Pulse Stream Research Articles

Steady-state free precession experiments and exact treatment of diffusion in a uniform gradient

We derive an analytic solution for the magnetization of spins diffusing in a constant gradient field while applying a long stream of rf pulses, which is known as the steady-state free precession (SSFP) sequence. We calculate the diffusion-dependent amplitude of the free induction decay (FID) and higher order echoes for pulses with arbitrary flip angle α and pulse spacing TR. Stopped-SSFP experiments were performed in a permanent gradient field and the amplitudes of the first three higher order echoes were measured for a range of values of α and TR. Theoretical results are in excellent agreement with experimental results, using no adjustable parameters. We identify various diffusion regimes in a rather large parameter space of pulsing and relaxation times, diffusion coefficient, and flip angle and discuss the interplay of the relevant time scales present in the problem. This “phase diagram” provides a road map for designing experiments which enhance or suppress the sensitivity to diffusion. We delineate the limits of validity of the widely used ansatz put forth by Kaiser, Bartholdi, and Ernst in their seminal paper.

Optical regeneration using a feedforward semiconductor optical amplifier with chirp‐controlled filtering

AbstractOptical pulse regeneration and reshaping using nonlinear optical pulse amplification followed by a chirp‐controlled filtering is simulated. The nonlinear amplification and chirp control are obtained by feedforward current injection in a semiconductor optical amplifier. The nonlinear amplification provides pulse regeneration, and the controlled chirp, in association with an optical frequency discriminator, can be used to optimize the shape of several Gbit/s pulse streams. The simulator model has been confirmed experimentally using eye diagram techniques. In addition, the switching action of an optical semiconductor amplifier with a dynamically arbitrarily injected current can be simulated. © 2001 John Wiley & Sons, Inc. Microwave Opt Technol Lett 30: 438–442, 2001.

New phase modulation technique based on spatial soliton switching

A phase modulation technique able to increase the transmission capacity of an optical channel is presented. It is based on spatial soliton switching properties. The modulator device accepts as inputs two streams of amplitude modulated pulses and generates an output stream of phase modulated pulses whose phase values depends on the different input combinations, coding properly the input streams and increasing the transmission capacity of the optical channel that carries this information. The modulator device can be properly cascaded, generating a unique stream of pulses capable of carrying the information of a certain number of input channels. A proper demodulator device is also presented. It is capable of accepting as input a phase modulated stream of pulses, generating as outputs the original amplitude modulated pulse streams.

A novel scheme for optical pulse multiplication using fiber coupler loop-connecting configuration

Connecting an input port and an output port of a single-mode fiber coupler to form a fiber-loop, and making the time delay of the fiber-loop odd multiple of the half-period of the input optical pulse stream, we can obtain a pulse stream with multiplying repetition rate at the other output port of the coupler. Utilizing three loop-connecting fiber couplers cascaded, a pulse stream with eight times the repetition rate is achieved.

Pulse Streams and Problems of Grouping and Metrical Dissonance in Bartök’s “With Drums and Pipes”

Polyphony has many interesting rhythmic properties that do not obtain in textures that are modeled by most rhythmic theories. This paper invokes the concept of pulse streams to demonstrate how phenomenal accent and grouping are organized in a extended two-voice polyphony by Bartók to create convincing form and process. The pulse-stream analysis is manifested audibly by Quicktime examples that combine audio playback, a scrolling annotated score, and the pulses played by percussion instruments.

Temporal self-imaging effects: theory and application for multiplying pulse repetition rates

A time-domain equivalent of the spatial Talbot or self-imaging phenomenon appears when a periodic temporal signal propagates through a dispersive medium under first-order dispersion conditions. The effect is of great interest because it can be applied for multiplying the repetition rate of an arbitrary periodic pulse train without distorting the individual pulse features and essentially without loss of energy. In this way, pulse sequences in the terahertz regime can be generated from typical mode-locked pulse streams (with a few gigahertz repetition rates). The Talbot-based repetition-rate-multiplication technique can be implemented by using a linearly chirped fiber grating (LCFG) as the dispersive medium. As compared with other alternatives, an LCFG can be designed to provide the required bandwidth and dispersion characteristics in significantly more compact forms. Here, by using a signal-theory-based approach, we carry out a general theoretical analysis of the temporal self-imaging phenomenon and derive analytical expressions for all cases of interest (integer and fractional self-imaging effects). We also show how to design a LCFG for implementing the repetition-rate-multiplication technique and discuss the impact of nonidealities in the grating's response on the multiplication process. Results of our study are relevant from both a physical and a practical perspective.

System impact of intra-channel nonlinear effects in highly dispersed optical pulse transmission

We provide accurate analytical estimates of timing- and amplitude-jitter caused by nonlinear pulse interaction in systems based on highly dispersed optical pulses, both for coherent and noncoherent pulse streams. We show that the system penalties reduce monotonically with pulsewidth and with increasing fiber dispersion. We demonstrate that proper dispersion pre-compensation can result in a significant reduction of the nonlinear inpairments and provide analytical tools for obtaining the optimal pre-compensation parameters.

Optically controlled delay lines by pulse self-trapping in parametric waveguides with distributed feedback

Quadratically nonlinear waveguides are studied in the presence of a Bragg grating resonant with one of the involved frequencies. In the case of second-harmonic generation and distributed feedback at the fundamental, we investigate the existence and features of two-color gap-solitary waves, focusing on controlling their propagation speed. This leads to the conception of optically controlled delay lines and retiming schemes for pulse streams in communication systems.

Automating parallel implementation of neural learning algorithms.

Neural learning algorithms generally involve a number of identical processing units, which are fully or partially connected, and involve an update function, such as a ramp, a sigmoid or a Gaussian function for instance. Some variations also exist, where units can be heterogeneous, or where an alternative update technique is employed, such as a pulse stream generator. Associated with connections are numerical values that must be adjusted using a learning rule, and and dictated by parameters that are learning rule specific, such as momentum, a learning rate, a temperature, amongst others. Usually, neural learning algorithms involve local updates, and a global interaction between units is often discouraged, except in instances where units are fully connected, or involve synchronous updates. In all of these instances, concurrency within a neural algorithm cannot be fully exploited without a suitable implementation strategy. A design scheme is described for translating a neural learning algorithm from inception to implementation on a parallel machine using PVM or MPI libraries, or onto programmable logic such as FPGAs. A designer must first describe the algorithm using a specialised Neural Language, from which a Petri net (PN) model is constructed automatically for verification, and building a performance model. The PN model can be used to study issues such as synchronisation points, resource sharing and concurrency within a learning rule. Specialised constructs are provided to enable a designer to express various aspects of a learning rule, such as the number and connectivity of neural nodes, the interconnection strategies, and information flows required by the learning algorithm. A scheduling and mapping strategy is then used to translate this PN model onto a multiprocessor template. We demonstrate our technique using a Kohonen and backpropagation learning rules, implemented on a loosely coupled workstation cluster, and a dedicated parallel machine, with PVM libraries.

Design of efficient all-optical code-division multiplexing systems supporting multiple-bit-rate and equal-bit-rate transmissions

We present the design of efficient all-optical code-division multiplexing (AOCDM) systems that can transmit multiple-bit-rate (MBR) data signals over a common optical fiber. This is achieved when the proposed strict optical orthogonal code (OOC) of autocorrelation and cross-correlation constraints of 1 are used but without performance degradation compared with the use of conventional OOC. We describe the design of various strict OOC's by employing the useful concept of slot distances, and methods of code construction are also presented. Moreover, we give the principle of MBR data transmissions in an AOCDM system. It is shown that AOCDM systems using the proposed OOC can effectively transmit multiuser MBR and equal-bit-rate (EBR) data with no increase of system complexity. In principle, optimal strict OOC's need the same or a slightly larger system bandwidth compared with optimal conventional OOC's for EBR operation, whereas the former can require a smaller system bandwidth and have a better system performance than the latter for MBR transmissions. A new, to our knowledge, family of strict OOC's is also introduced, whose code words can have nonconstant weights to support multiuser communications with different transmission quality. Furthermore, we design low-cost AOCDM transmitters that are based on an efficient gain-switching scheme that does not require an electro-optic intensity modulator to on-off modulate an optical clock pulse stream at each transmitter. The basic operation principle is also experimentally demonstrated.

Generation of a 40-GHz pulse stream by pulse multiplication with a sampled fiber Bragg grating

A sinc-sampled fiber Bragg grating is used to achieve multiplication of the repetition rate of a pulse stream from 10 to 40 GHz. The spectral characteristics of the grating ensure that the resultant pulses-solitons of 3.4-ps width-have the same individual pulse characteristics as those of the input.

Stabilised 10 GHz rationally mode-locked erbium fibre laser and its use in a 40 Gbit/s RZ data transmission system over 240 km of standard fibre

Mode-locked fibre lasers are a promising pulse source for use in high-speed data communication systems. Optical pulses at high repetition rates can be achieved by mode-locking the laser cavity at a rational harmonic of the fundamental mode-locking frequency. Using this technique, optical pulse streams at very high rates can be obtained by using longer laser cavities and slower optoelectronic components. A 10 GHz rationally mode-locked laser is presented, which is the fastest stabilised rationally mode-locked laser reported. The pulse source produces stable soliton-like pulses, with a full-width half-maximum pulse width of 5 ps and a back-to-back receiver sensitivity of -19.7 dBm for a bit error rate (BER) of 10/sup -9/. Using optical time division multiplexing, a 40 Gbit/s data stream is generated from the 10 GHz optical pulse source with a back-to-back receiver sensitivity of -18.9 dBm. This 40 Gbit/s transmitter is used for transmitting data over 240 km of dispersion-managed fibre. This incurs a sensitivity penalty of only 0.4 dB for a BER of 10/sup -9/.

Analog implementation of pulse-coupled neural networks

This paper presents a compact architecture for analog CMOS hardware implementation of voltage-mode pulse-coupled neural networks (PCNN's). The hardware implementation methods shows inherent fault tolerance specialties and high speed, which is usually more than an order of magnitude over the software counterpart. A computational style described in this article mimics a biological neural network using pulse-stream signaling and analog summation and multiplication. Pulse-stream encoding technique uses pulse streams to carry information and control analog circuitry, while storing further analog information on the time axis. The main feature of the proposed neuron circuit is that the structure is compact, yet exhibiting all the basic properties of natural biological neurons. Functional and structural forms of neural and synaptic functions are presented along with simulation results. Finally, the proposed design is applied to image processing to demonstrate successful restoration of images and their features.

FUNDAMENTAL BIOROBOTICS OF INHERENTLY LIFELIKE MACHINES

An original way to define, analyze and design mechanical systems with inherently lifelike dynamic properties is presented. The construction of elementary actuating mechanisms for robotic manipulators which embody a complete set of technologically relevant biological principles is outlined. The ultimate objective is to develop a new class of mobile, autonomous, and interactive machines which dynamically emulates live musculoskeletal systems. This study introduces the mathematical models and algorithms to transform and synthesize the results of research in musculoskeletal physiology into explicit engineering design specifications. The application of a new contractile muscle-like viscoelastic motor as a servomechanical drive for articulated rigid link mechanisms is investigated. Key features of the neuromuscular force control by twitch summation are combined to formulate a pulse stream control method suitable for fluid powered mechanisms.

On Computation with Pulses

We explore the computational power of formal models for computation with pulses. Such models are motivated by realistic models for biological neurons and by related new types of VLSI (“pulse stream VLSI”). In preceding work it was shown that the computational power of formal models for computation with pulses is quite high if the pulses arriving at a computational unit have an approximately linearly rising or linearly decreasing initial segment. This property is satisfied by common models for biological neurons. On the other hand, several implementations of pulse stream VLSI employ pulses that are approximately piecewise constant (i.e., step functions). In this article we investigate the relevance of the shape of pulses in formal models for computation with pulses. The results show that the computational power drops significantly if one replaces pulses with linearly rising or decreasing initial segments by piecewise constant pulses. We provide an exact characterization of the latter model in terms of a weak version of a random access machine (RAM). We also compare the language recognition capability of a recurrent version of this model with that of deterministic finite automata and Turing machines.

Polarisation-independent all-optical circulating shift register based on self-phase modulation of semiconductor optical amplifier

The authors propose and experimentally demonstrate a polarisation-independent all-optical circulating shift register (CSR) based on self-phase modulation of a semiconductor optical amplifier, which is very insensitive to the polarisation of the input pulse train and requires very low switching energy (~176 fJ). An all-optical 91 bit CSR was constructed for an input pulse stream at 2.897 GHz, and a high extinction ratio of 20 dB or more was obtained.

Non-Gaussian kernel circuits in analogue VLSI: implications for RBF network performance

The authors present a cascadable circuit for the distance metric and nonlinear functions required by radial basis function (RBF) neural networks. The distance metric is a quadratic approximation to the Euclidean distance between two voltages, and the nonlinearity is produced using two MOS transistors. This circuit has been developed for pulse stream neural systems. The operation of the circuit is described and suggestions are made for its practical implementation in pulsed analogue VLSI. Since the nonlinearity generated by the circuit has not been used in RBFs before, software results are presented to demonstrate that the circuit can produce good classification performance. However, software simulations show that the shape of the nonlinearity has implications for the performance of RBFs using the circuit. The authors consider the implications of these results to the development of pulsed analogue RBF chips in the limited precision environment of analogue VLSI. Based on their findings, they make suggestions for the shape and range of future centre circuits to make them robust to this limited precision and which they believe will help ensure good classification performance is obtained in hardware.

The optimum dynamics of preset count digital rate meter algorithms

The concept of “quality of measurement,” Q, of the count rate digital measurement algorithm has been applied to the class of preset count digital rate meter algorithms. The best quality corresponds to the minimum of (τ×ε) product, where τ is the response time of the algorithm to a sudden change in the average count rate and ε is the fractional standard deviation of the steady-state measurement of a constant average count-rate pulse stream. Analytical expressions for τ and ε of digital rate meter algorithms are used to calculate maximum values of Q for both weighted moving average and exponential algorithms. The values of the basic algorithm parameters, k for the moving average, and a for the exponential algorithm to achieve these maximum values of Q are derived for both algorithms. A comparison of the present results with the corresponding results obtained for preset time algorithms of these types has been carried out.

Self-Generation of Microwave Magnetic Envelope Soliton Trains in Yttrium Iron Garnet Thin Films

The self-generation of microwave magnetic envelope solitons in magnetic films has been realized for the first time. Solitons with a width of 20 ns and a carrier frequency of 5.1555 GHz were generated for the magnetostatic backward volume wave (MSBVW) configuration in a $5.2\ensuremath{\mu}\mathrm{m}$ thick yttrium iron garnet film. The film and MSBVW propagation structure were part of a ring with an overall gain of $+4\mathrm{dB}$. Pulse modulation at 10 MHz provided the interrupted feedback and the active mode locking which was needed to produce a stable and continuous stream of self-generated soliton pulses. Pulse width and phase measurements confirmed the soliton nature of the self-generated pulses.

A comparison of self-organizing neural networks for fast clustering of radar pulses

Four self-organizing neural networks are compared for automatic deinterleaving of radar pulse streams in electronic warfare systems. The neural networks are the Fuzzy Adaptive Resonance Theory, Fuzzy Min–Max Clustering, Integrated Adaptive Fuzzy Clustering, and Self-Organizing Feature Mapping. Given the need for a clustering procedure that offers both accurate results and computational efficiency, these four networks are examined from three perspectives – clustering quality, convergence time, and computational complexity. The clustering quality and convergence time are measured via computer simulation, using a set of radar pulses collected in the field. Estimation of the worst-case running time for each network allows for the assessment of computational complexity. The effect of the pattern presentation order is analyzed by presenting the data not just in random order, but also in radar-like orders called burst and interleaved.