Wavelength selectivity of the complex grating structure formed in a photorefractive phase conjugator

Experimental results show that the wavelength selectivity and the shape of the detuning curve of the complex grating structure formed in a photorefractive self-pumped BaTiO3 phase conjugator depend strongly on the coherence length of the incident beam. It is observed that the wavelength selectivity becomes larger as the coherence length of the incident beam is decreased. With a coherence length much longer than the dimensions of the conjugator crystal a wavelength selectivity of 0.026 nm is measured. It is shown that this very small wavelength selectivity and the experimental detuning curves are in agreement with a simple model that takes into account the interference between light reflected in 2k gratings and light that has traveled one round trip in the internal loop of the conjugator crystal. The findings explain how the optical feedback generated by the dynamic gratings in the phase conjugator can increase the coherence length of a multimode laser.