The Effect of Gamma Irradiation on Optical Isolators

The effects of gamma radiation on optical isolators have been investigated. This study has included the simultaneous irradiation and measurement of the individual emitters and detectors making up the isolators. In this manner, the net effect of irradiation on the isolators could be attributed to the degradation of either the emitter or detector, or both. As expected, isolators containing photodiodes are more radiation resistant than those containing phototransistors. In the photodiode isolator the LED is responsible for essentially all the gamma-induced isolator degradation. The performance of phototransistor isolators depends strongly on the phototransistor bias, VCE, and the LED input current, ILED. At high ILED and low VCE where gamma-induced surface effects in the phototransistor are minimized, the degradation of the isolator is due primarily to the LED which is more sensitive than the LED in the photodiode isolator. In contrast, at low ILED and high VCE, gamma-induced surface damage in the phototransistor is the dominant effect and the isolator is quite sensitive to irradiation.