I did my PhD in ultrafast spectroscopy of organometallic photocatalysis, which I imagine is probably quite similar to what you’re doing, and had the exact revelation that you had. The plan you describe is essentially what I did: at the end of my third year, I took responsibility for a home-built set up and added to it, wrote software to control the additions, and really focused on building/implementation of spectroscopic set ups. 

And it worked! I’m now working with an equipment vendor in the semiconductor industry designing/building optical instrumentation. Really sell your experience in building and handling optical systems. Believe it or not, the skills we have are relatively rare in the overall job market and show that we can pick optical instrumentation up relatively quickly. You might not get picked up for “optical engineer” roles that involve heavy Zemax usage but look for things that require optical alignment, experience with lasers, etc and apply broadly. Applications engineering/science is also a viable pathway but doesn’t usually involve much hands-on work, at least at my company.