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Optics Letters

Nondegenerate four-wave-mixing measurements of a resonantly induced refractive-index grating in a Nd:YAG amplifier

Oleg L. Antipov, A.S. Kuzhelev, Dmitry V. Chausov

Published Mar 15, 1998• 8 citations
Publisher

Abstract

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.

DOI
10.1364/ol.23.000448

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Nondegenerate four-wave mixing by resonantly induced refractive-index grating in inverted Nd:YAG crystal

Oleg L. Antipov, A.S. Kuzhelev, Dmitry V. Chausov

Summary form only given. The authors experimental and theoretical investigations confirmed existence of refractive index grating (RIG) accompanied by population grating in inverted Nd:YAG laser. This result is in good agreement with our recent investigations of degenerate two-wave mixing and stimulated resonant backscattering of the optical beam in Nd:YAG laser amplifiers. The RIG in inverted Nd:YAG must be taken into account for understanding the nonlinear interactions of optical waves in laser amplifiers and oscillators.

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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 .

Quantum Electronics•Oct 31, 1998•18 citations
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.
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.
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.
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.
Summary form only given. The authors experimental and theoretical investigations confirmed existence of refractive index grating (RIG) accompanied by population grating in inverted Nd:YAG laser. This result is in good agreement with our recent investigations of degenerate two-wave mixing and stimulated resonant backscattering of the optical beam in Nd:YAG laser amplifiers. The RIG in inverted Nd:YAG must be taken into account for understanding the nonlinear interactions of optical waves in laser amplifiers and oscillators.
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.
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 .