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 <i>pin</i> diodes and tested in a
surface-normal geometry. They exhibit low dark current density of 5 mA/cm<sup>2</sup> 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.
Silicon photonics is envisioned as a promising solution to address the interconnect bottleneck
in large-scale multi-processor computing systems, owing to advantageous attributes such as wide
bandwidth, high density, and low latency. To leverage these advantages, optical proximity coupler is
one of the critical enablers. Chip-to-chip, layer-to-layer optical proximity couplers with low loss,
large bandwidth, small footprint and integration compatibility are highly desirable. In this paper, we
demonstrate chip-to-chip optical proximity coupling using grating couplers. We report the
experimental results using grating couplers fabricated in a photonically-enabled commercial 130nm
SOI CMOS process.
Ring waveguide resonating structures with high quality factors are the key components servicing silicon
photonic links. We demonstrate highly efficient spectral tunability of the microphotonic ring structures
manufactured in commercial 130 nm SOI CMOS technology. Our rings are fitted with dedicated heaters
and integrated with silicon micro-machined features. Optimized layout and structure of the devices result in
their maximized thermal impedance and increased efficiency of the thermal tuning.
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.
Ring waveguide resonating structures with high quality factors are the key components in the silicon photonics portfolio
boosting up its functionality and circuit performance. Due to a number of manufacturing reasons their peak wavelengths
are often prone to deviate from designed values. In order to keep the ring resonator operating as specified, its peak
wavelength then needs to be corrected in a reliable and power efficient way. We demonstrate the performance of the
thermally tunable mux/demux filter ring structures fabricated in the commercial 130 nm SOI CMOS line.
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.
In this paper we present a computing system that uniquely leverages the bandwidth, density, and
latency advantages of silicon photonic interconnects to enable highly compact supercomputerscale
systems. We present the details of an optically enabled "macrochip" which is a set of
contiguous, optically-interconnected chips that deploy wavelength-division multiplexed (WDM)
enabled by silicon photonics. We describe the system architecture and the WDM point-to-point
network implementation of a "macrochip" providing bisection bandwidth of 10 TBps and discuss
system and device level challenges, constraints, and the critical technologies needed to implement
this system. We present a roadmap to lowering the energy-per-bit of a silicon photonic
interconnect and highlight recent advances in silicon photonics under the UNIC program that
facilitate implementation of a "macrochip" system made of arrayed chips.
We review the progress and challenges in scaling computing systems; discuss the
potential benefits and challenges for achieving optical-interconnects to the chip via the
native integration of silicon photonics components with VLSI electronics; and introduce
the "macrochip" - a collection of contiguous silicon chips enabled by optical proximity
We introduce a novel approach to interconnect multiple chips together with a silicon
photonic WDM point-to-point network enabled by optical proximity communications to act as a
single large piece of logical silicon much larger than a single reticle limit. We call this structure a
macrochip. This non-blocking network provides all-to-all low-latency connectivity while
maximizing bisection bandwidth, making it ideal for multi-core and multi-processor
interconnections. We envision bisection bandwidth up to TBps for an 8x8 macrochip design. And a
5-6x improvement in latency can be achieved when compared to a purely electronic implementation.
We also observe better overall performance over other optical network architectures.
We review 10Gb/sec Optical Proximity Communication realized with packaged chips that carry SOI
optical waveguides and reflecting mirrors micromachined in silicon. The high precision chip to chip
alignment and placement was enabled by a new packaging concept based on the co-integration of
pyramidal pit features defined by anisotropic silicon etch and matching high precision micro-spheres. We
support this novel packaging approach with measured optical transmission data and discuss the extent of it
towards other applications of Proximity Communication.
An electroabsorption modulator (EAM) is designed to optimize dynamic range performance over 20
GHz bandwidth. The single stripe waveguide enables an extremely compact and integrated package to
be fabricated with single mode fiber pigtails. The transfer function's shape permits suppression of
higher order intermodulation products, yielding a spur-free dynamic range exceeding that of Mach-
Zehnder designs. A dilute optical core diverts energy flow from absorbing layers into low loss
waveguide; the 20 dBm optical power tolerance is significantly higher than that of commercially
available electroabsorption devices. The tunable performance over 20 GHz is characterized and
applications are discussed. New approaches to the broadband impedance matching requirements are
calculated and the impact on system performance is assessed.
Externally coupled electroabsorption modulators (EAM) are commonly used in order to transmit RF signals on
optical fibers. Recently an alternative device design with diluted waveguide structures has been developed.  Bench
tests show benefits of lower propagation loss, higher power handling (100 mW), and higher normalized slope efficiency.
This paper addresses the specific issues involved in packaging the diluted waveguide EAM devices. An evaluation
of the device requirements was done relative to the standard processes. Bench tests were performed in order to
characterize the optical coupling of the EAM. The photo current maximum was offset from the optical power output
maximum. The transmissions vs. bias voltage curves were measured, and an XY scanner was used to record the mode
field of the light exiting from the EAM waveguide in each position. The Beam Propagation Method was used to simulate
the mode field and the coupling efficiency. Based on the bench tests and simulation results, a design including
mechanical, optical and RF elements was developed. A Newport Laser Welding system was utilized for fiber placement
and fixation. The laser welding techniques were customized in order to meet the needs of the EAM package design.
We demonstrate the use of an area selective zinc in-diffusion technique as a simple and efficient technique for the
fabrication of integrated photonic devices. In this work, the zinc in-diffusion process has a two fold application. It is well
known that the diffusion of zinc in InP follows an interstitial-substitutional diffusion mechanism. This provides a
concentration dependent diffusion profile, which allows us to control the sharpness of the diffusion front by controlling
the background doping concentration of the semiconductor wafer. By controlling the zinc depth combined with a sharp
diffusion front, the insertion losses of the devices can be minimized. In addition, this results in selective definition of p-n
junctions across the semiconductor wafer and therefore offers the potential for integration with electronic devices. Using
this technique an integrated 2x2 Mach-Zehnder modulator/switch was fabricated. The semiconductor wafer is based on
InGaAsP multiple quantum wells. To selectively define p-n regions for the contacts, we use a 200-nm thick silicon
nitride mask during the diffusion. The Mach-Zehnder structure is then patterned using photolithography and dry etching.
After a cyclotene planarization process, p-type contacts are deposited on top of the diffused regions by evaporation and
lift-off. Our experimental results demonstrate that on-chip losses on the order of 4-dB are obtained, which is
significantly lower compared to the use of isolation trenches. The device response as a modulator requires an additional
insertion loss of 3-dB for voltage controlled operation, with an extinction ratio better than 16 dB. In the case of electrical
current operation, better than 20 dB extinction ratio was obtained with only 8 mA.
Design for high efficiency, high power traveling wave electroabsorption modulator using Intra-Step-Barrier Quantum Well (IQW) and Peripheral Coupled waveguide (PCW) designs are presented. Both of these designs have separately yielded EAMs with high optical power handling and low Vp properties, in an analog fiber link configuration. The IQW EAM has low Vp (~0.73 V) and high power handling (100 mW). The lumped element IQW EAM has achieved a link gain of -16 dB, a multi-octave SFDR of 110 dB-Hz2/3 and a single-octave SFDR of 121dB-Hz4/5 at the 1543 nm wavelength. The PCW MQW EAM with lumped element configuration can achieve a low link low, a high multi-octave SFDR at the same wavelength. The traveling wave properties of these EAMs are under investigation.