Fix fibre port placed off the fibre axis in get_simulation_grating_fiber

[Alisama20]

What

get_simulation_grating_fiber places the fibre port off the fibre.

fiber_port_x_offset_from_angle is computed from a fiber_port_y that omits
pml_thickness, substrate_thickness and box_thickness. Forty lines later
fiber_port_y is reassigned to the full sum and used for fiber_port_center,
but the x offset is never recomputed — so the port's x is the x of a height the
port is not at.

The fibre core block is built centred on x=0 and rotated by fiber_angle, so
its axis crosses height y at x = y*tan(fiber_angle). Getting y wrong
displaces the port along x by y_error * tan(fiber_angle), growing with the
fibre angle.

Evidence

Measured from the simulation's own epsilon, sampled along the fibre port line
(defaults, fiber_angle_deg=20):

	x (um)
port centred at	0.3239
fibre core centre on that line	1.0701
fibre axis at that height predicts	1.0592
port offset from the core	-0.7462

The measured core centre and the axis prediction agree to 0.011 um, which is
what confirms the port — not the geometry — is the thing that is misplaced.
The stale offset also feeds sxy, so the cell comes out narrower than the
xmargin that was asked for.

Fix

Compute fiber_port_y once, correctly, and delete the duplicate. Net effect is
three fewer lines of logic and one definition instead of two.

After the fix the port lands 0.0728 um from the core centre. That residue is
not error: it is the monitor's deliberate 0.2 um standoff in y, projected onto
the tilted axis, 0.2 * tan(20 deg) = 0.0728 — agreeing to four decimals.

Test

This adds the first test for this module. It finds the fibre core by probing
the built dielectric along the port line, so it reads no port variable and
cannot agree with a misplaced port by construction. The tolerance is derived,
not tuned: the monitor's y standoff projected on the axis, plus two pixels for
the resolution of the core edges.

Verified to fail on the unfixed code and pass on the fixed one, at two angles:

# before
E  AssertionError: fibre port centred at x=0.3239 um but the fibre core on that
   line is centred at x=1.0701 um (off by -0.7462 um, tolerance 0.2061 um)
FAILED test_get_simulation_grating_fiber.py::test_fiber_port_sits_on_the_fiber_axis[10.0]
FAILED test_get_simulation_grating_fiber.py::test_fiber_port_sits_on_the_fiber_axis[20.0]
2 failed in 5.07s

# after
test_get_simulation_grating_fiber.py::test_fiber_port_sits_on_the_fiber_axis[10.0] PASSED
test_get_simulation_grating_fiber.py::test_fiber_port_sits_on_the_fiber_axis[20.0] PASSED
2 passed in 6.15s

Run in the meep environment (meep 1.34.0, gdsfactory 9.34.2).

Scope

This does not touch the S-parameter extraction. It came up while investigating
the fibre port for a separate issue about s22, which I am filing separately.

Summary by Sourcery

Align the grating-fibre port with the fibre axis and add regression coverage for angled fibre configurations.

Bug Fixes:

• Correct the fibre port placement so it remains aligned with the tilted fibre axis across the full simulation geometry.

Tests:

• Add angle-parametrized geometry tests that verify the fibre port is centred on the simulated fibre core.

[sourcery-ai]

Reviewer's Guide

Fixes fibre-port misalignment by calculating its vertical position once from the full layer stack before deriving the angle-dependent x offset, and adds independent epsilon-based regression coverage at two fibre angles.

Sequence diagram for fibre port axis regression test

sequenceDiagram
    participant Test
    participant Builder as get_simulation_grating_fiber
    participant Epsilon as Built dielectric
    Test->>Builder: get_simulation_grating_fiber(fiber_angle_deg)
    Builder-->>Test: Simulation with fibre port
    Test->>Epsilon: Probe dielectric along port line
    Epsilon-->>Test: Fibre core edge positions
    Test->>Test: Compare core centre with port centre
    alt within derived tolerance
        Test-->>Test: Pass at 10 and 20 degrees
    else outside tolerance
        Test-->>Test: Fail for stale port offset
    end

Loading

Flow diagram for corrected fibre port placement

flowchart TD
    A[Build full layer stack] --> B[Compute fiber_port_y once]
    B --> C[Compute fiber_port_x_offset_from_angle]
    C --> D[Create fiber_port_center]
    D --> E[Fibre port follows tilted fibre axis]

Loading

File-Level Changes

Change	Details	Files
Compute the fibre port height once using the complete stack below the port, ensuring the x offset and port center refer to the same physical height.	Includes PML, substrate, box, core, cladding, air gap, and port offset in the initial height calculation. Removes the later duplicate height calculation so the existing angle-derived x offset remains consistent with the port position.	gplugins/gmeep/get_simulation_grating_fiber.py
Add an angle-parametrized geometry test that validates the port against the simulated fibre dielectric rather than port-construction variables.	Samples epsilon along the fibre monitor line to locate the core center independently. Checks port alignment at 10° and 20° using a tolerance derived from monitor standoff and spatial resolution. Initializes the simulation before probing its built geometry.	gplugins/gmeep/test_get_simulation_grating_fiber.py

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[sourcery-ai]

Hey - I've reviewed your changes and they look great!

Sourcery assessment

Approved.

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[Alisama20]

@joamatab could you add a label here when you get a chance? require_label
is the only failing check - it needs one of bug/enhancement/maintenance
and I do not have permission to set labels on this repo. bug seems right.

Everything else is green, including Test meep, which runs the new test this
PR adds.