Near-field imaging of interference pattern of counterpropagating evanescent waves

It is generally accepted that measurement of of the contrast of the intensity interference pattern formed by two counterpropagating evanescent waves can be used to characterize the resolving power of a collection near-field microscope. We argue that, if the light collected by a fiber probe propagates toward a detector in the form of guided fiber modes, then this perception is wrong. Using symmetry arguments and simple calculations, we show that, if an extended fiber probe is axially symmetric and oriented perpendicular to the propagation direction, then the measured contrast should always be equal to the contrast of the interference pattern.