We have developed models describing the sensitivity and sampling volume of various remote fiber-optic Raman probes—single-fiber, lensed, dual-fiber beveled-tip, dual-fiber flat-tipped, and multi-fiber flat-tipped. The models assume clear samples and incorporate radii, separation, bevel angle, and numerical aperture of the fibers; overlap geometry of illumination and excitation light cones; and refractive index of immersion medium. For the Raman spectra of solid samples in air, single-fiber and lensed probes are predicted to yield the highest Raman signal. Beveled probes should provide greater Raman signal strength than do flat-tipped probes because beveled probes can collect light from a restricted volume closer to the probe end. Although multiple collection fibers improve Raman signal strength, progressively distant concentric fiber rings contribute less and sample material further from the probe.
Thomas F. Cooney, H. Trey Skinner, and S. M. Angel, "Comparative Study of Some Fiber-Optic Remote Raman Probe Designs. Part I: Model for Liquids and Transparent Solids," Appl. Spectrosc. 50, 836-848 (1996)