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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Cooperation effect of radiation in Nd:YAP

Michael P. Koselja, Jiří Kubelka, Jiří Kvapil

Published Jun 17, 1993• 0 citations
Publisher

Abstract

Behavior of radiation of Nd:YAP and Nd:YAG in a resonator with extremely low Q-value have been studied. The light pulses of the form discrete spikes around 1.08 micrometers have been observed using a relatively low pumping energies in Nd:YAP. The formation of these pulses has threshold character. Additional measurement such as second harmonic generation, far field distribution and spectral behavior have shown that emitted pulses have a typical laser character. Derived value of threshold gain parameter with material losses (g0 - (alpha) ) for threshold energy is 0.581 cm-1. No similar effect was observed for Nd:YAG, even for four timeshigher value of threshold energy than Nd:YAP. Obtained results demonstrate the cooperative effects of Nd3+ ions in Nd:YAP through a correlation between electromagnetic and crystal field.

DOI
10.1117/12.146886

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Behavior of radiation of Nd:YAP and Nd:YAG in a resonator with extremely low Q-value have been studied. The light pulses of the form discrete spikes around 1.08 micrometers have been observed using a relatively low pumping energies in Nd:YAP. The formation of these pulses has threshold character. Additional measurement such as second harmonic generation, far field distribution and spectral behavior have shown that emitted pulses have a typical laser character. Derived value of threshold gain parameter with material losses (g0 - (alpha) ) for threshold energy is 0.581 cm-1. No similar effect was observed for Nd:YAG, even for four timeshigher value of threshold energy than Nd:YAP. Obtained results demonstrate the cooperative effects of Nd3+ ions in Nd:YAP through a correlation between electromagnetic and crystal field.
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.
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 .
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 report the observation of coherent emission from three rare-earth ions in electro-optic crystals. Oscillation is observed from Nd3+ ions in Bi4Ge3O12, LiNbO3, and Ba0.75Ca0.25Nb26O at room temperature, and from Tm3+ and Ho3+ ions in LiNbO3 at 77°K. Second-harmonic radiation is obtained from LiNbO3:Tm3+ and electric-field modulation of threshold is observed for certain crystals of LiNbO3:Nd3+. The latter effect is believed to arise from crystal strain. The propagation angle for phase-matched second-harmonic generation is calculated to lie at 43° to the c axis for both LiNbO3:Tm3+ and LiNbO3:Ho3+ at 77°K. Phasematched second-harmonic generation from LiNbO3:Nd3+ is not possible, since the maser line is an extraordinary wave. However, the polarization of the maser line in LiNbO3:Nd3+ is appropriate for phasematched parametric frequency conversion in the near infrared. Attempts to observe such radiation were not successful due to the limited power available in the fundamental.
Abstract YAlO 3 (YAP) shows lower phonon energy than Y 3 Al 5 O 12 (YAG), which implies a lower temperature can be obtained in Yb 3+ -doped YAP according to the single-frequency phonon model. Here, a 2% Yb 3+ : YAP crystal was refrigerated by anti-Stokes fluorescence and compared to YAG crystal with the same doping concentration in experiments. Yb 3+ exhibited a broad spectral cooling range in YAP from 1020nm to 1070nm, which was broader than in YAG from 1020 nm to 1055 nm. A minimum achievable temperature (MAT) dropped to ~ 249.7 K at 1040 nm for Yb 3+ : YAP, which was lower than that of ~ 259.3 K obtained in 2% Yb 3+ : YAG. Further analysis predicted that the cooling limit of two crystals can reach ~167 K for Yb 3+ : YAP and ~171 K for Yb 3+ : YAG from room temperature. This work exhibited the excellent cooling properties of Yb 3+ : YAP compared with Yb 3+ : YAG in experiment and provided a unique perspective for the practical application of high-power Radiation balance lasers (RBLs).