Karsten Buse

Advanced wavelenth division multiplexing with thermally fixed volume-phase gratings in iron-doped lithium niobate crystals

Fiber communication networks utilize wavelength-division- multiplexing (WDM) to enhance the transmission capacity of fiber-optical networks. This technique requires narrowband wavelength filters for multiplexing and de-multiplexing of the channels. We report on realization of an advanced multiplex/demultiplex device based on superimposed volume- phase gratings in lithium-niobate crystals. The gratings are recorded via the photorefractive effect by interference of two green laser beams. Thermal fixing is employed to increase the lifetime of the recorded gratings. Infrared light in the telecommunication wavelength region around 1500 nm is diffracted from the gratings. Each grating reflects light of a certain WDM channel. The selected wavelengths and the propagation directions of the diffracted beams are determined by spatial frequency and orientation of the gratings in the crystal. We will present the basic concept of this technology as well as recent advances : (1) construction and testing of a two-channel demultiplexer prototype (fiber to fiber insertion loss 5-6 dB, crosstalk less than -25 dB, channel spacing 0.8 nm), (2) simultaneous demultiplexing of 8 channels (separation 0.8 nm).

Lifetime of polarons in lithium-niobate crystals

Pulsed illumination of lithium-niobate crystals with green light excites electrons from deep traps into the intrinsic defect NbLi5+ (Nb on Li site in the valence state 5+) and creates NbLi4+ centers (small polarons). The electrons trapped in this more shallow center increase the light absorption in the red and near-infrared. The dark decay of the polaron concentration is observed by monitoring the relaxation of these absorption changes. Iron- doped lithium-niobate crystals with different concentrations of NbLi are investigated for various illumination conditions. The relaxation shows a stretched-exponential behavior which is in disagreement with the predictions of the standard rate-equation model. The observed lifetimes of the polarons range from tens of nanoseconds to some milliseconds. Computer simulations reveal that all results can be explained considering distance-dependent excitation and recombination rates, i.e. the lifetime of an individual polaron depends on the distance to the next available deep electron trap. Based on the new insights, tailoring of lithium-niobate crystals for non-volatile holographic storage becomes possible.