Validate that bend radius is not smaller than half the mode plane size

tests/test_components/test_monitor.py

@@ -296,6 +296,24 @@ def test_monitor_num_modes(log_capture, num_modes, log_level):
     assert_log_level(log_capture, log_level)
 
 
+def test_mode_bend_radius():
+    """Test that small bend radius fails."""
+
+    with pytest.raises(ValueError):
+        mnt = td.ModeMonitor(
+            size=(5, 0, 1),
+            freqs=np.linspace(1e14, 2e14, 100),
+            name="test",
+            mode_spec=td.ModeSpec(num_modes=1, bend_radius=1, bend_axis=1),
+        )
+        _ = td.Simulation(
+            size=(2, 2, 2),
+            run_time=1e-12,
+            monitors=[mnt],
+            grid_spec=td.GridSpec.uniform(dl=0.1),
+        )
+
+
 def test_diffraction_validators():
     # ensure error if boundaries are not periodic
     boundary_spec = td.BoundarySpec(

tests/test_components/test_source.py

@@ -46,6 +46,22 @@ def test_dir_vector():
     assert DirectionalSource._dir_vector.fget(S) is None
 
 
+def test_mode_bend_radius():
+    """Test that small bend radius fails."""
+
+    with pytest.raises(ValueError):
+        src = td.ModeSource(
+            size=(1, 0, 5),
+            source_time=ST,
+            mode_spec=td.ModeSpec(num_modes=1, bend_radius=1, bend_axis=0),
+        )
+        _ = td.Simulation(
+            size=(2, 2, 2),
+            run_time=1e-12,
+            sources=[src],
+        )
+
+
 def test_UniformCurrentSource():
     g = td.GaussianPulse(freq0=1e12, fwidth=0.1e12)
 

tests/test_plugins/test_mode_solver.py

@@ -1034,3 +1034,14 @@ def test_modes_eme_sim(mock_remote_api, local):
         _ = msweb.run(solver.to_fdtd_mode_solver())
 
     _ = solver.reduced_simulation_copy
+
+
+def test_mode_bend_radius():
+    """Test that small bend radius fails."""
+
+    with pytest.raises(ValueError):
+        ms = ModeSolver(
+            plane=PLANE,
+            freqs=np.linspace(1e14, 2e14, 100),
+            mode_spec=td.ModeSpec(num_modes=1, bend_radius=1, bend_axis=0),
+        )

tidy3d/components/simulation.py

@@ -85,7 +85,11 @@
 from .structure import MeshOverrideStructure, Structure
 from .subpixel_spec import SubpixelSpec
 from .types import TYPE_TAG_STR, Ax, Axis, FreqBound, InterpMethod, Literal, Symmetry, annotate_type
-from .validators import assert_objects_in_sim_bounds, validate_mode_objects_symmetry
+from .validators import (
+    assert_objects_in_sim_bounds,
+    validate_mode_objects_symmetry,
+    validate_mode_plane_radius,
+)
 from .viz import (
     PlotParams,
     add_ax_if_none,
@@ -3245,6 +3249,24 @@ def _post_init_validators(self) -> None:
         self._validate_tfsf_nonuniform_grid()
         self._validate_nonlinear_specs()
         self._validate_custom_source_time()
+        self._validate_mode_object_bends()
+
+    def _validate_mode_object_bends(self) -> None:
+        """Error if any mode sources or monitors with bends have a radius that is too small."""
+        for imnt, monitor in enumerate(self.monitors):
+            if isinstance(monitor, AbstractModeMonitor):
+                validate_mode_plane_radius(
+                    mode_spec=monitor.mode_spec,
+                    plane=monitor.geometry,
+                    msg_prefix=f"Monitors[{imnt}] ",
+                )
+        for isrc, source in enumerate(self.sources):
+            if isinstance(source, ModeSource):
+                validate_mode_plane_radius(
+                    mode_spec=source.mode_spec,
+                    plane=source.geometry,
+                    msg_prefix=f"Sources[{isrc}] ",
+                )
 
     def _validate_custom_source_time(self):
         """Warn if all simulation times are outside CustomSourceTime definition range."""

tidy3d/components/validators.py

@@ -396,3 +396,21 @@ def freqs_not_empty(cls, val):
         return val
 
     return freqs_not_empty
+
+
+def validate_mode_plane_radius(mode_spec: "ModeSpec", plane: "Box", msg_prefix: str):  # noqa: F821
+    """Validate that the radius of a mode spec with a bend is not smaller than half the size of
+    the plane along the radial direction."""
+
+    if not mode_spec.bend_radius:
+        return
+
+    # radial axis is the plane axis that is not the bend axis
+    _, plane_axs = plane.pop_axis([0, 1, 2], plane.size.index(0.0))
+    radial_ax = plane_axs[(mode_spec.bend_axis + 1) % 2]
+
+    if np.abs(mode_spec.bend_radius) < plane.size[radial_ax] / 2:
+        raise ValueError(
+            f"{msg_prefix} bend radius is smaller than half the mode plane size "
+            "along the radial axis, which can produce wrong results."
+        )

tidy3d/plugins/mode/mode_solver.py

@@ -46,7 +46,11 @@
     PlotScale,
     Symmetry,
 )
-from ...components.validators import validate_freqs_min, validate_freqs_not_empty
+from ...components.validators import (
+    validate_freqs_min,
+    validate_freqs_not_empty,
+    validate_mode_plane_radius,
+)
 from ...components.viz import make_ax, plot_params_pml
 from ...constants import C_0
 from ...exceptions import SetupError, ValidationError
@@ -173,6 +177,11 @@ def plane_in_sim_bounds(cls, val, values):
             raise SetupError("'ModeSolver.plane' must intersect 'ModeSolver.simulation'.")
         return val
 
+    def _post_init_validators(self) -> None:
+        validate_mode_plane_radius(
+            mode_spec=self.mode_spec, plane=self.plane, msg_prefix="Mode solver"
+        )
+
     @cached_property
     def normal_axis(self) -> Axis:
         """Axis normal to the mode plane."""