Design Parameters for Multimode Fluoride Fibers: Effects of Coating and Bending on Loss
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		    DESIGN PARAMETERS FOR MULTIMODE FLUORIDE FIBERS: COATING AND BENDING ON LOSS
 
 EFFECTS OF
 
 L. E. BUSSE AND I. D. AGGARWAL Naval Research Laboratory, Code 6505,
 
 Washington, DC
 
 20375-5000
 
 ABSTRACT Numerical
 
 calculations
 
 of
 
 losses
 
 due
 
 to
 
 polymer
 
 coatings
 
 and
 
 macrobending have been made for step index multimode fluoride fibers. avoid
 
 such
 
 losses,
 
 fiber
 
 core
 
 and
 
 cladding
 
 diameters
 
 and
 
 To
 
 numerical
 
 aperture must be chosen carefully.
 
 Introduction Fluoride
 
 single mode optical fibers provide the potential
 
 for very
 
 long length communications networks with their predicted minimum loss of
 
 0.01 dB/km at 2.5 pm.
 
 Many fibers currently being made are step index
 
 multimode,
 
 to their relative ease
 
 however,
 
 due
 
 of fabrication.
 
 To the
 
 authors' knowledge, there are no published criteria for parameters such as core and clad diameters and numerical aperture. In this paper it will be
 
 shown
 
 that
 
 substantial
 
 such
 
 induced
 
 parameters losses
 
 due
 
 must to
 
 be
 
 chosen
 
 typical
 
 carefully
 
 polymer
 
 to
 
 avoid
 
 coatings
 
 and
 
 macrobending.
 
 Coating Losses The theory applied [1]
 
 involves numerically calculating the induced
 
 loss due to an outer jacket, core and cladding.
 
 assuming no mode coupling and a lossless
 
 The absorption loss of a typical polymer coating was
 
 determined to be 107 dB/km at 2.5 pm. Table I lists core diameters (2a),
 
 numerical apertures (N.A.)
 
 resultant values for V, the normalized frequency (at A-2.5 pm),
 
 and for step
 
 index multimode fluoride fibers and a typical silica fiber (at A-1.3 pm).
 
 Mat. Res. Soc. Symp. Proc. Vol. 172. e1990 Materials Research Society
 
 178
 
 TABLE I:
 
 Step index multimode fiber parameters
 
 179
 
 Fig. 1 shows,
 
 for various values of V, the dependence of the fraction
 
 of modes with calculated induced losses > 0.01 dB/km on the ratio of clad to core diameter for each value is
 
 (b/a).
 
 In Table
 
 I the values of b/a and 2b are given
 
 of V such that only 10% of the modes have such loss.
 
 apparent that such parameters
 
 of the silica fiber. of 56 pm and an N.A.
 
 For example, of 0.2 (V=14)
 
 preclude coating induced
 
 losses,
 
 a fluoride fiber with a core diameter needs an overall diameter of 181 jim to compared to the 50 um/125 pm structure
 
 suitable for silica fiber.
 
 801 0
 
 660ov=10 0 -
 
 = 0U
 
 40
 
 0
 
 20
 
 "AV=14 1-)
 
 n
 
 It
 
 are significantly different from those
 
 * V = 20 0V=24 L
 
 b/a (Clad dia./Core dia.) Fig. 1: Modes with loss > 0.01 dB/km
 
 180
 
 The which
 
 is
 
 total
 
 induced
 
 loss
 
 depends
 
 strongly
 
 often assumed to be Gaussian
 
 cases b/a - 2 and 3)
 
 on
 
 [2,3].
 
 the mode Fig.
 
 distribution
 
 2a shows
 
 (for
 
 the
 
 the calculated losses for a fluoride fiber with V-14
 
 assuming the Gaussian distributions shown in
 
 Fig. 2b characterized by the
 
 parameter G, i.e.:
 
 P(9)/9 - exp[-(O/GOc) where 8c = the critical
 
 angle.
 
 There
 
 is
 
 a
 
 2
 
 ]
 
 (1)
 
 strong dependence on the mode
 
 distribution, at this point unknown for fluoride fiber. the calculated loss with G =o
 
 By comparison,
 
 for the silica fiber was only 0.007 dB/km
 
 at 1.3 jm.
 
 m %0
 
 02 101
 
 "W 0 00 •j 10 1 -(., 10_2		
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