How about heating the tip of a straight pin with a lighter and push the heated tip into the broken optical cable, melting the plastic as it penetrates. Let it cool and pull it out. Even if it doesn't hold, you'll now have a hole in the broken optical cable that's the same size as the pin. At that point, you can dip the tip of the pin in some low viscosity super glue like Jet or Aaron Alpha. Jet is available at most model supply stores. Aaron Alpha is available at most Ace Hardware stores. Both are extremely low viscosity cyanoacrylate glues and will fill any narrow void. Careful... it's very easy to glued things together that you did not mean to glue. Things like your fingers. Also, the glue will run a long way into crevices. Be careful only to use enough glue to get the job done or you'll also glue everything else in the general vicinity. That's the reason I suggest dipping the end of the pin in the glue. That way, you'll get only enough glue to get the job done. DON'T try to apply directly to the broken piece on the optical cable or you'll end up just gluing that piece in place, guaranteed.