Instability of the Multimode Fiber Frequency Response Beyond the Baseband for Coherent Sources

Fluctuations of the multimode fiber (MMF) frequency response beyond the baseband are observed experimentally when highly coherent light sources are used. This behavior is caused by the variation of the mode group powers caused by the relative modal phase fluctuation combined with spatial filtering at elements such as nonideal connectors. The observed effect may have vital influence on subcarrier multiplexing systems operation outside the baseband of the MMF. The effect is investigated both theoretically and experimentally.      

Switched WLAN-wideband tapered slot antenna

A switched band antenna that has a capability to operate in dual-band WLAN mode (2.4-2.485 GHz and 5.725-5.875 GHz), a single band WLAN mode of 2.4-2.485 GHz and a wideband mode (2.2-7.2 GHz) is presented. The antenna is based on a tapered slot antenna, which has a defected ground structure containing two switchable ring and two rectangular slots to provide a switched band property. Measurement results on a prototype antenna, with an ideal switch, fully demonstrate the performances of the proposed designs. The proposed antenna is suitable for use in a base station or an access point for WLAN and wideband applications and may be also useful in future multimode radio systems.     

An integrated equivalent circuit model for relative intensity noise and frequency noise spectrum of a multimode semiconductor laser

Relative intensity noise (RIN) and the frequency/phase noise spectrum (FNS) equivalent circuit of a multimode semiconductor laser diode are derived from multimode rate equations with the inclusion of noise Langevin sources. FNS is an important parameter in optical communication systems, and its circuit model is presented, for the first time, in this paper. Both circuit models for RIN and FNS are integrated in one circuit. RIN and FNS are calculated as functions of frequency, output power, and mode number. It is shown that the RIN of the main mode is increased in the multimode lasers with higher mode numbers. Furthermore, we show that RIN and FNS are enhanced for higher output power. The dependency of a multimode laser diode linewidth on output power is also analyzed using the model.     

Investigation on the Multimode Dynamics of InGaAsP–InP Microring Lasers

A multimode model based on the rate equation approximation is presented in order to simulate the mode dynamics of 1.55-mum InGaAsP-InP microring lasers. A detailed characterization is performed including calculated optical spectra, time traces and relative intensity noise spectra. Different operation regimes are observed, bidirectional multimode with/without alternate oscillations, unidirectional single mode, bidirectional single mode and mode hopping. The boundaries of the above operation regimes are investigated with respect to the current level, bus waveguide reflectivity and ring radius. Varying the current level a transition from multimode to single mode and eventually mode hopping operation is observed. Increasing the bus waveguide reflectivity a transition from unidirectional to bidirectional operation is revealed, while the use of nonequal reflectivities between the two facets, promotes unidirectional operation. Moreover, the ring radius is proved to be a critical parameter for the extent of each operation regime since it directly influences the modal wavelength separation     

The output power of a 6328-Å He-Ne gas laser

Expressions are derived for the output power of a gas laser in single-mode operation. This theory is then extended to multimode operation where the mode spacing is less than the pressure-broadened "natural" atomic linewidth. Using measured values for the linewidth and saturation parameters, these formulas are used to predict 6328-Å output power for a He-Ne laser as a function of laser gain, cavity loss, and output coupling. Experiments are reported which show good agreement with the theoretical predictions, and it is concluded that the theory presented here is adequate to describe the main features of the power output behavior of a 6328-Å He-Ne gas laser.     

Syntactic Modeling and Signal Processing of Multifunction Radars: A Stochastic Context-Free Grammar Approach

Multifunction radars (MFRs) are sophisticated sensors with complex dynamical modes that are widely used in surveillance and tracking. This paper demonstrates that stochastic context-free grammars (SCFGs) are adequate models for capturing the essential features of the MFR dynamics. Specifically, MFRs are modeled as systems that ldquospeakrdquo a language that is characterized by an SCFG. The paper shows that such a grammar is modulated by a Markov chain representing radar's policy of operation. The paper also demonstrates how some well-known statistical signal processing techniques can be applied to MFR signal processing using these stochstic syntactic models. We derive two statistical estimation approaches for MFR signal processing-a maximum likelihood sequence estimator to estimate radar's policies of operation, and a maximum likelihood parameter estimator to infer the radar parameter values. Two layers of signal processing are introduced in this paper. The first layer is concerned with the estimation of MFR's policies of operation. It involves signal processing in the CFG domain. The second layer is concerned with identification of tasks the radar is engaged in. It involves signal processing in the finite-state domain. Both of these signal processing techniques are important elements of a bigger radar signal processing problem that is often encountered in electronic warfare applications-the problem of the estimation of the level of threat that a radar poses to each individual target at any point in time.     

System design methodology for power budgeting of 62.5 μmmultimode fiber systems

The authors present the data to support a recommendation of a new system loss budgeting rule applicable to high-component-count systems using 62.5 μm core diameter multimode fiber. The limited phase space or restricted launch characterization of fiber is demonstrated again to give the most accurate system loss prediction. However, for this fiber size, components such as connectors, switches, and couplers are shown to exhibit loss equivalent to the overfilled launch loss. Two mechanisms are found to explain why the overfilled launch component loss best characterizes their actual loss in common multimode fiber systems. Both are related to the large numerical aperture (NA) of the power distribution launched by the light-emitting diode (LED) system sources. It is important to note that these mechanisms only have been observed on components made with 62.5 μm multimode fiber. This behavior is likely to be different in other fiber sizes     

Air interface identification for Software Radio systems

Reconfigurable Software Radio (SR) equipment is considered as the next evolutionary step in the mobile communications. One of the most crucial properties of a SR terminal is that it is capable of using a wide range of air interface standards, providing a seamless interoperability between different standards and an enhanced roaming capability, paving way to a more flexible and efficient use of spectral resources. This multimode operation has to be supported by a number of key functionalities, one of which is the air interface identification. A SR terminal, when switched on, has to be able to locate and identify the air interfaces available in the frequency environment, and while connected to a network, it has to monitor the presence of alternative air interfaces to perform interstandard handover if necessary. In our work, we propose exploiting the distinct cyclostationary properties of signals from different air interfaces as features for air interface identification.     

Fiber-optical transmission between 0.8 and 1.4 µm

The present state of the art and expected development in discrete components for fiber-optic transmission systems are reviewed. Predicted performance of fiber systems in the 0.85, 1.06, and 1.27 µm regions is presented, and the advantages of longer wavelength operation quantified. It is concluded that operation near 1.27 µm is particularly attractive for a) moderate data rate systems employing LED's and multimode fibers whose chromatic dispersion and attenuation are greatly reduced compared with 0.85 and 1.06 µm, and b) high data rate systems employing lasers and monomode fibers. In systems employing lasers and graded index multimode fibers, the advantage of 1.27 µm versus 1.06 µm operation is not as pronounced, although transmission distances at both of these longer wavelengths are significantly increased from those at 0.85 µm.     

Fiber-Optical Transmission Between 0.8 and 1.4 /spl mu/m

The present state of the art and expected development in discrete components for Fiber-optic transmission systems are reviewed. Predicted performance of fiber systems in the 0.85, 1.06, and 1.27 /spl mu/m regions is presented, and the advantages of longer wavelength operation quantified. Itisconcluded that operation near 1.27 /spl mu/m is particularly attractive for a) moderate data rate systems employing LED's and multimode fibers whose chromatic dispersion and attenuation are greatly reduced compared with 0.85 and 1.06 /spl mu/m, and b) high data rate systems employing lasers and monomode fibers. In systems employing lasers and graded index multimode fibers, the advantage of 1.27/spl mu/m versus 1.06 /spl mu/m operation is not as pronounced, although transmission distances at both of these longer wavelengths are significantly increased from those at 0.85 /spl mu/m.     

2×2 InP/InGaAsP MMI-MZI型光开关设计与实验

主要研究基于多模干涉耦合器(MMI)的2×2 InP/InGaAsP马赫曾德型(MMI-MZI)光开关.开关的特性采用BPM(光束传输法)进行器件建模、参数分析与性能优化.开关的结构按照传输波导保证单模传输、低偏振敏感要求进行了设计.光开关通过载流子电注入产生的载流子吸收和带填充效应改变移相臂传输光相位.实验测得光开关在控制电压为6.4 V时可实现交叉态到直通态的倒换,开关和关态串扰分别为-20.49 dB、-19.19 dB.这种开关具有结构紧凑、制作容差大和偏振无敏感等优点,它可以很方便地和其它半导体有源器件集成,在未来DWDM系统中有着非常广泛的应用前景.     

Fault Diagnosis of Time-Varying Parameter Systems With Application in MEMS LCRs

Multiple-model adaptive estimation (MMAE) is a well-known technique used for model matching of deterministic parameter systems. This technique can be used in fault diagnosis by allocating a model to each type of fault. In each contingency, the model that represents the behavior of the actual system can indicate the type of fault occurrence. Kalman filters are generally used in modeling and residual-signal generation of time-invariant systems. Slowly time-varying parameter systems, however, require a system identification unit in addition to the model-matching core. This paper utilizes the least square forgetting-factor technique in parameter identification of slowly time-varying systems and combines it with MMAE for fault-diagnosis applications in microelectromechanical-systems (MEMS) lateral comb resonators (LCRs). Prescheduled faults were designed for simulations and experimentally examined in real-time implementations of estimation-based diagnosis technique for two fabricated MEMS LCRs. It is shown that the application of a system identification unit significantly increases the performance of the fault diagnosis in MEMS devices.     

稳定光学谐振腔高斯光束的发散角理论

当考虑到高斯光束在通过界面时参数的变化后,光学谐振腔输出高斯光束的远场发散角(外发散角)一般不等于自由空间中高斯光束的远场发散角(定义为内发散角),使用ABCD矩阵,g-和g-参数表示,导出了在基模和多模工作时内、外发散角的一般公式并对(1)固体激光腔;(2)输出镜为平面镜;(3)激光棒端面直接镀膜成镜;(4)热透镜腔等四个典型例证作了比较和讨论。     

Enhanced recognition in hybrid systems

Identifying the mode of operation of a hybrid system while simultaneously tracking the state is difficult without a direct modal measurement. Even with such a measurement, subtle biases have been observed in applications. It is shown here that for one estimation algorithm, modal identification can be improved with a simple adjustment in the measurement update     

An Adaptive Digital Front-End for Multimode Wireless Receivers

The ongoing evolution from 2G to 3G and beyond requires multimode operation of wireless cellular transceivers. This paper discusses the design of an adaptive wireless receiver for multimode operation and power efficiency. This is achieved by sharing digital signal processing stages for all operation modes and adapting them such that performance requirements are met at a minimum power consumption. The flexibility a digital front-end introduces in combination with an analog zero-intermediate-frequency receiver is studied with respect to the main cellular communication standards (Global System for Mobile Communication, Enhanced Data Rates for Global Evolution, Wide-Band Code-Division Multiple Access (WCDMA)/High-Speed Downlink Packet Access, and CDMA2000 and satellite navigation systems (Galileo), with considerations of impacts on the capability of an implementation of a software-defined radio. The proposed approach is verified by simulations exhibiting an error-vector magnitude of 2.9% in Universal Mobile Telecommunications System mode while the estimated power consumption can be reduced by 62% versus full featured mode at reasonable degradation in modulation quality.     

Switching a multistable solid-state laser from one stable steady state to another-dynamic stability analysis

The switching of a multistable laser from one stable steady state to the next has been studied using a longitudinally pumped laser with a nonlinear mirror. To achieve switching, use has been made of a second mode that is oscillating at the pump frequency. This pump mode is kept just below its threshold for oscillation. We find through an analysis of the dynamic gain functions of both the laser and pump modes that the steady state of the laser mode is very sensitive to changes in the steady state of the pump mode that are brought about by fluctuations in the pump intensity. It is found that for switching to be possible, the difference in frequency between the pump and laser mode should not be much larger than the linewidth of the laser atomic medium. We conclude that a conventionally pumped multistable and multimode solid-state laser can have one of its modes switched from one of its stable steady states to another such state, of a lower or higher intensity, by longitudinally injecting a pulse of suitable magnitude, frequency, and duration as a perturbation. This can happen provided that one of the other N-1 modes, which are kept below oscillation threshold, has been pushed by such a perturbation to just above threshold for oscillation in order that it couples with the oscillating laser mode. We have estimated the switching time to be less than the laser cavity photon decay time but of the order of the round-trip time in the Fabry-Perot cavity. The analysis also shows that the laser threshold intensity and its stability are not dependent on the difference between the frequencies of the pump and laser modes. However, the threshold is found to be sensitive to changes in the overlap parameter which is defined as a ratio of the beam waists.     

Fast and accurate identification of thermal dynamics for precision motion control: Exploiting transient data and additional disturbance inputs

Thermally induced deformations are becoming increasingly important for the control performance of precision motion systems. The aim of this paper is to identify the underlying thermal dynamics in view of precision motion control. Identifying thermal systems is challenging due to strong transients, large time constants, excitation signal limitations, large environmental disturbances, and temperature dependent behavior. An approach for non-parametric identification is developed that is particularly suitable for thermal aspects in mechatronic systems. In particular (1) reduced experiment time is achieved by utilizing transient data in the identification procedure. (2) an approach is presented that exploits measured ambient air temperature fluctuations as additional inputs to the identification setup. (3) the non-parametric model, obtained through (1) and (2), is used as a basis for parameter estimation of a grey-box parametric model. The presented methods form a complete framework that facilitates the implementation of advanced control techniques and error compensation strategies by providing high-fidelity models, enabling increased accuracy and throughput in high precision motion control.     

Influence of instantaneous mode competition on the dynamics ofsemiconductor lasers

Comprehensive theoretical investigations of the influence of instantaneous mode-competition phenomena on the dynamics of semiconductor lasers are introduced. The analyzes are based on numerical simulations of the multimode rate equations superposed by Langevin noise sources that account for the intrinsic fluctuations associated with the spontaneous emission. Numerical generation of the Langevin noise sources is performed in such a way as to keep the correlation of the modal photon number with the injected electron number. The gain saturation effects, which cause competition phenomena among lasing modes, are introduced based on a self-consistent model. The effect of the noise sources on the mode-competition phenomena is illustrated. The mode-competition phenomena induce instantaneous coupling among fluctuations in the intensity of modes, which induce instabilities in the mode dynamics and affect the state of operation. The dynamics of modes and the characteristics of the output spectrum are investigated over wide ranges of the injection current and the linewidth enhancement factor in both AlGaAs-GaAs and InGaAsP-InP laser systems. Operation is classified into stable single mode, stable multimode, hopping multimode, and jittering single mode. Based on the present results, the experimental observations of multimode oscillation in InGaAsP-InP lasers are explained as results of the large value of the linewidth enhancement factor     

Terahertz Frequency- and Mode-Insensitive Broadband Quasi-optical Converter Antenna System

Recently emerged multimode gyrotron, a high-power broadband terahertz radiator, encounters the challenge of efficiently converting a series of operating whispering-gallery modes (WGMs) into free-space Gaussian beams. To this demand, we propose a frequency- and mode-insensitive antenna capable of broadband multimode converting. For a single mode, to achieve broadband operation, special reflector configuration and large-radius launcher guarantee the system high robustness to frequency-induced wave number variation. Furthermore, for a series of operating WGMs, in order to achieve multimode operation, high-order mode indices guarantees familiar field patterns and ray trajectories. In particular, high-purity Gaussian beams are simultaneously achieved in different WGMs of broad continuous bands, including 351–361 GHz for TE_11,2 mode, 375–385 GHz for TE_12,2 mode, and 398–410 GHz for TE_13,2 mode. The results are verified by both the vector diffraction theory and the method of momentum. This kind of mode converter will promote the development of multimode gyrotrons and other antenna-feeder systems for high-power terahertz applications.