Fiber illumination and pulse shape

4-23 Ray transit times dependent on both invariants 4-24 Material dispersion 

In the previous chapter we described pulse spreading by the maximum temporal width possible, — tn n. Now we investigate pulse spreading more closely by examining the shape of the pulse as it propagates, i.e. the distribution of energy within the pulse, or impulse response. We derive the pulse shape at time t due to an impulse of initially zero width, from which we can describe the response of a pulse of arbitrary initial shape by convolution. The analysis includes only boimd rays, since they characterize the transmission properties of long fibers. 

Multimode fibers are typically Uluminated by diffuse sources. For such sources, the impulse response of a step or dad parabolic profile is a virtually rectangular pulse of width — t i . Hence, the maximum temporal width is an accurate description of the response for these cases. Nevertheless, we emphasize that pulse shape is acutely sensitive to small deviations from the parabolic profile. 

A source is specified by the distribution of power among all the ray directions emitted from each differential element dA of its surface area. For example, the source in Fig. 4-2 emits light within a cone of half-angle 6. Light emitted at 

A diffuse or Lambertian source [2] is one where each differential area dA of source area emits light in all directions, i.e. 0g = 7t/2 in Eq. (4-1). The cross-section of such a source is illustrated schematically in Fig. 4-3(a). This is the most typical source in practice and approximates the output of a light-emitting 

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Pulsed curing systems are widely used in the manufacture of medical devices, electronics, semiconductors, and optical fibers. Pulsed xenon lamps can be made in a variety of shapes to fit specific requirements, such as 360° illumination. Examples of different designs are in Figures 3.5 and 3.6. 

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