feat: add validation for units

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+- Validation check for unit error in grid spacing.
+
 ### Changed
 - Supplying autograd-traced values to geometric fields (`center`, `size`) of simulations, monitors, and sources now logs a warning and falls back to the static value instead of erroring.
 - Attempting to differentiate server-side field projections now raises a clear error instead of silently failing.

tests/test_components/test_grid_spec.py

@@ -1,6 +1,7 @@
 """Tests GridSpec."""
 
 import numpy as np
+import pydantic.v1 as pydantic
 import pytest
 import tidy3d as td
 from tidy3d.exceptions import SetupError
@@ -515,3 +516,28 @@ def test_domain_mismatch():
         boundary_spec=td.BoundarySpec.pml(),
     )
     z = sim.grid.boundaries.z
+
+
+@pytest.mark.parametrize(
+    ("dl", "expect_exception"),
+    [
+        (1e-8, True),  # Below 1e-7 => fail
+        (1e-7, False),  # Exactly at lower bound => pass
+        (0.0, True),  # Zero => fail
+    ],
+)
+def test_uniform_grid_dl_validation(dl, expect_exception):
+    """Test the validator that checks 'dl' is between 1e-7 and 3e8 µm."""
+    if expect_exception:
+        with pytest.raises(pydantic.ValidationError):
+            _ = td.Simulation(
+                size=(1, 1, 1),
+                grid_spec=td.GridSpec.uniform(dl=dl),
+                run_time=1e-12,
+            )
+    else:
+        _ = td.Simulation(
+            size=(1, 1, 1),
+            grid_spec=td.GridSpec.uniform(dl=dl),
+            run_time=1e-12,
+        )

tidy3d/components/grid/grid_spec.py

@@ -276,6 +276,19 @@ class UniformGrid(GridSpec1d):
         units=MICROMETER,
     )
 
+    @pd.validator("dl", always=True)
+    def _validate_dl(cls, val):
+        """
+        Ensure 'dl' is not too small.
+        """
+        if val < 1e-7:
+            raise SetupError(
+                f"Uniform grid spacing 'dl' is {val} µm. "
+                "Please check your units! For more info on Tidy3D units, see: "
+                "https://docs.flexcompute.com/projects/tidy3d/en/latest/faq/docs/faq/What-are-the-units-used-in-the-simulation.html"
+            )
+        return val
+
     def _make_coords_initial(
         self,
         axis: Axis,