  We proposed an optomechanical system to quantify the net force on a strand of
cleaved silica optical fiber in situ as the laser light was being guided
through it. Four strands of the fiber were bond to both sides of a macroscopic
oscillator, whose movements were accurately monitored by a Michelson
interferometer. The laser light was propagating with variable optical powers
and frequency modulations. Experimentally, we discovered that the driving force
for the oscillator consisted of not only the optical force of the light exiting
from the cleaved facets but also the tension along the fiber induced by the
light guided therewithin. The net driving force was determined only by the
optical power, refractive index of the fiber, and the speed of light, which
pinpoints its fundamental origin.
