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 10-12. In addition, we will discuss our Mux/Demux strategy and present our devices with small footprints and low tuning energy.
We report the first complete 10G silicon photonic ring modulator with integrated ultra-efficient CMOS driver and closed-loop wavelength control. A selective substrate removal technique was used to improve the ring tuning efficiency. Limited by the thermal tuner driver output power, a maximum open-loop tuning range of about 4.5nm was measured with about 14mW of total tuning power including the heater driver circuit power consumption. Stable wavelength locking was achieved with a low-power mixed-signal closed-loop wavelength controller. An active wavelength tracking range of > 500GHz was demonstrated with controller energy cost of only 20fJ/bit.
A highly efficient silicon (Si) hybrid external cavity laser with a wavelength tunable ring reflector is fabricated on a complementary metal-oxide semiconductor (CMOS)-compatible Si-on-insulator (SOI) platform and experimental results with high output power are demonstrated. A III-V semiconductor gain chip is edge-coupled into a SOI cavity chip through a SiN(x) spot size converter and Si grating couplers are incorporated to enable wafer-scale characterization. The laser output power reaches 20 mW and the highest wall-plug efficiency of 7.8% is measured at 17.3 mW in un-cooled condition. The laser wavelength tuning ranges are 8 nm for the single ring reflector cavity and 35 nm for the vernier ring reflector cavity, respectively. The Si hybrid laser is a promising light source for energy-efficient Si CMOS photonic links.
We present the design of a silicon microsystem enabled by silicon photonic interconnects. We review our recent progress in developing key building blocks toward sub pJ/bit optical links for inter/intra-chip interconnects, including ultra-low power silicon photonic transceivers and WDM components with low tuning power.
Electroabsorption from GeSi on silicon-on-insulator (SOI) is expected to have promising potential for optical modulation due to its low power consumption, small footprint, and more importantly, wide spectral bandwidth for wavelength division multiplexing (WDM) applications. Germanium, as a bulk crystal, has a sharp absorption edge with a strong coefficient at the direct band gap close to the C-band wavelength. Unfortunately, when integrated onto Silicon, or when alloyed with dilute Si for blueshifting to the C-band operation, this strong Franz-Keldysh (FK) effect in bulk Ge is expected to degrade. Here, we report experimental results for GeSi epi when grown under a variety of conditions such as different Si alloy content, under selective versus non selective growth modes for both Silicon and SOI substrates. We compare the measured FK effect to the bulk Ge material. Reduced pressure CVD growth of GeSi heteroepitaxy with various Si content was studied by different characterization tools: X-ray diffraction (XRD), atomic force microscopy (AFM), secondary ion mass spectrometry (SIMS), Hall measurement and optical transmission/absorption to analyze performance for 1550 nm operation. State-of-the-art GeSi epi with low defect density and low root-mean-square (RMS) roughness were fabricated into pin diodes and tested in a surface-normal geometry. They exhibit low dark current density of 5 mA/cm2 at 1V reverse bias with breakdown voltages of 45 Volts. Strong electroabsorption was observed in our GeSi alloy with 0.6% Si content having maximum absorption contrast of Δα/α ~5 at 1580 nm at 75 kV/cm.
Microresonators are basic building blocks for compact photonic integrated circuits (PICs). The performance of the microresonators depends on their intrinsic and loaded quality factors (Q). Here we demonstrate the optical characterization of a single crystalline silicon microtoroidal resonator with integrated MEMS-actuated tunable optical coupler. The device is realized on a two-layer silicon-on-insulator (SOI) structure. It is fabricated by combining hydrogen annealing and wafer bonding processes. The device has been demonstrated to be operated in all three coupling regimes: under-coupling, critical coupling, and over-coupling. At an actuation voltage of 114 V, the extinction ratio at the resonant wavelength of 1548.18 nm reaches 22.4 dB. To characterize these quality-factor tunable microtoroidal resonators, we have also developed a comprehensive model based on time-domain coupling theory. The intrinsic and loaded Qs are extracted by fitting the experimental curves with the model. The intrinsic Q is 110,000. And the loaded Q is continuously tunable from 110,000 to 5,400. This device has potential applications in variable bandwidth filters, reconfigurable add-drop multiplexers, and optical sensors.
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 4-μm-diameter laser mode.
A wavelength- and bandwidth- tunable filter with microelectromechanical-system (MEMS)-actuated waveguides is first demonstrated on a silicon-based microdisk resonator. Integrated microheaters enabled wavelength tuning up to 125 GHz. Bandwidth can be tuned from 12.0 to 41.2 GHz by coupling control.
We report an ultra-low power 80 Gbps arrayed silicon photonic transceiver for dense, large bandwidth inter/intra-chip interconnects. The hybrid CMOS transceiver consists of eight 10 Gbps WDM channels with total consumed power below 6 mW/channel.
We present slow light pulse propagation in MEMS-tunable microdisks at telecom wavelength for the first time. Furthermore we obtain delays up to 94 ps, a slowdown factor of 700, and a delay-bandwidth product of 0.5.
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