Optical modulator devices in silicon have experienced dramatic improvements over the last
decade, with data rates demonstrated up to 50Gb/s and ultra-lower power consumption with a
few fJ/bit. However a significant need exist for high speed low power devices with a small
footprint and broadband characteristics with extinction ratio above 5dB. Here we describe the
work within the UK silicon photonics program, which has led to the fabrication and
preliminary results of novel nano cavity optical architecture as well as self-aligned pn
junction structures embedded in a silicon rib waveguide with an active length in the
millimetre range producing high-speed optical phase modulation whilst retaining a high
In this work we present results from high performance silicon optical modulators produced within the two largest silicon
photonics projects in Europe; UK Silicon Photonics (UKSP) and HELIOS. Two conventional MZI based optical
modulators featuring novel self-aligned fabrication processes are presented. The first is based in 400nm overlayer SOI
and demonstrates 40Gbit/s modulation with the same extinction ratio for both TE and TM polarisations, which relaxes
coupling requirements to the device. The second design is based in 220nm SOI and demonstrates 40Gbits/s modulation
with a 10dB extinction ratio as well modulation at 50Gbit/s for the first time. A ring resonator based optical modulator,
featuring FIB error correction is presented. 40Gbit/s, 32fJ/bit operation is also shown from this device which has a 6um
radius. Further to this slow light enhancement of the modulation effect is demonstrated through the use of both
convention photonic crystal structures and corrugated waveguides. Fabricated conventional photonic crystal modulators
have shown an enhancement factor of 8 over the fast light case. The corrugated waveguide device shows modulation
efficiency down to 0.45V.cm compared to 2.2V.cm in the fast light case. 40Gbit/s modulation is demonstrated with a
3dB modulation depth from this device. Novel photonic crystal based cavity modulators are also demonstrated which
offer the potential for low fibre to fibre loss. In this case preliminary modulation results at 1Gbit/s are demonstrated.
Ge/SiGe Stark effect devices operating at 1300nm are presented. Finally an integrated transmitter featuring a III-V
source and MZI modulator operating at 10Gbit/s is presented.
We calculate the conduction band electron scattering rates from the Γ-valley into the indirect valleys in germanium,
and use this to determine the strength of the indirect absorption in Ge/SiGe quantum well heterostructures.
This is done as a function of the in-plane compressive strain in the Ge quantum wells, which results from pseudomorphic
growth on a SiGe virtual substrate. This compressive strain results in the Δ valleys becoming available
as destination states for scattering, which leads to a reduction in the Γ-valley lifetime. We calculate the indirect
absorption and lifetime broadening of excitonic peaks, and show that indirect absorption decreases as the Ge
fraction in the virtual substrate increases. We conclude that the Ge fraction of the SiGe virtual substrate should
be approximately 95% or larger for optimum electroabsorption performance of Ge/SiGe quantum wells.
The project is a consortium based activity involving researchers from the UK institutions of the
Universities of Surrey, St. Andrews, Leeds, Warwick, and Southampton, as well as the commercial
research institution QinetiQ. The aims of the project are to progress the state of the art in Silicon
Photonics, in the areas of waveguides, modulators, couplers, detectors, Raman processes, and integration
with electronics. Thus the field is vast, and impossible to cover comprehensively in one project, nor
indeed in one paper. The programme is run on a truly collaborative basis, with members from each
institution running one or more work packages within the project, each co-ordinating work from their
own plus other institutions. To date, the most well developed work has emerged from the activity on basic
waveguides and their characteristics, the modulator activity, optical filters, and work on Raman
Amplifiers. This work will be the main focus of this paper, but an attempt will be made to update the
audience on the remaining activities within the project. By the nature of the project, much of the work is
medium term, and hence some activities are not expected to yield viable results until at least next year,
hence the concentration on some activities rather than all activities at this stage.
A simulation technique for modeling optical absorption in Ge/SiGe multiple quantum well (MQW) heterostructures
is described, based on a combined 6 × 6 k • p hole wave-function a one-band effective mass electron wavefunction
calculation. Using this model, we employ strain engineering to target a specific applications-oriented
wavelength, namely 1310 nm, and arrive at a design for a MQW structure to modulate light at this wavelength.
The modal confinement in a proposed device is then found using finite-element modeling, and we estimate the
performance of a proposed waveguide-integrated electroabsorption modulator.
Silicon Photonics has the potential to revolutionise a whole raft of application areas. Currently, the main focus is on
various forms of optical interconnects as this is a near term bottleneck for the computing industry, and hence a number
of companies have also released products onto the market place. The adoption of silicon photonics for mass
production will significantly benefit a range of other application areas. One of the key components that will enable
silicon photonics to flourish in all of the potential application areas is a high performance optical modulator. An
overview is given of the major Si photonics modulator research that has been pursued at the University of Surrey to
date as well as a worldwide state of the art showing the trend and technology available. We will show the trend taken
toward integration of optical and electronic components with the difficulties that are inherent in such a technology.
A review will be presented of recent work on Si/SiGe heavy-hole to heavy-hole quantum cascade emitters showing
progress towards a laser using the bound-to-continuum design for the active region. The sample was grown by
low energy plasma enhanced chemical vapour deposition in significantly less time than comparable structures
and designs in III-V or Si/SiGe technology using molecular beam epitaxy or more standard chemical vapour
deposition techniques. Clear intersubband electroluminescence is demonstrated at 4.2 K between 6.7 and 10.1
THz. This is inside the III-V restrahlung band where III-V materials cannot lase, unlike Group IV materials.
A review of waveguide losses will also be presented and some ideas of how to design an active region with gain
higher than the waveguide losses will be discussed.