The specification describes optical fibers that are constructed to prevent theft of optical signals. One construction is designed to block access of the core (61) of the fiber to the "writing" radiation necessary to form a grating tap. In this embodiment the optical fiber cladding (62-66) is provided with a highly absorbing UV layer (65). In a variation of this embodiment, one or more additional optical paths (68) are provided in the optical fiber to accommodate monitoring signals. The added optical paths (68) allow monitoring signals to be transmitted in the optical fiber, separate from the information signal, to signal an attempt to breach the outer coating or the cladding (62-66) of the optical fiber. A second case of intrusion is addressed by increasing the sensitivity of the optical fiber to microbending loss to the extent that bends in the fiber cause such high attenuation of the signal that the bends do not go undetected at the receiving station.