Illumination single-mode fibers

Fig. 3. Typical setup of a scanning near-field optical microscope. Excitation light is coupled into a single-mode fiber with a metal coated taper at its far end. The light emitted by the aperture illuminates a region of the samples whose size is determined by the aperture diameter and the distance between probe and sample. Light from the interaction region is collect using a conventional optical microscope.

The partially diffuse source of Fig. 20-6(a) illuminates a single-mode fiber. We showed in Sections 20-7 and 20-12 that the fundamental mode is most efficiently excited by on-axis beams. Thus, it is intuitive that the more diffuse the source, i.e. the larger 0 , the lower the efficiency of the source in exciting the fundamental mode. To demonstrate this behavior quantitatively, we consider a source with a Gaussian intensity... 

Consider an infinite array of parallel, single-mode fibers which are identical and uniformly spaced. We assume the fibers are weakly guiding and have an otherwise arbitrary refractive-index profile. At the beginning of the array, z = 0, alternate fibers are illuminated with power P, (0) or Pj (0) < Pj (0), denoted by -I- or —, respectively, in the examples of Fig. 29-3. The periodic illumination means that the amplitudes 2>i (z) and biiz) of the fundamental-mode fields, defined in Eq. (29-1), are the same for eveiy... 

Consider an infinite, one-dimensional array of identical, single-mode fibers which are weakly guiding. They are labelled by n = — co,.. ., — 1,0,1. .., oo, as shown in Fig. 29-4(a). Only the center fiber n = 0 is illuminated with unit power at r = 0. As its fundamental mode propagates, power will be coupled to the fundamental modes of the neighboring fibers n = 1. These two fibers in turn couple to then = 2fibersandto the n = 0 fiber, and so on. Thus power is distributed over an increasing number of fibers farther into the array. 

In addition to cross-talk, we can also take into account the effects of absorption in arrays. This is useful in the study of visual photoreceptors, which in many animals are absorbing fibers, often single moded and weakly guiding [2,3], The image encoded by a photoreceptor array is determined by the distribution of absorbed light across the array. Here we examine the image deterioration due to cross-talk when the photoreceptors have the alternate illumination of Fig. 29-3. 

So far in this chapter we have considered mode excitation due to a single beam directed at a particular angle to the fiber axis. Let us now consider the effect on mode excitation when the illumination is composed of a family of beams which differ in their angles of incidence at the endface, e.g. the source of diffuse illumination depicted in Fig. 20-6(a). Such sources are known as (partially)... 

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