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Optics Letters

Observation of a high Raman gain induced by a pulsed pump laser in a Λ system of three fine-structure levels

Myoung‐Kyu Oh, Jai-Hyung Lee, Kyungwon An

Published Jun 19, 2007• 0 citations
Publisher

Abstract

We have observed a high Raman gain in a quasi-stationary state realized by a single strong-pulsed-pump laser. The Raman gain, originating from the population inversion in the dressed state picture, was as large as 1700% in a Lambda system of three fine-structure levels of atomic samarium, corresponding to a gain cross section comparable with the linear absorption cross section for the same transition.

DOI
10.1364/ol.32.001809

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Physical Review A•Mar 1, 1996•46 citations
We have observed a high Raman gain in a quasi-stationary state realized by a single strong-pulsed-pump laser. The Raman gain, originating from the population inversion in the dressed state picture, was as large as 1700% in a Lambda system of three fine-structure levels of atomic samarium, corresponding to a gain cross section comparable with the linear absorption cross section for the same transition.
Measurements of stimulated Raman gain in rubidium vapor are made and compared with a detailed model. Amplification of a weak probe laser is observed in a \ensuremath{\Lambda}-type three-level system that is driven by a strong cw coupling laser. The intensity of the coupling laser is sufficiently strong that it both optically pumps the atomic levels, causing a population inversion in the ground-state hyperfine levels, and drives the stimulated Raman process producing gain. A theory that models all aspects of the gain mechanism is presented. The model includes all interactions between the external fields and the atomic transitions, including those processes that involve multiple fields interacting with the same transition. Numerical calculations are compared with gain measurements made at several coupling laser intensities and detunings from the D1D1D1 line in 85Rb.{}^{85}\mathrm{Rb}.85Rb.
We consider a four-level atomic system with a degenerate or quasidegenerate ground-state doublet and two excited states. One of the upper levels is coupled to the ground doublet by a coherent field of variable strength, which we refer to as the Raman field, whose role is to create atomic coherence between the driven atomic states. The other higher lying state is coupled to the ground states when laser action develops. This model includes an incoherent pump mechanism to create some population in the top lasing state. In appropriate conditions, for the incoherent pump rate and the Raman field strength, the absorption profile of the laser transition develops gain features. We discuss the physical origin of this gain which we find to be the consequence of a population inversion between dressed atomic states. It is interesting to compare the results of this analysis with a similar study of a three-level lambda system which was previously recognized to yield lasing without inversion.1 In this case the appearance of gain can be attributed to the induced atomic coherence.
Recently, there has been much interest in the Raman gain observed in a Λ system with closely spaced ground states such that both ground states are initially equally populated and are both coupled to the excited state by a single frequency pump. This system is of interest for applications such as frequency shifting and self phase conjugation and as an example of a system that exhibits large optical gain for very small excited state populations. We have studied the mechanism of this gain theoretically and have found qualitative agreement with previously observed experimental results. Briefly, a single frequency pump interacts with both legs of the system. The gain spectrum is interpreted in terms of the energy intervals of the dressed states of the pumped system. The gain mechanism is analogous to the process of sideband amplification in a strongly driven two level system. However, the width of the gain is not limited by the linewidth of the excited state and, for low pump power, is determined by the transit time only. This is consistent with the extremely narrow gains (less than 1 kHz) observed in sodium vapor.
The emergence of laser action from an atomic system without population inversion is beginning to attract theoretical and experimental attention as a possible way of generating useful coherent radiation at unusual wavelengths. The goal of this communication is to clarify the physical origin of this phenomenon using a four-level system whose ground state consists of two quasidegenerate levels that are coupled to a higher-lying state by a Raman-like transition. In the presence of the ground-state coherence induced by the Raman field, we calculate the absorption spectrum of a weak probe whose tunable frequency scans across the transition frequencies between the two ground-state levels and another higher-lying state that may or may not be weakly excited by an incoherent pump mechanism. Our calculation, based on a dressed atomic state approach, clarifies the conditions that must be satisfied to produce gain, identifies the frequency (or frequencies) at which the gain is maximum, and elucidates the functions of the incoherent pump, the Raman field, and the coherence between the ground-state levels in this process. An appropriate set of Maxwell- Bloch equations for a ring-cavity model of this system confirms the appearance of laser action when the small-signal gain conditions are satisfied.
We report the observation of steady-state amplification of a weak probe laser without population inversion in the bare states in a Doppler-broadened \ensuremath{\Lambda}-type Rb atomic system. The probe amplification was observed with the Rabi frequency of a coupling laser much smaller than the Doppler width and can be viewed as due to the dressed-state inversion. A 10% single-pass amplification of a weak probe laser was obtained, and the measured linewidth of the gain spectral feature approaches the Rb natural linewidth. A theoretical analysis based on a three-level Doppler-broadened \ensuremath{\Lambda} system is presented, and qualitative agreement with the experiment results is achieved. \textcopyright{} 1996 The American Physical Society.