Sources modal excitation

The modal fields of an optical waveguide depend on all of the physical quantities which define the waveguide, i.e. the parameters which describe the refractive-index profile and the cross-sectional geometry, together with the frequency or wavelength of the source of excitation. From these parameters. 

Given a source of excitation, either at the waveguide endface or within the waveguide, the modal amplitudes aj Q) of radiation modes are found by analogy with the bound-mode amplitudes aj. 

We wish to add that there exists a wide variety of literature that considers the opposite case of monochromatic excitation by an infinitely narrow line causing velocity selection, such as [261, 268, 269, 320, 362] and the sources quoted therein. This description has been developed basically in connection with laser theory it refers most often to stabilized single-mode excitation. The intermediate case between monochromatic and broad line excitation is the most complex one, requiring integration over the modal structure of the laser inside the bounds of the absorption contour [28, 231, 243]. 

In prior chapters we looked at subtractive synthesis techniques, such as modal synthesis (Chapter 4) and linear predictive coding (Chapter 8). In these methods a complex source is used to excite resonant fQters. The source usually has a flat spectnun, or exhibits a simple roll-off pattern like f or ip (6 dB or 12 dB per octave). The filters, possibly time-varying, shape the spectrum to model the desired sound. 

The other important development, based on the fact that the ROS diffusion distance is limited to hundreds of nanometres, utilises the fact that the irradiated volume in PDT can be as small as 1 pm, when femtosecond pulsed lasers are used as an excitation source. Precise localisation of excitation light should, in principle, allow the treatment of tissues without any damage to surrounding structures, which is crucial in the treatment of sensitive tissues such as those found in the eye and the brain of the patient. See Sect. 9.11 for more details of this emerging treatment modality two-photon excited PDT (TPE PDT). 

In the example above, we established the equivalence of the geometric optics and modal analysis for the total, guided power excited in a weakly guiding, step-profile fiber by a totally incoherent source when the fiber parameter F-> 00. To determine the error in the geometric optics analysis when V is... 

As we now have orthogonality relations and normalization expressions for leaky modes, results which were derived for bound modes in earlier chapters can simply be extended to apply to leaky modes. These include the perturbation expressions of Chapter 18, the modal amplitudes due to illumination in Chapter 20, and the excitation and scattering effects of current sources in Chapters 21 to 23. We give an example of leaky-mode excitation by a source in Section 24—23. 

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