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Abstract
Passive calcium silicate and fluorosilicate glass optical fibers are fabricated using the molten core method, and their properties investigated. Reductions up to −11.5 dB in Brillouin gain coefficient, −2 dB in Raman gain coefficient, and −2.5 dB in thermo-optic coefficient, relative to silica, are measured. These results support the continued study of alkaline earth oxide silicates as host materials in high energy fiber-based applications since they offer less optical nonlinearity-limited properties for power-scaling than do current analogs. Other properties, such as refractive index and acoustic velocity, are also reported and discussed. Finally, modeling work is performed to study and identify the contributions of CaO and CaF2 glass materials on the fiber properties.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Continued scaling to higher optical powers in high energy fiber-based laser systems presently is plagued by numerous parasitic effects that include stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), Kerr nonlinearities (e.g., four wave mixing), and the more recently discovered transverse mode instabilities (TMI) [1–3]. These parasitics conspire to limit the achievable power, which is of detriment to applications ranging from manufacturing (e.g., cutting, drilling, welding), to defense and security, to medical applications [4].
The conventional approach adopted by the laser community to mitigate these nonlinearities has been to employ large mode area (LMA) optical fibers, where the fiber core is engineered such that the optical power is spread out over a larger effective modal area, permitting operation below nonlinear power thresholds. Recently, the Authors (JB, PD, MC) have advocated a different approach to mitigate these performance-limiting effects, namely a unified materials approach, in which nonlinearities are attacked at their fundamental origin: the material from which they originate [5–8]. The attractiveness of this method is that simple core/clad fiber designs are employed in contrast to the more complex micro-structured LMA fibers, reducing manufacturing difficulty.
In this context, alkaline earth (including Ba, Sr, Mg) aluminosilicate fibers have been fabricated and have shown great potential to mitigate SBS and SRS [9–11]. More recently, the fabrication of passive and (Yb-doped) active strontium fluoro-aluminosilicate fibers with concomitant reductions of Brillouin gain coefficient (BGC), Raman gain coefficient (RGC) and the thermo-optic coefficient (TOC), have shown the capability to mitigate SBS, SRS, and TMI through a judicious choice of materials, while keeping low values of linear and nonlinear refractive indices [12,13]. In this paper, simple circular core-clad fibers comprising calcium silicate and fluorosilicate cores surrounded by pure silica cladding, fabricated using the molten core method [14], are investigated and reported as a complement to the previous Ba, Sr, and Mg analogs. Hence, two fibers containing CaO-Al2O3 and CaF2-Al2O3 as initial precursor materials are fabricated and their composition/structure to property relationships are studied, in a context of fiber processing. Finally, the role played by each glass dopant into the silica matrix is extracted from the aggregate glass properties by virtue of established additivity models.
2. Experimental procedure
2.1 Fiber fabrication
The fibers investigated herein where fabricated using the molten core method. More details regarding the fabrication technique can be found in Ref. [14]. A first fiber, referred to as “CaAlSi” throughout this article, was fabricated as follows. A powder mixture of 5CaO-8Al2O3 (molar ratio) was mixed and pressed into 3 mm diameter pellets. The pellets then were sleeved inside a custom-made (Heraeus Tenevo, Buford, GA) fused silica capillary tube of 3 mm and 30 mm inner and outer diameters. This preform was drawn into fiber at a temperature of about 1950°C. At this temperature, the cladding softens and, as the core precursors melt, silica from the preform dissolves into the molten core. The fiber was drawn to a cladding diameter of 125 µm and the molten core is quenched into a glassy state as the fiber cools due to the high cooling rates (>2000K/s). A conventional acrylate fiber coating was deposited on-line. A second fiber, denoted “CaAlSiF,” was fabricated using a 3CaF2-1Al2O3 powder mixture as previously described, except for being drawn at 2000°C. One fiber segment of the CaAlSi fiber was collected and studied. For the CaAlSiF fiber, two segments with different compositions, from distant positions along the draw, were taken for analysis. These two samples are denoted as CaAlSiF-A and CaAlSiF-B. Fiber draw conditions are summarized in Table 1.
Table 1. Fiber drawing parameters and conditions
2.2 Fiber characterization
Compositional analysis of the cores from each fiber segment was performed using wavelength-dispersive x-ray (WDX) spectroscopy (Geller MicroAnalytical Laboratory, Inc., Topsfield, MA, USA). Raman spectra of the characterized fiber cores where taken using a commercial Raman microscope (alpha300, WItech) in backscattering geometry, with a 532 nm laser source similar to that discussed in Ref. [13]. All of the spectra are corrected similarly to those in Ref. [15] and normalized relative to the pure silica cladding, which is taken as a standard reference. Attenuation losses are measured at a wavelength of 1534 nm using a conventional cutback method, and the refractive index profiles (RIPs) were measured by Interfiber Analysis LLC (Sharon, MA, USA), at 950 nm using a spatially resolved Fourier transform interferometer [16]. Since the dispersion characteristics for the fibers investigated herein should not be markedly different from other silicate optical fibers [17–19], index difference (Δn) values in the 15XX nm region are expected to be somewhat similar. This is essential since the attenuation losses and Brillouin properties are measured at 1534 nm. The heterodyne apparatus to measure Brillouin-related properties such as BGC, Brillouin frequency νB and linewidth ΔνB, longitudinal acoustic velocity Va, thermal and strain Brillouin coefficients dν/dT and dν/dɛ is the same as used in Refs. [9,13,20]. Finally, thermo- and/or strain-optic coefficients (TOC = dn/dT, and SOC = $- \frac{2}{{{n^3}}}\frac{{dn}}{{d\varepsilon }}$, respectively) for each fiber is determined using an erbium-doped ring laser, as detailed in Refs. [20,21], where the test fiber is an integral part of the laser cavity. Upon heating or straining of the test fiber, the free spectral range (FSR) of the laser is modified, from which the TOC or SOC can be deduced. It is worth mentioning that the launched light is coupled to the fundamental mode (FM) of the test fiber, and, therefore, the measured values are that of the FM, and not necessarily that of the glass directly.
3. Results and discussions
3.1 Properties of the fibers
Elemental compositions and other fiber properties for CaAlSi, CaAlSiF-A and CaAlSiF-B fiber segments are reported in Table 2. The elemental compositions, reported in atomic percentage (At. %), are taken at the fiber core centers. The fiber core diameter is defined to be from the radial position starting at the onset of increasing refractive index.
Table 2. Fiber compositions (At. % at the core center), as well as typical fiber properties
The azimuthally-averaged RIPs are reported in Fig. 1(a). Although from the same draw, CaAlSiF-A and CaAlSiF-B fiber segments exhibit different compositions and, hence, RIPs. CaAlSiF-A fiber was taken from the beginning of the draw, and therefore exhibits higher F, Ca, and Al concentrations than CaAlSiF-B, while Si and O concentrations are found to be lower. An example of the elemental composition profile measured by WDX spectroscopy for fiber segment CaAlSiF-A is provided in Fig. 1(b), showing the compositional gradient of each element across the fiber core, resulting in a graded-index profile fiber. Additionally, it is worth mentioning that in Table 2 the initial Ca:Al ratio (that is, the one of the initial precursor composition from Table 1) is not always preserved post draw. More specifically, for CaAlSi fiber segment, the initial Ca:Al ratio is 0.31, while for CaAlSiF-A and CaAlSiF-B fiber segments this initial ratio is 1.5. After drawing, the ratio for CaAlSi fiber segment is preserved (0.31), while it is found lower for CaAlSiF-A and CaAlSiF-B (∼1.43 and ∼1.39 respectively). Additional experiments are needed to better comprehend how and why this ratio evolves.
Fig. 1. a) Refractive index profile (RIP) for CaAlSi, CaAlSiF-A and CaAlSiF-B fibers. The RIPs are relative to the silica cladding, for which Δn = 0. For completeness, the refractive index dip visible for each fiber at the core/cladding interface is an artifact of the apparatus measurement. b) Elemental concentration profile measured by WDX spectroscopy for one of the fibers investigated herein (CaAlSiF-A): not represented here for clarity is the At. % of oxygen [O], however [O] = 100 – ([Si] + [Al] + [Ca] + [F]). c) Core diameter (µm) as a function of Si content (At.%).
Higher dopant concentrations (i.e., non-silica glass constituents) are typically accompanied by an increase of the core size, as observed in Fig. 1(c), which is consistent with observations in other aluminosilicate fibers [13,22]. As the fiber is drawn, the precursor continuously reacts with the silica cladding and more of the latter is incorporated into the core, resulting in a higher silica concentration and core size diminution. This is well exemplified by CaAlSiF-B, which presents a higher Si content and a smaller core size compared to CaAlSiF-A, as the segment was collected at the end of the draw length.
Interestingly, the refractive index difference (Δn) between the core and the cladding for both CaAlSiF fiber segments differ only by 1.08×10−4, this regardless of the significant differences in compositions. Such a similar Δn can be explained as follows. CaAlSiF-A contains more Al and Ca than CaAlSiF-B, which are known to increase the refractive index relative to silica [22,23], but also has more F, which decreases the refractive index [13,24]. If CaAlSi contains less silica (lower [Si]) and, therefore, exhibits a higher Δn value, the absence of fluorine in the core composition strongly participates in an increase of that value, and Δn is found to be almost a factor of two higher than for the fluorine-containing fiber segments. This new generation of calcium (and strontium [13]) fluorosilicate fibers, similarly (as opposed to all-oxide silicate based fibers) permits the fabrication of fewer-moded optical fibers. For completeness and as discussed in Ref. [12], pedestal fiber designs or specialty glass claddings (e.g., SiO2-Al2O3-La2O3 compositions) can be implemented to further reduce index difference between core and cladding if single mode operation is mandatory.
Acousto-optic and other properties for the fibers investigated are reported in Table 3. Reductions of ∼11.5 dB and ∼2 dB in BGC and RGC relative to silica for the CaAlSi fiber is reported. Unfortunately, the TOC could not reliably be measured in this fiber. This is likely the result of the relatively high Δn in this fiber. Such problems were not encountered for the SOC. For CaAlSiF segments, BGC, RGC, and TOC are decreased by ∼7 dB, ∼1 dB, and ∼2.5 dB, respectively, relative to silica. While the SOC also is decreased in all of the fibers relative to silica, the purpose of measuring this quantity is to enable the estimation of the p11 photoelastic constant, and therefore no detailed discussion of this quantity will be presented. That being said, however, an ancillary application of these glass compositions could be in fibers with reduced susceptibility to environmental strain.
Table 3. Acousto-optic properties of the fibers considered and fused SiO2 for comparison
The low BGC of the fibers are due principally to the intrinsically low Brillouin response of the fiber cores, while a lesser proportion can be attributed to their acoustically anti-guiding nature. Materially, BGC = 2πn7p122/cλ02ρVaΔνB [25], where n is the refractive index, p12 the transverse photoelastic coefficient, c the speed of light, λ0 the wavelength, ρ the density, Va the longitudinal acoustic velocity, and ΔνB the Brillouin linewidth. The addition of alumina into silica reduces p12, while increasing n, ρ, Va, ΔνB, and, accordingly, promotes glasses with reduced Brillouin response. CaO (from CaAlSi but also from oxidation of CaF2 in CaAlSiF segments during fiber processing like in SrF2 fibers) is expected to behave similarly to the other alkaline earth oxides BaO, SrO and MgO [9–11], and, consequently, promote glasses with intrinsically low BGC. Finally, the role of fluorine has already been discussed in Ref. [13], and is not expected to further participate in the reduction of BGC. More details regarding the role of each dopant on the glass properties are provided later in the modeling related section (Section 3.2).
The second contribution to the reduction of the BGCs, the acoustic anti-guiding nature of the fibers investigated, is due to the higher acoustic velocity of the core relative to the silica cladding. This results in an increase of the Brillouin linewidth (see “ΔνB” versus “intrinsic ΔνB” in Table 3), further decreasing the BGC. The higher acoustic velocity is attributed to the Al2O3 [22] and possibly CaO [27] glass dopants (however findings here indicate that Va(CaO) < Va(SiO2)). F, on the other hand, is expected to decrease Va relative to silica [28]. Comparing with Ref. [13], strontium fluorosilicate fibers were found to be acoustically guiding. This can be explained by the fact that that as one goes up the Group II column, the acoustic velocities of the alkaline earth oxides steadily increases (Va(BaO) < Va(SrO) < Va(MgO) [7]), with MgO possessing an acoustic velocity contribution almost as significant as Al2O3 [11]. As such, CaO-derived fibers are expected to promote glasses with higher Va than their BaO- and SrO-containing analogs, favoring acoustic anti-guiding operation in silica-clad fibers, especially when Al2O3 is present. Comparing fluorides, a similar trend to the oxides is observed, and, especially, Va(CaF2) > Va(SrF2) [29]. Therefore CaF2-derived glass optical fibers are considered advantageous over SrF2-derived fibers with respect to fabricating acoustically anti-guiding fibers.
Based on further examination of the data in Table 3, it is noted that CaAlSi exhibits a nearly null Brillouin frequency dependency with temperature (dν/dT = +0.067), and can potentially be utilized for athermal distributed sensor applications [10,19,30].
The thermo-optic coefficients for the fibers also are reported in Table 3. As previously mentioned, the TOC could not reliably be measured for the CaAlSi fiber. The CaAlSiF segments present much lower TOC values than silica (∼2.5 dB lower), and are comparable to strontium fluorosilicate fibers fabricated using the molten core method [13]. These results confirm the benefit in using fluorosilicates to drastically mitigate dn/dT-driven thermally-induced effects (e.g., TMI, thermal lensing). On a side note, other silicates, such as boron-doped phosphosilicate glass systems also efficiently can mitigate dn/dT values [31]. Interestingly, TOC values for CaAlSiF-A and CaAlSiF-B are very close, this even with differing concentrations. CaAlSiF-A has higher F concentration than CaAlSiF-B, and should present lower TOC values. To first order, this would suggest that TOC(CaF2) > TOC(CaO).
Finally, the Raman spectra for each fiber sample, taken at the core center, are displayed in Fig. 2a, along with the spectrum of fused silica taken as a reference. For each fiber segment, the normalized Raman gain coefficient (RGC), provided in Table 3, corresponds to the relative strength of the highest intensity scattering peak (and is equal to 1 for SiO2). The Raman features of these calcium silicate and fluorosilicate systems resemble those of their strontium analogs [12,13]. The peak intensity situated around 440 cm-1 in Fig. 2(a), attributed to Si-O-Si linkages [32], is found to linearly increase as a function of Si content (Fig. 2(b)) over the range investigated. Quantitatively, an increase of 1 At.% of Si yields to a ∼3.2% higher RGC value.
Fig. 2. a) Normalized and corrected Raman spectra of CaAlSi, CaAlSiF-A and CaAlSiF-B fiber segments, relative to fused SiO2 (taken in CaAlSi cladding) and b) Raman gain coefficient (RGC) as a function of Si concentration (At. %); the linear fit serves as a guide-to-the-eye.
The CaAlSi fiber segment exhibits very different Raman features compared to the other two fiber segments, principally due to its lower Si content and the much higher Al concentration of this glass. The observed broadening of the peaks is consistent with Raman spectra of aluminosilicate and similar strontium aluminosilicate fibers [12,33]. The reader is referred to the cited references for more information regarding band assignments and trends in these glass systems.
Interestingly, the only differences observed between CaAlSiF-A and CaAlSiF-B spectra are i) the slight reduction of the 440 cm-1 peak for CaAlSiF-A and ii) a slight increase and redshift of the peaks situated around 1000 cm-1 for CaAlSiF-A, characteristic of non-bridging oxygens (NBOs) present in the glass [32]. This former difference is due to the higher silica content for CaAlSiF-B segment, while the latter is attributed to the higher calcium content in CaAlSiF-A segment, resulting in an increase of the number of NBOs. In addition, no characteristic peaks providing information on the bonding nature of F in these glasses is detected from either fluorosilicate fiber. It is possible that if the Ca-F bonds were preferentially formed, the peak would be hidden under the 440 cm-1 silica peak as CaF2 has a single Raman peak situated around 330 cm-1 [34]. This peak would likely be significantly broader in the amorphous glass environment, thereby reducing its spectral density.
3.2 Modeling of CaO and CaF2 as glass constituents
Modeling work is now performed to further investigate how calcium affects the glass properties when introduced, either as an oxide (CaO) or as a fluoride (CaF2), to the optical fiber core. Reasons for specifically identifying only the fluoride of calcium will be discussed later. The modeling methodology used here is similar to what is described in Refs. [7,20,26]. In short, each fiber graded index profile is approximated to a series of adjacent step index layers; using 4 layers in the core and 1 layer in the cladding. Each layer has a homogenized and unique composition, determined from the WDX compositional profile, to which is associated a set of properties (e.g., index, acoustic velocity). Calculation of these properties is done using an additive (or rule of mixtures) technique, where an aggregate glass value most generally is an average of the constituent (e.g. SiO2, Al2O3, CaO) values taken with respect to the total volume fraction occupied by each constituent. With the approximation to the profile, values for the waveguide mode (either optical or acoustic) can be determined. Additionally, since each layer can possess a strain or temperature dependence, modal response values (SOC, TOC, SAC, TAC) may also be calculated for the fiber. In order to determine the individual CaO values, silica and alumina physical properties first are assumed (see Table 4). Then, the model is used with the CaO values as fitting parameters. The fitting values then are adjusted until the modeled (modal) value matches that of the measurement. It should be noted that the acoustic attenuation coefficient also is calculated from this approximate profile using the simple model found in Ref. [35]. The placement of the various boundaries introduces some uncertainty in the calculated value.
Table 4. Properties CaO glass constituent calculated from CaAlSi fiber. Values of SiO2 and Al2O3 glass constituents, needed to compute the effect of CaO, are also reported
The WDX measurement exhibited some nonuniformity in the azimuthal direction which was at a maximum of ± 5%. It is not clear whether this is a real compositional variation or if this can be attributed to a drift in the measurement. Either way, the error in this measurement is consequently taken to be ± 5%. To determine uncertainties in the individual CaO values, the CaO concentration was varied from 95% to 105% of its measured value and the resultant new fitting parameters formed the endpoints of the range of uncertainty. Any uncertainties found in Table 3 also were added in this way. In the case of the photoelastic coefficients, it was more meaningful to provide absolute error rather than a percentage.
The calculated property values for CaO are reported in Table 4, and with this the values of SiO2 and Al2O3 used to perform the modeling. Comparatively to its crystal counterpart, amorphous CaO is found to exhibit a lower refractive index (1.711 vs. ∼1.83 at 0.65 µm [40]), density (2608 kg/m3 vs. ∼3300 kg/m3 [39]), and acoustic velocity (5120 m/s vs. ∼8000 m/s [39]). These values are consistent with other glass vs. crystal values, for instance such as SiO2 and BaO (Ref. [9]). Interestingly, there exist many similarities between properties of Al2O3 and CaO when compared with those of SiO2. More specifically, they exhibit very similar Pockels photoelastic coefficients (p11 and p12), both negatively contribute to the thermo-acoustic, and strain-acoustic coefficients (TAC, SAC) of the aggregate silicate glass, and finally present low strain-optic coefficients (SOC).
To further illustrate the effect of CaO on the various fiber properties, in Table 5 these results are directly compared with those from the others alkaline earth oxides which have already been modeled, namely MgO, SrO, and BaO. With respect to the other alkaline earth oxides, the lower refractive index for CaO may be used when a lower core NA is desired. Moreover, and as suggested above, its substantially higher acoustic velocity compared to BaO or SrO can ease the development of acoustically anti-guiding optical fibers when co-doped with Al2O3, resulting in low SBS fibers, but CaO appears to come with the drawback that it has less of an impact on p12. On a more general note, it is clear that, based on Table 5, alkaline earth oxides are promising candidates when it comes to engineering optical fibers with desired optical properties, as they enable tailorability of physical properties over a large range of values.
Table 5. Comparison between alkaline earth oxide properties
Finally, attention is now turned to the fluorosilicate fibers. Fluorine plays a determinant role in the concomitant reduction of n, n2, and TOC [12], which makes it an essential dopant in the mitigation of optical nonlinearities, notably wave-mixing phenomena and TMI. Therefore, an attempt was made to model the effect of the fluoride glass compound (here CaF2) to the aggregate glass properties. Because in strontium fluoro-aluminosilicate fibers fabricated using the molten core method F was found to be principally located around the Sr atoms [43], it is likely that the glass would act in a chemically similar fashion if Sr atoms were substituted by Ca atoms. As a consequence to this, the core glass is taken to be the quaternary sum of the stoichiometric compounds CaO, CaF2, Al2O3, and SiO2, and where all the fluorine is attached to the Ca atoms. It is worth noting that even if the latter composition does not fully and adequately reflect the actual glass structure, it renders the modeling very convenient.
Having determined the properties of CaO, and with Al2O3, and SiO2 already known (from Table 4), the contribution by a fictitious CaF2 glass compound can be calculated for both CaAlSiF-A and CaAlSiF-B fiber segments. The methodology was the same as that for calculations for CaO from CaAlSi fiber segment, with the results for CaF2 reported in Table 6 (CaF2-A and CaF2-B calculated from CaAlSiF-A and CaAlSiF-B segments, respectively). Properties of CaF2 crystal and the previous findings for CaO also are reported for comparison. While the measured TOCs for the fluorosilicate fibers come with a high degree of confidence, the additive model did a poor job of fitting to data. The best effort was to coarsely set TOC(CaO) and adjust TOC(CaF2) to minimize the error in the calculated fiber (modal) TOCs. This process was iterated until the error between the measured and modeled TOCs for the fluorosilicate fibers was minimized, but equal for both fibers, and that the average calculated TOC fell between the measured values. This resulted in a ∼10% error between the measured and modeled values, which leads to ∼25% error in the TOC, though a rough order-of-magnitude estimation can be made. As with fibers whose fiber cores have Brillouin frequencies that are independent of temperature [19], the coefficient of thermal expansion (CTE) of the core may be playing a significant role in the current observations and poor fittings. Additionally, the spectral width fittings produced non-physical results, namely suggesting that the spectral width for CaF2 is negative-valued. This may suggest that some structural change to the glass, which the additive model cannot predict, is acting to decrease the acoustic damping loss of one or more of the other constituents.
It is important to point out that the error associated with CaF2 still includes the uncertainty in the composition, but additionally the error in CaO propagates through the calculation. In the case of the refractive index (with the lowest uncertainty) they are calculated to be 8.9% and 17.7% for CaF2-A and CaF2-B, respectively. The latter has roughly double the uncertainty since its CaF2 concentration is lower (also about double). For the acoustic velocity, these are 12.4% and 18.6%. For parameters where CaO uncertainty is greater, uncertainty in CaF2 would be expected to scale correspondingly. Clearly, uncertainties approaching 100% may greatly limit the meaning of a measurement, however continuing with this analysis considering only the central CaF2 values still has intuitive benefit. First, many of the calculated quantities for CaF2 follow the same trends as its oxide analog, that is, a lower refractive index, density, and acoustic velocity than its crystal counterpart. Second, among the various properties calculated, the TOC of CaF2 is found to be very negative, and confirms the potential of fluorides as candidates in the mitigation of TMI when introduced into a SiO2 glass matrix. Third, similar to its crystalline counterpart, CaF2 in the silicate structure contributes a positive p12. From the standpoint of SBS suppression, this is undesirable as it raises the BGC (relative to the contribution by CaO). So, while CaF2 brings an advantage in decreasing Δn and Kerr nonlinearities, its effect on p12 is far weaker. Fourth, much like with crystalline CaF2, $|{{p_{12}}} |\gg |{{p_{11}}} |$ for CaF2 in silica. Fifth, CaF2 in silica, unlike CaO, appears to have a positive TAC, but like CaO has a negative SAC. As such, CaF2 may prove to be a useful dopant in achieving atensic (Brillouin frequency is independent of strain) fibers for Brillouin-based temperature sensing. Finally, and importantly, the analysis presented here does not contradict the argument that the predominant fluoride compound to be found in the glass is that of calcium.
4. Conclusion
This work (Ca) continues previous efforts (Sr, Ba) to characterize the effect of doping alkaline earth oxides on a number of optical fiber properties, both linear and nonlinear. Fibers exhibiting concomitant reduction of BGC, RGC, and TOC, relative to conventional silica, have been demonstrated using calcium silicate and fluorosilicate glasses, and mark these glasses as potential host materials for high energy laser applications. A modeling section was provided that attempted to quantify the contributions of CaO and CaF2 to properties of the silicate system. First, a fiber segment in the ternary calcium aluminosilicate system was studied to determine the effect of CaO, and those results then were extended to the quaternary calcium fluoro-aluminosilicate system (two fibers segments investigated) to determine the effect of CaF2. The results, while not necessarily case-closed conclusive, do support the idea that the preponderance of fluorine in the glass is to be found as a calcium compound. Reduction of the uncertainties in the quantities presented here would require greater concentrations of both CaO and CaF2. As such, work is underway to decrease the SiO2 content in these fibers while also lowering the NA. In addition, work is underway to understand how the CTE of CaF2 may be added in a silicate glass [47] such that the related effects may be taken into consideration.
Funding
Air Force Office of Scientific Research (AFOSR) (FA9550-16-1-0383FA); U.S. Department of Defense (DOD) High Energy Laser Joint Technology Office (N00014-17-1-2546); J. E. Sirrine Foundation.
Acknowledgement
The Authors wish to thank Dr. Andrew Yablon (Interfiber Analysis) for the RIP measurements as well as Jianan Tang for help with the Raman spectroscopy.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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