Reliability study of photodiodes for their potential use in future fusion reactor environments

Future thermonuclear fusion reactors need remote-handled equipment for maintenance tasks, since the stringent environmental conditions prohibit direct human interventions. Fiber-optic technology is considered since many years as a potential reliable alternative to conventional electronic transmission lines. Recently, we demonstrated the feasibility of transmitting analog data with a hybrid opto-electronic link at 850 nm, up to total doses of several MGy. For bidirectional communications under these severe conditions, however, we still need to characterize the corresponding photo detector' response and design an adapted radiation tolerant amplifier. Moreover, the neutron sensitivity needs to be assessed as well, since this could prevent them to be stored inside the bioshield during the plasma bums. We therefore assessed the radiation response of commercially available p-i-n type Si-detectors, on line, at a constant temperature of about 60°C, first under gammas rays up to a total dose of about 10 MGy, and also under neutrons up to a fluence of about 7·10 ¹⁵/cm². Similar tests were also performed with InGaAs photodiodes, in order to assess their use in radiation tolerant coarse wavelength division multiplexing (CWDM) transmission architectures. Our results indicate that the response of the detectors under gamma radiation remains almost unchanged. The displacement damage caused by the neutrons steadily increases with the neutron fluence, both in terms of an increased dark current and a reduced photo response. In this paper we also present temperature dependent measurements and hence assess to what extend both ionizing and particle radiation affect the photodiodes reliability.