Dedicated multi-project wafer (MPW) runs for photonic integrated circuits (PICs) from Si foundries mean that researchers and small-to-medium enterprises (SMEs) can now afford to design and fabricate Si photonic chips. While these bare Si-PICs are adequate for testing new device and circuit designs on a probe-station, they cannot be developed into prototype devices, or tested outside of the laboratory, without first packaging them into a durable module. Photonic packaging of PICs is significantly more challenging, and currently orders of magnitude more expensive, than electronic packaging, because it calls for robust micron-level alignment of optical components, precise real-time temperature control, and often a high degree of vertical and horizontal electrical integration. Photonic packaging is perhaps the most significant bottleneck in the development of commercially relevant integrated photonic devices. This article describes how the key optical, electrical, and thermal requirements of Si-PIC packaging can be met, and what further progress is needed before industrial scale-up can be achieved.

We demonstrate how to optimize the performance of PAM-4 transmitters based on lumped Silicon Photonic Mach-Zehnder Modulators (MZMs) for short-reach optical links. Firstly, we analyze the trade-off that occurs between extinction ratio and modulation loss when driving an MZM with a voltage swing less than the MZM's V_π. This is important when driver circuits are realized in deep submicron CMOS process nodes. Next, a driving scheme based upon a switched capacitor approach is proposed to maximize the achievable bandwidth of the combined lumped MZM and CMOS driver chip. This scheme allows the use of lumped MZM for high speed optical links with reduced RF driver power consumption compared to the conventional approach of driving MZMs (with transmission line based electrodes) with a power amplifier. This is critical for upcoming short-reach link standards such as 400Gb/s 802.3 Ethernet. The driver chip was fabricated using a 65nm CMOS technology and flip-chipped on top of the Silicon Photonic chip (fabricated using IMEC's ISIPP25G technology) that contains the MZM. Open eyes with 4dB extinction ratio for a 36Gb/s (18Gbaud) PAM-4 signal are experimentally demonstrated. The electronic driver chip has a core area of only 0.11mm2 and consumes 236mW from 1.2V and 2.4V supply voltages. This corresponds to an energy efficiency of 6.55pJ/bit including Gray encoder and retiming, or 5.37pJ/bit for the driver circuit only.

The thermal management of electronics is becoming an increasing concern as industry continues to simultaneously push performance while shrinking the size of electronic devices. Microchannel cooling is a promising technology to accommodate the heat dissipation rates and associated fluxes projected for future generations of electronics while also satisfying the need for a reduced footprint to accommodate ever-shrinking device sizes. One shortfall of microchannel cooling, however, is the large pressure drop associated with pumping liquids through microchannels, i.e., channels in which the smallest dimension is between about 1 micron and 1 mm. Superhydrophobic surfaces combine roughness features with low surface energy coatings to create materials with substantially decreased wettability and drag resistance in laminar flows and represent a promising technology for reducing the flow resistance of microchannels. The presence of an (insulating) air layer that is trapped within the superhydrophobic surface, and which separates the microchannel wall from the working fluid, gives rise to a low shear-stress region responsible for the observed reduction in flow resistance. There have been a limited number of studies on the fluid mechanics in superhydrophobic microchannels and, to our knowledge, heat transfer has not been examined. Quantifying the trade-off between the enhanced heat transfer due to pressure drop reduction versus the insulating characteristics of the air layer is of paramount importance for determining the viability of superhydrophobic surfaces as a technology for enhancing microchannel heat transfer. In this work we compute friction factors and Nusselt numbers for the fully-developed (with respect to energy and momentum) flow of a fluid in a parallel-plane microchannel with different heat flux and momentum boundary conditions at the upper and lower channel walls. Two approximations are taken for modeling the superhydrophobic microchannel. In the first case we study the single-phase flow of a fluid in a microchannel where one or both microchannel walls is assumed to be superhydrophobic and where the superhydrophobicity is modeled via application of Navier’s slip model at the microchannel wall. Solutions for the velocity profiles are then employed to calculate theoretical friction factors and Nusselt numbers for the constant heat flux condition. This analysis is then extended to examine the implications on the thermal resistance of a superhydrophobic surface due to the presence of a purely conductive air layer. In the second case we model the fluid flow in the presence of a recirculating air layer that separates the fluid from the microchannel wall. In this instance the low-viscosity air layer gives rise to apparent fluid slip for the working fluid which is dependant on the thickness of the air layer and the viscosity ratio of the two working fluids. This case represents an upper apparent-slip limit as the characteristic spacing of the surface roughness becomes large relative to the channel height and air-layer thickness.

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Intramedullary nailing is a routine orthopedic procedure used for treating fractures of femoral or tibial shafts. A critical part of this procedure involves the drilling of pilot holes in both ends of the bone for the placement of the screws that will secure the IM rod to sections of the fractured bone. This step introduces a risk of soft tissue damage because the drill bit, if not stopped in time, can transverse the bone-tissue boundary into the overlying muscle, causing unnecessary injury and prolonging healing time due to periosteum damage. In this respect, detecting the bone-tissue boundary before breakthrough can reduce the risks and complications associated with intramedullary nailing. Hence, in the present study, a two-wavelength diffuse reflectance spectroscopy technique was integrated into a surgical drill to optically detect bone-tissue boundary and automatically trigger the drill to stop. Furthermore, Monte-Carlo simulations were used to estimate the maximum distance from within the bone at which the bone-tissue boundary could be detected using DRS. The simulation results estimated that the detection distance, termed the "look-ahead-distance" was ∼1.5 mm for 1.3 mm source-detector fiber separation. Experimental measurements with 1.3 mm source-detector fiber separation showed that the look-ahead-distance was in the order of 250 µm in experiments with set drill rate and in the range of 1 mm in experiments where the holes were drilled by hand. Despite this difference, the automated DRS enhanced drill successfully detected the approaching bone tissue boundary when tested on samples of bovine femur and muscle tissue.