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 �m 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