You can use some software for free with limitations eg OSLO https://lambdares.com/oslo-edu-download https://lambdares.com/oslo-edu-download FRED also has a de

You can use some software for free with limitations eg OSLO https://lambdares.com/oslo-edu-download https://lambdares.com/oslo-edu-download FRED also has a demo mode https://photonengr.com/fred-software/demo-request/ https://photonengr.com/fred-software/demo-request/ That might be enough for simple designs. Beam Four is also decent and allows you to use most common surface types including gratings. I'm interested in Optica (for Mathematica) but I don't know anyone that's used it.