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Physical Review A

Lasers without inversion in a Doppler-broadened medium

Andrzej Karawajczyk, Jakub Zakrzewski

Published Jan 1, 1995• 31 citations
Publisher

Abstract

Three-level models of lasers without inversion in V and \ensuremath{\Lambda} configurations are studied in a realistic Doppler-broadened gas medium. The intensity of a generated coherent radiation is numerically calculated for a wide range of experimental parameters and the role of the laser cavity is fully taken into account. Different geometrical arrangements of the experiment are compared. Significant differences in lasing properties of the studied models are shown and discussed. An amplification mechanism based on the Doppler shift tuning to dressed-state inverted transitions is discussed.

DOI
10.1103/physreva.51.830

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Physical Review A•Oct 1, 1995•15 citations
Three-level models of lasers without inversion in V and \ensuremath{\Lambda} configurations are studied in a realistic Doppler-broadened gas medium. The intensity of a generated coherent radiation is numerically calculated for a wide range of experimental parameters and the role of the laser cavity is fully taken into account. Different geometrical arrangements of the experiment are compared. Significant differences in lasing properties of the studied models are shown and discussed. An amplification mechanism based on the Doppler shift tuning to dressed-state inverted transitions is discussed.
The theory of amplification and lasing without population inversion in a three-level medium with inhomogeneous broadening via the formation of an open V configuration is elaborated. The conditions for energy transfer from the infrared into the visible spectral range, i.e., the conditions of up-conversion n b >n c >n a , and the external field required for saturation of the b⇄a transition are established. Two-photon resonant Raman transitions in ensemble of mobile atoms of a gas-discharge plasma are analyzed. The frequency shift of the probe field spectrum as a whole is shown to be governed by the frequency shift of the pump field multiplied by the ratio of the wave numbers of the probe amplification field and the pump field. The interaction of atoms through Ne transitions with the pump field (λ=1.15 εm, 2p 4-2s 2 transition) and the lasing field (λ=0.6328 εm, 3s 2-2p 2 transition) with an increase in the lasing frequency by a factor of 1.82 with respect to the absorbed radiation is calculated.
Summary form only given. We study a model for an incoherently pumped laser operating with a Doppler-broadened three-level system placed in a doubly resonant cavity and producing two laser fields, one of them without population inversion. Each laser field couples to only one of the transitions of the three-level medium. We have studied our model for parameters appropriate to two real atomic systems (i) a cascade scheme in atomic /sup 138/Ba looking for lasing with (without) inversion at /spl lambda//sub /spl beta//=554 nm (/spl lambda//sub /spl alpha//=821 nm), and (ii) a V-scheme in atomic /sup 85/Rb with ordinary lasing at /spl lambda//sub /spl alpha//=780 nm and lasing without inversion (LWI) at /spl lambda//sub /spl alpha//=4216 nm. These two cases are qualitatively different in that two-photon processes are responsible for inversionless gain in case (i) while LWI in case (ii) arises from one-photon processes. Moreover, the frequency up-conversion ratio is R=0.67 for the case of /sup 138/Ba and it is R=1.88 for the case of /sup 85/Rb. In spite of the fact that the frequencies of the two laser fields are relatively different, our results show that dual-wavelength lasing extends up to Doppler broadening values typical for optical transitions of atoms in a vapor cell. In case (i) /spl alpha/-lasing occurs without two-photon inversion only for approximately the lower half of the domain of Doppler-broadening values. However, in case (ii) one has LWI in all the domains. Some considerations for a practical realization of the dual-wavelength inversionless-conventional laser are discussed.
We have studied an incoherently pumped laser operating with a Doppler-broadened three-level system placed in a doubly resonant cavity. This system generates two laser fields, one of them without population inversion. Both ladder and V-type three-level schemes are considered with a ratio $R={\ensuremath{\omega}}_{\ensuremath{\alpha}}/{\ensuremath{\omega}}_{\ensuremath{\beta}}$ of inversionless laser frequency ${\ensuremath{\omega}}_{\ensuremath{\alpha}}$ to ordinary laser frequency ${\ensuremath{\omega}}_{\ensuremath{\beta}}$ of R=0.67R=0.67R=0.67 and R=1.88,R=1.88,R=1.88, respectively. Dual-wavelength lasing extends up to Doppler-broadening values for optical transitions of atoms in a vapor cell. Some considerations for the practical realization of this dual-wavelength laser are discussed.
This thesis is concerned with the amplification of coherent radiation from an active medium under conditions which require no population inversion between the energy levels. Gain without population inversion may be obtained by creating a sufficiently strong correlation between pairs of atomic states, usually with the help of external sources of resonant electromagnetic radiation. Our main objective is to investigate two possible selections for the active medium: a three level atom in a lambda configuration and a Raman-driven four level model. For both we identify the conditions that lead to inversionless laser action, we calculate the small signal gain and investigate the physical mechanisms which are responsible for the emergence of gain. While the ultimate cause of the gain is quantum interference, we propose attractive alternative pictures based on the dressed states of the coupled atom-driving field systems. We address the important practical issue of the effect of atomic motion and related Doppler broadening on the inversionless gain profile and we also analyze the transient response of each laser model with the help of the appropriate Maxwell-Bloch equations. Finally we develop a fully quantum mechanical description of the lambda model and find that the radiated field can display unique quantum features which manifest themselves with sub-Poissonian photon fluctuations.
Lasing without inversion (LWI) in specific models of closed three-level systems is analyzed in terms of nonlinear dynamics. From a linear stability analysis of the trivial nonlasing solution of the homogeneously broadened systems with on-resonance driving and laser fields, we find that, near lasing threshold, resonant closed Λ and V schemes yield continuous-wave LWI while resonant cascade schemes can give rise to self-pulsing LWI. The origin of this different behavior is discussed. For parameters of a real cascade system in atomic 138Ba we check numerically that the self-pulsing solution is stable in a broad range of nonzero detunings. It is shown that the self-pulsing emission can still be observed when the typical residual Doppler broadening of an atomic beam is taken into account.
We discuss the effect of Doppler broadening on the gain and refractive index experienced by an optical probe when passing through a collection of atoms in the absence of a population inversion. We calculate the gain-absorption spectrum of the medium when the atoms have a Maxwell velocity distribution and compare the atomic response when the pump and probe beams are arranged in copropagating and counterpropagating configurations. The results in these two cases can be qualitatively very different from each other, as we show with the help of numerical calculations. A physical interpretation and the conditions for the validity of the results are provided.