better error when flux monitor used in autograd

CHANGELOG.md

@@ -19,6 +19,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - `DataArray` interpolation failure due to incorrect ordering of coordinates when interpolating with autograd tracers.
 - Error in `CustomSourceTime` when evaluating at a list of times entirely outside of the range of the envelope definition times.
 - Improved passivity enforcement near high-Q poles in `FastDispersionFitter`. Failed passivity enforcement could lead to simulation divergences.
+- More helpful error and suggestion if users try to differentiate w.r.t. unsupported `FluxMonitor` output.
 
 ## [2.7.2] - 2024-08-07
 

tests/test_components/test_autograd.py

@@ -1415,3 +1415,21 @@ def objective_multi(params, structure_key) -> float:
 
     assert not np.any(np.isclose(grad_indi, 0))
     assert not np.any(np.isclose(grad_multi, 0))
+
+
+def test_error_flux(use_emulated_run, log_capture):
+    """Make sure proper error raised if differentiating w.r.t. FluxData."""
+
+    def objective(params):
+        structure_traced = make_structures(params)["medium"]
+        sim = SIM_BASE.updated_copy(
+            structures=[structure_traced],
+            monitors=[td.FluxMonitor(size=(1, 1, 0), center=(0, 0, 0), freqs=[FREQ0], name="flux")],
+        )
+        data = run(sim, task_name="flux_error")
+        return anp.sum(data["flux"].flux.values)
+
+    with pytest.raises(
+        NotImplementedError, match="Could not formulate adjoint source for 'FluxMonitor' output"
+    ):
+        g = ag.grad(objective)(params0)

tests/utils.py

@@ -1004,13 +1004,21 @@ def make_mode_data(monitor: td.ModeMonitor) -> td.ModeData:
             grid_expanded=simulation.discretize_monitor(monitor),
         )
 
+    def make_flux_data(monitor: td.FluxMonitor) -> td.FluxData:
+        """make a random ModeData from a ModeMonitor."""
+
+        coords = dict(f=list(monitor.freqs))
+        flux = make_data(coords=coords, data_array_type=td.FluxDataArray, is_complex=False)
+        return td.FluxData(monitor=monitor, flux=flux)
+
     MONITOR_MAKER_MAP = {
         td.FieldMonitor: make_field_data,
         td.FieldTimeMonitor: make_field_time_data,
         td.ModeSolverMonitor: make_mode_solver_data,
         td.ModeMonitor: make_mode_data,
         td.PermittivityMonitor: make_eps_data,
         td.DiffractionMonitor: make_diff_data,
+        td.FluxMonitor: make_flux_data,
     }
 
     data = [MONITOR_MAKER_MAP[type(mnt)](mnt) for mnt in simulation.monitors]

tidy3d/components/data/monitor_data.py

@@ -1943,6 +1943,20 @@ class FluxData(MonitorData):
         ..., title="Flux", description="Flux values in the frequency-domain."
     )
 
+    def make_adjoint_sources(
+        self, dataset_names: list[str], fwidth: float
+    ) -> List[Union[CustomCurrentSource, PointDipole]]:
+        """Converts a :class:`.FieldData` to a list of adjoint current or point sources."""
+
+        raise NotImplementedError(
+            "Could not formulate adjoint source for 'FluxMonitor' output. To compute derivatives "
+            "with respect to flux data, please use a 'FieldMonitor' and call '.flux' on the "
+            "resulting 'FieldData' object. Using 'FluxMonitor' directly is not supported as "
+            "the full field information is required to construct the adjoint source for this "
+            "problem. The 'FluxData' does not contain the information necessary for gradient "
+            "computation."
+        )
+
     def normalize(self, source_spectrum_fn) -> FluxData:
         """Return copy of self after normalization is applied using source spectrum function."""
         source_freq_amps = source_spectrum_fn(self.flux.f)