The Characterization of Infrared Transmitting Optical Fibers
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THE CHARACTERIZATION OF INFRARED TRANSMITTING OPTICAL FIBERS R. D. DRIVER, G. M. LESKOWITZ, L. E. CURTISS D. E. MOYNIHAN & L. B. VACHA IRIS Fiber Optics, 40 Nagog Park, Acton, MA 01720
ABSTRACT The authors will review the methods used to characterize of FTIR optical fibers. The use infrared transmitting spectroscopic techniques for the measurement of fiber loss will be discussed. Fiber end preparation and fiber cleaving techniques will be reviewed. Data will be presented on silica fiber in the extended IR region, heavy metal fluoride, chalcogenide, silver halide fibers and sapphire crystalline fiber. INTRODUCTION The growing interest in IR transmitting optical fibers for remote spectroscopy and laser power delivery has created a need for strong low loss fiber [1-3]. The standard techniques of fiber loss measurement involve the use of fiber cutback in combination with scanning monochromators and lock-in amplification signal processing techniques [4]. Such techniques become inefficient above 2 Am due to the relatively low detectivities D* of IR detectors. The method of FTIR fiber-optic spectrometry applied to fiber loss
characterization allows spectra of high spectral resolution and low noise to be measured on timescales of less than one minute. The enhanced signal to noise of FTIR generated spectra arises from the Fellgett advantage: an FTIR spectrometer continuously collects spectral information over the complete optical spectrum during accumulation of the interferogram [5]. The parallel nature of FTIR spectroscopy also has the advantage of smoothing out effects due to source fluctuations. OPTICAL CHARACTERIZATION OF GLASSES AND POLYCRYSTALINE MATERIALS USING FTIR The FTIR System The fiber-optic FTIR system used for measuring optical loss in fluoride, silica, chalcogenide, and silver halide fibers is shown schematically in Figure 1. All data were obtained using a Digilab FTS-40 FTIR spectrometer equipped with a calcium fluoride beamsplitter and a fiber optic stage. For the silica and fluoride fiber measurements, the spectrometer was equipped with a tungsten halogen source and a cryogenically-cooled small area indiumantimonide detector. For the chalcogenide and silver halide fiber measurements, a Globar source and a cryogenically-cooled area MCT detector were used.
Mat. Res. Soc. Symp. Proc. Vol. 172. ©1990 Materials Research Society
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FIBER Figure 1. FTIR fiber measurement system.
The Fiber Cutback Technique Typically, glass and crystalline fibers are optically characterized using a cutback technique in which a length of fiber is optically scanned at a range of discrete wavelengths (with an FTIR spectrometer in our case), cut shorter and scanned again. The launch end should be left undisturbed during this process to minimize variations due to varying launch conditions. If necessary, a mode stripping technique should be used to remove any cladding modes present in the fiber. The attenuation coefficient (typically expressed in dB per meter or dB per kilometer) is a function of wavenumber thus; Attn(.)
= -10
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