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In this paper, we put forward a proof of concept for sixth generation (6G) Terabit infrared (IR) laser-based indoor optical wireless networks. We propose a novel double-tier access point (AP) architecture based on an array of arrays of vertical cavity surface emitting lasers (VCSELs) to provide a seamless grid-of-beam coverage with multi-Gb/s per beam. We present systematic design and thorough analytical modeling of the AP architecture, which are then applied to downlink system modeling using non-imaging angle diversity receivers (ADRs). We propose static beam clustering with coordinated multi-beam joint transmission (CoMB-JT) for network interference management and devise various clustering strategies to address inter-beam interference (IBI) and inter-cluster interference (ICI). Non-orthogonal multiple access (NOMA) and orthogonal frequency division multiple access (OFDMA) schemes are also adopted to handle intra-cluster interference, and the resulting signal-to-interference-plus-noise ratio (SINR) and achievable data rate are derived. The network performance is studied in terms of spatial distributions and statistics of the downlink SINR and data rate through extensive computer simulations. The results demonstrate that data rates up to 15 Gb/s are achieved within the coverage area and a properly devised clustering strikes a balance between the sum rate and fairness depending on the number of users., Comment: 15 pages, 13 figures, 1 table

Optical circuit switching is connection-oriented, being deterministic through the reservation of a complete wavelength channel or spatial path for a certain period. However, this comes at a trade-off against link dynamics, and overall capacity can thus be constrained by the time slot reservations, especially for switches with microsecond- to millisecond-scale reconfiguration times. For data-intensive applications, the communication patterns associated with random data sets typically yield short-lived flows. This situation calls for a new multi-dimensional switching paradigm that fully exploits not only the space and wavelength domains but also with nanosecond-scale reconfigurable capability in the time domain to enable ultra-dynamic links. In this work, we focus on the exploitation of micro-ring resonant phase shifters (RPSs) that are wavelength selective for optical switching in a single plane. By proposing an innovative analytical method with transmission circle chart, we fully unlock the power of RPS with nanosecond-scale reconfigurability and the capability to arbitrarily manipulate its phase and amplitude. Such a compact model offers fresh insights into designs with under and critically coupled RPSs beyond the commonly explored over-coupling condition. This creates not only versatile switch elements but also perfect absorbers for robust multi-wavelength operations. The proposed device can bring about a breakthrough in the optical switching capacity that potentially addresses the challenges faced by modern data center networks, as well as other photonic circuits for high-throughput signal processing., Comment: 10 pages

Continuous-variable quantum key distribution (CV QKD) using optical coherent detectors is practically favorable due to its low implementation cost, flexibility of wavelength division multiplexing, and compatibility with standard coherent communication technologies. However, the security analysis and parameter estimation of CV QKD are complicated due to the infinite-dimensional latent Hilbert space. Also, the transmission of strong reference pulses undermines the security and complicates the experiments. In this work, we tackle these two problems by presenting a time-bin-encoding CV protocol with a simple phase-error-based security analysis valid under general coherent attacks. With the key encoded into the relative intensity between two optical modes, the need for global references is removed. Furthermore, phase randomization can be introduced to decouple the security analysis of different photon-number components. We can hence tag the photon number for each round, effectively estimate the associated privacy using a carefully designed coherent-detection method, and independently extract encryption keys from each component. Simulations manifest that the protocol using multi-photon components increases the key rate by two orders of magnitude compared to the one using only the single-photon component. Meanwhile, the protocol with four-intensity decoy analysis is sufficient to yield tight parameter estimation with a short-distance key-rate performance comparable to the best Bennett-Brassard-1984 implementation., Comment: 27 pages, 6 figures, 6 tables. Comments are welcomed

Optical wireless communication (OWC) has recently received massive interest as a new technology that can support the enormous data traffic increasing on daily basis. Laser-based OWC networks can provide terabits per second (Tbps) aggregate data rates. However, the emerging OWC networks require clusters of optical transmitters to provide uniform coverage for multiple users. In this context, multi-user interference (MUI) is a crucial issue that must be managed efficiently to provide high spectral efficiency. Rate splitting (RS) is proposed as a transmission scheme to serve multiple users simultaneously by splitting the message of a given user into common and private messages, and then, each user decodes the desired message following a certain procedure. In radio frequency (RF) networks, RS provides higher spectral efficiency compared with orthogonal and non-orthogonal transmission schemes. Considering the high density of OWC networks, the performance of RS is limited by the cost of providing channel state information (CSI) at transmitters and by the noise resulting from interference cancellation. In this work, a user-grouping algorithm is proposed and used to form multiple groups, each group contains users spatially clustered. Then, an outer precoder is designed to manage inter-group interference following the methodology of blind interference alignment (BIA), which reduces the requirements of CSI at RF or optical transmitters. For intra-group interference, RS is applied within each group where the users belonging to a given group receive a unique common message on which their private messages are superimposed. Furthermore, an optimization problem is formulated to allocate the power among the private messages intended to all users such that the sum rate of the network is maximized.

The design of a compact high-speed and wide field of view (FOV) receiver is challenging due to the presence of two well-known trade-offs. The first one is the area-bandwidth trade-off of photodetectors (PDs) and the second one is the gain-FOV trade-off due to the use of optics. The combined effects of these two trade-offs imply that the achievable data rate of an imaging optical receiver is limited by its FOV, i.e., a rate-FOV trade-off. To control the area-bandwidth trade-off, an array of small PDs can be used instead of a single PD. Moreover, in practice, a large-area lens is required to ensure sufficient power collection, which in turn limits the receiver FOV (i.e., gain-FOV trade-off). We propose an imaging receiver design in the form of an array of arrays. To achieve a reasonable receiver FOV, we use individual focusing lens for each PD array rather than a single collection lens for the whole receiver. The proposed array of arrays structure provides an effective method to control both gain-FOV trade-off (via an array of lenses) and area-bandwidth trade-off (via arrays of PDs). We first derive a tractable analytical model for the SNR of an array of PDs where the maximum ratio combining has been employed. Then, we extend the model for the proposed array of arrays structure and the accuracy of the analytical model is verified based on several Optic Studio-based simulations. Next, we formulate an optimization problem to maximize the achievable data rate of the imaging receiver subject to a minimum required FOV. The optimization problem is solved for two commonly used modulation techniques, namely, OOK and direct current biased optical orthogonal frequency division multiplexing with variable rate quadrature amplitude modulation. It is demonstrated that a data rate of ~ 24 Gbps with a FOV of 15 is achievable using OOK with a total receiver size of 2 cm by 2 cm., Comment: 30 pages, 15 Figures and 6 Tables

The recently proposed phase-matching quantum key distribution offers means to overcome the linear key rate-transmittance bound. Since the key information is encoded onto the phases of coherent states, the misalignment between the two remote reference frames would yield errors and significantly degrade the key generation rate from the ideal case. In this work, we propose a reference-frame-independent design of phase-matching quantum key distribution by introducing high-dimensional key encoding space. With encoded phases spanning the unit circle, the error statistics at arbitrary fixed phase reference difference can be recovered and treated separately, from which the misalignment angle can be identified. By naturally extending the binary encoding symmetry and complementarity to high dimensions, we present a security proof of this high-dimensional phase-matching quantum key distribution and demonstrate with simulation that a 17-dimensional protocol is completely immune to any degree of fixed misalignment and robust to slow phase fluctuations. We expect the high-dimensional protocol to be a practical reference-frame-independent design for general phase-encoding schemes where high-dimensional encoding is relatively easy to implement., Comment: 20 pages, 8 figures

In this paper, the design of a multiple-input multiple-output (MIMO) optical wireless communication (OWC) link based on vertical cavity surface emitting laser (VCSEL) arrays is systematically carried out with the aim to support data rates in excess of 1 Tb/s for the backhaul of sixth generation (6G) indoor wireless networks. The proposed design combines direct current optical orthogonal frequency division multiplexing (DCO-OFDM) and a spatial multiplexing MIMO architecture. For such an ultra-high-speed line-of-sight (LOS) OWC link with low divergence laser beams, maintaining alignment is of high importance. In this paper, two types of misalignment error between the transmitter and receiver are distinguished, namely, radial displacement error and orientation angle error, and they are thoroughly modeled in a unified analytical framework assuming Gaussian laser beams, resulting in a generalized misalignment model (GMM). The derived GMM is then extended to MIMO arrays and the performance of the MIMO-OFDM OWC system is analyzed in terms of the aggregate data rate. Novel insights are provided into the system performance based on computer simulations by studying various influential factors such as beam waist, array configuration and different misalignment errors, which can be used as guidelines for designing short range Tb/s MIMO OWC systems., Comment: Accepted for publication in the IEEE Transactions on Communications

Light amplification by stimulated emission of radiation (laser) sources have many advantages for use in high data rate optical wireless communications. In particular, the low cost and high-bandwidth properties of laser sources such as vertical-cavity surface-emitting lasers (VCSELs) make them attractive for future indoor optical wireless communications. In order to be integrated into future indoor networks, such lasers should conform to eye safety regulations determined by the international electrotechnical commission (IEC) standards for laser safety. In this paper, we provide a detailed study of beam propagation to evaluate the received power of various laser sources, based on which as well as the maximum permissible exposure (MPE) defined by the IEC 60825-1:2014 standard, we establish a comprehensive framework for eye safety analyses. This framework allows us to calculate the maximum allowable transmit power, which is crucial in the design of a reliable and safe laser-based wireless communication system. Initially, we consider a single-mode Gaussian beam and calculate the maximum permissible transmit power. Subsequently, we generalize this approach for higher-mode beams. It is shown that the M-squared-based approach for analysis of multimode lasers ensures the IEC eye safety limits, however, in some scenarios, it can be too conservative compared to the precise beam decomposition method. Laser safety analyses with consideration of optical elements such as lens and diffuser, as well as for VCSEL array have been also presented. Skin safety, as another significant factor of laser safety, has also been investigated in this paper. We have studied the impacts of various parameters such as wavelength, exposure duration and the divergence angle of laser sources on the safety analysis by presenting insightful results., Comment: 54 pages, 24 figures Submitted to IEEE journal

We demonstrate how the 5G network slicing model can be extended to address data security requirements. In this work we demonstrate two different slice configurations, with different encryption requirements, representing two diverse use-cases for 5G networking: namely, an enterprise application hosted at a metro network site, and a content delivery network. We create a modified software-defined networking (SDN) orchestrator which calculates and provisions network slices according to the requirements, including encryption backed by quantum key distribution (QKD), or other methods. Slices are automatically provisioned by SDN orchestration of network resources, allowing selection of encrypted links as appropriate, including those which use standard Diffie-Hellman key exchange, QKD and quantum-resistant algorithms (QRAs), as well as no encryption at all. We show that the set-up and tear-down times of the network slices takes of the order of 1-2 minutes, which is an order of magnitude improvement over manually provisioning a link today., Comment: 9 pages, 7 figures

Optical interconnects play a key role in the implementation of high-speed short-reach communication links within high-performance electronic systems. Multimode polymer waveguides in particular are strong candidates for use in passive optical backplanes as they can be cost-effectively integrated onto standard PCBs. Various optical backplanes using this technology and featuring a large number of multimode polymer waveguide components have been recently demonstrated. The optimisation of the loss performance of these complex waveguide layouts becomes particularly important at high data rates (>=25 Gb/s) due to the associated stringent power budget requirements. Moreover, launch conditions have to be carefully considered in such systems due to the highly-multimoded nature of this waveguide technology. In this paper therefore, we present thorough loss and bandwidth studies on siloxane-based multimode waveguides and waveguide components (i.e. bends and crossings) that enable the implementation of passive optical backplanes. The performance of these components is experimentally investigated under different launch conditions for different waveguide profiles that can be readily achieved through fabrication. Useful design rules on the use of waveguide bends and crossings are derived for each waveguide type. It is shown that the choice of waveguide parameters depends on the particular waveguide layout, assumed launch conditions and desired link bandwidth. As an application of these studies, the obtained results are employed to optimise the loss performance of a 10-card shuffle router and enable >=40 Gb/s data transmission., Comment: 7 pages, 12 figures

Optical interconnects have attracted significant research interest for use in short-reach board-level optical communication links in supercomputers and data centres. Multimode polymer waveguides in particular constitute an attractive technology for on-board optical interconnects as they provide high bandwidth, offer relaxed alignment tolerances, and can be cost-effectively integrated onto standard printed circuit boards (PCBs). However, the continuing improvements in bandwidth performance of optical sources make it important to investigate approaches to develop high bandwidth polymer waveguides. In this paper, we present dispersion studies on a graded-index (GI) waveguide in siloxane materials designed to deliver high bandwidth over a range of launch conditions. Bandwidth-length products of >70 GHzxm and ~65 GHzxm are observed using a 50/125 um multimode fibre (MMF) launch for input offsets of +/- 10 um without and with the use of a mode mixer respectively; and enhanced values of >100 GHzxm are found under a 10x microscope objective launch for input offsets of ~18 x 20 um^2. The large range of offsets is within the -1 dB alignment tolerances. A theoretical model is developed using the measured refractive index profile of the waveguide, and general agreement is found with experimental bandwidth measurements. The reported results clearly demonstrate the potential of this technology for use in high-speed board-level optical links, and indicate that data transmission of 100 Gb/s over a multimode polymer waveguide is feasible with appropriate refractive index engineering., Comment: 8 pages, 10 figures

Dispersion studies are conducted on 1m long multimode polymer spiral waveguides with different refractive index profiles. Bandwidth-length products >40GHzxm are obtained from such waveguides under a 50/125 um MMF, indicating the potential of this technology., Comment: 3 pages, 2 figures, IEEE Optical Interconnects Conference (OIC), paper MD2, 2015

Bandwidth measurements are conducted on a 1.4 m long spiral polymer multimode waveguide for a SMF and 50/125 um MMF launch and for different input offsets. The waveguide exhibits a bandwidth of at least 30 GHz for all input types, yielding a bandwidth-length product of at least 42 GHzxm, while no impact is observed on the waveguide performance due to the different spatial input offsets. The results indicate that data transmission at data rates even higher than 25 Gb/s can be achieved over such structures, thereby demonstrating the potential of multimode polymer waveguide technologies in short-reach board-level datacommunication links., Comment: 3 pages, 1 figure

Dispersion studies demonstrate that waveguide layout can be used to enhance the bandwidth performance of multimode polymer waveguides for use in board-level optical interconnects, providing >40 GHzxm without the need for any launch conditioning., Comment: 4 pages, 4 figures, Optical Fiber Communication Conference (OFC)

This paper reports the observation of ultra-superluminal pulse propagation in GaAs/AlGaAs multiple contact heterostuctures in a superradiant emission regime, and shows definitively that it is a different class of emission from conventional spontaneous or stimulated emission. It is shown that coherent population gratings induced in the semiconductor medium under strong electrical pumping have great impact in causing a decrease of the group refractive index in the range of 5-40%. This decrease is much greater than that which would be observed due to conventional carrier depletion or chirp mechanisms. The decrease in refractive index in turn causes faster-than-c propagation of femtosecond pulses. The measurement also shows unequivocally the exist of coherent amplification of electromagnetic pulses in semiconductors at room temperature, the coherence being strongly enhanced by interactions of the light with coherent transient gratings locked to carrier gratings. This pulse generation technique can be anticipated to have great potential in applications where highly coherent femtosecond optical pulses must be generated on demand, Comment: 22 pages, 8 Figs

We report on a gated single-photon detector based on InGaAs/InP avalanche photodiodes (APDs) with a single-photon detection efficiency exceeding 55% at 1550 nm. Our detector is gated at 1 GHz and employs the self-differencing technique for gate transient suppression. It can operate nearly dead time free, except for the one clock cycle dead time intrinsic to self-differencing, and we demonstrate a count rate of 500 Mcps. We present a careful analysis of the optimal driving conditions of the APD measured with a dead time free detector characterization setup. It is found that a shortened gate width of 360 ps together with an increased driving signal amplitude and operation at higher temperatures leads to improved performance of the detector. We achieve an afterpulse probability of 7% at 50% detection efficiency with dead time free measurement and a record efficiency for InGaAs/InP APDs of 55% at an afterpulse probability of only 10.2% with a moderate dead time of 10 ns., Comment: 10 pages, 4 figures