A team at CoreCom (Milan, Italy) has demonstrated how volume holograms recorded in lithium niobate could be used in telecommunications systems. Their technique uses holography`s inherent and very strong wavelength selectivity to demultiplex signals in the 1550-nm spectral window, while at the same time storing patterns that can be useful for the signal`s onward journey through the network.
Iron-doped lithium niobate (LiNbO3:Fe) is a good photorefractive material but not sensitive in the near-infrared. Researchers on the Italian team realized that this was not a problem if the holographic gratings were designed carefully. By using a technique called the two-lambda (or dual-wavelength) method, they were able to work the problem through backwards. Given the wavelength to be diffracted and the angle it should be diffracted by, they were able to determine the required angle between the two recording beams and the crystal at the recording wavelength.
One of the requirements of this technique is that the incoming angle of the readout beam (at the readout wavelength) must have precisely the right relationship to the angle of the reference beam (at 488 nm) during recording. It turns out that the demultiplexing task is particularly suited to another technique, where the holographic medium is rotated between exposures, because this angle-of-incidence condition is wavelength-dependent.
The holograms are made for each wavelength without changing the recording geometry, but with the material rotation (change of recording angle) in between. During readout, each wavelength in the incoming WDM beam will "see" a different hologram precisely because their incident angles are identical (at different wavelengths, these would have to be different to read out the same grating). Thus, each component is diffracted by a different angle.
In another experiment, the CoreCom team recorded 8-bit words-patterns of light and dark stripes encoded using transparencies-into the holograms (see figure). These might be used, for instance, to separate each signal into a few identical channels: either to allow narrowcasting or to build redundancy into the network. By using a rotation of 0.03° between exposures, they were able to read out components spaced 0.8 nm apart, thus conforming to a common WDM standard (ITU). For more information, contact C. Ubaldi at firstname.lastname@example.org.
1. P. Boffi et al., IEEE Phot. Tech. Lett. 12 (10), 1355 (Oct. 2000).
Sunny Bains is a scientist and journalist based in London, England.