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Quantum Electronics

On the role of thermal nonlinearity in degenerate interactions in saturated laser media

V V Tumorin, Nikolai N Il'ichev

Published Sep 30, 2007• 1 citation
Publisher

Abstract

The production of the temperature grating of the refractive-index variation upon recording a gain grating in solid laser media is considered. The dependence between the amplitudes of the temperature grating and saturated-gain grating is found. Based on the dependence obtained, the phase component of the inverse-population grating in a Nd:YAlO 3 crystal is determined.

DOI
10.1070/qe2007v037n09abeh013476

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Quantum Electronics•Oct 31, 1998•18 citations
The production of the temperature grating of the refractive-index variation upon recording a gain grating in solid laser media is considered. The dependence between the amplitudes of the temperature grating and saturated-gain grating is found. Based on the dependence obtained, the phase component of the inverse-population grating in a Nd:YAlO 3 crystal is determined.
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.
Nonlinear optics normally involves the change in the susceptibility of a material due to the presence of optical radiation. In many cases, the light-matter interaction produces a change in the refractive index of the medium, as occurs in photorefractive media, as well as in Kerr-like media, Brillouin-active media and absorbing media with thermallyinduced refractive index changes. Such nonlinear refractive index variation corresponds to a change in the real part of the optical susceptibility %. In resonant materials, the optical radiation induces stimulated emission (or absorption) causing transfer of population between different energy states of the medium. This can lead to saturation of the amplification (or absorption) corresponding to changes in the imaginary part of the susceptibility x= i2odk where the amplitude gain coefficient a(I) is a saturable function of the intensity /, and k is the magnitude of the wavevector of the optical radiation. The interference of two (or more) coherent beams I(x) in the laser amplifier can cause a modulation of the population inversion N(x). This modulation of the inversion is commonly known as spatial hole burning in lasers and leads to formation of a gain-grating a(x) = 1/2GN(X) where G is the stimulated emission cross-section. The gain grating can act as a very efficient diffractive optical element exhibiting high diffraction efficiency that may be even greater than unity [16] and also very high phase conjugate reflectivity using four-wave mixing (FWM) geometries [5-7].
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.
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 .