Semiclassical Laser Theory Research Articles

Quantum corrections to semi-classical laser theory

We suggest a method for successive quantum corrections (in powers of the Planck constant ħ) to laser field characteristics. The Lagrangian formalism and the representation of the effective potential for the laser have been used. The analogy between the laser and the λø 4 theory has been established: the laser below, at and above the threshold corresponds to a relativistic particle with positive, zero and imaginary mass, respectively. For a sample calculation, corrections to linewidth, output power and phase diffusion which are due to the internal quantum nature of the laser light have been analytically derived. The bifurcation on the threshold turns out to be a semi-classical effect because the internal quantum fluctuations do not change the value of the threshold.

Low-intensity limit of the laser cooling of a multistate atom.

We adapt an earlier semiclassical theory of laser cooling of an arbitrary multistate atom [J. Javanainen, Phys. Rev. A 44, 5857 (1991)] to the limit of low light intensity. The formal theory is implemented analytically on a computer using mathematica. Expressions of light-pressure force and diffusion are provided for ${\mathit{j}}_{1}$=1/2\ensuremath{\rightarrow}${\mathit{j}}_{2}$=3/2 and ${\mathit{j}}_{1}$=1\ensuremath{\rightarrow}${\mathit{j}}_{2}$=2 atoms in a one-dimensional optical confinement area in which the pair of counterpropagating waves has orthogonal polarizations. In our models the cooling temperature decreases as the level degeneracy increases.

Gain of TE-TM modes in quantum-well lasers

Semiclassical laser theory is rigorously applied to semiconductor lasers in order to obtain both the complete TE and TM linear gain. The resulting expressions for the modal gain in heterostructure lasers differ in form from those conventionally accepted. In particular, the conventional modal gain written as the product of a confinement factor and a bulk gain is only an approximation of the true modal gain derived. The conventional expression relies on an explicit definition of the active region of the laser, which can be ambiguous when certain heterostructures, such as parabolic quantum wells, are to be treated. This ambiguity is eliminated by the gain expressions as a more natural active region defined by the product of electron and hole wave functions emerges. The relevant approximations which allow the newly derived gain equations to be written in forms similar to the conventional expressions for single quantum well, multiquantum well (MQW), and in wide active region lasers are explicitly shown. >

Magneto-optic effects induced by laser fields in atomic J = 1 → J = 2 → J = 1 systems

Conventional three level probe spectroscopy is used to investigate magnetically induced level crossings. Pump laser induced signals in the probe laser response are investigated when the atomic systems are tuned magnetically. This has been carried out in both a co-propagating and a counter-propagating configuration. The level scheme considered is a simplified J = 1 → J = 2 → J = 1 system. The theoretical calculations are based on the semiclassical laser theory and the equations obtained are solved numerically. Experimental verification of the theoretical lineshapes is carried out in a neon gas absorber. The agreement between the calculated results and the measurements is good.

Forward scattering induced by laser fields in atomic J=1→ J=0 systems

We investigate resonant forward scattering of laser radiation. Calculations based on the semiclassical laser theory are performed up to the second order in the laser intensity. The response of the atomic J=1→ J=0 transition to a laser beam is considered. Experimental verification of the theoretical lineshapes is carried out using a neon gas absorber. The agreement between the calculated results and the measurements is excellent and indicates that higher order nonlinearities such as hexadecapole couplings are not easily discerned in the lineshapes.

Polarisation properties of the stimulated zero-field level crossing

The polarisation properties of the single mode linearly polarised laser beam interaction with a Doppler broadened absorber has been investigated. The absorber is placed in a longitudinal tunable magnetic field. The laser beam is analysed with a polariser after it has traversed the absorber. With various analyser rotation angles the lineshapes of the nonlinear resonances known as the population effect and the Zeeman coherence effect have been studied near zero magnetic field tuning. Theoretical predictions derived from the semi-classical laser theory are compared with experimental results. The experiment has been carried out with a low power thermally stabilised 633 nm He-He laser. The measured lineshapes of the resonances of the 2p4-3s2 transition in neon agree well with the theoretical predictions. By using an analyser in the beam an improved signal-to-noise ratio is shown to result.

Filamentation in conventional double heterostructure and quantum well semiconductor lasers

A wave-optical model is used to investigate filamentation of the optical field in conventional double heterostructure and quantum well semiconductor lasers. The field is propagated by the Crank-Nicholson method, and the gain model is based on a semiclassical laser theory that contains the effects of cross relaxation due to electron-electron and electron-phonon collisions. The model predicts that for broad-area devices, quantum well lasers are less affected by filamentation than conventional double heterostructure lasers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Effects of spatial gain variation in an index-guided semiconductor laser array

This paper is written with two goals in mind. The first goal involves the extension of semiclassical laser theory to include the effects of gain guiding. The second goal involves the application of the more general semiclassical laser theory developed in this paper to investigate the effects of spatially varying the excitation geometry on the intracavity field of an index-guided semiconductor laser array.

The semiclassical theory of laser cooling

This paper reviews the basic theory of the mechanical action of light in resonant interaction with atoms. At present the main application is laser cooling, but the approach is applicable to a broader range of phenomena. It is based on an adiabatic elimination philosophy, which turns out to give the lowest-order quantum corrections to the behavior found when the photon momentum goes to zero. Hence it is called a semiclassical theory. In this manner a subjective but consistent approach can be presented; other treatments are incorporated or mentioned at the appropriate places. Both the classical and the quantum-mechanical approach are discussed. Those readers who wish to obtain only a heuristic overview of the phenomena can concentrate on Sec. III, which treats both the photon momentum effects and their connection with photon counting statistics. The detailed theoretical treatment utilizes Wigner functions and Fokker-Planck techniques. The ensuing theory is applied both to the cooling of free particles and trapped ones. The paper ends with an extensive bibliography, where the author lists most papers of interest for research into the mechanical manifestations of light. For completeness, many papers are included that are not explicitly mentioned in the text.

Frequency locking in an index-guided semiconductor laser array

A theory is developed for investigating frequency locking in an index-guided semiconductor laser array. This theory, which is based on the expansion of the laser field in terms of the passive array eigenmodes, and on the use of semiclassical laser theory to treat the interaction of these modes among themselves, is valid for all values of coupling between lasers and for all array sizes. To illustrate the application of this theory, we use it to determine the lockband of a two-laser array with dissimilar waveguides.

Semi-classical theory of a free electron laser

In this paper, a semi-classical theory of a free electron laser is presented. The semi-classical theory agrees with the classical theory of a free electron laser and gives more knowledge.

Raman and line centre oscillation selection in coherently pumped four-level fir lasers

Competition between Raman and line-centre oscillation in multilevel lasers pumped with coherent radiation off resonance is studied by means of semi-classical laser theory. In a four level system consisting of a combination of a Λ and V system, it is shown that simultaneous Raman and line centre oscillation is expected on one of the transitions contrary to the case of a simple three level system where the Raman line completely suppresses the line centre oscillation. The crucial importance of a new highly nonlinear phase locking effect for the behaviour of the system is demonstrated.

Competition between Raman and line-center oscillations in optically pumped far-infrared lasers

Competition between Raman and line-center oscillations in off-resonantly pumped far-infrared lasers is studied by means of semiclassical laser theory. The observed preferential Raman line operation is shown to be caused by an inversion of the line-center gain when the Raman mode reaches intensities well below full saturation. This inversion is partly due to simple population effects. A further enhancement in the cross saturation at line center is caused by population pulsations. Numerical examples of mode competition are presented.

Two-mode optical pumping of a laser

The coherent optical pumping of a laser is investigated by means of semiclassical laser theory with the assumption that either the pump source or the laser output is multimode. All radiation modes are allowed to reach saturating levels. A multitude of interesting effects result from induced population pulsations. As a particular application we study numerically the optical pumping of a single-mode homogeneously broadened FIR laser with a two-mode pump. We find that at least for symmetric pumping higher FIR intensities can be obtained with two, instead of one, pump modes.

Multistable laser through a nonlinear mirror

In this paper we show that a multistable laser is realizable by the use of a nonlinear medium as one of the resonant cavity mirrors. Using the semiclassical laser theory we derive a gain function with many maxima. The resulting gain equation has many real solutions. We discuss the possible application in switching by use of a suitable external perturbation.

Theory of line narrowing and frequency selection in an injection locked laser

We apply the multimode semiclassical laser theory to the problem of line narrowing and frequency selection in an injection locked laser. Our goal is to predict from the spectroscopic properties of a given active medium, the behavior of a broad-band laser that is under the influence of a narrow-band injected signal. Analytic expressions for the injected signal strength necessary for single-mode operation are obtained for both homogeneously and inhomogeneously broadened lasers, and for small-signal and strong-signal regimes. We found that under certain experimental conditions, an injection locked inhomogeneously broadened laser behaves significantly different from a homogeneously broadened lasel.

Changing the quality factor of a laser with a glass plate

A scheme is described for changing the quality factor of a laser cavity by rotating a glass plate around the laser axis. In order to check the setup, the output power of a laser is measured, and good agreement is obtained with the semiclassical laser theory.

A SEMICLASSICAL THEORY OF MODE COUPLING IN DYE LASERS

The semiclassical laser theory proposed by Lamb for two-level system is extended to the case of dye lasers. Mode coupling equations are deduced, and a discussion is given.

Metastable state relaxation time in dye lasers

In this paper we use the semi-classical laser theory to establish the equivalence of a dye laser to a conventional laser with an intracavity saturable absorber. Using an analogy with the Landau theory — the field acts as the order parameter — we obtain a bistable pseudo free-energy function. We add an external random noise term to the field equation in order to initiate a transition from one state to the other and use first passage time analysis to calculate the relaxation time from a metastable state to a state of absolute stability. We evaluate the relaxation time dependence on the strength of the random noise force and the triplet state parameter.

Semiclassical laser theory in the stochastic and thermodynamic frameworks

The thermodynamic properties of the laser distribution in the steadily oscillating state are investigated to determine the minimum characteristic of the entropy production. First, the laser Langevin equation for five random variables is treated in the light of the stochastic calculus to deduce the photon-number rate equationn = − C+(n − nc) + [A/(1 + sn)](n−nA), where nn and n4 are the two constants of the fluctuation attributed to the noise forces subject to the usual fluctuation-dissipation theorem, withn4 < 0 for the inverted atomic population. We then combine the dynamics of the lasing mode with a model open system of the Lebowitz type with two reservoirs for which the entropy productionσ(p) is expressed and made subject to a variational principle: The modified variation scheme, the same as Prigogine's local potential method, is shown to give the exact lasing distributionp as the optimum between two distributions of thermal type with temperatures far from each other.