Implementation of high speed optical universal logic gates using the electro-optic effect-based Mach–Zehnder interferometer structures

The universal logic gates are the most important logic gates responsible for optimized design of different types of complex digital logic circuits. It is of great interest to implement the function of universal logic gates such as NAND and NOR logic gates using the concepts of electro-optic effect. The smart use of electro-optic effect can provide very effective optical power switching devices. The implementation of universal logic gates operation in the optical domain can improve the performance of the devices and includes the advantages of the optical communication system. The proper configuration of Mach–Zehnder interferometer working on the principle of electro-optic effect can provide the optical responses equivalent to the NAND and NOR logic gates. The proposed devices can be analyzed to check the various performance affecting parameters in order to specify the physical parameters.     

Advanced all-optical logic gates on a spectral bus

We present experimental results on a new method for ultrafast all-optical logic, which utilizes four-wave mixing on polarization-modulated signals. The technique allows advanced operations such as exclusive-or and three-bit addition with carry bit. Furthermore, we show that on-the-fly error-correction encoding and decoding of a simple Hamming code is achieved when these gates are used on the bits of a spectrally structured word. These gates may be suitable for logic operations in an optoelectronic front end, which moves some of the necessary computation of data to the optical domain, before detection.     

A scheme of developing frequency encoded tristate-optical logic operations using semiconductor optical amplifier

The ever increasing demand for very fast and agile optical networks requires very fast execution of different optical and logical operations as well as large information handling capacities at the same time. In conventional binary logic based operations the information is represented by two distinct states only (0 and 1 state). It limits the large information handling capacity and speed of different arithmetic and optical logic operations. Tristate based logic operations can be accommodated with optics successfully in data processing, as this type of operation can enhance the speed of operation as well as increase the information handling capacity. Here in this communication the author proposes a new method to implement all-optical different logic gates with tristate logic using the frequency-encoding principle. The frequency encoding/decoding based optical communication has distinctly great advantages because the frequency is the fundamental character of an optical signal and it preserves its identity throughout the communication. The principle of the rotation of the state of polarization of a probe beam through semiconductor optical amplifier (SOA), frequency routing property of an optical add/drop multiplexer (AD) and high frequency conversion property of reflecting semiconductor optical amplifiers (RSOA) have been exploited here to implement the desired AND, OR, NAND and NOR logic operations with tristate logic.     

Tolerancing of board-level-free-space optical interconnects

For optical interconnects to become a mature technology they must be amenable to electronic packaging technology. Two main obstacles to including free-space optical interconnects are alignment and heat-dissipation issues. Here we study the issues of alignment tolerancing that are due to assembly and manufacturing variations (passive-element tolerancing) over long board-level distances (>10 cm) for free-space optical interconnects. We also combine these variations with active optoelectronic device variations (active-element tolerancing). We demonstrate a computer-aided analysis procedure that permits one to determine both active- and passive-element tolerances needed to achieve some system-level specification, such as yield or cost. The procedure that we employ relies on developing a detailed design of the system to be studied in a standard optical design program, such as code v. Using information from this model, we can determine the integrated power falling on the detector, which we term optical throughput, by performing Gaussian propagation or general Fresnel propagation (if significant vignetting occurs). This optical throughput can be used to determine system-level performance criteria, such as bit-error rate. With this computer-aided analysis technique, a sensitivity analysis of all the variations under study is made on a system with realistic board-level interconnect distances to find each perturbation's relative effects (with other perturbations set to 0) on the power falling on the detector. This information is used to set initial tolerances for subsequent tolerancing analysis and design runs. A tolerancing analysis by Monte Carlo techniques is applied to determine if the yield or cost (yield is denned as the percentage of systems that have acceptable system performance) is acceptable. With a technique called parametric sampling, a subsequent tolerancing design run can be applied to optimize this yield or cost with little increase in computation. We study a design example and show that most of the tolerances can be achieved with current technology.     

Compact and low-power optical logic NOT gate based on photonic crystal waveguides without optical amplifiers and nonlinear materials

An optical logic NOT gate (OLNG) is presented based on photonic crystal (PhC) waveguides without nonlinear materials and optical amplifiers. Also, a way of determining the operating parameters is presented. It is demonstrated through finite-difference time-domain simulations that the structure presented can operate as an OLNG. The optimized contrast ratio, defined as the logic-"1" output power divided by the logic-"0" output power, is found to be 297.07 or 24.73 dB. The size of the OLNG can be as small as 7a×7a, where a is the lattice constant of the PhC. Further, the OLNG presented in this paper can operate at a bit rate as high as 2.155 Tbit/s, which is much higher than that of electronic or optical logic gates developed until now. Moreover, as it is not based on the nonlinear effect, the OLNG can operate at very low powers and a relatively large operating bandwidth. This is favorable for large-scale optical integration and for developing multiwavelength parallel-processing optical logic systems.     

Demonstration of optically controlled data routing with the use of multiple-quantum-well bistable and electro-optical devices

We present experimental results on a 1-to-64-channel free-space photonic switching demonstration system based on GaAs/GaAlAs multiple-quantum-well active device arrays. Two control schemes are demonstrated: data transparent optical self-routing usable in a packet-switching environment and direct optical control with potential signal amplification for circuit switching. The self-routing operation relies on the optical recognition of the binary destination address coded in each packet header. Address decoding is implemented with elementary optical bistable devices and modulator pixels as all-optical latches and electro-optical and gates, respectively. All 60 defect-free channels of the system could be operated one by one, but the simultaneous operation of only three channels could be achieved mainly because of the spatial nonhomogeneities of the devices. Direct-control operation is based on directly setting the bistable device reflectivity with a variable-control beam power. This working mode turned out to be much more tolerant of spatial noises: 37 channels of the system could be operated simultaneously. Further development of the system to a crossbar of N inputs and M outputs and system miniaturization are also considered.     

Spatial soliton angular deflection logic gates

A generalized interaction geometry between orthogonally polarized optical spatial solitons is presented in which a weak signal soliton induces a small angular deflection of a stronger power supply, or pump, soliton, resulting in a spatially resolved shift of the pump at the gate output. This geometry allows for the all-optical realization of true three-terminal, inverting and restoring logic devices with gain, which can serve as building blocks for more complex logic operations. In addition, the effects of linear and nonlinear material absorption, which degrades the performance of the angular deflection gates, are considered. Even in the presence of realistic absorption, the angular deflection logic gates can still produce large-signal gain (>2) sufficient for general logic. Finally, by use of a modified gate transfer function approach, these optical logic gates are shown to possess large noise margins for robust operation.     

Sensitive photonic crystal phase logic gates

Total optical phase logic gates are reported in this paper. They are constructed by coupled-defect photonic crystal after two problems are overcome by a heterostructure or an asymmetric structure. Both half and all-phase logic gates are discussed. The sensitivity of these total optical phase logic gates not only are two orders sensitive than those using amplitude-signal, but also have many other advantages: such as very low energy cost. By using such phase logic gates, only a continued wave laser with one frequency is sufficient to operate the phase logic gate or the whole optical integrated circuit.     

Study of all-optical logic XNOR gate based on XGM in linear optical amplifier

Future digital optical communication cannot develop without all-optical high-speed optical devices, especially in the field of high speed large capacity optical transmission, all-optical packet switching and optical computing, and thus optical logic devices are becoming a hotter spot of research. Based on the cross-gain modulation (XGM), a novel scheme of all-optical logic XNOR gate using linear optical amplifier (LOA) is presented in this paper. LOA results show a good gain characteristic, which can get better output logic operation than traditional semiconductor optical amplifier (SOA). Choosing suitable injection current, wavelength scope of the input signal and CW power can achieve better logic operation effect.     

Field-programmable logic devices with optical input-output

A field-programmable logic device (FPLD) with optical I/O is described. FPLD's with optical I/O can have their functionality specified in the field by means of downloading a control-bit stream and can be used in a wide range of applications, such as optical signal processing, optical image processing, and optical interconnects. Our device implements six state-of-the-art dynamically programmable logic arrays (PLA's) on a 2 mm x 2 mm die. The devices were fabricated through the Lucent Technologies-Advanced Research Projects Agency-Consortium for Optical and Optoelectronic Technologies in Computing (Lucent/ARPA/COOP) workshop by use of 0.5-mum complementary metal-oxide semiconductor-self-electro-optic device technology and were delivered in 1998. All devices are fully functional: The electronic data paths have been verified at 200 MHz, and optical tests are pending. The device has been programmed to implement a two-stage optical switching network with six 4 x 4 crossbar switches, which can realize more than 190 x 10(6) unique programmable input-output permutations. The same device scaled to a 2 cm x 2 cm substrate could support as many as 4000 optical I/O and 1 Tbit/s of optical I/O bandwidth and offer fully programmable digital functionality with approximately 110,000 programmable logic gates. The proposed optoelectronic FPLD is also ideally suited to realizing dense, statically reconfigurable crossbar switches. We describe an attractive application area for such devices: a rearrangeable three-stage optical switch for a wide-area-network backbone, switching 1000 traffic streams at the OC-48 data rate and supporting several terabits of traffic.     

Analysis of intrachip electrical and optical fanout

We examine the benefits of electrical isolation in intrachip optical signaling. We calculate the delay and energy metrics of an optical interconnect with fanout driving an electrical load. By examining fanout and including load drivers into delay equations, we make a shift from the general trend of looking at optical interconnects as a replacement for long parasitic wires. Our calculations show that optical fanout provides a large improvement in an Etau2 (energy delay squared) metric and improves performance even at very short intrachip distances. The break-even length corresponds to the wiring length of 250 minimum-size inverters that are compactly laid out. These results provide a compelling reason to further examine the implementation of optical interconnects.     

Adaptive optical transmitter and receiver for space communication through thin clouds

Optical space communication from satellite to ground or air to air consists of clouds as part of communication channels. Propagation of optical pulses through clouds causes widening and deformation in the time domain and attenuation of the pulse radiant power. These effects decrease the received signal and limit the information bandwidth of the communication system. Having dealt with the other effects previously, here we concentrate on pulse broadening in the time domain. We derive a mathematical model of an adaptive optical communication system with a multiscattering channel (atmospheric cloud). We use knowledge about the impulse response function of the cloud to adapt the communication parameters to the transfer function of the cloud. The communication system includes a receiver and a transmitter. We adapted the transmitter to atmospheric conditions by changing the bit error rate. One can adapt the receiver to the atmospheric condition by changing the parameters of the detector and the filter. An example for a practical communication system between a low Earth orbit satellite and a ground station cover by cloud is given. Comparison and analysis of an adaptive and semiadaptive system with cloud channels are presented. Our conclusion is that in some cases only by such adaptive methods is optical communication possible.     

Submicrosecond speed optical coherence tomography system design and analysis by use of acousto-optics

A novel high-speed no-moving-parts optical coherence tomography (OCT) system is introduced that acquires sample data at less than a microsecond per data point sampling rate. The basic principle of the proposed OCT system relies on use of an acousto-optic deflector. This OCT system has the attractive features of an acousto-optic scanning heterodyne interferometer coupled with an acousto-optic (AO) variable optical delay line operating in a reflective mode. Fundamentally, OCT systems use a broadband light source for high axial resolution inside the sample or living tissue under examination. Inherently, AO devices are Bragg-mode wavelength-sensitive elements. We identify that two beams generated by a Bragg cell naturally have unbalanced and inverse spectrums with respect to each other. This mismatch in spectrums in turn violates the ideal autocorrelation condition for a high signal-to-noise ratio broadband interferometric sensor such as OCT. We solve this fundamental limitation of Bragg cell use for OCT by deploying a new interferometric architecture where the two interfering beams have the same power spectral profile over the bandwidth of the broadband source. With the proposed AO based system, high (e.g., megahertz) intermediate frequency can be generated for low 1/f noise heterodyne detection. System issues such as resolution, number of axial scans, and delay-path selection time are addressed. Experiments described demonstrate our high-speed acousto-optically tuned OCT system where optical delay lines can be selected at submicrosecond speeds.     

Design methods for space-variant optical interconnections to achieve optimum power throughput

Optoelectronic systems based on space-variant optics give great freedom to the system designer in terms of interconnect topologies. One feature of space-variant systems is that they can achieve a high interconnect density. However, this density is achieved by having large arrays of diffractive elements with very small apertures relative to the propagation distances involved. Thus diffraction losses from the finite apertures can significantly affect power throughput for these types of systems, regardless of the diffractive efficiencies of the optical elements involved. Therefore it is desirable that this loss be minimized. We present several space-variant optical interconnect design methods (for both one-to-one and fan-out interconnects) and compare them in terms of power throughput for diffraction-limited interconnect distances. Both numerical simulations and experimental results are presented.     

Effect of the header pressure drop induced flow maldistribution on the microchannel evaporator performance

This paper presents an experimental and numerical investigation of the flow maldistribution caused by the pressure drop in headers and its impact on the performance of a microchannel evaporator with horizontal headers and vertically oriented tubes. Experimental results show that the flash gas bypass method almost eliminates the quality induced maldistribution. However, refrigerant flow maldistribution caused by the header pressure drop still exists. This is mainly because the pressure drop along the headers results in uneven pressure difference and therefore non-uniform liquid refrigerant mass flow rate across each microchannel tube. A microchannel evaporator model validated by experimental results is employed to quantify header pressure drop induced flow maldistribution. Parametric analysis reveals that such maldistribution impact is significantly reduced by enlarging the outlet header size, increasing heat exchanger aspect ratio, or reducing the microchannel size while other parameters are kept constant. When ratio of outlet header to the total evaporator pressure drop is less than 30%, the cooling capacity reduction is limited below 3%.     

Adaptive beam shaping by controlled thermal lensing in optical elements

We describe an adaptive optical system for use as a tunable focusing element. The system provides adaptive beam shaping via controlled thermal lensing in the optical elements. The system is agile, remotely controllable, touch free, and vacuum compatible; it offers a wide dynamic range, aberration-free focal length tuning, and can provide both positive and negative lensing effects. Focusing is obtained through dynamic heating of an optical element by an external pump beam. The system is especially suitable for use in interferometric gravitational wave interferometers employing high laser power, allowing for in situ control of the laser modal properties and compensation for thermal lensing of the primary laser. Using CO(2) laser heating of fused-silica substrates, we demonstrate a focal length variable from infinity to 4.0 m, with a slope of 0.082 diopter/W of absorbed heat. For on-axis operation, no higher-order modes are introduced by the adaptive optical element. Theoretical modeling of the induced optical path change and predicted thermal lens agrees well with measurement.     

Parallel optical interconnects may reduce the communication bottleneck in symmetric multiprocessors

We start with a detailed analysis of the communication issues in today's symmetric multiprocessor (SMP) architectures to study the benefits of implementing optical interconnects (OI) in these machines. We show that the transmission of block addresses is the most critical communication bottleneck of future large SMPs owing to the need to preserve the coherence of data duplicated in caches. An address transmission bandwidth as high as 200-300 Gb/s may be necessary in ten years from now; this requirement will represent a difficult challenge for shared electric buses. In this context we suggest the introduction of simple point-to-point OIs for a SMP cache-coherent switch, i.e., for a VLSI switch that would emulate the shared-bus function. The operation might require as much as 10,000 input-outputs (IOs) to connect 100 processors, particularly if one maintains the present parallelism of transmissions to preserve a large bandwidth and a short memory access latency. The interest for OIs comes from the potential increase of the transmission frequency and from the possible integration of such a high density of IOs on top of electronic chips to overcome packaging issues. Then we consider the implementation of an optical bus that is a multipoint optical line involving more optical technology. This solution allows multiple simultaneous accesses to the bus, but the preservation of the coherence of caches can no longer be maintained with the usual fast snooping protocols.     

Zero-static-power nonvolatile logic-in-memory circuits for flexible electronics

Flexible logic circuits and memory with ultra-low static power consumption are in great demand for battery-powered flexible electronic systems.Here,we show that a flexible nonvolatile logic-in-memory circuit enabling normally-off computing can be implemented using a poly(1,3,5-trivinyl-1,3,5-trimethyl cyclotrisiloxane) (pV3D3)-based memristor array.Although memristive logic-in-memory circuits have been previously reported,the requirements of additional components and the large variation of memristors have limited demonstrations to simple gates within a few operation cycles on rigid substrates only.Using memristor-aided logic (MAGIC) architecture requiring only memristors and pV3D3-memristor with good uniformity on a flexible substrate,for the first time,we experimentally demonstrated our implementation of MAGIC-NOT and-NOR gates during multiple cycles and even under bent conditions.Other functions,such as OR,AND,NAND,and a half adder,are also realized by combinations of NOT and NOR gates within a crossbar array.This research advances the development of novel computing architecture with zero static power consumption for batterypowered flexible electronic systems.     

Beam width and transmitter power adaptive to tracking system performance for free-space optical communication

The basic free-space optical communication system includes at least two satellites. To communicate between them, the transmitter satellite must track the beacon of the receiver satellite and point the information optical beam in its direction. Optical tracking and pointing systems for free space suffer during tracking from high-amplitude vibration because of background radiation from interstellar objects such as the Sun, Moon, Earth, and stars in the tracking field of view or the mechanical impact from satellite internal and external sources. The vibrations of beam pointing increase the bit error rate and jam communication between the two satellites. One way to overcome this problem is to increase the satellite receiver beacon power. However, this solution requires increased power consumption and weight, both of which are disadvantageous in satellite development. Considering these facts, we derive a mathematical model of a communication system that adapts optimally the transmitter beam width and the transmitted power to the tracking system performance. Based on this model, we investigate the performance of a communication system with discrete element optical phased array transmitter telescope gain. An example for a practical communication system between a Low Earth Orbit Satellite and a Geostationary Earth Orbit Satellite is presented. From the results of this research it can be seen that a four-element adaptive transmitter telescope is sufficient to compensate for vibration amplitude doubling. The benefits of the proposed model are less required transmitter power and improved communication system performance.     

Adaptive divergence and power for improving connectivity in free-space optical mobile networks

A significant challenge for free-space optical (FSO) links is the restrictive alignment requirements, especially when the transceivers are moving. For moderate distances and rapid unpredictable motion, the receiver's field of view and the positioning system's dynamics become factors. We explore the use of adaptive transmitter power and beam divergence to improve the likelihood of maintaining a mobile FSO link by using Gaussian beam propagation theory and link budgets. We calculate the allowable misalignment between the transceivers' optical axes as a function of power, divergence, and transceiver distance. The maximum allowable error is independent of the distance, except when the field of view is a limiting factor. Certain combinations of divergence and power, while suboptimal for one distance, provide a relaxed misalignment limit for many distances. Based on the calculations, we make initial suggestions for system design.