Metal interconnections are expected to become the limiting factor for the performance of electronic systems as transistors continue to shrink in size. Replacing them by optical interconnections, at different levels ranging from rack-to-rack down to chip-to-chip and intra-chip interconnections, could provide the low power dissipation, low latencies and high bandwidths that are needed. The implementation of optical interconnections relies on the development of micro-optical devices that are integrated with the microelectronics on chips. Recent demonstrations of silicon low-loss waveguides, light emitters, amplifiers and lasers approach this goal, but a small silicon electro-optic modulator with a size small enough for chip-scale integration has not yet been demonstrated. Here we experimentally demonstrate a high-speed electro-optical modulator in compact silicon structures. The modulator is based on a resonant light-confining structure that enhances the sensitivity of light to small changes in refractive index of the silicon and also enables high-speed operation. The modulator is 12 micrometres in diameter, three orders of magnitude smaller than previously demonstrated. Electro-optic modulators are one of the most critical components in optoelectronic integration, and decreasing their size may enable novel chip architectures.

Micrometre-scale silicon electro-optic modulator

We examine the current performance and future demands of interconnects to and on silicon chips. We compare electrical and optical interconnects and project the requirements for optoelectronic and optical devices if optics is to solve the major problems of interconnects for future high-performance silicon chips. Optics has potential benefits in interconnect density, energy, and timing. The necessity of low interconnect energy imposes low limits especially on the energy of the optical output devices, with a ~ 10 fJ/bit device energy target emerging. Some optical modulators and radical laser approaches may meet this requirement. Low (e.g., a few femtofarads or less) photodetector capacitance is important. Very compact wavelength splitters are essential for connecting the information to fibers. Dense waveguides are necessary on-chip or on boards for guided wave optical approaches, especially if very high clock rates or dense wavelength-division multiplexing (WDM) is to be avoided. Free-space optics potentially can handle the necessary bandwidths even without fast clocks or WDM. With such technology, however, optics may enable the continued scaling of interconnect capacity required by future chips.

Device Requirements for Optical Interconnects to Silicon Chips

An electronic–photonic microprocessor chip manufactured using a conventional microelectronics foundry process is demonstrated; the chip contains 70 million transistors and 850 photonic components and directly uses light to communicate to other chips. The rapid transfer of data between chips in computer systems and data centres has become one of the bottlenecks in modern information processing. One way of increasing speeds is to use optical connections rather than electrical wires and the past decade has seen significant efforts to develop silicon-based nanophotonic approaches to integrate such links within silicon chips, but incompatibility between the manufacturing processes used in electronics and photonics has proved a hindrance. Now Chen Sun et al. describe a 'system on a chip' microprocessor that successfully integrates electronics and photonics yet is produced using standard microelectronic chip fabrication techniques. The resulting microprocessor combines 70 million transistors and 850 photonic components and can communicate optically with the outside world. This result promises a way forward for new fast, low-power computing systems architectures. 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 overcome1,2,3 by using optical communications based on chip-scale electronic–photonic systems4,5,6,7 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 chips9,10,11 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 photonics12, 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 microprocessors13,14,15,16. 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.

/pdf/single-chip-microprocessor-that-communicates-directly-using-3e2ii9mb9i.pdf

Single-chip microprocessor that communicates directly using light

Silicon photonics could enable a chip-scale platform for monolithic integration of optics and microelectronics for applications of optical interconnects in which high data streams are required in a small footprint. This paper discusses mechanisms in silicon photonics for waveguiding, modulating, light amplification, and emission. These mechanisms, together with recent advances of fabrication techniques, have enabled the demonstration of ultracompact passive and active silicon photonic components with very low loss.

Guiding, modulating, and emitting light on Silicon-challenges and opportunities

A system, method, and computer program product are provided for a memory system. The system includes a first semiconductor platform including at least one first circuit, and at least one additional semiconductor platform stacked with the first semiconductor platform and including at least one additional circuit.

System, method, and computer program product for improving memory systems

This paper describes both a near term and a long term optical interconnect solution, the first based on a packaging architecture and the second based on a monolithic photonic CMOS architecture. The packaging-based optical I/O architecture implemented with 90 nm CMOS transceiver circuits, 1 × 12 VCSEL/detector arrays and polymer waveguides achieves 10 Gb/s/channel at 11 pJ/b. A simple TX pre-emphasis technique enables a potential 18 Gb/s at 9.6 pJ/b link efficiency. Analysis predicts this architecture to reach less than 1 pJ/b at the 16 nm CMOS technology node. A photonic CMOS process enables higher bandwidth and lower energy-per-bit for chip-to-chip optical I/O through integration of electro-optical polymer based modulators, silicon nitride waveguides and polycrystalline germanium (Ge) detectors into a CMOS logic process. Experimental results for the photonic CMOS ring resonator modulators and Ge detectors demonstrate performance above 20 Gb/s and analysis predicts that photonic CMOS will eventually enable energy efficiency better than 0.3 pJ/b with 16 nm CMOS. Optical interconnect technologies such as these using multi-lane communication or wavelength division multiplexing have the potential to achieve TB/s interconnect and enable platforms suitable for the tera-scale computing era.

Optical I/O technology for tera-scale computing

We have proposed and demonstrated the principle of optical decoupling in a vertical-cavity surface-emitting laser (VCSEL)-electroabsorption modulator device by use of a duo-cavity device architecture. This technique promises ultrahigh-frequency modulation of the VCSEL output due to the elimination of the resonance influences on the intrinsic modulation transfer function. By adjusting spectral detuning between the resonances of the duo-cavity configuration, optical decoupling of output modulation by an electroabsorption modulator can be achieved. A flat ( 3 dB) response up to 20 GHz has been measured by proper detuning of the resonances.

Optically Decoupled Loss Modulation in a Duo-Cavity VCSEL

Systems and methods may couple on-chip optical circuits to external fibers. An SOI waveguide structure may include mirror structures and tapered waveguides to optically couple optical circuits to fibers in a vertically oriented external connector. The mirror structure(s) may be angularly disposed at the ends of the silicon waveguide structure. An oxide layer may cover a buried oxide layer and the silicon waveguide structure. The tapered waveguide(s) may have a narrow end and a wide end. The narrow end of the tapered waveguide(s) may be disposed above the mirror structures. The tapered waveguide(s) may extend through the oxide layer from the narrow end in a direction perpendicular to the silicon waveguide structure. An external connector may fit over the tapered waveguide(s) and uses a fiber array traveling through a connector body to optically couple to the external fiber.

Optical photonic circuit coupling

Self alignment of Optoelectronic (OE) chips, such as photodiode (PD) modules and vertical cavity surface emitting laser (VCSEL) modules, to external waveguides or fiber arrays may be realized by packaging the OE chips directly in the fiber optic connector.

Surface mount (smt) connector for vcsel and photodiode arrays

We have studied the modulation properties of VCSEL with intracavity multiple quantum well (MQW) electroabsorption
modulator integrated into the top distributed Bragg reflector (DBR) [1]. Small signal analysis of rate equations for loss
modulation shows an intrinsic high-frequency roll-off slope of 1/o instead of 1/o 2 in directly modulated laser diodes, and
consequently bandwidths in excess of 40 GHz are obtainable with this configuration [2]. Possible limiting factors to high
bandwidth were examined by fitting high frequency characteristics to a multi-pole transfer function, and include RC
delay and carrier drift-limited time of flight (TOF) in the modulator intrinsic region. Intracavity loss modulation shows a
strong (+20dB) relaxation oscillation resonant feature in both theory and experiment. As demonstrated, this feature can
be significantly reduced in amplitude using parasitics. We have extracted relative contribution of TOF and parasitic
capacitance by varying the modulator intrinsic region width (105 and 210 nm) and lateral size of the modulator (18 and
12mm). It was estimated that the small size modulator exhibits parasitics f -3dB at 8GHz. To estimate the carrier TOF
contribution to bandwidth limits, low temperature growth of a 210 nm absorber i-region and MQW was employed to
reduce photogenerated carrier lifetime. Bandwidth limitations were found to be mostly due to diode and metallization
capacitances, in addition to one pole set by the optoelectronic resonance frequency. We have used p-modulation doping
of the gain region to increase the relaxation frequency. Pronounced active Q-switching was observed, yielding pulse
widths of 40 ps at a 4 GHz rate.

Modulation properties of VCSEL with intracavity modulator

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