LAUSR

All-fiber filters with multiple narrow, flat-top channels are designed and fabricated, by incorporating both the phase sampling technique and multiple $\pi $ phase shifts into a fiber Bragg gratings. The phase sampling yields multiple reflection bands with high uniformity, while the combination of multiple phase shifts allows a narrow, flat-top transmission channel in each reflection band identically, leading to multiple narrow, flat-top channels. Such filters with nine channels are then fabricated and characterized with excellent comprehensive specifications, including a 3-dB bandwidth less than 800 MHz, a rectangular shape factor close to 2.5, an insertion loss less than 1 dB, and an out-of-band rejection greater than 25 dB.