Description and theory of a fiber-optic confocal and super-focal Raman microspectrometer

A fiber-optic bundle, placed in the imaging plane of a microspectrometer, functions as a variable-size pinhole. This arrangement allows for conventional confocal measurements to be made by collecting the signal from the central fiber. On the other hand, measurements arising from a larger focal volume are made by integrating the signal from the entire bundle. This new ''super-focal'' imaging technique yields larger imaging depth without any loss in spectral resolution. The instrument design and performance are described, as well as geometric optics calculations which accurately predict the depth resolution and oscillations in the super-focal depth response. Raman scattering from a three-component layered sample is used to illustrate the extension of this technique to more complicated systems.