Erbium-doped optical fiber prepared using erbium-doped nanoparticles

Abstract

An erbium-doped optical fiber comprises an erbium-doped core radially surrounded by a glass cladding, where the erbium-doped core comprises (a) a glassy matrix comprising silica and (b) nanoscale regions dispersed within the glassy matrix. The nanoscale regions comprise erbium ions and alkaline earth ions, and the erbiumdoped optical fiber exhibits a quantum efficiency (QE) above 50%.

Description

ERBIUM-DOPED OPTICAL FIBER PREPARED USING ERBIUM-DOPED NANOPARTICLES RELATED APPLICATION

[0001] The present patent document claims the benefit of priority under 35 U.S.C. 119(e) to U.S. Provisional Patent Application No.63 / 566,983, which was filed on March 19, 2024, and is hereby incorporated by reference in its entirety. TECHNICAL FIELD

[0002] The present disclosure relates generally to optical fiber fabrication and more particularly to optical fibers prepared using erbium-doped nanoparticles and devices comprising the fibers. BACKGROUND

[0003] Rare-earth (RE) core-doped optical fibers are valued for their use in lasing, sensing, and amplification. Currently, usage of these RE fibers ranges from Yb- and Tm- doped high-power fiber lasers to Er-doped fiber amplifiers (EDFAs) and to co-doped fibers, such as Yb-sensitized Er-doped fibers. The three most common RE ions used for active optical fiber doping are Yb3+, Tm3+, and Er3+. When doped into silicate glasses, emissions from these RE ions cover a broad range of IR wavelengths, from approximately 1000-2100 nm, which are valuable for telecommunications and lasing applications. While RE dopants are effective for lasing and amplification applications, the concentration of ions that can be incorporated into the fiber has been limited due to their poor solubility in silica leading to ion clustering and emission quenching, which limits their lasing and amplification efficiency.

[0004] Erbium-doped optical fibers in particular have been investigated for several decades for their excellent capabilities in optical amplification and lasing media. Erbium- doped fibers are primarily valuable for systems operating in the telecommunications band centered at a wavelength of 1550 nm. Access to this wavelength range is possible due to the Er3+emission from the4I13 / 2 →4I15 / 2 electronic levels. However, erbium has two disadvantages as a dopant in silica. First, the absorption cross-sections are relatively low (compared, for example, to Yb3+). Second, erbium solubility in silica is low and the ionsare therefore prone to cluster. As erbium ions cluster, the probability for non-radiative decay, known as quenching, increases, leading to a loss in the production of photons and therefore lower lasing efficiency and output. This propensity to cluster and quench increases as the concentration of Er3+increases. Quenching can, in part, manifest through cooperative upconversion. In this case, energy is transferred from one excited erbium ion to another, leading to one in the ground state and another excited to a higher energy level. That excited ion will relax back down to the4I15 / 2 electronic level and most energy is lost as heat in this process. However, some ions will produce spontaneous emission, dominated by luminescence in the green. Therefore, erbium doped fibers with significant cooperative luminescence glow a characteristic green when pumped near 980 nm. Fibers free from these ion-ion interactions will have minimal luminescence in the green when pumped near 980 nm. Therefore, there is a great need to prevent ion-ion interactions in erbium doped optical fiber.

[0005] Reducing these negative quenching effects has been approached from a variety of directions. One is keeping the concentration of erbium low and using other co-dopants to stimulate the excited state process, such as in Yb-sensitization, where excitation of the Er3+ 4I13 / 2 band begins with pumping (i.e., absorption of a photon by a) Yb3+and eventual energy transfer to an erbium ion. Another is to use passive co-dopants such as alumina (Al2O3), which improves the solubility of REs into silica to increase the concentration limit within silicate fibers before amplifier or laser efficiency diminishes significantly. Despite decades of research, ytterbium-free erbium-doped fibers remain limited in their available doping concentration. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Embodiments of the invention may be better understood with reference to the following drawings and description. Components in the figures, which may be schematic representations, are not necessarily to scale.

[0007] FIG.1 is a schematic of part of an erbium-doped optical fiber produced using rare-earth-doped barium fluoride nanoparticles as precursors; the as-drawn optical fiber has an erbium-doped core that extends along a length of the optical fiber and is overlaid by a glass cladding.

[0008] FIG.2A plots wavelength dispersive x-ray spectroscopy (WDS) scans for exemplary erbium-doped optical fibers showing erbia concentration.

[0009] FIG.2B plots WDS scans for the exemplary erbium-doped optical fibers showing baria concentration.

[0010] FIG.2C plots WDS scans for the exemplary erbium-doped optical fibers showing alumina concentration.

[0011] FIG.2D plots refractive index profiles measured at 976 nm for the exemplary erbium-doped optical fibers.

[0012] FIG.3 shows ground state absorption spectra taken near 976 nm for the exemplary erbium-doped optical fibers prepared using erbium-doped barium fluoride nanoparticles.

[0013] FIG.4 shows ground state absorption spectra near 1530 nm for the exemplary erbium-doped optical fibers prepared using erbium-doped barium fluoride nanoparticles and also for commercial fibers.

[0014] FIGS.5A and 5B show measured excited state→4I15 / 2) lifetime decay when pumped at 976 nm and 1480 nm, respectively; the lifetime is plotted on a logarithmic scale to display the effects due to non-radiative terms in the complete lifetime function.

[0015] FIG.6 shows a quenching model as a function of erbium ion density; specifically, τslowis plotted as a function of the average erbium ion concentrations for each NP fiber when pumped at 976 nm. These points were used to generate the fit for the model.

[0016] FIG.7 shows a diagram of a three-stage optical amplifier setup for measuring slope efficiency, where optical isolators to block backscattered light from the pump and probe lasers are marked “ISO,” “EDF” indicates commercial erbium-doped fibers used to boost the signals through to the fiber under test, and the output from the fiber under test is sent through a lens for collimation before being collected on an integrating sphere detector to be read out.

[0017] FIG.8 shows gain curves for a select set of the nanoparticle-based fibers and the commercial erbium-doped fibers, where each nanoparticle-based fiber presented correlates to a different erbia concentration.DETAILED DESCRIPTION OF THE DRAWINGS

[0018] Described in this disclosure is a nanoparticle-based approach to incorporate erbium into optical fibers at relatively high concentrations (e.g., higher than 0.04 mol. %) without or with minimal erbium ion quenching. Specifically, optical fibers may be formed using erbium-doped nanoparticles, such as erbium-doped barium fluoride nanoparticles, where the rare earth doping level in the nanoparticles and the amount of nanoparticles incorporated into the fibers may be controlled independently. Within the nanoparticle structure, erbium ions may be separated by barium ions in such a way to prevent quenching effects in the final drawn fiber. Additionally, pre-fabricating the nanoparticles, prior to their addition into the preform from which the fiber is drawn, ensures that the dopant (e.g., Er) and the glass modifier (e.g., Ba) are necessarily spatially proximate to promote higher solubility. Using the approach described in this disclosure, it is possible to produce optical fibers with more erbium than conventional fibers while retaining amplifier efficiency. Consequently, shorter fibers can be used for amplifiers, more amplifiers can be made per fiber draw, and better fibers can be produced for high power lasers.

[0019] Although this disclosure is focused on active fibers employing erbium-doped barium fluoride nanoparticles as precursors, it is recognized that the results may be extended to other rare-earth:alkaline-earth doped nanoparticle systems. For example, other alkaline earths besides barium (Ba), such as calcium (Ca), magnesium (Mg), or strontium (Sr), may be suitable for the nanoparticles, which may be fluorides and / or oxides, and in some examples rare earth dopants besides erbium (Er), such as ytterbium (Yb) or thulium (Tm), may also or alternatively be employed.

[0020] Below, a method of making an erbium-doped optical fiber utilizing a suspension of erbium-doped alkaline earth fluoride nanoparticles is discussed, and then the composition and properties of the final erbium-doped optical fiber are described.

[0021] The fabrication method includes preparing a suspension that contains erbium- doped nanoparticles and then incorporating this suspension into an optical fiber preform. In this example, the erbium-doped nanoparticles comprise erbium-doped barium fluoride (“Er:BaF2 nanoparticles”), but it is contemplated that other rare earth-doped nanoparticles may alternatively be used, as indicated above. The erbium-doped nanoparticles may insome examples include, in addition to the alkaline earth fluoride, an aluminum fluoride and / or a rare earth fluoride, such as cerium fluoride, lanthanum fluoride, gadolinium fluoride, lutetium fluoride, and / or yttrium fluoride. The alkaline earth fluoride may form a solid solution with the aluminum / rare earth fluoride. The Er:BaF2or other rare earth- doped nanoparticles, which may include an aluminum fluoride and / or rare earth fluoride, may be dispersed and suspended in a carrier liquid, which may comprise an organic solvent such as diethylene glycol, alcohol and / or water. The carrier liquid may further include alumina or an alumina precursor, e.g., an aluminum salt, such as aluminum nitrate or aluminum chloride, in some examples.

[0022] The doping of the nanoparticles with erbium may be carried out using a solution (liquid) chemistry process as described below and may be understood to involve the chemical incorporation of erbium into the atomic lattice of the nanoparticle; as demonstrated for barium fluoride nanoparticles, suitable concentrations of erbium may lie in a range from 4 mol. % Er to 16 mol. %. Generally speaking, the concentration of erbium in the alkaline earth fluoride nanoparticles may be about 2 mol. % or more, 4 mol.% or more, about 7 mol.% or more, about 10 mol.% or more, or about 13 mol.% or more, and / or up to about 16 mol.%. These concentrations are much higher than can be added to silica, or the conventional alumina-doped silica (aluminosilicate). The as- synthesized erbium-doped alkaline earth fluoride nanoparticles may have an average particle size of 100 nm or less, e.g., in a range from 10 nm to 50 nm, or from 20 nm to 30 nm.

[0023] The suspension including the erbium-doped alkaline earth nanoparticles may be used in a “suspension-doping” process during fabrication of an optical fiber preform, as described below. Typically, the concentration of the erbium-doped nanoparticles (e.g., Er:BaF2 nanoparticles) in the suspension is from about 0.02 M to about 0.2 M. For example, the concentration may be 0.02 M or more, 0.05 M or more, 0.08 M or more, 0.11 M or more, and / or up to 0.12 M, or up to 0.2 M. Once fabricated, the optical fiber preform may undergo a drawing process to prepare the final optical fiber. The amount of erbium incorporated into the final optical fiber may be controlled by manipulating the concentration of erbium in the nanoparticles and / or the concentration of doped nanoparticles in the suspension. More specifically, the concentration of nanoparticles inthe suspension and the erbium dopant concentration may be tuned to ensure a large amount of erbium (e.g., > 1 wt.% on average) in the final optical fiber without deleterious changes to the erbium spectroscopy. For example, a lower concentration of erbium in the nanoparticles (e.g., 2 mol.%) may be compensated for by a larger concentration of nanoparticles in the suspension (e.g., 0.2 M). Experiments described below show that optical fibers prepared as set forth in this disclosure with relatively high erbium concentrations show improved optical properties compared to commercially available fibers of similar erbium concentrations.

[0024] To fabricate the optical fiber preform, porous layer(s) comprising particulate silica may be formed on an inner surface of a tubular silica substrate that ultimately becomes the cladding of the subsequently drawn optical fiber. The tubular substrate may be positioned on a rotatable lathe, and a modified chemical vapor deposition (MCVD) process involving the introduction of gaseous precursors into an end of the tubular silica substrate may be used to form the porous coating. For example, a volatile halide, e.g., SiCl4, may be employed to form a porous silica layer. As the lathe is rotated during MCVD, fine silica or silicate particulates, called “soot,” may be deposited on the inner surface of the tubular silica substrate and then partially sintered to form a mechanically robust yet porous layer, e.g., by exposure to an external flame or other heat source that may be translated along a length of the tubular substrate as it rotates. It is noted that the phrase “comprising silica” used in this disclosure may encompass silica (SiO2) and / or silicate(s), as set forth below. After MCVD, the suspension including the (pre- synthesized) erbium-doped nanoparticles may be infiltrated into the porous coating to accomplish the suspension doping. For example, the tubular substrate may be filled with the suspension, such that the carrier fluid and suspended erbium-doped nanoparticles are adsorbed into the porous soot by capillary action. As indicated above, the suspension including the erbium-doped nanoparticles may also include an oxide or oxide precursor such as an aluminum salt to incorporate an oxide such as alumina into the porous silica coating. Oxides such as alumina may further help prevent clustering of the erbium atoms and / or may be used to control the light-guiding properties of the resultant optical fiber. Typically, other rare earths are deliberately excluded from the suspension, with theexception of their possible presence as an unavoidable or unintended impurity at trace levels.

[0025] After suspension doping, the porous layer may undergo passive or active drying (e.g., heating) to remove the carrier liquid while the erbium-doped nanoparticles remain embedded in the porous layer, along with any oxide species. Chlorine (Cl2) may be used during fabrication, such as during drying, to reduce hydroxyl content. The porous layer and tubular substrate may be heated to a temperature (e.g., about 2000°C or higher) sufficient to cause densification of the porous layer and, eventually, collapse of the tubular substrate into a dense rod, resulting in a fiber preform that can be thermally drawn into the finished optical fiber. Typically, upon heating to a temperature of 2400°C or higher, collapse occurs, resulting in a diameter reduction (to about 10-20 mm) and formation of the fiber preform, where the porous coating becomes the preform core and the tubular substrate becomes the preform cladding. The fiber preform may then undergo a drawing process, also at an elevated temperature (e.g., from 1800°C to 2000°C) to form an erbium-doped optical fiber having an erbium-doped glass core. A commercially available optical fiber draw tower, such as a Heathway draw tower, may be employed for drawing. The final drawn optical fiber may have a diameter in a range from about 100- 150 µm. In some examples, fiber drawing may be followed by an annealing step at a temperature in a range from about 800-1100°C. The effect of the annealing step may be to further reduce the quenching of the erbium ions in the fiber. An additional polymeric coating may be applied to the drawn fiber to provide mechanical / strength protection.

[0026] Referring to the schematic of FIG.1, the optical fiber 110 obtained upon drawing has an erbium-doped core 100 that extends along a length of the optical fiber 110 and which is overlaid by a glass cladding 120 having a lower index of refraction, such that light propagates predominantly in the glass core. The optical fiber 110 may be a single-clad fiber or a double-clad fiber. The erbium-doped core 100 includes (a) a glassy matrix comprising silica, that is silica and / or one or more silicates, and (b) nanoscale regions dispersed throughout the glassy matrix, where the nanoscale regions include erbium ions and alkaline earth (AE) ions. The nanoscale regions are formed from the precursor erbium-doped (e.g., Er:AEF2) nanoparticles incorporated into the fiber preform during its fabrication and subsequent drawing into the optical fiber, and are believed to belargely or entirely sub-100 nm in size. Due to the high temperatures employed to fabricate the optical fiber preform, the erbium-doped nanoparticles may undergo structural and / or chemical changes during processing, including oxidation and / or amorphization, such that the erbium-doped nanoparticles present in the precursor suspension may not be identifiable as such in the final drawn optical fiber. Instead, the erbium ions and / or the alkaline earth ions in the nanoscale regions 104 may be present in the form of one or more oxides, such as, when Er:BaF2 nanoparticles are used as precursors, Er2O3, BaO, and / or erbium-doped barium silicates. When other erbium-doped alkaline earth fluoride nanoparticles are used as precursors, the nanoscale regions may contain other alkaline earth oxide(s) and / or alkaline earth silicates, such as calcium oxide, calcium silicate, magnesium oxide, magnesium silicate, strontium oxide, strontium silicate. In some examples, the nanoscale regions may also or alternatively include aluminum oxide, aluminum silicate, lanthanum oxide, lanthanum silicate, gadolinium oxide, gadolinium silicate, lutetium oxide, lutetium silicate, yttrium oxide, yttrium silicate, cerium oxide, and / or cerium silicate.

[0027] It is postulated that the alkaline earth ions may serve to separate or block the interaction between erbium ions within the nanoscale regions, inhibiting or preventing the quenching that may lead to a deterioration of optical properties. This results from the intimate co-location of the dopant (e.g., erbium) and glass modifier (e.g., Ba) afforded by the use of pre-synthesized doped nanoparticles. Accordingly, the alkaline earth ions may be present in the nanoscale regions in an amount sufficient to provide the desired separation. It is believed that a minimum amount of the alkaline earth ions is needed for the desired separation / inhibition of quenching, but too high of a concentration of alkaline earth ions may lead to phase separation, an undesirable form of devitrification, or other deleterious effects. Experimental data suggest the ratio of alkaline earth, in particular barium, to erbium in the erbium-doped core may be, on average, in a range from 1:1 to 7:1 for the desired inhibition of quenching while avoiding possible negative effects of an over-abundance of alkaline earth ions. The nanoscale regions may be amorphous or glassy, meaning the molecular structure about the dopant is not crystalline and is devoid of long-range order. Accordingly, at least 90%, at least 95%, or at least 98%, and / or up to 100% of the nanoscale regions may be amorphous. The glass core of the erbium-dopedoptical fiber may comprise, on average, at least 0.3 wt.% Er. By controlling the erbium- doping of the nanoparticles and the concentration of nanoparticles in the precursor suspension, the amount of erbium incorporated into the glass core may be at least 0.5 wt.% Er, at least 1 wt.% Er, and as high as about 2 wt.% Er. As discussed below, the erbium-doped optical fiber may be double clad or triple clad. The erbium-doped glass core is preferably devoid of ytterbium.

[0028] The glassy matrix in which the nanoscale regions are dispersed includes silica and may further include one or more other phases, e.g., oxides of aluminum, phosphorus, and / or boron. As explained above, the nanoscale regions are formed from the precursor erbium-doped (e.g., Er:AEF2) nanoparticles incorporated into the fiber preform during high-temperature fabrication of the optical fiber, which may involve structural and / or chemical changes (e.g., oxidation and / or amorphization) such that the erbium-doped nanoparticles present in the precursor suspension may not be identifiable as such in the glassy matrix of the final drawn optical fiber. When the glassy matrix further includes alumina in addition to silica, the matrix may be referred to as an aluminosilicate matrix. Generally speaking, the glassy matrix comprising silica may in some examples refer to aluminosilicate(s), phosphosilicate(s), borophosphosilicate(s), alumino-boro- phosphosilicate(s), borosilicate(s), and / or fluorosilicate(s). As indicated by the name, the glassy matrix, and thus the silica and the one or more other phases, may be amorphous or glassy, that is, noncrystalline and devoid of long-range order.

[0029] Due to the molecular structure about the dopant described above, the erbium- doped optical fiber may exhibit a quantum efficiency (QE) above 50% for the exemplar4I13 / 2 →4I15 / 2 emission at a wavelength around 1550 nm. In some examples, the quantum efficiency may be above 60%, above 70%, and / or as high as 84%, or as high as 87%. For some applications, such as in telecommunications, the molar concentration of erbium maybe at least 2 x 1025 m-3 and the QE may exceed 80%. As discussed in the Examplesbelow, the erbium-doped optical fiber may further exhibit an absorption of at least 60 dB / m at a wavelength of approximately 1530 nm (e.g., 1532.1 nm). In some examples, the absorption at ~1530 nm may be at least 80 dB / m, at least 100 dB / m, and / or as high as about 120 dB / m, or as high as about 200 dB / m, or as high as about 400 dB / m, or higher. The slope efficiency of the erbium-doped optical fiber, which may be determined using a1550 nm signal laser and 976 nm pump laser which is coupled into the core of the optical fiber at a power of around 500 mW, may be at least 48% at an optimal length of operation, L0, of about 85 cm or less. Advantageously, a laser or amplifier may utilize the erbium-doped optical fiber described in this disclosure. The erbium-doped optical fiber may achieve wider L-Band amplification, e.g., in a range from 1565 nm to 1630 nm, which extends beyond the range of conventional aluminosilicate fibers, e.g., in a range from 1565 nm to 1625 nm. To be clear, the range of L-Band amplification achieved by the erbium-doped optical fiber may exceed 1625 nm, which is not possible with conventional optical fibers. Amplifiers based on the erbium-doped optical fibers may be employed in data communications, such as in subsea applications which may benefit from the larger gain width and / or in data centers where a high erbium doping level without sacrificing efficiency would be advantageous. The fibers may also be useful for high power lasers.

[0030] As described above, the erbium-doped optical fiber may be fabricated using a MCVD process and suspension doping, where a porous layer comprising silica is formed from gaseous precursors on an inner surface of a tubular substrate, and then infiltrated with a suspension comprising erbium-doped alkaline earth fluoride nanoparticles in a liquid carrier. The infiltration is followed by heating to sinter and ultimately collapse the porous layer / tubular substrate and form an optical fiber preform, that then is thermally drawn into an erbium-doped optical fiber. The suspension employed to form the erbium- doped optical fiber may comprise erbium-doped alkaline earth fluoride nanoparticles in a liquid carrier, where the erbium-doped alkaline fluoride nanoparticles include erbium at a concentration from 2 mol. % to 16 mol. %. The concentration of the erbium-doped alkaline earth fluoride nanoparticles in the suspension may be in the range from 0.02 M to about 0.2 M, as discussed in more detail above. The liquid carrier used in the suspension may comprise water and / or alcohol and / or diethylene glycol, or another suitable solvent. The as-synthesized erbium-doped alkaline earth fluoride nanoparticles may have a sub- 100 nm average particle size, e.g., in a range from 10 nm to 50 nm.

[0031] The optical fiber may further comprise an outer buffer layer. This buffer material may be an acrylate, a polyimide, polyvinyl, or other material. This material may be selected from one that has a refractive index higher than that of a fiber claddingmaterial. In this configuration, the fiber may sometimes be referred to as single clad. The buffer layer may instead be configured to have an index lower than a cladding layer and therefore be considered to be double clad. This fiber configuration allows for pump light to be guided in the cladding layer, consistent with known high-power fiber laser systems. The cladding layer may further be shaped, such as into a hexagon, octagon, or D, to scramble the pump modes, breaking the circular symmetry for those pump modes that may tend to propagate helically around the core. The fiber may instead have a glass outer layer, such as a fluorinated silica layer, surrounding an outer cladding layer, also serving as a cladding for guiding pump light. This glass outer layer may also be further coated in a protective material. This protective material may have a refractive index less than the glass outer layer, thereby forming what is known as a triple clad fiber. The fiber may also possess an inner cladding layer whose index is greater than that of an outer cladding but less than that of the core. This configuration is often referred to as a ‘pedestal fiber’ and promotes effective single mode operation when the core refractive index exceeds the cutoff condition for single- or few-mode operation. In some cases, the fiber may possess stress applying parts or be elliptically shaped giving the fiber the ability to maintain a polarization state.

[0032] The fiber may further be configured to suppress nonlinearities. For example, the core may be doped with a material that lowers any one or more of Brillouin, Raman, nonlinear refractive index (n2), and thermo-optic (dn / dT) coefficients of the glass. The fiber may also be configured to possess a core whose waveguide characteristics give rise to reduced nonlinearities. For example, the core may be acoustically structured to lower the overlap integral between an optical and acoustic mode. It may also be structured to possess acoustically antiguiding propagation loss. It may also be structured in way that expands the optical mode size, for example as a large mode area fiber. The large mode area fiber may be single- or few-moded at the desired operating wavelength. Shorter fiber lengths via increased rare earth dopant concentrations also serve to increase the power thresholds for the onset of nonlinearities.

[0033] The doping profile (including the distribution of erbium ions, alkaline earth ions, erbium oxide, alkaline earth oxide, alkaline earth silicate, and / or other material) may be one that is uniform across the core region. In other examples, the doping profile maybe nonuniform across the core region. The fiber may be doped, for example, with a ring- shaped distribution. The distribution of erbium in the core may also occupy a region smaller than the core. The latter configuration promotes selective gain on the fundamental optical mode, providing some suppression to the propagation of higher order modes in a multimode core.

[0034] The doped optical fiber described in this disclosure may be pumped into either the core or into the cladding. Pumping into the core comprises configuring pump light to be coupled directly into, confined by, and propagated in the core of the fiber. The optical fiber core may have a brightness value associated with its diameter and numerical aperture. However, semiconductor laser pumps may be limited in that they usually have much lower brightness than a typical single mode core. As a result, a limited amount of pump power, usually less than a few Watts, can be coupled into the core, especially if it is single mode, limiting laser and amplifier power. An alternative is the use of a double cladding structure as described above. In this case, the pump is coupled into the much larger cladding possessing a much larger numerical aperture. This cladding has much lower brightness than the core and facilitates the coupling of much more pump power into the fiber. Thus, the pump power can be increased to more than several Watts, greater than 100 W, or greater than 1000 W. Since the pump light is guided in the cladding of a double clad fiber, the overlap with the doped core is greatly reduced, resulting in a lower absorption coefficient. In some cases, it is desirable to set the cladding to core diameter ratio in a double clad fiber to be as small as possible to maximize the pump absorption coefficient. As the pump power is slowly absorbed, the double clad fiber slowly transforms the brightness of the pump source, and such fibers are often referred to as ‘brightness converters.’

[0035] In pulsed applications, it may be desirable for the core diameter to be large, such as greater than 20 μm in diameter, greater than 30 μm in diameter, or even greater than 50 μm in diameter to extract sufficient pulse energy.

[0036] There may be an overlap factor between the pump light and the active ion, such as erbium, distribution in the optical fiber. The fiber absorption coefficient is the product of the active ion number density, the absorption cross section, and the overlap factor. The highest absorption coefficients for a given core composition are where theoverlap factor is equal to one. However, the overlap factor may be smaller, such as 0.1 or 0.01 in a double cladding fiber.

[0037] The erbium doped optical fiber of the present disclosure preferably has a substantially reduced degree of cooperative upconversion. The nanoscale regions including the alkaline earth ions (e.g., in the form of alkaline earth oxides, for example) and the rare earth ions (e.g., in the form of rare earth oxides, for example) are configured to lower the ion-ion interactions typically observed in conventional glasses. These ion-ion interactions decrease the quantum efficiency of the rare earth dopant. Preferably, these interactions are at least two times weaker than in a typical rare earth doped aluminosilicate fiber. More preferably, the cooperative upconversion process is more than 10 times weaker.

[0038] The pumping wavelength may be selected such that the number of erbium ions involved in lasing or optical amplification is less than the doping concentration. For example, in some cases, the number of rare earth ions involved in the gain process may be ½ of the doping concentration, and in some cases less than 1 / 10 of the doping concentration.

[0039] The optical fiber of the present disclosure has an emission and absorption cross section spectrum characteristic of the rare earth that may be impacted by the host and distribution of rare earth ions. The fiber also has an associated upper state lifetime. The upper state lifetime, preferably, is dominated by radiative processes, where the nonradiative or quenching contributions and / or effects are minimized. In some cases, the upper state lifetime is greater than about 9 ms in the erbium-doped fiber or greater than about 10 ms.

[0040] EXAMPLES

[0041] This section describes experimental work on the doping of optical fibers withprecursor Er:BaF2nanoparticles. With barium as a co-dopant, the aim is to employ themaximum amount of erbium that can be incorporated into the nanoparticles before the quantum efficiency of the optical fiber declines to a point unacceptable for a given application, device or system. Also of interest is the amount of nanoparticles that can be incorporated into a silicate fiber without losing efficiency. By controlling these two independent variables, it may be possible to increase the concentration of erbium insilicate fibers while additionally improving laser gain beyond the current benchmark for 976 nm pumping. These new fibers demonstrate quantum efficiencies similar to or greater than those of commercially available erbium-doped fibers (EDFs) while having higher slope efficiencies for 976 nm pumping.

[0042] The4I15 / 2 →4I11 / 2 transition at 976 nm employed here is a common pumping configuration for erbium doped material systems. A drawback to pumping at this wavelength is the large quantum defect when lasing near 1550 nm using the4I13 / 2 →4I15 / 2 transition, or within the gain bandwidth of the Er3+ions. It will also be known to those skilled in the art that the erbium ion may be pumped in-band, sometimes referred to as ‘resonantly pumped,’ using the4I15 / 2 →4I13 / 2 transition. For example, Er3+may be pumped near 1470 nm, 1490 nm, or 1530 nm. This will greatly reduce the quantum defect relative to pumping near 976 nm, enhance the slope efficiency, may also increase the quantum efficiency, and reduce the thermal load on the active fiber.

[0043] Fiber fabrication

[0044] Er:BaF2nanoparticles were synthesized in a liquid chemistry process where two solutions were mixed to form nanoparticles suspended in a solvent. All precursors used were reagent grade or better. The first solution was the fluorinating solution, which contained ammonium fluoride dissolved in diethylene glycol (DEG) at 90⁰C under an N2 atmosphere. The second solution (Er / Ba solution) contained, in a DEG / water mixture, a barium precursor (in this example, barium acetate hydrate) and an erbium precursor (in this example, erbium chloride hexahydrate) at stoichiometric amounts to form Er-doped BaF2nanoparticles with 4 mol.%, 8 mol. % and 16 mol.% Er doping. The Er / Ba solution was added to the fluorinating solution, reacted at 90⁰C under a N2 atmosphere for 2 h, and cooled to room temperature. This formed the mother solution (or suspension), which contains the Er-doped BaF2 nanoparticles.

[0045] Two versions of the nanoparticle suspension were produced for each Er concentration: a “high” concentration doping suspension was synthesized with the mother solution and aluminum chloride hexahydrate to yield 0.1 M Al2O3 concentration and 0.114 M Er:BaF2in the final suspension, and a “low” concentration doping suspension comprised of the mother solution diluted with equal parts DEG and doped to 0.05 M Al2O3and 0.057 M Er:BaF2. Fibers were fabricated using the high and lowconcentrations of Er:BaF2 nanoparticles in the doping suspension. The higher concentrated version was made to allow for double the nanoparticles in the final precursor, creating the “Hi” variants of the three fibers (Hi 4, Hi 8, Hi 16).

[0046] The resultant suspensions (hereafter called NP 4, NP 8, and NP 4 Hi correlating to their doping concentration and nanoparticle number density, respectively) were doped into fiber preforms on an SG Controls modified chemical vapor deposition (MCVD) lathe using a solution / suspension doping process as described above. Chlorine was used to reduce the hydroxyl content during preform fabrication in this example. The preforms were collapsed to ~15.5 mm in diameter on the MCVD lathe and then drawn at ~1925⁰C to ~125 μm diameter fiber with a Heathway fiber draw tower.

[0047] Post-draw fiber composition analysis

[0048] Developing the means for scalability in erbium-doped fibers prepared using Er:Ba nanoparticles as described above (“nanoparticle fibers”) would be aided by an understanding of the amount of Er3+in the optical fiber after the draw process.

[0049] Characteristic analysis was performed to track the amount of Er2O3, BaO, and Al2O3 contained in the core of each fiber via both spectroscopy and ground state absorption measurements. Wavelength dispersive X-ray spectroscopy (WDS) was performed on each fiber core using electron probe microanalysis (EPMA) to isolate the weight concentration of each oxide. Paired with the WDS analysis, refractive index profiles (RIPs) were taken for each fiber at 976 nm to further understand the size and geometry of the core. Finally, ground state absorption measurements were taken near 976 nm and 1530 nm. With higher absorption peaks at these wavelengths, it is expected that there is more Er3+contained in the fiber.

[0050] FIGS.1A-1C plot the WDS scans for erbia (Er2O3), baria (BaO), and alumina (Al2O3), respectively. From FIG.1A, there is an unsurprising trend that with an increase in the concentration of nanoparticles in the precursor suspension the erbia retained in the core also increases. Additionally, FIG.1B and FIG.1C show that with higher alumina contents in the core as well as a higher nanoparticle density, the amount of baria retained is also increased. This suggests that the alumina and nanoparticle concentrations control the baria concentration. However, of note is the high correlation between the resulting alumina and baria concentrations. The distribution of erbium and barium within thenanoparticles, on the other hand, sets the amount of erbia. In other words, as the mol.% of Er in the precursor increased, the amount of erbia in the core also increased. The alumina concentration is higher in the “Hi” version of the fibers than the original set, which is likely due to incorporating twice the amount of Al2O3 in the precursor suspension in the “Hi” variants. FIG.1D plots the refractive index profile (RIP) for each fiber at 976 nm. The general shapes of the fiber cores are reasonably consistent, with a slight refractive index dip in the center due to burn-out of more volatile components (e.g., BaO) which react with the Cl2used for drying during the preform collapse and close stages. Table 1 shows oxide distributions in the core of each fiber, presented as an average along the line scan of the core. Number densities of Ba2+and Er3+were used to calculate the ratio shown in column four, signifying the number of barium ions for each erbium ion in the respective fiber core. Table 1. Oxide Distributions and Ba / Er Ratio in Fiber Cores Er2O3 BaO Al2O3 Ratio of Average Er3+Fiber (wt.%) (wt.%) (wt.%) Ba / Er ion density ρavg (m−3) Lo 4 0.42% 1.13% 2.44% 3.4 2.69 × 1025Hi 4 0.41% 2.31% 2.42% 7.0 3.59 × 1025Lo 8 0.65% 1.27% 1.91% 2.4 4.34 × 1025Hi 8 1.08% 2.94% 3.10% 3.4 8.76 × 1025Lo 16 1.45% 1.20% 1.93% 1.1 8.86 × 1025Hi 16 2.09% 2.78% 3.09% 1.7 1.68 × 1026

[0051] To optimize the operation of the nanoparticle-based fibers, it would be beneficial to understand the relationship between the Er3+ions and the barium. It is expected that there is a minimum amount of barium needed to effectively keep the erbium from quenching. A goal for optimizing these fibers is to have the least amount of barium possible while increasing the Er concentration to realize EDFAs and ytterbium-free, erbium-doped high power fiber lasers of greater efficiency and shorter lengths.

[0052] To validate that the concentration of the erbium was increasing with each change to the fiber, the ground state absorption was measured for each of the erbium-doped optical fibers. FIGS.2 and 3 shows the absorption spectra for each fiber. The4I15 / 2→4I11 / 2spectra are shown in FIG.2 and the4I15 / 2→4I13 / 2spectra are in FIG.3. When paired with the WDS plots, the characteristic absorption peaks for erbium at 976 nm and 1530 nm are unsurprisingly higher with increasing Er2O3concentration. For comparison, two commercially available fibers were analyzed as a reference for ion concentration and efficiency: LIEKKI's Er110-4 / 125 and Er30-4 / 125 (hereafter called Er 110 and Er 30, respectively). The4I15 / 2 →4I13 / 2 peak absorption for the Er 110 is 110 dB / m, while for the Er 30 it is 30 dB / m. Since absorption directly scales with ion concentration, the amount of erbium contained in the Lo 4 is approximately double that of the Er 30 due to having a4I15 / 2 →4I13 / 2 peak absorption of 62 dB / m. Likewise, the Lo 8 is similar to the Er 110, with a4I15 / 2→4I13 / 2peak absorption of 110 dB / m.

[0053] Excited state lifetimes and minimum quenching models

[0054] Performance of EDFs as an amplifier for lasing systems is estimated by how much quenching is occurring in the fiber. The amount of ion quenching occurring was determined through calculation of quantum efficiency (QE). The QE was determined by measuring the4I13 / 2 →4I15 / 2 upper state transition lifetime.

[0055] Detrimental ion quenching occurring in the fibers was estimated through measurement of the4I13 / 2 →4I15 / 2 excited state transition lifetime. The lifetime was measured by pumping short (!1 mm) fiber samples to saturation with a pulsed laser. This sample length was chosen to inhibit the reabsorption of spontaneous emission. A long pass filter (1400 nm and 1500 nm for 976 nm and 1480 nm pumping, respectively) was used to prevent pump detection, then the emitted 1550 nm signal was focused onto an InGaAs avalanche photodiode. The measurement was taken as a function of time between two pulses. Two pumps were used to compare characteristics when pumping both out-of- band (OOB) at 976 nm and in-band (IB) at 1480 nm.

[0056] FIGS.4A and 4B plot the excited state transition lifetime as a normalized logarithmic decay for each fiber, including the commercial counterparts (Er 30 and Er 110). FIG.4A corresponds to OOB pumping, while FIG.4B comes from IB pumping. It is important to note that the curvature close to 0 ms indicates that there is not just one type of Er3+site that contributes to the excited state decay. This means that there is at leastone quenching site occurring in each fiber, with more quenching indicated by the level of the curvature in the lifetime.

[0057] To determine the number of Er3+sites in these fibers, the lifetime data sets werefit with a multi-term exponential model: " = "# + $' &() %&In this model, the Ai corresponds to the distribution of ions in each site, τiis the site's lifetime, and y0is a constant offset in the measurement. For this work, the number of sites determined to fit closest to the experimental data was three (n = 3). The first term (i = 1) correlates to the slowest lifetime and thus the mostly radiative component (hereafter called τslow and Aslow). The remaining two terms (i = 2,3) are subject to significant quenching (hereafter called τf1, Af1and τf2, Af2, respectively).

[0058] Table 2 lists the excited state lifetimes for OOB pumping determined from fitting to the measured lifetime curves. Table 3 lists these extracted lifetime values when pumped IB. Of note is that theand Aslow are similar to each other for both pumping schemes. Another observation in these results is that the slow lifetimes are shorter at the highest concentrations, indicating that significantly more quenching is occurring in those fibers compared to their lighter-doped versions. This is also apparent in the Aslowvalues, which decrease as ion concentration increases. Moreover, while not monotonically, the fast components also generally become faster with increasing erbium concentration. Despite this, the Lo 4, Hi 4, and Lo 8 still maintain an Aslow above 0.7, with a significant gap between the similarly concentrated Lo 8 and Er 110. From these results, we expect that these three NP fibers will have higher lasing efficiency than the Er 110 counterpart as well as the highest concentrated NP fibers (Lo 16 and Hi 16). Table 2. Excited State (4I13 / 2 →4I15 / 2) Lifetimes for Fibers When Pumped at 976 nm Fiber τslow (ms) (Aslow) τfast1 (ms) (Afast1) τfast2 (ms) (Afast2) Er 30 10.24 (0.79) 3.43 (0.16) 0.48 (0.05) Er 110 9.21 (0.47) 2.61 (0.35) 0.41 (0.17) Lo 4 10.53 (0.85) 3.20 (0.15) 0.36 (0.14) Hi 4 10.77 (0.78) 3.76 (0.18) 0.58 (0.05) Lo 8 10.60 (0.72) 3.51 (0.25) 0.56 (0.12) Hi 8 9.54 (0.56) 2.68 (0.32) 0.43 (0.12)Fiber τslow (ms) (Aslow) τfast1 (ms) (Afast1) τfast2 (ms) (Afast2) Lo 16 9.66 (0.45) 2.66 (0.43) 0.42 (0.36) Hi 16 8.90 (0.31) 2.22 (0.43) 0.37 (0.26) Table 3. Excited StateLifetimes for Fibers When Pumped at 1480 nm Fiber τslow (ms) (Aslow) τfast1 (ms) (Afast1) τfast2 (ms) (Afast2) Er 30 10.53 (0.77) 5.12 (0.16) 1.55 (0.07) Er 110 9.32 (0.51) 2.58 (0.32) 0.39 (0.14) Lo 4 11.11 (0.75) 5.61 (0.18) 1.81 (0.06) Hi 4 11.10 (0.80) 4.03 (0.17) 0.72 (0.04) Lo 8 10.74 (0.69) 3.88 (0.23) 0.84 (0.07) Hi 8 10.22 (0.57) 3.23 (0.32) 0.59 (0.10) Lo 16 9.52 (0.53) 2.52 (0.35) 0.43 (0.12) Hi 16 8.44 (0.39) 2.07 (0.41) 0.36 (0.19)

[0059] To understand the limits to which these nanoparticle-based optical fibers can be doped with erbium before quenching becomes a limiting factor in lasing operation, a model was used to fit to their ion densities with respect to their lifetime components. The function for this model comes from previously published work, where the lifetime of an ion in a

[0060]

[0061] The fitting parameters are τ0, the longest lifetime possible associated with no quenching, and ρq, the ion density referred to as the quenching concentration. FIG. 5 plots this model using the densities for all nanoparticle-based fibers with their respective τslow taken from the values in Table 2. The fit of the model indicates that the lifetime correlated to a NP fiber with zero quenching is 11.32 ms, while the quenching concentration is 1.25 ×However, due to the limited number of available data points there is a margin of error in the fit for the model. Future investigation into newer variants of these NP fibers to add to this model is currently underway. This is importantsince it was difficult to control the relative proportion of barium to erbium in these proof- of-concept fibers.

[0062] Slope efficiency, optical fiber length, and quantum efficiency

[0063] In addition to quantifying the ion quenching occurring in the fibers through the QE the second analysis to determine the fibers’ effectiveness as a laser and / or fiber amplifier is through measurement of the slope efficiency (SE). The SE is calculated as the slope of the curve of output signal power as a function of input pump power.

[0065] In the quantum limit, quantum efficiency and slope efficiency have a linear relationship as

[0067] where 1>?@?A-refers to the quantum defect of the signal and pump photons.

[0069] This means that for a pumping wavelength of 976 nm and a signal of 1550 nm the quantum defect and therefore the highest possible SE is 63%.

[0070] With QE approximately the same as the Er 30 fiber, it is expected that the SE of the nanoparticle fibers will be similar to the lower-concentration commercial version. However, due to having higher 1530 nm absorption values than the Er 30, the nanoparticle fibers are expected to have a shorter optimal length of operation, L0when core pumped.

[0071] A contextual diagram of the experimental setup used to measure slope efficiency is shown in FIG.6. Two amplification stages were used to guarantee the input signal was deeply saturating, while a final third stage was used to perform the measurement. The first stage combines a 1550 nm signal with a 976 nm pump through a wavelength division multiplexer (WDM). The combined output is sent into a commercial EDF for amplification and on to stage two. Like stage one, the second stage combines the output of the first with another 976 nm pump and propagates through another commercial EDF and into the third stage. In the third stage, the output of the first two stages is combined with the pump for testing. The signal power launched to stage 3 from stages 1 and 2 is at least 30 mW. Both OOB and IB pumping schemes were used for testing. Thecombined output was sent through the fiber under test and collimated onto an integrating sphere power detector with a long-pass filter attached to remove residual pump power.

[0072] The slope efficiency measurement involved cutback on the length of fiber under test to maximize the SE value possible for each test sample. This means that two data points were collected on every fiber: the maximum measured SE and the corresponding length of fiber (hereafter L0, or “optimal length”). Identifying the SE also required accounting for losses within the experimental setup. In the system, there were three points of loss incorporated into the final SE calculation. One comes from the scattered light from the collimating lens that fails to be picked up by the detector. Another is the amount of residual pump power from the testing stage that is not converted into 1550 nm light. The last is the loss due to splice mismatch (namely at the pumping wavelength) between the fiber under test and the output fiber of the third stage. To measure the mismatch, a sample of the test fiber is pumped with a saturated pump signal from the third stage of the amplifier. The sample tested is slightly shorter than the determined L0 from the SE test to mitigate effects due to reabsorption of the pump. The output pump from the sample is then measured as a function of cutback towards the splice point with the amplifying stage. Splice mismatch is extrapolated from the linear fit of the cutback measurement at the y-intercept. Table 4 lists the final calculated slope efficiency values for all fibers, including the commercial counterparts, for the OOB pumping scheme. For each length, the adjusted slope efficiency after accounting for losses due to splicing, loss at the collimating lens, and pump leakage is presented. The third column lists the optimal lengths of operation for each EDF. The last column (ηq) is the quantum efficiency calculated from ηs. From these results, it is clear that the Lo 4, Hi 4, and Lo 8 have SEs that either meet or exceed those of the commercial EDFs. In particular, the Lo 4 value is higher than the Er 30 value by 4% (0.48 / 0.46), while the Lo 8 is greater than the Er 110 by 27% (0.47 / 0.37). Additionally, due to the higher erbium concentration, the nanoparticle-based erbium- doped fibers have optimal lengths that are, in some cases, significantly shorter than the commercial versions. As an example, the Hi 4 has a similar SE to the Er 30, yet the L0 is less than half as long at 60.3 cm. This means that the nanoparticle-based erbium-dopedfibers are less susceptible to detrimental nonlinear effects (e.g., Brillouin scattering) with equal efficiency. Table 4. Slope efficiency data for all fibers when pumped at 976 nm. Fiber ηs L0 (cm) ηq Er 30 0.46 164.3 0.73 Er 110 0.37 38.2 0.59 Lo 4 0.48 84.7 0.76 Hi 4 0.46 60.3 0.74 Lo 8 0.47 39.7 0.74 Hi 8 0.35 19.6 0.56 Lo 16 0.27 14.6 0.43 Hi 16 0.23 7.2 0.37

[0073] When compared to previous results in the literature, there have been records of SE as high as 0.47 in fibers produced using alternative nanoparticle techniques. However, the erbia concentration of these EDFs were significantly lower (on the order of 0.047 wt.%) than in the NP fibers presented in the current investigation. This indicates that, if scaled down to match the previously reported fibers that these new EDFs would have SE that exceed these previous benchmarks. When comparing to records of shortest EDFA schemes, fibers as short as 6.5 cm were previously reported with SE as high as 0.20 for similar quantum defect values. On the lowest end, the Hi 16 demonstrates a comparable L0at 7 cm and a higher SE at 0.23. In addition, the Hi 16 has a higher absorption than EDFs in previously published work, which have comparable fiber core sizes, indicating that the Er3+concentration is greater. In both cases, the nanoparticle- based fibers are able to match previously set records while having higher ion densities.

[0074] Table 5 lists the final calculated SE values for the fibers pumped IB at 1480 nm. Note that this table only contains results from one commercial (Er 110) and one of the NP fibers (Lo 4). Currently, the remaining fibers are still under analysis and data will be forthcoming. With the data available from the Er 110 and Lo 4, there is a clear increase in both the SE and L0 when compared to OOB pumping. This is expected for theformer due to the change in limiting term ηd, which is related to the quantum defect as ηd=λpump / λsignal. This limiting factor determines the maximum SE achievable for each pumping wavelength. With longer λpumpthe limit increases, so the SE should increase as the pumping wavelength approaches the signal. The increase in length is attributed to the difference in absorption values at the respectiveReferring back to FIGS.2 and 3, the absorption at 976 nm for the nanoparticle-based fibers is slightly larger than at 1480 nm, and thus the optimal operating length is expected to be shorter when pumping OOB. Table 5. Slope efficiency data for a subset of the investigated fibers when pumped at 1480 nm. Fiber ηs L0 (cm) ηq Er 110 0.47 80.1 0.59 Lo 4 0.68 131.2 0.72

[0075] From the calculated SE, the quantum efficiency (QE) can also be calculated via the relationshipBoth Table 4, Table 5 have the calculated QE for each fiber at the respective pumping wavelength. The quantum efficiency is directly related to the amount of quenching occurring when the erbium ions are pumped in the4I15 / 2 →4I13 / 2 transition. From Table 4, the Lo 4, Hi 4, and Lo 8 have QEs that are greater than 0.7 and therefore at least the majority of the ions are being used for producing 1550 nm photons. On the contrary, the highest concentrated NP fibers (Lo 16 and Hi 16) have QEs less than 0.5, which is indicative of the significant amount of ion quenching occurring in these fibers. Investigations correlating the QE calculated from the SE and the QE that can be calculated from the lifetimes are currently underway.

[0076] Gain characteristics

[0077] Together with the absorption characteristics around 1530 nm and the optimal lengths observed for lasing at 1550 nm, it is important to understand the wavelengths at which these EDFs are able to provide gain for use in fiber amplifiers. Gain measurements across the C-band and into the L-band regimes (roughly 1450–1650 nm) were measured by coupling light from a broadband light source produced by the spontaneous emission of approximately 80 cm of Lo 4 fiber and a 976 nm pump into approximately L0 samplelengths of each EDF. Each sample was pumped with 280 mW of 976 nm light for these measurements. The resulting gain curve was the difference between the small signal broadband source input with no pump on and the broadband source input with the pump off and pump on. FIG.8 plots these gain curves for the standard NP density set of fibers (Lo 4, Lo 8, and Lo 16) as well as for both commercial fibers. When testing the Hi variants of the NP fibers, each displayed significant oscillations due to effects of mode beating and were therefore excluded from the final results.

[0078] It should be noted that the fiber core diameters differ somewhat, such that a one-to-one comparison of the absolute gain in each fiber is difficult. However, when paired with the concentration values from Table 1, FIG.8 shows that with increasing erbia the gain across the C-band both decreases and narrows. Much of the decrease in gain can be attributed to quenching of the erbium ion, which continues into the L-band. The reason for this may be related to the ratio of erbia to alumina in the core, which is the smallest of all the fibers in the set. In addition, again referring to Table 1, the Ba / Er ratio decreases going from Lo 4 to Lo 16.

[0079] Comparing the NP fibers to the commercial EDFs, there is a difference in the shape of the gain curve between 1540 and 1560 nm. This is believed to be due to the incorporation of baria, though it is still unclear how adjusting the baria concentration affects this shape. This may be related to the ionic interaction between the host and the Er3+ions. It is also important to note that the gain in the Er 30 appears to be higher across most wavelengths than both nanoparticle-based fibers. This is due to the Er 30 having a smaller core size (4 μm diameter according to the data sheet) than the nanoparticle-based fibers.

[0080] There are two main applications in which these EDFs can be tailored for development. First is power scalability to kW-level lasing, which has yet to be achieved in erbium-based systems. By continuing to develop these fibers beyond the current benchmarks of the ion quenching limit, the next step is to adjust the geometry of the fiber to test high power lasing capabilities. A double-clad version of these fibers for testing is currently under development. Alternatively, these fibers can serve as a new means for developing erbium-doped fiber amplifiers (EDFAs) for telecommunications. Recently, there has been an increased interest in improving the performance of EDFAs to meet therapid scaling of the internet. To meet the rising demand, approaches have been taken to expand the operational band of EDFAs. For these new EDFs, testing of the gain bandwidth further into the L-band and development of a high QE version are currently under investigation.

[0081] The above working examples show that using a unique nanoparticle suspension allows for higher densities of Er3+ions in the core before limiting quenching effects inhibit lasing efficiency. This is attributed to the combination of baria in the fiber and its innate proximity to the erbium dopants afforded their pre-synthesis into a common nanoparticle that limits ion-ion (Er-Er) interactions. Analysis of these new fibers demonstrates average Er3+densities as high as 1.7 × 1026ions. Some of these fibers have slope efficiencies up to 0.48 (for OOB pumping) and 0.68 (for IB pumping), which meet and, in some cases, exceed that of reported and commercial EDFs with similar or lower erbium ion densities. Based on the fitting model for ion quenching with density, there is potential to adjust the precursor suspension such that the erbium concentration and efficiencies are maximized. This indicates that these new EDFs are capable of achieving new benchmarks in amplification and high-power lasing at 15XX nm.

[0082] The present disclosure also includes the following aspects:

[0083] A first aspect relates to an erbium-doped optical fiber comprising: an erbium- doped core radially surrounded by a glass cladding, the erbium-doped core comprising: a glassy matrix comprising silica; and nanoscale regions dispersed within the glassy matrix, the nanoscale regions comprising erbium ions and alkaline earth ions, wherein the erbium-doped optical fiber exhibits a quantum efficiency (QE) above 50%.

[0084] A second aspect relates to the erbium-doped optical fiber of the first aspect being prepared using a suspension of erbium-doped alkaline earth fluoride nanoparticles, and / or wherein the alkaline earth ions are selected from the group consisting of: Ba, Ca, Mg, and Sr.

[0085] A third aspect relates to the erbium-doped optical fiber of any preceding aspect, wherein the quantum efficiency is above 60%, above 70%, and / or as high as 84%, or as high as 87%, and / or wherein an erbium concentration in the erbium-doped core is at least 2 x 1025m-3and the QE exceeds 80%.

[0086] A fourth aspect relates to the erbium-doped optical fiber of any preceding aspect exhibiting an absorption of at least 30 dB / m at a wavelength of approximately 1530 nm.

[0087] A fifth aspect relates to the erbium-doped optical fiber of any preceding aspect having a slope efficiency of at least 48% and an optimal length of operation, L0, of about 85 cm or less.

[0088] A sixth aspect relates to the erbium-doped optical fiber of any preceding aspect, wherein a ratio of alkaline earth to erbium in the erbium-doped core is, on average, in a range from 1:1 to 7:1.

[0089] A seventh aspect relates to the erbium-doped optical fiber of any preceding aspect, wherein the alkaline earth ions are present in the nanoscale regions in an amount sufficient to inhibit or prevent quenching of the erbium ions.

[0090] An eighth aspect relates to the erbium-doped optical fiber of any preceding aspect, wherein at least 90%, at least 95%, or at least 98%, and / or up to 100% of the nanoscale regions are amorphous.

[0091] A ninth aspect relates to the erbium-doped optical fiber of any preceding aspect, wherein the nanoscale regions about the original (as-suspension doped) nanoparticle comprise one or more oxides.

[0092] A tenth aspect relates to the erbium-doped optical fiber of the preceding aspect, wherein the one or more oxides include Er2O3, BaO, erbium-doped barium silicate, calcium oxide, calcium silicate, magnesium oxide, magnesium silicate, strontium oxide, strontium silicate, aluminum oxide, aluminum silicate, lanthanum oxide, lanthanum silicate, gadolinium oxide, gadolinium silicate, lutetium oxide, lutetium silicate, cerium oxide, cerium silicate, yttrium oxide, and / or yttrium silicate.

[0093] An eleventh aspect relates to the erbium-doped optical fiber of any preceding claim, wherein the erbium-doped core comprises on average at least 1 wt.% Er, and / or wherein the erbium-doped optical fiber is configured to be double clad or triple clad.

[0094] A twelfth aspect relates to the erbium-doped optical fiber of any preceding aspect, wherein the glassy matrix further comprises oxides of aluminum, phosphorus, and / or boron.

[0095] A thirteenth aspect relates to the erbium-doped optical fiber of any preceding aspect being purposefully devoid of ytterbium.

[0096] A fourteenth aspect relates to the erbium-doped optical fiber of any preceding aspect, wherein the glass cladding comprises silica.

[0097] A fifteenth aspect relates to the erbium-doped optical fiber of any preceding claim having an L-Band amplification in a range from 1565 nm to 1630 nm, and / or wherein the L-Band amplification range exceeds 1625 nm.

[0098] A sixteenth aspect relates to a laser or amplifier comprising the erbium-doped optical fiber of any preceding claim.

[0099] A seventeenth aspect relates to a method of making an erbium-doped optical fiber, the method comprising: forming a porous layer comprising silica on an inner surface of a tubular substrate; infiltrating a suspension including erbium-doped alkaline earth fluoride nanoparticles in a carrier liquid into the porous layer; optionally drying to remove the carrier liquid from the porous layer, the erbium-doped alkaline earth fluoride nanoparticles remaining in the porous layer; heating the tubular substrate to a temperature sufficient to sinter the porous layer and collapse the tubular substrate, thereby forming a fiber preform; and drawing the fiber preform into an erbium-doped optical fiber, wherein the erbium-doped optical fiber exhibits a quantum efficiency above 50%.

[0100] An eighteenth aspect relates to the method of the preceding aspect, wherein, during the heating, the erbium-doped alkaline earth fluoride nanoparticles undergo structural and / or chemical changes, resulting in formation of nanoscale regions comprising erbium ions and alkaline earth ions in an erbium-doped core of the erbium- doped optical fiber, and / or wherein the temperature sufficient to cause collapse is at least about 2400 °C.

[0101] A nineteenth aspect relates to the method of any preceding aspect, wherein drawing of the fiber preform occurs at a temperature in a range from 1800°C to 2000°C.

[0102] A twentieth aspect relates to the method of any preceding aspect, wherein forming the porous layer comprising silica on the inner surface of the tubular substrate comprises modified chemical vapor deposition (MCVD).

[0103] A twenty-first aspect relates to the method of any preceding aspect, further comprising preparing the suspension, and / or wherein a concentration of the erbium-doped alkaline earth fluoride nanoparticles in the suspension is in a range from 0.02 M to 0.2 M.

[0104] A twenty-second aspect relates to the method of any preceding aspect, wherein an average size of the erbium-doped alkaline earth fluoride nanoparticles is in a range from 10 nm to 50 nm.

[0105] A twenty-third aspect relates to the method of any preceding aspect, wherein the erbium-doped alkaline earth fluoride nanoparticles in the suspension include erbium at a concentration from 2 mol. % to 16 mol. %, and / or wherein the erbium-doped alkaline earth nanoparticles further comprise, in addition to erbium and alkaline earth fluoride, an aluminum fluoride and / or a rare earth fluoride, the aluminum fluoride and / or the rare earth fluoride optionally being in solid solution with the alkaline earth fluoride.

[0106] A twenty-fourth aspect relates to the method of any preceding aspect, wherein the erbium-doped optical fiber has any or all of the features, characteristics or properties recited in any preceding aspect or elsewhere in this disclosure.

[0107] A twenty-fifth aspect relates to a suspension for use in forming an erbium- doped optical fiber, the suspension comprising: erbium-doped alkaline earth fluoride nanoparticles in a carrier liquid, wherein the erbium-doped alkaline earth fluoride nanoparticles include erbium at a concentration from 2 mol. % to 16 mol. %, and wherein a concentration of the erbium-doped alkaline earth fluoride nanoparticles in the carrier liquid is in a range from 0.02 M to 0.2 M.

[0108] A twenty-sixth aspect relates to the suspension of the previous aspect, wherein an average size of the erbium-doped alkaline earth fluoride nanoparticles is in a range from 10 nm to 50 nm.

[0109] A twenty-seventh aspect relates to the suspension of any preceding aspect, wherein the carrier liquid comprises water, alcohol, and / or diethylene glycol.

[0110] A twenty-eighth aspect relates to the suspension of any preceding aspect, wherein the carrier liquid further comprises an aluminum salt.

[0111] A twenty-ninth aspect relates to the suspension of any preceding aspect, wherein the erbium-doped alkaline earth nanoparticles further comprise an aluminum fluoride and / or a rare earth fluoride.

[0112] A thirtieth aspect relates to the suspension of any preceding aspect, wherein the aluminum fluoride and / or rare earth fluoride is in solid solution with the alkaline earth fluoride.

[0113] To clarify the use of and to hereby provide notice to the public, the phrases "at least one of , , … and <N>" or "at least one of , , … or <N>" or "at least one of , , … <N>, or combinations thereof" or ", , … and / or <N>" are defined by the Applicant in the broadest sense, superseding any other implied definitions hereinbefore or hereinafter unless expressly asserted by the Applicant to the contrary, to mean one or more elements selected from the group comprising A, B, … and N. In other words, the phrases mean any combination of one or more of the elements A, B, … or N including any one element alone or the one element in combination with one or more of the other elements which may also include, in combination, additional elements not listed. Unless otherwise indicated or the context suggests otherwise, as used herein, "a" or "an" means "at least one" or "one or more."

[0114] While various embodiments have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible. Accordingly, the embodiments described herein are examples, not the only possible embodiments and implementations.

[0115] In addition to the features mentioned in each of the independent aspects enumerated above, some examples may show, alone or in combination, the optional features mentioned in the dependent aspects and / or as disclosed in the description above and shown in the figures.

Claims

CLAIMS 1. An erbium-doped optical fiber comprising: an erbium-doped core radially surrounded by a glass cladding, the erbium-doped core comprising: a glassy matrix comprising silica; and nanoscale regions dispersed within the glassy matrix, the nanoscale regions comprising erbium ions and alkaline earth ions; wherein the erbium-doped optical fiber exhibits a quantum efficiency (QE) above 50%.

2. The erbium-doped optical fiber of claim 1 being prepared using a suspension of erbium-doped alkaline earth fluoride nanoparticles.

3. The erbium-doped optical fiber of claim 1, wherein the alkaline earth ions are selected from the group consisting of: Ba, Ca, Mg, and Sr.

4. The erbium-doped optical fiber of claim 1, wherein the quantum efficiency is above 60%, above 70%, and / or as high as 84%, or as high as 87%.

5. The erbium-doped optical fiber of claim 4, wherein an erbium concentration in the erbium-doped core is at least 2 x 1025m-3and the quantum efficiency exceeds 80%.

6. The erbium-doped optical fiber of claim 1 exhibiting an absorption of at least 30 dB / m at a wavelength of approximately 1530 nm.

7. The erbium-doped optical fiber of claim 1 having a slope efficiency of at least 48% and an optimal length of operation, L0, of about 85 cm or less.

8. The erbium-doped optical fiber of claim 1, wherein a ratio of alkaline earth to erbium in the erbium-doped core is, on average, in a range from 1:1 to 7:

1.

9. The erbium-doped optical fiber of claim 1, wherein the alkaline earth ions are present in the nanoscale regions in an amount sufficient to inhibit or prevent quenching of the erbium ions.

10. The erbium-doped optical fiber of claim 1, wherein at least 90%, at least 95%, or at least 98%; and / or up to 100% of the nanoscale regions are amorphous.

11. The erbium-doped optical fiber of claim 1, wherein the nanoscale regions comprise one or more oxides.

12. The erbium-doped optical fiber of claim 11, wherein the one or more oxides include Er2O3, BaO, erbium-doped barium silicate, calcium oxide, calcium silicate, magnesium oxide, magnesium silicate, strontium oxide, strontium silicate, aluminum oxide, aluminum silicate, lanthanum oxide, lanthanum silicate, gadolinium oxide, gadolinium silicate, lutetium oxide, lutetium silicate, cerium oxide, cerium silicate, yttrium oxide, and / or yttrium silicate.

13. The erbium-doped optical fiber of claim 1, wherein the erbium-doped core comprises on average at least 1 wt.% Er.

14. The erbium-doped optical fiber of claim 13 configured to be double clad or triple clad.

15. The erbium-doped optical fiber of claim 1, wherein the glassy matrix further comprises oxides of aluminum, phosphorus, and / or boron.

16. The erbium-doped optical fiber of claim 15, wherein the glassy matrix includes aluminosilicate(s), phosphosilicate(s), borophosphosilicate(s), alumino-boro- phosphosilicate(s), borosilicate(s), and / or fluorosilicate(s).

17. The erbium-doped optical fiber of claim 1 being devoid of ytterbium.

18. The erbium-doped optical fiber of claim 1, wherein the glass cladding comprises silica.

19. The erbium-doped optical fiber of claim 1 exhibiting L-Band amplification in a range from 1565 nm to 1630 nm, and / or wherein the L-Band amplification range exceeds 1625 nm.

20. A laser or amplifier comprising the erbium-doped optical fiber of claim 1.

21. A method of making an erbium-doped optical fiber, the method comprising: forming a porous layer comprising silica on an inner surface of a tubular substrate; infiltrating a suspension into the porous layer, the suspension including erbium- doped alkaline earth fluoride nanoparticles in a carrier liquid; heating the tubular substrate to a temperature sufficient to sinter the porous layer and collapse the tubular substrate, thereby forming a fiber preform; and drawing the fiber preform into an erbium-doped optical fiber, wherein the erbium-doped optical fiber exhibits a quantum efficiency above 50%.

22. The method of claim 21, further comprising preparing the suspension prior to infiltration.

23. The method of claim 21, further comprising, prior to heating, drying to remove the carrier liquid from the porous layer, the erbium-doped alkaline earth fluoride nanoparticles remaining in the porous layer.

24. The method of claim 21, wherein, during the heating, the erbium-doped alkaline earth fluoride nanoparticles undergo structural and / or chemical changes, resulting in formation of nanoscale regions comprising erbium ions and alkaline earth ions in an erbium-doped core of the erbium-doped optical fiber.

25. The method of claim 21, wherein the temperature sufficient to cause collapse is at least about 2400 °C.

26. The method of claim 21, wherein drawing of the fiber preform occurs at a temperature in a range from 1800°C to 2000°C.

27. The method of claim 21, wherein forming the porous layer comprising silica on the inner surface of the tubular substrate comprises modified chemical vapor deposition (MCVD).

28. The method of claim 21, wherein an amount of erbium in the erbium-doped optical fiber is controlled by (a) changing a concentration of the erbium-doped alkaline earth fluoride nanoparticles in the suspension, and / or (b) changing a concentration of erbium in the erbium-doped alkaline earth fluoride nanoparticles.

29. The method of claim 21, wherein a concentration of the erbium-doped alkaline earth fluoride nanoparticles in the suspension is in a range from 0.02 M to 0.2 M.

30. The method of claim 21, wherein the erbium-doped alkaline earth fluoride nanoparticles in the suspension include erbium at a concentration from 2 mol. % to 16 mol. %.

31. The method of claim 21, wherein an average size of the erbium-doped alkaline earth fluoride nanoparticles is in a range from 10 nm to 50 nm.

32. The method of claim 21, wherein the erbium-doped alkaline earth fluoride nanoparticles further comprise, in addition to erbium and alkaline earth fluoride, an aluminum fluoride and / or a rare earth fluoride.

33. The method of claim 32, wherein the aluminum fluoride and / or the rare earth fluoride is in solid solution with the alkaline earth fluoride.

34. An erbium-doped optical fiber made according to the method of claim 21.

35. A suspension for use in forming an erbium-doped optical fiber, the suspension comprising: erbium-doped alkaline earth fluoride nanoparticles in a carrier liquid, wherein the erbium-doped alkaline earth fluoride nanoparticles include an alkaline earth fluoride and erbium at a concentration from 2 mol. % to 16 mol. %, and wherein a concentration of the erbium-doped alkaline earth fluoride nanoparticles in the carrier liquid is in a range from 0.02 M to 0.2 M.

36. The suspension of claim 35, wherein an average size of the erbium-doped alkaline earth fluoride nanoparticles is in a range from 10 nm to 50 nm.

37. The suspension of claim 35, wherein the carrier liquid comprises water, alcohol, and / or diethylene glycol.

38. The suspension of claim 35, wherein the carrier liquid further comprises an aluminum salt.

39. The suspension of claim 35, wherein the erbium-doped alkaline earth nanoparticles further comprise an aluminum fluoride and / or a rare earth fluoride.

40. The suspension of claim 39, wherein the aluminum fluoride and / or rare earth fluoride is in solid solution with the alkaline earth fluoride.

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