Narrow-band filter using multiple photorefractive gratings

In this paper, we propose a novel high SNR tunable filter using a photorefractive single crystal fiber with multiple holographic gratings. Since the single crystal fiber which is grown by the laser heated pedestal growth (LHPG) technique can be more than 100 mm long, volume gratings with extremely large number of period can be obtained in the fiber. Thus, the wavelength selectivity of the grating can be very high on the order of 10⁵ so that one can achieve very narrow band filter (<0.01 nm). To increase the signal-to-noise ratio (SNR) of the filter, a method of using multiple holographic gratings is proposed. The SNR can be improved by more than 100 times as compared with a conventional volume grating filter. Finally, the electro-optic effect can be used to change the refractive index of the crystal fiber, which in turn changes the wavelength response of the stored volume grating. Since the response time of the electro-optic effect is on the order of 1 ns, one can very rapidly tune the central-wavelength of the filter.