Effect of Cross-Sectional Structure on Optical Properties of Metal-Filled Side-Hole Fiber

The effect of the cross-sectional structure of metal-filled side-hole fiber on the birefringence and thermooptic properties was investigated by the fiber-optic Sagnac loop interferometry. The optical fibers having different cross-sectional structures filled with indium metal were fabricated by drawing a fiber preform with two side holes at different temperatures followed by metal infiltration. The original circular core and two identical holes in the fiber preform were deformed into the fiber core with various core ellipticities and different hole diameters after the drawing. It was found that the fiber birefringence (
${\boldsymbol B}$
) and the temperature dependence were strongly influenced by the deformation of the fiber cross section, the birefringence increased from
$5.68 \,{\times}\, 10 ^{-5}$
to
$1.16 \,{\times}\, 10^{-4}$
with the increase of the core ellipticity from
$0.273$
to
$0.412$
. Additional birefringence was found to be induced after the infiltration of indium into holes due to the large thermal expansion mismatch between indium and silica glass of the fiber. The temperature sensitivities of the unfilled side-hole fibers (
d(
δλ
$_{n}$
)/dT
) were almost the same and found to be constant of ∼−
$0.60$
 nm/K regardless of the fiber cross sections. On the other hand, the sensitivities of the indium-filled side-hole fibers (
d(
δλ
$_{m}$
)/dT
) were enhanced 10–15 times larger than those of the unfilled side-hole fibers, increased from −
$5.98$
to −
$11.03$
 nm/K with the increase of the filler indium diameter from ∼ 
$18.5$
to
$24.1$
 μm, respectively.