Silicon Nanophotonic Packaging - Fiber Assembly
A schematic of parallelized fiber assembly in standard high-throughput microelectronic tools is presented below. We start with a short but standard MT ferrule with a standard cleaved fiber array. A polymer lid is attached to the bare fibers and the resulting fiber stub is picked and placed by the polymer lid on a V-groove array integrated on the photonic chip. Each V-groove individually re-aligns each fiber to its respective fiber coupler on chip. The polymer lid enables vacuum pick-tip handling of the fiber stub in high-throughput microelectronic tools and coarsely maintains the bare fiber pitch within the re-alignment range of the V-groove array on chip.
The result is a standard fiber connector interface providing the optical inputs and outputs. A standard flip-chip solder ball interface is shown here for electrical inputs and outputs although wirebonds could be used as well if found cost-efficient for a given device.
A picture of a parallelized fiber assembly along with cross-sectional micrographs taken across the on-chip V-groove array is presented below. The 12 fibers are well seated in their corresponding V-grooves, which warrants vertical and lateral fiber self-alignment.
A Monte Carlo tolerance analysis presented at ECTC 2015 [manuscript in PDF] investigated the self-alignment accuracy at manufacturing when inherent fabrication and fiber dimensional tolerances are taken into account. We found that a 3 sigma self-alignment accuracy to better than 1.3 um is expected between fiber core and waveguide coupler. This alignment is sufficient for low-loss connections to waveguide couplers that are mode matched to standard cleaved fibers. However, such positioning accuracy would not be adequate if specialty small-mode fibers were used. Thus, in addition to their high cost, small-mode fibers are not compatible with the inherent tolerances of V-groove self-alignment. Expanding the mode on chip to match a standard fiber is desired.
In addition to lateral and vertical alignment, the fiber ends need to be butted on the fiber couplers for best optical performance. In fact, this is the most challenging part of fiber assembly in high-throughput microelectronic tools as these tools do not include the ability of a horizontal pressure-sensing movement. Only vertical pressure-sensing movement is necessary for chip joins. As shown above, we use a sliding base to trigonometrically transform part of the vertical pressure sensing movement into a controlled butting force. A cross-sectional micrograph taken along the fiber length is shown above on the right. The fiber is well butted on the waveguide coupler. Our coupler is embedded in a suspended oxide membrane and surrounded with low-index adhesive at assembly. This is further explained below.
As mentioned above, expending the optical mode on chip to match a standard fiber is desired to enable low-cost self-alignment. To be clear, any design of such in-plane coupler can be used with the high-throughput self-aligned assembly described above. As a front-up option, we have demonstrated an in-plane mode converter between a waveguide and a standard cleaved fiber that requires only a single silicon layer and shows excellent bandwidth, coupling efficiency, and tolerance to fabrication control. It is based on a metamaterial guiding layer. The silicon layer is patterned with sufficiently fine periodic structures not to allow light diffraction. The light propagates as in a homogeneous medium but with engineered optical properties. This enables better mode matching to fiber and improved tolerances to dimensional control. This approach is similar to subwavelength gratings. We prefer the metamaterial terminology as it correctly infers index homogenization while subwavelength gratings technically include widespread diffractive elements such as Bragg gratings.
A top-view schematic of the fiber interface is presented above. The fiber sits in a V-groove and butts on a suspended oxide membrane in which the metamaterial converter is embedded. The waveguide is suspended to improve optical isolation to the substrate. Expanding the mode to match a standard cleaved fiber would not be possible otherwise due to the limit on buried oxide thickness in typical silicon on insulator wafers. The transmission spectrum of an O-band metamaterial converter is shown above on the right and was reported at OFC 2015 [manuscript in PDF]. An excellent peak efficiency to standard fiber of -1.3 dB is shown with small polarization dependence and a spectral penalty of 0.8 dB over a 100 nm bandwidth.
Our goal is the broad enablement of low-cost silicon photonic packaging. Please contact us if you would like to make use of any of the described technology.
- The fiber stubs used in this work are provided by AFL Telecommunications under the technical leadership of Ted Lichoulas and Eddie Kimbrell.
Tymon Barwicz et al.