A hollow optical fiber (HOF) has an unique modal distribution of a central evanescent field due to its structure.
The HOF consists of a central air hole, a Ge-doped ring core placed at the inmost layer, and silica cladding,
which induces the weak evanescent field at the central hole. By the structure with geometric symmetry, it is
possible to inject a refractive fluid into the hole and to modify the modal distribution. When a refractive index of
the fluid is same with or higher than the core's, guiding of light becomes dominant at the center and the ringshaped
field turns into a LP01 mode. During the process, optical force is induced and the net momentum of the
fluid is changed. The direction of optical force is opposite to that of light propagation, and the fluid come to be
dragged along the central channel in the HOF. In order to further investigate the phenomenon, we have changed
the refractive index of the fluid and measured resultant optical force. The direction and strength of the optical
force was dependent on the refractive index of the central fluid, which shows ample potential of the HOF as a
refractive index sensor.