Development of a high total radiation dose resistant vertical-cavity surface-emitting laser driver with discrete COTS components

Radiation tolerant fiber optic transmission links are desired for their potential use in future thermonuclear fusion reactors and other harsh radiation fields. As part of it, we design and test a radiation resistant opto-electronic transmitter based on vertical-cavity surface-emitting lasers (VCSELs) and dedicated driver electronics consisting of discrete components. VCSELs have already demonstrated their high radiation acceptance level. We confirm this by on-line irradiation experiments on such devices up to a 10 MGy total dose. For the design of the driver circuit, we rely on discrete commercial-off-the-shelf (COTS) bipolar transistors. When the radiation induced degradation of these components is considered within the design of the circuits, total dose levels larger than 1 MGy can be tolerated. The driver uses standard TTL input signals and delivers a forward current of 12 mA to a pigtailed 840 nm VCSEL. SPICE simulations show that the driver still delivers a sufficient forward current to the VCSEL, in spite of the radiation induced degradation of the h_fe and V_CEsat values of the transistors. These simulations are verified by our experiments. At a total dose of 1 MGy, the measured decrease of the forward current is only about 8 %. This induces an optical output power decrease that can still be tolerated with irradiated VCSELs, as shown by our experiments. We conclude that a high total dose hardened optical transmitter for use in nuclear instrumentation systems can be fabricated using discrete COTS bipolar transistors, COTS vertical-cavity surface-emitting lasers and COTS optical fiber.