fibers

Article Fabrication and Characteristics of Yb-Doped Silica Fibers Produced by the Sol-Gel Based Granulated Silica Method

Ali El Sayed 1,2,*, Soenke Pilz 1, Hossein Najafi 1, Duncan T. L. Alexander 3,4 , Martin Hochstrasser 5 and Valerio Romano 1,2,*

1 Institute for Applied Laser, Photonics and Surface Technologies (ALPS), Bern University of Applied Sciences, Pestalozzistrasse 20, 3400 Burgdorf, Switzerland; [email protected] (S.P.); hossein.najafi@bfh.ch (H.N.) 2 Institute of Applied Physics (IAP), University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland 3 Electron Spectrometry and Microscopy Laboratory (LSME), Institute of Physics (IPHYS), Ecole Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland; duncan.alexander@epfl.ch 4 Interdisciplinary Centre for Electron Microscopy (CIME), Ecole Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland 5 Reseachem GmbH, Pestalozzistrasse 20, 3400 Burgdorf, Switzerland; [email protected] * Correspondence: [email protected] (A.E.); [email protected] (V.R.); Tel.: +41-34-426-4347 (A.E.); +41-31-631-8940 (V.R.)


Received: 7 September 2018; Accepted: 19 October 2018; Published: 23 October 2018


Abstract: Combining the sol-gel method for fiber material production with the granulated silica method for preform assembly results in a robust method that offers a high degree of freedom regarding both the composition and the geometry of the produced fiber. Using this method, two types of Yb-doped silica glass composition, that feature an excess in P concentration with respect to Al, have been prepared. The elemental distributions in a fiber core were analyzed by scanning transmission electron microscopy (STEM). The elemental mapping shows a similar localization of Al, P and Yb through the microstructure. In addition, the influence of the variation in the co-dopant concentration, with respect to Yb, on the fiber properties has been investigated. The results show an increase in the refractive index step and in the fiber’s transmission loss as the excess concentration of P increases. A significant contribution to the losses can be assigned to the existence of impurities such as iron, which was detected in our samples by mass spectrometer. Single exponential fluorescence decays with lifetimes of around 0.88 ms were measured for the two compositions. Finally, pumping at 976 nm a laser slope efficiency of 67% at 1031 nm was achieved for one of the fiber compositions.

Keywords: fiber lasers; Yb-doped materials; rare-earth (RE) clusters; optical properties; sol-gel; sol-gel based granulated silica method

1. Introduction Optical fibers doped with ytterbium (Yb) ions have attracted increasing interest for the development of high-power laser systems for applications requiring bright and high power beams, such as materials processing, medicine and telecommunications [1–6]. In comparison with other rare-earth (RE) ions, Yb is the favored lasing ion for emission at around 1 µm due to its numerous 2 advantages, such as a simple energy-level diagram with only two electronic multiplets ( F5/2 and 2 F7/2), a long fluorescence lifetime which provides the benefit of energy storage, and broad absorption and emission bands [7–9]. However, Yb and RE ions in general tend to form microstructural clusters when doped in silica glass and therefore co-doping with high field strength ions such as aluminum

Fibers 2018, 6, 82; doi:10.3390/fib6040082 www.mdpi.com/journal/fibers Fibers 2018, 6, 82 2 of 17

(Al) and/or phosphorus (P) is very important in order to improve the atomic distribution of Yb ions and to suppress their cluster formation in the silica matrix [10,11]. The main focus of this work is to investigate the influence of a fixed Al to P concentration ratio and varying content concentration with respect to Yb on the optical and laser properties of the fibers. Due to its simplicity and flexibility in tailoring the Al and P co-dopants’ concentration, the sol-gel based granulated silica method has been used to prepare two Yb-doped fibers with different Al and P co-dopant concentrations As a host for Yb ions, silica glasses are of high importance because of their special properties such as high mechanical strength, high purity and low optical losses [12,13]. However, because of the low solubility of Yb ions in pure silica glass, the RE cations with their high field strength require a high number of non-bridging oxygens to coordinate with, in order to compensate for the positive charge [10]. This is difficult to achieve in a network that has few non-bridging oxygen hole centers, like that of SiO2 [14,15]. As a result, the RE ions can be forced to escape the rigid network in order to form clusters where they share non-bridging oxygen ions RE-O-RE [10,16]. With poor solubility of RE ions in molten silica glass, these RE microstructural clusters form even at very low concentrations [11,17]. Fortunately, co-doping silica glass with a second high field strength ion such as Al and/or P improves the solubility of RE ions in the matrix, which results in the suppression of RE ion formation, thereby improving the fluorescence properties of the glass [12,15,16]. The improved distribution of RE ions by co-doping with Al could be due to the formation of a solvation shell around the RE ions, since RE ions are soluble in alumina but not in silica [18]. The AlO4/2 and/or AlO6/2 coupled with SiO4/2 surround the RE ions to accommodate them in the glass − network without sacrificing excess enthalpy. One model proposes that [AlO4] anion compensates for the positive charge of the RE cations [16]. In this case, three Al per RE would be needed, but experimentally a ratio of 10:1 was found to be necessary to dissolve RE in silica [10]. Also, density functional theory-based energy calculations indicated a higher energy state for dissolved RE when co-doped with Al than for the clustered RE ions [19]. Another model proposes an entropic effect of Al co-doping that compensates for the higher enthalpy of the dissolved RE ions. The entropic effect is caused because Al adopts an octahedral state coordinated with six oxygen atoms, similar to a RE ion in silica. In this state Al could share sites with RE ions, resulting in more sites containing less RE ions than without Al co-doping [20]. Because the P co-dopant changes the fluorescence spectrum in a similar way to Al, an analogous solvation shell model is proposed for the influence of the P co-dopant on the RE dispersion in glass [10]. Co-doping with P creates orthophosphate, pyrophosphate, metaphosphate and phosphoryl groups [21]. These introduce negative charge and non-bridging oxygen into the glass network and thereby increase the solubility of the RE ions. In contrast to the Al effect, the direct interaction between P and RE ions supports a solvation shell model and, therefore, an enthalpic mechanism for the solubility improvement [20]. In this paper, we present two fibers with a fixed Al to P concentration ratio (Al/P = 0.86) but varying absolute content concentration, with respect to an Yb concentration of 0.3 at.%. The production method of the realized fibers, and its advantages and disadvantages in comparison with the standard production methods, are described. The effect of co-doping with Al and P on the atomic distribution of Yb in the silica matrix is examined by atomic resolution analytical scanning transmission electron microscopy (STEM). In addition, since the additional material processing (required by the sol-gel powder approach) has pronounced effects on the transmission properties of the fiber, one fiber composition was treated at two different temperatures in order to study its influence on the fiber transmission properties. Furthermore, variations in the fiber properties caused by the different Al-P concentrations with respect to Yb have been investigated by measuring their optical characteristics, such as refractive index profiles, transmission losses, and fluorescence lifetime. Finally, a laser slope efficiency of 67% was achieved at 1031 nm by pumping one of the fibers at 976 nm with a fiber-pigtailed laser diode (LD). Fibers 2018, 6, 82 3 of 17

2. Fiber Production Method Most RE doped silica glass fiber preforms for industrial applications are fabricated using the standard modified chemical vapor deposition (MCVD) method, in combination with solution doping, because it gives an excellent degree of achievable purity and low optical loss [22,23]. However, the MCVD method has geometrical limitations; it is mainly suited to producing fiber shapes with a cylindrical symmetry and limited in core size, because it is difficult to prepare large fiber cores while maintaining the homogeneity of the dopants and co-dopants [24]. These limitations make it challenging to fabricate RE-doped silica based fibers with large mode area (LMA) using the standard MCVD method. Furthermore, with MCVD it is difficult to maintain proper control over the preform/fiber parameters such as the dopant concentration, the usable dopants, and the dopant distribution [25,26]. In comparison, the sol-gel based granulated silica method has the potential to overcome some of the fabrication constraints of the conventional techniques by combining the benefits of the sol-gel based granulate/powder with the advantages of the powder-in-tube technique [27]. The advantages of the sol-gel method are mild reaction conditions, such as ambient temperature and pressure, that can be easily maintained [28,29]. In addition, since the process starts from a solution, materials of high homogeniety can be produced. However, if the sol-gel method is used only to coat the interior of the preform tubes, although one exploits the compositional freedom offered by the method, it does not afford the structural flexibility needed to make micro-structured fiber concepts, such as photonic crystal fibers (PCF) and leakage channel fibers (LCF). The alternative preform production technology that we have been studying is the granulated silica method. A straightforward approach for this method is the so-called “oxides approach”, which is based on mixing the dopant and co-dopant powders with coarse grains of pure silica oxide powder, which offers unprecedented flexibility with respect to the fiber geometry and microstructure [30]. However, this approach suffers from high scattering losses due to the formation of micro-bubbles and glass inhomogeneities as a result of inhomogenously doped powder grains and incomplete diffusion. Nevertheless, due to the attractive flexibility of the granulated silica method, several processes and additional techniques have been introduced to reduce the scattering losses and to obtain a more homogeneous distribution of the dopants and co-dopants in the glass [31–36]. Among these techniques is the REPUSIL (reactive powder sintering of silica) process, which mixes fine silica powder with dopants and co-dopants in liquid form [24]. This technique leads to homogeneously doped glasses. However, the efficient and practical way for evacuation from the top of the preform while drawing the fiber, as a result of using a coarse grain size, is lost. Another technique is homogenization by means of stack-and-draw [37]. This method yields very low scattering losses; however, the homogenization is achieved after several cycles of drawing-stacking-consolidation which can be very time consuming. Our approach is to combine the sol-gel method with the granulated silica method into the so called sol-gel granulated silica approach [38–40]. This combination offers a high degree of freedom concerning the composition and concentration of dopants and co-dopants, while also giving more control over the fiber geometry and microstructure [30,34,40]. This freedom enables a very high level of control over the refractive index of the core, allowing fabrication of optical fibers with a desired refractive index contrast. This approach is very attractive for matching passive to active fibers, and for production of fibers such LMA fibers and LCFs, where a small refractive index contrast between core (LMA) and cladding or core and microstructure (LCFs) is needed. On the counter side, using the sol-gel approach for fiber material production requires an additional material processing step in order to remove all the used solvents and organic material. During such processes, diffusion may occur, resulting in dopant concentration gradients and crystallization, which result in higher transmission losses in comparison with the standard methods. Nevertheless, since we are targeting applications for active fibers with a fiber length of a few meters, not kilometers, the higher losses of the fiber can be afforded and can be compensated for by the gain of the active fiber. Fibers 2018, 6, 82 4 of 17

2.1.Fibers Method 2018, 6, x DescriptionFOR PEER REVIEW 4 of 18

2.1. MethodThe basic Description principle and development of the sol-gel based granulated silica method for active fiber production has been previously described in detail in [27,38,41]. It starts with mixing silica, dopant, The basic principle and development of the sol-gel based granulated silica method for active fiber and co-dopant material in the liquid phase. This results in the homogenous dissolving of dopant and production has been previously described in detail in [27,38,41]. It starts with mixing silica, dopant, co-dopant precursors, and permits high dopant concentrations (up to several at.%) [30]. The simplicity and co-dopant material in the liquid phase. This results in the homogenous dissolving of dopant and and flexibility of the sol-gel process also enables freedom for adopting varied dopant and co-dopant co-dopant precursors, and permits high dopant concentrations (up to several at.%) [30]. The simplicity materials. Furthermore, it has been reported that silica powder based on the sol-gel method can show and flexibility of the sol-gel process also enables freedom for adopting varied dopant and co-dopant lowermaterials. impurities Furthermore, compared it has tobeen the reported natural that quartz silica powder powder which based on is commonlythe sol-gel method used for can producing show purelowersilica impurities glass compared [42]. Therefore, to the natural the sol-gel quartz method powder is which very is well commonly suited toused making for producing doped materialpure forsilica the glass production [42]. Therefore, of rare-earth the sol-gel doped method fibers. is Thevery sol-gelwell suited procedure to making process doped is shownmaterial in for Figure the 1, andproduction described of asrare-earth follow: doped fibers. The sol-gel procedure process is shown in Figure 1, and described as follow: • Taking place at room temperature, the sol-gel process starts from a liquid condition by mixing an • Takingalkoxide place precursor at room (TEOS) temperature, with other the sol-gel dopant process precursors. starts from a liquid condition by mixing an • alkoxideBy adding precursor distilled (TEOS) water with while other stirring dopant atprecursors. moderate temperature, the solution undergoes • Byhydrolysis adding anddistilled condensation, water while forming stirring a gel at atmoderate the end. temperature, the solution undergoes hydrolysis and condensation, forming a gel at the end. • This gel is then dried, resulting in a powder where every grain is homogenously doped. • This gel is then dried, resulting in a powder where every grain is homogenously doped.

Figure 1. 1. SchematicSchematic of of the the sol-gel sol-gel based based granulated granulated silica silica method method process. process.

Thereafter, several several post-processing post-processing steps steps are are appl appliedied in inorder order to touse use the the powder powder and and the thefinal final granulate together together with with a aslightly slightly adapted adapted powder powder-in-tube-in-tube preform preform assembly assembly for fiber for fiber drawing drawing [27]: [27]: • In a first first processing processing step, step, the the powder powder is isheat heat treated treated into into a powder a powder block, block, in order in order to eliminate to eliminate most OH-groups. OH-groups. Choice Choice of of correc correctt heat heat treatment treatment temperature temperature is of is high of high importance importance in order in order to to prevent the the formation formation of of crystall crystallineine structures structures which which could could lead lead to high to high transmission transmission losses. losses. • • The powder powder block block is is then then milled milled into into granulates granulates an and,d, by by sieving sieving the the granulates, granulates, the thegrain grain size size can can be selected in in order order to to facilitate facilitate the the evacuation evacuation ofof the the preform preform while while drawing. drawing. The The selected selected grain grain size is in the range of 150 µm to 1 mm. size is in the range of 150 µm to 1 mm. After obtaining the doped granulated silica by the sol-gel method, the following steps take place (see FigureAfter obtaining2): the doped granulated silica by the sol-gel method, the following steps take place (see Figure2): • The doped powder is vitrified into a core rod in the drawing tower, before fiber drawing [38,39]. • The preform doped powder is then assembled is vitrified by into placing a core the rod vitrifie in thed drawingcore rod in tower, the center before of fiber a large drawing silica tube [38, 39]. • andThe filling preform the is gap then with assembled pure silica by powder. placing the vitrified core rod in the center of a large silica tube • Finally,and filling the thepreform gap withis drawn pure into silica a fiber powder. at around 2000 °C. • Finally, the preform is drawn into a fiber at around 2000 ◦C. Fibers 2018, 6, 82 5 of 17 Fibers 2018, 6, x FOR PEER REVIEW 5 of 18

Fibers 2018, 6, x FOR PEER REVIEW 5 of 18

FigureFigure 2. Schematic 2. Schematic of powder of powder vitrification vitrification and preform and preform assembly. assembly.

2.2.2.2. Fabrication Fabrication of Yb-Doped, of Yb-Doped, Al and Al and P Co-Doped P Co-Doped Silica Silica Fibers Fibers by the by Sol-Gel the Sol-Gel Granulated Granulated Silica SilicaMethod Method Figure 2. Schematic of powder vitrification and preform assembly. ForFor the the sol-gel sol-gel process, process, we we used used tetraethyl tetraethyl orthosilicate orthosilicate (TEOS: (TEOS: SiOC Si(HOC2)H as5) 4silica) as silicaprecursor, precursor, ytterbium(III)ytterbium (III) nitrate nitrate pentahydrate pentahydrate (YbNO (Yb(·5NOH O)) as·5H YbO dopant) as Yb precursor, dopant precursor,and for optically and forpassive optically 2.2. Fabrication of Yb-Doped, Al and P Co-Doped 3Silica3 Fi2bers by the Sol-Gel Granulated Silica Method co-dopants,passive co-dopants, the precursors the precursors aluminum aluminum nitrate nitratenonahydrate nonahydrate (Al NO (Al (NO·9 H3)O3·)9 Hand2O )phosphorus and phosphorus For the sol-gel process, we used tetraethyl orthosilicate (TEOS: SiOCH) as silica precursor, pentoxidepentoxide (P (PO2O). 5). ytterbium(III) nitrate pentahydrate (YbNO ·5H O) as Yb dopant precursor, and for optically passive InIn previous previous work work [43], [43 we], wehave have studied studied the effects the effects of different of different Al to P Alconcentration to P concentration ratios on ratios the on co-dopants, the precursors aluminum nitrate nonahydrate (Al NO ·9 H O ) and phosphorus performancethe performance of Yb-doped of Yb-doped silica fibe silicars prepared fibers preparedby the sol-gel by thebased sol-gel granulated based silica granulated method. silica Favored method. P O performancepentoxide was ( achieved). for the composition with a slightly higher P concentration than Al (Al/P = Favored performanceIn previous work was [43], achieved we have for studied the composition the effects of with different a slightly Al to higherP concentration P concentration ratios on thanthe Al 0.82). In this work, two core compositions have been prepared with Al to P concentration ratio fixed (Al/Pperformance = 0.82). In of this Yb-doped work, two silica core fibe compositionsrs prepared by havethe sol-gel been based prepared granulated with Al silica to P method. concentration Favored ratio at this optimum ratio (Al/P = 0. 82), but with two different absolute concentrations with respect to 0.3 fixedperformance at this optimum was achieved ratio (Al/P for the = 0.composition 82), but with with two a slightly different higher absolute P concentration concentrations than Al with (Al/P respect = at.% of Yb concentration. The two compositions were treated at a maximum applied temperature of to 0.30.82). at.% In of this Yb concentration.work, two core compositions The two compositions have been wereprepared treated with at Al a to maximum P concentration applied ratio temperature fixed 1550 °C before drawing, and then drawn into fibers named A1550 and B1550. The powder derived of 1550at this◦C optimum before drawing, ratio (Al/P and = 0. then 82), drawnbut with into two fibers different named absolute A1550 concentrations and B1550. with The respect powder to derived 0.3 from the sol-gel of one of the compositions was also treated at a maximum temperature of 1200 °C to fromat.% the of sol-gel Yb concentration. of one of the The compositions two compositions was alsowere treatedtreated at at a a maximum maximum applied temperature temperature of 1200 of ◦C yield the fiber named B1200 after drawing. The core precursor compositions (given in at.% while to yield1550 the °C fiberbefore named drawing, B1200 and afterthen drawn drawing. into The fibers core named precursor A1550 compositionsand B1550. The (given powder in derived at.% while neglectingfrom thethe oxygensol-gel of content) one of theand compositions heat treatment was te alsomperatures treated at for a maximum the fibers temperatureare listed in ofTable 1200 1. °C to neglecting the oxygen content) and heat treatment temperatures for the fibers are listed in Table1. Whileyield ourthe fiberproduction named methodB1200 after gives drawing. no limitation The core on precursorthe core size,compositions the fibers (given were indrawn at.% withwhile core/claddingneglecting diameters the oxygen of content)10/125 µmand correspondingheat treatment tetomperatures LMA fibers, for theas afibers benchmark are listed geometry in Table 1. for Table 1. Core precursor composition and maximum heat treatment temperature. testing theirWhile different our productioncompositions/heat method treatmentgives no limitation effects. The on thecore core and size, cladding the fibers diameters were drawnvary from with one piececore/cladding to another diameters by 1 and 2Fiberof µm 10/125 respectively. µm corresponding The fibers to were LMA A1550 then fibers, coated as awith B1550benchmark a low index geometry B1200 coating for in ordertesting to obtain their differentdouble-clad compositions/heat Yb-doped fibers. treatment Figure e ffects.3 shows The an core image and claddingof the cross diameters section vary of fiber from Yb/at.% 0.3 0.3 0.3 one piece to another by 1 and 2 µm respectively. The fibers were then coated with a low index coating B1200. Al/at.% 9 4.5 4.5 in order to obtain double-cladP/at.% Yb-doped fibers. Figure 3 shows11 an image of 5.5the cross section 5.5 of fiber B1200. Table 1. Core precursorSi/at.% composition and maximum heat79.7 treatment temperature. 89.7 89.7 Maximum applied temperature before drawing/◦C 1550 1550 1200 Table 1. Core precursorFiber composition and maximum heat treatmentA1550 temperature.B1550 B1200 Yb/at.% 0.3 0.3 0.3 Fiber A1550 B1550 B1200 While our production methodAl/at.% gives no limitation on the core9 size, the4.5 fibers were4.5 drawn with Yb/at.% 0.3 0.3 0.3 core/cladding diameters of 10/125P/at.% µm corresponding to LMA fibers,11 as a benchmark5.5 5.5 geometry for Al/at.% 9 4.5 4.5 testing their different compositions/heatSi/at.% treatment effects. The core79.7 and cladding89.7 diameters89.7 vary from P/at.% 11 5.5 5.5 Maximum applied temperatureµ before drawing/°C 1550 1550 1200 one piece to another by 1 and 2 mSi respectively./at.% The fibers were then coated79.7 with89.7 a low index89.7 coating in order to obtainMaximum double-clad applied Yb-doped temperature fibers. before Figure drawing3 shows/°C an image1550 of the cross1550 section 1200 of fiber B1200.

Figure 3. Image of the cross section of fiber B1200 illuminated from the bottom side. FigureFigure 3. Image 3. Image of of the the cross cross section section of of fiberfiber B1200 illuminated illuminated from from the the bottom bottom side. side. Fibers 2018, 6, 82 6 of 17

Fibers 2018, 6, x FOR PEER REVIEW 6 of 18 3. Measurements and Results 3. Measurements and Results 3.1. Structural Imaging and Analysis 3.1.The Structural atomic Imaging distribution and Analysis of Yb ions, and general elemental distributions in the vitrified core rod of A1550The sample, atomic were distribution observed of with Yb ions, aberration-corrected and general elemental STEM distributions in combination in the with vitrified energy-dispersive core rod of X-rayA1550 spectroscopy sample, were (EDXS). observed The with analysis aberration-corre was performedcted STEM on Titan in combination Themis 60-300 with (FEI/Thermo energy-dispersive Fisher Scientific,X-ray spectroscopy Eindhoven, (EDXS). The Netherlands) The analysis operated was perfor atmed 200 kVon highTitan tensionThemis and60-300 using (FEI/Thermo probe currents Fisher of 85Scientific, pA for the Eindhoven, atomic imaging The Netherlands) and 250 pA operated for the imaging at 200 kV with high chemical tension analysis.and using The probe high currents sensitivity of Super-X85 pA for EDXS the detectorsatomic imaging (FEI/Thermo and 250 FisherpA for Scientific, the imaging Eindhoven, with chemical The Netherlands) analysis. The ofhigh the sensitivity instrument allowedSuper-X mapping EDXS detectors of the distribution (FEI/Thermo of Fisher different Scientific, elements Eindhoven, in the fiber The Netherlands) core sample. of the instrument allowedTo prepare mapping the of samples the distribution for STEM of imaging,different elements the core ofin thethe fibersfiber core was sample. first mechanically crushed into powderTo prepare using the a diamondsamples for tip. STEM The powderimaging, was the core then of dropped the fibers onto was a first Quantifoil mechanically carbon crushed grid, i.e., perforatedinto powder amorphous using a diamond carbon films tip. The supported powder on was Cu th grids.en dropped No Argon onto sputteringa Quantifoil was carbon applied grid, during i.e., theperforated sample preparation. amorphous carbon films supported on Cu grids. No Argon sputtering was applied during theFigure sample4 preparation.shows example high-angle annular dark-field (HAADF) STEM images of the vitrified core ofFigure sample 4 A1550.shows example Because high-ang of the highle annular atomic dark-field number of (HAADF) the Yb ions STEM relative images to thatof the of vitrified the silica core of sample A1550. Because of the high atomic number of the Yb ions relative to that of the silica matrix, there is strong scattering of electrons from their nuclei to the HAADF detector. Combining matrix, there is strong scattering of electrons from their nuclei to the HAADF detector. Combining this this scattering with the STEM imaging using an aberration-corrected sub-Å diameter electron probe, scattering with the STEM imaging using an aberration-corrected sub-Å diameter electron probe, the the images show the atomic distribution of Yb ions in the silica matrix. Individual ions are visible as images show the atomic distribution of Yb ions in the silica matrix. Individual ions are visible as bright bright dots on the darker background of the silica matrix, similar to the analysis in reference [44]. In the dots on the darker background of the silica matrix, similar to the analysis in reference [44]. In the images,images, the the Yb Yb ions ions are are clearly clearly grouped grouped intointo clustersclusters of ~3–6 nm nm in in diameter. diameter. Some Some of of the the clusters clusters are are outlinedoutlined by by red red ellipses ellipses in in orderorder toto helphelp the reader distinguish distinguish them. them.

FigureFigure 4. 4.Aberration-corrected Aberration-corrected high-anglehigh-angle annular dark dark-field-field (HAADF) (HAADF) scanning scanning transmission transmission electron electron microscopymicroscopy (STEM) (STEM) images images ofof thethe fiberfiber corecore of sample A1550, taken taken from from different different regions regions at at slightly slightly differentdifferent magnifications. magnifications. TheThe whitewhite dotsdots inin thethe imagesimages correspond to to individual individual Yb Yb cations cations contained contained inin the the silica silica matrix; matrix; the the latter latter is is seen seen asas aa moremore homogeneous,homogeneous, darker background. background. (Note (Note that that the the dots dots areare sometimes sometimes distorted distorted because because of of smallsmall dynamicdynamic motionsmotions of the the cations cations under under the the electron electron beam beam used used forfor imaging.) imaging.) The The Yb Yb cations cations are are seenseen toto formform clustersclusters ~3–6 nm nm in in diameter, diameter, rath ratherer than than being being uniformly uniformly distributed.distributed. Some Some ofof thesethese clustersclusters areare approximatelyapproximately outlined outlined by by red red ellipses, ellipses, to to help help indicate indicate their their presencepresence to to the the reader. reader.

FigureFigure5 shows5 shows the the chemical chemical analysis analysis ofof aa regionregion ofof samplesample which is similar to to those those shown shown in in FigureFigure4, but4, but chosen chosen to to be be slightly slightly thicker thicker in in order order to to increase increase the the X-ray X-ray signal. signal. In In the the HAADF HAADF image image of Figureof Figure5a, clouds 5a, clouds of white of white dots dots indicate indicate the the Yb Yb ion ion clusters. clusters. Confirming Confirming thatthat this image image interpretation interpretation is correct, the EDX spectrum in Figure 5b integrated from signal acquired over a region containing a is correct, the EDX spectrum in Figure5b integrated from signal acquired over a region containing a cluster, as indicated with the red box on Figure 5a, shows a clear Yb signal in the form of a small Yb- cluster, as indicated with the red box on Figure5a, shows a clear Yb signal in the form of a small Yb-L α Lα peak. In contrast Figure 5c, a spectrum taken from a region away from the Yb clusters, as indicated Fibers 2018, 6, 82 7 of 17

peak. InFibers contrast 2018, 6, x FOR Figure PEER5 REVIEWc, a spectrum taken from a region away from the Yb clusters, as7 of indicated18 on Figure5a with a blue box, shows no Yb-L α peak signal. Panels Figure5d–i show elemental EDXS maps ofon the Figure whole 5a with of the a blue region box, shownshows no in Yb-L theα HAADF peak signal. image Panels Figure Figure5a. 5d–i show elemental EDXS maps of the whole of the region shown in the HAADF image Figure 5a.

FigureFigure 5. Chemical 5. Chemical analysis analysis of of a region region of ofsample sample A1550. A1550. (a) aberration-corrected (a) aberration-corrected HAADF STEM HAADF image STEM of the region mapped by energy-dispersive X-ray spectroscopy (EDXS). The region contains clouds of image of the region mapped by energy-dispersive X-ray spectroscopy (EDXS). The region contains white dots corresponding to clusters of Yb cations, similar to the clusters shown in Figure 4. (b) cloudsintegrated of white dotsEDX spectrum corresponding from a region to clusters containing of Yb a Yb cations, cluster; similar the region to theis indicated clusters by shown the red inbox Figure 4. (b) integratedon image EDX(a). (c) spectrum EDX spectrum from from a region a region containing without Yb aions Yb as cluster; indicated the on region image ( isa) by indicated the blue by the red boxbox. on On image both (spectraa). (c) the EDX main spectrum peaks are from indicate a regiond and labeled without with Yb their ions corresponding as indicated elemental on image X- (a) by the blueray box. emissions. On both (Note spectra that the theCu-K mainα peak peaks is fluoresced are indicated from the Cu and transmission labeled with electron their microscope corresponding elemental(TEM) X-ray grid.) emissions. Because of (Notethe very that low the signal Cu-K fromα peakthe small is fluoresced number of Yb from atom thes, in Cu order transmission to show the electron weak Yb-Lα peak, the intensity of the right-hand part of the spectrum is multiplied by a factor of 20 microscope (TEM) grid.) Because of the very low signal from the small number of Yb atoms, in order compared to the left-hand part of the spectrum. (d–i) elemental maps for Yb, Si, O, Al, P and composite to showAl-P, the across weak the Yb-L wholeα peak,of the region the intensity shown in ofimage the ( right-handa). Each map partis calculated of the by spectrum integrating is the multiplied area by a factorunder of 20 the compared corresponding to the elemental left-hand peaks part indicated of the spectrum.on the EDX (spectrad–i) elemental (b,c); a 2D maps Gaussian for blur Yb, Si,is O, Al, P and compositeapplied to each Al-P, map across to reduce the noise. whole When of the interpreted region showntogether inwith image the HAADF (a). Each image map (a), is the calculated maps by integratingshow thethat areathe Al under and P theare co-l correspondingocalized with the elemental Yb clusters. peaks indicated on the EDX spectra (b,c); a 2D Gaussian blur is applied to each map to reduce noise. When interpreted together with the HAADF image (a), the maps show that the Al and P are co-localized with the Yb clusters. Fibers 2018, 6, 82 8 of 17

Map Figure5d shows the chemical distribution of Yb, which, although noisy because of the very small Yb signal, correlates well with the Yb clusters seen as clouds of white dots in the HAADF image Figure5a. In contrast, maps for Si and O, Figure5e,f respectively, confirm that these two main matrix elementsFibers are 2018 much, 6, x FOR more PEER homogeneouslyREVIEW distributed, according with the background intensity8 of 18 from the matrix in the HAADF image Figure5a. In comparison, maps Figure5g,h for Al and P, and the composite Al-PMap mapFigure in 5d Figure shows5 thei, show chemical that distribution these two of dopants Yb, which, are although both localized noisy because to each. of the Furthermore, very comparisonsmall Yb of signal, their distributioncorrelates well towith the the Yb Yb clusters clusters se seenen as in clouds the HAADFof white dots image in the shows HAADF that image the three Figure 5a. In contrast, maps for Si and O, Figure 5e,f respectively, confirm that these two main matrix elements are co-localized together; that is, the Al and P are associated with the Yb clusters. As a final elements are much more homogeneously distributed, according with the background intensity from confirmationthe matrix of this in the co-localization, HAADF image the Figure EDX 5a. spectrum In comparison, Figure maps5b from Figure the 5g,h cluster for Al region and P, shows and the a much higher signalcomposite from Al-P P map and in Al Figure relative 5i, show to the that matrix these two Si signaldopants than are both that localized for the to spectrum each. Furthermore, Figure5c from the Yb-freecomparison region of of their the matrix.distribution to the Yb clusters seen in the HAADF image shows that the three elements are co-localized together; that is, the Al and P are associated with the Yb clusters. As a final 3.2. Refractiveconfirmation Index of Profiles this co-localization, the EDX spectrum Figure 5b from the cluster region shows a much higher signal from P and Al relative to the matrix Si signal than that for the spectrum Figure 5c from Thethe refractive Yb-free region index of the profile matrix. is one of the essential guiding parameters to determine various properties of optical fiber, such as the numerical aperture (NA) and the single mode cutoff. Bubnov [45], studied3.2. the Refractive effect Index of Al Profiles and P concentrations on the refractive index in silica glass showing that, individually,The refractive each index co-dopant profile wouldis one of increase the esse thential refractive guiding parameters index, but, to determine in combination, various the P compensatesproperties for of the optical refractive fiber, such index as changethe numerical arising aperture from the(NA) Al and [46 the]. single mode cutoff. Bubnov To[45], confirm studied the the effect effect of Al the and Al P andconcentrations P concentration on the refractive on the index refractive in silica index glass showing steps, an that, accurate individually, each co-dopant would increase the refractive index, but, in combination, the P measurement of the refractive index profile is required. Therefore, the refractive index profiles compensates for the refractive index change arising from the Al [46]. were measuredTo confirm by two the differenteffect of the methods. Al and P Theconcentrat firstion one on is the aself-built refractive inverseindex steps, refracted an accurate near-field technique,measurement which is of based the refractive on the refractedindex profile near-field is required. technique Therefore, but the modifiedrefractive index in such profiles a way were to allow a two-dimensionalmeasured by two (2-D) different measurement methods. The of first the one refractive is a self-built index inverse profile refracted [47–49 near-field]. The second technique, setup is a self-builtwhich reflection-based is based on the refractive refracted indexnear-field mapping technique setup but withmodified high in measurementsuch a way to accuracyallow a two- [50]. dimensional (2-D) measurement of the refractive index profile [47–49]. The second setup is a self-built Figure6 shows the 2-D refractive index profile of fiber A1550 and Figure7 shows the refractive reflection-based refractive index mapping setup with high measurement accuracy [50]. index profileFigure of a line6 shows scan the over 2-D therefractive fiber’s index cross profile section. of fiber The A1550 refractive and Figure index 7 stepsshows were the refractive obtained from the 2-Dindex refractive profile index of a line profiles scan over by the subtracting fiber’s cross the se averagection. The refractive refractive index index steps of a were selected obtained area from within the core fromthe the2-D averagerefractive refractive index profiles index by ofsubtracting a selected the area average within refractive the cladding. index of a The selected refractive area within index steps were measuredthe core from at 630 the nmaverage and, refractive therefore, index the of NAs a sele werected estimatedarea within atthe 630 cladding. nm. The The measuredrefractive index values and their uncertaintiessteps were measured with 95% at 630 confidence nm and, therefore, bounds arethe listedNAs were in Table estimated2. The at measured630 nm. The refractive measured index values and their uncertainties with 95% confidence bounds are listed in Table 2. The measured steps correspond well with the Al and P co-dopant concentrations. The smaller the difference between refractive index steps correspond well with the Al and P co-dopant concentrations. The smaller the Al anddifference P concentrations, between Al the and smaller P concentrations, the refractive the smalle indexr the contrast refractive to index the pure contrast silica to the cladding, pure silica and thus the smallercladding, the NA.and thus the smaller the NA.

FigureFigure 6. Two 6. Two dimensional dimensional refractive refractive index profile profile of A of1550 A1550 fiber obtained fiber obtained from the from2-D refractive the 2-D index refractive index profilingprofiling setup. The The core core and and cladding cladding region region are arecircled circled in white in white for clarity. for clarity. The black The blacktriangles triangles represent dust particles. represent dust particles. Fibers 2018, 6, x FOR PEER REVIEW 9 of 18

Table 2. Refractive index step between doped core and pure silica cladding.

Fiber A1550 B1550 B1200 Index step Δn (measured at 630 nm) 0.00262 0.00207 0.00205 Uncertainty of Δn ±1.5 × 10–4 ±0.6 × 10–4 ±1.5 × 10–4 NA (estimated at 630 nm) 0.089 0.077 0.076 Fibers 2018, 6, 82 9 of 17 Uncertainty of NA ±2.6 × 10–4 1.2 10 ±2.6 × 10–4

FigureFigure 7. 7. One-dimensional refractive refractive index index profile profile of A1550 of A1550 fiber fiber (cladding (cladding was set was to setbe zero). to be The zero). Thescanning scanning line line is indicated is indicated in Figure in Figure 6, in6 red., in red.

3.3. TransmissionTable Losses 2. Refractive index step between doped core and pure silica cladding. The transmissionFiber losses of the fiber cores are A1550determined by a B1550cutback method. B1200Light from a helium-neonIndex steplaser∆ atn (measured630 nm is atmodulated 630 nm) using a0.00262 chopper and launched 0.00207 into one end 0.00205 of the fiber being tested. TheUncertainty light emitted of ∆fromn the other end of±1.5 the× fiber10–4 is collected±0.6 by× 10a large-area–4 ± 1.5photo× 10 detector–4 and the resultingNA (estimated signal is amplified at 630 nm) by a lock-in amplifier.0.089 The fiber is then 0.077 cut back by a known 0.076 length –4 −3 –4 and the cleavedUncertainty end reinserted of NA into the detector ±as2.6sembly.× 10 The loss±1.2 measurement× 10 of± 2.6each× fiber10 was performed over a fiber length of 3.5–4 m. In order to obtain accurate loss measurements, five 3.3.measurement Transmission points Losses (cutbacks) over the fiber length were obtained for each fiber. The transmission loss of each measurement point was then calculated using the following equation: The transmission losses of the fiber cores are determined by a cutback method. Light from a helium-neon laser at 630 nm is modulated using a10 chopper and launched into one end of the fiber Transmission loss dB⁄ m log⁄ (1) being tested. The light emitted from the other end∆ of the fiber is collected by a large-area photo detectorwhere andthe and resulting are signal the detected is amplified transmitted by a lock-inpower throug amplifier.h the Thefiber fiberbefore is and then after cut the back fiber by a knownis cut respectively, length and theand cleaved ∆ is the end length reinserted of the cut into piece. thedetector The average assembly. transmission The loss loss measurement for each fiber of eachwas fiber then wasobtained performed by linearly over fitting a fiber the length five ofmeasurement 3.5–4 m. In orderpoints to and obtain calculating accurate its loss slope. measurements, five measurementSince the fibers points being (cutbacks) tested are over double the fibercladding length fibers, were stripping obtained the for cladding each fiber. modes The transmissionis essential lossfor ofobtaining each measurement accurate measurements point was then of the calculated transmissi usingon losses the followingof the fiber equation: core. In order to do that, a specific length of the coating was removed and the uncoated part was placed in index-matching oil. Since the fibers had big scattering centers which could lead10 to additional cladding modes as the light Transmission loss (dB/m) = log P /P (1) propagates in the fiber, the coating was removed towards∆ Lthe fiber10 endshort underlong measurement after every cutback. This allows most of the cladding light (due to in-coupling and scattering) to be refracted away where P and P are the detected transmitted power through the fiber before and after the fiber is from theshort fiber. Additionallong spatial filtering is applied by using a pinhole that is placed between the fiber cut respectively, and ∆L is the length of the cut piece. The average transmission loss for each fiber was end and the detector, in order to make sure that only the guided light from the core reaches the thendetector. obtained The bymeasured linearly transmission fitting the five losses measurement of the fibers points are summarized and calculating in Table its 3. slope. Since the fibers being tested are double cladding fibers, stripping the cladding modes is essential for obtaining accurate measurements of the transmission losses of the fiber core. In order to do that, a specific length of the coating was removed and the uncoated part was placed in index-matching oil. Since the fibers had big scattering centers which could lead to additional cladding modes as the light propagates in the fiber, the coating was removed towards the fiber end under measurement after every cutback. This allows most of the cladding light (due to in-coupling and scattering) to be refracted away from the fiber. Additional spatial filtering is applied by using a pinhole that is placed between the fiber end and the detector, in order to make sure that only the guided light from the core reaches the detector. The measured transmission losses of the fibers are summarized in Table3.

Table 3. Transmission losses of fibers derived from the sol-gel granulated silica method.

Fiber A1550 B1550 B1200 Losses @630nm/(dB/m) 1.86 ± 0.16 0.73 ± 0.05 0.69 ± 0.03 Fibers 2018, 6, 82 10 of 17

In our fibers, the main sources of optical loss are optical scattering and impurity absorption. Optical scattering could be a result of doping inhomogeneity and/or scattering centers, such as bubbles or unvitrified powder grains. By lowering the heat treatment temperature to 1200 ◦C during the post-processing of the powder (derived from the sol-gel), the number of scattering centers can be reduced. Although that is not well reflected by the measured transmission losses of the two fibers (B1550 and B1200) with the two different heat treatment temperatures (1550 ◦C and 1200 ◦C), the scattering centers were nevertheless reduced in the latter fiber. This was already evident with the naked eye, with the B1550 fiber visibly showing more scattering centers. Impurity absorption mainly originates from iron (Fe), which can dramatically increase the optical losses of the Yb-doped silica fibers owing to its strong and broad absorption band centered at around 1200 nm. To check for Fe impurities in our fibers, Fe content was measured using a laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS). The measured Fe concentrations are summarized in Table4. The significant Fe concentrations detected in our fibers, and especially in A1550, could be the reason (in addition to the optical scattering) for the high transmission losses. In comparison with the loss level of the fibers produced by the state-of-the-art methods, the measured transmission losses of our fibers are higher than the loss level that can be achieved in for example, MCVD, sol-gel, and powder sinter technology, where losses in the range of 0.02–0.05 dB/m can be achieved at 1200 nm [51,52]. However, due to significant Fe concentration in our fibers which could lead to high absorption at 1200 nm, transmission loss measurements at 1200 nm are in progress in order to have a direct comparison with the other state-of-the-art methods.

Table 4. Fe concentrations in the fibers derived from the sol-gel based granulated silica method.

Fiber A1550 B1550 B1200 Fe concentration/ppm 18 ± 1.9 9 ± 1.28 11 ± 1.34

3.4. Fluorescence Lifetime One of the most important features of Yb is its associated long upper-level excited state lifetime that allows the storage of large energies in the excited state. The excited state lifetime of Yb ions can be as long as 2 ms [53]. However, the lifetime is strongly dependent on the host material; depending on this, several mechanisms can lead to the broadening of the spectral line-width of transitions which shortens the fluorescence lifetime of the Yb ions. The shortening of the excited state lifetime is a result of homogeneous broadening of the spectral line-width due to interactions between the Yb ions with the host lattice via phonons, and of inhomogeneous broadening due to site-to-site variation of the ligand field around the Yb ions [54]. In silica glass, the excited state lifetime is in the order of 1 ms. Nevertheless, additional unwanted mechanisms, such as energy transfer between Yb ions, can result in quenching of the overall intensity of the fluorescence [55]. These quenching processes can be apparent as a further reduction in the fluorescence lifetime of the excited Yb ions in silica [56]. Therefore, the measurement of the excited state lifetime is of high importance, as it indicates whether there is significant quenching of the excited Yb ions, which could consequently influence the laser performance of an Yb-doped fiber. To measure the fluorescence lifetimes at the maximum Yb emission at 976 nm, short fiber pieces were excited with a pulsed diode laser (915 nm, 500 mW) with rectangular pulses of 100 ms and a trailing edge slope of less than 5 µs. The pump light was filtered out by using several long pass filters centered at 950 nm and the generated and transmitted fluorescence emission at 976 nm was detected using a photomultiplier with a large sensitive area. Fluorescence decay curves were measured and evaluated. The decay curves were described well by a single exponential fit as shown in Figure8, and fluorescence lifetimes of around 0.88 ms were obtained for both compositions. The measured lifetimes of our fibers were comparable with the fluorescence lifetime of a commercial fiber (Nufern: LMA-YDF-25/250-VIII) (Nufern, East Granby, CT, USA), for which a lifetime of 0.91 ms was measured. Fibers 2018, 6, x FOR PEER REVIEW 11 of 18

of our fibers were comparable with the fluorescence lifetime of a commercial fiber (Nufern: LMA-YDF- 25/250-VIII) (Nufern, East Granby, CT, USA), for which a lifetime of 0.91 ms was measured. This indicates that there is no significant quenching in the overall fluorescence intensity of Yb due to clustering. The lifetime results are summarized in Table 5. Fibers 2018, 6, 82 11 of 17 Table 5. Fluorescence lifetime of fibers derived from the sol-gel granulated silica method.

Fiber A1550 B1550 B1200 This indicates that there is no significant quenching in the overall fluorescence intensity of Yb due to Lifetime/ms 0.86 0.87 0.86 clustering. The lifetime results are summarized in Table5.

100

B1200, 0.86 ms A1550, 0.87 ms B1550, 0.86 ms

10-1 Normalized intensity Normalized

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 -3 Time /s 10 FigureFigure 8. Fluorescence 8. Fluorescence lifetime lifetime decays decays of of fibers fibers derived derived from from the the sol-gel sol-gel granulated granulated silicasilica methodmethod (log scale).scale). The The fluorescence fluorescence decays decays of of the the fibers fibers are are separated separated by by 1 1 ms ms forfor clarity.clarity.

3.5. LaserTable Properties 5. Fluorescence lifetime of fibers derived from the sol-gel granulated silica method. Due to its better optical Fiberproperties, namely A1550 the B1550lowest transmission B1200 loss, the B1200 fiber was characterized with a continuous-waveLifetime/ms (CW)0.86 free-space 0.87 laser setup. 0.86 The fiber features a core NA of 0.073 and was coated with low-index coating, resulting in a double cladding fiber with an inner- cladding NA of 0.48. For pumping the Yb-doped fiber, we used cladding-pumping, which relaxes the 3.5. Laser Properties requirements on pump beam quality and launch alignment. Another reason for the use of cladding- pumpingDue to itsis to better allow optical a much properties, higher incident namely pump the power lowest than transmission that which loss, could the be B1200tolerated fiber by wasthe characterizedcore alone, withfor damage a continuous-wave reasons. (CW) free-space laser setup. The fiber features a core NA of 0.073 and wasFor coated the cladding-pumping with low-index coating, configuration, resulting the in aabsorption double cladding per unit fiber length with of anpump inner-cladding light travelling NA of 0.48.in the For inner pumping cladding, the without Yb-doped furt fiber,her measures, we used cladding-pumping,can be estimated by a which factor relaxes approaching the requirements the ratio of on pumpthe areas beam of the quality inner andcladding launch and alignment. the core, compared Another to reason direct for core-pumping. the use of cladding-pumping However, this reduced is to allowabsorption a much higherper unit incident length can pump be simply power compensated than that which by using could a becombination tolerated byof longer the core fiber alone, length for damageand a reasons.pump wavelength where there is strong pump absorption. In Yb-doped silica fiber, the highest peakFor absorption the cladding-pumping occurs at 976 configuration,nm. the absorption per unit length of pump light travelling The experimental setup is shown in Figure 9. We used a forward pumping scheme with a fiber- in the inner cladding, without further measures, can be estimated by a factor approaching the ratio of pigtailed LD with a silica fiber (NA 0.22) operating at 976 nm with output power up to 15 W. The the areas of the inner cladding and the core, compared to direct core-pumping. However, this reduced yellow and red arrows in the experimental setup represent the pump beam and the signal beam, absorption per unit length can be simply compensated by using a combination of longer fiber length respectively. The pump beam was coupled into the double-clad Yb-doped fiber using collimating and and a pump wavelength where there is strong pump absorption. In Yb-doped silica fiber, the highest a focusing lenses, through a dichroic mirror (M) with high transmission (HT) at the pump wavelength peakat absorption976 nm andoccurs high reflection at 976 nm. (HR) at the signal wavelength at around 1030 nm. On the output arm of theThe laser experimental cavity, a feedback setup was is shown provided in Figureby the Fresnel9. We reflection used a forward from the pumping cleaved fiber scheme end. with a fiber-pigtailedThe output LD withsignal a from silica the fiber Yb-doped (NA 0.22) fiber operating was separated at 976 nmfrom with the pump output beam power using up toanother 15 W.

Thedichroic yellow andmirror red (M arrows). Additional in the experimental long pass filters setup centered represent at 1000 the nm pump were beam used andto make the signalsure that beam, no respectively.residual pump The pump wavelength beam wasis in coupledthe signal into beam. the Fi double-cladnally, the spectrum Yb-doped of fiberthe laser using output collimating signal was and a focusingmeasured lenses, by an through Ocean aOptics dichroic USB4000 mirror spectromet (M1) with higher (Ocean transmission Optics, Dunedin, (HT) at the FL, pump USA). wavelength The laser at 976output nm andspectrum high reflectionof the fiber (HR) is shown at the in signal Figure wavelength 10. at around 1030 nm. On the output arm of the laser cavity, a feedback was provided by the Fresnel reflection from the cleaved fiber end. The output signal from the Yb-doped fiber was separated from the pump beam using another dichroic mirror (M2). Additional long pass filters centered at 1000 nm were used to make sure that no residual pump wavelength is in the signal beam. Finally, the spectrum of the laser output signal was measured by an Ocean Optics USB4000 spectrometer (Ocean Optics, Dunedin, FL, USA). The laser output spectrum of the fiber is shown in Figure 10. Fibers 2018, 6, 82 12 of 17 Fibers 2018, 6, x FOR PEER REVIEW 12 of 18

Fibers 2018, 6, x FOR PEER REVIEW 12 of 18

FigureFigure 9. Yb-doped 9. Yb-doped fiber fiber (YDF) (YDF) free-space free-space laser laser arrangement, arrangement, with fiber-pigtailed fiber-pigtailed pump pump diode diode source, source, Figure 9. Yb-doped fiber (YDF) free-space laser arrangement, with fiber-pigtailed pump diode source, collimatingcollimating and couplingand coupling lenses lenses (L, (L, L1 ,L and1, and L2 L),2), in-coupling in-coupling dichroicdichroic mirror mirror (M (M),1 ),dichroic dichroic mirror mirror (M (),M 2), collimating and coupling lenses (L, L1, and L2), in-coupling dichroic mirror (M), dichroic mirror (M), and aand set ofaand set long aof set long pass of long pass filters pass filters filters centered centered centered at at 1000 at 1000 1000 nm. nm.nm.

Figure 10. FigureLaser 10. output Laser output spectrum spectrum of of B1200 B1200 fiberfiber derived derived from from the sol-gel the sol-gelgranulated granulated silica method, silica with method, an emission peak at around 1031 nm (log scale). with anFigure emission 10. Laser peak output at around spectrum 1031 of B1200 nm (log fiber scale). derived from the sol-gel granulated silica method, with an emissionBy cladding-pumping peak at around at1031 976 nm nm (login 2.13 scale). m of fiber length of fiber B1200, an absorbed pump power By cladding-pumpingof 55% (including losses) at was 976 obtained. nm in 2.13This gives m of an fiber absorption length coefficient of fiber of 1.63 B1200, dB/m. an Considering absorbed pump power ofBy 55%the cladding-pumping Yb (including concentration losses) in the at fiber976 was nm and obtained. in the 2.13 overlap m of fa This fiberctor between giveslength anof the fiberabsorption pumped B1200, and an doped coefficient absorbed area (cladding pump of 1.63 power dB/m. Consideringof 55%pumped), (including the Yb the concentration losses)absorption was crossobtained. in thesection fiber This was andgives estimated the an overlapabsorption to be factoraround coefficient between1.45 pm of 1.63, thewhich dB/m. pumped is in Considering good and doped agreement with the absorption cross section of other Yb-doped alumino-phospho-silicate glasses area (claddingthe Yb concentration pumped), in the the absorption fiber and the cross overlap section factor was between estimated the pumped to be around and doped1.45 areapm2 (cladding, which is in pumped),found thein literature absorption [57,58]. cross With sect respection towas the estimatedabsorbed pump to be power around in 2.13 1.45 pm m of fiber , length,which a isslope in good good agreementefficiency with of 67% the was absorption achieved for cross fiber sectionB1200. The of laser other output Yb-doped power at alumino-phospho-silicate the signal wavelength versus glasses agreement with the absorption cross section of other Yb-doped alumino-phospho-silicate glasses found in literaturethe absorbed [57 pump,58]. Withpower respectis shown toin Figure the absorbed 11. pump power in 2.13 m of fiber length, a slope found in literature [57,58]. With respect to the absorbed pump power in 2.13 m of fiber length, a slope efficiencyefficiency of 67% of 67% was was achieved achieved for for fiber fiber B1200. B1200. The The laser laser outputoutput power at at the the signal signal wavelength wavelength versus versus the absorbedthe absorbedFibers pump2018, 6pump, x FOR power PEER power REVIEW is shown is shown in in Figure Figure 11 11.. 13 of 18 Output Signal @1030nm / W

Figure 11.FigureLaser 11. output Laser output power power at 1031 at 1031 nm nm versus versus absorbed pump pump power power at 976 atnm 976 of fiber nm B1200. of fiber B1200.

4. Discussion Two Yb-doped silica glass compositions with fixed Al to P concentration ratio (Al/P = 0. 82) and varying absolute concentration with respect to 0.3 at.% of Yb concentration were prepared by the sol- gel based granulated silica method. The compositions were drawn into fibers, whose optical characteristics were investigated. As mentioned in the introduction, the incorporation of co-dopants like Al and P can improve the solubility of Yb and RE ions in general in silica glass, by forming a solvation shell around the RE ions that can be merged and dissolved in the silica network. To support this, the atomic distribution of the dopant (Yb) and co-dopants (Al and P) was observed using high-resolution atomic imaging and chemical analysis (Figures 4 and 5). The elemental mappings obtained and their overlaps support this model, as they show the similar preferable localization of Al, P and Yb through the microstructure (Figure 5a,d,g–i). This is in very good agreement with Arai’s suggestion [10]. In addition, the EDX spectra comparing the compositions of cluster-containing and cluster-free zones (Figure 5b,c) confirm the similar localization of Al, P, and Yb. These nano-scale clusters of Yb, Al and P are supposed to be one of the major causes of the optical loss in the manufactured fibers. The influence of variation in the co-dopant concentration with respect to Yb on the optical properties of the fibers was also investigated. For the fiber B1550, which has a lower excess of P concentration with respect to Al (P—Al = 1 at.%), we observed a decrease in the refractive index compared to the fiber A1550 having P—Al = 2 at.%. This is due to there being a better compensation of the refractive index change arising from the Al by the P, consistent with Bubnov’s study on the influence of Al and P on the optical properties of silica fibers [46]. The transmission losses of the fibers were measured at 630 nm using a cutback method. A large loss increase is observed in the fiber containing a higher excess P concentration. Similar effects have been also observed in [59,60], where an additional loss component that extends from the visible to the near infrared region was observed for the compositions that contain excess in P concentration with respect to Al, and it disappears as Al exceeds P concentration. Bubnov in [45,46] suggests that this

increase in the losses is not due to the intrinsic properties of the PO-AlO-SiO system containing excess in P, but its due to Fe impurities. In silica glass, Fe can be present in the form of Feand Fe. The Fe ion leads to strong absorption at wavelengths lower than 400 nm [46]. Fe in silica glass is

characterized by a broad absorption band around 1200 nm, very similar to the one observed in PO- AlO-SiO fibers containing an excess of PO in [45,46,59,60,61]. The valence of Fe is dependent on many factors, one of them being the fiber composition [46]. Gambling et al. [62] pointed out that Fe impurities in phosphosilicate glasses were particularly undesirable because they had a valence of 2+. Therefore, having an excess in P could enhance the formation of Fe that leads to a broad absorption band around 1200 nm. To test whether Bubnov’s study supports the trend observed in our samples Fibers 2018, 6, 82 13 of 17

4. Discussion Two Yb-doped silica glass compositions with fixed Al to P concentration ratio (Al/P = 0. 82) and varying absolute concentration with respect to 0.3 at.% of Yb concentration were prepared by the sol-gel based granulated silica method. The compositions were drawn into fibers, whose optical characteristics were investigated. As mentioned in the introduction, the incorporation of co-dopants like Al and P can improve the solubility of Yb and RE ions in general in silica glass, by forming a solvation shell around the RE ions that can be merged and dissolved in the silica network. To support this, the atomic distribution of the dopant (Yb) and co-dopants (Al and P) was observed using high-resolution atomic imaging and chemical analysis (Figures4 and5). The elemental mappings obtained and their overlaps support this model, as they show the similar preferable localization of Al, P and Yb through the microstructure (Figure5a,d,g–i). This is in very good agreement with Arai’s suggestion [ 10]. In addition, the EDX spectra comparing the compositions of cluster-containing and cluster-free zones (Figure5b,c) confirm the similar localization of Al, P, and Yb. These nano-scale clusters of Yb, Al and P are supposed to be one of the major causes of the optical loss in the manufactured fibers. The influence of variation in the co-dopant concentration with respect to Yb on the optical properties of the fibers was also investigated. For the fiber B1550, which has a lower excess of P concentration with respect to Al (P—Al = 1 at.%), we observed a decrease in the refractive index compared to the fiber A1550 having P—Al = 2 at.%. This is due to there being a better compensation of the refractive index change arising from the Al by the P, consistent with Bubnov’s study on the influence of Al and P on the optical properties of silica fibers [46]. The transmission losses of the fibers were measured at 630 nm using a cutback method. A large loss increase is observed in the fiber containing a higher excess P concentration. Similar effects have been also observed in [59,60], where an additional loss component that extends from the visible to the near infrared region was observed for the compositions that contain excess in P concentration with respect to Al, and it disappears as Al exceeds P concentration. Bubnov in [45,46] suggests that this increase in the losses is not due to the intrinsic properties of the P2O5-Al2O3-SiO2 system containing excess in P, but its due to Fe impurities. In silica glass, Fe can be present in the form of Fe3+ and Fe2+. The Fe3+ ion leads to strong absorption at wavelengths lower than 400 nm [46]. Fe2+. in silica glass is characterized by a broad absorption band around 1200 nm, very similar to the one observed in P2O5-Al2O3-SiO2 fibers containing an excess of P2O5 in [45,46,59–61]. The valence of Fe is dependent on many factors, one of them being the fiber composition [46]. Gambling et al. [62] pointed out that Fe impurities in phosphosilicate glasses were particularly undesirable because they had a valence of 2+. Therefore, having an excess in P could enhance the formation of Fe2+ that leads to a broad absorption band around 1200 nm. To test whether Bubnov’s study supports the trend observed in our samples [46], the impurity concentration of Fe was measured by the means of LA-ICP-MS. Significant Fe concentration ranges from 9 to 18 ppm was found in both compositions, and is already contained within the precursors (i.e., not introduced due to any powder post-processing step). While this supports Bubbnov’s suggestion, further measurements and investigations are needed for confirmation. Further improvement in the transmission properties were obtained when the temperature of the heat treatment of the powder derived from the sol-gel was reduced from 1550 ◦C to 1200 ◦C. This is ◦ most probably due to the growth of crystals (e.g., cristobalite for SiO2) at 1550 C which results in higher scattering losses. The fluorescence lifetime of the upper-level excited state of Yb has been measured for the two compositions. The similar behavior in the fluorescence lifetime for the two compositions shows that the atomic distribution of the Yb dopant is not significantly influenced by the variation of a fixed Al to P concentration ratio with respect to Yb concentration, as long as the Al to Yb and/or P to Yb concentration ratio is greater than 10 and 15, respectively. Finally, the laser characteristics of the B1200 fiber were measured. Using cladding-pumping at 976 nm in 2.13 m of fiber length, 55% of absorbed pump power at 976 nm and a slope efficiency of Fibers 2018, 6, 82 14 of 17

67% with respect to the absorbed pump power at 1031 nm were achieved. The limited slope efficiency could be a result of the high transmission losses in the fiber due to impurities, in combination with non-radiative transitions that occur due to the Yb nano-clusters shown in the STEM-HAADF images.

5. Conclusions and Outlook We have shown the advantages of the sol-gel based granulated silica method regarding the flexibility and simplicity of incorporating dopant and co-dopants at room temperature, by preparing Yb-doped, Al and P co-doped silica fibers. The dependence of the fiber properties on the Al and P concentrations, as well as on the powder heat treatment condition, have been studied. With the results presented here and the previous investigation reported in [43], first comparisons can be made and conclusions drawn concerning the behavior of Yb in the glass host P2O5-Al2O3-SiO2 with different Al and P concentrations. Improved Yb solubility, which leads to a longer excited state lifetime and better laser characteristics, have been obtained for compositions that contain a slight excess in P concentration with respect to Al [43]. However, as the excess in P concentration increases, higher transmission losses were observed. The reasons behind this increase in the transmission losses is not clear yet, but one valid assumption could be due to Fe2+ impurities in the samples. Additional losses due to large scattering centers along the fiber were present in the composition that was heat treated at 1550 ◦C. By lowering the temperature of the heat treatment to 1200 ◦C, less scattering centers were observed. These components are featured in B1200 fiber with composition of 0.3 at.% Yb, 4.5 at.% Al and 5.5 at.% P, and heat treated at 1200 ◦C. When pumping this fiber with a LD at 976 nm, a slope efficiency of 67% at around 1031 nm was achieved. Finally, additional work is currently going on in order to further improve the distribution of the dopant and co-dopants in our fibers. This would prevent the formation of the nano-clusters, in turn reducing the probability of the non-radiative transitions, and thus improving the laser slope efficiency. In addition, further measures are to be taken in order to reduce the transmission losses in our fibers, such as reducing and eliminating the impurities that we have detected in our samples (namely the iron), by using high purity starting materials.

Author Contributions: Conceptualization, A.E. and V.R.; Methodology, A.E. and V.R.; Validation, A.E., S.P. and V.R.; Formal Analysis, A.E., H.N. and D.T.L.A.; Investigation, A.E., S.P., M.H., H.N. and D.T.L.A.; Resources, A.E. and V.R.; Data Curation, A.E. and S.P.; Writing-Original Draft Preparation, A.E.; Writing-Review and Editing, A.E., S.P., M.H., H.N., D.T.L.A. and V.R.; Visualization, A.E., M.H. and S.P.; Supervision, S.P. and V.R.; Project Administration, V.R.; Funding Acquisition, V.R. Funding: This research was partially funded by Swiss Commission for Technology and Innovation CTI under project numbers 18613.1 and 17133.1. Acknowledgments: We are very thankful to J. Scheuner, Ch. Bacher and D. Lopez for active help in powder post-processing and in fiber drawing. Many thanks to T. Feurer and H. Kim for discussions, and to D. Kummer, S. Berger, A. Ruefenacht, and G. Karametaxas for their help in sol-gel material production. We also thank T. Petke for providing the LA-ICP mass spectrometer. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Petit, V.; Tumminelli, R.P.; Minelly, J.D.; Khitrov, V. Extremely Low NA Yb Doped Preforms (<0.03) Fabricated by MCVD; Ballato, J., Ed.; SPIE: Bellingham, WA, USA, 2016; p. 97282R. 2. Beier, F.; Hupel, C.; Kuhn, S.; Hein, S.; Nold, J.; Proske, F.; Sattler, B.; Liem, A.; Jauregui, C.; Limpert, J.; et al. Single mode 43 kW output power from a diode-pumped Yb-doped fiber amplifier. Opt. Express 2017, 25, 14892–14899. [CrossRef][PubMed] 3. Xu, W.; Lin, Z.; Wang, M.; Feng, S.; Zhang, L.; Zhou, Q.; Chen, D.; Zhang, L.; Wang, S.; Yu, C.; et al. 50 µm core diameter Yb3+/Al3+/F− codoped silica fiber with M2 <11 beam quality. Opt. Lett. 2016, 41, 504–507. [CrossRef][PubMed] Fibers 2018, 6, 82 15 of 17

4. He, W.; Leich, M.; Grimm, S.; Kobelke, J.; Zhu, Y.; Bartelt, H.; Jäger, M. Very large mode area ytterbium fiber amplifier with aluminum-doped pump cladding made by powder sinter technology. Laser Phys. Lett. 2015, 12.[CrossRef] 5. Li, W.; Zhou, Q.; Zhang, L.; Wang, S.; Wang, M.; Yu, C.; Feng, S.; Chen, D.; Hu, L. Watt-level Yb-doped silica glass fiber laser with a core made by sol-gel method. Chin. Opt. Lett. 2013, 11, 091601–91603. [CrossRef] 6. Wang, S.; Li, Z.; Yu, C.; Wang, M.; Feng, S.; Zhou, Q.; Chen, D.; Hu, L. Fabrication and laser behaviors of Yb3+doped silica large mode area photonic crystal fiber prepared by sol-gel method. Opt. Mater. 2013, 35, 1752–1755. [CrossRef] 7. Moghaddam, M.R.A.; Harun, S.W.; Ahmad, H. Comparison between Analytical Solution and Experimental Setup of a Short Long Ytterbium Doped Fiber Laser. Opt. Photonics J. 2012, 2, 65–72. [CrossRef] 8. Liu, S.; Li, H.; Tang, Y.; Hu, L. Fabrication and spectroscopic properties of Yb3+-doped silica glasses using the sol-gel method. Chin. Opt. Lett. 2012, 10, 8–11. [CrossRef] 9. Wang, S.; Feng, S.; Wang, M.; Yu, C.; Zhou, Q.; Li, H.; Tang, Y.; Chen, D.; Hu, L. Optical and laser properties 3+ of Yb -doped Al2O3–P2O5–SiO2 large-mode-area photonic crystal fiber prepared by the sol–gel method. Laser Phys. Lett. 2013, 10, 115802. [CrossRef] 10. Arai, K.; Namikawa, H.; Kumata, K.; Honda, T.; Ishii, Y.; Handa, T. Aluminum or phosphorus co-doping effects on the fluorescence and structural properties of neodymium-doped silica glass. J. Appl. Phys. 1986, 59, 3430–3436. [CrossRef] 11. Sen, S.; Rakhmatullin, R.; Gubaidullin, R.; Pöppl, A. Direct spectroscopic observation of the atomic-scale mechanisms of clustering and homogenization of rare-earth dopant ions in vitreous silica. Phys. Rev. B 2006, 74, 2–5. [CrossRef] 12. Unger, S.; Lindner, F.; Aichele, C.; Leich, M.; Schwuchow, A.; Kobelke, J.; Dellith, J.; Schuster, K.; Bartelt, H. A highly efficient Yb-doped silica laser fiber prepared by gas phase doping technology. Laser Phys. 2014, 24. [CrossRef] 13. Gambling, W.A. The rise and rise of optical fibers. IEEE J. Sel. Top. Quantum Electron. 2000, 6, 1084–1093. [CrossRef] 14. Warren, B.E.; Pincus, A.G. Atomic Consideration of Immiscibility in Glass Systems. J. Am. Ceram. Soc. 1940, 23, 301–304. [CrossRef] 15. Glasser, F.P.; Warshaw, I.; Roy, R. Liquid immiscibility in silicate systems. Phys. Chem. Glasses 1960, 1, 39. 16. Sen, S. Atomic environment of high-field strength Nd and Al cations as dopants and major components in silicate glasses: A Nd LIII-edge and Al K-edge X-ray absorption spectroscopic study. J. Non-Cryst. Solids 2000, 261, 226–236. [CrossRef] 17. Savel, E.A.; Krivovichev, A.V.; Golant, K.M. Clustering of Yb in silica-based glasses synthesized by SPCVD. Opt. Mater. 2016, 62, 518–526. [CrossRef] 18. Arai, K.; Yamasaki, S.; Isoya, J.; Namikawa, H. Electron-spin-echo envelope-modulation study of the distance 3+ 3+ between Nd ions and Al ions in the co-doped SiO2 glasses. J. Non-Cryst. Solids 1996, 196, 216–220. [CrossRef]

19. Lægsgaard, J. Dissolution of rare-earth clusters in SiO2 by Al codoping: A microscopic model. Phys. Rev. B 2002, 65, 174114. [CrossRef] 20. Funabiki, F.; Kamiya, T.; Hosono, H. Doping effects in amorphous oxides. J. Ceram. Soc. Jpn. 2012, 120, 447–457. [CrossRef] 21. Canevali, C.; Mattoni, M.; Morazzoni, F.; Scotti, R.; Casu, M.; Musinu, A.; Krsmanovic, R.; Polizzi, S.; Speghini, A.; Bettinelli, M. Stability of Luminescent Trivalent Cerium in Silica Host Glasses Modified by Boron and Phosphorus. J. Am. Chem. Soc. 2005, 127, 14681–14691. [CrossRef][PubMed] 22. Nagel, S.R.; MacChesney, J.B.; Walker, K.L. An Overview of the Modified Chemical Vapor Deposition (MCVD) Process and Performance. IEEE Trans. Microw. Theory Tech. 1982, 30, 305–322. [CrossRef] 23. Poole, S.B.; Payne, D.N.; Mears, R.J.; Fermann, M.E.; Laming, R.I. Fabrication and Characterization of Low-Loss Optical Fibers Containing Rare-Earth Ions. J. Lightwave Technol. 1986, 4, 870–876. [CrossRef] 24. Langner, A.; Schötz, G.; Such, M.; Kayser, T.; Reichel, V.; Grimm, S.; Kirchhof, J.; Krause, V.; Rehmann, G. A new material for high power laser fibers. Proc. SPIE 2008, 6873, 687311. [CrossRef] 25. Wang, S.; Lou, F.; Wang, M.; Yu, C.; Feng, S.; Zhou, Q.; Chen, D.; Hu, L. Characteristics and Laser Performance of Yb3+-Doped Silica Large Mode Area Fibers Prepared by Sol–Gel Method. Fibers 2013, 1, 93–100. [CrossRef] Fibers 2018, 6, 82 16 of 17

26. Montiel i Ponsoda, J.J.; Norin, L.; Ye, C.; Bosund, M.; Söderlund, M.J.; Tervonen, A.; Honkanen, S. Ytterbium-doped fibers fabricated with atomic layer deposition method. Opt. Express 2012, 20, 25085. [CrossRef][PubMed] 27. Pilz, S.; Najafi, H.; Ryser, M.; Romano, V. Granulated Silica Method for the Fiber Preform Production. Fibers 2017, 5, 24. [CrossRef] 28. MacChesney, J.B.; Johnson, D.W., Jr.; Bhandarkar, S.; Bohrer, M.P.; Fleming, J.W.; Monberg, E.M.; Trevor, D.J. Optical fibers by a hybrid process using sol–gel silica overcladding tubes. J. Non-Cryst. Solids 1998, 226, 232–238. [CrossRef] 29. Locher, M.; Romano, V.; Weber, H.P.Rare-earth doped sol-gel materials for optical waveguides. Opt. Lasers Eng. 2005, 43, 341–347. [CrossRef] 30. Romano, V.; Pilz, S.; Etissa, D. Sol-gel-based doped granulated silica for the rapid production of optical fibers. Int. J. Mod. Phys. B 2014, 28, 1442010. [CrossRef] 31. Auguste, J.L.; Humbert, G.; Leparmentier, S.; Kudinova, M.; Martin, P.O.; Delaizir, G.; Schuster, K.; Litzkendorf, D. Modified powder-in-tube technique based on the consolidation processing of powder materials for fabricating specialty optical fibers. Materials 2014, 7, 6045–6063. [CrossRef][PubMed] 32. Ballato, J.; Snitzer, E. Fabrication of fibers with high rare-earth concentrations for Faraday isolator applications. Appl. Opt. 1995, 34, 6848. [CrossRef][PubMed] 33. Dauliat, R.; Gaponov, D.; Jamier, R.; Schuster, K.; Grimm, S.; Valette, S.; Benoit, A.; Salin, F.; Roy, P. Powder technology and innovative fiber design enabling a new generation of high-power single-mode fiber laser sources. Adv. Laser Technol. 2012.[CrossRef] 34. Renner-Erny, R.; Di Labio, L.; Lüthy, W. A novel technique for active fibre production. Opt. Mater. 2007, 29, 919–922. [CrossRef] 35. Neff, M.; Romano, V.; Lüthy, W. Metal-doped fibres for broadband emission: Fabrication with granulated oxides. Opt. Mater. 2008, 31, 247–251. [CrossRef] 36. Wilhelm, B.; Romano, V.; Weber, H.P. Fluorescence lifetime enhancement of Nd3+-doped sol-gel glasses by

Al-codoping and CO2-laser processing. J. Non-Cryst. Solids 2003, 328, 192–198. [CrossRef] 37. Velmiskin, V.V.; Egorova, O.N.; Mishkin, V.; Nishchev, K.; Semjonov, S.L. Active material for fiber core made by powder-in-tube method: Subsequent homogenization by means of stack-and-draw technique. Proc. SPIE 2012, 8426.[CrossRef] 38. Pilz, S.; Najafi, H.; El Sayed, A.; Boas, J.; Kummer, D.; Scheuner, J.; Etissa, D.; Ryser, M.; Raisin, P.; Berger, S.; et al. Progress in the Fabrication of Optical Fibers by the Sol-Gel-Based Granulated Silica Method. In Proceedings of the SPIE—The International Society for Optical Engineering; Kalli, K., Mendez, A., Eds.; SPIE: Brussels, Belgium, 2016; Volume 9886, p. 988614. 39. Scheuner, J.; Heidt, A.M.; Pilz, S.; Raisin, P.; El Sayed, A.; Najafi, H.; Ryser, M.; Feurer, T.; Romano, V.; Bacher, C.; et al. Advances in optical fibers fabricated with granulated silica. In Proceedings of the 2017 Optical Fiber Communications Conference and Exhibition (OFC), Los Angeles, CA, USA, 19–23 March 2017. 40. Pedrazza, U.; Romano, V.; Lüthy, W. Yb3+:Al3+: sol-gel silica glass fiber laser. Opt. Mater. 2007, 29, 905–907. [CrossRef] 41. Bacher, C.; Scheuner, J.; Pilz, S.; El Sayed, A.; Ryser, M.; Heidt, A.; Romano, V. Spectroscopy of a heated Yb-doped optical fiber with high aluminum content. In Proceedings of the SPIE—The International Society for Optical Engineering; Kalli, K., Kanka, J., Mendez, A., Peterka, P., Eds.; SPIE: Prague, Czech Republic, 2017; Volume 10232, p. 102320J. 42. Tomozawa, M.; Kim, D.-L.; Lou, V. Preparation of high purity, low water content fused silica glass. J. Non-Cryst. Solids 2001, 296, 102–106. [CrossRef] 43. El Sayed, A.F.; Pilz, S.; Scheuner, J.; Najafi, H.; Feurer, T.; Romano, V. Properties of Yb doped silica fibers with different Al and P co-dopants concentrations produced by the Sol-Gel based granulated silica method. In Micro-Structured and Specialty Optical Fibres V; Bunge, C.-A., Kalli, K., Mendez, A., Eds.; SPIE: Strasbourg, France, 2018; Volume 1068105, p. 49. 44. Mizoguchi, T.; Findlay, S.D.; Masuno, A.; Saito, Y.; Yamaguchi, K.; Inoue, H.; Ikuhara, Y. Atomic-Scale Identification of Individual Lanthanide Dopants in Optical Glass Fiber. ACS Nano 2013, 7, 5058–5063. [CrossRef][PubMed] Fibers 2018, 6, 82 17 of 17

45. Bubnov, M.M.; Vechkanov, V.N.; Gur’yanov, A.N.; Zotov, K.V.; Lipatov, D.S.; Likhachev, M.E.; Yashkov, M.V.

Fabrication and optical properties of fibers with an Al2O3-P2O5-SiO2 glass core. Inorg. Mater. 2009, 45, 444–449. [CrossRef] 46. Bubnov, M.M.; Gur’yanov, A.N.; Zotov, K.V.; Iskhakova, L.D.; Lavrishchev, S.V.; Lipatov, D.S.; Likhachev, M.E.; Rybaltovsky, A.A.; Khopin, V.F.; Yashkov, M.V.; et al. Optical properties of fibres with aluminophosphosilicate glass cores. Quantum Electron. 2009, 39, 857–862. [CrossRef] 47. Raine, K.W.; Baines, J.G.N.; Putland, D.E. Refractive index profiling-state of the art. J. Light. Technol. 1989, 7, 1162–1169. [CrossRef] 48. Young, M. Optical fiber index profiles by the refracted-ray method (refracted near-field scanning). Appl. Opt. 1981, 20, 3415–3422. [CrossRef][PubMed] 49. El Sayed, A.; Pilz, S.; Ryser, M.; Romano, V. Two-Dimensional Refractive Index Profiling of Optical Fibers by Modified Refractive Near-Field Technique. In Proceedings of the SPIE—The International Society for Optical Engineering; Kalli, K., Mendez, A., Eds.; SPIE: San Francisco, CA, USA, 2016; Volume 9744, p. 98861N. 50. Raisin, P.; Scheuner, J.; Romano, V.; Ryser, M. High-precision confocal reflection measurement for two dimensional refractive index mapping of optical fibers. In Proceedings of the SPIE—The International Society for Optical Engineering; Kalli, K., Kanka, J., Mendez, A., Eds.; Kalli, K., Kanka, J., Mendez, A., Eds.; SPIE: Prague, Czech Republic, 2015; Volume 9507, p. 95070O. 51. Wang, S.; Xu, W.; Wang, F.; Lou, F.; Zhang, L.; Zhou, Q.; Chen, D.; Feng, S.; Wang, M.; Yu, C.; et al. Yb3+-doped silica glass rod with high optical quality and low optical attenuation prepared by modified sol-gel technology for large mode area fiber. Opt. Mater. Express 2017, 7, 2012–2022. [CrossRef] 52. Leich, M.; Just, F.; Langner, A.; Such, M.; Schötz, G.; Eschrich, T.; Grimm, S. Highly efficient Yb-doped silica fibers prepared by powder sinter technology. Opt. Lett. 2011, 36, 1557–1559. [CrossRef][PubMed] 53. Krishnaiah, K.V.; Kumar, K.U.; Agarwal, V.; Murali, C.G.; Chaurasia, S.; Dhareshwar, L.J.; Jayasankar, C.K.; Lavin, V. Fluorescence and Spectroscopic Properties of Yb3+-Doped Phosphate Glasses. Phys. Procedia 2012, 29, 109–113. [CrossRef] 54. Dong, L.; Samson, B. Optical Fibers: Materials and Fabrication. In Fiber Lasers; CRC Press and Taylor & Francis Group: Boca Raton, FL, USA, 2016; pp. 61–82. 55. Paschotta, R.; Nilsson, J.; Barber, P.R.; Caplen, J.E.; Tropper, A.C.; Hanna, D.C. Lifetime quenching in Yb-doped fibres. Opt. Commun. 1997, 136, 375–378. [CrossRef] 56. Wang, X.; Zhang, R.; Ren, J.; Vezin, H.; Fan, S.; Yu, C.; Chen, D.; Wang, S.; Hu, L. Mechanism of cluster dissolution of Yb-doped high-silica lanthanum aluminosilicate glass: Investigation by spectroscopic and structural characterization. J. Alloys Compd. 2017, 695, 2339–2346. [CrossRef] 57. Jetschke, S.; Unger, S.; Schwuchow, A.; Leich, M.; Kirchhof, J. Efficient Yb laser fibers with low photodarkening by optimization of the core composition. Opt. Express 2008, 16, 15540. [CrossRef][PubMed] 58. Melkoumov, M.A.; Bufetov, I.A.; Bubnov, M.M.; Kravtsov, K.S.; Semjonov, S.L.; Shubin, A.V.; Dianov, E.M.

Ytterbium Lasers Based on P2O5- and Al2O3-doped Fibers. In Proceedings of the Europeen Conference on Oprical Communication, Stockholm, Sweden, 5–9 September 2004. 59. Unger, S.; Schwuchow, A.; Jetschke, S.; Reichel, V.; Leich, M.; Scheffel, A.; Kirchhof, J. Influence of aluminum-phosphorus codoping on optical properties of ytterbium-doped laser fibers. Proc. SPIE 2009, 7212, 72121B. [CrossRef] 60. Unger, S.; Schwuchow, A.; Dellith, J.; Kirchhof, J. Codoped materials for high power fiber lasers: Diffusion behaviour and optical properties. Proc. SPIE 2007, 6469, 646913. [CrossRef] 61. Unger, S.; Schwuchow, A.; Jetschke, S.; Reichel, V.; Scheffel, A.; Kirchhof, J. Optical properties of Yb-doped laser fibers in dependence on codopants and preparation conditions. Proc. SPIE 2008, 6890, 689016. [CrossRef] 62. Gambling, W.A.; Payne, D.N.; Hammond, C.R.; Norman, S.R. Optical fibres based on phosphosilicate glass. Proc. Inst. Electr. Eng. 1976, 123, 570–576. [CrossRef]

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