph of the experimental setup using a 16-channel angle-polished silica waveguide block. (b) Schematic diagram of the experimental setup using a 16-channel SWB device aligned to forward-oriented grating couplers. (c) Schematic diagram of the experimental setup using a 1-channel SWB and a vertically-placed single-mode optical fiber.
For tolerance measurements, we first found the best coupling condition with respect to the fixed silicon grating couplers, and then the variance in output power was subsequently measured while changing the SWB position. To demonstrate the versatility of our coupling approach, our 1-channel SWB was designed to couple a negative radiation angle (negative θoffset), while the 16-channel SWB was designed for a positive radiation angle. The detailed coupling scheme and the orientations of the grating couplers are shown in Figs. 4(b) and 4(c). The 1-dB alignment tolerance for the single-channel SWB was measured to be over 5 µm, 6 µm, and 9 µm in the x-, y-, and z-direction, respectively (Figs. 5(a-c)). For the 16-channel device, the 1-dB alignment tolerances were 2.5 µm, 2.5 µm, and 5.5 µm along the x-, y-, and z-direction, respectively (Figs. 5(d-f)direction, respectively. The measured values for the single-channel case are almost twice the 16-channel case because the input and output waveguide positions simultaneously change for 16-channel SWB. The simulation results and the experimental results are in good agreement, and show the possibility of utilizing the SWBs for surface-parallel, lateral packaging with relaxed alignment tolerances.
The measured wavelength-dependent fiber-to-PIC coupling efficiencies are shown in Fig. 6 (solid curves). The minimum optical coupling loss for the SWB-based lateral coupling was 3.53 dB (red solid curve), which is very close to 3.57 dB when using a standard 8° APC fiber nearly vertically aligned to the PIC (black solid curve). Both measurement results are also comparable to the 3D FDTD simulation results for vertical coupling whose minimum coupling loss is -3.26 dB (black dashed curve). This shows that the intermediate SWBs for planar packaging scheme (red curve) are highly effective in suppressing light divergence and additional coupling losses, and provide compatibility for the existing grating coupler designs (black curves). The wavelength shifts in the coupling efficiency spectra could result from the angular uncertainties of angle-polishing fabrication and inevitable misalignments in experiments. We believe that this spectral shift can be resolved by carefully adjusting the polishing angle to compensate for the fabrication error and improving the positioning accuracies.
Fig. 5. (a-c) Measured alignment tolerance using the 1-channel device in the (a) x-direction; (b) y-direction; (c) z-direction; (d-f) Measured alignment tolerance using the 16-channel device in the (d) x-direction; (e) y-direction; (f) z-direction.
Fig. 6. Measured optical coupling efficiencies as a function of input wavelengths for the SWB-based lateral coupling scheme (red solid curve) and the vertically coupling scheme (black solid curve). The black dashed curve represents the simulated coupling efficiency for the vertically-aligned fiber case.
4. Conclusions
We have demonstrated a silicon photonic chip-to-fiber packaging scheme that utilizes an intermediate angle-polished SWB between the silicon-based diffraction grating coupler and the optical fiber. The demonstrated method can decrease the distance between the silica waveguide core and the grating coupler, and successfully suppress the additional insertion loss from unnecessary light divergence.
The simulation results and the experimental results showed that lateral misalignment parallel to the PIC plane is allowed over an about 5-µm range, and out-of-plane misalignment vertical to the chip plane is allowed over an about 9-µm range with 1 dB excess optical losses. Although not experimentally demonstrated here, it is also feasible to decrease the silica waveguide pitch much smaller than the fiber diameter to significantly improve the optical port density for high port-count applications.
Funding
Institute for Information and Communications Technology Promotion grant funded by the Korean government (MSIT) (2011-2019-00006, Development of 200Gb/s optical transceiver for metro-access network).
Disclosures
The authors declare no conflicts of interest.
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