PNNL

Abstract

Multilayer tuned infrared absorber/emitter coatings were applied to fiber optic infrared screen projectors. The coatings consisted of a three layer Si3N4/Cr/Si3N4 absorber tuned at the 1.06 µm laser wavelength, and a six layer Cr/dielectric/Cr/dielectric/Cr/dielectric coating which emitted strongly in either the 3-5 µm or the 8-12 µm infrared wavelength bands. All coatings were deposited by reactive DC and RF magnetron sputtering onto 2.5 inches fiber optic faceplates. The fibers were potted in the square array, planarized, and then the potting material removed to a depth of 20 µm to prevent cross talk between the fibers. Either Si3N4, Si, or ZnS thin film dielectric materials were used in the emitter coatings. Si3N4 was used in the 3-5 µm emitter designs, and was not usable for the 8-12 µm designs because of the Si-N Rhestrahlen band near 8 µm. Si and ZnS were used for both wavelength ranges. The absorption of the three layer coating was >0.99 at 1.06 µm. The average emittance of the six layer coatings was >0.95 at the design wavelengths. With an input laser power of 15 W, the coatings emitted at a black body temperature 529 K, which compares well with black body radiation predictions. The coatings were analyzed after thermal cycling, and some delamination occurred at the edges of the fiber surfaces. This was attributed to residual potting material left on the edges of the fibers, and to surface defects in the fibers. The selection of the metal reflective layer and a nonabsorbing dielectric layer was critical to the performance of the coating, which let to general design rules for this type of coating