I found this:

The Atlec used laser weapons on their ships and once targeted them on the Enterprise-D in an attempt to acquire the Straleb Jewel of Thesia and Thadiun Okona. Picard noted that such lasers would not even penetrate their navigation shields. Starfleet regulations required that a vessel targeted with laser weapons go to yellow alert; it was considered a very old regulation. (TNG: "The Outrageous Okona")