Amplitude of the dynamic phase gratings in saturable Er-doped fibers

Serguei I. Stepanov, Eliseo Hernández

Serguei I. Stepanov, C. Nuñez Santiago

C. M. Nascimento, Maria José Valenzuela Bell

Serguei I. Stepanov, Andrei A. Fotiadi, Patrice Mégret

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 lambda = 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.

Daniel García Casillas, Serguei I. Stepanov, Marcos Plata Sánchez

"Because of the unshifted type of recorded amplitude gratings, adaptive interferometers based on dynamic population gratings in Er-doped fibers with saturable absorption demonstrate an essentially quadratic response and, as a result, are not sensitive enough to detect a small object displacement ( < ¿/4), and cannot distinguish the displacement direction. We propose, and experimentally demonstrate, two techniques for their linearization which are based on externally introduced phase off-set between the recording interference pattern and the recorded grating. In the transient technique, the application of a fast ¼/2 phase jump in the detected wave makes the system response linear during the transient time of the grating formation T g (typically of a millisecond-submillisecond scale in Er-doped fibers)..."

Serguei I. Stepanov, E. Hernández, M. Plata

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.