Pierre Glorieux

Repetitive passive Q-switching and bistability in lasers with saturable absorbers

We report detailed experimental data on the passive Q-switching operation in a CO2 laser with CH3I saturable absorber, and on the transient behaviour in the near-Q-switching situation. Under suitable operating conditions, we found bistability in the output power. In some cases, we observed the simultaneous presence of bistability and passive Q-switching. The theoretical part of the paper starts from the four-level model of laser with saturable absorber, as formulated by other authors. By adiabatically eliminating the variables of the resonant levels, we reduce the problem to a set of three differential equations, from which we derive explicit analytical conditions for the rise of passive Q-switching. These conditions turn out to be in good qualitative and partially quantitative agreement with our experimental findings as well as with other experimental data previously obtained by other authors. Finally we classify the possible combinations of passive Q-switching and bistability that one can find in this type of experiments.

Shilnikov Dynamics in a Passive Q-Switching Laser

The laser containing a saturable absorber (LSA) is known to present optical instabilities in the form of regular pulses of various shapes. We report on the first experimental observation of optical chaos through different scenarios in the CO2 + CH3I LSA. By varying the absorber pressure and/or the laser frequency, the LSA output follows a period-doubling cascade to chaos or makes a transition to Shilnikov-type chaos.

Basic transverse dynamics of a photorefractive oscillator

The basic phenomena of transverse dynamics of a two-wave mixing photorefractive oscillator (PRO) in the weakly multimode case are considered. The evolution of the transverse patterns as a function of two control parameters, namely, the cavity length and the PRO gain, is investigated. The behavior is described in terms of the modes of the empty cavity. It is found that the mode interactions occur differently, depending on whether the modes belong to the same frequency-degenerate family or not, although in both cases the observed behavior is periodic.

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.

Pulse shape and phase diagram in the passive Q switching of CO 2 lasers

Investigations of the theoretical and experimental features of the passive Q-switching operation of CO2 lasers containing intracavity saturable absorbers are reported. The pulse shapes observed in experiments with SF6, CF3Br, and CH3I gases are presented as a function of the laser-control parameters across the phase diagram. Through a numerical analysis based on a four-level model, the laser-operation regions and the pulse shapes were investigated for absorption of these gases. The main experimental features are reproduced by the theory, but a quantitative agreement could not be reached.

Observation of bistability in a CO2 laser exhibiting passive Q-switching

Abstract Optical bistability has been investigated experimentally in a CO 2 laser containing CF 3 Br as a saturable absorber. The frequency dependence of the hysteresis cycle of this laser containing a saturable absorber (LSA) has been studied. It has also been shown that the LSA may undergo simultaneously hysteresis cycles and Q-switching. The phase diagram of the LSA has been extended towards regions where this behavior has been observed.

Nonlinear polarization dynamics in anisotropic lasers

Abstract Instabilities are observed in the polarization of lasers with controlled anisotropies when double external dynamics are induced by both static and time periodic nonlinear Faraday effects, revealing two kinds of behavior: (i) in lasers with loss anisotropy, quasiperiodicity and Arnold tongues are exhibited and (ii) in lasers with phase anisotropy, additional period-doubling and intermittency occur. These different behaviors are in agreement with models in which the atomic variables are adiabatically eliminated.

Behavior of a CO2 laser under loss modulation : critical analysis of different theoretical models

Abstract Various theoretical rate equation models describing the dynamics of the CO 2 laser are developed and analysed. In the frame-work of these models, the response of a CO 2 laser subjected to cavity loss modulation is investigated. A critical analysis of these descriptions is performed by comparison with experimental data. It is shown that both the so-called two- and three-level models are inadequate to reproduce, even qualitatively, the experimental data. On the other hand, the four-level model which includes the rovibrational level manifolds fits the experiments correctly.

Spatio-temporal dynamics of a weakly multimode CO2 laser

Abstract The behavior of a slightly asymmetric weakly multimode CO 2 laser is studied experimentally and compared with that of perfectly cylindrically symmetrical lasers. It is found that in most cases, the system symmetry breaking destroys the vortices, although it appears that for a very small loss of symmetry, the behavior of the laser remains symmetrical.

Heterodyne detection of tunable FIR sidebands

Continuously tunable far infrared (FIR) laser sidebands have been generated in a Schottky diode by nonlinear mixing the radiation from an optically pumped FIR laser operating between 0.6 and 3 THz with that from a microwave source in the 2-4 GHz range. A very sensitive heterodyne detection is adapted to detect the sidebands generated not only at the sum or difference frequency of the two radiations, but also, for the first time in the FIR region, those related to the mixing of the FIR radiation with the second and third harmonics of the microwaves. The high selectivity of the superheterodyne detection allows easy separation of the various sidebands. The global tunability of this system is 2 × 10 GHz for the more powerful FIR emission lines. To demonstrate the tunability of the device absorption signals of several molecules are presented in the whole range of operation of the system (0.6-3 THz) using either direct or lock-in detection techniques.