Figure 8-4. Example of a small on-axis index dip obtained from a fiber made by the outside vapor phase oxidation process. intended to show the diverse behavior of index dips found in commercially available graded index fibers. The outside chemical vapor deposition process generally gives smaller dips than the inside process. Figure 8-4 is an example of a fiber made by an outside process and exhibits the smallest dip we have observed. Figure 8-5 shows the largest dip observed with the system to date. The depth is about half of the peak height of the curve. In many of the index dips measured, the widths are close to the system resolution. Consequently, the dips are deeper than the near-field measurements indicate. Other methods for determining index profile such as refracted near field offer better resolution for these kinds of measurement. 9. CONCLUSION The determination of radiation angle (numerical aperture) from far-field data at the 5 percent intensity points should result in good measurement precision and accuracy when applied to near-parabolic, index fibers. In these fibers, leaky modes are contained within the meridionally defined numerical aperture so they should not affect the intensity at the largest observed angles. However, in step index fibers the situation is different. leaky modes cause uncertainty if meridionally defined numerical aperture is desired. present, standards groups have not made recommendations for step index fibers. At More work is needed if core diameter is to be determined from a near-field measurement. Correlation between the near field and index profile at the core-cladding boundary must be established. The use of a low index "barrier layer" may cause differences between core diameter determined by near-field and index profile measurements. By judiciously choosing definitions, it should be possible to reduce systematic errors between these two measurements. Technically, near-field measurements are much easier to impliment than index profile measurements which, for the most part, rely on interferometry over small dimensions. 10. REFERENCES [1] Cathey, W. T. Optical information processing and holography. New York: John Wiley; 1974. [2] Sladen, F. M. E., et al. Determination of optical fiber refractive index profiles by a near-field scanning technique. A.P.L. 28(5):255-258: 1976 March. [3] Adams, M. J., et al. Leaky rays on optical fibers of arbitrary (circularly symmetric) index profiles. Electronics Letters, 11(11):239-240; 1975 May. [4] Holmes, G. T. Propagation parameter measurements of optical waveguides. Proceedings of the SPIE, Fiber Optics for Communication and Control, Vol. 224: 1980. [5] Kitayama, K., et al. Impulse response prediction based on experimental mode coupling coefficient in a 10-km long graded-index fiber. IEEE J. Quant. Elect. QE-16(3):356-362; 1980 March. [6] Procedures for determining radiation angle (numerical aperture) and core diameter from far and near-field patterns respectively are under consideration by Committee P6.6 of the Electronic Industries Association. [7] Hanson, A. G., et al. Optical waveguide communications glossary. NTIA Special publication, NTIA-SP-79-4; 1979 September. [8] Electronics Industries Association Committee P6.6 is currently recommending that radiation angle be based on the far-field pattern half width at the 5 percent intensity points. [9] Miller, S. E.; Chynoweth, A. G. Optical Fiber Telecommunications. Press; 1979. New York: Academic [10] Matsumura, H. The light acceptance angle of a graded index fibre. Optical and Quantum Electronics 7:81-86; 1975. [11] Eriksrud, M., et al. Comparison between measured and predicted transmission characteristics of 12 km spliced graded-index fibres. Optical and Quantum Electronics. 11:517-523; 1979. [12] Tateda, M. Optical loss measurements in graded-index fiber using dummy fiber. Applied Optics. 18(19):3272-3275: 1979 October. [13] Cherin, A. H.; Gardner, W. B. Standardization of optical fiber transmission measurements. Laser Focus. 16(8):60-65; 1980 August. [14] Adams, M. J., et al. Length-dependent effects due to leaky modes on multimode graded index optical fibres. Optics Communications. 17(2):204-209; 1976 May. [15] Petermann, K. Uncertainties of the leaky mode correction for near-square-law optical fibres. Electronics Letters. 13(17):513-514; 1977 August. [16] Barrell, K. F.; Pask, C. Leaky ray correction factors for elliptical multimode fibres. Electronics Letters. 16(14):532-533; 1980 July. [17] Hazan, J. P. 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