Numerical modeling of the route‐to‐chaos of semiconductor lasers under optical feedback and its dependence on the external‐cavity length

This paper investigates numerically influence of the external‐cavity length on the type of the route‐to‐chaos of semiconductor lasers under external optical feedback. The study is based on numerical solution of a time‐delay model of rate equations, and the solutions are employed to construct bifurcation diagrams and to examine the Fourier frequency spectrum of the laser output. The ratio of the relaxation frequency to the external‐cavity resonance frequency is employed to measure the influence of the length of the external cavity. The route‐to‐chaos is period doubling when this frequency ratio is less than unity. The route is sub‐harmonic when the frequency ratio increases up to 2.25. When the frequency ratio increases further, the transition to chaos becomes quasi‐periodic characterized by the compound‐cavity frequency and the relaxation frequency as well as their difference. Copyright © 2009 John Wiley & Sons, Ltd.     

Numerical analysis of optical feedback phenomenon and intensity noise of fibre‐grating semiconductor lasers

This paper demonstrates numerical analysis of the dynamics and intensity noise of fibre‐grating semiconductor lasers (FGSLs). The induced phenomenon of strong optical feedback (OFB) is analysed. The simulations are based on an improved time‐delay rate equations model of a single‐mode laser that takes into account the multiple round‐trips of the lasing field in the fibre cavity. The analyses are performed in terms of the temporal trajectory of the laser intensity, bifurcation diagram and relative intensity noise (RIN). We explore influence of the fibre‐cavity length on the dynamics and RIN. The results show that when the fibre cavity is short, the regime of strong OFB is characterized by either continuous‐wave (CW) operation or periodic pulsation. The pulsation frequency is locked at the frequency separation of either the compound‐cavity modes or the external fibre‐cavity modes. The corresponding RIN level is close to or higher than the level of the solitary laser depending on pulse symmetry. When the fibre cavity is long, the laser exhibits unstable dynamics over wider range of OFB. Moreover, the strong‐OFB pulsation becomes beating quasi‐periodic at the relaxation oscillation frequency and the fibre‐cavity mode‐separation frequency. Copyright © 2007 John Wiley & Sons, Ltd.     

A simple and accurate dynamical modeling of quantum‐dot semiconductor optical amplifiers

In this article, quantum‐dot semiconductor optical amplifiers (QD‐SOAs) have been modelled using state space method. To derive this model, we have manipulated the rate equation model of the QD‐SOA, where the average values of the occupation probabilities along the QD‐SOA cavity are considered as the state variables of the system. Using these variables, the distance dependence of the rate equations is eliminated. The derived state space model gives the optical gain and output signal of the amplifier with a high accuracy. Simulation results show that the derived model is not only much simpler and faster than conventional rate equation models, but also the optical gain and output signal of the investigated QD‐SOA are calculated with a higher precision compared to the rate equation model. Copyright © 2013 John Wiley & Sons, Ltd.     

Dynamics of PFC power converters subject to time‐delayed feedback control

Power factor correction converters are power electronics circuits used as AC‐DC power supplies. These systems are well known to exhibit nonlinear phenomena such as subharmonic oscillations and chaotic regimes. These undesirable behaviors increase the THD and therefore can jeopardize enormously the system performances. In this paper, time delay feedback control is applied to stabilize a two‐stage power factor correction AC‐DC converter when it exhibits these instabilities under traditional controllers. This control technique introduces many advantages to the most and widely used average current mode control through widening the stability domain of the system. By appropriately selecting the time delay feedback gain and the time delay period, the undesirable subharmonic components are eliminated, whereas the desired ones remain unchanged. A harmonic balance approach is used for studying the dynamics of the system under the new control scheme and to obtain the stabilization domain. Copyright © 2010 John Wiley & Sons, Ltd.     

Experience replay–based output feedback Q‐learning scheme for optimal output tracking control of discrete‐time linear systems

This paper focuses on solving the adaptive optimal tracking control problem for discrete‐time linear systems with unknown system dynamics using output feedback. A Q‐learning‐based optimal adaptive control scheme is presented to learn the feedback and feedforward control parameters of the optimal tracking control law. The optimal feedback parameters are learned using the proposed output feedback Q‐learning Bellman equation, whereas the estimation of the optimal feedforward control parameters is achieved using an adaptive algorithm that guarantees convergence to zero of the tracking error. The proposed method has the advantage that it is not affected by the exploration noise bias problem and does not require a discounting factor, relieving the two bottlenecks in the past works in achieving stability guarantee and optimal asymptotic tracking. Furthermore, the proposed scheme employs the experience replay technique for data‐driven learning, which is data efficient and relaxes the persistence of excitation requirement in learning the feedback control parameters. It is shown that the learned feedback control parameters converge to the optimal solution of the Riccati equation and the feedforward control parameters converge to the solution of the Sylvester equation. Simulation studies on two practical systems have been carried out to show the effectiveness of the proposed scheme.     

A design method of strong stability self‐tuning controller based on on‐demand type feedback control

This paper proposes a design method of strong stability self‐tuning controller based on on‐demand type feedback control. For safety in industrial applications, although it is important to consider on‐demand type feedback control system, the previous papers about on‐demand type feedback control did not consider the influence of noise and fixed the design parameter to constant value. Therefore, this paper extends the design parameter of on‐demand type feedback control as stable rational function through the design method of strong stability system using coprime factorization. Moreover the self‐tuning controller of the proposed method is given and the control result with noise is shown by numerical example.     

Transmission characteristics of WDM signals amplified by semiconductor optical amplifiers in the 1.3‐µm wavelength band

Applications of semiconductor optical amplifiers to wavelength division multiplexing systems in a 1.3‐µm wavelength band are studied. The measured characteristics of gain and noise figure of the SOA, with small dependency on polarization states, confirm that it is applicable to repeater amplifiers and preamplifiers. When it is used as the preamplifier of a nonrepeated two‐channel 10 Gb/s system, an allowable line loss of 32 dB is attained with the aid of another SOA used for the postamplifier. The measured bit‐error rate of a system with one repeater amplifier is well simulated from the individual characteristics of the SOA and the optical receiver. Using a similar technique, it is predicted that more than 10 amplifiers can be applied in a system with cascaded repeater amplifiers, on the assumption that the polarization dependency of the gain is ignored. © 1998 Scripta Technica, Electr Eng Jpn, 126(1): 23–33, 1999     

Generation of pseudo‐random numbers by chaos‐type function and its application to cryptosystems

The logistic map is known to be one of the nonlinear difference equations as a chaos map, and to generate pseudo‐random numbers. However, since the chaos has a highly sensitive dependence on initial conditions and accumulates inevitable round‐off errors caused by iterating the map, the numerical generation of exact chaotic time series is said to be impossible. The aim of this paper is, first, to propose an algorithm to generate exact chaotic time series of a chaos‐type function derived from the exact chaos solution. Next, the pseudo‐random numbers are evaluated by four tests and the accumulation of chi‐square values. Also, an application to cryptosystems, which do not need the synchronization in usual computer environments, is considered. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 163(2): 67–74, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20436     

A DC stabilized log‐domain nth‐order multifunction filter based on the decomposition of nth‐order HP filter function to FLF topology

The design of high‐order log‐domain filters can be easily accomplished by transposing already known linear‐domain G_m‐C filter topologies to their counterparts in the log‐domain through the employment of a set of complementary operators. To achieve the G_m‐C filter topologies, the multiple feedback approach is widely used due to its accrued advantages. In this paper a synthesis approach for the development of an nth‐order multifunction log‐domain filter comprising lowpass (LP), highpass (HP) and bandpass (BP) filter functions is proposed. The approach is based on the decomposition of nth‐order HP filter function to follow‐the‐leader‐feedback (FLF) topology. The design is simple and simultaneously achieves nearly all of the chief advantages. The design offers superior performance factors vis‐à‐vis the ones recently reported. To verify the high‐order behavior of the topology, a 5th‐order multifunction filter was designed and the achieved simulated results verify the theory. Copyright © 2008 John Wiley & Sons, Ltd.     

Effects of spontaneous and piezoelectric polarization fields on the electronic and optical properties in GaN/AlN quantum dots: multimillion‐atom sp3d5s* tight‐binding simulations

GaN quantum dots (QDs) grown on AlN substrates are strong candidates for UV and near‐infrared applications. The wurtzite crystal symmetry in these dots induces internal fields arising from the following: (i) crystal atomicity;(ii) strained active region; (iii) piezoelectricity; and (iv) spontaneous polarization (pyroelectricity). Accurate modeling of electronic and optical properties of these QDs must capture the interplay of these atomistic and long‐range fields and the size quantization on an equal footing. In this work, single‐particle electronic structure and interband optical transition rates of a GaN/AlN QD grown along the c‐axis are studied using a coupled molecular mechanics‐atomistic 20‐band sp³d⁵s^* tight‐binding (VFF‐TB) framework. To calculate piezoelectricity, a recently reported model that takes into account both the linear and the nonlinear dependence of polarization on the strain tensors has been employed. The simulated GaN/AlN dot is realistically sized (containing ~3 million atoms) and of hexagonal disk shape having height and base length of 4.1 and 17 nm, respectively. It is found that, in contrast to the well‐studied InN/GaN systems, the pyroelectric potential in GaN/AlN dot is larger than the piezoelectric counterpart, and the effects of piezoelectric and pyroelectric fields add up. The internal fields result in a large redshift in the electronic states near the Brillouin zone center (known as quantum confined Stark effect), pronounced non‐degeneracy in the excited states, strongly suppressed optical transition, and anisotropic emission spectra. Copyright © 2014 John Wiley & Sons, Ltd.