Results of investigation of transient two-wave mixing via dynamic population gratings in Yb-doped fibers with saturable absorption are reported. The recorded gratings are characterized by submillisecond formation times and at wavelength λ = 1064 nm need cw recording light power of 1 – 10 mW scale. What is important for different applications, the dynamic gratings were found to be predominantly of a phase type with an admixture of a significantly weaker amplitude component only. As in Er-doped fibers, the amplitude grating component proved to be essentially inferior to the theoretical estimation based on the fiber optical density and saturation power.

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 $\lambda \approx 1549 \mathrm{nm}$ 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 (${\tau }_g\approx 3 \mathrm{ms}$ 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.

Sagnac fiber interferometer with the dynamic population grating formed in the rare-earth doped fiber is proposed for homodyne adaptive detection of optical phase modulation. The configuration is shown to be a simple all-optical fiber sensor suitable for linear high sensitivity detection of mechanical vibrations, acoustic signals, thermo-optic effect etc. Theoretical consideration shows that the quadratic response of this interferometric configuration associated with the amplitude dynamic grating is observed in the reflected wave mainly, while the recorded phase grating results in the linear energy exchange between the transmitted and reflected from the Sagnac loop light waves. Experiments with the erbium- and ytterbium-doped fiber based adaptive Sagnac configurations (with the operation wavelengths 1485 and 1064nm respectively) of the fiber accelerometers confirmed these general conclusions and demonstrated sensitivity of the fiber based interferometric configurations (~3*10⁻⁵ rad/Hz^1/2) governed basically by the noise of the utilized lasers.

The influence of spatial migration of excitation among neighboring ${\mathrm{Er}}^{3+}$ 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 $\lambda =1549\mathrm{nm}$ in the configuration of transient two-wave mixing with two similar single-mode Er-doped fibers with significantly different erbium concentrations ($\approx 640$ and $\approx 5600$ 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 $\approx 2.3\times {10}^{-9}{\mathrm{cm}}^2∕\mathrm{s}$, which ensures effective migration of the excitation by $\approx 48\mathrm{nm}$, i.e., by $\approx 18$ average inter-ion distances in this fiber.