tion of DC isolation between circuit blocks, and find widespread application on spacecraft. The primary performance metric is the current transfer ratio (CTR), which is the ratio of the photodetector collector current to the LED forward (i.e., drive) current. There are many different optocoupler designs and applications (digital and linear), and the radiation response is highly dependent on both factors. As a result of on- orbit failures of these devices in military and civilian spacecraft, laboratory investigations have been performed that confirm the important role of displacement damage in device degradation. The failure in the TOPEX/Poseidon mission was due to CTR degradation at equivalent TID levels of 10-20 krad(Si) where about 1/3 of the TID was contributed by protons [Rax96]. On-orbit errors may also occur as a result of single event transients (SETs) as discussed in Part IVA of this course. The experience to date has been that SETs are more likely to be an issue for high speed applications (> 1 MHz), whereas displacement damage effects have been most pronounced in optocouplers with a type of LED used in lower speed circuits. Investigations of the radiation response of current optocoupler technology include [Lisc93, John96, DOrd97, Reed98]. To date, optocouplers with amphoterically doped AlGaAs LEDs have displayed the highest sensitivity to proton damage, and modern devices have proved significantly more susceptible than earlier generations [Rose82]. The radiation response of optocouplers is complicated by several factors that have been considered in the recent work cited, and continue to be investigated. First, the devices are hybrid modules that may exhibit large part to part variability. A given commercial hybrid may have internal components (such as LEDs) that cannot be traced and may from several sources. Second, the observed radiation induced degradation results from a combination of TID and displacement damage mechanisms, and the relative importance depends on the optocoupler design and application