Polarization-independent two-channel MUX/DEMUX using a photorefractive lithium niobate crystal

Abstract

We present the design and construction of a two-channel MUX/DEMUX at 1310 nm with a channel spacing of 0.8 nm, using a photorefractive LiNbO3: Fe crystal. To achieve polarization independence, four holographic gratings are superimposed in the crystal, two per wavelength, in order to satisfy the Bragg condition for both polarization-modes of the birefringent LiNbOs. Theoretical studies investigating the feasibility of matched diffraction efficiencies for each polarization component are presented. The gratings are written in transmission geometry with the 514 nm line of an Argon-ion laser; the infrared readout is carried out in reflection geometry. Thermal fixing is used to achieve non-destructive readout, and the results of several modified fixing procedures are compared. Preliminary experimental results with a HeNe readout beam in transmission geometry are discussed. Fixed diffraction efficiencies of about 50% per channel for the superposition of 4 gratings in a 0.05 wt.% iron-doped LiNbO3 crystal are achieved. We describe similar experiments in a 0.1 wt.% iron-doped LiNbO3 and a BaTiO3 crystal for comparison purposes. An active phase stabilization setup is incorporated in order to stabilize the interference pattern during the writing process.