Amir H. Atabaki

Single-chip microprocessor that communicates directly using light

Data transport across short electrical wires is limited by both bandwidth and power density, which creates a performance bottleneck for semiconductor microchips in modern computer systems—from mobile phones to large-scale data centres. These limitations can be overcome by using optical communications based on chip-scale electronic–photonic systems enabled by silicon-based nanophotonic devices8. However, combining electronics and photonics on the same chip has proved challenging, owing to microchip manufacturing conflicts between electronics and photonics. Consequently, current electronic–photonic chips are limited to niche manufacturing processes and include only a few optical devices alongside simple circuits. Here we report an electronic–photonic system on a single chip integrating over 70 million transistors and 850 photonic components that work together to provide logic, memory, and interconnect functions. This system is a realization of a microprocessor that uses on-chip photonic devices to directly communicate with other chips using light. To integrate electronics and photonics at the scale of a microprocessor chip, we adopt a ‘zero-change’ approach to the integration of photonics. Instead of developing a custom process to enable the fabrication of photonics, which would complicate or eliminate the possibility of integration with state-of-the-art transistors at large scale and at high yield, we design optical devices using a standard microelectronics foundry process that is used for modern microprocessors. This demonstration could represent the beginning of an era of chip-scale electronic–photonic systems with the potential to transform computing system architectures, enabling more powerful computers, from network infrastructure to data centres and supercomputers.

High quality planar silicon nitride microdisk resonators for integrated photonics in the visible wavelength range

High quality factor (Q approximately 3.4 x 10(6)) microdisk resonators are demonstrated in a Si(3)N(4) on SiO(2) platform at 652-660 nm with integrated in-plane coupling waveguides. Critical coupling to several radial modes is demonstrated using a rib-like structure with a thin Si(3)N(4) layer at the air-substrate interface to improve the coupling.

Systematic design and fabrication of high-Q single-mode pulley-coupled planar silicon nitride microdisk resonators at visible wavelengths

High quality (Q approximately 6 x 10(5)) microdisk resonators are demonstrated in a Si(3)N(4) on SiO(2) platform at 652-660 nm with integrated in-plane wrap-around coupling waveguides to enable critical coupling to specific microdisk radial modes. Selective coupling to the first three radial modes with >20dB suppression of the other radial modes is achieved by controlling the wrap-around waveguide width. Advantages of such pulley-coupled microdisk resonators include single mode operation, ease of fabrication due to larger waveguide-resonator gaps, the possibility of resist reflow during the lithography phase to improve microdisk etched surface quality, and the ability to realize highly over-coupled microdisks suitable for low-loss delay lines and add-drop filters.

Optimization of metallic microheaters for high-speed reconfigurable silicon photonics

The strong thermooptic effect in silicon enables low-power and low-loss reconfiguration of large-scale silicon photonics. Thermal reconfiguration through the integration of metallic microheaters has been one of the more widely used reconfiguration techniques in silicon photonics. In this paper, structural and material optimizations are carried out through heat transport modeling to improve the reconfiguration speed of such devices, and the results are experimentally verified. Around 4 micros reconfiguration time are shown for the optimized structures. Moreover, sub-microsecond reconfiguration time is experimentally demonstrated through the pulsed excitation of the microheaters. The limitation of this pulsed excitation scheme is also discussed through an accurate system-level model developed for the microheater response.

Vertical integration of high-Q silicon nitride microresonators into silicon-on-insulator platform

We demonstrate a vertical integration of high-Q silicon nitride microresonators into the silicon-on-insulator platform for applications at the telecommunication wavelengths. Low-loss silicon nitride films with a thickness of 400 nm are successfully grown, enabling compact silicon nitride microresonators with ultra-high intrinsic Qs (~ 6 × 10(6) for 60 μm radius and ~ 2 × 10(7) for 240 μm radius). The coupling between the silicon nitride microresonator and the underneath silicon waveguide is based on evanescent coupling with silicon dioxide as buffer. Selective coupling to a desired radial mode of the silicon nitride microresonator is also achievable using a pulley coupling scheme. In this work, a 60-μm-radius silicon nitride microresonator has been successfully integrated into the silicon-on-insulator platform, showing a single-mode operation with an intrinsic Q of 2 × 10(6).

Fully reconfigurable compact RF photonic filters using high-Q silicon microdisk resonators.

We present a fully reconfigurable four-pole, four-zero SOI RF photonic filter with a tunable 3dB bandwidth of 0.9 - 5 GHz, more than 38 dB out-of-band rejection, FSR larger than 600 GHz, and compact size (total area 0.15 mm2) using high-Q resonator-based components.

Accurate post-fabrication trimming of ultra-compact resonators on silicon

We experimentally demonstrate an accurate post-fabrication trimming technique for the correction of the optical phase of silicon photonic devices using a single fabrication step. Using this technique, we reduce the random resonance wavelength variation of ultra-compact silicon resonators by a factor of 6 to below 50 pm.

A 45 nm CMOS-SOI Monolithic Photonics Platform With Bit-Statistics-Based Resonant Microring Thermal Tuning

The microring resonator is critical for dense wavelength division multiplexed (DWDM) chip-to-chip optical I/O, enabling modulation and channel selection at the μm-scale suitable for a VLSI chip. Microring-based links, however, require active tuning to counteract process and thermo-optic variations. Here, we present a bit-statistical tuner that decouples tracking of optical one and zero-levels to realize non-dc-balanced data transmission, an “eye-max”-locking controller, and self-heating cancellation without need for a high-speed sensing frontend. We implement the tuner on a 45 nm CMOS-SOI process with monolithically integrated photonic devices and circuits. The tuner consumes 0.74 mW in the logic while achieving a record 524 GHz (> 50 K temperature) tuning range at 3.8 μW/GHz heater efficiency. To our knowledge, this is the highest range and heater efficiency reported by an on-chip closed-loop thermal tuner to date. The tuner integrates with a 5 Gb/s 30 fJ/bit monolithic microring transmitter, achieving wavelength-lock and immunity to both tracking failures and self-heating events caused by arbitrary, nondc-balanced bitstreams. In addition, the tuner provides critical functionality for an 11-λ DWDM transmitter macro capable of 11 × 8 Gb/s bandwidth on a fiber. Together with the transmitter, a 10 Gb/s on-chip monolithic optical receiver with 10-12 BER sensitivity of 9 μA at 10 Gb/s enables a sub-pJ/bit 5 Gb/s optical chip-to-chip link, with the bit-statistical tuner providing thermally robust microring operation.

Tuning of resonance-spacing in a traveling-wave resonator device

In this work a traveling-wave resonator device is proposed and experimentally demonstrated in silicon-on-insulator platform in which the spacing between its adjacent resonance modes can be tuned. This is achieved through the tuning of mutual coupling of two strongly coupled resonators. By incorporating metallic microheaters, tuning of the resonance-spacing in a range of 20% of the free-spectral-range (0.4nm) is experimentally demonstrated with 27mW power dissipation in the microheater. To the best of our knowledge this is the first demonstration of the tuning of resonance-spacing in an integrated traveling-wave-resonator. It is also numerically shown that these modes exhibit high field-enhancements which makes this device extremely useful for nonlinear optics and sensing applications.

Sub-100-nanosecond thermal reconfiguration of silicon photonic devices.

One of the limitations of thermal reconfiguration in silicon photonics is its slow response time. At the same time, there is a tradeoff between the reconfiguration speed and power consumption in conventional reconfiguration schemes that poses a challenge in improving the performance of microheaters. In this work, we theoretically and experimentally demonstrate that the high thermal conductivity of silicon can be exploited to tackle this tradeoff through direct pulsed excitation of the device silicon layer. We demonstrate 85 ns reconfiguration of 4 µm diameter microdisks, which is one order of magnitude improvement over the conventional microheaters. At the same time, 2.06 nm/mW resonance wavelength shift is achieved in these devices, which is in a par with the best microheater architectures optimized for low-power operation. We also present a system-level model that precisely describes the response of the demonstrated microheaters. A differentially addressed optical switch is also demonstrated that shows the possibility of switching in opposite directions (i.e., OFF-to-ON and ON-to-OFF) using the proposed reconfiguration scheme.