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Acta Optica Sinica

Theory of FIR tunable Raman lasers

C Wang

Published Jan 1, 1987• 0 citations
OpenAlex

Abstract

In this paper, we use the dressed-atom approach to derive the analytical expression of the gain of a FIR laser with a four-level system. The results indicate that the FIB laser gain spectrum generally consists of four peaks. For some ranges of pump detunings, it is possible to obtain two positive gain maxima: one at the Raman frequency and the other at the line center frequency.

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Physical Review A•Jul 1, 1997•6 citations
In this paper, we use the dressed-atom approach to derive the analytical expression of the gain of a FIR laser with a four-level system. The results indicate that the FIB laser gain spectrum generally consists of four peaks. For some ranges of pump detunings, it is possible to obtain two positive gain maxima: one at the Raman frequency and the other at the line center frequency.
Optically pumped FIR lasers give only discrete spectrums. This property restricts the practical use of lasers. Since 1977, many research works have been done to tune the output frequency of FIR laser, of which some are tried to find the possibility of tuning the laser lines by changing the pumping frequency continuously. But it is not easy to get an accurate result due to the complexity of theoretical calculation especially for quantum system of more than three energy levels. In this research work, two photon processes in four levels system have been studied by means of graph theory, which was developed by T. A. DeTemple et al. It was found that the output frequency of optically pumped FIR laser could be tuned with a frequency range equal to the frequency difference of two FIR transition frequencies. The tuning curves of FIR frequency and the curves of system gain coefficient vs. pump detuning frequencies were given.
The semiclassical density matrix treatment is applied to serial four-level systems for the steady state. Phase and energy relaxations as well as Doppler effects andMdegeneracy are accounted for. The calculations are restricted to parallel polarized fields, avoiding problems of quantum interference. The formalism is used to calculate the gain spectra of different four-level far-infrared (FIR) laser schemes: double resonance pumped laser, Raman laser, two-photon pumped laser, and hyper-Raman-laser.
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.