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Annual Meeting Optical Society of America

Incoherent pump effects in AM spectroscopy

Stephen H. Chakmakjian, Karl W. Koch, Carlos R. Stroud, Lloyd W. Hillman

Published Jan 1, 1987• 1 citation
Publisher

Abstract

The response of three- and four-level homogeneous laser systems to an amplitude modulated field in the presence of an incoherent pump is treated using a rate equation model. A spectral hole due to the finite bandwidth of the population oscillations is observed with a modulated field at either the pump or the laser frequency. The spectral width of the hole broadens at a rate proportional to the sum of the dimensionless intensities of the pump and laser fields. The power levels required to broaden the 1/T1 hole in various laser systems is presented. The experimental implications of this result are discussed.

DOI
10.1364/oam.1987.md6

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The response of three- and four-level homogeneous laser systems to an amplitude modulated field in the presence of an incoherent pump is treated using a rate equation model. A spectral hole due to the finite bandwidth of the population oscillations is observed with a modulated field at either the pump or the laser frequency. The spectral width of the hole broadens at a rate proportional to the sum of the dimensionless intensities of the pump and laser fields. The power levels required to broaden the 1/T1 hole in various laser systems is presented. The experimental implications of this result are discussed.
We report the observation of a spectral feature in the gain and absorption profiles of a multilevel system. The gain experienced by the sidebands of a weakly modulated laser field Is measured as a function of modulation frequency. For small modulation frequencies the sidebands experience diminished absorption or gain depending on the time-averaged inversion of the system. The width of the spectral feature depends on spontaneous as well as stimulated population transition rates. This paper Is an extension of single-field modulation experiments In collisionally broadened media.1,2 Two lasers are used in our experiment. One laser is used as an incoherent pump field that provides population for the upper level of the other optical transition. The second laser stimulates this population to the lower or ground level depending on the system configuration. In the experiment we modulate the second laser field. Using alexandrite as an active medium we are able to study both three- and four-level atomic systems.3 The stability of a single-mode laser to the growth of neighboring modes is related to the gain experienced by the sidebands. The competition between modes is strongest when their frequency separation is less than the width of this spectral feature.
A theory is developed to describe the way in which modulations in the pump intensity produce modulations in the gain of a four-level, homogeneously broadened laser amplifier. The theory is tested by carrying out an experiment using an alexandrite crystal pumped by a cw dye laser. A second dye laser is used to measure the gain in the inverted laser transition. The dependence of the magnitude and the bandwidth of the gain on the pumping rate is determined. Agreement between theory and experiment is good.
The amplification of noise in a laser amplifier is treated theoretically. The model for the active medium and its description using density-matrix techniques, are taken from the theory of laser operation. The fundamental concern of this investigation is the spectral behavior of the radiation in the nonlinear regime; hence, the formalism is written from the onset in the frequency domain. The statistics of the light are gradually modified by the nonlinear amplification process, and expressions are derived for the rate of change of fluctuations in intensity as a measure of statistical changes. These expressions should be easily susceptible to detailed experimental observations. In addition, the range of validity of Litvak's Gaussian-statistics approximation is discussed with some detail. In the homogeneous-broadening case, the evolution of initially broadband Gaussian radiation toward quasimonochromatic oscillations with laserlike statistics is extensively explored in several numerical examples. The connections of this study with the time-domain work of Risken and Nummedal, on self-pulsing in a ring-laser configuration, are clearly established. Finally, spectral-narrowing and -rebroadening effects in Doppler-broadened media are discussed both analytically and with numerical examples. These examples show the distinct contribution of pulsations in the population ("Raman-type terms"), and saturation phenomena. The predicted narrowing-rebroadening rate is compared with expressions found in previous literature.
Pomeau et al.1 have shown that a two-level atom strongly driven by a single 100% amplitude modulated (AM) field can show broadband spectral behavior when the two-frequency components of the field are incommensurate. The equations describing the atomic response to a monochromatic field without making the rotating wave approximation (RWA) are identical to form to the equations describing a 100% amplitude modulated field within the RWA. This fact makes the observation of the predictions of Pomeau et al. experimentally realizable in an optical resonance experiment. We present theoretical and numerical results of a pair of 100% AM fields interacting with a single atomic resonance. The effects on the atomic response of commensurate and incommensurate modulation frequencies are discussed as are the experimental implications.
We report a mechanism that leads to the suppression of line broadening effects. We show that a strong competition between the effects of coherent and incoherent pumps can lead to suppression of spontaneous emission or to narrowing of the spectral profiles. Consider the interaction of a system with a broadband incoherent pump, the effects of which can be accounted for by changing the relaxation times T1, T2 in the Bloch equations. For example, for radiative relaxation (rate 2γ) we have where | x | is the Rabi frequency associated with the broadband pump, and Γ is its bandwidth. The spontaneous emission spectrum from such a system in the presence of a coherent drive is the Mollow spectrum with the broadband pump contributing an amount β to the widths of the peaks. We demonstrate that the effects of β can be suppressed if the strength of the coherent pump is such that many Rabi oscillations are possible within the coherence time of the incoherent pump. We show this explicitly by obtaining exact results for spontaneous emission from a model system.
A theoretical study of amplitude (AM) and frequency (FM) modulation components of an optical wave interacting resonantly with a homogeneously broadened two-level system is presented. It is shown that the AM component is usually less attenuated than the FM component; however, under conditions which depend on the modulation frequency, relaxation times, and the saturation parameter, the reverse may occur. In strongly saturated media the AM component can be amplified, a process limited by the depletion of the carrier intensity as the wave propagates in the absorber. The two- and three-wave cases of unidirectional saturation spectroscopy are considered and the effects of depletion are discussed. In the two-wave (single-probe) case a third wave is generated which substantially modifies the absorption profile for the probe. An analysis is included of the sideband evolution in a laser with an intracavity saturable absorber. By analyzing the problem in the frequency domain, some new insights concerning the role of the saturable absorber in passively mode-locked lasers are presented.