In optical systems, Fresnel reflections from an optical surface have a variety of undesirable effects. These may include reduced transmittance, feedback into laser systems, and stray reflections.
In bulk optics, Fresnel reflections are traditionally reduced using thin film dielectric stacks of materials with alternating high and low refractive indices.
As a result of thin film interference effects, these stacks may be designed to behave as anti-reflective (AR) coatings for a range of wavelengths. Such coatings, however, may have several problems associated with them. For example, they may exhibit laser-induced damage thresholds (LIDTs) significantly lower than those of the bulk optics, may be subject to environmental degradations and delamination under thermal cycling and may perform well only for a limited optical bandwidth and angular range.
Navy scientists and engineers have developed an approach for reducing Fresnel reflections while simultaneously reducing the problems associated with traditional AR coatings. The process involves direct nano-patterning of anti-reflective surface structure (ARSS) on the surface of an optical material. Processing of these structures does not involve a permanent coating on the optic. Nano-patterning of the surface results in antireflective performance of ARSS comparable to that of the traditional AR coatings while adding significant advantages such as higher laser damage thresholds, wide spectral bandwidths, and large acceptance angles.
- ARSS could provide a gradual transition in refractive index from one medium to another such that the effective index in a given plane increases
- ARSS on fiber tips could provide AR performance while increasing LIDT and environmental stability
- Patterning ARSS bay be done by an etch or stamp process
- U.S. Patent 10,126,504 available for license
- Potential for collaboration with Navy researchers