# Bragg grating — transmission and reflection stopband

<span class="validation-badge">validated · 2026-08-28</span>

<div class="case-meta"><span>forward</span><span>resolution 1</span><span>device</span><span>real_world</span><span>pic</span><span>bragg</span><span>filter</span><span>tidy3d_golden</span><span>official_notebook</span></div>

This case compares a local FDTDX result with pinned reference data generated by Tidy3D. The originating Tidy3D example is [`BraggGratings.ipynb`](https://github.com/flexcompute/tidy3d-notebooks/blob/c37c785d52e9258c9d048a781524b8e8d7c758ca/BraggGratings.ipynb), pinned at commit `c37c785d52e9`.

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:::{grid-item-card} Open the complete executed tutorial →
:link: ../notebooks/bragg_grating
:link-type: doc
:class-card: device-ladder-card

Parameters → exact geometry → voxelized material → local FDTDX run → simulated field → quantitative analysis. Every code cell and retained output is visible, and the `.ipynb` source is downloadable.
:::

::::

This page is the compact benchmark record. The linked notebook is the primary scientific documentation and contains the actual simulation evidence.


## What is exercised

Twenty-four 324 nm periods form a side-corrugated silicon guide. The three wavelengths sample both shoulders and the 1.532 um stopband. The reflection monitor lies in the uniform access guide rather than intersecting a tooth, and the connected grating is rasterized as one polygon on a 25 nm grid.

This case runs at its declared full benchmark resolution (resolution factor 1.0).

## Reconstruction choices

The official device dimensions, material stack, excitations, and measured observable are retained where they determine the physics. The local scene uses **416 × 136 × 72 cells** and the common port/source normalization machinery. The shared energy-decay cutoff stopped the run after **16,811 of 62,940** allowed steps. Geometry is rasterized as connected material before placement when touching polygons would otherwise introduce grid-snapping seams.

The field panel above is explicitly a schematic of the device and propagation path. The curves and scalar values in the result panel are executed benchmark outputs: solid circles are FDTDX; dashed crosses are the frozen Tidy3D reference.


## Recorded result

| Metric | Observed / error | Reference / limit | Error | Effective budget | Result |
|---|---:|---:|---:|---:|---:|
| `transmitted` | 0.0790785 | 0.12 | 0.0790785 | 0.12 | <span class="metric-pass">PASS</span> |
| `reflected` | 0.107065 | 0.12 | 0.107065 | 0.12 | <span class="metric-pass">PASS</span> |
| `total` | 0.0199027 | 0.05 | 0.0199027 | 0.05 | <span class="metric-pass">PASS</span> |

The table is rendered from `progress.json`; it is not a hand-written success claim. For metrics that report an error directly, the “observed” column repeats that error and the reference column is the acceptance threshold.

## Reproduce

```bash
uv run fdtdx-bench run --case device_bragg_grating
```

Implementation and provenance: `benchmarks/cases/device_bragg_grating/case.yaml`, `benchmarks/cases/device_bragg_grating/run.py`, `benchmarks/goldens/device_bragg_grating.npz`.

[Return to the example atlas](../index.md)