If you are looking for stand-alone open source software for simulation of photonic components or circuits, then there is plenty to find on the web. Each with their own interface, which makes using them not always straightforward.
A number of tools (without any claim of completeness or even usefulness) is listed here:
- Manfred Hammer’s “simulations in integrated optics”, http://siio.eu (not open source but free for use)
- CAMFR – full-vectorial Maxwell solver (open source)
- mpb – MIT photonic bands (open source)
- meep MIT electromagnetic equation propagation, FDTD (open source)
- openEMS – EM field solver, FDTD (open source)
- SMU photonics group – different tools for photonic simulations (free for use or open source)