Twisted Optical Microfibers for Refractive Index Sensing

A simple, compact, and robust refractive index sensor is developed, by twisting a pair of silica microfibers to form a coupling device. The transmission spectrum of the device is highly sensitive to the surrounding refractive index and the highest sensitivity obtained is
$-$
2377 nm/refractive index unit (RIU), at the refractive index value of
$-$
1.3680, for the fibers with diameter of
$\sim$
4.2
$\mu\hbox{m}$
, and the sensitivity can be further increased to
$-$
2735 nm/RIU for a smaller fiber diameter of
$\sim$
2.8
$\mu\hbox{m}$
. Such a device is easy to construct, of low cost, and compatible with the optical fiber system.