Anti-reflective Surface for Infrared Optical Fiber Endfaces
Navy SBIR 2019.2 - Topic N192-067
NAVAIR - Ms. Donna Attick - [email protected]
Opens: May 31, 2019 - Closes: July 1, 2019 (8:00 PM ET)
TECHNOLOGY AREA(S): Air Platform, Materials/Processes ACQUISITION PROGRAM: PMA272 Tactical Aircraft Protection Systems
OBJECTIVE: Develop an anti-reflective surface for use on bare and connectorized infrared fiber optic cable assembly endfaces.
DESCRIPTION: Fiber optic cables are used to deliver traditional optical communication data and signals; however, they can also be used to transmit high intensity light. Several high intensity light transmission applications require wavelengths beyond the near infrared, extending into the short and mid-wave spectral regions. The wavelength range of interest is 1.4 to 5 micrometers. In the 1.4 to 5 micrometer wavelength region, optical materials with a large index of refraction are often used. According to the Fresnel equations, reflection loss increases significantly when the difference between the index of the exit medium and the index of the entrance medium is 1 or greater.
In addition to the need for low reflectivity, anti-reflective surfaces must be tolerant to high optical power. For fiber optic applications, optical power is focused on a microscopic fiber optic core resulting in large irradiance.
Traditional antireflection coatings are advertised to withstand 3-10 Joules per centimeter squared (J/cm^2) with 10 nanosecond (ns) pulses. Optical damage thresholds are lower when defects are present on anti-reflective surfaces. When illuminated with a high intensity light source, heat accumulates at these defects, causing the surface to be damaged. In some cases, the surface damage will be due to melting, vaporization, or sublimation. Some fiber optic cables may produce hazardous particulates or fumes when damage occurs. Also, in some cases, the temperatures produced by anti-reflective surface damage can induce ignition in an explosive atmosphere or nearby flammable material.
Anti-reflective surfaces that improve upon traditional anti-reflective coating damage thresholds are needed to withstand at least 10 J/cm^2 with 10 ns pulses within the operating wavelength range. The surface must also withstand at least 1 megawatt per centimeter squared (1 MW/cm^2) average power with continuous wave light sources. The anti-reflective surface is intended to also transmit 1.4 to 5 micrometers light throughout the range of angles defined by the selected fiber’s numerical aperture and should not be damaged by misalignment of the light source with the fiber core.
As a threshold, the anti-reflective surface should be capable of producing less than 2.5% reflectivity when designed for simultaneous emission of any three laser wavelengths selected within the 1.4 to 5 µm region. The wavelength separation between laser outputs should not be less than 350 nanometers. This reflectivity threshold allows wavelength sensitive solutions to be considered as long as the anti-reflective surface design can be optimized to support simultaneous transmission of three wavelengths. The anti-reflective surface should have a minimum reflectivity of less than 1% at a single optimized wavelength.
The anti-reflective surface should be realizable on non-silica optical fiber. The infrared fiber types of interest include indium fluoride, chalcogenide, tellurite, and ZBLAN.
Fiber optic cables should be designed to assemble with SubMiniature Version A (SMA) 905 connectors and be compatible with short and mid-wave laser sources. The fiber optic cable assembly must pass thermal, vibration, and humidity environmental testing. Vibration testing should assume operation within a helicopter environment, and MIL-STD-810G should be used as the basis for environment testing of fiber optic cable assemblies.
The anti-reflective surface designs should be validated via modeling, simulation and/or laboratory testing. During laboratory testing, specular transmission power, spatial beam stability, and diffuse scatter (hemispheric angular
losses) should be captured. Once the design is mature, the approach should be implemented and tested on fiber optic cable assemblies. The end result of this project is an anti-reflective surface with an improved damage threshold that is able to be manufactured.
PHASE I: Design, model, and demonstrate a proof of concept anti-reflective surface for short and mid-wave spectral region optical fibers and fiber optic cables. The Phase I effort will include prototype plans to be developed under Phase II.
PHASE II: Optimize the anti-reflective surface design from Phase I. Fabricate, test, and deliver fiber with the prototype anti-reflective surface. Demonstrate and test an infrared fiber optic cable assembly and quantify the damage threshold and transmission properties. If necessary, perform root cause analysis of anti-reflective surface failures, and remediate anti-reflective surface failures. Establish a plan for full volume production and a commercialization strategy for this technology in preparation for Phase III.
PHASE III DUAL USE APPLICATIONS: Qualify the anti-reflective surface on aircraft representative short and mid-wave fiber optic cable assembly designs. Integrate the anti-reflective surface technology into DoD systems that use short and mid-wave fiber. Initiate manufacturing technology development to improve cable assembly producibility using the anti-reflective surface technology.
This technology would improve the reliability of commercial fiber optic cables. Additionally, the anti-reflective surface may be compatible with non-fiber optic surfaces. The commercial market for anti-reflective surfaces includes lens manufacturing, light emitting diode (LED) fabrication, laser fabrication, and other technologies requiring surfaces that efficiently transfer light.
1. “High-Power Multimode Fiber Optic Patch Cables: SMA to SMA.” Thorlabs. https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=4393
2. Lyngnes, O., Ode, N. and Ness, D. “Anti Reflection Coating Damage Threshold Dependence on Substrate Material.” Precision Photonics Corporation: Boulder, CO. https://pdfs.semanticscholar.org/7347/046b9d32577ce2d7a416e7579df197bf7aeb.pdf
3. Ronian, S., Hanson, C., & Erdogan, T. “Laser Induced Damage Threshold of Optical Coatings.” An IDEX Optics & Photonics White Paper, 2013. CVI Laser Optics: Albuquerque, NM. http://www.masbonfante.it/download/cvi/CVI_LIDT_WhitePaper_FIN.pdf
4. “The Complexities of High-Power Optical Coatings.” Edmund Optics Worldwide. https://www.edmundoptics.com/resources/application-notes/optics/the-complexities-of-high-power-optical-coatings/
5. MIL-STD-810G, Department of Defense Test Method Standard: Environmental Engineering Considerations and Laboratory Tests. http://everyspec.com/MIL-STD/MIL-STD-0800-0899/MIL-STD-810G_12306/
KEYWORDS: Anti-Reflection; Fiber Optics; Lasers; Photonics; Optics; Infrared