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.
Owing to the high Bragg selectivity and capacity of multiple recording, volume holography has been widely investigated
for devices such as wavelength filters in wavelength-division multiplexing fiber communication systems in recent years.
Transmission-Writing and Orthogonal-Readout scheme (TWOR) has been proposed for making wavelength filters in
holographic media. In this paper, experiments on wavelength selectivity of volume gratings with TWOR scheme are
successfully conducted for the first time to our knowledge. Readout by wavelength-tuning showed that a 3-dB bandwidth
of 0.5nm was achieved. The experimental results agreed well with the expectation of 2-D coupled-wave theory, and
verified the feasibility of TWOR scheme.