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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Three-Ion Upconversion Processes Evidenced in the Kinetics of 4/11/2 Level in ErAG

Serban Paul Georgescu, Voicu Lupei, Thomas J. Glynn, R. J. Sheriock

Published Jan 1, 1998• 0 citations
Publisher

Abstract

A detailed analysis of the non-exponential behaviour of the kinetics of the initial laser level for three-micron generation 4I11/2, in ErAG (Erbium Aluminium Garnet) crystals (laser transition 4I11/2 →4I13/2), excited in the 4H11/2 level with short laser pulses of variable intensity, is performed. The observed non-exponentiality of the 4I11/2 decay and its dependence on the energy of the exciting pulse cannot be satisfactorily described with a rate equation modelling which includes only two-ion energy transfer processes among Er3+ ions as up-conversion from the terminal laser level (4I13/2), up-conversion from the initial one. and cross-relaxation from 4I9/2 and 4S3/2 (thermalised, at room temperature, with 2H11/2) Our analysis has shown that the inclusion of three-ion up-conversion processes from both initial and terminal three-micron laser levels leads to an excellent agreement between experimental and theoretical non-exponentiality of the 4I11/2 decay for various pumping intensities and allows the estimation of the three-ion up-conversion rates The best agreement with the experimental non-exponentialities was obtained ω111 = ω222 = 2 x 10-34 cm6s-1 and q2 = 0.9, where ω111 and ω222 represent the rates of three-ion upconversion from 4I13/2 and 4I11/2, respectively, and q2 is the ratio o two-ion up-conversio rates.

DOI
10.1364/cleo_europe.1998.cwf33

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A detailed analysis of the non-exponential behaviour of the kinetics of the initial laser level for three-micron generation 4I11/2, in ErAG (Erbium Aluminium Garnet) crystals (laser transition 4I11/2 →4I13/2), excited in the 4H11/2 level with short laser pulses of variable intensity, is performed. The observed non-exponentiality of the 4I11/2 decay and its dependence on the energy of the exciting pulse cannot be satisfactorily described with a rate equation modelling which includes only two-ion energy transfer processes among Er3+ ions as up-conversion from the terminal laser level (4I13/2), up-conversion from the initial one. and cross-relaxation from 4I9/2 and 4S3/2 (thermalised, at room temperature, with 2H11/2) Our analysis has shown that the inclusion of three-ion up-conversion processes from both initial and terminal three-micron laser levels leads to an excellent agreement between experimental and theoretical non-exponentiality of the 4I11/2 decay for various pumping intensities and allows the estimation of the three-ion up-conversion rates The best agreement with the experimental non-exponentialities was obtained ω111 = ω222 = 2 x 10-34 cm6s-1 and q2 = 0.9, where ω111 and ω222 represent the rates of three-ion upconversion from 4I13/2 and 4I11/2, respectively, and q2 is the ratio o two-ion up-conversio rates.
Yttrium Aluminum Garnet (YAG, Y_3 〖Al〗_5 O_12) crystals doped with Erbium have posed an interesting position in the field of rare earth solid state lasers as they possess the property of self-saturation, in which the upper energy level in a laser has a much shorter lifetime than that of the lower level. This is the case of the 2.94μm transition in Er:YAG. That property at first seems to make this material unfit for use as a laser gain medium, however further research into these classes of rare earth materials revealed interesting non-liner energy transfer mechanisms that allow it to be a useful mid wave infra-red (MWIR) coherent source in the continuous wave (CW) and pulsed regimes. This came to fruition due to decades of modeling and spectroscopic research investigating the non-linear energy transfer mechanisms of excited state absorption (ESA), energy transfer up-conversion (ETU), and cross relaxation (XR). These effects show up in the rise and fall times of the levels observed through fluorescence. Resulting in the non-exponential rise and fall characteristics and having a squared or even cubed relation to the population. The population evolution for each of the lasing levels is now affected by ions recycling energy and in turn cause the lifetimes of the levels to “effectively” change to a point where simple linear models do not adequately describe the system and its performance. The non-linear energy transfer dynamics of Er:YAG are modeled under high resonant pump conditions. By pumping with selective resonant pumps, the interaction dynamics of the (_ ^4)I_(11/2) and the (_ ^4)I_(13/2) levels in the Erbium ion reveal the contributions from generally ignored non-linear energy transfer mechanisms. Specifically, the multi-photon effect known as excited state absorption (ESA) is modeled by measuring and characterizing the cross section in single crystal Er:YAG samples utilizing a pump and probe technique coupled with transient fluorescence measurements. The measured ESA cross section is then included in the rate equation modeling.
Yttrium Aluminum Garnet (YAG) crystals doped with Erbium have posed an interesting position in the field of rare earth solid state lasers as they possess the property of self-saturation, in which the upper energy level in a laser has a much shorter lifetime than that of the lower level.The non-linear energy transfer dynamics of Er:YAG are modeled under high resonant pump conditions. Specifically the multi-photon effect known as excited state absorption (ESA) is modeled by measuring the cross section in a single crystal Er:YAG sample utilizing a pump and probe technique. The measured ESA cross section is then included in the rate equation modeling that has typically been ignored such an effect when modeling the non-linear ion to ion interactions. By pumping with selective resonant pumps, the interaction dynamics of the upper and the lower laser levels in the Erbium ion reveal the contributions of the non-linear energy transfer mechanisms towards the populations of the levels respectively. This thesis work shows the added accuracy of the rate equation modeling by its inclusion of the measured ESA cross-section and its effect on the population evolution in the Er:YAG system. The model will be tested with measurements made with select Er:YAG crystals being pumped by combinations of resonant infra-red lasers with a chopper spinning at a rate on the order of magnitude of the lower lasing level lifetime.
In this work, we have investigated experimentally and theoretically the process of frequency upconversion at low temperatures in rare earth doped vitreous samples. A sample of chalcogenide glass doped with erbium ions (Er3+) was pumped with a laser source at 1.54 μm. We have observed, at room temperature, three different emission bands around 530 nm, 555 nm and 670 nm, originating from the excited state erbium levels 2H11/2, 4S3/2 and 4F9/2 to the 4I15/2 ground-state, respectively. A cryogenic system cooled the sample down to 4 K, and we have recorded many spectra in this range of temperature. We have observed relative increase in the intensity of the green fluorescence, which comes from the level 4S3/2, and also the vanishing emission of the 530 nm line around 210 K. For all emission lines, the frequency upconversion process has been accounted for with three photons absorbed from the pumping source, but saturation effects were observed above 10 mW of incident power. We have made a theoretical treatment of the experimental data and a numeric simulation, which has revealed that the observed behaviors are related to processes that involve step-wise photon absorption and phonon assisted nonradiative decay, both dependent on the temperature and phonon energy of the sample. We also present, in the third chapter, an analytic treatment of pulse propagation in nonlinear media. We have used a variational approach to predict the behavior of an optical pulse propagating in the vicinity of a two-photon resonance, where the third order nonlinearities may be enhanced and limits optical devices development. A possible solution for the problem is the propagation of a pulse with suitable profile and frequency chirp resulting in a balancing of the nonlinear effects.