long pass filter

Abstract

Abstract PURPOSE:To reduce the ripple and to increase the transmittane of an infrared long pass filter comprising the basic periods of three layers of high and low refractive index layers, by providing the prescribed three layer films of different optical thicknesses and the reflection preventing film of a single layer. CONSTITUTION:The alternate layers 2, 3 and 4 of L, H and L respectively comprising a high refractive index layer 3 of germanium and low refractive index layers 2 and 4 of zinc sulfide or silicon monoxide are formed on a germanium substrate 1 in the form of the basic periods. The optical film thickness of the layer L is set at 1/2 thickness of the layer H. Then the matching layers 5, 6 and 5 to the upper layer and a reflection preventing single layer film 7 are formed on these basic periods. The thickness of the layers 5, 6 and 5 are set at alpha(L, H, L) with 0.75<alpha<0.85 if the basic periods are set at L, H and L respectively. And the thickness of the film 7 is set at betaL within a range of 1.6< beta<1.8. As a result, the ripple is reduced with the improvement ensured for the rise of transmittance for an infrared long pass filter.

Inventors

• 👤 Tetsuo Kuwabara
• 👤 鉄夫 桑原

Bibliographic Details