Contrast phase imaging at infrared wavelengths is achieved through an extrinsic Fabry-Perot cavity in optical fiber. The micro-cavity is realized by approaching a cleaved fiber to a distance of about few tens of microns from the surface under test. The probe is a single mode fiber and is fed by a low-coherence source. The information is extracted from the reflected spectrum, that starts to be modulated by the interference when the fiber begins to interact with the sample. The measurement of the reflected optical intensity provides a map of the sample reflectivity, whereas from the analysis of the spectrum in the time/spatial domain, it is possible to extract topography and refractive index variations. This information is entangled in the contrast phase image obtained. In this work we review the system proposed in [19] in order to extract topography and local surface permittivity of biological samples. The system displays tridimensional images with a transverse resolution that is not limited by the numerical aperture NA of the scanning probe (as suggested by the Rayleigh limit), but it is related to the transverse field behavior of the electromagnetic field inside the micro-cavity. Differently, the source bandwidth, demodulation algorithm and optical spectrum analyzer resolution affect the resolution in the normal direction.
