Rare-Earth-Doped Glass Fiber for Background Rejection in Remote Fiber-Optic Raman Probes: Theory and Analysis of Holmium-Bearing Glass THOMAS F. COONEY,* CHRISTIAN L. SCHOEN, SHIV K. SHARMA, and DAVID M. CAREY Hawaii Institute o/Geophysics, School of Ocean and Earth Science and Technology, University o[ Hawaii at Manoa, Honolulu, Hawaii 96822 (T.F.C., C.L.S., S.K.S.); and Rensselaer Polytechnic Institute, Troy, New York 12180 (D.M.C.) We have investigated the feasibility of using rare-earth-doped glasses in "self-filtering" optical fibers used for remote Raman spectral collec- tion. 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-wave- length transmission cut-on for the doped glass. Index Headings: Remote Raman spectroscopy; Optical fiber; Optical absorption; Rare-earth-doped glass. INTRODUCTION In recent years, the use of optical fibers in the devel- opment of probes capable of performing remote Raman spectroscopic chemical analyses has increased. 1-9 How- ever, the useful length of such probes has generally been limited by the fact that the glass comprising the fiber itself produces a Raman spectral background that in- creases with length. Special filters must be used to at- tenuate the fiber background in long (>10 m) remote Raman probes, x° Such filters were utilized in the devel- opment of a 100-m dual-fiber remote Raman probe (RRP) for identification of weak Raman scatterers2 x Figure 1 shows a schematic of the RRP from Schoen et al. 11 One of the two legs in this probe is used for excitation; the other leg is used for spectral collection and transmission. The most important components of this design are the custom-made filters for rejection of interference from Raman scattering from Si02 glass in both the excitation and collection fibers. The "bandpass" (BP) filter positioned in front of the excitation fiber transmits the laser line while blocking Raman (and flu- orescence) background generated in the excitation fiber. The "longpass" (LP) filter positioned in front of the collection fiber transmits the Raman spectrum of the sample while blocking the laser wavelength from entering the collection fiber, where it could produce additional unwanted background. The primary consideration of the Received 29 December 1992;revision received9 April 1993. * Author to whom correspondenceshould be sent. design is that, to obtain the required filtering, the ray path of the light passing through both the BP and LP filters must make an angle of no greater than approxi- mately six degrees with a line perpendicular to the re- spective filter surface. The optical densities of these ill- ters at the unwanted wavelengths decrease rapidly with angles greater than this value. Thus, the light must be collimated upon passage through each of the filters. The graded-index (GRIN) and bi-convex focusing lenses cre- ate the proper ray path and collimation. The need for accessory optics (Fig. 1) adds bulk and complexity to the design of the dual-fiber RRP because special housing and alignment features need to be in- corporated. Furthermore, overall efficiency of light col- lection (e.g., numerical aperture) from the sampled point by the collection fiber is sacrificed to optimize the per- formance of the filters; attempts to increase the numer- ical aperture would decrease the signal-to-background ratio because of the decreased angle the paths of some rays at the excitation wavelength would make with the filters. The collection efficiency of the RRP described by Schoen et al. xl diminishes in proportion to the factor R 2, where R is the distance from the sample surface to the bi-convex focusing lens. The consequent problem of low signal level was overcome by using a highly light-sensitive CCD Raman detector21 New capabilities in the synthesis of glass media with relatively high levels of rare-earth dopants have raised the possibility that the fibers themselves might be used as filters for certain laser excitation wavelengths. This consideration arises from the fact that small proportions of rare earth and possibly other types of chemical dopant produce intense, sharp optical absorption (and related fluorescence) bands within the glass used to produce the fiber. For instance, the optical properties of erbium-doped optical fibers have already been used in the development of optical amplifiers and lasers.X2-2~The existence of these sharp absorption bands means that the transmission characteristics of the fiber itself might be utilized to ad- vantage to reject background features caused by Raman scattering within the fiber. If such "self-filtering" optical fibers could be incorporated into the collection fiber of the RRP, then the requirement of filters and accessory optics associated with the collection fiber could be elim- inated, thereby miniaturizing and simplifying the design. This enhancement would also enable the fibers to be positioned closer to the sampling point for improved signal collection. This capability would allow sampling Volume 47, Number 10, 1993 0003-7028/93/4710.168352.00/0 APPLIED SPECTROSCOPY 1683 © 1993 Society for Applied Spectroscopy