Improvement of the wavelength selectivity of volume holographic gratings for optical communication

The wavelength selectivity of volume holographic gratings with different size ratios is presented. The gratings are made in a doubly doped LiNbO3 photorefractive crystal by using the transmissive-writing and orthogonal-readout scheme. The gratings are recorded at the short wavelength of 532 nm and read out at the long communication wavelength, which is tunable between 1520 and 1630 nm. It is demonstrated that the wavelength selectivity of gratings can be improved by choosing larger grating size ratios. A 3 dB bandwidth of 0.43 nm is achieved with the grating size ratio of 2.5. The experimental results agree well with the prediction of two-dimensional coupled-wave theory.