Can gravitational waves be verified by optical signals? I thought the whole point of LIGO is that it's a method not relying on electromagnetic signals

Yes. In fact that this is the exact (very exciting and somewhat recent) experiment that proved that if gravity does propagate in higher dimensions, then the dimensions must be very thin.

https://youtu.be/3HYw6vPR9qU

Edit: To be clear: the experiment I’m referring to was the observation of two neutron stars merging. Since there was no event horizon prior to the merger, we’d expect a lot of EM energy to be radiated away. This new observation is neat because we’d expect much less EM energy out of a black hole merger.

Depending on the source, we may or may not expect photon's to accompany the gravity waves. The most note worthy one so far was a gravity wave signal that we associate with a neutron star merger[1] that we also managed to catch a lot of photon signals which also matched what we think should come out of such a collision. Most black hole mergers that I know of would be dark. I imagine if they had a large accretion disks associated with them they might generate some light as well as gravity waves? But I'm not sure, I get most of my astronomy over lunch with colleagues.

[1] https://www.sciencemag.org/news/2017/10/merging-neutron-star...

I recommend this video of the press conference from the neutron star merger discovery.

https://www.youtube.com/watch?v=mtLPKYl4AHs

Its an amazing example of collaboration where it stars with gravitational waves, that helped locate it and quickly half the world of astronomy was looking at it on every wave length. Its an unbelievable example of cooperation and using different observatories to look at the same event.

It also shows how LIGO can be used to find events, and then others can point there to get more information.