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Third Generation Sensor Anti-reflecting Surface Textures for Maximum Transmission through Infrared Optical Surfaces

Description:

 
 

TECHNOLOGY AREA(S): Materials/Processes

The technology within this topic is restricted under the International Traffic in Arms Regulation (ITAR), which controls the export and import of defense-related material and services. Offerors must disclose any proposed use of foreign nationals, their country of origin, and what tasks each would accomplish in the statement of work in accordance with section 5.4.c.(8) of the solicitation.

OBJECTIVE: Second Generation Infrared Sensors have a single band focal plane with one F# and 5 or 6 lenses. Third Generation Sensors have a dual band focal plane with two F#s and a plethora of refractive lenses and mirrors. As a result the transmission through this optical system is about 65% of second generation sensors with the resultant increase in noise equivalent temperature difference. It is the objective of this topic to develop anti-reflecting surface textures to have transmission values equal to or better than Second Gen. These surface textures must include both the MWIR and LWIR bands. Innovative solutions to this important problem are being sought after such as the motheye or structured gradient meta-materials. Successful completion of this project would have an overwhelming positive impact on the performance of the 3rd Gen Sensor.

DESCRIPTION: Surface treatments have been optimized for both the LWIR and MWIR spectral band for systems where only a single band is being used. The next generation of systems being developed by the Army will be imaging simultaneously over a much broader band. There are currently surface treatments available that cover this broad spectral range, however, their performance is insufficient to meet all the needs of the Third Gen Sensor systems. The surface treatments are not limited to just one component of the optical system, each set of components needs improvement in transmission to see the largest improvement in performance. The components are listed below in the order of the potential for most significant impact on overall system performance. Radioactive surface treatments are not an acceptable solution.

Refractive lenses:

 

Typically these surface treatments are degraded by 3-5% versus a standard surface treatment for just the MWIR or LWIR individually. Overall system performance is degraded both because of the lower transmission surface treatment and due to the increased complexity in the design of these systems which result in an increased number of optical elements. The goal is to obtain greater than 99.2% average transmission per lens with high yield for the 3.5 -5.0 µm and 7.8 – 10.5 µm spectral bands (based on a 1mm material thickness). At a minimum, materials to consider are Germanium, Zinc Selenide (ZnSe), Zinc Sulfide (ZnS), Barium Fluoride, and Gallium Arsenide, but also chalcogenide glasses such as AMTIR, GASIR lenses, and any other materials that will meet the transmission spectrum with environmental stability.

Mirror surface treatments:

 

Mirror surface treatments have two different requirements depending on their location in the system. Typical 3rd Generation FLIR systems will consist of a reflective afocal that will be required to pass light from 0.5 µm to 11 µm tend to have a reflective component to them creating a demand for high reflectivity surface treatments over an extremely broad range. Greater than 99% reflectivity is desired in the 3.5 -5.0 µm and 7.8 – 10.5 µm spectral bands while maintaining greater than 97% reflectivity over the remainder of the spectral band. In addition to the afocal surface treatments, fold mirrors will exist that do not require transmission beyond the 3.5 -5.0 µm and 7.8 – 10.5 µm spectral bands. These surface treatments can be further optimized due to the limited spectral bands.

Beamsplitter surface treatments:

 

In order to pass multiple spectral bands through the same aperture, it is required to combine the light paths prior to the afocal. This is accomplished via a beamsplitter that will be required to both pass the 3.5 -5.0 µm and 7.8 – 10.5 µm spectral bands and reflect the 0.5 – 2.0 µm or reflect the 3.5 -5.0 µm and 7.8 – 10.5 µm spectral bands and pass the 0.5 – 2.0 µm spectral band. It is desired to be able to achieve greater than 95% for the 3.5 -5.0 µm and 7.8 – 10.5 µm and greater than 92% for the 0.5 – 2.0 µm spectral band.

Cold filter surface treatments:

 

An important component of the 3rd Gen FLIR Dewar is the cold filter located inside the cold shield. This filter is at nearly the same temperature as the focal plane (~80K) and controls the amount of out of band radiation that reaches the detector. It is desired to have very high transmission in band while rejecting the out of band spectrum. A transmission of greater than 95% within the 3.5 -5.0 µm and 7.8 – 10.5 µm spectrum is desired.

Windows:

 

Broadband windows are another significant component that needs improvement. Windows offer an additional challenge in that they require a surface treatment that is not only highly transmissive, but also durable. In addition, windows may be required to pass light from 0.5 µm to 11 µm in order to maintain the desired common aperture between sensors that is present in 3rd Gen systems. It is desired that a 3rd Gen window be able to meet a 95% transmission over the spectral band while maintaining a severe abrasion resistance.

Environmental Conditions:

 

The surface treatments shall meet specified performance after being stored in temperatures IAW ATPD-2404A, 5.2.2.1 and temperature shock IAW ATPD-2404A, 5.2.4. The surface treatments shall meet the Operational Humidity IAW ATPD-2404A, 5.1.3.3. The surface treatments shall meet the exposure to blowing sand IAW ATPD-2404A, 5.1.6.2.

PHASE I: Create theoretical anti-reflecting surface texture designs that will exceed the performance of the current state of the art as described in the topic description. Develop a plan for improved processes to increase yield in surface quality to achieve as built surface treatments closer to that of theoretical. The cost for this innovative approach should not be greater than what is used in second generation FLIRs.

PHASE II: Revise surface treatment designs from phase I as needed and provide witness samples for all IR materials and components suggested in the topic description meeting each requirement. Develop surface treatments for other materials that are applicable for dual band applications. Implement improved surface treatment processes developed in Phase I to increase yield of dual band surface treatments. Provide samples for testing of transmission and durability.

PHASE III DUAL USE APPLICATIONS: Military Application: Successful demonstration of this technology will lead to its insertion into the Third Gen FLIR program that will be fielded by PMdGS. The success of this technology will immediately improve the performance of 3rd Gen technology (and other dual band infrared technologies), and be immediately inserted without impacting any other system components. Commercial Application: The same impact would be expected for commercial applications that would utilize infrared focal planes. Applications include law enforcement, search and rescue, and high sensitivity broad-band radiometric measuring devices.

REFERENCES:

  • "Army applications for Multi-spectral Windows," John Hall, SPIE Proceedings 3060, 1997
  • "Dual f/number optics for third generation FLIR systems," Jay Vizgaitis, Proceedings of SPIE -- Volume 5783, Infrared Technology and Applications XXXI, Bjorn F. Andresen, Gabor F. Fulop, Editors, May 2005, pp. 875-886.
  • "Third Generation Infrared Imagers," Paul R. Norton, James B. Campbell III, Stuart B. Horn, and Donald A. Reago, Proceedings of SPIE -- Volume 4130, Infrared Technology and Applications XXVI, Bjorn F. Andresen, Gabor F. Fulop, Marija Strojnik, Editors, December 2000, pp. 226-236.
  • “Design, Fabrication, and Measured Performance of Anti-Reflecting Surface Textures in Infrared Transmitting Materials” Douglas S. Hobbs and Bruce D MacLeod Proceedings of SPIE Volume 5786-40
  • “Perfect anti-reflection from first principles” Kyoung-Ho Kim & Q-Han Park; Scientific Reports 3, Article number: 1062, January 2013

KEYWORDS: Electronics, infrared, dual band, anti-reflection, motheye, Meta materials

 

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