Gdsfactory Component Designer

gdsfactory Component Designer Skill

This skill lets an LLM agent generate, visualize, and iteratively modify photonic-IC components using the gdsfactory Python library.

When to use this skill

Activate this skill when the user:

• Asks to create or generate a photonic component (waveguide, MMI, ring resonator, coupler, grating coupler, spiral, taper, MZI, etc.).
• Wants to see what a component looks like (requests a plot, image, or preview).
• Wants to tweak component parameters (width, length, radius, gaps, layers, cross-sections, etc.) and see the result.
• Mentions GDS, photonic layout, or a specific PDK.
• Asks to compose multiple components together into a larger circuit.

1 — Setting up the environment

Choosing the right Python command

The examples below use bare python, but you must adapt the invocation to whatever Python environment the user has set up. Common alternatives:

Probe the environment first (e.g. check for a pyproject.toml with [tool.uv], or an active virtualenv) and pick the appropriate command. When in doubt, try python -c "import gdsfactory" — if it fails, fall back to uv run python or ask the user.

Activating the PDK

Before generating any component, make sure the PDK is activated. The generic PDK ships with gdsfactory and is always available:

import gdsfactory as gf

# Activate the built-in generic PDK (always available)
gf.gpdk.PDK.activate()

If the user specifies a third-party PDK (e.g. cspdk, ubcpdk, sky130, gf45spclo), import and activate it instead.

Always clear the cell cache between independent component generations to avoid stale state: gf.clear_cache().

2 — Generating a component

2.1 From the built-in component library

gdsfactory ships with 300+ parametric component factory functions under gf.components. To instantiate a component, call its factory function:

import gdsfactory as gf
gf.gpdk.PDK.activate()

# Example: 1×2 MMI splitter
c = gf.components.mmi1x2(width_mmi=5.0, length_mmi=25.0, gap_mmi=0.25)

2.2 Inspecting component parameters

Every component factory function is a standard Python callable with typed parameters. Use help() or inspect.signature() to discover the parameters.

3 — Visualizing a component (Proactive Workflow)

Visualization is essential: render and inspect the component after creating or modifying it to verify the result.

3.1 Save a plot image to disk and display it

Use the helper script bundled with this skill for reliable headless rendering. From a bash tool or Python subprocess:

python .agents/skills/gdsfactory-component-designer/scripts/visualize_component.py \
    "gf.components.mmi1x2(width_mmi=5.0, length_mmi=25.0)" \
    /tmp/mmi1x2.png

After saving, always import the image into context so you and the user can see it.

• Be Proactive: If the user's intent is clear (e.g., "Create an MMI then change its width"), do not stop for permission after the first step. Perform the generation, visualization, and modification in a single turn if possible.
• Concise Reporting: Describe the component briefly (ports, size). If the change is minor, just confirm the update and point to the new image. Avoid repeating the entire component description multiple times.

4 — Modifying a component

4.1 Adjusting parameters

The simplest modification is changing the factory-function arguments. Always call gf.clear_cache() before regenerating to avoid stale data.

4.2 Composing components

Build a custom component by placing and connecting sub-components. Use the @gf.cell decorator for proper naming and caching.

5 — Exporting the component

# Write to GDS file
gdspath = c.write_gds("/tmp/my_component.gds")

6 — Proactive Design Loop (Best Practices)

When the user asks for a component, follow this streamlined loop:

1. Understand & Execute: Identify the component and parameters. If the user asks for a sequence of steps, execute them as a batch where logical.
2. Generate & Visualize: Create the component and render the PNG.
3. Show & Tell: Share the image and a short summary of what changed.
4. Anticipate: If the next step is obvious, offer to perform it or just do it and show the result.
5. Iterate Concisely: For small tweaks, don't repeat the full initial explanation. Just show the new image and highlight the specific change.

7 — Common component quick-reference

8 — When you are unsure: consult the docs and samples

The full gdsfactory docs are at https://gdsfactory.github.io/gdsfactory/. Browse tutorial notebooks under docs/notebooks/ or over 100 sample Python scripts under gdsfactory/samples/ for worked examples.

Don't guess – search the repo for examples first.

9 — KLayout API: transformations with DCplxTrans

gdsfactory's geometry backend is kfactory, built on KLayout's Python db module. Full API docs: https://www.klayout.de/doc-qt5/code/module_db.html

9.1 DCplxTrans constructor

klayout.db.DCplxTrans represents a rotation + mirror + translation on µm-unit coordinates. Positional and keyword arguments are both supported:

import klayout.db as kdb

# positional: DCplxTrans(mag, angle, mirror, u)
t = kdb.DCplxTrans(1.0, 45.0, True, kdb.DVector(10.0, 5.0))

# keyword (preferred for clarity):
t = kdb.DCplxTrans(mag=1.0, angle=45.0, mirror=True, u=kdb.DVector(10.0, 5.0))

Parameters (application order: mirror → rotate → translate):

⚠️ Never set mag != 1.0. No foundry accepts scaled instances. Use component factory parameters to create differently-sized variants.

ℹ️ ComponentReference.dcplx_trans snaps to the manufacturing grid via ICplxTrans, so off-grid placements are silently adjusted.

9.2 Setting ComponentReference.dcplx_trans

import gdsfactory as gf
import klayout.db as kdb

circuit = gf.Component("circuit")
ref = circuit.add_ref(gf.components.mmi1x2())

# rotate 45°, mirror, translate to (10, 5) µm
ref.dcplx_trans = kdb.DCplxTrans(mag=1.0, angle=45.0, mirror=True, u=kdb.DVector(10.0, 5.0))

9.3 Composing transformations

DCplxTrans objects compose with * (right-hand operand applied first):

rotate      = kdb.DCplxTrans(angle=90.0)
translation = kdb.DCplxTrans(u=kdb.DVector(20.0, 0.0))

ref.dcplx_trans = translation * rotate   # rotate first, then translate

9.4 Quick-reference