Tunable Ultranarrow Dual-Channel Filter Based on Sampled FBGs

A new kind of ultranarrow dual-bandpass fiber Bragg grating is proposed and demonstrated, for the first time to the author's best knowledge, with a symmetrical technique in which the first and second half gratings are sampled with the period of 0-and-pi and pi-and-0, respectively. This kind of grating can achieve dual-wavelength operation with a symmetrical spectrum. The unique spectral property of the proposed gratings is analyzed by means of Fourier theory and confirmed experimentally. By integrating the proposed grating with the 0-and-pi sampling grating, an ultranarrow dual-channel filter is designed in principle and demonstrated experimentally whose 3-dB bandwidth is less than 1 pm. By stretching the gratings, the two wavelengths of the proposed filter can be tuned whereas their channel spacing remains unchanged.