Experiments on transverse detection of transient fluorescence excited by oscillating interference pattern in Er-doped fiber in spectral range 1480-1600 nm are reported. This technique is used for evaluation of local amplitude of the recorded population grating.
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.
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."