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A robust finite-time control (FTC) framework using continuous terminal sliding-mode control (SMC) and high-order sliding-mode observer (HOSMO) is discussed to realize the trajectory tracking of flexible-joint robots in this article. Control performances of the robots always suffer from unknown matched and mismatched time-varying disturbances. Traditional SMC exists with a chattering phenomenon and cannot cope with mismatched time-varying disturbances due to its inherent structure property. For this reason, two HOSMOs are devised to estimate the time-varying disturbances on the link and motor side, respectively. Then, by fusing the states and disturbance estimations into a novel terminal sliding-mode surface, a continuous robust FTC scheme is developed. The proposed control strategy can not only handle both matched and mismatched time-varying disturbances but also obtain a finite-time convergence performance. The rigorous finite-time stability analysis of the closed-loop system under the proposed control method is guaranteed. The results are illustrated to verify the effectiveness and robustness of the proposed design approach. [ABSTRACT FROM AUTHOR]

The tunable laser based on distributed feedback (DFB) laser array has a good single-longitudinal-mode property, high wavelength stability, narrow spectrum linewidth, and feasibility for monolithic integration. The tunable in-series DFB laser array (TIDLA) was proposed to avoid the large extra power loss by the optical combiner. But in the TIDLA, a large laser count will cause a large chip length, which brings about low utilization ratio of material, poor mechanical strength, and large power consumption. In this paper, we demonstrated a 4-channel TIDLA with duplex DFB sections. The total chip length was greatly shortened owing the duplexing of DFB sections. A long linearly chirped Bragg grating is implemented in the laser cavity to reduce the grating interference among different lasers. Multiple π phase shifts and two reflector sections are introduced to improve the mode stability. The designed channel spacing is 100 GHz, and the reconstruction-equivalent-chirp technique is used to provide high precision control of the grating phase. Noted that a variation of 0.1 nm in grating period will cause that of 0.65 nm in wavelength. Output power of above 4 mW, SMSR of above 42 dB, and small wavelength deviation of within 0.15 nm was obtained. Besides, fast channel switches of less than 1 μs are obtained by laser on/off switching. The proposed TIDLA might be applied in the NG-PON2 network where the dense wavelength division multiplexing technique and fast channel switching are required. [ABSTRACT FROM AUTHOR]

Fast optical switches have the potential of lowering the cost, power, and latency of today’s data-center networks. The on-off switching of DFB lasers in arrays is poised to provide extremely fast switching because the on-off time of a DFB laser is the level of sub-nanosecond. However, the key of the DFB laser array based tunable laser lies in the high precision control of the lasing wavelength. In this letter, we proposed and experimentally demonstrated a tunable laser by integrating 16 DFB lasers based on the reconstruction-equivalent-chirp (REC) technique. Without using the conventional e-beam lithography, an equivalent $\lambda $ /4 phase-shift is fabricated in each DFB laser to ensure the single-longitudinal-mode (SLM) yield. Moreover, high precise wavelength spacing is realized. Wavelength deviations of 89.3% DFB lasers are within ±0.2 nm. The output power of all the channels is above 7 mW, and the relative intensity noise is below −130 dB/Hz. By on-off switching the lasers, sixteen channels with 2.4-nm spacing can be fast switched. The switching time between any two channels is less than 50 ns. In addition, a wide-band continuous wavelength tuning range of 38.4 nm can be obtained with a small temperature variation of 18 °C. Owing to the REC technique, the tunable laser via high-density integration of DFB lasers is demonstrated with simple fabrication, high SLM yield, and high wavelength precision, which opens up a path for wide-band and especially fast tunable lasers. [ABSTRACT FROM AUTHOR]

In this study, a tunable in-series multi-wavelength distributed feedback laser array (DFB-MLA) based on reconstruction-equivalent-chirp (REC) technique with simple wavelength tuning scheme is presented and experimentally demonstrated. Unlike traditional DFB laser tuned by thermal-electric cooler (TEC), the proposed laser tuned by injection current can realize faster wavelength-tuning and reduce TEC power consumption. By comparing the thermal influence of different structures of heatsink blocks, several optimized block structures are selected for appropriate thermal conductivity and controllable heat dissipation, resulting in a broad and continuous wavelength tuning range. The proposed in-series DFB-MLA on optimized heatsink achieves better tuning performance with a wavelength tuning range of ~2.4 nm per channel from 40 to 120 mA and stable output power of over 20 mW compared to the laser mounted on the standard heatsink block with the tuning range of 1.4 nm per channel. Furthermore, the tunable laser keeps good single-mode stability, with a side mode suppressing ratio (SMSR) of over 40 dB. Thus, this study provides a new perspective for designing a low-cost continuous tunable semiconductor laser with a simple tuning mechanism in the future. [ABSTRACT FROM AUTHOR]

We proposed a new kind of add-drop multiplexer based on $\pi $ phase-shifted antisymmetric Bragg grating ($\pi $ -ASBG). Light resonance is formed in the $\pi $ -ASBG with mode conversion between backward fundamental transverse electric (TE0) mode and the forward first order transverse electric (T1) mode. With the aid of the TE1 mode generated in the $\pi $ -ASBG, add-drop multiplexer can be designed. As examples, two multiplexer structures based on single- and dual-bus resonator, on silicon-on-insulator (SOI) platform have been studied in detail. Besides, no light is reflected to the input port, if the structure is carefully designed with critical coupling condition satisfied. For the fabricated add-drop multiplexer based on single-bus resonator, the extinction ratio is 23.90 dB and full width at half maximum (FWHM) is 0.33 nm at the through port. For the fabricated add-drop multiplexer based on dual-bus resonator, the extinction ratio is 17.22 dB and FWHM is 0.29 nm at through port. The results show that proposed structures provide a new method to realize add-drop multiplexer and other integrated optical filters for optical communications and sensors. [ABSTRACT FROM AUTHOR]

Multi-wavelength distributed feedback laser array (DFB-MLA) based tunable semiconductor lasers have been widely used owing to the high mode stability and simple wavelength tuning scheme. In this paper, we proposed and experimentally demonstrated a matrix-grating tunable DFB laser array. And the reconstruction-equivalent-chirp (REC) technique is utilized to simplify grating fabrication and realize precise control of the grating phase. Traditionally, the laser is tuned by thermal-electric cooler (TEC), with large power consumption, slow wavelength tuning capability, and added device. To improve the wavelength tuning speed of laser, we varied laser’s injection current instead of adjusting TEC’s temperature. In addition, we analyzed the thermal influence of submount on the laser and then chose the SiO2 as the submount material for its low thermal conductivity and poor heat dissipation, resulting in wide wavelength tuning range via injection current. The DFB-MLA chip achieves a better tunable effect as the lasers can be tuned at 3.5–3.7 nm per channel from 50 to 120 mA with stable output power of over 3 mw as contrasted with the laser on the AlN submount being tuned at 1.4–1.5 nm per channel. And the chip has a side-mode suppressing ratio (SMSR) of over 35 dB and strong stability of wavelength in the repeated experiments. The proposed method may give a new way to realize the wide tuning range tunable DFB laser with a simple wavelength tuning mechanism in the future, especially for the applications of uncooled low-cost tunable lasers. [ABSTRACT FROM AUTHOR]

Multi-wavelength distributed feedback laser array based tunable semiconductor lasers have been widely used owing to their high mode stability and simple tuning scheme. Comparing with the in-parallel laser array, the in-series one can avoid a large power loss caused by the optical combiner. However, the single-mode stability of an in-series laser array is vulnerable to the reflections of other lasers. Here we experimentally demonstrated a tunable in-series laser array with high single-mode stability and high uniformity of wavelength spacing. To reduce the impact of such reflections, the Bragg gratings in different lasers were designed in phase, and the grating phase error was reduced by utilizing the reconstruction-equivalent-chirp technique. Besides, a three-section laser structure was applied in each laser to increase the priority of the dominant lasing mode. Four lasers with small wavelength-spacings of 2.5 nm were integrated in the in-series laser array. According to the measurement results of 40 in-series laser arrays (160 lasers), wavelength deviations of 90.6% lasers were within ±0.2 nm, average wavelength spacings of 97.5% of the measured laser arrays were deviated from the design within ±0.2 nm, and SMSRs of 96.3% lasers were above 45 dB. The output power was above 25 mW and the relative intensity noise was below −130 dB/Hz. Like the in-parallel laser array, the proposed laser array can be applied in either continuous wavelength tuning or fast channel switching. For the continuous wavelength tuning applications, the 10-nm tuning range was obtained with a temperature tuning range of less than 20 °C. For the fast channel switching applications, four channels with 2.5-nm spacing are available, and the switching time between two channels was less than 300 ns. [ABSTRACT FROM AUTHOR]

Hybrid precoding based on the sub-connected structure is one of promising technologies for millimeter-wave communications. Whereas, the existing hybrid designs of the sub-connected structure are all based on the linear precoding, which may suffer from a noise enhancement and performance deterioration. To tackle this issue, this letter proposes a novel nonlinear hybrid Tomlinson-Harashima (TH) precoding algorithm. In the proposed algorithm, a column-wise refinement procedure is firstly developed to update the radio frequency (RF) precoding and combining matrices column by column via Schur complement. Such a refinement procedure can effectively obtain the optimal solutions of each columns of the RF precoding and combining matrices, however, it requires matrix inversions with high complexity. Thus a recurrence relation of the matrix inversion is further established, which can reduce the complexity of the aforementioned refinement procedure greatly. Finally, the digital precoding and combining matrices are given by QR decomposition. Simulations show that the proposed algorithm can effectively improve the spectral efficiency with lower complexity. [ABSTRACT FROM AUTHOR]

A four-channel wavelength-selectable in-series DFB laser array with good single-longitudinal-mode (SLM) property is proposed and theoretically analyzed. To realize good SLM property, a linearly chirped Bragg grating with multiple π phase shifts are implemented in the laser cavity. In addition, two reflector sections with chirped Bragg grating are specially designed for reflection compensation at both ends. In our simulation, the split-step time-domain model with coupled-wave equations and carrier rate equations are used. Based on the simulation results, all the channels work with low threshold currents and the SLM properties are considerably enhanced compared with the conventional in-series laser array. Notably a high precision control of grating phase is required in the practical fabrication due to the ultra-small variation of grating period. Therefore, the reconstruction-equivalent-chirp technique is discussed to provide the fabrication of high-precision grating. In addition, the complex heterogeneous integration or butt-joint technique is not required during the whole fabrication process, which simplifies the fabrication and reduces the cost. The proposed wavelength-selectable in-series laser array is deemed to be applied in the dense wavelength division multiplexing technology, especially when fast wavelength switching is needed, such as in the application of NG-PON2. [ABSTRACT FROM AUTHOR]

We designed and fabricated a π equivalent phase-shifted sampled Bragg grating (SBG) semiconductor laser with a controlled distributed phase shift. The phase shift is equivalently realized with reconstruction-equivalent-chirp (REC) technique. The laser is divided into three sections with the same length. By injecting different currents into the side and middle sections, a distributed phase shift can be introduced. The lasing wavelength can be continuously tuned by the injection currents. In our experiment, when the total current keeps 130 mA, the lasing wavelength can be continuously tuned by 1.2 nm. During the tuning, the side mode suppression ratio keeps above 40 dB and the output power varies only 1.3 dB. Therefore, the proposed method provides an alternative solution for the multiwavelength laser arrays in dense wavelength-division multiplexing (DWDM) systems. [ABSTRACT FROM AUTHOR]

There is a growing demand for positioning actuators with a higher resolution, accuracy, speed, and driving force. Various piezoelectric actuators have been proposed to meet these requirements; however, they all have inherent limitations. This paper presents a novel high-performance piezoelectric actuator that can overcome the limitations of existing designs. It is based on the stick–slip actuation principle and makes use of coupling motions of the proposed triangular driving mechanism to generate a clamping action during the “stick” phase and a releasing action during the “slip” phase. Unlike existing driving mechanisms based on similar principles, the proposed actuator can employ its unique structure to amplify the clamping force and the related driving force by using a large design triangular angle. Apart from its superior performance in driving force, it is interestingly found that its driving speed performance also improves when the design angle is increased. Finite-element analysis and experiments are carried out to justify the superior performance of the proposed actuator. In comparison with existing actuator prototypes based on similar principles, a prototype of the proposed actuator, even driven with a lower input voltage, achieves an 11 times larger driving load and a 3 times higher free-load driving speed. [ABSTRACT FROM AUTHOR]

Under the noncascade structure, a permanent-magnet synchronous motor (PMSM) regulates the speed and current in one loop. On the one hand, fast dynamic performance requires large transient current to provide a high torque. On the other hand, unlike a cascade control, the q-axis current is no longer governed by a reference current signal. In such a system, the nominal controllers cannot guarantee that the q-axis current is within the required range. But an excessively large transient current may threaten the circuit safety. To solve the overcurrent protection problem, the ordinary solution is to choose conservative control parameters, but the dynamic performance inevitably suffers a certain degree of loss. In order to improve this drawback, a simple and effective control scheme is introduced with a current-constrained technique. By constructing a special nonlinear gain, the punishment mechanism for the q-axis current is established in the control action directly. The proposed control approach does not impose limitations on the initial state. Moreover, it has good robustness to load torque uncertainty and undergoes rigorous theoretical analysis. Besides, the proposed current-constrained controller has a very concise structure. It yields a higher reliability of the PMSM control system. Comparative simulation and experimental results between the classic PID and the current-constrained controller on the PMSM servo system verify the feasibility of the presented control scheme. [ABSTRACT FROM AUTHOR]

Permanent magnet synchronous motors are extensively used in high-performance industrial applications. However, plenty of practical factors (e.g., cogging torques, load torques, friction torques, measurement error effects, dead-time effects, and parameter perturbations) in the closed-loop servo system inevitably bring barriers to the high-performance speed regulation, which can be regarded as generalized disturbances. Most of the existing control approaches only focus on one single kind of disturbances. However, the practical servo system is affected by multiple sources of disturbances simultaneously and these disturbances enter into the system through different channels. To this end, this paper systematically analyzes several representative disturbances, particularly including their features and distribution in the practical servo system, and then, specifically puts forward a novel disturbance rejection framework based on a noncascade structure. Under this framework, a comprehensive disturbance observer is proposed to simultaneously and accurately estimate multiple disturbances such that a composite controller can be designed to correspondingly compensate disturbances. Rigorous analysis of stability is established. Comparative experimental results demonstrate that the proposed method achieves a better speed dynamic response and a higher accuracy tracking performance even in the presence of multiple sources of disturbances. [ABSTRACT FROM PUBLISHER]

This paper presents an effective direction-of-arrival (DOA) estimation method based on a new computation method of the \ell \infty -norm for a complex matrix. Making use of the new computation method of the \ell \infty -norm, the DOA estimation problem can be efficiently formulated as a linear programming (LP) problem. Thus, the proposed method can effectively give the DOA estimation by finding the sparse coefficients which are obtained by solving the LP problem. Except for the first singular value decomposition of the data matrix, the proposed method is effectively implemented in the light of the LP theory based on real-valued computation. It avoids solving the complicated second-order cone programming problem. Furthermore, it can primely suppress spurious peaks in DOA estimation. Simulation results demonstrate the efficiency of the presented approach. [ABSTRACT FROM PUBLISHER]