Silicon photonics has undergone significant development in the last decade with some commercial successes for optical transceivers in telecommunication and data center applications. Here, we review and discuss the most successful silicon photonics devices which have been already implemented in the products, the remaining device challenges in the coming 400G transceivers, and the future of silicon photonics.
Silicon-based optical interconnects are expected to provide high bandwidth and low power consumption solutions for
chip-level communication applications, due to their electronics integration capability, proven manufacturing record and
attractive price volume curve. In order to compete with electrical interconnects, the energy requirement is projected to be
sub-pJ per bit for an optical link in chip to chip communication. Such low energies pose significant challenges for the
optical components used in these applications. In this paper, we review several low power photonic components
developed at Kotura for DARPA's Ultraperformance Nanophotonic Intrachip Communications (UNIC) project. These
components include high speed silicon microring modulators, wavelength (de)multiplexers using silicon cascaded
microrings, low power electro-optic silicon switches, low loss silicon routing waveguides, and low capacitance
germanium photodetectors. Our microring modulators demonstrate an energy consumption of ~ 10 fJ per bit with a drive
voltage of 1 V. Silicon routing waveguides have a propagation loss of < 0.3 dB/cm, enabling a propagation length of a
few tens of centimeters. The germanium photodetectors can have a low device capacitance of a few fF, a high
responsivity up to 1.1 A/W and a high speed of >30 GHz. These components are potentially sufficient to construct a full
optical link with an energy consumption of less than 1 pJ per bit.
We present a hybrid integration technology platform for the compact integration of best-in-breed VLSI and photonic
circuits. This hybridization solution requires fabrication of ultralow parasitic chip-to-chip interconnects on the candidate
chips and assembly of these by a highly accurate flip-chip bonding process. The former is achieved by microsolder bump
interconnects that can be fabricated by wafer-scale processes, and are shown to have average resistance <1 ohm/bump
and capacitance <25fF/bump. This suite of technologies was successfully used to hybrid integrate high speed VLSI chips
built on the 90nm bulk CMOS technology node with silicon photonic modulators and detectors built on a 130nm
CMOS-photonic platform and an SOI-photonic platform; these particular hybrids yielded Tx and Rx components with
energies as low as 320fJ/bit and 690fJ/bit, respectively. We also report on challenges and ongoing efforts to fabricate
microsolder bump interconnects on next-generation 40nm VLSI CMOS chips.
Scaling of high performance, many-core, computing systems calls for disruptive solutions to provide ultra energy
efficient and high bandwidth density interconnects at very low cost. Silicon photonics is viewed as a promising solution.
For silicon photonics to prevail and penetrate deeper into the computing system interconnection hierarchy, it requires
innovative optical devices, novel circuits, and advanced integration. We review our recent progress in key building
blocks toward sub pJ/bit optical link for inter/intra-chip applications, ultra-low power silicon photonic transceivers. In
particular, compact reverse biased silicon ring modulator was developed with high modulation bandwidth sufficient for
15Gbps modulation, very small junction capacitance of ~50fF, low voltage swing of 2V, high extinction ratio (>7dB)
and low optical loss (~2dB at on-state). Integrated with low power CMOS driver circuits using low parasitic microsolder
bump technique, we achieved record low power consumption of 320fJ/bit at 5Gbps data rate. Stable operation with biterror-
rate better than 10<sup>-13 </sup>was accomplished with simple thermal management. We further review the first hybrid
integrated silicon photonic receiver based on Ge waveguide photo detector using the same integration technique, with
which high energy efficiency of 690fJ/bit, and sensitivity of ~18.9dBm at 5Gbps data rate for bit-error-rate of 10<sup>-12 </sup>were
We report a very compact (1.6μmx10μm) and low dark current (20nA) Germanium p-i-n photodetector integrated on
0.25μm thick silicon-on-insulator (SOI) waveguides. A thin layer of Germanium was selective-epitaxially grown on top
of SOI waveguides. Light is evanescently coupled into Germanium layer from the bottom SOI waveguide. The device
demonstrates superior performance with demonstrated responsivity of 0.9A/W and 0.56A/W at wavelength of 1300nm
and 1550nm, respectively, and dark current less than 20nA at -0.5V bias. The 3dB bandwidth of the device is measured
to be 23GHz at -0.5V bias.
The Ultra-performance Nanophotonic Intrachip Communication (UNIC) project aims to achieve unprecedented high-density,
low-power, large-bandwidth, and low-latency optical interconnect for highly compact supercomputer systems.
This project, which has started in 2008, sets extremely aggressive goals on power consumptions and footprints for
optical devices and the integrated VLSI circuits. In this paper we will discuss our challenges and present some of our
first-year achievements, including a 320 fJ/bit hybrid-bonded optical transmitter and a 690 fJ/bit hybrid-bonded optical
receiver. The optical transmitter was made of a Si microring modulator flip-chip bonded to a 90nm CMOS driver with
digital clocking. With only 1.6mW power consumption measured from the power supply voltages and currents, the
transmitter exhibits a wide open eye with extinction ratio >7dB at 5Gb/s. The receiver was made of a Ge waveguide
detector flip-chip bonded to a 90nm CMOS digitally clocked receiver circuit. With 3.45mW power consumption, the
integrated receiver demonstrated -18.9dBm sensitivity at 5Gb/s for a BER of 10<sup>-12</sup>. In addition, we will discuss our
Mux/Demux strategy and present our devices with small footprints and low tuning energy.
Nonlinear optical effects for frequency conversion require a phase-matching condition to efficiently generate a coherent field at the new wavelength. We find that the phase-matching condition can be replaced by a resonance condition when the nonlinear effect takes place in a waveguide directional coupler. We have previously reported this novel resonance phenomenon when the effect of second-harmonic generation occurs in waveguide directional couplers. In this paper, we present our detailed theory. An example of the design of such waveguide directional coupler is presented.
A study of the modal reflection, transmission and radiation properties of deeply etched second-order Bragg waveguide gratings in silicon-on-insulator (SOI) slab waveguide is presented. Both the eigenmode expansion method and the finite-difference time-domain method have been used to model this structure. High reflection can be obtained with a very short grating structure. The out-of-plane coupling efficiency and directionality can be controlled by the grating depth and groove shape. This structure has potential for use in micro-cavity laser, compact outcouplers and incouplers, and surface-emitting lasers. We propose an application example which involves a double-grating coupler to realize efficient coupling between vertically integrated SOI waveguides over short distances. Experimental results will be presented.
A wide variety of integration strategies for micro-optical systems have been employed. Here we review some of these and comment on their relative strengths and weaknesses. In particular we compare approaches that are based on monolithic fabrication with those that make use of discrete components. As applications we consider free-space optical interconnects, telecommunications optical space switches and radiation mode interconnects for optical waveguides.
The area of integrated optical circuits has been undergoing rapid development due to the important applications of fiber communication systems and optical interconnects. A significant challenge of photonic circuits is to increase circuit density and to miniaturize these devices. The vertical integration of stacked waveguides for photonic circuits onto a single substrate is a promising configuration to enable the dense monolithic integration of three-dimensional photonic devices. Application of high-index-contrast waveguides, such as silicon-on-insulator waveguides, is another important way to increase the density of optical circuits due to their small sizes. These waveguides produce high confinement in the guiding layers and have the advantages of compactness and immunity of cross-talk between different waveguides. It is thus expected that efficient coupling of light between vertically integrated waveguides where no direct field-overlap of guided modes exists is a key issue. We propose a compact double-grating coupler to realize efficient coupling through radiation modes between two vertically stacked SOI waveguides. The grating is strong enough to be considered as a onedimensional photonic bandgap structure which facilitates a very short coupling length. Simulations suggest that a 22% efficiency is achievable in coupling light from one waveguide to another with a 12.9μm long grating. We find that the coupling efficiency is enhanced by Fabry-Perot resonance between two gratings. Coupling efficiency can be dramatically increased by incorporating a reflective under-layer structure or using blazed grating.