Silicon photonics (SiPh) has emerged as the predominant platform across a wide range of integrated photonics applications, encompassing not only mainstream fields such as optical communications and microwave signal processing but also burgeoning areas such as artificial intelligence and quantum processing. A vital component in most SiPh applications is the optical phase shifter, which is essential for varying the phase of light with minimal optical loss. Historically, SiPh phase shifters have primarily utilized the thermo-optic coefficient of silicon for their operation. Thermo-optic phase shifters (TOPSs) offer significant advantages, including excellent compatibility with complementary metal–oxide–semiconductor technology and the potential for negligible optical loss, making them highly scalable. However, the inherent heating mechanism of TOPSs renders them power-hungry and slow, which is a drawback for many applications. We thoroughly examine the principal configurations and optimization strategies that have been proposed for achieving energy-efficient and fast TOPSs. Furthermore, we compare TOPSs with other electro-optic mechanisms and technologies poised to revolutionize phase shifter development on the SiPh platform.
The interest in the development of silicon photonic integrated devices is rapidly expanding from the telecom/datacom sector to new emerging application domains such as artificial intelligence or quantum photonics. Silicon benefits from CMOS-compatible fabrication processes and a high index contrast. However, the implementation of new functionalities or the achievement of a superior performance necessarily requires the integration of new materials in current silicon photonics platforms. In this context, phase change materials have been established as promising material technologies for optical switching. In this work, the benefits and challenges for enabling optical switching with VO2/Si and GST/Si devices will be analyzed and discussed.
Reconfigurable photonics enable the realization of diverse functionalities using a single photonic device. Such devices could play a prominent role in a large variety of applications, including neuromorphic computing, telecommunications, data communication networks, or optical sensing. The silicon photonics platform is the ideal candidate to implement those devices due to its unique capability for handling large scalability and mass-manufacturing at low cost. However, switching and modulation functionalities offered by current silicon platforms are based on the plasma dispersion effect or the thermo-optic effect, which yields devices with large footprints or reduced bandwidth, preventing scalability. Therefore, combining silicon photonics with materials with unique optoelectronic properties is emerging as the most promising path towards developing ultra-compact devices with competitive performance. In this context, new functionalities not yet offered by current silicon platforms may be implemented by the integration of phase change materials (PCMs) and transparent conducting oxides (TCOs) in silicon structures. Hybrid PCM/Si and TCO/Si devices will be presented for implementing weighting operation, reconfigurable activation functions, and optical storage, which are crucial functionalities in artificial neural networks and the emerging field of neuromorphic computing.
We numerically study the chaotic synchronization of microresonator frequency combs. The chaotic state of microresonators could be a key factor in optical communications because the modulation instability state (chaotic) has a larger output than the soliton state (stable), which may enable us to realize a higher signal-to-noise ratio. In addition, it will allow secure communication. We show that two microresonator frequency combs in a leader-follower configuration can be synchronized in chaotic regimes. Interestingly, the follower comb synchronizes even when some longitudinal modes of the leader comb are absent. We also show that the Turing pattern comb in the follower ring becomes chaotic and synchronizes when we inject the leader’s modulation instability comb.
There is a natural trade-off between spacecraft size and functionality in all current satellite applications, independently of orbit and mission. Therefore, advances in both miniaturization and integration technologies are required to increase satellites’ lifetime and performance, simultaneously reducing their cost. In case of the next generation of Earth Observation satellites, one of the key development areas is synthetic aperture radar (SAR) antennas, where expected progress will be to increase the operating bandwidth - requiring, for instance wideband true-time delay (TTD) beamformers - and miniaturization, drastically reducing the mass and volume compared to current implementations. In this scenario, the use of photonic integrated circuits (PIC) technology in the beamforming network, in combination with an optical fibre harness, are obvious key enabling technologies for future SAR instruments. Optically implemented TTD beamforming structures achieve orders-of-magnitude improvements in size and mass compared with coaxial cable and RF switch based alternatives. Photonic technology also brings easy routing thanks to wavelength-division multiplexing, antenna and RF system integration due to the EMI -free characteristic of the optical fibre and a reduction of the risks associated with the in-orbit antenna deployment. Additionally, the inherent broadband characteristic of photonic technology, related to the transport and processing of RF signals, simplifies the beamforming network and signal distribution design for different frequencies, applications and missions. In the H2020 RETINA project (H2020-SPACE-2018-821943) a consortium formed by DAS Photonics, Airbus Italia, AMO GmbH, STFC Rutherford Appleton Laboratory and Universitat Politècnica de València is developing a miniaturised photonic front-end for next-generation X-band space SAR applications. In this article we present advances in design and fabrication of PIC for TTD, the design and predicted performance of multi element, dual polarisation antenna building blocks and photoreceivers for phase and amplitude controlled optical to RF conversion.
This paper reports and analyses the hybrid 5G NR (new-radio) fronthaul technology with emphasis in the different functional splitting points, and the associated silicon-photonics technologies which enable a completely integrated electro-optical transceiver. Different silicon photonics solutions have been proposed so far targeting an integrated solution combining electronics and photonics, where electronics deal with digital coding and MAC forming, and photonics implement modulation (enabling QPSK, QAM and PAM4 transmissions above 100 Gbit/s) and switching functionalities (reducing the switching time to the ns range). High-speed transceivers integrated in bulk CMOS would enable Tbit/s optical interconnects. In particular, this work firstly summarizes the recent advances and challenges of silicon photonics technology that will have an impact on 5G applications in a near future. Next, an experimental demonstration of a radio-over-fiber fronthaul for the simultaneous provision of multiple radio-access technologies including 2G, 3G and 4G is included. Enhanced capabilities enabled by silicon photonics including MIMO and beamforming are also evaluated.
Programmable switching nodes supporting Software-Defined Networking (SDN) over optical interconnecting technologies arise as a key enabling technology for future disaggregated Data Center (DC) environments. The SDNenabling roadmap of intra-DC optical solutions is already a reality for rack-to-rack interconnects, with recent research reporting on interesting applications of programmable silicon photonic switching fabrics addressing board-to-board and even on-board applications. In this perspective, simplified information addressing schemes like Bloom filter (BF)-based labels emerge as a highly promising solution for ensuring rapid switch reconfiguration, following quickly the changes enforced in network size, network topology or even in content location. The benefits of BF-based forwarding have been so far successfully demonstrated in the Information-Centric Network (ICN) paradigm, while theoretical studies have also revealed the energy consumption and speed advantages when applied in DCs. In this paper we present for the first time a programmable 4x4 Silicon Photonic switch that supports SDN through the use of BF-labeled router ports. Our scheme significantly simplifies packet forwarding as it negates the need for large forwarding tables, allowing for its remote control through modifications in the assigned BF labels. We demonstrate 1x4 switch operation controlling the Si-Pho switch by a Stratix V FPGA module, which is responsible for processing the packet ID and correlating its destination with the appropriate BF-labeled outgoing port. DAC- and amplifier-less control of the carrier-injection Si-Pho switches is demonstrated, revealing successful switching of 10Gb/s data packets with BF-based forwarding information changes taking place at a time-scale that equals the duration of four consecutive packets.
The change in the optical properties of VO2 across its insulator to metal to transition could give rise to novel photonic devices with beyond state-of-the art performance in terms of footprint, power consumption and speed operation. However, the final performance strongly depends on the control and efficiency of the phase transition in the VO2. For electro-optical applications, such control and efficiency is highly dependent on the electrode configuration. In this work, the influence of the electrodes is experimentally analyzed and an alternative approach is proposed to switch the VO2 to the metallic state by minimizing the electrical power consumption. The electro-optical performance of the proposed short-circuited electrode is also experimentally demonstrated. An extinction ratio of 12 dB is achieved with a 20μm long hybrid VO2/Si waveguide with an electrical power of only 11mW. The power consumption could be further reduced by decreasing the distance between the electrode and the silicon waveguide, which in this work has been fixed to 1.5μm, without affecting the optical losses.
The electro-optic Pockels effect may be generated in silicon photonics structures by breaking the crystal symmetry by means of a highly stressing cladding layer (typically silicon nitride, SiN) deposited on top of the silicon waveguide. In this work, the influence of the waveguide parameters on the strain distribution and its overlap with the optical mode to enhance the Pockels effect has been analyzed. The optimum waveguide structure have been designed based on the definition and quantification of a figure of merit. The fabrication of highly stressing SiN layers by PECVD has also been optimized to characterize the designed structures. The residual stress has been controlled during the growth process by analyzing the influence of the main deposition parameters. Therefore, two identical samples with low and high stress conditions were fabricated and electro-optically characterized to test the induced Pockels effect and the influence of carrier effects. Electro-optical modulation was only measured in the sample with the high stressing SiN layer that could be attributed to the Pockels effect. Nevertheless, the influence of carriers were also observed thus making necessary additional experiments to decouple both effects.
Electron beam evaporation was employed in order to fabricate Al- and Tb-codoped Si oxide multilayers via the delta-doping approach. This methodology permits the control of the rare-earth (RE) separation along the growth direction with nanometric resolution. To investigate the control of the RE separation in the growth direction, different SiO2 thicknesses were studied. After deposition, the samples were submitted to different annealing processes for 1 h in N2, at temperatures ranging from 700 to 1100 °C. Photoluminescence experiments reveal narrow emissions ascribed to Tb3+ ions in all samples, with an intensity variation depending on the oxide thickness and annealing temperature. In addition, the incorporation of Al under different spatial configurations produced an enhancement of more than one order of magnitude in the photoluminescence intensity, in respect to the best sample without Al. Finally, time-resolved measurements were carried out in order to determine the 5D4→7F5 transition dynamics, obtaining a decay time of ~1.6 ms ascribed to the Tb3+ ions.
A novel method to significantly decrease power consumption in a silicon switch based on an asymmetric Mach-Zehnder interferometer (MZI) structure is proposed and experimentally demonstrated. A radical power consumption reduction up to 50% is achieved for switching digital data at bit rates from 10 to 30Gbps with respect to a conventional switch based on a symmetric MZI. Furthermore, the broadband performance of the proposed silicon MZI comb switch is also demonstrated by transmitting a 120 Gbps DWDM data stream.
Silicon-photonic 2×2 electro-optical switching elements and modulators based on the carrier depletion mechanism using both dual-resonator and MZI layout configurations have been developed. The passive photonic structures were developed and optimized using a fast design-fabrication-characterization cycle. The main objective is to deliver smallfootprint, low-loss and low-energy silicon photonic electro-optical switching elements and modulators equipped with standard input-output grating couplers and radio-frequency electrical contact tips to allow their characterization in highspeed probe-station setups. The insertion losses, crosstalk, power consumption and BER performance will be addressed for each electro-optical structure. The fabrication steps, including low loss waveguide patterning, pn junction and low resistive ohmic contact formation have been optimized to produce high performance devices with relaxed fabrication tolerances, employing both optical and electron-beam lithography.
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.
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.
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.
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.
A simple configuration for achieving a radio frequency transparent 90° hybrid, for broadband QAM wireless systems
using silicon photonics is proposed. The device consists of a high Q ring resonator which induces an optical 90° phase
shift between two adjacent resonant wavelengths. When these optical carriers are modulated by an RF carrier the
resulting device behaves as an RF 90° hybrid. Numerical simulations of the phase shift were performed on a 40 GHz
carrier, and to demonstrate the frequency transparency phase shift simulations was also performed at a carrier frequency
of 60 GHz. One of the main applications of such a device is the generation of millimeter wave 10 Gb/s wireless based on
quadrature amplitude modulation.
Generic packaging concepts for silicon photonics have been developed in the frame of EU-funded Network of
Excellence ePIXnet (FP6). Three approaches for Silicon photonic packaging will be presented within this paper. Two
concepts provide solutions for fiber array coupling to high-index contrast photonic wire waveguide gratings. Third
concept is the integration of inverted taper-based fiber coupling structure with silicon etched V-grooves. Using
standardized SOI chip designs and commercial available assembly parts, the packaging concepts allow for small
footprint or flexible use in an R&D environment. The work presented here has resulted from cooperation within the
European Network of Excellence ePIXnet.
Optical nonlinear effects have been widely studied in III-V semiconductor photonics. However, nonlinear performance
in silicon photonics is still inefficient. An alternative silicon-based waveguide configuration, which is known as slot
waveguide, has been recently proposed to improve the nonlinear performance in a very efficient way. In the slot
waveguide, the fundamental mode light is highly confined in a very small region, which is called slot, of a low index
contrast material between two silicon high index contrast layers. This enables the introduction of new silicon photonic
devices in which the characteristics of active optical materials can be efficiently exploited for modulation, switching,
sensing, and other applications. Horizontal and vertical slot waveguides for optimum nonlinear performance have been
recently proposed. However, the horizontal slot waveguide is more feasible for nonlinear applications. To increase
nonlinear performance in the horizontal slot region, silicon nanocrystals (Si-nc) embedded in silica (SiO2) have been
proposed to fill the slot region between the two silicon layers. It is achievable nonlinear performance in the horizontal
slot region for down to 50nm thick slots. However, the lower the slot thickness is, the more difficult the coupling to fiber
results. One of the most developed silicon photonics efficient vertical coupling techniques is the grating coupler. We
demonstrate grating couplers for efficient coupling between horizontal slot waveguides and standard single mode fibers.
Broadband and highly efficient horizontal slot waveguide grating couplers have been obtained by means of simulations.
These grating couplers configuration are suitable for nonlinear performance in silicon photonics. It is achieved 61%
maximum coupling efficiency for λ=1550 nm and TM polarization. Furthermore, a 35 nm 1dB-bandwidth is achievable
for the designed grating couplers.
In this work, several building blocks for high-performance all-optical switching on silicon are addressed. The FP6-PHOLOGIC approach is based on exploiting the nonlinear properties of silicon nanocrystals embedded in slot
waveguides, in which propagating light is highly confined.
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