[4/n][Adjoint Module] Launch a mode through a stratified stack into the simulation (and backprop) - #3284
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incident_fields is the time reverse of spectrum: a Mode is specified where it is physically meaningful -- at the fiber facet, on the far side of the stack -- and carried back down to the monitor plane, where Meep can inject it. The scalar transfer function of a stratified medium is the same in both directions, so the same product of transmission and propagation phases applies. The result is purely in-going by construction, since it inverts decompose for a spectrum with no outgoing part. equivalent_sources then hands those fields to mp.get_equiv_sources, which already applies the equivalence principle and emits sources carrying amp_data. Because amp_data is differentiable, a beam injected this way backpropagates to its own waist, tilt and offset. from_volume builds a propagator without a monitor, which the injection path needs: a DFT monitor requires the field components to be allocated, and that does not happen until sources are added -- which is what is being built. Verified end to end: the injected field is 99.997% in-going, the radiated beam leaks 4e-3 in amplitude the wrong way (1.6e-5 in power), and the beam measured downstream is still 99.99% travelling the direction it was sent.
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