Abstract
We investigate numerically the optical field in the region immediately behind the input facets of dielectric step-index single-mode slab and fiber waveguides. Visualization of the intensity distributions gives insight into the formation of the fundamental mode and of radiation modes. For a more quantitative characterization we determine the amount of optical power and mode purity of the field in core vicinity as a function of propagation distance. The investigation assists in designing and optimizing waveguides being employed as modal filters, e.g. for astronomical interferometers.
Highlights
In a single-mode optical waveguide, the field distribution commonly called fundamental mode constitutes a steady-state solution of Maxwell's equations
Received 10 Jul 2007; revised Aug 2007; accepted Aug 2007; published 4 Sep 2007 17 September 2007 / Vol 15, No 19 / OPTICS EXPRESS 11817 In Fig. 6 we present mode purity graphs for the four cases of input coupling covered in Fig. 3 for two different wavelengths of the input radiation, i.e. for the design wavelength λ = λ0 and for a wavelength increased by 50% (λ = 1.5 λ0)
Using the Fourier Modal Method we have calculated the transient fields after the input facets of both slab and fiber step-index waveguides
Summary
In a single-mode optical waveguide, the field distribution commonly called fundamental mode constitutes a steady-state solution of Maxwell's equations. Speaking, it exists only if the radiation at the guide's input facet already perfectly matches the mode in both amplitude and phase, or after infinitely long guidance. The swift convergence of the field distribution towards the one of the fundamental mode is highly desirable It will depend on waveguide geometry and material properties, and on the form of the incident wave. We examine to a lesser extent the computationally much more extensive three-dimensional fiber waveguide, and in particular look for any fundamental differences in field behavior, power distribution and mode purity compared to the slab waveguide.
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