Integrated optical proton exchanged TM-pass polarizers in LiNbO/sub 3/: modelling and experimental performance

Abstract

Integrated optical TM-pass polarizers operating at wavelengths around X =1.5 pm 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.

Photonic applications of lithium niobate crystals

“…(b) Linear polarizers can also be implemented using the anisotropic index changes produced by proton exchange. The device presented by Hemppelmann et al [67], and reproduced in Fig. 8, works by coupling away the TE polarized light propagating in a Ti waveguide by means of coupling to PE lateral regions, in a X-cut sample with Y propagation.…”

Electric Field Poling of Lithium Niobate Crystals after Proton-Exchanged Channel Waveguide Fabrication

“…PE is usually carried out in a molten benzoic acid in a closed reactor with a subsequent temperature annealing. The PE corresponds to a substitution of lithium ions in LN crystal lattice for hydrogen ions from the melt (proton source) [4,5]. As a result of a subsequent annealing, α phase with the same symmetry as LN is forming at the surface [6].…”

Untitled

“…Among the variety of proposals are overlays of strongly absorbing, metallic (Sletten and Seshadri 1990;Saini et al 1995) or amorphous (Veasey et al 1991(Veasey et al , 1994 materials, the inclusion of metallic thin ®lms (Johnstone et al 1990;Thyagarajan et al 1990;Nakano et al 1994), or thin composite, highly dichromatic metallic layers (Bloemer andHaus 1992a, b, 1996), and highly refracting or birefringent transparent overlays (Findakly et al 1983;Thyagarajan et al 1991;Pe rez et al 1999). Alternatively, the overlays can be substituted by parallel waveguide segments (Hempelmann et al 1995;Oh et al 1996). A number of interferometric concepts for polarization splitters employ directional couplers (Han et al 1990;Bersiner et al 1991), Mach±Zehnder interferometers (Shani et al 1990;Han et al 1991), other multiwaveguide geometries (Thyagarajan and Pilevar 1992;Trutschel et al 1995;Lohmeyer et al 1999b), or recently a birefringent Y -junction (Lee et al 1999).…”