Design and characterization of a radiation tolerant optical transmitter using discrete COTS bipolar transistors and VCSELs

In this paper, we design and test a radiation tolerant opto-electronic transmitter based on vertical-cavity surface-emitting lasers (VCSELs) and dedicated driver electronics consisting of discrete components. VCSELs have already demonstrated their good radiation tolerance 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 %, as measured for three driver circuits. 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.