Optical Fiber Extrinsic Micro-Cavity Scanning Microscopy

An extrinsic Fabry-Perot cavity in optical fiber is used to achieve surface imaging at infrared wavelengths. The micro-cavity is realized by approaching a single mode fiber optic with a numerical aperture NA to a sample and it is fed by a low-coherence source. The measurement of the reflected optical intensity provides a map of the sample reflectivity, whereas from the analysis of the reflected spectrum in the time/spatial domain, we disentangle the topography and contrast phase information, in the limit of nearly homogeneous sample with complex permittivity having Im(ε) << Real(ε). The transverse resolution is not defined by the numerical aperture NA of the fiber and consequently by the conventional Rayleigh limit (about 0.6λ/NA), but it is a function of the transverse field behavior of the electromagnetic field inside the micro-cavity. Differently, the resolution in the normal direction is limited mainly by the source bandwidth and demodulation algorithm. The system shows a compact and simple architecture. An analytical model for data interpretation is also introduced.