A probe comprises an excitation source and a double-core optical fiber. A
pulsed laser signal (20) of the excitation sources supplied to the outer
core (42) at one end of the optical fiber. The other end is provided with
an interferometer film (18). An excitation signal (22) produced in the
sample (10) modulates the thickness of the film (18). This provides an
interferometer signal (26, 28) detected from the inner core (40).
