  We present a fiber-diffraction interferometer using a coherent fiber optic
taper for optical testing in an uncontrolled environment. We use a coherent
fiber optic taper and a single-mode fiber having thermally-expanded core. Part
of the measurement wave coming from a test target is condensed through a fiber
optic taper and spatially filtered from a single-mode fiber to be reference
wave. Vibration of the cavity between the target and the interferometer probe
is common to both reference and measurement waves, thus the interference fringe
is stabilized in an optical way. Generation of the reference wave is stable
even with the target movement. Focus shift of the input measurement wave is
desensitized by a coherent fiber optic taper.
