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Tecnical efects in frequency upconversion of the Er3+ and pulse propagation in nonlinear media.

Oliveira, Jonathas Matias de

Published Aug 12, 2002• 0 citations

Abstract

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.

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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.

The dependence of the negative nonlinear absorption effect at 1.5μm on the sample temperature, the modulation frequency and the sample length were investigated in glasses containing amounts higher than 20mol% Er3+. Reversed-phase waveforms were observed in the temperature range -260-600°C and at modulation frequencies from 1kHz to 10GHz in an erbium-doped phosphate glass. With increasing temperature, the amplitude of the reversed-phase waveforms decreases exponentially according to two separate steps. Optical bistability in the transmitted laser was observed in the range of incident laser intensity of 30μW/cm2-2.4mW/cm2. The transmitted laser intensity propagating through the glass decreased exponentially according to the well-known exponential law. However, the amplitude of the optical bistability increased logarithmically as the sample length was increased. The negative nonlinear absorption effect can be explained by considering an enhanced absorption model for a four-level system of the Er3+ ion.

Here we describe a class of cw upconversion lasers in which inversion is due exclusively to cooperative interactions of rare earth ions in highly doped solids, resulting in unique opportunities for output wavelength agility, large upconversion factors, and high quantum efficiencies through nonlinear dynamics. We have isolated capabilities of the cooperative mechanism by performing experiments in Er:CaF2 and Er:LiYF4 with infrared pumping of the first Er3+ resonance at 1.53 μm. At liquid helium temperatures, laser action at 544 nm was achieved in an astigmatically compensated, three-mirror cavity with 5% efficiency in LiYF4. The upconversion factor (ratio of pump to laser wavelength) was 2.81, to our knowledge the highest ever reported for cw upconversion lasers. At 77 K, laser action at 551 nm was observed. Surprisingly, output at both 544 and 551 nm was truly cw with cooperative pumping, unlike earlier green erbium lasers, which exhibited self-pulsing. Also at 77 K, near infrared laser action was obtained at 0.855 pm with exceptionally high efficiency in CaF2(28%) and LiYF4 (20%) on self-quenching transitions. Successful demonstration of cw laser action on such transitions also suggests a means whereby quantum efficiencies may exceed unity. The lower laser level in such cases is the origin of cooperative pumping to the upper laser level, but also receives population as the result of stimulated emission. Hence population can be recycled for enhanced quantum efficiency. Additionally, population pulsations can occur and sustained oscillations in laser output have been observed in a cooperative erbium upconversion laser at 2.8 μm.

Erbium-doper fiber (EDF) is a flexible and promising model medium for investigation of the slow/fast light propagation in saturable optical materials. The experiments are usually performed in the spectral range 1480-1570 nm of the absorption/gain of Er3+ ions using the input power of a sub-mW scale. Conventional experimental configuration allows one to observe, however, the input and the output pulse profiles only. We report an original nondestructive technique for observation of a spatial propagation of the pulses via observation of the transient fluorescence excited by the propagating light-pulses at the fiber side, from which we are able to reconstruct how does the fractional delay and the amplitude of the propagating pulses change along the fiber. Results of a numerical simulation of the nonlinear pulse propagation performed for a saturable two-level medium in low contrast approximation proved to be in a reasonable agreement with the experimental observations.

Dependence of the negative nonlinear absorption effect on both modulation frequency and sample temperature was investigated in an erbium yttrium aluminum garnet crystal. The phase of transmitted laser was opposite to that of the incident laser at modulation frequencies lower than 1 MHz. However, the transmitted laser waveform was asymmetrical above 5 MHz and the time-dependence characteristics agreed with the lifetime of the 4S3/2 level in Er3+. At temperatures lower than 25 K, enhanced absorption was weakened because the number of phonons decreased due to low temperature. It is suggested that the negative nonlinear absorption effect occurred due to enhanced absorption in three excited states. © 1998 Scripta Technica. Electron Comm Jpn Pt 2, 80(6): 43–48, 1997

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.

We describe a general numerical method for calculating short-pulse laser propagation in rare-earth-doped materials, which are very important as gain materials for solid-state lasers, fiber lasers and optical amplifiers. The split-step, finite difference method simultaneously calculates changes in the laser pulse as it propagates through the material and calculates the dynamic populations of the rare-earth energy levels at any position within the material and for times during and after the laser pulse has passed through the material. Many traditional theoretical and numerical analyses of laser pulse propagation involve approximations and assumptions that limit their applicability to a narrow range of problems. Our numerical method, however, is more comprehensive and includes the processes of single- and multi-photon absorption, excited state absorption (ESA), energy transfer, upconversion, stimulated emission, cross relaxation, radiative relaxation and non-radiative relaxation. In the models, the rare-earth dopants can have an arbitrary number of energy levels. We are able to calculate the electron population density of every electronic level as a function of, for example, pulse energy, dopant concentration and sample thickness. We compare our theoretical results to published experimental results for rare-earth ions such as Er3+, Yb3+, Tm3+ and Ho3+.