Fix: Bug in WavePort when requesting more than one mode in the ModeSpec

Jira-Ticket: FXC-3371

Author: dmarek-flex

CHANGELOG.md

@@ -16,7 +16,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 - More robust `Sellmeier` and `Debye` material model, and prevent very large pole parameters in `PoleResidue` material model.
-
+- Bug in `WavePort` when more than one mode is requested in the `ModeSpec`.
 
 ## [v2.10.0rc2] - 2025-10-01
 

tests/test_plugins/smatrix/test_terminal_component_modeler.py

@@ -780,6 +780,25 @@ def test_run_coaxial_component_modeler_with_wave_ports(
                 s_matrix.sel(**coords_in).sel(**coords_out).values.shape == shape_both_ports
             ), "monitor index not present in S matrix"
 
+    # Another run with more modes in the mode spec
+    mode_spec = td.ModeSpec(num_modes=2)
+    modeler = modeler.updated_copy(path="ports/0/", mode_spec=mode_spec)
+    s_matrix = get_terminal_port_data_array(monkeypatch, modeler)
+
+    shape_one_port = (len(modeler.freqs), len(modeler.ports))
+    shape_both_ports = (len(modeler.freqs),)
+    for port_in in modeler.ports:
+        for port_out in modeler.ports:
+            coords_in = {"port_in": port_in.name}
+            coords_out = {"port_out": port_out.name}
+
+            assert np.all(s_matrix.sel(**coords_in).values.shape == shape_one_port), (
+                "source index not present in S matrix"
+            )
+            assert np.all(
+                s_matrix.sel(**coords_in).sel(**coords_out).values.shape == shape_both_ports
+            ), "monitor index not present in S matrix"
+
 
 def test_run_mixed_component_modeler_with_wave_ports(monkeypatch, tmp_path):
     """Checks the terminal component modeler will allow mixed ports."""

tidy3d/plugins/smatrix/ports/wave.py

@@ -103,8 +103,6 @@ def _mode_voltage_coefficients(self, mode_data: ModeData) -> FreqModeDataArray:
         to the total port voltage.
         """
         flux_sign = 1 if self.direction == "+" else -1
-
-        mode_data = mode_data._isel(mode_index=[self.mode_index])
         if self.voltage_integral is None:
             flux_sign = 1 if mode_data.monitor.store_fields_direction == "+" else -1
             current_coeffs = self.current_integral.compute_current(mode_data)
@@ -118,7 +116,6 @@ def _mode_current_coefficients(self, mode_data: ModeData) -> FreqModeDataArray:
         to the total port current.
         """
         flux_sign = 1 if self.direction == "+" else -1
-        mode_data = mode_data._isel(mode_index=[self.mode_index])
         if self.current_integral is None:
             flux_sign = 1 if mode_data.monitor.store_fields_direction == "+" else -1
             voltage_coeffs = self.voltage_integral.compute_voltage(mode_data)
@@ -221,20 +218,20 @@ def to_absorber(
 
     def compute_voltage(self, sim_data: SimulationData) -> FreqDataArray:
         """Helper to compute voltage across the port."""
-        mode_data = sim_data[self._mode_monitor_name]
+        mode_data = sim_data[self._mode_monitor_name]._isel(mode_index=[self.mode_index])
         voltage_coeffs = self._mode_voltage_coefficients(mode_data)
         amps = mode_data.amps
-        fwd_amps = amps.sel(direction="+").squeeze()
-        bwd_amps = amps.sel(direction="-").squeeze()
+        fwd_amps = amps.sel(direction="+", mode_index=self.mode_index).squeeze()
+        bwd_amps = amps.sel(direction="-", mode_index=self.mode_index).squeeze()
         return voltage_coeffs * (fwd_amps + bwd_amps)
 
     def compute_current(self, sim_data: SimulationData) -> FreqDataArray:
         """Helper to compute current flowing through the port."""
-        mode_data = sim_data[self._mode_monitor_name]
+        mode_data = sim_data[self._mode_monitor_name]._isel(mode_index=[self.mode_index])
         current_coeffs = self._mode_current_coefficients(mode_data)
         amps = mode_data.amps
-        fwd_amps = amps.sel(direction="+").squeeze()
-        bwd_amps = amps.sel(direction="-").squeeze()
+        fwd_amps = amps.sel(direction="+", mode_index=self.mode_index).squeeze()
+        bwd_amps = amps.sel(direction="-", mode_index=self.mode_index).squeeze()
         # In ModeData, fwd_amps and bwd_amps are not relative to
         # the direction fields are stored
         sign = 1.0