ter, and others) employ IR sensors this will change. IR sensors are generally fabricated as a hybrid of the IR detector array and a silicon readout circuit, and both elements must be considered. The IR detector elements are sensitive to both TID and, in principle, displacement damage effects, but the materials usually have high enough defect levels to mask proton induced damage to any realistic level expected on-orbit. Even if a linear Si CCD is employed for the readout circuitry, proton displacement damage effects are not generally important. The low operating temperature minimizes dark current and CTE changes, and CTE degradation is also reduced as a result of fast readout rates and relatively high signal levels. 3.3.4 Lasers and Light Emitting Diodes Photonic subsystems are increasingly widespread on satellite systems because of their performance advantages. Lasers and LEDs are employed in fiber optic communications links [Mars92, LaBe93, Mars94], optocouplers [Rax96, Reed98, Barn98], position encoders, etc. Note that laser modules employed in data links may include both lens and photodiodes, and the radiation response of the passive elements may be significant as discussed in [Mars92, Lisc93]. To date, laser diodes have proven to be relatively insensitive to proton displacement damage effects, even as technology development continues to reduce the threshold currents. Radiation does introduce nonradiative recombination centers that lower the quantum efficiency, resulting in an increased threshold current (and therefore degrades the optical power) [Barnes84]. If the threshold shift is large enough, the laser may fail to operate. Proton measurements on emerging laser technologies such as vertical cavity surface emitting lasers (VCSELs) [Paxt97] and multi-quantum well (MQW) laser diodes [Zhao98, Evan93] demonstrate very robust behavior. As seen in figure 23, MQW lasers exposed to 200 MeV proton fluences of 10 cm -2 of (corresponding to almost 1 Mrad(Si) TID), show very minimal degradation [Zhao98]. The current th