Rare-Earth-Doped Glass Fiber for Background Rejection in Remote Fiber-Optic Raman Probes: Theory and Analysis of Holmium-Bearing Glass

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Abstract
We have investigated the feasibility of using rare-earth-doped glasses in “self-filtering” optical fibers used for remote Raman spectral collection. We have derived a theoretical treatment and have used the measured relevant sample and glass parameters (e.g., optical absorption of the doped glass and ratio of the intensities of inelastic scattering to elastic scattering plus reflection) to evaluate the usefulness of such fibers. With the use of these parameters, the optical absorption bands of Ho-doped glass, in particular, are found to be sufficiently intense and sharp to enable this glass to be used in the collection fiber of a remote Raman probe. Ho-doped glass fiber of as little as 2 cm in length is sufficient to filter undesirable laser radiation while permitting a high proportion of the sample Raman signal to pass. Use of the 488.0-nm Ar+ laser line or green or red laser wavelengths from a “tunable” laser can ensure that the excitation is within an absorption band and close to the long-wavelength transmission cut-on for the doped glass.