Stephanie Morris

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.

Pockels’ coefficients of alumina in aluminosilicate optical fiber

The photoelastic constants of the alumina component in aluminosilicate optical fibers are evaluated and determined to be p11=−0.237±0.020 and p12=−0.027±0.012, thus confirming that the low and negative pij characteristics of bulk alumina are conserved as part of a binary aluminosilicate glass system in optical fiber form. In order to enumerate these values, the strain- and stress-optic coefficients of two fibers (one with an aluminosilicate core and one with a pure silica core) were measured by applying mechanical tension or twist, respectively, to the fibers and measuring changes to an optical system as a function of the mechanical deformation. In the former, the strain-optic coefficient (eOC) is measured directly by recording changes to the free spectral range of a ring fiber laser. In the latter, the stress-optic coefficient (σOC) is found by measuring the change in polarization angle after linearly polarized light propagates through a segment of twisted test fiber. To the best of our knowledge, this is the first such measurement of its type, i.e., the retrieval of the component photoelastic constants, with their signs, of a multicomponent glass. Binary glass compositions wherein the constituents have opposite signs of the photoelastic constant (such as the aluminosilicates) have the potential to give rise to extremely low values of the Brillouin gain coefficient.

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.

Annealing of silicon optical fibers

The recent realization of silicon core optical fibers has the potential for novel low insertion loss rack-to-rack optical interconnects and a number of other uses in sensing and biomedical applications. To the best of our knowledge, incoherent light source based rapid photothermal processing (RPP) was used for the first time to anneal glass-clad silicon core optical fibers. X-ray diffraction examination of the silicon core showed a considerable enhancement in the length and amount of single crystallinity post-annealing. Further, shifts in the Raman frequency of the silicon in the optical fiber core that were present in the as-drawn fibers were removed following the RPP treatment. Such results indicate that the RPP treatment increases the local crystallinity and therefore assists in the reduction of the local stresses in the core, leading to more homogenous fibers. The dark current-voltage characteristics of annealed silicon optical fiber diodes showed lower leakage current than the diodes based on as-draw...

Mass density and the Brillouin spectroscopy of aluminosilicate optical fibers

Provided herein is a detailed analysis of the Brillouin properties of alumina-doped silica optical fiber. The acoustic velocity of alumina in silica is shown to be a very strong function of its mass density, which can vary significantly from sample-to-sample and likely originates from the observed linear relationship between the longitudinal elastic modulus and the mass density. Further, the refractive index versus the alumina concentration provides a very sensitive probe of this mass density, and can be used to derive other structural details about the alumina. For example, for the first time to the best of our knowledge measurements of the thermo- and strain-acoustic coefficients (TAC and SAC, respectively) of the alumina dopant in silica-based fiber are presented and it is shown that these quantities are not strongly influenced by the density of alumina. Further, the material acoustic damping does not appear to be strongly influenced by the density. The TAC and SAC, or the dependence of the acoustic velocity on temperature or strain, respectively, are both found to be negative and large for alumina, in fact much larger than those for silica. Alumina thus represents a unique and potentially very useful material for the compositional tuning of the Brillouin scattering characteristics of optical fibers for distributed sensing and other applications. Conversely, these properties of alumina reduce the effectiveness of using applied temperature or strain gradients to fiber in order to suppress Brillouin scattering in fiber laser systems.

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.

Sapphire-Derived All-Glass Optical Fibers

If we could eliminate the phenomenon of stimulated Brillouin scattering (SBS), we could create pulsed laser sources for spectroscopic and coherent lidar applications, as well as multi-kW narrow-linewidth fiber laser sources for coherently phased laser arrays. With that in mind, we used a novel fiber fabrication method known as the moltencore method to realize a new class of optical fibers: all-glass optical fibers free from the effects of SBS.

Novel optical fibers for Brillouin-based distributed sensing

Optical fiber sensors utilizing Brillouin scattering rely on the principle that the Brillouin frequency shift is a function of the local temperature or strain. Conventional optical fibers, such as standard telecommunications single-mode fibers, have been successfully used in these applications, and most typically in the time domain, such as with BOTDR. Such conventional fibers however are susceptible simultaneously to both temperature and strain, requiring either at least two fibers or specialized cabling to distinguish the effects of a local stress from those of a local change in temperature. Recently, methods utilizing fibers possessing at least two Brillouin frequency shifts, each with different temperature or strain coefficients have been proposed. However, realizing such fibers is challenging, requiring fibers with regions of very different compositions, all of which must have substantial overlap with the optical field, posing significant manufacturing challenges. We present several new specialty optical fibers based on novel and unconventional fabrication techniques with significant potential for use in distributed fiber sensor systems. First, we describe a class of fibers fabricated from materials whose Brillouin frequency shifts are immune to either temperature or strain, with a demonstration of the former using fiber derived from sapphire crystal, and modeling and measurements predicting the latter. The ‘Brillouin-athermal’ fiber enables the measurement of a local strain, independent of the local temperature. Second, we describe and demonstrate a novel group of longitudinally graded (chirped) fibers enabling easily-implemented frequency-domain systems; affording the potential to simplify and reduce the cost of Brillouin-based distributed sensors.

Intrinsically-low Brillouin gain optical fibers

Numerous methods to increase the stimulated Brillouin scattering (SBS) threshold have been previously implemented. Some are passive, based on acousto-optic fiber designs that incorporate longitudinally- or radially-tailored optical and/or acoustic index profiles, leading to broadened Brillouin gain spectra (BGS) with reduced peak gain. Some are active, relying on an applied temperature or strain distribution, also resulting in broadened BGS. Broadening the laser spectrum still represents the most effective method to-date to obtain large-scale (> 20 dB) decreases in the gain, but the suitability of this method depends largely on the application and system requirements on the laser spectrum. Despite these technologies, some introduced only in the last decade, the vast majority of high-energy, narrow-linewidth fiber laser systems are still limited by SBS rather than the availability of pump power. We present an alternative approach; rather than focusing on ‘suppressing’ SBS in waveguide or other designs, we propose implementing materials with intrinsically low Brillouin gain. We focus on high-density, high-soundvelocity, large acoustic-damping-coefficient, and low-photoelastic-constant materials wherein the correct balancing of physical characteristics gives rise to extremely low Brillouin gain. In general, the approach requires the use of compositions that would be considered to be highly unconventional and unachievable utilizing standard fiber fabrication methods. For example, we describe recent results on sapphire-derived fibers (among other compositions) wherein a Brillouin gain nearly 20 dB lower than those of more conventional fibers has been realized. Other compositions will also be presented, including new results on a novel baria doped fiber, including others predicted to have zero-valued photoelastic constants, and therefore zero Brillouin gain.

All-glass optical fibers derived from sapphire

Increasing power levels and novel applications are demanding from fibers performance capabilities that have, to date, not been realized. One such example arises from the nascent push towards the 10-kW power threshold for narrow linewidth fiber lasers designed for applications including coherently-phased laser arrays and spectroscopic lidars. It is well-known that Brillouin scattering still restricts continued power scaling in these systems, despite several recent advances in acoustic-wave Brillouin management. Accordingly, novel fibers possessing a Brillouin gain coefficient 10 dB or more less than previously demonstrated would be of great practical benefit if they comprise novel materials in simple geometries and are manufactured using industry-accepted methods. Introducing a new and effective approach to the management of Brillouin scattering, we present on all-glass optical fibers derived from silica-clad sapphire with alumina concentrations up to 55 mole percent; considerably greater than conventionally possible enabling the design of optical fiber possessing a series of essential properties. Markedly, a Brillouin gain coefficient of 3.1 × 10-13 m/W was measured for a fiber with an average alumina concentration of 54 mole percent. This value is nearly 100 times lower than standard commercial single-mode fiber and is likely the lowest ever specified value. This reduction in Brillouin gain is enabled by a number of key material properties of the alumina-silica system, amazingly even leading to a predicted, but not yet demonstrated, composition with zero Brillouin gain. Optical fiber materials with these and other crucial properties will be discussed in the context high energy fiber laser systems.