  We experimentally demonstrate that side-coupling of coplanar bent optical
fibers can induce a high Q-factor whispering gallery mode (WGM) optical
microresonator. To explain the effect, we consider WGMs with wavelengths close
to the cutoff wavelengths (CWs) of these fibers which slowly propagate along
the fiber axes. In the vicinity of the touching region, WGMs of adjacent fibers
are coupled to each other, and CWs experience sub-nanoscale axial variation
proportional to the coupling strength. We show that in certain cases the CW
variation leads to full localization of the WGMs and the creation of an optical
microresonator. By varying the characteristic curvature fiber radius from the
centimeter order to millimeter order, we demonstrate fully mechanically
reconfigurable high Q-factor optical microresonators with dimensions varying
from the millimeter order to 100-micron order and free spectral range varying
from a picometer to hundreds of picometers. The new microresonators may find
applications in cavity QED, microresonator optomechanics, frequency comb
generation with tunable repetition rate, tunable lasing, and tunable processing
and delay of optical pulses.
