3 days ago5 min read

Passively Aligned Glass Micro-Optic Bridge for Expanded-Beam Vertical Coupling and Pluggable Silicon Photonics

Introduction

As the demand for artificial intelligence and machine learning grows, the performance of the underlying compute hardware is increasingly limited by the bandwidth and power consumption of the interconnects between the chips. Significant challenges exist in scaling memory bandwidth and off-package I/O bandwidth to meet the corresponding scaling of compute integrated circuits (ICs). Optical compute interconnects offer a route to overcome these limitations, facilitating high-speed, low-power, and low-latency data transmission.

However, the design and implementation of optical interconnects pose several challenges, such as the alignment and packaging of optical components, the integration of optical and electronic devices, and the compatibility with existing fabrication processes. This is particularly the case for future architectures requiring die-to-die optical interconnects within the same package.

Achieving reliable, low-loss optical coupling in the confined space of electronic packages is a key challenge for high-yield co-packaged photonics. While a common method to relax alignment tolerances involves beam expansion, conventional approaches often entail attaching large micro-optic die to the silicon photonic integrated circuit (PIC), significantly impacting the overall assembly thickness. Furthermore, precise alignment systems are typically necessary for positioning the micro-optic die with sub-micron tolerances.

This tutorial introduces a novel solution to address these challenges: utilizing a glass micro-optic bridge passively aligned to a silicon PIC through existing mechanical alignment features. The micro-optic bridge contains curved micromirrors which allow for vertical coupling in and out of the PIC, enabling the use of existing low-loss edge couplers along with facilitating high-speed optical test. The optical bridges contain mechanical alignment features that allow for passive placement onto the PIC, allowing for a simplified assembly process with relaxed tolerances.

Micro-Optic Bridge Design

The device presented in this work makes use of an existing PIC design containing V-grooves for passive placement and coupling of optical fibers, and makes use of these existing mechanical alignment features to allow for the passive placement of the micro-optic bridge.

Figure 1. 3D render of glass micro-optic bridge attached to PIC.

Figure 1 shows a 3D render of the glass micro-optic bridge design, containing curved micromirrors, cylindrical protrusions for passive alignment into PIC V-grooves and additional topographical features to fit the existing PIC design and ensure a consistent bond line across the assembly.

The glass micro-optic bridge is formed using ultrashort pulse laser direct writing, allowing for arbitrary 3D shapes to be micromachined directly into glass wafers with high precision. The glass substrate material used was a Borosilicate glass with a coefficient of thermal expansion well matched to Silicon. The ultrashort pulse laser patterning and etching is carried out at the wafer scale and the patterned glass wafers are then singulated to form individual micro-optic bridge die.

The cylindrical protrusions are designed to mimic conventional optical fibers, but are sized such that only the outer two protrusions make contact with the PIC, in order to reduce mechanical over-constraint and minimize positional variations and susceptibility to particle contamination between the PIC and glass bridge.

Figure 2. Cross-section of the Micro-optic bridge assembled to the PIC.

Figure 2 shows a to-scale cross sectional diagram of the micro-optic bridge when assembled to the PIC. The dimensions of the micro-optic bridge are 5.2 x 2.5 x 0.5 mm, with approximately 0.35 mm protruding above the PIC surface after assembly.

Figure 3. Photograph of fabricated micro-optic bridge.

Figure 3 shows a photograph of the underside of a micro-optic bridge, showing the micromirrors and cylindrical protrusions.

The choice of collimated beam diameter is defined by a tradeoff between increased lateral alignment tolerance due to the increased beam size, and a decrease in pitch and yaw angular alignment tolerance due to the reduced numerical aperture of the optical system. A target beam diameter of 35 μm was chosen to balance the amount of lateral alignment relaxation with the amount of achievable angular alignment.

The parabolic optical surface of the micromirror was designed with an image distance of 189.4 μm in order to form a collimated beam of approximately 35 μm diameter from the PIC edge coupler which has a mode field diameter (MFD) of approximately 7 μm.

Micro-Optic Bridge Fabrication and Assembly

The curved micromirror surfaces were measured using an optical profiler, and the resulting point cloud data from the surface measurement was subtracted from the designed parabolic profile to obtain the residual design-to-manufacturing error. Figure 4 shows a 2D plot of the residual error across the surface of one of the mirrors, showing a maximum residual error of 51.7 nm within a 53.2 μm x 73.4 μm ellipse which corresponds to the region containing 99% of the optical power.

Figure 4. Residual error plot of example micromirror surface profile. The 1/e2 beam diameter contour is shown in red, and the contour corresponding to the edge of the region containing 99% of the optical power is shown in white. The Y axis in this measurement is defined in the plane of the mirror and is therefore rotated with respect to the X-Y plane of the bridge.

Four micro-optic bridges were assembled to PICs containing 16 V-grooves, with 8 pairs of loopbacks across adjacent channels. The assembly was carried out passively using a manually operated motorized assembly tool, with the bridge placed into the PIC V-grooves and then moved forwards to bring the bridge close to the target distance of 14 μm from the edge coupler facet by butting up against a mechanical hard-stop present on the PIC.

Figure 5. Side view of the micro-optic bridge during assembly to the PIC.

Figure 5 shows a side view of the optical bridge during assembly, prior to application of the index matching epoxy at the coupling interface.

Figure 6. Top-down photograph of assembled micro-optic bridge on to PIC

Figure 6 shows a top-down photo of one assembly following attach of the optical bridge to the PIC. The inset image shows a close-up of the PIC edge couplers and micromirrors through the top of the bridge die.

Optical Characterization

As the PIC contained only loopback waveguides, it was not possible to independently characterize the channel-by-channel optical performance, or directly measure the beam profile emitted from the micro-optic bridge. The characterization of the optical performance was carried out by using an optical bridge with curved micromirrors coupled to waveguides fabricated using ultrashort pulse laser direct writing.

Figure 7 shows a schematic of the test configuration used. The mirror surface on the waveguide bridge was of the same optical design as that used on the micro-optic bridge attached to the PIC, with the combined system forming a periscope with 1:1 image magnification.

The collimated beam profile from the waveguide bridge was measured, using a calibrated infrared imaging system. The beam profile at 1310 nm is shown in Figure 8, with a measured beam diameter of 34.7 x 31.6 μm and a near-Gaussian profile.

Figure 8. Normalized intensity profile of the free space beam in the waveguide bridge.

A Gaussian fit is shown in blue, with 1/e2 beam diameters labelled on the line plots.

Single pass excess losses for the dual-mirror periscope system were calculated by taking half the full loopback insertion loss and subtracting the contributions from various sources. Figure 9 shows measured single pass excess loss for four assemblies, with average losses of 1.62, 1.23, 1.13 and 1.29 dB respectively.

In order to analyze the contributions to these loss values, a measurement of the sensitivity to misalignment was carried out to provide insight on the collimated beam diameter achieved by the micro-optic bridge. Figure 10 shows the 0.25 dB roll-off points for the four assemblies, with lines corresponding to calculated beam diameters for the amount of roll-off observed based on a Gaussian coupling model.

Figure 10. Loopback loss roll-off due to misalignment induced at the waveguide bridge to micro-optic bridge interface.

The variations in the extrapolated beam diameter values from the micro-optic bridge are likely due to variations in the numerical aperture of the edge couplers in the vertical (Z) axis, as well as potential contributions from vertical and longitudinal misalignment of the micro-optic bridge to the PIC.

Simulation results in Figure 11(a) show that a significant amount of angle deviation can be induced from an offset of the Z axis position of the micro-optic bridge, and Figure 11(b) shows that such angle deviations can contribute significantly to the observed excess losses.

Conclusion

Glass micro-optic bridges were demonstrated, enabling expanded beam vertical coupling from existing edge-coupled PICs. The glass micro-optic bridges were passively aligned to the silicon PICs and allow for low-cost, high-volume wafer based assembly of low profile collimating micro-optics. Beam expansion of the PIC edge coupler was achieved with a beam diameter of approximately 30 μm, and an average excess loss of 1.32 dB was measured when coupled through two curved micromirrors into a waveguide bridge in a periscope configuration.

The technology presented in this work offers a promising solution for enabling reliable, low-loss optical coupling in the confined space of electronic packages, addressing the key challenges faced in high-yield co-packaged photonics.

Reference

[2] N. Psaila, Z. Chaboyer, J. Macdonald and P. Tadayon, "Passively aligned glass micro-optic bridge for expanded-beam vertical coupling and pluggable Silicon Photonics," in Journal of Lightwave Technology, doi: 10.1109/JLT.2024.3386033.