Holographic Reflection Filters in Photorefractive LiNbO3 Channel Waveguides

Abstract. Permanent refractive-index gratings in waveguide devices are of consid-erable interest for optical communication systems that make use of the high spec-tral selectivity of holographic filters, e.g., dense wavelength division multiplexing (DWDM) or narrow-bandwidth mirrors for integrated waveguide lasers in LiNbO3. Other possible applications include grating couplers and optical sensors. In this con-tribution we investigate such holographic wavelength filters in Fe- and Cu-doped LiNbO3 channel waveguides. Permanent refractive-index gratings are generated by thermal fixing of holograms in the waveguides. The samples are fabricated by suc-cessive in-diffusion of Ti stripes and thin layers of either Fe or Cu. After high temperature recording with green light, refractive-index changes up to ∆n ≈ 10−4 for infrared light (1.55µm) are obtained, resulting in a reflection efficiency well above 99 % for a 15mm-long grating. Several gratings for different wavelengths can be superimposed in the same sample, which may enable more complex filters, laser mirrors or optical sensors. By changing the sample temperature the reflection wave-length can be tuned by thermal expansion of the grating, and wavelength filters can be switched on and off by applying moderate voltages using the electrooptic effect. Furthermore, we report on a new thermal fixing mechanism that does not need any additional development by homogeneous light illumination and therefore does not suffer from the non-vanishing dark conductivity of the material.