Stricter validation for mode solver bend radius

Author: caseyflex

CHANGELOG.md

@@ -16,6 +16,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 
 ### Fixed
+- Stricter validation for `bend_radius` in mode simulations, preventing the bend center from coinciding with the simulation boundary.
 
 
 ## [v2.10.0rc1] - 2025-09-11

tests/test_plugins/test_mode_solver.py

@@ -1171,6 +1171,14 @@ def test_mode_small_bend_radius_fail():
             mode_spec=td.ModeSpec(num_modes=1, bend_radius=1, bend_axis=0),
             simulation=simulation,
         )
+    # also error when bend radius is exactly aligned with simulation boundary
+    with pytest.raises(ValueError):
+        ms = ModeSolver(
+            plane=PLANE,
+            freqs=np.linspace(1e14, 2e14, 100),
+            mode_spec=td.ModeSpec(num_modes=1, bend_radius=1.5, bend_axis=0),
+            simulation=simulation,
+        )
     # should work for infinite mode plane
     ms = ModeSolver(
         plane=td.Box(center=(0, 0, 0), size=(td.inf, 0, td.inf)),

tidy3d/components/mode/mode_solver.py

@@ -60,7 +60,7 @@
     validate_freqs_not_empty,
 )
 from tidy3d.components.viz import make_ax, plot_params_pml
-from tidy3d.constants import C_0
+from tidy3d.constants import C_0, fp_eps
 from tidy3d.exceptions import SetupError, ValidationError
 from tidy3d.log import log
 from tidy3d.packaging import supports_local_subpixel, tidy3d_extras
@@ -299,7 +299,7 @@ def _validate_mode_plane_radius(cls, mode_spec: ModeSpec, plane: Box, sim_geom:
         _, plane_axs = mode_plane.pop_axis([0, 1, 2], mode_plane.size.index(0.0))
         radial_ax = plane_axs[(mode_spec.bend_axis + 1) % 2]
 
-        if np.abs(mode_spec.bend_radius) < mode_plane.size[radial_ax] / 2:
+        if np.abs(mode_spec.bend_radius) <= mode_plane.size[radial_ax] / 2 + fp_eps:
             raise ValueError(
                 "Mode solver bend radius is smaller than half the mode plane size "
                 "along the radial axis, which can produce wrong results."