There was a download of OptSim 3.6 on the developer's website when we last checked. We cannot confirm if there is a free download of this software available. This program is a product of RSoft. We recommend checking the downloaded files with any free antivirus. Commonly, this program's installer has the following filename: optsim.exe. The software is included in Photo & Graphics Tools. The most popular version of the OptSim is 3.6.
From the developer:
OptSim is RSoft's award-winning software tool for the design and simulation of optical communication systems at the signal propagation level. With state-of-the-art simulation techniques, an easy-to-use graphical user interface and lab-like measurement instruments, OptSim provides unmatched accuracy and usability.
You may want to check out more software, such as Block CAD or BabyaCAD, which might be related to OptSim.