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EXTENDING ENLIGHTENMENT AREA OF SMALL-SIZE OPTICAL ELEMENT BY COATING WITH A SPECIFIED THICKNESS DISTRIBUTION

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Anti-reflection coating formed by layers with specified distribution of geometric thickness on the surface of optical elements of a very small radius (2—12 mm), is investigated. Distribution of the energy reflection coefficient along the optical element surface is studied. It is revealed that for the given coating structure, the relative size of the enlightenment area defined as the ratio of the radius for the optical element surface where the reflection is less than a certain value (ρ) to the element radius (r), is independent of the curvature radius of the optical element made of homogeneous material. The relative size ρ/r depends of the refractive index of the optical element material and of the antireflection coating design. For the reflection coefficient of less than 1 %, it is possible to obtain the maximum enlightenment area of relative size ρ/r = 80 % with single layer coating, ρ/r = 82 % with double-layer coating, and ρ/r = 81.5 % with three-layer coating. It is shown that multiplying the number of coating layers allows for insignificant extension of the enlightenment area, but makes the coating applicable in a wider range of wavelengths. It is concluded that for extending the enlightenment area, formation of coating layers with a specified distribution of geometric thickness is required.

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