D.B. Keck’s research while affiliated with Corning Incorporated and other places

Frequency response measurements on six high-bandwidth optical waveguides made by the doped deposited silica process are reported. One of these had an extrapolated bandwidth in excess of 3 GHz-km. From plane wave measurements the actual or optimum waveguide profile may be determined given the other. Data taken on a germanium borosilicate waveguide is in agreement with interference measurements.

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 αc which characterizes the optimal profile. Using the best available refractive index data, it is found that in highsilica waveguides, the dispersive properties of the glasses significantly influence the pulse broadening of near-parabolic fibers, and that the parameter ac 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 αc, but demonstrates an ability to predict pulse broadening in fibers having general index gradients.

The technology of optical fibers is shifting toward solving identified problems of utilization and toward reducing cost. As a consequence, effort has expanded into many special areas. A limited review of the present status in fiber fabrication, transmission and packaging will serve to indicate the evolutionary trend of the technology.

This study correlates the physical structure of glass optical waveguides with pulse broadening observations in fibers several kilometers long. Some of the techniques used in fabricating low-loss optical waveguides are reviewed. The refractive index profile can be deduced from electron microprobe scans of the guide for correlation with pulse broadening measurements. Three types of dispersions in a multimode waveguide are discussed: intermodal dispersion, variation of propagation constant within an individual mode of the waveguide with source wavelength, and basic material dispersion. It is shown that intermodal dispersion presently provides the dominant pulse broadening contribution. Pulse broadening can be reduced at least to the nanosecond region by proper control of the index gradient, and good agreement between gradient theory and pulse measurement is demonstrated. Waveguides can be made up to 3 km in length with negligible mode coupling.

Experimental measurements of the spatial and temporal transfer of power of a 225-m length of low-loss optical waveguide have been made. In particular, measurement of the angular attenuation showed substantial loss of the high order modes, which reflected itself in an ~8.2 nsec/km decrease in measured dispersion. Additionally there was a reduction of the effective numerical aperture from 0.15 to 0.12. Negligible mode coupling was observed in this particular waveguide, which allowed a phenomenological calculation of temporal output for an assumed uniform excitation of all modes. This agreed well with experimental measurements. Calculation of this output from knowledge of the index profile is presently not in agreement, and some possible reasons are indicated.

Externally controlled mode coupling was observed in a 0.7-km optical waveguide fiber resulting in a significant pulse narrowing. The method of mode coupling generation requires that care be exercised in any mode coupling measurement. Observations on a 3-km waveguide failed to show evidence of mode coupling, allowing the possibility of practical external control.

Previous measurements of pulse broadening in the CGW-Bell-10 optical fiber showed very low dispersion ( less than 2 ns/km). The authors recently measured pulse spreading as a function of length in this fiber. The data indicate a (length)**1**/**2 dependence, at wavelengths of 0. 6328, 0. 9, and 1. 06 mu m, for long fiber lengths ( greater than 550 m) which are of practical significance. It is emphasized that these results pertain only to this particular fiber and should not be extended to include all multimode waveguides when designing optical communication systems. Mode mixing effects and the unique core refractive index profile of this particular fiber together determined the fiber dispersion.

The refractive-index profile and impulse response of three low-loss multimode optical fibers have been measured. Pulse dispersion of about 1. 5 ns or less was observed in two of the fibers, each 1 km long. The fibers were 1 km, 1 km, and 0. 266 km long, respectively, all made of the same material and by the same processes. All had doped-silica cores 80-100 mu m in diameter with relatively low transmission loss (4 to 10 dB/km) at some wavelength of interest. The impulse response of the fibers was measured with suitable pulsed lasers operating at 1. 06-, 0. 9-, and 0. 6- mu m wavelength. The results, essentially the same at all three wavelengths and insignificantly affected by the launching angle, are summarized.