THE USE OF ULTRASONIC IMAGING FOR NON-DESTRUCTIVE EVALUATION OF RADIATION DAMAGE TO DIELECTRIC OPTICAL COATINGS

摘要

Certain space borne optical system designs require that dielectric coated lenses be exposedto the energetic electrons and protons associated with the space environment. The dielectriccoating can charge to a potential that causes electrostatic discharge to occur within thecoating layers. This can cause melting and re-crystallization of a portion of the coating andlens, but the damage may not be apparent in a visual or microscope inspection if the lens isintended for use with light outside of the visible wavelength range. Inspecting the lens with amicroscope that operates in the wavelength range for which the lens is designed is alsodifficult, because the specular surface of the lens makes it difficult to know when themicroscope is focused at the depth where damage will be found. Experiments wereconducted with an ultrasonic imaging system to find out if this technique is useful fordetecting radiation-induced damage within a lens. The experimental technique provedsuccessful. The ultrasonic microscope selects a portion of the echo signal that is defined tobe within a specific time interval after the echo from the front surface of the coating, and usesthat measurement to paint one pixel in an overall inspection picture of the lens. By using atransducer with a frequency appropriate to the thickness of the reflective coating, andexamining the trailing edge portion of the echo signal reflected from the depth of the coatinglensinterface, it was possible to form an image that revealed the electrostatic dischargedamage. This made it possible to characterize the radiation damage with a non-destructivetest, to see how total dose and energy affect the amount of damage, and to correlate thedamage with the optical properties of the lens.