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Journal of the Optical Society of America B

Effects of pump modulation on a four-level laser amplifier

Stephen H. Chakmakjian, Karl W. Koch, Stephanos Papademetriou, Carlos R. Stroud

Published Sep 1, 1989• 4 citations
Publisher

Abstract

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.

DOI
10.1364/josab.6.001746

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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 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 comparison is reported of the observed spectral narrowing of amplified spontaneous emission from dye amplifiers with predictions from a dynamical model based on rate equations. This is employed to develop a generally applicable method of determining the unsaturated gain within the dye medium. Comparison is made with another technique for determining gain over the restricted pumping range for which it is applicable. The matching of theory to experiment is sufficiently sensitive that good values can also be determined for the wavelength dependence of the excited singlet state absorption cross section.
A theoretical study is presented of the gain and the noise spectra of laser amplifiers at pumping rates both above and below the oscillation threshold. The theory is valid for both class-A and class-B laser systems, where the decay rate of the collective atomic dipole moment greatly exceeds the decay rates of the atomic population inversion and the laser cavity. The gain and the noise properties are shown to be strongly influenced by a four-wave mixing of the signal and image frequencies via a modulation of the population inversion. Particular attention is paid to the residual noise flux that persists in the laser output above threshold and to the typical characteristics of a second-order phase transition displayed by the gain in the threshold region. The results for the above-threshold laser amplifier are compared with experimental measurements of both gain and noise for the argon-ion (class-A) and CO2{\mathrm{CO}}_{2}CO2​ (class-B) lasers. The signal-to-noise ratios for both heterodyne and direct-detection measurements are evaluated, and the conditions are determined under which optical amplification can lead to an enhancement of sensitivity.
A new method for determining the gain in high gain amplifiers is reported. The technique only requires measurements of the relative intensity and the output spectral width of the amplified spontaneous emission from the amplifier as a function of input pumping. The results of an experimental test of this technique using a flashlamp-pumped rhodamine 6G dye amplifier are described. Simple approximate analytical models are used to compare with experiment thereby determining the gain. The results are found to be in good agreement with both more accurate gain measurements and an alternative technique which can be applied over a restricted range of operational parameters.
Introduction When an electromagnetic wave propagates through amedium stimulated emissions increase the intensity of the wave, while absorptions diminish it. The overall intensity will increase if the number of stimulated emissions can be made larger than the number of absorptions. If we can create such a situation then we have built an amplifier that operates through the mechanism of stimulated emission. This laser amplifier , in common with electronic amplifiers, has useful gain only over a particular frequency bandwidth. Its operating frequency range will be determined by the lineshape of the transition, and we expect the frequency width of its useful operating range to be of the same order as the width of the lineshape. It is very important to consider how this frequencywidth is related to the various mechanisms by which transitions between different energy states of a particle are smeared out over a range of frequencies. This line broadening affects in a fundamental way not only the frequency bandwidth of the amplifier, but also its gain. A laser amplifier can be turned into an oscillator by supplying an appropriate amount of positive feedback. The level of oscillation will stabilize because the amplifier saturates. Laser amplifiers fall into two categories, which saturate in different ways. A homogeneously broadened amplifier consists of a number of amplifying particles that are essentially equivalent, whereas an inhomogeneously broadened amplifier contains amplifying particles with a distribution of amplification characteristics. Homogeneous line broadening All energy states, except the lowest energy state of a particle (the ground state) cover a range of possible energies.