Hollow-core MWIR Optical Fibers
Abstract
Mid-infrared optical sources are very important for applications such as LADAR seekers, target illuminators, designators, target trackers, infrared counter measures, and standoff chemical/biological agent detection. This is especially so in the atmosphere transmission windows. In addition to defense, mid-infrared sources also have a wide range of applications in molecular spectroscopy, biomedicine, and environmental and industrial sensing. Optical fiber lasers have proven to be an excellent platform for high-power lasers, largely due to the closeness of the heat-dissipating surface to the active core and large surface-to-volume ratio. Conventional silica optical fibers, however, do not transmit lights in mid-infrared due to strong phonon absorption. Heavy metal fluorides provide good transmission up to a wavelength of ~6µm and chalcogenides to a wavelength of ~10µm due to low maximum phonon energies as a result of the weak ionic bonds in these glasses. These weak bonds, however, also lead to poor strength and stability. This makes them prone to degradation in humid environments at room temperature, let alone the more corrosive environments at elevated temperatures possibly required for industrial process monitoring, health and safely monitoring, environmental monitoring, and countermeasures. Recently, negative curvature design has enabled record low losses in silica optical fibers at mid-infrared not traditionally associated with silica fibers. The fiber loss demonstrated is ~0.1dB/m at ~4µm where the material loss of silica is ~1000dB/m. Further more, these fiber losses are much lower even than the lowest loss ever demonstrated at mid-infrared in fibers made from low-phonon-energy glasses such heavy metal fluorides and chalcogenides! We now, for the first time, have a low-loss mid-infrared optical fibers which provides high strength, robustness, and high reliability, all at the same time, thus being much better suited for extreme environments than any low-phonon-energy glass we know. The fact that the light is mostly propagated in air also makes these fibers much less prone to optical damage and to have much higher nonlinear thresholds, making these fibers also ideal for high-power operations. Hollow-core optical fiber can potentially be used to combine fiber optic and gas laser technologies to efficiently generate mid-infrared sources at the higher powers. The hollow-core optical fiber combines the compactness and long interaction length features of fiber lasers with the potential for high output power, excellent thermal management, high damage threshold and narrow line width operation of gas lasers. Our simulations have shown much superior first-bandgap designs exist with much lower MWIR losses than state-of-art fibers which are based in the third bandgap. The loss at 5µm for the first-bandgap design can be 0.32dB/m, over 3 orders of magnitudes lower than that for the corresponding third-bandgap design used in the existing state-of art hollow-core fibers. An optimized design is already identified, which can be appropriately coiled to provide optimal suppression of higher-order modes. In this work, we plan to develop a plan to fabricate the hollow-core fibers optimized for MWIR transmission, to optimize the fabrication process and to demonstrate hollow-core fiber with record low loss in the MWIR. These fibers are critical for highly efficient high-power MWIR source generation from 3-5µm. The proposed work will significantly advance education and knowledge in the areas of hollow-core fiber designs and fabrications, MWIR fiber lasers, and high-power MWIR sources.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jul 24, 2019
- Source ID
- W911NF1910409
Entities
People
- Liang Dong
Organizations
- Army Contracting Command
- Clemson University
- United States Army