Collett-Wolf sources and multimode lasers

It is shown than an exact realization of a Collett-Wolf source is given by a laser oscillating on a suitable superposition of (infinitely many) transverse modes. Good approximation can be obtained with a finite number of modes.     

Excitation efficiency measurement in multimode solid-state lasers

Designing solid-state laser systems for various applications is an important subject in the laser industry. Excitation efficiency and the losses of such a laser are the main parameters sought. Both parameters can be measured using a method first published by Findlay and Clay, but measurements of efficiency and losses of several laser cavities showed that the applied technique may fail sometimes. In this note a detailed investigation is reported about the measuring technique for the losses and excitation efficiency of a solid-state laser. It is shown that limitations of the model mislead, and give false results under different experimental conditions, and it is described how to overcome the difficulties outlined here.     

Intracavity spectroscopy with modulated multimode lasers

The output of a cw multimode dye laser with an intracavity narrow-band absorber and its pump power modulated shows spectral condensation on both wings of the absorption line. This indicates phase locking of two groups of laser modes. The dispersion of the absorber modifies the mode spacing of the laser such that mode groups on both sides of the absorption line get into resonance with the modulation. These mode groups feel smaller loss and acquire the total laser power. Spectra of the laser output reveal the total absorption coefficient, the homogeneous broadening of the absorber, and the spectral width of individual laser modes.On leave from Lebedev Physical Institute, Academy of Sciences of the USSR, SU-117924 Moscow, USSR     

Gain and saturation in semiconductor lasers

Several approaches have been developed for modelling the gain and saturation behaviour of semiconductor lasers, and this study includes a historical review of these various approaches. Special attention is given to rate equation models and to the possibility of so-called gain nonlinearities that might affect the CW and modulation performance of semiconductor lasers. Discussions are mainly limited to the GaAs/GaAIAs and InGaAsP/InP systems, for which the gain and gain nonlinearities have been most extensively studied.     

Frequency stabilized and tunable multimode He-Ne lasers

A simple modified technique for stabilizing an internal mirror multimode laser has been experimentally studied. The laser was locked with respect to different points on the gain curve. Lasers with different types of polarization configurations have been stabilized and short-term variations of about 2 MHz and a long-term drift of ±5 MHz over a period of two hours have been observed.     

Gain competition effect in dual-wavelength dye lasers

In this paper, we present a theoretical model for the gain competition in dual-wavelength dye lasers. For this purpose the rate equations under the steady-state conditions have been used. The degree of the gain competition is shown to depend on the polarization of the two laser beams. The dependence of the gain competition on various working parameters such as pumping power, dye concentration and the reorientation rate of the molecules or solvent viscosity is studied for two different cases of parallel and orthogonal polarizations. The results of our model show dramatically different behaviors in the two proposed cases.     

Polarization properties of multimode internal mirror He-Ne lasers

The polarization behaviour of internal mirror multimode He-Ne lasers oscillating at 632.8 nm has been experimentally studied. It has been found that phase anisotropy of the cavity plays a dominant role in determining this behavior and fixing polarization configuration of the modes.     

Regular and chaotic behaviour of multimode gas lasers

The build-up of multimode gas laser spectra is studied on the basis of the full system of Lamb's equations of motion. Comparison is made with the results of the phase-averaged equations that constitute the so-called free-running approximation. It turns out that the range of validity of the latter is approximately given byr?0.5 wherer is the ratio of the homogeneous linewidth and the mode spacing (provided the former is mainly due to the decay of both the upper and the lower level). However, forr?2, the free-running approximation which generally predicts a steady-state spectrum to occur, breaks down. Actually, in this case multimode oscillation of a chaotic character takes place, and no steady-state regime is attained.     

Bistable operation of multimode lasers containing saturable absorbers

Continous-wave operation of many longtudinal modes in a unidirectional ring cavity containing a laser medium and a saturable absorber is investigated. The phases of the modes are assumed to be randomly distributed. The system is described by a set of rate equations including noise terms from both media. The steady-state total intensity (as an average value) and the linewidth of the laser output are calculated for the region where bistable operation and hysteresis can occur. Moreover, the hysteresis cycles of individual modes are studied.     

Gain lineshapes studies in optically pumped dimer lasers

Experimental studies of gain lineshapes in an optically pumped Na₂ dimer laser system are reported. All the phenomena predicted by the three level model are observed and gain in four-level systems induced by collision is demonstrated experimentally.     

Mode-competition noise associated with microwave modulation of multimode semiconductor lasers

We analyze the mode-competition (MC) phenomenon and the associated noise in multimode semiconductor lasers at microwave modulation. The study is based on the multimode rate-equation model, which takes into account the mechanisms of modal gain suppression. The MC is evaluated by the correlation coefficients between oscillating modes in the laser cavity. We show that an increase in the modulation depth changes the mode coupling from anticorrelation to positive correlation and then to complete coupling, which corresponds to emission of periodic pulses. The frequency spectra of relative intensity noise (RIN) exhibit sharp peaks at the modulation frequency and higher harmonics. The increase in the modulation depth is associated with suppression of the total and modal RIN under high-frequency modulation and with noise enhancement under low-frequency modulation.     

Optimal intensity noise reduction in multimode vertical-cavity surface-emitting lasers by polarization-selective attenuation

We study in detail the effect of the anticorrelation of the polarization modes on the total intensity noise of a vertical-cavity surface-emitting laser. We show that small polarization-dependent losses can greatly improve the total intensity noise, and we determine and experimentally demonstrate the conditions necessary for optimal intensity noise reduction.     

Bistable operation and spatial hole burning in multimode lasers containing saturable absorbers

Continuous-wave operation of many longitudinal modes with randomly distributed phases in a Fabry-Perot cavity containing a laser medium and a saturable absorber is investigated theoretically. The atomic linewidths of both media are assumed to be homogeneously broadened. The system is described by a coupled set of rate equations including noise terms as well as the dependence of the population difference on the spatial coordinate (along the laser axis) due to standing-wave pattern. The minimum ratio of the saturation intensities of both media which is necessary to get bistability is calculated for different places of the amplifier and the absorber within the cavity and for different spectral widths of the laser field. Due to hole burning which influences the strength of saturation in the media this ratio can vary notably. Moreover, the intensities on the branches of the hysteresis cycles associated with bistable operation are evaluated.     

Gain recovery dynamics and photon-driven transport in quantum cascade lasers

Quantum cascade lasers are semiconductor devices based on the interplay of perpendicular transport through the heterostructure and the intracavity lasing field. We employ femtosecond time-resolved pump-probe measurements to investigate the nature of the transport through the laser structure via the dynamics of the gain. The gain recovery is determined by the time-dependent transport of electrons through both the active regions and the superlattice regions connecting them. As the laser approaches and exceeds threshold, the component of the gain recovery due to the nonzero lifetime of the upper lasing state in the active region shows a dramatic reduction due to the onset of quantum stimulated emission; the drift of the electrons is thus driven by the cavity photon density. The gain recovery is qualitatively different from that in conventional lasers due to the superlattice transport in the cascade.     

Irreversible circulation and the undamped spiking in lasers

Our new theory of fluctuation was applied to the undamped spiking in lasers with single mode saturable absorption. At the marginal points, which lead to limit cycles, it was found that the irreversible circulation α and the residual circulation α′ diverge together with the variance σ. The model system of this article has effectively three degrees of freedom; however, it was demonstrated that the dominant motion and the fluctuation have two dimensional character in the neighbourhood of a hard mode instability.     

Irreversible circulation and the undampled spiking in lasers

Our new theory of fluctuation was applied to the undamped spiking in lasers with single mode saturable absorption. It was found that the irreversible circulation α diverges at the thresholds of hard mode instabilities, which leads to limit cycle.     

Theoretical analysis of multimode fiber amplifiers and lasers containing spatial filters

By the use of the coupling theory of the modes and the multimode rate equations, the performances of the multimode fiber amplifiers or lasers containing a spatial filter have been discussed. After the rate equations applicable to amplifiers are solved, the beam quality factors of the output from a multimode fiber amplifier with and without a single mode filter have been evaluated and compared. By the use of the analytical expression of the threshold pump power applicable to the multimode fiber lasers with the spatial filter and doped with quasi two-energy-level ions, and two example fibers had been selected which may guide 4 and 9 transversal modes, respectively, the threshold pump power for each transversal fiber mode has been evaluated. The results indicate that the single mode filter, does introduce a very strong discrimination for the modes between the high order modes and the fundamental mode. This should considerably favor the operation of the fundamental mode in the lasers with a homogeneously broadened active medium.