Reconfigurable Optical Interconnects Research Articles

Numerical investigation on Ge2Sb2Te5-assisted reconfigurable asymmetric directional coupler-based switches

The performance analysis on phase change material germanium-antimony-tellurium-Ge2Sb2Te5 (GST)-assisted silicon waveguide-based non-volatile 1 × 2 and 2 × 2 switches are proposed. The designed structure contains a hybrid silicon waveguide and a regular silicon waveguide. By perfectly choosing the design considerations of the proposed switches, the power of transverse electric mode is coupled and shifted to cross port as an output at the amorphous state of GST (aGST), and for the crystalline state of GST (cGST), the input mode passes through the bar port as an output. The performances of the switches are evaluated by numerical calculations with the help of mode analysis of 2D finite-element method (FEM), and it is further verified by 3D finite-difference time-domain method (FDTD). For the 1 × 2 and 2 × 2 proposed switches, the obtained insertion loss (IL) of both phases of GST is <1 dB. For 1 × 2 switch design, the calculated cross-talk (CT) and extinction ratio (ER) are −31.32 and 31.31 dB, respectively at aGST state and for the cGST state, the values of CT and ER are −20.77 and 20.73 dB at 1550-nm operating wavelength. Similarly, for 2 × 2 switch design, the observed CT and ER are −32.7 and 32.69 dB, correspondingly at aGST state and for the cGST state; they are −11.98 and 11.71 dB, respectively. An additional optimization is realized via variations in the wavelength, GST width, and the coupling gap with the help of FEM analysis. Hence, broadband (80 nm) and ultracompact (∼20 μm) switches are proposed with low IL, high ER, and low CT, which can be suitable for reconfigurable optical interconnects, optical field-programmable gate array applications, microwave photonics, and quantum computing.

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.

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.

Short-range optical wireless communications for indoor and interconnects applications

Optical wireless communications have been widely studied during the past decade in short⁃range applications, such as indoor high⁃ speed wireless networks and interconnects in data centers and high⁃performance computing. In this paper, recent developments in high⁃speed short⁃range optical wireless communications are reviewed, including visible light communications (VLCs), infrared in-door communication systems, and reconfigurable optical interconnects. The general architecture of indoor high⁃speed optical wire⁃ less communications is described, and the advantages and limitations of both visible and infrared based solutions are discussed. The concept of reconfigurable optical interconnects is presented, and key results are summarized. In addition, the challenges and potential future directions of short⁃range optical wireless communications are discussed.

Two-dimensional silicon annular photonic crystals for realizing polarization-independent unidirectional transmission

Optical diode is a device that can realize unidirectional transmission of light. Its function is similar to that of an electronic diode. It has important applications in the field of optoelectronic integration and all-optical communications. Unidirectional wave transmission requires either time-reversal or spatial inversion symmetry breaking. The magneto-optical effect and optical nonlinearity are usually utilized to break the time-reversal symmetry and obtain the unidirectional transmission. However, these schemes all need high light intensity or magnetic field strength to be realized, and limit the usage. Therefore, spatial inversion symmetry breaking is highly desirable because of totally linear materials under low intensities. Quit a lot of researchers have designed optical diodes based on the photonic crystals and achieved unidirectional transmission for TE-like or TM-like light. The early design realized light unidirectional transmission by PC structures for only one polarization state (TE-like or TM-like incident light). It limits the application for the high integration and reconfigurable optical interconnection. The structure which can achieve unidirectional transmission for both TE and TM polarizations needs to be designed. The annular PCs have been verified to realize polarization-independent phenomena, such as beam splitting, self collimation and waveguide. In this paper, an annular PC is proposed. The plane wave expansion method is used to calculate band structures. The results show that it exhibits a significant directional band gap for both TE and TM mode. Then, the triangular annular PC is constructed, and its transmission spectra and field distributions are calculated by the finite-different time-domain method. It is found that the structure can realize the polarization-independent unidirectional transmission, but the forward transmissivity is too low (about 20%). Moreover, another smaller size annular PC is further introduced to form annular PC heterojunction, which effectively improves the polarization-independent unidirectional transmission performance and the forward transmissivity has doubled. Through the adjustment of the interface structure, the forward transmissivity is further increased. The optimized annular PC heterostructure can realize polarization-independent unidirectional transmission, and the forward transmissivity reaches 44%. The heterostructure can be used to fabricate polarization-independent optical diode, and may have potential applications in complex all-optical integrated circuits.

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.

Space-Time-Coded High-Speed Reconfigurable Card-to-Card Free-Space Optical Interconnects

Space-time-coded, free-space-based card-to-card optical interconnects for data centers and high-performance computing are proposed and experimentally demonstrated in this paper. Free-space propagation, which enables flexible and reconfigurable interconnections, is used in conjunction with 2 χ 2 extended Alamouti space-time coding, which reduces the inter-channel crosstalk and improves the sensitivity of the interconnects. Experimental results show that, for 2 × 2 10 Gb/s reconfigurable optical interconnects, space-time coding extends the maximum interconnection range by more than 48% and improves the receiver sensitivity. In addition, the impact of air turbulence on the proposed space-time-coded, free-space-based reconfigurable optical interconnects is experimentally investigated and results show that the interconnect performance is slightly degraded even under relatively strong air turbulence, where the maximum interconnection range is reduced only by around 4 cm.

High-speed reconfigurable card-to-card optical interconnects based on hybrid free-space and multi-mode fiber propagations

In this paper, a high-speed reconfigurable card-to-card optical interconnect architecture based on hybrid free-space and multi-mode fiber (MMF) propagation is proposed. The use of free-space signal transmission provides flexibility and reconfigurability and the MMF extends the achievable interconnection range. A printed-circuit-board (PCB) based integrated optical interconnect module is designed and developed and proof-of-concept demonstration experiments are carried out. Results show that 3 × 10 Gb/s reconfigurable optical interconnect is realized with ~12 cm free-space propagation and a 10 m MMF length. In addition, since air turbulence due to high temperature of electronic components and heat dissipation fans always exists in typical interconnect environments and it normally results in system performance degradation, its impact on the proposed reconfigurable optical interconnect scheme is also experimentally investigated. Results indicate that even with comparatively strong air turbulence, 3 × 10 Gb/s optical interconnects with flexibility can still be achieved and the power penalty is <0.7 dB.

Performance of High-Speed Reconfigurable Free-Space Card-to-Card Optical Interconnects Under Air Turbulence

Free-space based optical interconnects are promising candidates for the prevision of parallel, high-speed, densely integrated and reconfigurable card-to-card interconnectivities in data centers and for high-performance computing clusters. However, air turbulence due to high temperature of electronic components and heat dissipation fans typically results in degradation of system performance through effects such as signal scintillation, beam broadening and beam wander. In this paper, the impact of air turbulence on our recently proposed reconfigurable free-space card-to-card optical interconnect scheme is investigated. Experiments are carried out with lab-emulated moderate and comparatively strong turbulence and results show that the bit-error-rate (BER) performance of proposed interconnect scheme is degraded. A power penalty of ~ 0.5 dB is observed at BER of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> in emulated moderate turbulence, and in emulated comparatively strong turbulence the BER performance suffers power penalties of ~ 1.6 dB. It should be noted that a BER of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> is not sufficient for typical card-to-card optical interconnects and FEC should be employed to improve the BER performance at the cost of some overhead. Nevertheless, experimental results show that our proposed 3×3 10 Gb/s reconfigurable optical interconnects with card-to-card spacing of up to 30 cm is still achieved even under considerably strong turbulence.

An experimental observation of a spatial optical soliton beam and self splitting of beam into two soliton beams in chiral nematic liquid crystal

In this paper, the torque, reorientation and the change in angular momentum of the light are firstly discussed. And moreover, the propagation of light beam (optical spatial soliton beam) at the distance of a few millimeters in chiral nematic liquid crystal (CNLC) will be experimentally under study. In spite of the complex structure of these crystals, due to a self-focusing effect, by injection of infrared laser beam with a power of the order of a few tenths of milliwatts into CNLC an optical spatial soliton beam is observed. In addition, we experimentally obtain the formation of splitting of a soliton beam into two soliton beams which have equal powers due to a saturation of the reorientational nonlinearity. As a matter of fact, this saturation of the reorientational nonlinearity can occur at one point of CNLC, due to the nonlocal nonlinearity. Hence this saturation of the reorientation nonlinearity can split one soliton beam to two soliton beams. This splitting can have a great potential interest for applications in all-optical signal processing, reconfigurable optical interconnects and switching.

High-Speed Reconfigurable Free-Space Card-to-Card Optical Interconnects

A reconfigurable free-space-based card-to-card optical interconnect architecture employing MEMS-based steering mirror arrays in conjunction with VCSEL and photodiode arrays is proposed and demonstrated in this paper. Theoretical studies and simulations indicate that error-free [bit error rate (BER) of <; 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> ] optical interconnects with a range on the order of tens of centimeters can be achieved, and the major factors limiting the performance are the VCSEL beam divergence and interchannel optical crosstalk. The tradeoff between the BER performance and the channel spacing of the receiver MEMS mirror array is also investigated. A proof-of-concept 3 × 10 Gb/s reconfigurable optical interconnect architecture is developed, demonstrating a BER of ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> and a receiver sensitivity better than ~ -11.5 dBm. Both the port-to-port and board-to-board reconfigurability of the proposed architecture are also experimentally demonstrated, opening the way for attaining higher throughputs through highly dense 3-D parallel optical interconnects.

Reconfigurable mesh-based inter-chip optical interconnection network for distributed-memory multiprocessor system

A 4×4 reconfigurable mesh-based inter-chip optical interconnection network is reported for distributed-memory multiprocessor system and the experiment confirmed that the data rate in each channel could reach above 3.125 Gbps, which would be a good solution to solve the communication bottlenecks between processors. Each node of this reconfigurable mesh could realize 15 internal connection patterns to complete the interconnections of processors. Besides, this mesh interconnect network via ultra-high bandwidth waveguides embedded in EOPCB can realize flexible multiprocessor system architecture options.

A novel reconfigurable optical interconnect architecture using an Opto-VLSI processor and a 4-f imaging system

A novel reconfigurable optical interconnect architecture for on-board high-speed data transmission is proposed and experimentally demonstrated. The interconnect architecture is based on the use of an Opto-VLSI processor in conjunction with a 4-f imaging system to achieve reconfigurable chip-to-chip or board-to-board data communications. By reconfiguring the phase hologram of an Opto-VLSI processor, optical data generated by a vertical Cavity Surface Emitting Laser (VCSEL) associated to a chip (or a board) is arbitrarily steered to the photodetector associated to another chip (or another board). Experimental results show that the optical interconnect losses range from 5.8dB to 9.6dB, and that the maximum crosstalk level is below -36dB. The proposed architecture is tested for high-speed data transmission, and measured eye diagrams display good eye opening for data rate of up to 10Gb/s.

Predicting the performance of reconfigurable optical interconnects in distributed shared-memory systems

New advances in reconfigurable optical interconnect technologies will allow the fabrication of low-cost, fast and run-time adaptable networks for connecting processors and memory modules in large distributed shared-memory multiprocessor machines. Since the switching times of these components are typically high compared to the memory access time, reconfiguration exploits low frequency dynamics in the network traffic patterns. These are however not easily reproduced using statistical traffic generation, a tool commonly used when doing a fast design space exploration. In this paper, we present a technique that can predict network performance, based on the traffic patterns obtained from simulating the execution of real benchmark applications, but without the need to perform these slow full-system simulations for every parameter set of interest. This again allows for a quick comparison of different network implementations with good relative accuracy, narrowing down the design space for more detailed examination.

Proposed Low-Power High-Speed Microring Resonator-Based Switching Technique for Dynamically Reconfigurable Optical Interconnects

The dynamic bandwidth re-allocation (DBR) technique balances traffic by re-allocating bandwidth from under-utilized links to over-utilized links in a network. In this letter, we propose a nonblocking, fast, low-power, and integratable optical switch that enables DBR. The basic building blocks of the proposed switch are silicon-on-insulator-based microring resonators. Analytical and numerical studies show that the active switch design gives similar performance in throughput and latency, while reducing cost (number of lasers) and area significantly when compared to implementation of DBR with only passive components. There is a slight increase in power (~0.4% for worst-case traffic pattern) using the active switch implementation.

High-speed (2.5 Gbps) reconfigurable inter-chip optical interconnects using opto-VLSI processors

Reconfigurablele optical interconnects enable flexible and high-performance communication in multi-chip architectures to be arbitrarily adapted, leading to efficient parallel signal processing. The use of Opto-VLSI processors as beam steerers and multicasters for reconfigurable inter-chip optical interconnection is discussed. We demonstrate, as proof-of-concept, 2.5 Gbps reconfigurable optical interconnects between an 850nm vertical cavity surface emitting lasers (VCSEL) array and a photodiode (PD) array integrated onto a PCB by driving two Opto-VLSI processors with steering and multicasting digital phase holograms. The architecture is experimentally demonstrated through three scenarios showing its flexibility to perform single, multicasting, and parallel reconfigurable optical interconnects. To our knowledge, this is the first reported high-speed reconfigurable N-to-N optical interconnects architecture, which will have a significant impact on the flexibility and efficiency of large shared-memory multiprocessor machines.

Routing of anisotropic spatial solitons and modulational instability in liquid crystals

In certain materials, the spontaneous spreading of a laser beam (owing to diffraction) can be compensated for by the interplay of optical intensity and material nonlinearity. The resulting non-diffracting beams are called 'spatial solitons' (refs 1-3), and they have been observed in various bulk media. In nematic liquid crystals, solitons can be produced at milliwatt power levels and have been investigated for both practical applications and as a means of exploring fundamental aspects of light interactions with soft matter. Spatial solitons effectively operate as waveguides, and so can be considered as a means of channelling optical information along the self-sustaining filament. But actual steering of these solitons within the medium has proved more problematic, being limited to tilts of just a fraction of a degree. Here we report the results of an experimental and theoretical investigation of voltage-controlled 'walk-off' and steering of self-localized light in nematic liquid crystals. We find not only that the propagation direction of individual spatial solitons can be tuned by several degrees, but also that an array of direction-tunable solitons can be generated by modulation instability. Such control capabilities might find application in reconfigurable optical interconnects, optical tweezers and optical surgical techniques.

Fast Electrooptic Bragg Grating Couplers for On-Chip Reconfigurable Optical Waveguide Interconnects

We present an array of electrooptic Bragg gratings for on-chip optical waveguide interconnect applications. These integrated gratings can serve as switchable couplers to couple high-speed optical signals from a vertical-cavity surface-emitting laser to on-chip polymeric waveguides and from waveguides to photodetectors. The unique switching feature provides a promising solution to facilitate dynamically reconfigurable on-chip optical interconnects architecture for intrachip, chip-to-chip, and board-to-board data communications. We demonstrate electrooptic grating couplers in a polymeric channel waveguides on a silicon substrate with an electrically tunable surface-normal coupling efficiency up to 8% and a switching speed of 75 μs.

Simulations and experiments on self-focusing conditions in nematic liquid-crystal planar cells

Owing to the nonlinear effect of optical field-induced director reorientation, self-focusing of an optical beam can occur in nematic liquid crystals and an almost diffraction-compensated propagation can be observed with milliwatts of light power and propagation lengths of several millimeters. This opens the way for applications in all-optical signal handling and reconfigurable optical interconnections. Self-focusing of an optical beam in nematic liquid-crystal cells has been studied experimentally and by means of numerical simulation. The relationships between bias voltage, cell thickness and required optical power have been examined, thus allowing the determination of the most favorable conditions for soliton-like beam propagation.

Three-dimensional self-organized microoptoelectronic systems for board-level reconfigurable optical interconnects-performance modeling and simulation

Self-organized microoptoelectronic system (SELMOS) built from three concepts - scalable film optical link multichip-module (S-FOLM), three-dimensional (3-D) microoptical switching system (3D-MOSS), and self-organized lightwave network (SOLNET)-is proposed. The feasibility of SELMOS for board-level reconfigurable optical interconnects is studied by the beam propagation method/finite difference time domain simulation focusing on three key issues; reducing size/cost of electrical to optical (E-O) and optical to electrical (O-E) signal conversion devices, tolerating alignment accuracy for optical coupling, and miniaturizing high-speed massive optical switching. S-FOLM, which consists of film-waveguide-based 3-D structures with embedded optoelectronic active elements and optical Z-connections for interplane links, enables drastic size/cost reduction of E-O and O-E conversion devices. 3D-MOSS, which is an S-FOLM with embedded microoptical switches, has a potentiality of 1024 /spl times/ 1024 switching with a system size of /spl sim/1.4 /spl times/ 0.6 cm/sup 2/ and an insertion loss of 29 dB. The switching rate of the 3D-MOSS is determined by the heat releasing speed to be /spl sim/2 /spl times/ 10/sup 5/ 1/s when PLZT waveguide-prism-deflector microoptical switches are used. By using advanced electrooptic materials, rates higher than 10/sup 8/ 1/s are expected. Twenty-five percent misalignment in waveguide assembly raises the insertion loss of the 3D-MOSS to 73 dB. The loss is reduced to 32 dB in SELMOS-based 3D-MOSS, where a self-organized 3-D microoptical network is implemented using SOLNET. Further loss reduction is expected by structural optimization of loss-inducing parts. Thus, SELMOS is found to be a solution of the three key issues for board-level reconfigurable optical interconnects. In addition, photolithographic packaging with selectively occupied repeated transfer (PL-Pack with SORT), which integrates different types of active elements into one substrate in desired arrangements using an all-photolithographic process, can contribute to cost and the coefficient of thermal expansion-mismatching reduction.