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 remotehandled 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 fibre 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 towards 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 fibre optic devices including WDM singlemode couplers, which remain operational up to MGy dose levels while the channel drift observed in narrowband couplers compromises its use as WDM multiplexer. The intrinsic wavelength-encoding of Fibre Bragg Grating (FBG) sensors make them ideal candidates for WDM fibre optic sensor networks. Therefore, we also investigated the γ-radiation response of FBGs written in germanosilicate fibres. 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 multi-component 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 fibre up to GGy dose levels. Keywords— Wavelength-division multiplexing, fibre Bragg grating sensor, optical fiber sensor, gamma radiation, radiation effects, nuclear robotics, ITER reactor.