Surface-corrugated microfiber Bragg grating

There are two steps to obtain as small as possible fiber Bragg gratings. First, it is to taper the fiber and reduce its diameter. A subwavelength-scale microfiber (MF) is the basic element of miniature fiber devices and sub-systems. Then it is to reduce the grating length. For short fiber grating, strong refractive index modulations are necessary. Strong refractive index modulations can be obtained inducing surface corrugation by alternating layers of different materials, one of which can be air. Several techniques have been proposed for the fabrication of surface-corrugated fiber gratings, including photorefractive inscription using etching, femtosecond lasers, and focused ion beam (FIB). So far, FIB is the most flexible and powerful tool for patterning, cross- sectioning or functionalizing a subwavelength circular MF due to its small and controllable spot size and high beam current density. In past two years, a number of ultra-compact surfacecorrugated microfiber Bragg gratings (SCMGs) have been successfully fabricated by FIB milling. The length of FIB milled SCGMs can be as small as tens of micrometers. In addition, there are several novel proposals on SCMG including wrapping a microfiber on a microstructure rod or put a microfiber on a surface-corrugated planar grating. In this paper, we will introduce recent advances in these ultra-small SCMGs and their characteristics and applications.