Free-space Optical Interconnects Research Articles

Transmissive and reflective type multi-nanolayer electro-optical modulators for chip-to-chip free space optical interconnection: wavelength-scale electromagnetic modelling by the method of single expression

Transmissive and reflective type multi-nanolayer electro-optical modulators for chip-to-chip free space optical interconnection: wavelength-scale electromagnetic modelling by the method of single expression

Improvement of misalignment tolerance in free-space optical interconnects

In this paper, the use of micro lenses with non-uniform transmittance apertures as an alternative to those with uniform transmittance apertures in optical communication systems is proposed. In particular, we consider the use of micro lenses with tapered Gaussian transmittance profiles to improve the misalignment tolerance in optical interconnects. We study the effects of utilizing Gaussian transmittance profiles on the propagation of light beams and the signal to crosstalk ratio of misaligned optical systems. Moreover, we consider the use of uniform transmittance profiles in optical systems for the sake of comparison. To this end, the crosstalk optical noise is modeled at the plane of the detectors array considering the two scenarios of uniform and Gaussian apertures. This was possible after finding the optical field for both scenarios at the of the detectors array. Numerical results clearly demonstrate the significant improvement in decreasing the crosstalk and increasing the signal to crosstalk ratio in the considered optical systems upon utilizing the Gaussian profiles.

Bandwidth Density Analysis of Coded Free-Space Optical Interconnects

The performance of free-space optical interconnects (FSOIs) system is significantly influenced by noise, similar to any wireless communication system. This noise has a notable impact on both the bandwidth density and data rate of FSOIs system. To address these challenges, this study proposes the utilization of vertical-cavity-surface-emitting laser (VCSEL) arrays on the transmitter side and photodetector arrays on the receiver side for FSOIs. The study investigates the bandwidth density of the system with and without coding while maintaining a specific bit error rate. An analysis is conducted in the presence of higher-order modes in the laser beams of the FSOIs system and a fundamental Gaussian operating mode. The presence of the higher-order modes leads to degradation in the performance of the FSOIs system in terms of bandwidth density. In addition, we examine the impact of the signal-to-noise ratio (SNR) on the system’s bandwidth density for each considered operating mode. The provided simulation results clearly demonstrate that coding significantly enhances the bandwidth density of the systems, with the extent of improvement being closely tied to the employed code rate and codeword length.

Highly efficient ultra-broad beam silicon nanophotonic antenna based on near-field phase engineering

Optical antennas are a fundamental element in optical phased arrays (OPA) and free-space optical interconnects. An outstanding challenge in optical antenna design lies in achieving high radiation efficiency, ultra-compact footprint and broad radiation angle simultaneously, as required for dense 2D OPAs with a broad steering range. Here, we demonstrate a fundamentally new concept of a nanophotonic antenna based on near-field phase-engineering. By introducing a specific near-field phase factor in the Fraunhofer transformation, the far-field beam is widened beyond the diffraction limit for a given aperture size. We use transversally interleaved subwavelength grating nanostructures to control the near-field phase. A Bragg reflector is used at the end of the grating to increase both the efficiency and the far-field beam width. The antenna has a compact footprint of 3.1 µm × 1.75 µm and an ultra-broad far-field beam width of 52° and 62° in the longitudinal and transversal direction, respectively, while the radiation efficiency reaches 82% after incorporating a bottom reflector to further improve the directionality. This unprecedented design performance is achieved with a single-etch grating nanostructure in a 300-nm SOI platform.

Bandwidth density optimization of misaligned optical interconnects

In this paper, the bandwidth density of misaligned free space optical interconnects (FSOIs) system with and without coding under a fixed bit error rate is considered. In particular, we study the effect of using error correction codes of various codeword lengths on the bandwidth density and misalignment tolerance of the FSOIs system in the presence of higher order modes. Moreover, the paper demonstrates the use of the fill factor of the detector array as a design parameter to optimize the bandwidth density of the communication. The numerical results demonstrate that the bandwidth density improves significantly with coding and the improvement is highly dependent on the used codeword length and code rate. In addition, the results clearly show the optimum fill factor values that achieve the maximum bandwidth density and misalignment tolerance of the system.

Non-diffracting and self-accelerating Bessel beams with on-demand tailored intensity profiles along arbitrary trajectories.

Owing to their robustness against diffraction, Bessel beams (BBs) offer special advantages in various applications. To enhance their applicability, we present a method to generate self-accelerating zeroth-order BBs along predefined trajectories with tunable z direction intensity profiles. The character of tunable z direction intensity profiles in non-diffracting self-accelerating BBs potentially can attract interest in the regimes of particle manipulation, microfabrication, and free-space optical interconnects.

10 Gb/s free space optical interconnect with broadcasting capability enabled by a silicon integrated optical phased array

We have proposed and experimentally demonstrated a reconfigurable free space optical interconnect with broadcasting capability based on an eight-channel silicon integrated optical phased array. By using the silicon integrated beam steering and broadcasting device, 10 Gb/s on–off keying data is transmitted over 15 cm in free space for up to three receivers located in three different cards. The experimental results show that the optical phased array can be used with broadcasting capability provided to multi-receivers in the card to card optical interconnects, which can significantly reduce device size, system complexity, and total costs.

Efficient non-segregated routing for reconfigurable demand-aware networks

More and more networks are becoming reconfigurable: not just the routing can be programmed, but the physical layer itself as well. Various technologies enable this programmability, ranging from optical circuit switches to beamformed wireless connections and free-space optical interconnects. Existing reconfigurable network topologies are typically hybrid in nature, consisting of static and a reconfigurable links. However, even though the static and reconfigurable links form a joint structure, routing policies are artificially segregated and hence do not fully exploit the network resources: the state of the art is to route large elephant flows on direct reconfigurable links, whereas the remaining traffic is left to the static network topology. Recent work showed that such artificial segregation is inefficient, but did not provide the tools to actually leverage the benefits on non-segregated routing.In this paper, we provide several algorithms which take advantage of non-segregated routing, by jointly optimizing topology and routing. We compare our algorithms to segregated routing policies and also evaluate their performance in workload-driven simulations, based on real-world traffic traces. We find that our algorithms do not only outperform segregated routing policies, in various settings, but also come close to the optimal solution, computed by a integer linear program formulation, also presented in this paper. Finally, we also provide insights into the complexity of the underlying combinatorial optimization problem, by deriving approximation hardness results.

High-speed reconfigurable free-space optical interconnects with carrierless-amplitude-phase modulation and filter-enhanced spatial modulation

Optical technologies have been widely studied to satisfy the interconnect demand in high-performance computers and data centers, and free-space optical interconnects have been investigated for flexible card-to-card applications. To increase the interconnect speed and range, in this Letter we propose a reconfigurable free-space optical interconnect with 16-carrierless-amplitude-phase (16-CAP) symbol modulation and filter-enhanced spatial modulation (FE-SM). The 16-CAP increases the interconnect speed in the symbol domain, and the FE-SM scheme increases the speed in the spatial domain, even under highly correlated optical interconnect channels. A 50 Gb/s reconfigurable optical interconnect is experimentally demonstrated. The results show that the impact of FE-SM on the bit error rate (BER) is negligible, while the speed is significantly improved. Compared with the conventional SM scheme, the BER is improved by over one order of magnitude. The proposed scheme is also capable of extending the interconnect range. The results show that over 70% range extension and 50% speed increase can be achieved simultaneously. The proposed scheme with a 16-CAP and FE-SM provides a promising solution in free-space optical interconnects.

Dual-Band Accelerating Beams Enabled Full Duplex Free-Space Optical Interconnection

High speed free-space optical interconnection (FSOI) is an effective strategy to overcome the bottleneck of short-reach electronic interconnection from chip scale to board level, due to its wide bandwidth, high energy efficiency, low latency and high interconnect density. Here, we propose an accelerating beam enabled reconfigurable FSOI scheme by harnessing its characteristics of diffraction-free, self-healing, and specifically designed trajectory. Then we experimentally demonstrate a proof-of-concept 100 Gbps full duplex FSOI enabled by dual-band 2D Airy beams over 40 cm free-space optical link with a receiver sensitivity of -17.3 dBm at bit error ratio (BER) of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> . Both O-band and C-band 2D Airy beams can be successfully generated and detected with the help of two C-band spatial light modulators (SLMs). In addition, we experimentally investigate the impact of an obstacle occurring at the free-space optical link on the BER performance. Our experimental results indicate that, the performance of accelerating beam based FSOI is superior to the conventional Gaussian beam enabled FSOI under the same free-space optical link condition.

A Review of Optical Neural Networks

Optical neural network can process information in parallel by using the technology based on free-space and integrated platform. Over the last half century, the development of integrated circuits has been limited by Moore's law. We know that neural network is based on the digital computer for successive calculation, most of which cannot be made into real-time processing system. Therefore, it is necessary to develop ONN for real-time processing and device miniaturization. In this paper, we review the progress of optical neural networks. Firstly, based on the principle of artificial neural networks, we elaborate the essence of optical matrix multiplier for linear operation. Then we introduce the optical neural network achieved by free-space optical interconnection and waveguide optical interconnection. Finally we talk about the nonlinearity in optical neural networks. With the gradual maturity of nanotechnology and the rapid advancement of silicon photonic integrated circuits, the progress of integrated photonic neural network has been promoted. Therefore, the construction of optical neural network on the future integrated photonic platform has potential application value.

Bessel Beams and Gaussian Beams as information carriers in free space optical interconnects systems: A comparison study

&lt;p&gt;&lt;span style="font-family: Times New Roman;"&gt;We introduce a comparison study for the performance of a lens-based free space optical interconnects system assuming Bessel Beams and Gaussian Beams as information carriers. The optical field at the detector plane was derived for the two beam profiles. In both cases the expressions for the output optical filed are expressed in terms of complex Gaussian functions. The performance of the system for the two beams is evaluated and compared. Using simulation results we show that the use of Bessel beam gives superior results to that of using Gaussian beam for large interconnects distance.&lt;/span&gt;&lt;/p&gt;

(Invited) Heterogeneous Integration of III-V Semiconductors for Imaging and High-Speed Communication

Heterogenous integration of III-V compound semiconductors with Si circuitry is an enabling technology for applications including high-bandwidth communications [1], focal plane arrays [2] and concentrating solar cells [3, 4]. Typical integration schemes include low temperature flip-chip bonding using indium bump bonds along with epoxy underfill and may include the integration of micro-optics for collimation and focusing. In this talk, I will discuss previous and ongoing compound semiconductor heterogenous integration efforts at Sandia National Laboratories and the challenges involved in forming high-yield devices. Specific programs to be discussed include the ARPA-E MOSAIC solar concentrator (Fig. 1), infrared focal plan arrays (Fig. 2), and the recently-awarded DARPA PIPES ultra-low-energy-per-bit photonic transceiver. References Ciftcioglu, et al, “3-D integrated heterogenous intra-chip free-space optical interconnect,” Opt. Express, vol. 20 2012.W. Peters, et al., “Resonant ultrathin infrared detectors enabling high quantum efficiency,” IEEE RAPID, 2018.Li, et al., “Wafer integrated micro-scale concentrating photovoltaics,” Prog. Photovoltaics, vol. 26, 2018.H. Jared, et al., “Integration of wafer-level solar cells and micro-optic concentrators for micro-scale concentrating photovoltaics,” ASPE Annual Meeting, 2017. Acknowledgment This work was supported in part by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government. Figure 1

Models for optical crosstalk in lens-based free-space optical interconnects system that use Gaussian beams as information carriers

We introduce three crosstalk models in lens-based free-space optical interconnects systems. In these models, we use the Gaussian beams as the information carriers. In the first and the second models, the resulted finite integral of the optical field at the detectors plane was solved by expanding the lens’ finite aperture in terms of complex Gaussian functions and in terms of an infinite series of Bessel functions, respectively. In the third proposed model, the resulted finite integral was solved by approximating the Gaussian function in the integrand by zeroth-order and first-order Bessel functions without the need to expand the aperture function. We show that the third proposed model compares favorably with the other models with superior computational efficiency.

High-Speed Reconfigurable Free-Space Optical Interconnects with Carrierless-Amplitude-Phase Modulation and Space-Time-Block Code

High-speed interconnects are highly demanded in data centers and high-performance computing, and the use of optical interconnects to replace electrical interconnects has been proposed and widely studied. For medium-range board-to-board communications, free-space based optical interconnects provide the additional advantage of reconfigurability and flexibility, compared with traditional waveguide or fiber-based counterparts. However, the bit rate and interconnection range in free-space optical interconnects are typically limited. In this paper, we propose a carrierless-amplitude-phase (CAP) modulated and space-time coded free-space optical interconnect scheme to overcome these limitations. Up to 80 Gb/s (2 × 40 Gb/s) interconnection with reconfigurability and flexibility is experimentally demonstrated for a maximum error-free distance exceeding 50 cm, showing about 65% improvement over previous demonstrations. The receiver sensitivity of the space-time coded system is measured as well, and results show that the space-time-block code (STBC) is capable of improving the receiver sensitivity and suppressing the inter-channel crosstalk. The impact of air turbulence, which exists in typical interconnect environments, is also experimentally investigated and results show that its impact on the proposed scheme is negligible. The proposed reconfigurable free-space optical interconnects with CAP and STBC can be further scaled up to provide higher speed and further extended interconnection range.

Free Space Intra-Datacenter Interconnects Based on 2D Optical Beam Steering Enabled by Photonic Integrated Circuits

Data centers are continuously growing in scale and can contain more than one million servers spreading across thousands of racks; requiring a large-scale switching network to provide broadband and reconfigurable interconnections of low latency. Traditional data center network architectures, through the use of electrical packet switches in a multi-tier topology, has fundamental weaknesses such as oversubscription and cabling complexity. Wireless intra-data center interconnection solutions have been proposed to deal with the cabling problem and can simultaneously address the over-provisioning problem by offering efficient topology re-configurability. In this work we introduce a novel free space optical interconnect solution for intra-data center networks that utilizes 2D optical beam steering for the transmitter, and high bandwidth wide-area photodiode arrays for the receiver. This new breed of free space optical interconnects can be developed on a photonic integrated circuit; offering ns switching at sub-μW consumption. The proposed interconnects together with a networking architecture that is suitable for utilizing those devices could support next generation intra-data center networks, fulfilling the requirements of seamless operation, high connectivity, and agility in terms of the reconfiguration time.

Crosstalk Modeling in Lens Based Free Space Optical Interconnects Systems with Circular Symmetry

We introduce convenient model and an optimization scheme to optimize the signal-to-crosstalk ratio (SCR) in a free space optical interconnects (FSOIs) system that uses microlenses with finite circular apertures. In this model, we consider both the stray light crosstalk and the crosstalk due to the diffraction at the microlens apertures to evaluate the SCR. Using cylindrical form of Collins diffraction integral and the Laguerre–Gaussian (LG) beam model, we derive an approximate closed form formula for the optical field of a multimode LG beam propagating through circular apertured FSOIs by expanding the hard edge circular aperture function of the microlens in terms of complex Gaussian functions. The analyses indicate that the size of the detector is an important factor to optimize the SCR for both the apertured and the unapertured misaligned FSOIs system. The effect of higher order mode of the laser source on the SCR is also considered.

Aberrated Microlenses to Reduce Crosstalk in Free Space Optical Interconnects Systems

A free space optical interconnects system that uses Gaussian beams and focusing microlenses with spherical aberration has been considered. Numerical results show that the use of microlenses with negative spherical improves the signal-to-crosstalk ratio of the interconnects system at some optimum positions of the detector array. An expression for output field at the detectors plane is derived and used to evaluate the signal-to-crosstalk ratio.

Air turbulence effects on performance of optical wireless communication with crosstalk in server backplane

Free space optical interconnections (FSOIs) are anticipated to become a prevalent technology for short-range high-speed communication. FSOIs use lasers in board-to-board and rack-to-rack communication to achieve improved performance in next generation servers and are expected to help meet the growing demand for massive amounts of inter-card data communication. An array of transmitters and receivers arranged to create an optical bus for inter-card and card-to-backplane communication could be the solution. However, both chip heating and cooling fans produce temperature gradients and hot air flow that results in air turbulence inside the server, which induces signal fading and, hence, influences the communication performance. In addition, the proximity between neighboring transmitters and receivers in the array leads to crosstalk in the received signal, which further contributes to signal degradation. In this Letter, the primary objective is to experimentally examine the off-axis crosstalk between links in the presence of turbulence inside a server chassis. The effects of geometrical and inter-chassis turbulence characteristics are investigated and first-and second-order statistics are derived.

Crosstalk in Free Space Optical Interconnects that Use Micro-Lenses Arrays: Practical Consideration

The effect of positive spherical aberration of micro-lens on the optical crosstalk in free space optical interconnects is considered. Using the derived field distribution at the detector array, the optical crosstalk level is evaluated and numerical results are provided. It is found that practical lenses with positive spherical aberration increases the crosstalk and degrade the system performance.