In this paper we present silicon and germanium-based material platforms for the mid-infrared wavelength region and we report several active and passive devices realised in these materials. We particularly focus on devices and circuits for wavelengths longer than 7 micrometers.
Silicon photonics has traditionally focused on near infrared wavelengths, with tremendous progress seen over the past decade. However, more recently, research has extended into mid infrared wavelengths of 2 μm and beyond. Optical modulators are a key component for silicon photonics interconnects at both the conventional communication wavelengths of 1.3 μm and 1.55 μm, and the emerging mid-infrared wavelengths. The mid-infrared wavelength range is particularly interesting for a number of applications, including sensing, healthcare and communications. The absorption band of conventional germanium photodetectors only extends to approximately 1.55 μm, so alternative methods of photodetection are required for the mid-infrared wavelengths. One possible CMOS compatible solution is a silicon defect detector. Here, we present our recent results in these areas. Modulation at the wavelength of 2 μm has been theoretically investigated, and photodetection above 25 Gb/s has been practically demonstrated.
We review our recent developments of the trimming techniques for correcting the operating point of ring resonator and Mach-Zehnder Interferometers (MZIs). This technology has been employed to fine-tune the effective index of waveguides, and therefore the operating point of photonic devices, enabling permanent correction of optical phase error induced by fabrication variations. Large resonance wavelength shift of ring resonators was demonstrated, and the shift can be tuned via changing the laser power used for annealing. A higher accuracy trimming technique with a scanning laser was also demonstrated to fine-tune the operating point of integrated MZIs. The effective index change of the optical mode is up to 0.19 in our measurements, which is approximately an order of magnitude improvement compared to previous work, whilst retaining similar excess optical loss.
In recent years, we have presented results on the development of erasable gratings in silicon to facilitate wafer scale testing of photonics circuits via ion implantation of germanium. Similar technology can be employed to develop a range of optical devices that are reported in this paper. Ion implantation into silicon causes radiation damage resulting in a refractive index increase, and can therefore form the basis of multiple optical devices. We demonstrate the principle of a series of devices for wafers scale testing and have also implemented the ion implantation based refractive index change in integrated photonics devices for device trimming.
A crucial component of any large scale manufacturing line is the development of autonomous testing at the wafer scale. This work offers a solution through the fabrication of grating couplers in the silicon-on-insulator platform via ion implantation. The grating is subsequently erased after testing using laser annealing without affecting the optical performance of the photonic circuit. Experimental results show the possibility for the realisation of low loss, compact solutions which may revolutionise photonic wafer-scale testing. The process is CMOS compatible and can be implemented in other platforms to realise more complex systems such as multilayer photonics or programmable optical circuits.
We present three main material platforms: SOI, suspended Si and Ge on Si. We report low loss SOI waveguides (rib, strip, slot) with losses of ~1dB/cm. We also show efficient modulators and detectors realized in SOI, as well as filters and multiplexers. To extend transparency of SOI waveguides, bottom oxide cladding can be removed. We have fabricated low loss passive devices in a suspended platform that employ subwavelength gratings. Ge on Si material can have larger transparency range than suspended Si. We have designed passive devices in this platform, demonstrated all optical modulation and carried out two photon absorption measurements. We have also investigated theoretically free carrier optical modulation in Ge.
Communication traffic grows relentlessly in today’s networks, and with ever more machines connected to the network, this trend is set to continue for the foreseeable future. It is widely accepted that increasingly faster communications are required at the point of the end users, and consequently optical transmission plays a progressively greater role even in short- and medium-reach networks. Silicon photonic technologies are becoming increasingly attractive for such networks, due to their potential for low cost, energetically efficient, high-speed optical components. A representative example is the silicon-based optical modulator, which has been actively studied. Researchers have demonstrated silicon modulators in different types of structures, such as ring resonators or slow light based devices. These approaches have shown remarkably good performance in terms of modulation efficiency, however their operation could be severely affected by temperature drifts or fabrication errors. Mach-Zehnder modulators (MZM), on the other hand, show good performance and resilience to different environmental conditions. In this paper we present a CMOS-compatible compact silicon MZM. We study the application of the modulator to short-reach interconnects by realizing data modulation using some relevant advanced modulation formats, such as 4-level Pulse Amplitude Modulation (PAM-4) and Discrete Multi-Tone (DMT) modulation and compare the performance of the different systems in transmission.
In this paper we present SOI, suspended Si, and Ge-on-Si photonic platforms and devices for the mid-infrared. We demonstrate low loss strip and slot waveguides in SOI and show efficient strip-slot couplers. A Vernier configuration based on racetrack resonators in SOI has been also investigated. Mid-infrared detection using defect engineered silicon waveguides is reported at the wavelength of 2-2.5 μm. In order to extend transparency of Si waveguides, the bottom oxide cladding needs to be removed. We report a novel suspended Si design based on subwavelength structures that is more robust than previously reported suspended designs. We have fabricated record low loss Ge-on-Si waveguides, as well as several other passive devices in this platform. All optical modulation in Ge is also analyzed.
We have demonstrated a bidirectional wavelength division (de)multiplexer (WDM) on the silicon-on-insulator platform. An excellent match of the peak transmission wavelength of each channel between the two AMMIs was achieved. This type of device is ideal for integrated optical transceivers where the transmission wavelengths are required to match with the receiving wavelengths. The device also benefits from simple fabrication (as only a single lithography and etching step is required), improved convenience for the transceiver layout design, a reduction in tuning power and circuitry, and efficient use of layout space.
This paper summarises our work on modulators for integration, either as a front end approach, or a co-location of custom electronic drivers, approaches that have yielded data rates up to 50Gb/s from a range of device variants. As well as more conventional depletion based devices, we also report photonic crystal cavity based modulators for very low power consumption, as well as other device variants aimed at improving device performance metrics.
Silicon-based optical modulator devices have experienced dramatic improvements over the last decade with data rates
up to 50Gbps for On-Off-Keying (OOK) consuming ultra low power in fJ/bit [1-3]. The ability to fully understand the
performances of these plasma dispersion effect-based devices from a simulation standpoint could be further improved
especially in the coupling of high-speed electrical and optical effects. Here, we report an accurate methodology to study
high-speed eye diagrams from the electrical and optical simulation data of individual silicon modulators. In particular,
we demonstrate the capacity of this simulation methodology by applying it to the current state-of-the-art experimental
demonstrated silicon optical modulator using OOK at 50Gbps .
In this paper we will discuss recent results in our work on Silicon Photonics. This will include active and passive devices for a range of applications. Specifically we will include work on modulators and drivers, deposited waveguides, multiplexers, device integration and Mid IR silicon photonics. These devices and technologies are important both for established applications such as integrated transceivers for short reach interconnect, as well as emerging applications such as disposable sensors and mass market photonics.
The angled multimode interferometer (AMMI) was recently demonstrated as a wavelength division (de)multiplexing
(WDM) structure employing the phenomenon of dispersive self-imaging in a multimode planar waveguide. It has
distinct advantages including very low insertion loss, ease of fabrication and good tolerance to fabrication error. Here,
we first demonstrated a further optimized single 4-channel AMMI on the silicon-on-insulator (SOI) platform with a low
insertion loss of < 1 dB, a low cross talk of < -23 dB and a non-uniformity of < 0.5 dB. However, a single AMMI is
limited to have small channel count with a relatively large channel spacing in order to maintain high-performance WDM
functionality. Therefore we subsequently demonstrated two structures derived from a single AMMI on the SOI platform:
An 8-channel WDM comprising of two 4-channel AMMIs interleaved by an imbalanced Mach-Zehnder interferometers
(MZIs) was fabricated. Low-insertion-loss and low-cross-talk operation was achieved without active tuning structures.
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.
Group IV mid-infrared photonics is attracting more research interest lately. The main reason is a host of potential
applications ranging from sensing, to medicine, to free space communications and infrared countermeasures. The field is,
however, in its infancy and there are several serious challenges to be overcome before we see progress similar to that in
the near-infrared silicon photonics. The first is to find suitable material platforms for the mid-infrared. In this paper we
present experimental results for passive mid-infrared photonic devices realised in silicon-on-insulator, silicon-on-sapphire,
and silicon on porous silicon. We also present relationships for the free-carrier induced electro-refraction and
electro-absorption in silicon and germanium in the mid-infrared wavelength range. Electro-absorption modulation is
calculated from impurity-doping spectra taken from the literature, and a Kramers-Kronig analysis of these spectra is used
to predict electro-refraction modulation. We examine the wavelength dependence of electro-refraction and electro-absorption,
finding that the predictions suggest longer-wave modulator designs will in many cases be different than those
used in the telecom range.
Slow light optical modulators are attracting ever more attention in the field of silicon photonics owing to their capacity
to shrink the footprint of conventional rib waveguide based carrier depletion modulators while maintaining similar drive
voltages. Nonetheless, the integration of future photonics components with advanced complementary-metal-oxide-semiconductor
(CMOS) electronics will require drive voltages as low as 1V. Here, we demonstrate that the use of slow
light provides an attractive solution to reduce the driving power of carrier depletion-based Mach-Zehnder modulators so
that they fulfill the consumption requirements of future CMOS electro-photonics transceivers. Preliminary
characterization results show that our 1mm-long slow light device features a data transmission rate of 5 Gbit/s with ~5.7
dB extinction ratio under a 1V drive voltage with 12dB insertion loss. Further measurements show that higher
transmission speeds are achievable while sustaining the drive voltage close to current CMOS requirements.
We experimentally demonstrate a simple but more efficient technique to modulate and multiplex multiple WDM
channels. Our design is based on a bus waveguide vertically coupled to multiple Photonic Crystal (PhC) resonator, each
of which modulates an individual channel in place. The Photonic crystal resonator modulator provide very low switching
energies (~fJ) while the bus waveguide can be made from a material with a low refractive index thereby allowing very
efficient coupling with an optical fiber.
In this paper we describe two silicon based optical modulators that have been
fabricated as part of two projects in which the Surrey group is involved, the "UK
Silicon Photonics project" funded by the UK Engineering and Physical Sciences
Research Council (EPSRC), and the European "HELIOS" project funded by the
European Union. The modulators exploit the carrier depletion effect in MZI structures,
but have different advantages and disadvantages. One has a performance that is
close to polarisation independence, whilst the other demonstrates a very high
extinction ratio for a 40Gb/s silicon modulator. Both are shown to operate at 40Gb/s.
The HELIOS project is a European funded program which focuses on the development and
integration of the different photonic and electronic building block components required to form high
performance photonic circuits with a variety of functionality. One of the key photonic building block
components central to most photonic applications is the optical modulator which is required to write
data onto an optical carrier. Within the project two designs of carrier depletion based phase
modulator are under development, together with a means of enhancing the modulation effect using
slow wave and ring resonator based structures. In this work modulation results from the two phase
shifters are presented along with passive results from related slow wave and resonator structures.
The workhorse of future high speed short reach interconnect technology will be the optical modulator.
These devices in silicon have experienced dramatic improvements over the last 6 years and the
modulation bandwidth has increased from a few tens of MHz to over 30 GHz. However, the demands
of optical interconnects are significant. Hence, the need for devices with compact real estate,
broadband characteristics, operating at high speed and working for both polarisation is of outmost
importance. Here we describe the approach taken at Surrey to meet these requirements from the early
days to the more recent work where some initial data are introduced. The recent all-silicon optical
modulator uses a CMOS compatible fabrication and demonstrates high data rate with large extinction
ratio for TE and TM polarisations. This technology is not only compatible with conventional
complementary MOS (CMOS) processing, but is also intended to facilitate a high yield, reliable
Silicon optical modulators have generated an increasing interest in the recent years, as their
performances are crucial to achieve high speed optical links. Among possibilities to achieve
optical modulation in silicon-based materials, index variation by free carrier concentration
variation has demonstrated good potentiality. High speed and low loss silicon modulators can
be obtained by carrier depletion inside lateral PN or PIPIN diodes. When the diode is reverse
biased, refractive index variations are obtained and then phase modulation of the guided wave
is obtained. Mach-Zehnder interferometers are used to convert phase modulation into
intensity modulation. Experimental results are presented for both PN and PIPIN diodes.
Silicon photonics has generated a growing interest with impressive results on active devices
like optical modulators and photodetectors in the last few years. In the framework of the
European project HELIOS, several research groups and industrial partners work on the main
building blocks to make high-speed optical links based on either silicon-based materials or
III-V components bonded on silicon. Here, we present an overview of the main achievements
on PN and PIPIN optical modulators based on carrier depletion and on germanium and III-V
photodetectors integrated with silicon waveguides.
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.
Silicon Photonics is a field that has seen rapid growth and dramatic changes in the past 5 years. According to the MIT
Communications Technology Roadmap , which aims to establish a common architecture platform across market
sectors with a potential $20B in annual revenue, silicon photonics is among the top ten emerging technologies. This has
in part been a consequence of the recent involvement of large semiconductor companies around the world, particularly in
the USA. Significant investment in the technology has also followed in Japan, Korea, and in the European Union. Low
cost is a key driver, so it is imperative to pursue technologies that are mass-producible.
Therefore, Silicon Photonics continues to progress at a rapid rate. This paper will describe some of the work of the
Silicon Photonics Group at the University of Surrey in the UK. The work is concerned with the sequential development
of a series of components for silicon photonic optical circuits, and some of the components are discussed here. In
particular the paper will present work on optical waveguides, optical filters, modulators, and lifetime modification of
carriers generated by two photon absorption, to improve the performance of Raman amplifiers in silicon.
As Silicon Photonics is developing further towards integration on a single platform, the need for precise fabrication is
paramount and no matter how developed a technology is, there is always potential for error at the wafer and chip level. In
combination with Focused Ion Beam (FIB) technology, we present direct write methods to fabricate and tailor Silicon
Photonic devices to offer the potential of prototyping, testing and correction in a post-processing environment. However,
inherent in most FIB processing is the introduction of large optical loss due to damage and implantation of Gallium,
because Gallium is typically the gas species used in FIBs. Therefore, methods of processing to minimise potential loss
and changes to the original device design will be presented alongside results and a discussion offering a comparison with
other potential methods.
We investigate the effects of silicon ion irradiation on free carrier lifetime and propagation loss in silicon rib
waveguides, and thus its ability to reduce the density of two-photon-absorption (TPA) generated free carriers, an
undesired effect of the Raman process in crystalline silicon. Our experimental results show that free carrier lifetime can
be reduced significantly by silicon ion implantation. Associated excess optical absorption from the implanted silicon ions
can be kept low if irradiation energy and dose are correctly chosen. Simulations of Raman amplification in silicon rib
waveguides suggest that net gain can be achieved in certain cases without the need for an integrated diode in reverse bias
to sweep out the photo-generated free carriers.
The field of Silicon Photonics has gained a significant amount of momentum in recent years. Announcements of high
speed modulators and cost-efficient light sources in the Silicon-on-insulator material system have helped to make Silicon
Photonics a viable contender as a low-cost active photonic platform. As a pioneer in the field, the University of Surrey
continues to investigate the prospects of silicon photonics. Herein we present a summary of our work on several key
areas such as ion implanted grating devices, high-speed modulators, switches and ring resonators. We conclude with a
discussion on an advanced fabrication technique, proton beam writing.
Silicon-on-Insulator (SOI) has emerged as promising material choice for various integrated optoelectronic devices. Two
issues make SOI attractive for complex optical systems: the cost reduction due to compatibility with CMOS technology
and high refractive index contrast between core and cladding, which is an important property for good confinement of
light and efficient guiding and coupling in sub-micron waveguides. However, for those devices that are intended to be
part of broadband optical networks, for example multiplexers and de-multiplexers, it is desirable to demonstrate a high
selectivity and a tunable response. Thus, it is necessary to provide wavelength selective elements with the ability to filter
input data streams producing a large Free Spectral Range (FSR), a small Full Width at Half Maximum (FWHM), and a
high quality factor (Q), all conditions set by communication standards. Owing to the generic and adaptable operation,
ring-resonator-types of filters in SOI are often considered as candidates to meet these demands. Herein two different
designs are investigated from both experimental and modelling standpoints in order to tailor the filter transfer function.
These are mutually coupled (Vernier) resonators and cascaded resonators based on small SOI photonic wires. Fabricated
filters designed to provide a large FSR and a polarisation independent (PI) response are analysed and improvements
proposed. Issues associated with temperature control of the transfer function have also been addressed.
The requirement of a precise and controllable reflection interface in total internal reflection type optical switches is
widely acknowledged. When these switches are based upon carrier injection such as those fabricated in silicon-oninsulator
the ability to set up a precise reflection interface becomes difficult due to the diffusion of carriers. This
diffusion of carriers across the reflection interface creates a refractive index gradient which is likely to cause the input
light to be imperfectly reflected into the output port, which is obviously less efficient than reflection from a precise
interface in terms of loss due to the absorption by the free carriers and the directivity of the reflected wave. In our work
we propose the use of a barrier positioned along the reflection interface, and around a completely enclosed injection
region to prevent diffusion of carriers, and therefore set up a precise reflection interface. The barrier will also improve
the injection efficiency since the carriers are being injected into a much smaller volume. This will, in turn, lead to a
reduced switching current and faster switching speeds. This paper reports the modeling of the device and predicts the
bandwidth performance for one specific switch design.
In this paper we report two novel fabrication techniques for silicon photonic circuits and devices. The techniques are
sufficiently flexible to enable waveguides and devices to be developed for telecommunications wavelengths or indeed
other wavelength ranges due to the inherent high resolution of the fabrication tools. Therefore the techniques are
suitable for a wide range of applications. In the paper we discuss the outline fabrication processes, and discuss how they
compare to conventional processing. We compare ease of fabrication, as well as the quality of the devices produced in
preliminary experimental fabrication results. We also discuss preliminary optical results from fabricated waveguide
devices, as measured by conventional means. In these preliminary results we discuss fundamental properties of the
waveguides such as loss and spectral characteristics, as it is these fundamental characteristics that will determine the
viability of the techniques. Issues such as the origins of the loss are discussed in general terms, as resulting fabrication
characteristics such as waveguide surface roughness (and hence loss), or waveguide profile and dimensions may be
traded off against cost of production for some applications. We also propose further work that will help to establish the
potential of the technique for future applications.
The single-mode optical rib waveguide is a fundamental building block for many, more complex optical circuits. Recent modelling has been provided in the literature that has investigated polarisation and modal properties of small, deeplyetched rib waveguides in SOI. In this paper we present work that has utilised a total of 160 directional couplers fabricated from rib waveguides of various waveguides dimensions, to investigate the validity of the published modelling. In particular 5 waveguide designs have been used to fabricate directional couplers of differing lengths, to map out the variation in coupling of power within the directional couplers. For a singlemode device, a characteristic sinusoidal variation is expected, but the sinusoid will be corrupted in the presence of higher order modes, each of which will have a different coupling length as compared to the fundamental mode. We have observed experimental results that are consistent with the modelling for each of the 5 waveguide designs, and hence we present experimental evidence of higher-order mode behaviour that is consistent with modelling.