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Quantum Electronics

Changes in the refractive index of an Nd:YAG laser crystal on excitation of the Nd3+ions

Oleg L. Antipov, A.S. Kuzhelev, A. Yu. Luk’yanov, A.P. Zinov'ev

Published Oct 31, 1998• 18 citations
Publisher

Abstract

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 .

DOI
10.1070/qe1998v028n10abeh001345

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Quantum Electronics•Oct 31, 2005•1 citation
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.
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 .
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.
It is shown experimentally that a weak optical signal in a Nd:YAG amplifier can have an extraordinarily large gain that is due to transient coherent nonlinear interaction with a strong laser beam. The effect is explained by the diffraction of the strong beam on the dynamic refractive-index grating induced in the interference field of the interacting optical waves and caused by the difference of polarizability of the excited and the unexcited Nd3+ ions. The numerical calculation confirmed the energy transfer from the strong beam to the weak one owing to the predominance of diffraction efficiency of the refractive-index grating over the gain grating in an inverted laser crystal. The investigations give the estimation of the ratio of the real and the imaginary parts of the resonant susceptibility of Nd:YAG at 1064 nm in the range 2–3.
Two optical waves (at 1064 nm) were used to resonantly induce a population grating in a flash-lamp-pumped Nd:YAG rod, and a nonresonant wave (at 1054 or 694 nm) was used to read (with a diffraction efficiency of as much as 1% or 0.3%, respectively) the refractive-index grating caused by the difference in the polarizability of the excited and the unexcited Nd ions. Comparisons of experimental results and numerical calculations gave the dependence of the ratio of the real and the imaginary parts of the resonant susceptibility of Nd:YAG at 1064 nm on the pump power.