Saturated Amplifier Research Articles

Experimental and Numerical Demonstration on the Gain and Power Saturation of Yb-Doped Double-Clad Fiber Amplifiers in Multiwavelength CW and Ultrafast Pulsed Operation

Experimental studies on gain and saturation of a Yb-doped double-cladding fiber amplifier accompanied with and interpreted by rigorous numerical simulations are presented. The saturation power, the value, and the spectral asymmetry of the output signal are experimentally characterized in both pulsed and continuous wave (CW) operation, and then, verified and interpreted numerically. Power conversion efficiency and quantum conversion efficiency of the amplifier are measured to be up to 55% and 58%, respectively. It is the first time, to the best of our knowledge, to demonstrate a numerical model for the pulsed operation based on the interaction of vector coupled nonlinear Schrodinger equations and the rate equations of the Yb-doped double-clad fiber taking into account all high-order dispersion parameters and the variation of gain coefficient during pulse propagation. A good agreement between numerical and experimental results is observed providing interpretations to the results. The numerical models and the experimental demonstrations provide guidance for the design and analysis of Yb-doped double-cladding fiber amplifiers both in pulsed and CW operation.

A non-saturated sliding-mode control of shaft deflection for magnetically suspended momentum wheel with coupled disturbance and saturated amplifier

The magnetically suspended momentum wheel (MSMW) expands its fresh functions through deflecting the rotary shaft. An improved nonsingular terminal sliding-mode control (NTSMC) method is proposed to achieve high precision tracking of shaft deflection for the MSMW under coupled disturbance and saturated amplifier. A novel structure designed for this MSMW is introduced initially. Its magnetic torque model and coupled disturbance are analyzed, and a tracking error dynamic model is established. Then a NTSMC method is applied to shaft tracking control. As the saturation of amplifier influences tracking performances, an improved NTSMC is designed to deal with saturation problem. Finally, several simulations are performed to validate the effectiveness of the proposed method. The results indicate the proposed method improves the tracking precision and velocity compared with the conventional integral sliding-mode method, and solves the saturation problem compared with existing NTSMC method.

Design of MC-CDMA receiver using radial basis function network to mitigate multiple access interference and nonlinear distortion

Multicarrier code division multiple access (MC-CDMA) is a novel wireless communication technology with high spectral efficiency and system performance. However, all multiple access techniques including MC-CDMA were most likely to have multiple access interference (MAI). So, this paper mainly aims at designing a suitable receiver for MC-CDMA system to mitigate such MAI. The classical receivers like maximal-ratio combining and minimum mean square error fail to cancel MAI when the MC-CDMA is subjected to nonlinear distortions, which may occur due to saturated power amplifiers or arbitrary channel conditions. Being highly nonlinear structures, the neural network (NN) receivers such as multilayer perceptron and radial basis function networks could be better alternative for such a case. The possibility NN receiver for a MC-CDMA system under different nonlinear conditions has been studied with respect to both performance and complexity analysis.

Atrial undersensing secondary to quiet timer blanking in pediatric and congenital heart disease patients.

Atrial undersensing despite normal atrial lead parameters and high amplitude electrograms has been described in adult patients as a consequence of atrial amplifier saturation. Repetitive inputs cause amplifier ringing and undersensing occurs as a consequence of quiet timer blanking. High amplitude ventricular electrograms can also result in atrial blanking secondary to cross-chamber effect. This has not been described in pediatric patients or epicardial pacing systems and specific risk factors are not known. We describe five cases of atrial undersensing in pediatric and congenital heart disease patients with epicardial dual-chamber systems and high ventricular lead outputs. This was a retrospective cohort study of all patients with dual-chamber pacing systems cared for at the Heart Institute at Children's Hospital Colorado. Five cases of atrial undersensing with normal atrial lead parameters were found. All were epicardial systems with acceptable lead parameters at implantation and Medtronic (Medtronic, Inc., Minneapolis, MN, USA) generators with unipolar leads. Ventricular pacing outputs were elevated at presentation, median 5.5 V (5-7.5). Presenting symptoms were exercise intolerance (four) and detection on routine monitoring (one). Successful interventions included reduction in ventricular lead pacing output (one), programming of rate response (two), and implantation of an alternative generator manufacturer (two). Atrial undersensing secondary to amplifier ringing and quiet timer blanking is an uncommon presentation but may be clinically important in patients with unipolar epicardial atrial leads and high pacing outputs from ventricular leads.

MC–CDMA receiver design using recurrent neural networks for eliminating multiple access interference and nonlinear distortion

SummaryMulticarrier code division multiple access (MC–CDMA) is a promising wireless communication technology with high spectral efficiency and system performance. However, all multiple access techniques including MC–CDMA were most likely to have multiple access interference (MAI). So this paper mainly aims at designing a suitable receiver for MC–CDMA system to mitigate such MAI. The classical receivers like maximal ratio combining, minimum mean square error, and iterative block–decision feedback equalization fail to cancel MAI when the MC–CDMA is subjected to severe nonlinear distortions, which may occur due to saturated power amplifiers or arbitrary channel conditions. Being highly nonlinear structures, the neural network receivers such as multilayer perceptron and recurrent neural network could be better alternative for such a case. The feasibility, efficiency, and effectiveness of the proposed neural network receiver are studied thoroughly for MC–CDMA system under different nonlinear conditions.

CE-OFDM Schemes: Spectral Characterization and Optimum Performance

Constant envelope orthogonal frequency division multiplexing (CE-OFDM) techniques have been recently proposed to allow the use of power-efficient saturated amplifiers, since those signals do not have the high peak-to-average ratio of OFDM signals, but maintain most of the OFDM advantages. However, the nonlinear nature of the phase modulation process inherent to CE-OFDM schemes gives rise to both theoretical challenges (analytical difficulties due to the nonlinear processes involved) and practical challenges (spectral widening and possible performance degradation due to nonlinear distortion effects). In this work, we present accurate expressions for the average power spectral density of CE-OFDM signals. Making use of recent results that indicate performance gains associated to the optimum detection of nonlinearly distorted multicarrier signals, we also investigate the optimum detection of CE-OFDM signals considering both additive white Gaussian noise channels and frequency-selective channels with Rayleigh fading, providing theoretical expressions for the asymptotic gains as well as the correspondent approximate bit-error-rate results.

Longitudinal mode selection in a delay-line homogeneously broadened oscillator with a fast saturable amplifier.

Homogeneously broadened delay-line oscillators such as lasers or optoelectronic oscillators (OEOs) can potentially oscillate in a large number of cavity modes that are supported by their amplifier bandwidth. In a continuous wave operating mode, the oscillating mode is selected between one or few cavity modes that experience the highest small-signal gain. In this manuscript, we show that the oscillation mode of a homogeneously broadened oscillator can be selected from a large number of modes in a frequency region that can be broader than the full width at half maximum of the effective cavity filter. The mode is selected by a short-time injection of an external signal into the oscillator. After the external signal is turned off, the oscillation is maintained in the selected mode even if this mode has a significantly lower small-signal gain than that of other cavity modes. The stability of the oscillation is obtained due to nonlinear saturation effect in the oscillator amplifier. We demonstrate, experimentally and theoretically, mode selection in a long cavity OEO. We could select any desired mode between 400 cavity modes while maintaining ultra-low phase noise in the selected mode and in the non-oscillating modes. No mode-hopping was observed during our maximum measurement duration of about 24 hours.

Feedback Linearization and Extended State Observer-Based Control for Rotor-AMBs System With Mismatched Uncertainties

To effectively reject the influence of moving gimbal effect and parameter variation on the control performance of the rotor-active magnetic bearings (AMBs) system in a double gimbal magnetically suspended control moment gyro (MSCMG), this paper proposes a feedback linearization and extended state observer (ESO) based control (FLESOBC) strategy, which can attenuate the matched and mismatched system uncertainties simultaneously. First, considering the voltage-type power amplifier used in the MSCMG for space applications, the rotor dynamics is modeled completely and then rearranged into translation and tilt motion subsystems according to modal separation control, where the lumped system uncertainties are adopted to represent the external disturbances and the parameter variations. Then, feedback linearization based on differential geometry theory is performed to transform the original system into the integral chain pseudo-linear system, making the mismatched uncertainties in the same channel as the new virtual input. Afterwards, the ESO is designed to estimate the lumped uncertainties. To combine the merits of compensation control and robust control, a PIDD controller (PID with second order differential) is designed to guarantee the closed loop stability and avoid the power amplifier saturation. Finally, simulations and experiments are carried out to demonstrate the effectiveness of the proposed method.

Low-Noise Single-Frequency 1.5- $\mu$ m Fiber Laser With a Complex Optical-Feedback Loop

A low-noise narrow-linewidth distributed Bragg reflector (DBR) single frequency 1.5- $\mu \text{m}$ fiber laser with a complex optical-feedback loop (COFL) is demonstrated. The COFL is used to reduce the relative intensity noise (RIN) and narrow the linewidth, which is composed of an erbium-doped fiber amplifier (EDFA), an unpumped Er80 fiber and a sagnac loop mirror. The COFL combines the nonlinear amplification effect of saturated optical amplifiers, the standing-wave saturation effects of saturable absorber (SA), and the self-injection locking technology at the same time. The effects of self-injection locking give a sufficient proof, which the RIN is reduced and the resonance peak shifts to lower frequency. The RIN of low frequency is greatly reduced by the EDFA. In addition, the SA narrows the linewidth. After the COFL suppression, the RIN of relaxation oscillation is reduced by 24.5 dB from 80.3 dB/Hz at 219 kHz to 104.8 dB/Hz at 87 kHz. Furthermore, the linewidth narrows from 7.47 kHz to 720 Hz. Even after two-stage fiber amplification, the RIN does not increase significantly, and a spectral linewidth well below 1.8 kHz is also achieved at the output power of 11.1 W.

Full quadrature regeneration of QPSK signals using sequential phase sensitive amplification and parametric saturation.

We demonstrate all-optical regeneration of both the phase and the amplitude of a 10 GBaud quadrature phase shift keying (QPSK) signal using two nonlinear stages. First we regenerate the phase using a wavelength converting phase sensitive amplifier and then we regenerate the amplitude using a saturated single-pump parametric amplifier, returning the signal to its original wavelength at the same time. We exploit the conjugating nature of the two processing stages to eliminate the intrinsic SPM distortion of the system, further improving performance.

Bee Searching Radar With High Transmit–Receive Isolation Using Pulse Pseudorandom Code

This paper presents an innovative radar architecture to improve the isolation of the harmonic radar between the transmitter and the receiver. The proposed radar transmits two closely located frequencies to the transponder, and the transponder transmits a response signal at a mixed frequency. Because the frequency of the response signal is different from those of the clutter and leakage signals, the isolation is significantly improved. For a traditional harmonic radar, the leakage signals should be suppressed to a sensitivity lower than −106 dBm to avoid interference. The proposed radar requires only attenuation of leakage signals to a level lower than −8 dBm to avoid low-noise amplifier saturation. Harmonic radar transponders are compatible with the proposed radar system without additional design. This paper also proposes a new method for maintaining the correlation of transmitter and receiver phase noise. Field test results demonstrate that leakage signal interference in the proposed radar is far lower than that in the harmonic radar. The significant improvement in isolation reveals the advantages of applying the frequency mixing concept in the proposed radar.

Multi-Objective Optimization Design of Nonlinear Magnetic Bearing Rotordynamic System

Nonlinear vibrations and their control are critical in improving the magnetic bearings system performance and in the more widely spread use of magnetic bearings system. Multiple objective genetic algorithms (MOGAs) simultaneously optimize a vibration control law and geometrical features of a set of nonlinear magnetic bearings supporting a generic flexible, spinning shaft. The objectives include minimization of the actuator mass, minimization of the power loss, and maximization of the external static load capacity of the rotor. Levitation of the spinning rotor and the nonlinear vibration amplitude by rotor unbalance are constraint conditions according to International Organization for Standardization (ISO) specified standards for the control law search. The finite element method (FEM) was applied to determine the temperature distribution and identify the hot spot of the actuator during steady-state operation. Nonlinearities include magnetic flux saturation, and current and voltage limits of power amplifiers. Pareto frontiers were applied to identify and visualize the best-compromised solutions, which give a most compact design with minimum power loss whose vibration amplitudes satisfy ISO standards.

Rotating and Precessing Dissipative-Optical-Topological-3D Solitons.

We predict and study a new type of three-dimensional soliton: asymmetric rotating and precessing stable topological-dissipative-optical localized structures in homogeneous media with saturable amplification and absorption. The crucial factor determining their dynamics is the ratio of the diffusion coefficients characterizing the frequency dispersion and angular selectivity (dichroism) of the scheme. These vortex solitons exist and are stable for overcritical values of the selectivity coefficients and can be realized in lasers of large sizes with saturable absorption.

Performance Analsis of Clipping Technique for Papr Reduction of MB-OFDM UWB Signals

Multiband Orthogonal Frequency Division Multiplexing (MB-OFDM) is used as efficacious procedure for ultra-wideband (UWB) wireless communication applications, which divides the spectrum into various subbands, whose bandwidth is approximately 500MHz. Major arduousness in multiband-OFDM is ,it have very large peak to average power ratio value which causes the signal to enter into dynamic region that consequence in the loss of orthogonal properties and results in the interference of the carrier signals which crops the amplifier saturation and finally limits the capacity of the system. Many PAPR amortize algorithms have reported in the survey and pre-coding is PAPR reduction which is inserted after modulation in the OFDM system. The Existing work presents the reduction of that value by different clipping techniques namely Classical-Clipping (CC), Heavy side-Clipping (HC), Deep-Clipping (DC) and Smooth-Clipping (SC) and their comparison analysis is done. Every clipping method is best at its own level .The proficiency of these strategies are evaluated in locutions of average power disparity, complete system decadence and PAPR reduction. Finally results show the MB OFDM yields better performance to reduce PAPR in effective way.

A hybrid hardware and software approach for cancelling stimulus artifacts during same-electrode neural stimulation and recording.

Recovering neural responses from electrode recordings is fundamental for understanding the dynamics of neural networks. This effort is often obscured by stimulus artifacts in the recordings, which result from stimuli injected into the electrode-tissue interface. Stimulus artifacts, which can be orders of magnitude larger than the neural responses of interest, can mask short-latency evoked responses. Furthermore, simultaneous neural stimulation and recording on the same electrode generates artifacts with larger amplitudes compared to a separate electrode setup, which inevitably overwhelm the amplifier operation and cause unrecoverable neural signal loss. This paper proposes an end-to-end system combining hardware and software techniques for actively cancelling stimulus artifacts, avoiding amplifier saturation, and recovering neural responses during current-controlled in-vivo neural stimulation and recording. The proposed system is tested in-vitro under various stimulation settings by stimulating and recording on the same electrode with a superimposed pre-recorded neural signal. Experimental results show that neural responses can be recovered with minimal distortion even during stimulus artifacts that are several orders greater in magnitude.

Direct current electrocorticography for clinical neuromonitoring of spreading depolarizations

Spreading depolarizations cause cortical electrical potential changes over a wide spectral range that includes slow potentials approaching the direct current (or 0 Hz) level. The negative direct current shift (<0.05 Hz) is an important identifier of cortical depolarization and its duration is a measure of potential tissue injury associated with longer lasting depolarizations. To determine the feasibility of monitoring the full signal bandwidth of spreading depolarizations in patients, we performed subdural electrocorticography using platinum electrode strips and direct current-coupled amplifiers in 27 patients with acute brain injury at two neurosurgical centers. While large baseline direct current offsets developed, loss of data due to amplifier saturation was minimal and rates of baseline drift throughout recordings were generally low. Transient negative direct current shifts of spreading depolarizations were easily recognized and in 306/551 (56%) cases had stereotyped, measurable characteristics. Following a standardized training session, novice scorers achieved a high degree of accuracy and interobserver reliability in identifying depolarizations, suggesting that direct current-coupled recordings can facilitate bedside diagnosis for future trials or clinical decision-making. We conclude that intracranial monitoring of slow potentials can be achieved with platinum electrodes and that unfiltered, direct current-coupled recordings are advantageous for identifying and assessing the impact of spreading depolarizations.

Saturation in cascaded optical amplifier free‐space optical communication systems

The performance of a free-space optical (FSO) communication system in a turbulent atmosphere employing an optical amplifier (OA) cascade to extend reach is investigated. Analysis of both single and cascaded OA FSO communication links is given and the implications of using both adaptive (to channel state) and non-adaptive decision threshold schemes are analysed. The benefits of amplifier saturation, for example in the form of effective scintillation reduction when a non-adaptive decision threshold scheme is utilised at the receiver for different atmospheric turbulence regimes, are presented. Monte Carlo simulation techniques are used to model the probability distributions of the optical signal power, noise and the average bit error rate due to scintillation for the cascade. The performance of an adaptive decision threshold is superior to a non-adaptive decision threshold for both saturated and fixed gain preamplified receivers and the ability of a saturated gain OA to suppress scintillation is only meaningful for system performance when a non-adaptive decision threshold is used at the receiver. An OA cascade can be successfully used to extend reach in FSO communication systems and specific system implementations are presented. The optimal cascade scheme with a non-adaptive receiver would use frequent low gain saturated amplification.

Multi-Way Lossless Outphasing System Based on an All-Transmission-Line Combiner

A lossless power-combining network comprising cascaded transmission-line segments in a tree structure is introduced for a multi-way outphasing architecture. This architecture addresses the suboptimal loading conditions in Chireix outphasing transmitters while offering a compact and microwave-friendly implementation compared to previous techniques. In the proposed system, four saturated power amplifiers (PAs) interact through an all-TL power-combining network to produce nearly ideal resistive load modulation of the branch PAs over a 10:1 range of output powers. This work focuses on the operation of the combining network, deriving analytical expressions for input-port admittance characteristics and an outphasing control strategy to modulate output power while minimizing reactive loading of the saturated branch amplifiers. A methodology for combiner design is given, along with a combiner design example for compact layout. An experimental four-way outphasing amplifier system operating at 2.14 GHz demonstrates the technique with greater than 60% drain efficiency for an output power range of 6.2 dB. The system demonstrates a W-CDMA modulated signal with a 9.15-dB peak-to-average power ratio with 54.5% average modulated efficiency at 41.1-dBm average output power.

Interference-aware self-optimizing Wi-Fi for high efficiency internet of things in dense networks

Wi-Fi is one of the candidate technologies for the Internet of Things (IoT), and today connects billions of devices world-wide in dense networks to offer Internet connectivity in a partially or fully automated manner. In order to provide seamless and high quality service, wireless local area networks (WLANs) can adopt dynamic channel access technologies such as dynamic bandwidth or channel hopping schemes in order to avoid interference for better link quality. However, in dense networks, the dynamic channel access leads to a higher probability of adjacent channel interference (ACI). The efficiency of IEEE 802.11-based WLANs using multi-channel and wide dynamic ranges is thus severely degraded by ACIs in dense networks. In this paper, we analyze the ACI effect on WLANs and propose an interference-aware self-optimizing (IASO) Wi-Fi design that incorporates a multi-channel multi-level carrier sense and adaptive initial gain control scheme. This scheme controls carrier sensing thresholds in each band for multi-level sensors, as well as initial gains for amplifiers. The proposed scheme reduces false carrier sensing and avoids saturation of amplifiers while simultaneously improving the dynamic range of the receiver. Our prototype evaluation results demonstrate that the proposed scheme can improve the dynamic range of the receiver by approximately 45dB and 30dB for a low data rate and a high data rate mode, respectively, compared with the conventional receiver designs. Furthermore, network emulation results demonstrate that the IASO Wi-Fi can improve the average throughput, latency, and energy efficiency by approximately 32% (24%), 41% (43%), and 13% (17%), respectively, compared with the conventional receiver designs (and channel hopping techniques) in dynamically varying interfered channel conditions.

Analog optical computing primitives in silicon photonics.

Optical computing accelerators help alleviate bandwidth and power consumption bottlenecks in electronics. We show an approach to implementing logarithmic-type analog co-processors in silicon photonics and use it to perform the exponentiation operation and the recovery of a signal in the presence of multiplicative distortion. The function is realized by exploiting nonlinear-absorption-enhanced Raman amplification saturation in a silicon waveguide.