IBM Release 1.93 manual Beam Sources, Modefile Sources

3.5.3. BEAM SOURCES

Beam sources are made up of sums of plane waves. They share the unidirectional property of plane sources. This is sometimes useful in waveguide simulations, because you can bounce the wave off a mirror at the far end and look at the field coming backwards through the plane wave surface. Since all of this field has travelled twice the length of the waveguide, it can make a good approximation of the waveguide mode. This is where the mode file used in 3.5.4 came from.

3.5.4. MODEFILE SOURCES

Modefile sources are actually implemented as arrays of point sources. TEMPEST can’t eat its own output, and furthermore it has a very limited number of point sources available (as few as 100 in the vanilla 6.0 release), so it’s hard to do a good job of representing a waveguide mode.

From the modefile generated by the postprocessor (via the MODEFILE statement of the POSTPROCESS group), POEMS computes NxN boxcar averages of the E field, and generates point_sources spaced N cells apart in each perpendicular direction. The decimation factor N is chosen to be at most 0.3λ/(n dx), so that the evanescent field dies out by a factor of 1000 in at most 0.8 λ/n.

Figure 2.9 Plot of the Z (axial) component of the Poynting vector in the simulation of Fig. 3.3, after 5 cycles. A PML absorbs the -Z wave (dark), leaving a clean +Z wave. Leading ripples are an artifact of the quadrature-field calculation.

Figure 2.8: The same mode source as in Figure 2.7, but showing a slice taken 0.8 µm downstream. By this point the mode has evened out completely because the strongly evanescent ripple components have died away.

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