【Packaging Technology】Several Common Fiber Coupling Schemes for Silicon Photonic Chips
Silicon photonic chips utilize CMOS semiconductor processes to integrate photonic devices such as waveguides, modulators, detectors, multiplexers, and demultiplexers onto a silicon platform. Compared with traditional discrete device solutions, silicon photonic integrated chips feature low cost, low power consumption, high integration density, and high transmission bandwidth.
To assemble silicon photonic integrated chips into micro-optical systems, fiber-to-silicon waveguide coupling is required. To achieve high integration, fiber arrays are preferably used for chip coupling. In addition, the waveguides on photonic chips are extremely small and cannot be directly coupled with conventional single-mode fibers, necessitating special treatment. There are two primary coupling methods between silicon chips and fibers: vertical coupling and horizontal coupling. This paper introduces several typical fiber array coupling schemes.
1. V-Groove Array Coupling
In this scheme, the end face of the silicon chip is etched into a V-groove array to accommodate the fiber array, as shown in the figure. The green section is a polymer lid that presses down on the fiber array, allowing the fibers to fit perfectly into the grooves. The position of each fiber can be further adjusted to ensure it settles completely into the V-groove, thereby achieving optimal coupling efficiency.
- Polymer Waveguide Coupling Scheme
As shown in Figure 3, the polymer waveguide acts as a bridge between the single-mode fiber array and the silicon waveguide. Light is coupled from the single-mode fiber array into the polymer waveguide, which then couples into the silicon waveguide.
- Mode Field Conversion Scheme
Due to the small mode field of the waveguide on the silicon chip, coupling with a standard single-mode fiber results in significant loss caused by mode field diameter (MFD) mismatch. In this scheme, a short segment of fiber with a small mode field diameter (MFD) is fusion-spliced to the single-mode fiber array, converting the larger MFD in the fiber to a smaller MFD that can be coupled into the silicon chip. As shown in Figure 4, the small-mode-field fiber acts as a mode spot converter.
Figure 4: MFD-Matched Fiber Array
- Lensed Fiber Scheme
A lensed fiber array is a highly efficient coupling scheme for photonic integrated chips. The mode field diameter (MFD) of a lensed fiber is only about 3 μm, enabling efficient coupling to silicon waveguides, as shown in Figure 5.
Figure 5: Lensed Fiber Array
- Quasi-Planar Coupling Scheme
First, waveguides for vertical grating coupling are etched on the photonic chip. Then, a fiber array with a 40-degree polished facet is used for coupling to achieve vertical coupling in the grating, as shown in Figure 6. This is also known as quasi-planar coupling.
45° Uncapped FA 45° Embedded Cap FA
Dual-Layer 45° FA 45° FA with Small-Diameter Fiber
- Vertical Fiber Array Scheme
A 90-degree vertical fiber array can be easily applied to different positions on a silicon photonic chip, facilitating flexible design of silicon photonic chips.
Figure 8: Example of 90° Fiber Array Coupling with SiPh Silicon Photonic Chip
Figure 9: 90° Fiber Array
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