Multichannel wavelength-division multiplexing with thermally fixed Bragg gratings in photorefractive lithium niobate crystals

The transmission capacity of fiber communication networks is enhanced by usage of dense wavelength division multiplexing (WDM). This technique requires wavelength filters for multiplexing of the channels. We report on the realization of a multiplexer device based on superimposed volume-phase gratings in a single lithium niobate crystal. The gratings are recorded through the photorefractive effect by interference of two green laser beams. Thermal fixing is employed to increase the lifetime of the gratings. Each grating diffracts light of a certain WDM channel (wavelengths of ∼1500 nm). Simultaneous multiplexing of many channels is achieved by suitable arrangement of the gratings in the crystal. We present the basic concept of this technology as well as recent advances: (1) refined experimental methods about tailored recording of many-channel multiplexers, (2) characterization of the multiplexers for up to sixteen WDM channels (1-dB bandwidth up to 0.1 nm, channel spacing down to 0.4 nm), and (3) construction of a two-channel multiplexer device.

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