A rod having an embedded optical fiber has a stiff composite outer layer (10) to make the rod self straightening to enable it to be pushed into a pipe or borehole from a spool. This can help enable a reduction of friction between rod and conduit and can enable longer reach. A barrier layer is provided to separate the fiber from the composite layer. The rod can be retrieved after use. The rod can be narrow enough to enable normal flow along the pipe or borehole, and can be injected and retrieved even when the pipe or borehole is pressurized. The fiber can be used for remote sensing of conditions along the conduit. Other tools can be inserted by the rod during the intervention.