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Optics Letters

Multiphoton absorption and cooperative upconversion excitation in Er^3+-doped fibers

Jan Thøgersen, J. Mark, Nis Bjerre

Published Feb 1, 1993• 35 citations
Publisher

Abstract

Multiphoton and cooperative upconversion excitation of Er3+ ions has been studied in two Er3+/Ge-doped optical fibers with different Er3+ concentrations by using 120-fs pulses at 1.51 μm. In the case of high Er3+ concentration it is demonstrated that the excitation of the excited levels is dominated by cooperative upconversion, whereas multiphoton absorption is mainly responsible for the excitation of the excited levels in the case of low Er3+ concentration. Lifetimes of the excited levels are measured to lie between 1 and 10 μs.

DOI
10.1364/ol.18.000197

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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.

Annual Meeting Optical Society of America•Jan 1, 1992•0 citations
Multiphoton and cooperative upconversion excitation of Er3+ ions has been studied in two Er3+/Ge-doped optical fibers with different Er3+ concentrations by using 120-fs pulses at 1.51 μm. In the case of high Er3+ concentration it is demonstrated that the excitation of the excited levels is dominated by cooperative upconversion, whereas multiphoton absorption is mainly responsible for the excitation of the excited levels in the case of low Er3+ concentration. Lifetimes of the excited levels are measured to lie between 1 and 10 μs.
Erbium-Doped Fiber Amplifiers (EDFAs) can very efficiently convert pump to signal energy. Quantum efficiencies higher than 80 % with 1.48 pm pump [1], and 93% when pumped at 980 nm [2] are reported. However, even higher values should be theoretically possible for an ideal EDFA. Various physical effects may lead to quantum efficiency (QE) reduction. Not all of them are quite well understood yet. At Erbium concentrations above 100 ppm by weight the pump loss is partly explained by energy transfer between two excited ions resulting in upcon- version process [3]. At low concentrations Excited-State Absorption (ESA) could be a more significant effect reducing QE. ESA of photons from 1.5 |im band have been predicted from absorption measurements [4] and also observed [5]. However, to our knowledge, no quantitative analysis has been made which would distinguish between the contributions of ESA and two-particle upconversion when measurements of short wavelength luminescence were performed.
The influence of spatial migration of excitation among neighboring Er3+ ions on the dynamics of population grating formation in Er-doped optical fibers is reported. The effect manifests itself in an additional increment in the grating formation rate compared with the fluorescence growth rate observed for the same spatially uniform average light power. The experiments, performed at lambda=1549 nm in the configuration of transient two-wave mixing with two similar single-mode Er-doped fibers with significantly different erbium concentrations (approximately 640 and approximately 5600 parts in 10(6)), demonstrated the essential contribution of this effect to the grating formation rate in the latter fiber. The evaluated diffusion coefficient proved to be approximately 2.3 x 10(-9) cm2/s, which ensures effective migration of the excitation by approximately 48 nm, i.e., by approximately 18 average inter-ion distances in this fiber.
"Experimental results on formation dynamics of population gratings in erbium-doped fiber (EDF) investigated via two-wave mixing (TWM) are reported. It is shown that at all three utilized essentially different recording wavelengths 1492, 1526 and 1568nm the transient TWM response in EDF with low erbium concentration (¼ 640 ppm) has the characteristic rate significantly larger than that of the population of the Er3+ meta-stable level evaluated from the fluorescence dynamics. Additionally in EDF with high erbium concentration (¼ 5600 ppm) the TWM response is faster (even at low recording power) and the relative TWM amplitude is lower than that in EDF with low concentration. General increase of the TWM response rate is explained by contribution of a transient decrease of the fiber average absorption level, while above-mentioned differences observed in heavily doped EDF - by spatial migration of the excited states. The spectral dependence of the later effect is attributed to the difference in the absorption/emission cross-section ratio of the Er3+ ions at the utilized wavelengths."
We report measurements of the metastable state lifetime in a high-concentration erbium-doped fiber. We use a single-wavelength pump-probe technique that allows us to measure the average lifetime over the entire fiber length. A reduction in the lifetime of approximately 20% caused by a cooperative upconversion effect is clearly observed when the input power is increased up to 10 mW. The input power dependence of the lifetime is analyzed using a simple model. The cooperative upconversion coefficient is determined based on this analysis.
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.