A Monolithic Integrated Device For Fast Radiation Detection With An Optical Output

Abstract

Detection and characterization of high speed x-ray, gamma ray or particle beam signals in an intense radiation environment pose problems for conventional electronic systems since the presence of the intense radiation field interferes with the electronic systems used in the diagnostics. It is therefore highly desirable to remote the diagnostic equipments away from the radiation environment, where radiation sensors convert the radiation signals into electrical signals being transported to the remote location. However, the transportation of these high speed signals with gigahertz bandwidths is not a trivial matter, due to its very high bandwidth and its total immunity to electromagnetic interference. It would therefore be necessary to convert the electrical signal from the radiation detector into an optical signal for subsequent transportation. This is schematically illustrated in Fig. 1, where the transmitter refers to an electrical-to-light conversion device, and the optical fiber can be many kilometers long before affected by bandwidth limitations. Semiconductor lasers with gigahertz modulation bandwidths have been developed in recent years for high speed analog signal transmission, and are ideal for the present application. Since these semiconductor lasers as well as the radiation detectors are fabricated from GaAs based compounds, it is logical to consider integrating them into a single monolithic device. The integrated device takes the form shown in Fig. 2, which shows a photo (radiation) detector connected in series with a laser diode. The DC bias current is for biasing the lasing into the linear region above lasing threshold, and the signal generated from the photoconductor modulates the current into the laser thereby modulating the optical output.