New technologies of fiber manufacture and a demand for unusual fiber qualities in communication systems have intensified the interest in a comprehensive theory of multimode fibers with nonuniform index distributions This paper deals with a general class of circular symmetric profiles which comprise the parabolic distribution and the abrupt core-cladding index step as special cases We obtain general results of useful simplicity for the impulse response, the mode volume, and the near- and far-field power distributions We suggest a modified parabolic distribution for best equalization of mode delay differences The effective width of the resulting impulse is more than four times smaller than that produced by the parabolic profile Of course, practical manufacturing tolerances are likely to influence this distribution A relation is derived between the maximum index error and the impulse response

Multimode theory of graded-core fibers

This paper reports on some theoretical and experimental investigations of the radial refractive index gradient that maximizes the information-carrying capacity of a multimode optical waveguide. The primary difference between this work and previous studies is that the dispersive nature of core and cladding materials is taken into consideration. This leads to a new expression for the index gradient parameter alpha(c) which characterizes the optimal profile. Using the best available refractive index data, it is found that in high-silica waveguides, the dispersive properties of the glasses significantly influence the pulse broadening of near-parabolic fibers, and that the parameter alpha(c) must be altered by 10-20% to compensate for dispersion differences between core and cladding glasses. These predictions are supported by pulse broadening measurements of two graded-index fibers. A comparison is made between the widths and shapes of measured pulses and pulses calculated using the WKB approximation and the near-field measurement of the index profiles. The good agreement found between theory and experiment not only supports the predictions made for the value of alpha(c), but demonstrates an ability to predict pulse broadening in fibers having general index gradients.

Pulse broadening in graded-index optical fibers.

Two situations in which self-imaging techniques can be applied to advantage are presented: the pinhole-array camera and transmission through an optical fiber. The experimental procedure and results are presented for the case of a pinhole array illuminated with an extended incoherent object distribution. In the Fresnel-image planes, more images are formed than there are pinholes in the array, which is in contrast to the case of the pinhole-array camera. An optical fiber or thin film working in the kaleidoscope mode may form an image, provided that its length fulfills the self-imaging condition.

Image formation using self-imaging techniques*

In complex optical systems comprising many individual components, precise mechanical alignment becomes a difficult problem. We propose to integrate free-space optical components using planar technologies for their fabrication.

Planar integration of free-space optical components

1. Introduction: Building a City of Light 2. Guiding Light and Luminous Fountains (1841-1890) 3. Fibers of Glass 4. The Quest for Remote Viewing: Television and the Legacy of Sword Swallowers (1895-1940) 5. A Critical Insight: The Birth of the Clad Optical Fiber (1950-1955) 6. 99 Percent Perspiration: The Birth of an Industry (1954-1960) 7. A Vision of the Future: Communicating with Light (1880-1960) 8. The Laser Stimulates the Emission of New Ideas (1960-1969) 9. "The Only Thing Left Is Optical Fibers" (1960-1969) 10. Trying to Sell a Dream (1965-1970) 11. Breakthrough: The Clearest Glass in the World (1966-1972) 12. Recipes for Grains of Salt: The Semiconductor Laser (1962-1977) 13. A Demonstration for the Queen (1970-1975) 14. Three Generations in Five Years (1975-1983) 15. Submarine Cables: Covering the Ocean Floor with Glass (1970-1995) 16. The Last Mile: An Elusive Vision 17. Reflections on the City of Light Appendix A. Dramatis Personae: Cast of Characters Appendix B. A Fiber-Optic Chronology

City of Light: The Story of Fiber Optics

A lenslike glass fiber guide with a parabolic variation of refractive index has been developed. This optical guide named SELFOC® has the following characteristics: simultaneous transmission of laser beams modulated by wide-band signals through narrow space; optical image transmission; realization of a lens with tiny aperture or with extremely short focal length; and the possibility of being bent with a small radius of curvature without spoiling transmission characteristics. In the case of a typical fiber guide with length 1 meter and diameter 0.3 mm, transmission loss is about 0.2 dB and depolarization is about 20 dB at wavelength 0.63μ. The mode pattern of a laser beam after passing through the fiber guide is scarcely deformed. The fiber guide can be used as a transmission line or lens, in optical communication, optical data processing, and optical instruments.

Optical characteristics of a light-focusing fiber guide and its applications

A light-focusing fiber guide

A Light-Focusing Fiber Guide Prepared by Ion-Exchange Techniques

Deformation of the spot of an off-axis Gaussian beam after passing through a lens-like glass fiber (SELFOC®) with the fourth-order aberration is investigated by using first-order perturbation theory and numerical calculation. A method for determining the coefficient of the fourth-order aberration is proposed. By this method, it is found that a SELFOC sample (0.5 mm in diameter and 150 mm in length) has a coefficient h of about 1.1, if the dielectric constant in the meridional plane of the sample is expressed as \epsilon(x) = \epsilon(0) (1 - g^{2}x^{2} + hg^{4}x^{4}) , where x is the distance from the axis of the medium and g is a positive constant.

I. Kitano

Papers

Optical characteristics of a light-focusing fiber guide and its applications

A light-focusing fiber guide

A Light-Focusing Fiber Guide Prepared by Ion-Exchange Techniques

Measurement of fourth-order aberration in a lens-like medium

New light-focusing glass fibers made by a continuous process