A novel WDM optical filter based on volume holographic gratings recorded in transmissive-writing and orthogonal-readout scheme

Using transmissive-writing and orthogonal-readout scheme, a dual-channel wavelength division multiplexer on the bases of two volume holographic gratings is demonstrated for the first time. The two gratings locate in different areas of a piece of doubly doped LiNbO3 photorefractive crystal, so that each grating can reach the maximal diffraction efficiency of the crystal. The gratings are recorded in the crystal by using green laser. During the readout, the input infrared light is incident on a surface of the material perpendicularly, and the light beam diffracted by one of the two channels emerge from a surface adjacent the input one. The two diffracted beams both propagate normally to the output surface. This makes it easier to couple the communication light beams into and out of the device. The filter works at the third window of optical communication with a channel space of 0.72 nm.