Studies of semitransparent optoelectronic position sensors

Semitransparent optoelectronic position sensors (ALMY sensors) have been developed for high-precision multipoint position and angle measurements of collimated laser beams over a large measurement range. The sensors provide a position resolution in the order of a micrometer over sensitive areas of several square centimeters. They consist of a thin film of amorphous silicon deposited on a glass substrate between two transparent layers of crossed strip electrodes. A transmittance of 80%-90% has been achieved for 780-nm laser light produced by diode lasers. We report about recent optimizations of the sensor performance and tests of the long-term stability under laser illumination and of the radiation tolerance at high neutron doses. As expected, the radiation hardness of the amorphous silicon sensors exceeds the one of crystalline silicon devices. The custom-designed readout electronics allow for operation at sufficiently low laser intensities in order to prevent significant degradation of the performance of the amorphous silicon sensors under illumination with laser light.