Abstract

Two-wave mixing (TWM) in a photorefractive crystal (PRC) is an efficient technique for demodulating small phase changes encrypted in a speckled wave such as reflected from a rough surface [1] or emerged from a multimode optical fiber [2]. Owing to self-diffraction of the recording beams from the dynamic hologram, this technique provides automatic adaptation of their wavefronts while only fast phase transients of the object wave are transformed to the intensity modulation in beams after transmission trough the PRC depending on the time constant of the crystal [3]. A critical requirement for the TWM is the achievement of quadrature condition which allows the linear regime in the phase-to-intensity demodulation. This phase shift depends on the physical mechanism of the hologram formation. As known, diffraction from the hologram recorded in the diffusion regime (without external electric field) does not provide the quadrature condition when the input beams have the same polarization states [3]. Nevertheless, mixing in cubic PRC in the geometry of anisotropic diffraction of two waves with different polarization states (for example, linear and elliptical) results in the linear phase demodulation [4]. However, the linear phase demodulation is impossible even in the later configuration when the object beam is unpolarized, which is typical for the wave emerged from a multimode fiber.

© 2009 IEEE