 the errors are due to direct ionization from protons [Mars-94a],. The solid lines in the curve follow the customary Weibull form, and the family of Weibull curves corresponds to different levels of optical power used in the operating link. IV-35 Data Rate Effects on Cross-Section ETX75 18 MeV He lons at 70° Data Rate 1.0 GHz BER is # bits lost 0.4 GHz 0.2 GHz per bit transmitted BER = 63 MeV p+ at 50° Data Rate Optical Power (dBm) Figure 16. The Bit Error Ratio (BER) cross section for proton strikes on a photodetector is approximately proportional to data rate irrespective of optical power or LET. The relations indicate that a cross section that is proportional to data rate results in a BER which is independent of data rate. 4.2.3 Optocouplers and Metal-Semiconductor-Metal (MSM) Photodiodes: The introduction and section 4.1 offered discussion and references to the extensive literature describing "conventional" proton effects involving nuclear reactions and the indirect upset mechanism. The section 4.2 then dealt with a newer formalism that applies to devices that exhibit behavior that is dominated by direct ionization from protons. Not surprisingly, some devices show characteristics of both behaviors. This section examines two such examples. Optocouplers have received a great deal of attention for their sensitivity to displacement damage from protons and this will be discussed at length in Section IVB on displacement effects. At the 1997 NSREC it was reported that high bandwidth optocouplers could also be sensitive to proton-induced transient effects [LaBe-97]. That study reported that proton initiated transients exhibited a rapid onset and then dissipated with a time constant governed by the bandwidth of the device. At first this appeared to be another example of the general problem of transient effects in linear devices, caused presumably by proton reaction recoil products. But on closer inspection, the angular dependence seen in figure 17 shows an enhancement in the cross section around the plane of the packa