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Journal of the Optical Society of America B

Laser amplification without population inversion on the D_1 line of the Cs atom with semiconductor diode lasers

Parminder S. Bhatia, George R. Welch, Marlan O. Scully

Published Nov 1, 2001• 11 citations
Publisher

Abstract

The importance of lasing without inversion (LWI) in comparison with the use of nonlinear optical techniques and techniques based on high-intensity femtosecond light pulses to generate high-frequency coherent radiation is outlined. The original development and utilization of an experimental facility based on semiconductor diode lasers to study the fundamental aspects of LWI on the D1 line of the Cs atom, which is rarely attempted, is described. This experimental facility is used to observe inversionless amplification on the D1 line of the Cs atom by use of a Λ atomic scheme. A combination of EIT and broadband pumping of atoms from the lower to the upper level of the probe transition allowed the observation of the inversionless gain. Justification for the absence of inversion in the probe transition is provided. We report a single-pass inversionless gain of ≈11% with relatively weak incoherent pumping. The results show improvement over the previously reported inversionless gain observed in Na, Rb, and Cs, illustrating the potential advantage of using the D1 line of Cs.

DOI
10.1364/josab.18.001587

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A new method of laser radiation generation without population inversion in the red wing of the $$D_{1}$$ -line of alkali metal atoms under resonant absorption of broadband radiation of pump laser diodes at the $$D_{1}$$ transition is studied theoretically. The appearance of laser generation is caused by the fact that the probability of stimulated emission in the red wing of the spectral line exceeds the probability of absorption if homogeneous broadening due to interaction between active particles and the buffer gas significantly exceeds the natural broadening (under high pressures of the buffer gas). Analytical formulas representing the operation of a transversely diode-pumped alkali metal vapor laser are obtained. It is found that cesium atoms are the most promising object for observation of laser generation without population inversion in the red wing of the $$D_{1}$$ -line of alkali metal atoms. In a sufficiently long active medium (the length of the medium is 50 times its width), the efficiency of pumping radiation conversion into laser radiation can reach 57% at a buffer gas pressure of 5 atm, a pump diode radiation intensity of 5 kW/cm $${}^{2}$$ , and a half-width of its spectrum of 1 cm $${}^{-1}$$ . The laser radiation frequency can be tuned by several tens of cm $${}^{-1}$$ .

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The importance of lasing without inversion (LWI) in comparison with the use of nonlinear optical techniques and techniques based on high-intensity femtosecond light pulses to generate high-frequency coherent radiation is outlined. The original development and utilization of an experimental facility based on semiconductor diode lasers to study the fundamental aspects of LWI on the D1 line of the Cs atom, which is rarely attempted, is described. This experimental facility is used to observe inversionless amplification on the D1 line of the Cs atom by use of a Λ atomic scheme. A combination of EIT and broadband pumping of atoms from the lower to the upper level of the probe transition allowed the observation of the inversionless gain. Justification for the absence of inversion in the probe transition is provided. We report a single-pass inversionless gain of ≈11% with relatively weak incoherent pumping. The results show improvement over the previously reported inversionless gain observed in Na, Rb, and Cs, illustrating the potential advantage of using the D1 line of Cs.
The most commonly used lasers operate with population inversion. This requires an incoherent pumping power (from lamps, electrical discharges, etc.) that rapidly scales with the frequency of the generated laser radiation. In practice, this is the main obstacle for the operation in the continuous-wave regime of short-wavelength lasers in the UV, X-rays, etc. But lasers can also operate without population inversion using atomic coherences excited in the laser medium by coherent pumping with another laser. We present in this paper a short review of the field of lasing without inversion (LWI). In particular, we summarize the basic physics of LWI in the framework of both semiclassical theory and quantumjump formalism to describe light-matter interaction. We also review the experimental state-of-the-art in LWI. Most proof-of-principle demonstrations of LWI have used atomic vapours, so far. But recently gain without inversion (a pre-requisite for lasing) has been observed also in a technologically relevant medium: semiconductor nanostructures that can be tailored to have “artificial atom” electron states.
We report a lasing without inversion (LWI) experiment on the basis of coherent population trapping, realized in low-pressure cadmium vapor. A single-pass inversionless amplification by more than a factor of 10 has been measured, a significant improvement over previous experiments. In addition, we report synchronously pumped laser oscillation without inversion. Using a simple model to fit the gain/absorption curves as a function of magnetic field, we have obtained values for the populations in the system, showing that there is no inversion in the atomic basis. Combined with a simple optical method to measure absolute densities of a gas, this model results in a completely optical method to determine the absolute populations and density in the system.
Atomic coherence gives rise to a series of interesting phenomena in quantum optics.1 In this work we investigate coherence effects within the Na Dl line, demonstrating coherent trapping and lasing without inversion, and study the dependence of these phenomena on buffer gas pressure. Two pump laser beams (I, and I2) are tuned to the frequencies υ1 and υ2 that correspond to the 3S1/2 (F=1, 2) → 3P1/2 transition and are right circularly polarized. Pockels cells are used to turn on or/and off the beams within approximately one nanosecond. To demonstrate lasing without inversion, a weak pulse of left polarized light tuned to t)2 was used to create a small population in the upper level (3P1/2) and the amplification of I, was observed. We found a significant gain in the absence of population inversion, which was verified by a direct experimental observation. These results were observed as a function of buffer gas pressure. The atomic coherence effects are shown to disappear in the presence of a helium buffer gas at a pressure of ≈ 1 Torr; whereas, complete optical pumping is only achieved at helium buffer gas pressures >10 Torr.
We demonstrate amplification without inversion in the 87Rb D1-line pumped by an incoherent light. A closed N-type, four-level atomic system has been considered to obtain an insight into the origin of the gain as well as the conditions under which the system exhibits gain without population inversion in bare states. We investigate the gain as a function of the incoherent pumping rate, the coupling laser detuning, and Rabi frequency. The experimental results are in good agreement with the numerical calculations considering the velocity distribution of atoms.
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.
Laser Generation without Population Inversion in the Red Wing of the D1{D}_{1}D1​-Line of Alkali Metal Atoms under Resonant Diode Pumping
A new method of laser radiation generation without population inversion in the red wing of the D1D_{1}D1​ -line of alkali metal atoms under resonant absorption of broadband radiation of pump laser diodes at the D1D_{1}D1​ transition is studied theoretically. The appearance of laser generation is caused by the fact that the probability of stimulated emission in the red wing of the spectral line exceeds the probability of absorption if homogeneous broadening due to interaction between active particles and the buffer gas significantly exceeds the natural broadening (under high pressures of the buffer gas). Analytical formulas representing the operation of a transversely diode-pumped alkali metal vapor laser are obtained. It is found that cesium atoms are the most promising object for observation of laser generation without population inversion in the red wing of the D1D_{1}D1​ -line of alkali metal atoms. In a sufficiently long active medium (the length of the medium is 50 times its width), the efficiency of pumping radiation conversion into laser radiation can reach 57% at a buffer gas pressure of 5 atm, a pump diode radiation intensity of 5 kW/cm 2{}^{2}2 , and a half-width of its spectrum of 1 cm −1{}^{-1}−1 . The laser radiation frequency can be tuned by several tens of cm −1{}^{-1}−1 .