Paul Foy

Silicon optical fiber

Described herein are initial experimental details and properties of a silicon core, silica glass-clad optical fiber fabricated using conventional optical fiber draw methods. Such semiconductor core fibers have potential to greatly influence the fields of nonlinear fiber optics, infrared and THz power delivery. More specifically, x-ray diffraction and Raman spectroscopy showed the core to be highly crystalline silicon. The measured propagation losses were 4.3 dB/m at 2.936 microm, which likely are caused by either microcracks in the core arising from the large thermal expansion mismatch with the cladding or to SiO(2) precipitates formed from oxygen dissolved in the silicon melt. Suggestions for enhancing the performance of these semiconductor core fibers are provided. Here we show that lengths of an optical fiber containing a highly crystalline semiconducting core can be produced using scalable fiber fabrication techniques.

Apoptosis of lung carcinoma cells induced by a flexible optical fiber-based cold microplasma.

Atmospheric pressure plasmas have been used as a therapy for cancer. However, the fairly large size and rigidity of present plasma-delivery systems obstructs the precise treatment of tumors in harder-to-reach internal organs such as the lungs, pancreas, and duodenum. In order to improve the targeted delivery of plasmas a highly flexible microplasma jet device is fabricated using a hollow-core optical fiber with an inner diameter of either 15 μm, 55 μm, or 200 μm. Described herein, based on this device, are results on lung carcinoma therapy using a microplasma cancer endoscope. Despite the small inner diameter and the low gas flow rate, the generated plasma jets are shown to be sufficiently effective to induce apoptosis, but not necrosis, in both cultured mouse lung carcinoma and fibroblast cells. Further, the lung carcinoma cells were found to be more sensitive to plasma treatment than the fibroblast cells based on the overall plasma dose conditions. This work enables directed cancer therapies using on highly flexible and precise hollow optical fiber-based plasma device and offers enhancements to microplasma cancer endoscopy using an improved method of plasma targeting and delivery.

Glass-clad single-crystal germanium optical fiber.

Long lengths (250 meters) of a flexible 150 microm diameter glass-clad optical fiber containing a 15 microm diameter crystalline and phase-pure germanium core was fabricated using conventional optical fiber draw techniques. X-ray diffraction and spontaneous Raman scattering measurements showed the core to be very highly crystalline germanium with no observed secondary phases. Elemental analysis confirmed a very well-defined core-clad interface with a step-profile in composition and nominally 4 weight-percent oxygen having diffused into the germanium core from the glass cladding. For this proof-of-concept fiber, polycrystalline n-type germanium of unknown dopant concentration was used. The measured infrared transparency of the starting material was poor and, as a likely outcome, the attenuation of the resultant fiber was too high to be measured. However, the larger Raman cross-section, infrared and terahertz transparency of germanium over silicon should make these fibers of significant value for fiber-based mid- to long-wave infrared and terahertz waveguides and Raman-shifted infrared light sources once high-purity, high-resistivity germanium is employed.

Single‐Cell‐Level Microplasma Cancer Therapy

A flexible microplasma endoscope based on a 15 μm hollow-core glass optical fiber is fabricated, and tumor cell apoptotic analysis supports its potential use in targeted cancer therapies. The optical-fiber microplasma jet reveals antitumor activity at a certain plasma dose in animal studies.

Confined propagation and near single-mode laser oscillation in a gain-guided, index antiguided optical fiber

The authors report laser oscillation in what appears to be a single transverse mode with very large mode area in optical fibers having heavily Nd-doped 100μm diameter cores with refractive index significantly lower in the core than in the surrounding cladding. Since fibers of this type cannot support conventional index-guided modes, their results appear to confirm a recent analysis which predicts gain-guided single-mode propagation in index antiguided fibers, provided the gain coefficient in the core exceeds a threshold value. Fibers of this type may be of significant interest for amplifiers and oscillators having large power outputs and/or small nonlinear pulse distortion.

On the fabrication of all-glass optical fibers from crystals

The highly nonequilibrium conditions under which optical fibers conventionally are drawn afford considerable, yet underappreciated, opportunities to realize fibers comprised of novel materials or materials that themselves cannot be directly fabricated into fiber form using commercial scalable methods. Presented here is an in-depth analysis of the physical, compositional, and selected optical properties of silica-clad erbium-doped yttrium aluminosilicate glass optical fibers derived from undoped, 0.25, and 50 wt % Er3+-doped yttrium aluminum garnet (YAG) crystals. The YAG-derived fibers were found to be noncrystalline as evidenced by x-ray diffraction and corroborated by spectroscopic measurements. Elemental analysis across the core/clad interface strongly suggests that diffusion plays a large role in this amorphization. Despite the noncrystalline nature of the fibers, they do exhibit acceptable low losses (∼0.15–0.2 dB/m) for many applications, broad-band emissions in the near-infrared, and enhanced thermal conductivity along their length while maintaining equivalent mechanical strength with respect to conventional silica optical fibers. Further, considerably higher rare-earth doping levels are realized than can be achieved by conventional solution or vapor-phase doping schemes. A discussion of opportunities for such approaches to nontraditional fiber materials is presented.The highly nonequilibrium conditions under which optical fibers conventionally are drawn afford considerable, yet underappreciated, opportunities to realize fibers comprised of novel materials or materials that themselves cannot be directly fabricated into fiber form using commercial scalable methods. Presented here is an in-depth analysis of the physical, compositional, and selected optical properties of silica-clad erbium-doped yttrium aluminosilicate glass optical fibers derived from undoped, 0.25, and 50 wt % Er3+-doped yttrium aluminum garnet (YAG) crystals. The YAG-derived fibers were found to be noncrystalline as evidenced by x-ray diffraction and corroborated by spectroscopic measurements. Elemental analysis across the core/clad interface strongly suggests that diffusion plays a large role in this amorphization. Despite the noncrystalline nature of the fibers, they do exhibit acceptable low losses (∼0.15–0.2 dB/m) for many applications, broad-band emissions in the near-infrared, and enhanced therm...

Binary III-V semiconductor core optical fiber

For the first time to the best of our knowledge a glass-clad optical fiber comprising a crystalline binary III-V semiconductor core has been fabricated. More specifically, a phosphate glass-clad fiber containing an indium antimonide (InSb) core was drawn using a molten core approach. The core was found to be highly crystalline with some oxygen and phosphorus diffusing in from the cladding glass. While optical transmission measurements were unable to be made, most likely due to free carrier absorption associated with the conductivity of the core, this work constitutes a proof-of-concept that optical fibers comprising semiconductor cores of higher crystallographic complexity than previously realized can be drawn using conventional fiber fabrication techniques. Such binary semiconductors may open the door to future fiber-based nonlinear devices.

Impact of fiber outer boundaries on leaky mode losses in leakage channel fibers.

In a leakage channel fiber, the desired fundamental mode (FM) has negligible waveguide loss. Higher-order modes (HOM) are designed to have much higher waveguide losses so that they are practically eliminated during propagation. Coherent reflection at the fiber outer boundary can lead to additional confinement especially for highly leaky HOM, leading to lower HOM losses than what are predicted by conventional FEM mode solver considering infinite cladding. In this work, we conducted, for the first time, careful measurements of HOM losses in two leakage channel fibers (LCF) with circular and rounded hexagonal boundary shapes respectively. Impact on HOM losses from coiling, fiber boundary shapes and coating indexes were studied in comparison to simulations. This work, for the first time, demonstrates the limit of the simulation method commonly used in the large-mode-area fiber designs and the need for an improved approach. More importantly, this work also demonstrates that a deviation from circular fiber outer shape may be an effective method to mitigate HOM loss reduction from coherent reflection from fiber outer boundary, even in double-clad fibers, with HOM losses in excess of 20 dB/m measured in the hexagonal LCF with ~50 µm core diameter while keeping FM loss negligible.

Mode area scaling with all-solid photonic bandgap fibers

There are still very strong interests for power scaling in high power fiber lasers for a wide range of applications in medical, industry, defense and science. In many of these lasers, fiber nonlinearities are the main limits to further scaling. Although numerous specific techniques have studied for the suppression of a wide range of nonlinearities, the fundamental solution is to scale mode areas in fibers while maintaining sufficient single mode operation. Here the key problem is that more modes are supported once physical dimensions of waveguides are increased. The key to solve this problem is to look for fiber designs with significant higher order mode suppression. In conventional waveguides, all modes are increasingly guided in the center of the waveguides when waveguide dimensions are increased. It is hard to couple a mode out in order to suppress its propagation, which severely limits their scalability. In an all-solid photonic bandgap fiber, modes are only guided due to anti-resonance of cladding photonic crystal lattice. This provides strongly mode-dependent guidance, leading to very high differential mode losses. In addition, the all-solid nature of the fiber makes it easily spliced to other fibers. In this paper, we will show for the first time that all-solid photonic bandgap fibers with effective mode area of ~920?m2 can be made with excellent higher order mode suppression.

Silica-clad crystalline germanium core optical fibers

Silica-clad optical fibers comprising a core of crystalline germanium were drawn using a molten core technique. With respect to previous fibers drawn using a borosilicate cladding, the present fibers exhibit negligible oxygen despite being fabricated at more than twice the melting point of the germanium. The counterintuitive result of less oxygen when the fiber is drawn at a higher temperatures is discussed. The measured propagation loss for the fiber was 0.7 dB/cm at 3.39 μm, which is the lowest loss reported to date.