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

On the influence of the excited state migration on the population gratings amplitude recorded in the transient two wave mixing configuration in Er-doped optical fibers

E. Hernández-Hernández, Liliana O. Martínez-Martínez, Serguei I. Stepanov

Published Aug 10, 2011• 0 citations
Publisher

Abstract

In this paper we analyze the influence of a spatial migration of the excited states on suppression of the population gratings recorded in erbium-doped optical fibers (EDF). Such dynamic Bragg gratings are formed via saturation of the optical transition between the fundamental and the meta-stable states of the active ions and are observed experimentally via transient two wave mixing (TWM) of the phase-modulated counter-propagating recording waves. The reported experiments were performed in the spectral range 1492-1568nm, which covers the absorption region of the fundamental transition 4I15/2→4I13/2 of Er3+ ions. Significantly stronger grating suppression is observed at the central/long-wavelength regions of the above-mentioned spectral region. A set of similar fibers with essentially different Er3+ ions concentrations with maximum attenuation of 4, 9, 17 and 35 ± 1 dB/m at 1531 nm was investigated and a clear concentration dependence of the grating suppression was observed. The TWM measurements are supported by the original experimental data on the fluorescence depolarization. The observed spectral dependence of the grating reduction factor is explained using the model of the vacant states limited spatial migration of the excitation among the Er3+ ions.

DOI
10.1117/12.902481

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Two-wave mixing in erbium-doped-fibers with spectral-hole burning at 77K

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Abstract Experimental results on effective two-wave mixing (TWM) via dynamic population gratings in saturable erbium doped fibers (EDFs) at liquid nitrogen temperature (77 K) are reported. It is shown that formation of the spectral holes (SHs) by the recording coherent waves in the wavelength region 1500–1540 nm results in a complex dynamics of the transient TWM response. In addition to conventional slow, even-type response due to amplitude grating formation via spectrally uniform EDF absorption saturation we also observed a dominating contribution due to SH formation. In particular, at the recording wavelength 1527 nm the relaxation time of the SH contribution to the TWM process was significantly shorter (< 4.7 ms) and with the effective recording power ∼4 times lower than that for conventional EDF population grating.

Journal of Optics•Jun 11, 2020•5 citations
In this paper we analyze the influence of a spatial migration of the excited states on suppression of the population gratings recorded in erbium-doped optical fibers (EDF). Such dynamic Bragg gratings are formed via saturation of the optical transition between the fundamental and the meta-stable states of the active ions and are observed experimentally via transient two wave mixing (TWM) of the phase-modulated counter-propagating recording waves. The reported experiments were performed in the spectral range 1492-1568nm, which covers the absorption region of the fundamental transition 4I15/2→4I13/2 of Er3+ ions. Significantly stronger grating suppression is observed at the central/long-wavelength regions of the above-mentioned spectral region. A set of similar fibers with essentially different Er3+ ions concentrations with maximum attenuation of 4, 9, 17 and 35 ± 1 dB/m at 1531 nm was investigated and a clear concentration dependence of the grating suppression was observed. The TWM measurements are supported by the original experimental data on the fluorescence depolarization. The observed spectral dependence of the grating reduction factor is explained using the model of the vacant states limited spatial migration of the excitation among the Er3+ ions.
"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."
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.
Detailed experimental data on the efficiency of two-wave mixing via population dynamic grating in erbium-doped fiber (EDF) with saturable optical absorption are presented. The measurements were performed for spectral region 1490–1560 nm of the EDF absorption/gain spectrum, both for absorption and for phase (refractive index) components of the dynamic grating. The reported data confirm the earlier results published for some particular recording wavelengths. In particular, the amplitude gratings recorded in EDF in the central part of this region (1520–1560 nm) are significantly less efficient (by more than 1 order of magnitude) than it can be estimated from measurements of a spatially uniform fiber absorption saturation. It is shown, however, that this effect cannot be explained by spatial migration of the Er3+ excited state. At the short-wavelength spectrum side around 1490 nm, the disagreement between the experimentally observed and the evaluated grating amplitude is minimum (∼0.3) and can, probably, be attributed to the polarization mismatch in the counterpropagating recording waves.
The contribution of the spatially uniform variation of average optical absorption to the dynamics of the transient two-wave mixing (TWM) response is considered. It is shown theoretically and confirmed experimentally that this transient effect, via dynamic population gratings in erbium-doped fibers (EDFs) can ensure a response nearly two times faster in such gratings as compared to the growth rate of fluorescence uniformly excited under similar conditions, and can also result in an additional overshot in the tail of the TWM response. This additional ‘accelerating’ contribution is of even type, and does not influence the odd transient TWM response for the refractive index component of such gratings in the EDFs reported earlier. It is also shown that this effect can be utilized to monitor the formation of the dynamic grating with an auxiliary probe wave of the essentially different non-Bragg wavelength.
We report a new, to the best of our knowledge, method for detection of dynamic population gratings in Er-doped fibers with saturable absorption that is based on observation of the transient fluorescence resulting from fast displacement of the recording interference pattern. The signal appears due to essentially different relaxation times of the fluorescence in bright and dark fringes of the light pattern. The experimental results obtained at the recording wavelength λ=1492 nm in a single-mode fiber with 5600 ppm Er demonstrate good agreement with the theoretical model based on a saturable two-level system. Unlike conventional diffraction-based techniques, the proposed method results in the local grating amplitude value and can be used for characterization of the grating amplitude spatial profile. It can also be suitable for characterization of the grating formation processes in other bulk and thin-film media with saturable absorption.
Abstract Experimental results on effective two-wave mixing (TWM) via dynamic population gratings in saturable erbium doped fibers (EDFs) at liquid nitrogen temperature (77 K) are reported. It is shown that formation of the spectral holes (SHs) by the recording coherent waves in the wavelength region 1500–1540 nm results in a complex dynamics of the transient TWM response. In addition to conventional slow, even-type response due to amplitude grating formation via spectrally uniform EDF absorption saturation we also observed a dominating contribution due to SH formation. In particular, at the recording wavelength 1527 nm the relaxation time of the SH contribution to the TWM process was significantly shorter (< 4.7 ms) and with the effective recording power ∼4 times lower than that for conventional EDF population grating.