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US-12618138-B2 - Anti-reflective coatings for IR-transmitting substrates

US12618138B2US 12618138 B2US12618138 B2US 12618138B2US-12618138-B2

Abstract

Optical elements including YbF 3 layers with high transmittance in the LWIR spectral range are described. The YbF 3 layer is produced by an ion-assisted deposition process under high voltage conditions. Dense, uniform, and nearly defect-free YbF 3 layers are formed. The improved material quality of the YbF 3 layers leads to low absorption in the LWIR spectral range, especially at wavelengths above 10.0 microns. The extinction coefficient of the YbF 3 layers is less than 0.0400 at a wavelength of 13.5 microns.

Inventors

  • Nicholas Levi Bredberg
  • Lance Changyong Kim
  • Yongli Xu

Assignees

  • CORNING INCORPORATED

Dates

Publication Date
20260505
Application Date
20221209

Claims (15)

  1. 1 . An optical element comprising: a substrate; and a coating in contact with the substrate, the coating comprising a layer of YbF 3 , the layer of YbF 3 having an absorption less than 2.2%/micron at a wavelength of 13.5 microns.
  2. 2 . The optical element of claim 1 , wherein the substrate is selected from the group consisting of ZnS, ZnSe, Ge, InSb, chalcogenide glasses, As 40 Se 60 , Ge 10 Se 50 As 40 , Ge 28 Sb 12 Se 60 , and Ge 33 Se 55 As 12 .
  3. 3 . The optical element of claim 1 , wherein the substrate comprises ZnSe, Ge, or InSb.
  4. 4 . The optical element of claim 1 , wherein the coating comprises a plurality of the layers of YbF 3 .
  5. 5 . The optical element of claim 1 , wherein the coating further comprises a second layer in direct contact with the layer of YbF 3 , the second layer having a higher refractive index than the layer of YbF 3 .
  6. 6 . The optical element of claim 5 , wherein the coating comprises a plurality of the second layers and a plurality of the layer of YbF 3 .
  7. 7 . The optical element of claim 6 , wherein the plurality of second layers and the plurality of layers of YbF 3 are arranged as an alternating sequence.
  8. 8 . The optical element of claim 1 , wherein the layer of YbF 3 has an absorption less than 1.9%/micron at a wavelength of 13.5 microns.
  9. 9 . The optical element of claim 1 , wherein the layer of YbF 3 has an absorption less than 0.8%/micron at a wavelength of 10.5 microns.
  10. 10 . The optical element of claim 1 , wherein the layer of YbF 3 has an absorption less than 0.8%/micron at a wavelength of 7.5 microns.
  11. 11 . The optical element of claim 1 , wherein the layer of YbF 3 has an extinction coefficient less than 0.0400 at a wavelength of 13.5 microns.
  12. 12 . The optical element of claim 1 , wherein the layer of YbF 3 has an extinction coefficient less than 0.0100 at a wavelength of 6.1 microns.
  13. 13 . The optical element of claim 1 , wherein the layer of YbF 3 has an extinction coefficient less than 0.0200 at a wavelength of 3.0 microns.
  14. 14 . The optical element of claim 1 , wherein the optical element has a transmittance greater than 80% at a wavelength of 13.5 microns.
  15. 15 . The optical element of claim 5 , wherein the second layer comprises ZnS or ZnSe.

Description

This application claims the benefit of priority under 35 U.S.C § 120 of U.S. Provisional Application Ser. No. 63/292,045 filed on Dec. 21, 2021, the content of which is relied upon and incorporated herein by reference in its entirety. FIELD The present disclosure generally relates to optical elements with high transmission in the infrared and to methods of making the optical elements. Particularly, the present disclosure relates to optical elements containing infrared-transmitting substrates and anti-reflective coatings that exhibit high transmission in the long wavelength infrared (LWIR) portion of the electromagnetic spectrum. More particularly, the present disclosure relates to anti-reflective coatings exhibiting low absorption at wavelengths above 10.0 microns. BACKGROUND The optical elements (lenses, windows, mirrors etc.) used in many types of optical instruments, telescopes, cameras, airborne detectors, satellite cameras, surveillance cameras, and missile domes typically include substrates and optical coatings designed to satisfy optical specifications needed to ensure acceptable performance for a particular application. Optical elements that are compatible with wavelengths in the atmospheric transmission window are important for many applications. FIG. 1 shows a transmission spectrum of the earth's atmosphere over a wavelength range extending up to 15 microns. The approximate absorption wavelengths of common atmospheric gases are also indicated. Optical communication and detection systems that operate over extended distances through the earth's atmosphere require optical elements that are compatible with wavelengths of high transmission through the earth's atmosphere. FIG. 1 depicts several such wavelength ranges. The region extending from 8 microns to 14 microns (referred to herein as the “LWIR” or “long wavelength infrared” region of the spectrum) offers many wavelengths of high transmission for optical communication and detection, but is a demanding region in which to operate because of the difficulty of identifying suitable materials for optical elements. Optical elements needed for optical communication and detection in the LWIR region include lenses and windows. The performance of optical elements in the LWIR requires high transmission of LWIR wavelengths. High transmission requires both high internal transmission and low surface reflection of the materials used to make optical elements for LWIR applications. Materials such as ZnS, ZnSe, InSb, Ge, As40Se60, Ge10Se50As40, Ge28Sb12Se60, and Ge33Se55As12 are known to exhibit high transmittance over at least portions of the LWIR region. These materials, however, exhibit high reflection of LWIR wavelengths, leading to a deterioration of overall performance. To improve LWIR performance, a material with high internal transmittance (referred to herein as an “infrared-transmitting material”) can be used as a substrate and an anti-reflection (AR) coating can be applied to the surface of the infrared-transmitting material to form an optical element. Anti-reflection coatings consist of a stack of alternating high index layers and low index layers. Each of the high index layer and low index layer must exhibit low absorption over the wavelength range of operation (e.g. LWIR region) of the optical element to maximize the transmission of LWIR wavelengths through the optical element. Suitable substrate materials have low absorption in the LWIR region and can also function as a material for the high index layer in an anti-reflection coating. It has heretofore proved challenging, however, to identify a suitable material for the low index layer of anti-reflection coatings for the LWIR region. Oxides are not suitable because they exhibit strong absorption in the LWIR region. Alkali halides such as CsI, KBr, KCl, NaCl, thallium bromoiodide (e.g. KRS-5) have low absorption in the LWIR region, but because of sensitivity to moisture and poor mechanical properties, they are not acceptable. KRS-5 has favorable transmission properties, but it is soft, toxic, and expensive. Fluorides are the only practical class of materials that can be used as the material for the low index layers of an anti-reflection coating in the LWIR region. LiF, for example, can transmit at wavelengths up to 30 microns, but it is not widely used because of its poor mechanical properties and high cost. Other common fluorides (e.g. CaF2, MgF2) are limited to wavelengths below 10 microns because the intrinsic absorption increases significantly at wavelength longer than 10 microns. YbF3 is a promising material for the low index layer of anti-reflection coatings in the LWIR region. Because Yb is a heavier element than Ca and Mg, the intrinsic absorption of YbF3 shifts to about 15 microns, thus offering the potential of high transmittance at wavelengths well above 10 microns. In practice, however, the measured absorption of YbF3 between 10 microns and 15 microns is relatively high and applications of YbF