Radiation-resistant WDM optical link for thermonuclear fusion reactor instrumentation

The future International Thermonuclear Experimental Reactor (ITER) is a complex installation that will require permanent monitoring and frequent maintenance operations. The high-gamma dose rates, the high neutron fluence, and other radiological hazards call for the use of remote-handled equipment. The management of heavy umbilicals connecting the control systems with the remote tools is therefore a key issue. Multiplexing signals can relieve the cable-handling difficulties. In this respect, the intrinsic wavelength division multiplexing (WDM) capabilities of fiber-optic technology make it a very promising candidate for integration in ITER instrumentation links. However, the radiation hardness of a complete WDM optical link still needs to be assessed. In this paper, as a first step toward the development of a rad-hard WDM optical link, we report on irradiations of different parts of a typical WDM optical link. We present our irradiation results on COTS fiber-optic devices, including WDM single-mode couplers, which remain operational up to MGy dose levels while the channel drift observed in narrow-band couplers compromises their use in WDM multiplexers. The intrinsic wavelength encoding of fiber Bragg grating (FBG) sensors makes them ideal candidates for WDM fiber-optic sensor networks. Therefore, we also investigated the γ-radiation response of FBGs written in germanosilicate fibers. We irradiated such sensors up to MGy dose levels. At a total dose of 0.1 MGy, saturation of the radiation-induced Bragg peak shift has been observed, evidencing the potential radiation hardness of FBG-based devices in highly radioactive environments. To illustrate wavelength multiplexing in sensing, we discuss our preliminary results on a new multicomponent force sensor design based on eight multiplexed FBG sensors intended for use at the end effector wrist of remote-controlled robots. Finally, we present the in-reactor irradiation results of standard Corning Ge-doped fiber up to GGy dose levels.