Not cheap, but 3M security film is pretty effective:
Combine this with a couple Sliding Door Security Bars (one high, one low), motion-activated lighting, and a loud alarm and I think you'd be safe from anything short of a deranged madman with a chainsaw.
Security film is nice, but for what it costs to armor a french door(~$100 per door leaf) I could install security screen doors. For that kind of money, I'd save it for hardening a designated "safe" room in the house.
Also, it's a french door, not a sliding glass door. The only type of security bar that's any good is the braced type.