We present a novel reconfigurable comb laser design that enables channel sparing, and demonstrate its functionality in a III-V/Si external cavity configuration. The laser is comprised of a single shared tunable Si ring mirror Vernier-paired with an array of tunable channel ring filters each coupled to their respective reflective semiconductor optical amplifier. This comb laser is not only tunable, capable of shifting all comb lines together, but it is flexible, able to change the spacing of channels in the comb within a fixed grid defined by the ring mirror. This flexibility results in features not previously demonstrated in a comb, such as mixed channel spacing and channel sparing capability. This is a practical candidate for flexible, scalable wavelength division multiplexed links targeting the next generation of data centers for the cloud.

The server compute landscape is changing. The traditional model of building general-purpose enterprise compute boxes, that end-users can use in various combinations with storage and networking to assemble their desired compute environments, has evolved to purpose-built systems optimized for specific applications. This tight integration of hardware and software components allows for unprecedented levels of switching and compute efficiencies and has fueled the penetration of optical interconnects deep "inside the box", particularly for switch scale-up. We present an 8.2Tb/s Infiniband packet switch with 41 ports running 100Gb/s per port interconnected by 12-channel mid-board optical transceivers with 25Gb/s per channel per direction of optical I/O. These components enable systems with up to 49Tb/s bandwidth in a 2U standard rack mount configuration.

A wafer-level, batch-fabricated, mechanically flexible interconnect (MFI) with a contact tip has been developed for rematable heterogeneous system integration. The contact tip, which exhibits a truncated-cone profile, enhances the scrubbing capability while maintaining the tip lifetime by avoiding tip plastic deformation. Electrical and mechanical characterization has been conducted on various testbeds to verify the performance of the assembled chip links with MFIs. The results indicate that a single MFI has an average electrical resistance of 103.21 mQ and up to 1 A current carrying capability, and can be successfully assembled on nonplanar surfaces with up to 45-μm surface variation.

We present a novel 3.3-μm radius ring modulator design with dramatically reduced resonance wavelength variations. By implementing a multi-mode waveguide design for the ring waveguides, phase errors from processing non-idealities in waveguide width and etching depth are significantly reduced. Measured resonance wavelengths from four 200-mm wafers fabricated in a commercial 130-nm CMOS foundry showed a total of ~5-nm peak-to-peak variation, which is only about 1/6 of the free spectral range of the ring design. With this tighter control over the absolute resonance positions, the tuning range and tuning power requirements of the ring modulators can be significantly reduced, which in turn enhances their functionality for applications in high density, energy-efficient high-performance computing systems.

We report a silicon interposer based 1×4 external-cavity hybrid III-V/Si laser array using a manufacturable back-end-of-the-line integration. All channels are individually-tunable and wavelength-stabilized with a threshold current of 14mA, output powers of >3mW, over 35dB side mode suppression ratio and less then 46kHz linewidth.

We demonstrate a surface-normal coupled tunable hybrid silicon laser array for the first time using passively-aligned, high-accuracy flip chip bonding. A 2x6 III-V reflective semiconductor optical amplifier (RSOA) array with integrated total internal reflection mirrors is bonded to a CMOS SOI chip with grating couplers and silicon ring reflectors to form a tunable hybrid external-cavity laser array. Waveguide-coupled wall plug efficiency (wcWPE) of 2% and output power of 3 mW has been achieved for all 12 lasers. We further improved the performance by reducing the thickness of metal/dielectric stacks and achieved 10mW output power and 5% wcWPE with the same integration techniques. This non-invasive, one-step back end of the line (BEOL) integration approach provides a promising solution to high density laser sources for future large-scale photonic integrated circuits.

Stable mode-hop-free operation for a Si/III-V hybrid external-cavity laser was demonstrated during 320mA bias current sweep using a real-time ring monitoring, a fast feedback control loop, and a simple bang-bang control algorithm, by both tunable micro-ring filter and cavity phase section.

We report the first closed-loop operation of a 100 Gbps polarization-insensitive, 4-channel wavelength-tracking WDM receiver in silicon photonics platform. Error-free operation is achieved with input polarization scrambling over input wavelength change of 4.5 nm using efficient thermal tuning of Si microring demux, corresponding to greater than 60°C fluctuation in temperature.

We present a real-time approach for stabilizing a III-V/Si hybrid external-cavity laser implemented using microring monitoring and a feedback control loop. Laser mode stabilization over bias current and stage temperature variations are experimentally demonstrated. We achieved single-mode and mode-hop-free laser operation as the bias current was swept across 320 mA. The same feedback control also enabled wavelength-locked laser operation using an intracavity phase control. The feedback control was also applied to an integrated on-chip hybrid laser, and mode-hop-free laser operation was demonstrated over 23 °C substrate temperature variation.

We demonstrate a low back reflection grating coupler on a SOI CMOS-compatible process designed for vertical integration of on-chip laser sources. It showed <;-26dB back reflection and 1.9dB coupling loss to a 4μm-diameter laser mode.

We demonstrate the first 200 GHz III-V/Si hybrid tunable, individually addressable four channel comb laser defined by a single shared tunable Si ring mirror, Vernier-paired with four rings filters using < 2.5 mW per channel.

Creating arrays of efficient optical sources to enable dense integration of silicon photonic transceivers with silicon very large-scale integration circuits remains a challenge. We review manufacturing strategies and challenges for back-end-of-line integration of tunable, external-cavity lasers using edge-coupled and surface-normal-coupled integration approaches showing that such laser arrays can be manufactured with a fab-less model. We demonstrate, for the first time, hybrid laser arrays using both integration approaches highlighting performance and design differences. We also discuss challenges and opportunities for each method and present experimental techniques to reduce alignment tolerance and improve laser stability and mode control.

We demonstrated wavelength locking for a high speed ring modulator using a low-power integrated balanced homodyne CMOS controller. Locking for wavelength change of 4 nm and substrate temperature change of 40°C was achieved.

A Si/III-V hybrid laser has been a highly sought after device for energy-efficient and cost-effective high-speed silicon photonics communication. We present a high wall-plug efficiency external-cavity hybrid laser created by integrating an independently optimized SOI ring reflector and a III-V gain chip. In our demonstration, the uncooled integrated laser achieved a waveguide-coupled wall-plug efficiency of 12.2% at room temperature with an optical output power of ~10 mW. The laser operated single-mode near 1550 nm with a linewidth of 0.22 pm. This is a tunable light source with 8 nm wavelength tuning range. A proof-of-concept laser wavelength stabilization technique has also been demonstrated. Using a simple feedback loop, we achieved mode-hop-free operation in a packaged external-cavity hybrid laser as bias current was varied by 60mA.

We demonstrate an external-cavity Si/III-V hybrid laser with a waveguide-coupled wall-plug efficiency of 12.2% at room temperature. The laser operates single-mode in C-band with a linewidth of 0.22 pm and is capable of wavelength tuning.

We report on a packaged prototype of a WDM photonic transceiver. It is an all-solid state hybrid assembly based on 130nm SOI photonic circuitry integrated with a 40nm CMOS VLSI driver. Our prototype supports eight tunable WDM channels operating at 10Gb/s, each capable of both transmitting and receiving data on the same chip. We discuss two options to close the link using the optical fiber or a waveguide bridge chip. We provide integration details and supporting link measurement data to describe packaged photonic module and its power efficient functionality with its on-chip power per channel averaging 1.3pJ/bit, excluding off-chip laser electrical power.

This focus issue is a collection of 17 invited papers showcasing the recent advances and the future challenges of optical interconnect technology. These papers represent multiple key areas that include the enabling devices and components, advances in the system and sub-system technologies as well as networking and its applications.

We present the packaging of a large multi-chip energyefficient WDM silicon photonic interconnect prototype enabled by hybrid integration, high-accuracy optical alignment and thermal-mechanical aware design and assembly.