Undamped Pulsations Research Articles

Flow Structure and Pressure Oscillations during the Interaction of a Supersonic Underexpanded Gas Jet with a Tubular Cavity

Supersonic jets are widely used in devices based on a self-oscillation process emerging during the interaction of a gas flow with tubular cavities (gas-jet acoustic emitters). We analyze the mechanisms for sustaining undamped pulsations of pressure and determine the flow field in a tubular cavity during the interaction of an underexpanded supersonic jet with it. We consider the physical pattern of the flow in the cavity of a gas-jet emitter, demonstrate the existence of odd longitudinal modes, and propose wave diagrams for describing the flow in odd longitudinal modes. Wave diagrams are constructed based on analysis of signals from piezoelectric sensors detecting pressure pulsations in a tubular cavity. The flow parameters in the tubular cavity in the longitudinal modes are calculated using the flow velocity–velocity of sound diagram.

Mechanism of formation of undamped pulsations around several equilibrium states in a laser with a saturable absorber

A CO2 laser emitting two longitudinal modes and containing a saturable absorber in its cavity is considered. Numerical simulation is used to predict regular generation of intensity pulsations and it reveals the possibility of occurrence of transitions between three equilibrium states.

Spontaneous coherent pulsations in standing-wave laser oscillators: stability criteria for homogeneous broadening

The stability criteria for single-mode standing-wave laser oscillators in the homogeneously broadened limit are reported, and two types of criteria are distinguished. The first type (type 1) corresponds to the minimum value of the threshold parameter for which an infinitesimal perturbation away from steady state grows into an oscillatory solution. A second type (type 2) corresponds to the minimum value of threshold parameter for which large-amplitude oscillations do not decay to the steady-state solution. Undamped pulsations in single-mode homogeneously broadened standing-wave laser oscillators are found to occur at a much higher excitation level than in ring-laser oscillators with homogeneous broadening. The effects of detuning on the stability criteria are also investigated.

Influence of phonons on stimulated emission from activated crystals

An analysis is made of the influence of distortion of the gain profile on the stimulated emission parameters of three-level solid-state lasers. It is shown that this may cause undamped pulsations of the radiation intensity. Estimates are made of the time dependence of the spectral characteristics of spiky stimulated emission from a ruby laser. Good agreement with the experimental data is reported.

Coherent pulsations in the vibrational fluorescence of CO and NO in solid N 2

Infrared laser pumping of the v = 2 level of CO and NO molecules dispersed in liquid-helium-cooled N 2 crystals results in vibrational stimulated emission (VSE) on the v = 2 → v = 1 transition. VSE is characterized by strong undamped pulsations whose intensity exceeds in some cases that of the regular spontaneous fluorescence by a factor of 100. Numerical computations have been carried out using a model describing the interaction of the field building up at the 2→1 band center with the inhomogenously broadened medium. The occurence of spontaneous coherent pulsations very similar to that experimentally observed is strongly related to the long dephasing time and to the strong spectral inhomogeneity of the vibrational lines of CO and NO in solid N 2.

Instabilities in a solid-state ring laser

In addition to a well-known type, two more types of relaxation oscillation are present in a solid-state ring laser. The frequencies of these specific oscillations depend on the difference in the eigenfrequencies of counterpropagating resonator modes (phase nonreciprocity). There are instability mechanisms that can convert these relaxation oscillations into undamped pulsations. The instability is responsible for the possibility of detecting low-frequency oscillation characteristic of a ring laser. In our experiment these oscillations are represented by resonant features in the intensity-fluctuation spectrum of a Nd:YAG laser operating in a stable traveling-wave regime although close to a self-modulation instability threshold.

Unsteady reflection of a shock wave from a body with a cylindrical recess

In a number of cases of supersonic flow past bodies with recesses pulsations in the flow arise [1–3]. Experiments [4, 5] indicate that stabilization of the steady supersonic flow past the body with a recess on which a shock wave is incident takes place after a series of oscillations of the bow wave. Numerical calculation of the interaction of a supersonic jet with a cylindrical cavity [6] reveals that damped pressure pulsations arise inside the cavity if the jet is homogeneous, and undamped pulsations it is inhomogeneous. The authors explain the damping of the pulsations by the influence of artificial viscosity. This paper investigates experimentally and theoretically (by numerical methods) the oscillations of the bow shock wave and the parameters of the flow behind it in the case of unsteady reflection of a shock wave from a body with a cylindrical recess turned towards the flow. The problem is posed as follows. A plane shock wave with constant parameters impinges on a cylinder with a cavity. The unsteady flow originating from this interaction is investigated.

Observation of undamped pulsations in a low-pressure, far-infrared laser and comparison with a simple theoretical model

Experimental details of undamped pulsations are presented for a low pressure, far-infrared (FIR) laser optically pumped by a single-frequency CO2 laser in a standing-wave configuration for both the pumping and the FIR lasing action. Spontaneous pulsing in the single-mode HCOOH FIR laser at 742 μm was observed when the cavity was tuned to the FIR resonance and when the CO2 laser of high intensity was sufficiently detuned from the resonance of the pump transition to excite two distinct velocity groups of molecules to contribute to the FIR gain. Theoretical results, based on studies of a model for a laser with two distinct resonant species forming the gain medium, provide a basis for understanding the origin of these instabilities. The model compares satisfactorily with the observed FIR instabilities including the variations in pulsing frequencies observed experimentally as the CO2 laser is detuned. Additional predictions from the model include details of transitions from cw to pulsing behavior and thresholds for deterministic chaos that may be tested in future experiments.

Hysteresis and irregular undamped pulsations in a linear laser

Hysteresis and irregular undamped pulsations in a linear laser

Pulsations of the radiation emitted by multimode solid-state lasers

The kinetics of multimode free oscillation is analyzed on the basis of rate equations allowing for an inhomogeneous distribution of the population inversion. The model of two overlapping modes is considered. It is shown analytically and confirmed by computer calculations that transient pulsations (spikes) of the radiation emitted by such a system should be damped out and the steady-state emission should be stable. In the case of external modulation of the losses, undamped oscillations should appear only if the modulation coefficient is sufficiently large and the modulation frequency is close to the pulsation frequency. It is shown that undamped pulsations appear when the mode composition is altered by slow changes in the losses or stimulated emission cross sections. Such conditions are encountered in pulsed lasers when the hearing causes the mode frequencies and center of the gain profile to shift with time at rates which differ from one mode to another. Criteria for the appearance of damped or undamped pulsations are established. The theoretical results are supported by experimentally obtained slit scans of the free-oscillation emission of ruby at 100°K which exhibits undamped regular pulsations.

Spatial inhomogeneity of inversion and undamped pulsations of solid-state lasers

An analogy is suggested between the emission of radiation from a solid-state laser and the frictional motion of a particle in a potential well. This analogy is used to demonstrate analytically that a system of rate equations which allows only for the spatial inhomogeneity of the population inversion fails to describe the undamped pulsation regime. It is shown that this inhomogeneity makes the emission much more sensitive to perturbations in a real resonator. A comparison of the theoretical and experimental results is used to demonstrate the existence of a mechanism which generates undamped pulsations but is not related to laser instabilites.

Influence of the refractive index nonlinearity on the dynamics of emission from semiconductor lasers

A mechanism is suggested for undamped self-switching of the radiation emitted from semiconductor lasers. Undamped pulsations of the emission intensity are attributed to switching of the resonator Q factor as a result of nonlinearity of the refractive index. Modified rate equations describing regular pulsations are derived. It is shown that the theoretically predicted dependence of the self-switching on the pumping level is in agreement with the experimental results.

Regular undamped pulsations in the output power of a laser with bleaching filters

Regular undamped pulsations in the output power of a laser with bleaching filters