Linearizing response of adaptive interferometer based on dynamic grating in erbium-doped fiber with saturable absorption

Abstract

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 ($\ll\lambda $/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 $\pi $/2 phase jump in the detected wave makes the system response linear during the transient time of the grating formation $\tau $$_{g}$ (typically of a millisecond-submillisecond scale in Er-doped fibers). In the continuous technique, the introduction of a permanent frequency off-set $\Delta \Omega $$_{0} \approx $ $\tau $$_{g}^{ - 1}$ into the detected wave linearizes the system response permanently.