Nanoscale Si-based photonics for next generation integrated circuits

Subwavelength conductor-gap-silicon plasmonic waveguides along with compact S-bends and Y-splitters were theoretically investigated and experimentally demonstrated on a silicon-on-insulator platform. A thin SiO2 gap between the conductor layer and silicon core provides subwavelength confinement of light while a long propagation length of 40 microm was achieved. Coupling of light between the plasmonic and conventional silicon photonic waveguides was also demonstrated with a high efficiency of 80%. The compact sizes, low loss operation, efficient input/output coupling, combined with a CMOS-compatible fabrication process, make these conductor-gap-silicon plasmonic devices a promising platform for realizing densely-integrated plasmonic circuits.

This paper presents the design and preliminary results of a low power, compact and high-speed modulator in the O-band featuring apodized slow-light structures and a slow-wave RF design. The device is a candidate for future single mode optical interconnects in large-scale data centers.

We present a multistack design of ultra-thin germanium-on-silicon (Ge-on-Si) photodetectors (PDs) featuring both backside-illuminated (BSI) and top-illuminated architectures, optimized for operation in the 1.0–1.4 μm wavelength range. Using Monte Carlo molecular dynamics simulations, we evaluate the effects of doping profiles and biasing on device performance. A Ge absorption layer with a thickness of 350 nm enables high-speed operation exceeding 60 GHz, while nanostructures integrated at the bottom of the Ge layer enhance optical absorption efficiency to as high as 80%. The BSI design supports vertical integration with electronic wafers, facilitating compact, low-latency signal processing and transmission—particularly beneficial for optical interconnects in data centers and other high-performance computing environments. We also perform a comparative analysis of BSI and top-illuminated designs, highlighting the performance trade-offs and integration advantages in high-speed, vertically stacked, co-packaged optics. These results offer a compact, CMOS (Complementary Metal-Oxide-Semiconductor) -compatible solution for high-speed optical interconnects in future hyperscale data centers.

For development of on-chip photonic circuits, a complementary metal-oxide-semiconductor (CMOS)-compatible hybrid plasmonic slot waveguide is proposed. The characteristics of its guided hybrid modes at a wavelength of 1.55 μm are investigated by the finite-element method. The combination of a Si gate and metallic features of a CMOS electronic device provides both subwavelength light confinement and long propagation length with high power intensity. The characteristics of the guided hybrid mode are improved by optimizing the configurations of the Si core. The proposed hybrid plasmonic slot waveguide is compatible not only with the silicon electronics fabrication processes but also with the metallic features of CMOS electronic devices.

Multimode interference effect in plasmonic subwavelength waveguides and an ultra-compact power splitter

The study of quantum cascade lasers (QCL) in the terahertz frequency region by the use of THz time-domain spectroscopy has gained a lot of interest within the last few years [1]. Good physical insight was obtained for THz QCLs employing a single plasmon waveguide (surface plasmon waveguide) [2,3]. In comparison to that the double plasmon or metal-metal waveguide QCL uses a metal/semiconductor/metal structure as a waveguide. THz QCLs with this type of waveguide are showing the highest operating temperatures so far [4]. The metal-metal THz QCLs confine the whole mode in the active region of subwavelength size. In order to investigate metal-metal THz QCLs by THz pulses an efficient method of THz pulse launching needs to be employed. Several methods for coupling of free space THz pulses into the subwavelength waveguide, such as horn antenna, gratings or lenses can be used. A further method is a waveguide emitter [5,6] which benefits from the near infrared excitation beam focus which is much smaller than the corresponding THz focus.

Public and Private Enterprise clouds are changing the nature of WAN data center interconnects. Data center WAN interconnects today are pre-allocated, static optical trunks of high capacity. These optical pipes carry aggregated packet traffic originating from within the data centers while routing decisions are made by devices at the data center edges. In this paper, we propose a software-defined networking enabled optical transport architecture (Transport SDN) that meshes seamlessly with the deployment of SDN within the Data Centers. The proposed programmable architecture abstracts a core transport node into a programmable virtual switch that leverages the OpenFlow protocol for control. A demonstration use-case of an OpenFlow-enabled optical virtual switch managing a small optical transport network for a big-data application is described. With appropriate extensions to OpenFlow, we discuss how the programmability and flexibility SDN brings to packet-optical data center interconnect will be substantial in solving some of the complex multi-vendor, multi-layer, multi-domain issues that hybrid cloud providers face.

An algorithm for network and data-aware placement of multi-tier applications in cloud data centers

Spectrum engineering in flexible grid data center optical networks

With continuing speed increases of communication links found both inside and outside the servers in a data center, link power management is becoming an important issue. In this paper we examine width control of such links and show that it provides substantial benefits over other methods for latency sensitive applications. The paper also presents the design of a specific width control algorithm called DWCA (dynamic width control algorithm) that is effective and easily implemented in hardware at high data rates.

We present a new graphene-based long-range surface plasmon waveguide in this paper. The plasmon wave in this waveguide is propagated in the core enclosed between two anisotropic clads which are in the form of photonic crystal consisting of graphene layers and dielectric. We show that, using the anisotropic clad causes flexibility in the design of waveguide. Also, with the change of chemical potential of graphene we can adjust the propagation length and the confinement of plasmon wave. The results of this research are presented in the terahertz frequency range. Finally, the presented waveguide has simultaneously an acceptable propagation length and confinement.

The ultrastrong light-matter interaction regime was investigated in metallic\nand superconducting complementary split ring resonators coupled to the\ncyclotron transition of two dimensional electron gases. The sub-wavelength\nlight confinement and the large optical dipole moment of the cyclotron\ntransition yield record high normalized coupling rates of up to\n$\\frac{\\Omega_R}{\\omega_c}=$ 0.87. We observed a blue-shift of both polaritons\ndue to the diamagnetic term of the interaction Hamiltonian.\n

The volume of traffic carried on the Optical Transport Network (OTN) is continuously growing, not only due to the increase of new services, new applications, and the number of both connected users and smart devices, but also owing to the rapidly growing Data Center Interconnect (DCI) market. As a result, telecom operators need to upgrade their transport networks to cope with these new traffic requirements. This tremendous increase in bandwidth demand will also introduce energy bottlenecks in OTNs. The telecommunication networks' significant Greenhouse Gas Emissions (GGE) is another challenge that must be addressed in OTN design and planning policies. Thus, energy-efficient OTN architectures are currently attracting the attention of telecom operators. The candidate OTN architectures are: (1) the conventional Dense Wavelength Division Multiplexing (DWDM) fixed grid with high Single Line Rates (SLRs); (2) the conventional DWDM fixed grid adopting Mixed Line Rates (MLRs); and (3) the Elastic Optical Network (EON) or flexigrid-based architecture which is enabled by the Optical Orthogonal Frequency Division Multiplexing (O-OFDM) technique. This paper evaluates the power consumption of the abovementioned OTN architectures under different traffic loads and patterns, as well as different network physical topologies. In order to perform the evaluation, new heuristic algorithms are specifically developed for the green design and planning of data center optical interconnects.

Integration of photonic devices with CMOS circuits has recently generated much interest for numerous applications ranging from Internet networks to sensor technology. For example, integrated optical interconnects have emerged as a strong contender to replace bandwidth-hungry and energy-guzzling electrical interconnects in data centers and inside microprocessors. However, the lack of group IV semiconductor materials suitable for light emission has thus far prevented the realization of such important photonic-integrated circuits. In this paper, we will present our research works on strain-engineered group IV light sources on silicon that will help enable photonic-integrated circuits.

Optical transmitters for four-level pulse amplitude modulation (PAM-4) have attracted a significant amount of research in recent years, in large part due to the standardization of the format for the 200 and 400 Gigabit Ethernet optical interconnects in data centers. However, combining low-power and linear operation of the electro-optical frontend with sufficiently large bandwidths has proven challenging, especially for the 100 Gb/s/lambda links (i.e., employing 50 Gbaud PAM-4). The most straightforward solution has been to deal with the non-idealities of the modulator in the electrical domain: predistorting the signal levels and/or equalizing the frequency response with the help of digital signal processing (DSP). However, this typically requires fast digital-to-analog converters (DACs), either capable of delivering large swings (>1 Vpp) or supplemented with an additional linear amplifier to drive the optical modulator. Both options substantially increase the power consumption and the complexity of the transceiver. Rather than allocating effort to linearize the electrical to optical conversion of a single modulator, we propose a topology that performs the DAC operation in the optical domain. Two compact electro-absorption modulators in an interferometer layout are driven with NRZ data to generate the four-level signal in the optical domain. Using this topology, we demonstrate the first real-time 128 Gb/s PAM-4 transmission with a silicon photonic transmitter in a chip-to-chip link. In a back-to-back setup, we obtained a bit-error ratio (BER) of 4 x 10(-10) without requiring any DAC, DSP, or modulators with large traveling wave structures. Over 1 km of standard single mode fiber a BER of 8 x 10(-6) is recorded, still well below the KP4 forward error-coding limit. These results correspond to the lowest BERs reported for any real-time PAM-4 link at 100 Gb/s or higher, illustrating the benefit of performing the DAC operation in the optical domain.