Optimizing Photodetectors for Digital Communication in Radiation Environments

A number of military applications require the operation of optoelectronic data links in ionizing environments. In a previous study, an analysis of the analog signal response of Si and other photodiode materials to light and to ionizing radiation for single-pass, two-pass, and multipass optical detectors was presented. The purpose of this paper is to extend the analysis to treat the digital signal response of single-pass and multipass Si and other PIN photodiode structures. The input optical power P0 required to give a signal-to-noise ratio of one is determined as a function of junction depletion width and ionizing dose rate. These calculations show that optimized photodiode structures require moderately low values of P0 to give a signal-to-noise ratio of one. For example, at dose rates of 106 rads/sec and a bandwidth of 7.5 MHz, values for P0 of 4.7 × 10-7 to 1.8 × 10-6 W are expected for realistic optimized photodiodes. In addition, the use of high optical absorption coefficient materials or multipass detector designs show two important improvements over single-pass Si photodiodes. First, the radiation-induced current is reduced, decreasing the possibility of saturating the following amplifier stage and, thus, relaxing constraints on amplifier design. Second, the dynamic response to ionizing radiation is significantly less. Responding to a change from zero to 107 rads/sec, the required input optical signal P0 increases only one to five times compared to single-pass Si photodiodes which require P0 increases of 10 to 100 times. Experiments are in progress to support the theoretical modelling.