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Quantum Electronics

Mechanisms of a change in the refractive index of an intensely pumped Yb:YAG crystal

Oleg L. Antipov, D. V. Bredikhin, O N Eremeikin, Е. В. Ивакин, Alexander P. Savikin, A. V. Sukhodolov, Ksenia A. Fedorova

Published May 31, 2006• 10 citations
Publisher

Abstract

Mechanisms of a change in the refractive index appearing in an intensely diode-pumped Yb:YAG-laser disk element are studied with the help of polarisation interferometry and dynamic grating testing. It is found that changes in the electronic component of the refractive index arising upon changing the populations of electronic levels of Yb 3+ ions (the ground 2 F 7/2 state and the upper 2 F 5/2 level of the laser transition) and caused by the difference in the polarisability of these levels are an order of magnitude greater than thermal changes in the refractive index. It is shown that the difference Δ p in the polarisability at the probe wavelength of 633 nm is 1.9 × 10 -26 cm 3 and at the laser transition wavelength of 1029 nm is 1.6 × 10 -26 cm 3 .

DOI
10.1070/qe2006v036n05abeh013230

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Mechanisms of a change in the refractive index appearing in an intensely diode-pumped Yb:YAG-laser disk element are studied with the help of polarisation interferometry and dynamic grating testing. It is found that changes in the electronic component of the refractive index arising upon changing the populations of electronic levels of Yb 3+ ions (the ground 2 F 7/2 state and the upper 2 F 5/2 level of the laser transition) and caused by the difference in the polarisability of these levels are an order of magnitude greater than thermal changes in the refractive index. It is shown that the difference Δ p in the polarisability at the probe wavelength of 633 nm is 1.9 × 10 -26 cm 3 and at the laser transition wavelength of 1029 nm is 1.6 × 10 -26 cm 3 .
A polarising interferometer was used to investigate changes in the refractive index of an Nd:YAG laser crystal associated with changes in the populations of the electronic energy levels characterised by different polarisabilities. The study was carried out on diode-pumped crystals as well as on crystals which were flashlamp-pumped under conditions of saturation of the gain experienced by the optical beam. The changes in the refractive index depended nonlinearly on the pump power. The ratio of the real and imaginary parts of the nonlinear susceptibility of an Nd:YAG laser amplifier was determined: this ratio rose from 0.2 to 1.2 when the gain increased from 0.2 to 0.45 cm -1 .
The mechanism for refractive-index changes that accompany changes in population during pumping and amplification of an optical beam is studied in a Nd:YAG laser crystal by both resonant four-wave mixing and interferometric techniques. It is found that these refractive-index changes (connected with the difference in polarizability of excited and unexcited Nd3+ ions) have two components: a small noninertial component that accompanies an amplified optical pulse and the major changes that are intensified with a delay time of ∼3 µs. The latter effect is explained by population (or depopulation) of a higher-lying energy level 2(F2)5/2 of the 4f electron shell of Nd3+ ions, which has large polarizability in near-IR and visible light. The real part of the nonlinear optical resonant susceptibility of the inverted Nd:YAG caused by the refractive-index changes is also discussed.
Absence of full knowledge on resonant susceptibility (RS) of laser crystals leads to underestimation of different nonlinear optical effects (such as holorgraphic grating formation and others) which are very important for understanding the complicated laser dynamics and construction of new laser architectures. In the majority of previous experiments the resonant wave mixings (using nonlinearity of saturation of the working transition) in inverted Nd:YAG are explained by the gain grating (GG) formation (sec. for example. [1]). On the other hand, it is well known, that pumped and unpumped laser crystals have different refractive indexes due to difference in polarizability of excited and unexcited Nd3 ions [2,3]. However. various investigation methods provide different values of the polarizability difference varying from 4.10−3nm3 [2] to 9.10−4 nm3 [3] for the well-studied Nd-dopcd materials.
We quantify the refractive index changes (RIC) in single-mode ytterbium-doped optical fibers induced by optical pulses at 980 nm and, for the first time, report details of the effect dynamics. The RIC dynamics is shown to follow that of the population of the excited/unexcited ion states with a factor proportional to their polarizability difference (PD). The absolute PD value is evaluated in the spectral range of 1460-1620 nm for different fiber samples and is found to be independent on the fiber geometry and on the ion concentration. The PD dispersion profile indicates to a predominant far-resonance UV rather than near-resonant IR transitions contribution to the RIC.
The formation of gratings accompanying a population grating in a flash-lamp pumped Nd:YAG laser amplifier has been studied by using both nondegenerate and degenerate four-wave mixing techniques. In our investigations the grating of population inversion was induced by optical waves at 1064 nm. The refractive index grating caused by the population grating was read by an optical wave at another wavelength (1054 nm or 694 nm) in nondegenerate four-wave mixing experiments. In the degenerate four-wave mixing experiments the total grating, i.e. the sum of the refractive index and gain gratings, was read by resonant wave (at 1064 nm wavelength). The dependence of the ratio of real and imaginary parts of the resonant susceptibility of Nd:YAG at 1064 nm on the pump power has been determined through comparison of experimental results and numerical calculations.