Abstract
To increase the manufacturing throughput and lower the cost of silicon photonics packaging, an alignment tolerant approach is required to simplify the process of fiber-to-chip coupling. Here, we demonstrate an alignment-tolerant expanded beam backside coupling interface (in the O-band) for silicon photonics by monolithically integrating microlenses on the backside of the chip. After expanding the diffracted optical beam from a TE-mode grating through the bulk silicon substrate, the beam is collimated with the aid of microlenses resulting in an increased coupling tolerance to lateral and longitudinal misalignment. With an expanded beam diameter of 32 μm, a ±7 μm lateral and a ±0.6° angular fiber-to-microlens 1-dB alignment tolerance is demonstrated at the wavelength of 1310 nm. Also, a large 300 μm longitudinal alignment tolerance with a 0.2 dB drop in coupling efficiency is obtained when the collimated beam from the microlens is coupled into a thermally expanded core single-mode fiber.
Highlights
Silicon photonics technology provides integrated die-level optical functionalities with advantages such as single-mode transmission, wavelength division multiplexing and compatibility with established CMOS-manufacturing facilities, leveraging advanced 3D integration schemes to meet high bandwidth demands with low power dissipation [1,2,3]
With an expanded beam diameter of 32 μm, a ±7 μm lateral and a ±0.6° angular fiber-to-microlens 1-dB alignment tolerance is demonstrated at the wavelength of 1310 nm
We have reported results for monolithic integration of microlenses directly at the backside of a silicon photonics chip with a goal to achieve alignment-tolerant expanded beam interface
Summary
Silicon photonics technology provides integrated die-level optical functionalities with advantages such as single-mode transmission, wavelength division multiplexing and compatibility with established CMOS-manufacturing facilities, leveraging advanced 3D integration schemes to meet high bandwidth demands with low power dissipation [1,2,3]. Chip-surface out-of-plane beam expansion and collimation from grating couplers has been a subject of great interest recently In this direction, a demonstration was made by coupling between a glass-based lens integrated fiber array and microlens hybrid-integrated on the "topside" of a standard grating coupler to result in a ±30 μm 1-dB lateral alignment tolerance [35]. The results presented here, have been achieved with the commercially available TEC fibers To realize such an expanded beam interface, there are some design parameters and boundary conditions (Fig. 3) that need to be evaluated based on the desired final metrics such as final beam diameter, 1-dB lateral and angular alignment tolerance, wavelength range of operation, coupling efficiency etc. In this paper, the system-level design is discussed for a specific case in O-band, but it should be noted that the generic design principles described can be applied for a different wavelength band (e.g. C-band), provided the grating design, beam diameter, beam divergence, chip substrate thickness, chip backside AR coating and microlens parameters are adjusted
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