Harmonic plasma switch

Abstract

A system for protecting a radiation-responsive device, such as an infrared sensor in an imaging system includes a plasma switch operative in response to amplitude of incident radiation. The protection system is suitable for protecting the infrared sensor from a high-intensity laser beam which might impinge upon receiving optics of the imaging system. The plasma switching responds differently to different portions of the electromagnetic spectrum, a lower frequency portion being either transparent or reflective of the infrared radiation, while an upper frequency portion absorbs radiation to initiate a high or low density of free-charge carriers in the plasma dependent on the intensity of photons injected into the plasma in the higher frequency band. Incoming infrared radiation is passed through a nonlinear crystal which serves as a harmonic generator for generating a harmonic component of the fundamental frequency, the fundamental frequency being below the plasma frequency and the harmonic component being above the plasma frequency. Thereby, photons injected at the harmonic frequency are absorbed into the plasma to raise the free-charge carrier density sufficiently to convert the plasma switch into a reflector of the fundamental component of the radiation under conditions of excessively high incident radiation.