Resonator-free tunable sub-MHz spectral dip in the Brillouin gain using spun birefringent fibers

Abstract

Ultra-narrow spectral features are desirable for a broad range of applications, and they are conventionally realized using ultrahigh Q resonant structures. These structures typically require precision fabrication processes, and moreover, since they are passive, they suffer from signal loss. Here, we demonstrate a novel way to achieve sub-MHz tunable spectral dip in the Brillouin gain spectrum of a spun birefringent fiber (SBF) without loss, and without using a resonator. We show that this dip is unique to SBF, where its polarization eigenmodes are elliptical and frequency-dependent, and the dip only occurs when these orthogonal polarization eigenmodes of the SBF (at the respective pump and signal frequencies) are launched in counter-propagating directions. We experimentally demonstrate a 0.72 MHz spectral dip in the Brillouin gain spectrum of a commercial SBF which is to our knowledge, the narrowest SBS spectral feature ever reported. Furthermore, the linewidth, depth, and spectral location of this dip are tunable on demand by controlling the pump frequency, pump power, and the input polarization of the signal. Its simplicity in implementation, its ultra-narrow linewidth, and its tunability can have a wide range of potential applications, from slow-light to microwave photonics.