Permanent refractive-index gratings in waveguide devices are of high potential for optical communication systems that make use of the high spectral selectivity of holographic filters, e.g. dense wavelength division multiplexing (DWDM), optical sensors, or narrow-bandwidth mirrors for integrated waveguide lasers. In this contribution we review our results on holographically recorded refractive-index gratings in Cu-doped LiNbO 3 channel waveguides. Elementary holograms are recorded with green light and read in reflection geometry in the insensitive infrared wavelength region around 1.5 µm. To enable long-term stability of the Bragg gratings a thermal fixing technique is applied. In this way strong and almost permanent refractive-index gratings are obtained and their application as narrow-bandwidth filters for DWDM applications is demonstrated. In comparison with Bragg gratings in silica fibres, the electro-optic effect in LiNbO 3 allows for a direct wavelength tuning and a fast, reliable electrical switching of these gratings.

Lithium niobate volume crystals ͑up to 1 mm thick͒ are doped by indiffusion of thin layers of evaporated copper. The obtained samples are investigated by conventional electrical and holographic methods. Saturation values ⌬n s of the refractive-index changes, bulk-photovoltaic current densities j phv , and photoconductivities ph are measured utilizing samples with copper concentrations in the range (2.2Ϫ145)ϫ10 24 m -3 . Comparison with experimental data for melt-doped lithium niobate shows that the photorefractive properties do not depend on the doping technique. Another important outcome is that the refractive-index changes saturate at 7 ϫ10 Ϫ4 ͑ordinary light polarization, green light͒ for copper concentrations larger than 60ϫ10 24 m -3 . This is caused by a strong increase of the photoconductivity for high doping levels.

We have obtained photovoltaic lenses and dark spatial solitons in planar optical waveguides in lithium niobate doped with iron and copper. For TE modes of lower indices the photovoltaic nonlinearity only partly decreased the width of a dark notch within the outcoupled image of the recording light beam. The corresponding time to reach a steady state of this light-induced change ranged from about 0.1 to 30 s depending on the waveguide sample. For higher modes we observed a full compensation of the divergence of the dark notch on a time scale of some minutes. In some cases this was followed by an extinction of the dark solitons because the light was over-defocused in the highest modes.

We present a narrow-bandwidth interference filter in an optical monomode LiNbO>(3):Ti:Fe channel waveguide operating in the infrared wavelength region around lambda = 1.55 microm . The filter consists of a thermally fixed refractive-index Bragg grating recorded with visible green light by use of a holographic technique. In a first approach we measure a reflectivity for the infrared light of 60% and a linewidth (FWHM) of 0.11 nm.

No abstract