reshold is found to degrade linearly with particle fluence, enabling the definition of a damage constant [Barnes 84]. LEDs are generally more sensitive to proton damage than laser diodes because they tend to have longer minority carrier lifetimes in the light emitting region, and are therefore more sensitive to the introduction of recombination centers [Barnes82, Barnes84]. LEDs may employ different lifetimes in the active volume in order to produce a device with specific qualities such as high light output or high speed, and therefore the resulting radiation sensitivities can be quite different. Although many types of LEDs are quite robust [Lisc92, Lisc93], amphoterically doped LEDs have been found to be quite sensitive to displacement damage [Barn76, Rose82, Rax96, Barr95, John98]. IV-90 Measurement Temperature: T=30°C Bias: Short Pre-radiation Fluence: 1x10 13 p/cm2 Fluence: 5x10 p/cm2 Fluence: 1x10 p/cm2 Laser Diode Current (mA) Figure 23 Optical power versus laser drive current for devices shorted during irradiation. The threshold current increases linearly with proton fluence. [After Zhao97]. In fact, such LEDs have been investigated for use as displacement damage monitors on board spacecraft [Barr89, Barr90], as have SiC LEDs [Hinr98]. Recent measurements by Rax et al. have shown that the highly efficient amphoterically doped 890 nm AlGaAs LEDs used in the low speed Optek and Micropac 4N49 optocouplers suffer severe degradation for proton fluences of interest to the satellite community [Rax96]. On the other hand, they found that the shorter wavelength (700 nm) GaAIP heterojunction LED used in the HP 6N140 optocoupler is very resistant to proton damage. Photonic devices fabricated from compound semiconductors may exhibit forward bias annealing after irradiation. This phenomenon has been observed in laser diodes [Barn70, Mind76, Lisc94, Paxt97, Zhao98] and also in InP solar cells [Ando86, Yama88, Walt91]. It cannot be explained by a rise in the diode junction temperature, and this is generally considered to 