Permanent narrow-band reflection holograms in copper-doped lithium niobate channel waveguides for optical communications

摘要

Lithium niobate is a widely used substrate material for integrated optics because of its outstanding acustooptical, electrooptical, and photorefractive properties. Holographic reflection gratings in this material may be well suited for different applications, e.g., dense wavelength division multiplexing (DWDM) or narrow-band mirrors for integrated DBR waveguide lasers. We investigate such holographic wavelength filters in copper-doped lithium niobate channel waveguides. Permanent refractive-index gratings are generated by thermal fixing of holograms. The waveguides are fabricated by successive indiffusion of titanium stripes and thin layers of copper. After high temperature recording with green light, refractive-index changes exceeding Δn = 8Xl0~(-5) for infrared light (1550 nm) appear without the need of an additional development process. We believe that these strong refractive-index modulations most likely do not originate from the photorefractive effect but from material changes. The gratings are stable in the dark for at least one year without degradation, and no compensation mechanism via dark conductivity is observed.