Relaxation-oscillation Phenomena Research Articles

Theoretical characteristics of mid-infrared gain switched pulsed iron-doped ZnSe laser

We report a four-level theoretical model revealing the laser kinetics of a gain-switched pulsed Fe2+:ZnSe laser that is established for the first time, to the best of our knowledge. The experimental results agree with the theoretical primary spike pulse and relaxation oscillation phenomenon of pulse. Furthermore, the effects of temperature, doping concentration, and the output coupler reflectivity on the performance of the pulsed Fe2+:ZnSe laser are simulated using the proposed model. The simulated results indicate that the laser efficiency is higher at low temperature and high Fe2+ concentrations, and the optimal output coupler reflectivity is ∼23% at 77 K.

Broadband laser chaos generation using a quantum cascade laser with optical feedback.

We propose a method to generate broadband laser chaos using a quantum cascade laser (QCL). Through numerical simulation, we give the evidence that the QCL with optical feedback can route to chaos through the quasi-periodic path. Furthermore, we investigate the influence of the feedback intensity and the bias current on the chaos bandwidth. Final results demonstrate that the chaos bandwidth can headily reach 43.1 GHz due to the lack of relaxation oscillation phenomena in QCLs.

EXOTIC DYNAMICS IN NETWORKS OF COUPLED RINGS OF CELLS

We study curious features appearing in three systems of two coupled rings of cells with Z3 × Z5 exact symmetry, and Z3 and Z5 interior symmetries. This study was motivated by previous work by Antoneli et al. [2008, 2010], on two rings of cells coupled through a "buffer" cell, with Z3 × Z5 and D3 × D5 exact and interior symmetry groups. There, quasi-periodic behavior was found through a sequence of Hopf bifurcations, and relaxation oscillation phenomena appearing further away from the third Hopf point seemed to be necessary for a curious dynamic feature to occur. In this paper, we analyze the dynamical behavior of three distinct networks of two coupled rings of cells, with no "buffer" cell, and find an analogous bifurcation scenario to explain the appearance of quasi-periodic motion in these systems and the curious exotic feature. We compute the relevant states numerically.

Quasi-periodic states in coupled rings of cells

We study some dynamical features of certain coupled cell networks that consist of two (unidirectional or bidirectional) rings of cells coupled through a ‘buffer’ cell. Depending on how the rings and the buffer cell are coupled, the full network may have a non-trivial group of symmetries or a non-trivial group of ‘interior’ symmetries. This group is Zp×Zq in the unidirectional case and Dp×Dq in the bidirectional case. We are interested in finding quasi-periodic motion in these networks, motivated by an example presented by Golubitsky, Nicol and Stewart (Some curious phenomena in coupled cell systems, J Nonlinear Sci 2004;14(2):207–36). In the examples considered here, we obtain quasi-periodic states through a sequence of Hopf bifurcations. Interestingly, we observe relaxation oscillation phenomena appearing further away from the last Hopf bifurcation point. We use XPPAUT and MATLAB to compute numerically the relevant states.

Experimental study on relaxation oscillation in a detuned FM harmonic mode-locked Er-doped fiber laser

We experimentally studied the relaxation oscillation characteristics in a FM harmonic mode-locked fiber laser when the modulation frequency has a small detuning. The relaxation oscillation can lower the laser output power and make the amplitudes of output pulses uneven both in normal and abnormal dispersion cavities. While combined with self-phase modulation and the optical filter, the abnormal intra-cavity dispersion can suppress the relaxation oscillation phenomenon.

Characterization of Relaxation–Oscillation Noise in Continuous-Wave Traveling Wave Tubes

Extremely low noise levels are frequently required in radar and communication systems that utilize microwave tubes. A noise waveform frequently found in continuous-wave (CW) linear-beam microwave tubes is a relaxation-type oscillation that has a period typically on the order of several milliseconds. This paper provides an extensive experimental characterization of this relaxation-oscillation phenomenon by recording the corresponding signal modulations on the output of several solenoid-focused CW traveling wave tubes. The characteristics of these amplitude and phase modulations were studied during systematic variations of tube operating parameters that included magnetic field, cathode voltage, depressed collector voltage, heater filament voltage, and input drive level. A measurement technique was also developed with the capability to obtain detailed information on the growth of the relaxation-oscillation noise waveforms along the beam interaction region by comparing reflected and output noise waveforms.

Nonlinear study of the curvature-driven parallel velocity shear–tearing instability

The nonlinear regime of the parallel velocity shear–tearing instability is studied numerically using a two-dimensional reduced, resistive magnetohydrodynamics model. In this instability, a sheared parallel velocity profile interacts with the perpendicular dynamics via the magnetic field curvature. Linearly, it has been shown [J. M. Finn, Phys. Plasmas 2, 4400 (1995)] that, in the inviscid limit, such interaction alters the classical behavior of the tearing instability, resulting in increased growth rates for classically tearing-unstable regimes (Δ′>0), and destabilizing classically tearing-stable regimes, leading to an electrostatic mode as Δ′→−∞. These trends are seen to hold with finite viscosity as long as the perpendicular plasma viscosity is of the order or smaller than the plasma resistivity. Nonlinearly, it is found that a self-consistent perpendicular shear flow and a reversed (stabilizing) density gradient develop. For favorable curvature, the latter implies an anomalous pinch effect. The shear flow generation mechanism is found, using quasilinear theory, to be related to asymmetries (tilting) introduced by the magnetic curvature coupling term. In the classically tearing-stable regime, the nonlinear behavior is very rich, including relaxation-oscillation phenomena and chaotic behavior. The potential implications of the nonlinear regime of this instability for the plasma confinement are also discussed.

Limit cycles for planar piecewise linear differential systems via first integrals

Using first integrals we introduce an alternative tool to the Poincare maps in order to study the limit cycles of piecewise linear differential systems. It is also shown that an usual relaxation oscillation phenomenon occurrs in a two-parameter family of planar piecewise linear vector fields.

Relaxation oscillation phenomena in cryogenic Si diodes

We present observations of unusual oscillatory behavior of Si diodes at low temperatures. The oscillations observed are stable, highly reproducible and obey a frequency-current relation that is linear over nearly eight decades in current strength allowing for precision measurement of currents over the whole range down to picoampere currents.

Plasma characteristics of long-pulse discharges heated by neutral beam injection in the Large Helical Device

Long-pulse neutral beam injection heating has been achieved in the large helical device (LHD). Two different confinement states are observed for different averaged densities in the long-pulse plasmas. A quasi-steady-state plasma was sustained for 21 s with an injection power of 0.6 MW, where the central plasma temperature was around 1 keV with a line-averaged electron density of 0.3 × 1019 m-3. The discharge duration can be so extended as to keep the plasma properties in the short-pulse discharge. The energy confinement time is nearly the same as that of the short-pulse discharge, which is 1.3 times as long as the international stellarator scaling ISS95. At higher densities, a relaxation oscillation phenomenon, observed as if the plasma would breathe, lasted for 20 s with a period of 1-2 s. The phenomenon is characterized with profile expansion and contraction of the electron temperature. The density oscillation is out of phase with the temperature oscillation and is related to the density clamping phenomenon. The observed plasma properties are shown in detail for the `breathing' oscillation phenomenon. Possible mechanisms for the breathing oscillation are also discussed, with a view of the screening effect near the last closed magnetic surface and the power balance between the heating and the radiation powers. The long-pulse heating results indicate unique characteristics of the LHD where no special feedback stabilization is required due to absence of disruption and no need for current drive.

Initial long-pulse plasma heating at reduced power with negative-ion-based neutral beam injector in large helical device

We have achieved long-pulse plasma heating using a negative-ion-based neutral beam injector (NBI) in the large helical device (LHD), where the confinement magnetic field is generated by only external superconducting coils. In the initial long-pulse experiments at lower power than that in short-pulse experiments, 80 keV–1.1 MW NBI heating lasted for 10 s with a little increase in the plasma density at the pulse end. Almost steady-state plasma heating was achieved for 21 s with 66 keV–0.6 MW NB injection. Plasma relaxation oscillation phenomena at a period of 1–2 s were also observed for 20 s. Above 1 keV plasma was easily sustained with a long-pulse NBI heating in LHD, without the current drive nor the disruption in tokamaks. Negative ion source operation was stable and the cooling water temperature rise of beam accelerator grids was nearly saturated with a temperature rise below 10 °C. For a higher power injection, the pulse duration is determined by the beam blocking, where the reionization loss is exponentially increased together with an increase in outgas in the injection port. The port conditioning by a careful repetition of injection is effective to the extension of the injection duration and the plasma maintenance duration. The initial long-pulse NBI heating at the reduced power has demonstrated an ability of steady-state operation in superconducting LHD.

Relaxation-oscillation phenomena in an injection-locked semiconductor laser

Relaxation-oscillation (RO) phenomena in a semiconductor diode laser with external optical injection are studied. We extend the concept of `locked dynamics' to include excited ROs. A two-variable scalar function W is constructed, from which the `slow' (transient) dynamics of the RO can be derived as well as an earlier published `potential' model. The function W is used to investigate bifurcations of the RO dynamics in the locking region. The location of the Hopf and the period-doubling bifurcations are in good, respectively fair, agreement with numerical simulations. A bistability is recovered and the dependence on the linewidth enhancement parameter (`the -parameter') is studied.

Semiconductor laser precision gain switching experiment for Gbaud ternary optical signaling

An experiment performed to gain switch a 1.3 mu m semiconductor laser at Gb/s rates and with sufficient electrical-drive precision to validate a previous theoretical study is discussed. The method takes advantage of the relaxation oscillation phenomenon to produce shortened, single optical pulses by careful control of the electrical bias, drive excursion and pulse duration. It was demonstrated that two-level and three-level (i.e ternary) optical data stream generation could be reliably achieved with only two electrical drive levels. The latter was performed by control of the drive pulse duration and is the first demonstration of a three-level PCM system based on two-level gain switching. Furthermore, the pulse shortening which results recommends this technique for optical TDM (time-division multiplexing) of multiple laser transmitters. It is suggested that this combination of formats may ultimately allow the 100-Gb/s target to be achieved.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>