R. Stolen

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.

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.

Nondestructive position-resolved measurement of the zero-dispersion wavelength in an optical fiber

We describe a new technique for mapping the distribution of the zero-dispersion wavelength in a span of optical fiber. With this technique, which is based on four-wave mixing of short pulses, we obtained a spectral accuracy of /spl plusmn/0.2 nm and a spatial resolution of 700 m. Theoretical analysis agrees well with experimental results and points to an order of magnitude improvement in spatial resolution using shorter pulses. Spatial resolutions less than 100 m may be necessary because we find, by destructive measurements, variations in lambda-zero by as much as 0.5 nm over spans of 500 m.

Reactive molten core fabrication of silicon optical fiber

Silicon optical fibers fabricated using the molten core method possess high concentrations of oxygen in the core [Opt. Express 16, 18675 (2008)] due to dissolution of the cladding glass by the core melt. The presence of oxygen in the core can influence scattering, hence propagation losses, as well as limit the performance of the fiber. Accordingly, it is necessary to achieve oxygen-free silicon optical fibers prior to further optimization. In this work, silicon carbide (SiC) is added to the silicon (Si) core to provide an in situ reactive getter of oxygen during the draw process. Scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX), and powder x-ray diffraction (P-XRD) are used to verify that the glass-clad silicon optical fibers possess very low oxygen concentrations and that the SiC is consumed fully during the reactive molten core fabrication. Optical measurements indicated a reduction in light scattering out of the silicon core as expected. However, the measured attenuation of about 10 dB/cm, which is consistent with existing low-oxygen-content silicon fibers, implies that scattering might not be the dominant source of loss in these molten core-derived silicon fibers. More generally, this work shows that the high temperature processing of optical fibers can be an asset to drive chemical reactions rather than be limited by them.

The Early Years of Fiber Nonlinear Optics

This paper looks back at the beginnings of nonlinear optics in fibers and some of the subsequent work that forms the basic underpinnings of the modern field. There is a brief look at some of the major applications and current trends.

On loss in silicon core optical fibers

Glass clad semiconductor core fibers have received much attention recently for their potential utility for nonlinear optics and infrared power delivery. As these fibers have progressed, it has become evident that a greater understanding as to the dominant sources of loss is needed. This work begins that discussion by investigating intrinsic and extrinsic sources of loss in silica glass clad crystalline silicon core optical fibers. Of particular interest are, to the best of our knowledge, the first lattice-fringe images of single and poly-crystalline regions of the silicon core optical fibers as well as scattering sources. Suggested herein are methods to further reduce the presence of impurities and defects that lead to scattering and dominate optical losses.

Reactive molten core fabrication of glass-clad amorphous and crystalline oxide optical fibers

Described herein are glass-clad optical fibers, fabricated using a molten core fiber draw process, comprising oxide cores in the Bi2O3 – GeO2 system. More specifically, the fibers utilized a borosilicate glass cladding with core compositions in the initial preform ranging from un-reacted crystalline Bi2O3-rich (Bi2O3 + GeO2) powders to stoichiometric crystalline Bi12GeO20. Fibers drawn from the as-purchased crystalline Bi2O3-rich powders were amorphous with a transmission of about 80% at 1.3 μm. Fibers drawn from the crystalline Bi12GeO20 core contained a mixture of crystalline bismuth germanate (Bi2GeO5) and bismuth oxide (δ-Bi2O3/BiO2-x). While representing an initial proof-of-concept, this work shows that commercially-relevant draw processing can be employed to yield fibers with core composition that are very difficult to fabricate using conventional methods and that the molten core method further enables in situ reactive chemistry to take place during fiberization resulting in amorphous or crystalline oxide core fibers depending on initial core composition. Perhaps more importantly is that optical fibers possessing acentric, hence optically nonlinear, oxide crystals can be realized in a scalable manufacturing manner though further optimization is required both of the core chemistry and process conditions in order to achieve a single phase and single crystalline fiber.

Single- and few-moded lithium aluminosilicate optical fiber for athermal Brillouin strain sensing.

Results are presented toward realizing a true single-mode fiber whose Brillouin frequency shift is independent of temperature, while its dependence on strain is comparable to conventional high-silica-content single-mode fibers. Demonstrated here is a fiber with a negative thermal sensitivity dν/dT of -0.26  MHz/K and a strain sensitivity of +406  MHz/%. The suppression of the Brillouin thermal response is enabled by the large thermal expansion coefficient of the group I oxide-containing silica glass network.

Direct experimental observation of stimulated thermal Rayleigh scattering with polarization modes in a fiber amplifier

Modal interference can lead to intensity modulations in optical fibers, which can produce refractive index gratings under the influence of quantum defect heating in a fiber laser. These gratings are perfectly phased matched for mode couplings, which can lead to transverse mode instabilities at high average powers in fiber lasers. A detailed understanding of this process is critical for further power scaling of fiber lasers. We have directly observed and characterized this quantum-defect-assisted mode coupling for the first time using polarization modes in a PM fiber amplifier, providing solid experimental evidence for this key mechanism for transverse mode instability in fiber lasers.

An Erbium Fiber Fabricated by the “Core-Suction” Technique

An erbium doped fiber preform having a lead germanate erbium glass core with silica cladding was fabricated by a newly developed technique named ¿core-suction¿. This preform was then drawn into fiber and fiber cross-section, loss spectrum and refractive index profile measured. A 30 cm piece of the manufactured fiber was spliced to a standard silica fiber using a commercial available splicer. This spliced fiber was then used to setup an erbium doped fiber amplifier (EDFA) and gain spectrum of the amplifier measured. Distributed gain of the manufactured erbium fiber was measured using an optical frequency domain reflectometry (OFDR) based optical backscatter reflectometer (OBR). It is demonstrated that the ¿core-suction¿ technique can be used to make a high-gain amplifier that is compatible with conventional silica fibers.