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Applied Optics and Photonics China (AOPC)

Study on time-frequency characteristics of transient response of the dynamic gratings in erbium-doped fiber

Pan Xu, Zhengliang Hu, Peng Gan, HU Yong-ming, Junjie Wang

Published Oct 24, 2017• 0 citations
Publisher

Abstract

The time-frequency characteristics of the transient response of dynamic gratings in erbium-doped fiber are different from stable gratings, which make it has great application value in the fields of fiber laser and optical fiber sensing. In consideration of the time domain characteristics, a time domain response model was built, which was based on metastable Er3+ population rate equation and light field transmission equation. The model simulated the process of establishing dynamic gratings in erbium-doped fiber with time sequence. And the theoretical results showed that the formation time of the dynamic grating gets shorter with higher pump power and shorter Erbium-doped fiber length within the effective length limits, which was consistent with the experimental results obtained by the so called “Strong light and weak light alternation” method except that the measured formation time decay ranges obviously migrated to lower pump power relative to the theoretical results. In consideration of the frequency domain characteristics, the transient two-wave mixing process was approximately simulated by using four-wave mixing equation. The transient reflectance spectra obtained through numerical calculation reflected the evolution rule of the dynamic gratings, which were coincident with the experimental results gotten by the rapid optical frequency modulation method. Under the condition of 1.8m Erbium-doped fiber, the experimental results showed that the turning saturated pump light power from the absorption type dynamic gratings to the gain type ones is about 0.05mW, the maximum relative reflectance variation of the gain dynamic gratings (3%) is 3 times of the absorption types. The measurement results have important references to the practical application of Erbium-doped fiber dynamic gratings.

DOI
10.1117/12.2285331

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Optics Letters•Jun 15, 2004•37 citations
The time-frequency characteristics of the transient response of dynamic gratings in erbium-doped fiber are different from stable gratings, which make it has great application value in the fields of fiber laser and optical fiber sensing. In consideration of the time domain characteristics, a time domain response model was built, which was based on metastable Er3+ population rate equation and light field transmission equation. The model simulated the process of establishing dynamic gratings in erbium-doped fiber with time sequence. And the theoretical results showed that the formation time of the dynamic grating gets shorter with higher pump power and shorter Erbium-doped fiber length within the effective length limits, which was consistent with the experimental results obtained by the so called “Strong light and weak light alternation” method except that the measured formation time decay ranges obviously migrated to lower pump power relative to the theoretical results. In consideration of the frequency domain characteristics, the transient two-wave mixing process was approximately simulated by using four-wave mixing equation. The transient reflectance spectra obtained through numerical calculation reflected the evolution rule of the dynamic gratings, which were coincident with the experimental results gotten by the rapid optical frequency modulation method. Under the condition of 1.8m Erbium-doped fiber, the experimental results showed that the turning saturated pump light power from the absorption type dynamic gratings to the gain type ones is about 0.05mW, the maximum relative reflectance variation of the gain dynamic gratings (3%) is 3 times of the absorption types. The measurement results have important references to the practical application of Erbium-doped fiber dynamic gratings.
"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 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.
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.
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 Dynamic population gratings formed by two‐wave mixing in an erbium‐doped fiber amplifier are reported. One of the counter‐propagating fields is intensity‐modulated to distinguish reflection from the grating from amplified transmission fields. The modulation‐frequency dependence of the reflection is studied under single‐wavelength and two‐wavelength input conditions to separate reflection from transient gain. The results are analyzed by measuring the gain saturation and recovery times using self‐probe and pump–probe techniques.
We present experimental results of two-wave mixing in single-mode Er-doped optical fibers for which dynamic Bragg reflectance gratings are formed as a result of saturation of fiber-optic absorption (i.e., by means of the effect of spatial hole burning). The gratings are probed by the same recording waves at lambda approximately = 1549 nm and are detected as periodic changes of the intensity of light reflected from a Sagnac interferometer (with a piece of the doped fiber included) observed when periodic phase modulation is induced in one of the waves. Both rectangular and sinusoidal modulation were used, which permitted evaluation of the grating recording time (tau(g) approximately = 3 ms for OFS-Fitel EDF-HG980 fiber) and the grating amplitude, which proved to be approximately 6-7 times lower than expected from measurements of saturation of fiber-optic absorption by one wave only.