Integrated optical proton exchanged TM-pass polarizers in LiNbO3: modelling and experimental performance

Integrated optical TM-pass polarizers operating at wavelengths around λ=1.5 μm have been realized by introducing proton exchanged regions adjacent to a Ti-indiffused waveguide on X-cut, Y-propagating lithium niobate. Structural investigations of the proton exchanged regions have been carried out by raster electron microscope and optical methods, in order to characterize the index profile created by the proton exchange. Several polarizers with different geometries of the proton exchanged regions have been analysed numerically by employing the beam propagation method (BPM) and the finite element method (FEM). The device performance is shown to depend strongly on the geometrical shape of the outer boundaries of the proton exchanged regions and on their distance to the channel waveguide. Experimental results are given for several samples with different gaps between the optical waveguide and the proton exchanged regions as well as for different annealing times. For a proper device design a TE-extinction of -26 dB and a TM-excess loss of only 1.2 dB have been obtained experimentally