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

摘要

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 LiNbO_3: 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 LiNbO_3. 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 LiNbO_3 crystal are achieved. We describe similar experiments in a 0.1 wt.% iron-doped LiNbO_3 and a BaTiO_3 crystal for comparison purposes. An active phase stabilization setup is incorporated in order to stabilize the interference pattern during the writing process.