Identical-dual-bandpass sampled fiber Bragg grating and its application to ultranarrow filters.

We have theoretically proposed and experimentally demonstrated a new kind of ultranarrow identical-dual-bandpass sampled fiber Bragg gratings (SFBGs) with a pi phase shift technique. The spacing of two bandpasses of the proposed grating can be flexibly adjusted by changing the sampled period, and any desired spacing can be achieved in principle. An experimental example shows that the transmission peaks of two narrow transmission-band are near 1549.1 and 1550.1 nm. Based on the proposed SFBG, an ultranarrow identical-dual-channel filter is designed. Two channels of the proposed filter have an equal bandwidth, an even strength, and the same group delay. The bandwidth of each channel of our filter is as small as 1 pm and up to 10(-3) pm (corresponding to approximately 0.1 MHz), which is less than the bandwidth of the conventional SFBG filters by a factor of 10(2)-10(4). The proposed grating and filter can find potential applications with slow light and dual-wavelength single-longitudinal-mode fiber lasers.