Development and research on narrowband spectral filters on a cascade of fiber Bragg gratings

Subject of study. Cascaded high-signal-attenuation narrowband fiber Bragg gratings connected via a fiber-optic isolator. Aim of study. This study aims to develop a narrowband (82-pm bandpass) spectral filter based on cascaded fiber Bragg gratings. Method. Long, narrow-bandwidth fiber Bragg gratings were written using a phase mask and a scanning beam from a Coherent COMPexPRo 102F krypton fluoride (KrF) excimer laser. Two gratings with identical central Bragg resonance wavelengths were cascaded via a fiber-optic isolator in order to avoid interference effects. Main results. Transmission spectra from the resulting fiber-Bragg-grating cascade had a measured 3-dB filter transmission bandwidth of 82 pm and a maximum attenuation of 46 dB. The filter had a central minimum-transmission wavelength of 1549.9 nm. For comparison, single gratings of the same length with similar attenuation value have a 3-dB bandwidth of 123 pm—much larger than that of the cascaded fiber Bragg grating filter. Practical significance. The narrowband filter based on cascaded fiber Bragg gratings obtained in this paper is suitable for separating carrier wavelengths from subcarrier wavelengths in subcarrier-wave quantum key distribution systems and for use in other communications-link applications requiring selection of a narrow spectral band.