Extended germano-gallate fiber drawing domain: from germanates to gallates optical fibers

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Théo Guérineau, Clément Strutynski, Téa Skopak, Steeve Morency, Anouar

Hanafi, Florian Calzavara, Yannick Ledemi, Sylvain Danto, Thierry Cardinal,

Younès Messaddeq, et al.

To cite this version:

Théo Guérineau, Clément Strutynski, Téa Skopak, Steeve Morency, Anouar Hanafi, et al.. Extended

germano-gallate fiber drawing domain: from germanates to gallates optical fibers. Optical Materials

Express, OSA pub, 2019, 9 (6), pp.2437-2445. �10.1364/OME.9.002437�. �hal-02188507�

Extended germano-gallate fiber drawing domain:

from germanates to gallates optical fibers

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1_{Institut de Chimie de la Matière Condensée de Bordeaux (ICMCB), CNRS, UPR 9048, Pessac, 33608,}

France

2_{International Associated Laboratory (LIA) LuMAQ: Centre d’Optique, Photonique et Laser (COPL),}

Université Laval, 2375 rue de la Terrasse, Québec, QC, G1 V 0A6, Canada

*_{sylvain.danto@u-bordeaux.fr}

Abstract: Here we report on the production of crystal-free light guiding fibers using a

preform-to-fiber approach in the germano-gallate glass system Ga2O3-GeO2-BaO-La2O3-Y2O3 for

various contents of gallium to germanium. For glasses in the system Ga2O3-GeO2

-BaO-K2O, where surface crystallization precludes fiber drawing from the preform, an open-crucible

technique enables the drawing of fiber samples tens of meters long. Cut-back optical attenuation measurements show the extended transmission in the mid-infrared of the produced fibers, up to 2.8 µm with minimum losses of 3.1 dB/m at 1310 nm from unpurified glass. These results show that the germano-gallate glasses represent promising mid-infrared materials over an extended fiber drawing domain.

© 2019 Optical Society of America under the terms of theOSA Open Access Publishing Agreement

1. Introduction

There is currently a critical need in designing new infrared (IR) optical materials for developing integrated IR optical fiber devices, for applications either as passive medium for telecom-munication, remote sensing and detection, or as active fiber-laser medium upon doping with luminescent-active elements. Based on their extended transmission in the infrared and their tailorable thermo-mechanical properties, fluoride, tellurite and chalcogenide glasses have been

extensively considered as optical components and devices [1]. However, despite their successful

technological developments into fibers [2], they still suffer from limitations due to either low

transmission in the visible range or relatively poor chemical and mechanical resistance, thus impeding their further effective integration into functional components.

In the meantime heavy metal germano-gallate oxide glasses offer wide optical transparency, extending from ∼280 nm in the UV up to ∼6 µm in the mid-IR, high glass transition temperature

(Tg∼700 °C) and superior mechanical strength and laser damage threshold [3,4]. Gallium oxide

insertion tends to increase the transmission into the infrared range and the refractive index of the

host matrix [5,6]. Wen et al. reported on the fabrication of fibers containing up to 19 mol.% of

Ga2O3in the Tm3+-doped BaO-Ga2O3-GeO2system [6,7]. Yet, to the best of our knowledge,

no optical fiber fabrication has been reported for rich Ga2O3-based glass compositions, a result

explained by their larger tendency to crystallization preventing their synthesis and shaping in large amounts.

Recently the authors have explored properties of germano-gallate glasses in the systems

Ga2O3-GeO2-Na2O [5], Ga2O3-La2O3-K2O [8] and Ga2O3-GeO2-BaO-K2O [9]. Extending this

work, we describe here a strategy to manufacture single-index glass fibers of optical quality in the

germano-gallate systems Ga2O3-GeO2-BaO-La2O3-Y2O3and Ga2O3-GeO2-BaO-K2O. First, we

#361417 https://doi.org/10.1364/OME.9.002437

present experimental results obtained by the standard preform-to-fiber method. The introduction

of lanthanide oxides La2O3and Y2O3instead of alkali oxide K2O circumvents the detrimental

glass surface crystallization observed previously [9].

In a second approach, we investigate the open-crucible technique for potassium-containing glasses. As it relies on the whole melting of the glass, this method, already largely employed in

fluoro-phosphate and chalcogenide glass fiber manufacturing [10,11], eliminates crystallization

issues during fiber drawing process. Using this technique, fabrication method for the production

of optical fibers in the Ga2O3-GeO2-BaO-K2O glass system is detailed and fiber losses are

presented.

2. Experimental details

Glass samples were prepared by the conventional melt-quenching technique from high purity

reagents (Ga2O3: 99.998%, Strem Chemical, GeO2: 99.999%, Strem Chemical, BaCO3: 99%,

ACS Merck, K2CO3: 99%, Sigma Aldrich, La2O3: 99.9%, Sigma Aldrich, Y2O3: 99.9%, Alfa

Aesar). The powders were mixed in a platinum crucible and heated up to 1500 °C for 2 hours. Glass chunks devoted to drawing experiments by crucible technique were obtained by quenching the melt on a stainless steel plate. Preform-to-fiber drawing experiments require cylindrical glass rods of 40 grams, 10 mm in diameter and approximately 10 cm in length. To that end the glass was melted again and quenched in a stainless steel mold with appropriate sizes, the resulting

glass rod being annealed at Tg-40 °C for 4 hours and slowly cooled down to room temperature to

remove residual mechanical stress. The thermal drawing ability of the preforms was assessed using dedicated optical fiber drawing towers as further detailed below.

The glass transition (Tg) and onset of crystallization (Tx) temperatures were determined by

DSC using a Netzsch Pegasus 404F3 apparatus with glass chunks inserted in a Pt pan at a heating rate of 10 °C/min with a precision of ± 2°C. The density ρ was measured by immersing a glass chunk in diethyl phthalate at room temperature on a Precisa XT 220A weighing scale

(estimated error of ± 0.005 g.cm−_{3). The refractive index was measured by prism coupling on a}

M-line Metricon 2010/M at 532 nm with an estimated error of ± 0.005. The optical transmission spectra in the UV-Visible-IR range were obtained from an Agilent Cary 5000 (UV-Visible) and a Bruker spectro-photometers. Powder X-ray diffraction (XRD) was performed on a PANalitycal X’pert PRO MPD diffractometer in Bragg-Brentano θ-θ geometry equipped with a secondary monochromator and X’Celerator multi-strip detector. The Cu-Kα radiation was generated at 45 kV and 40 mA (λ = 0.15418 nm). Micro-Raman spectra were recorded in backscattering mode on a confocal micro-Raman spectrometer HR (Horiba/Jobin Yvon) equipped with a Synapse CCD detector. A continuous wave laser operating at 532 nm was used for excitation. Typical

resolution used for Raman spectrum acquisition is 2.5 cm−1_.

Optical fiber attenuation was recorded using the cut-back method. Measurements were performed in two steps: (i) using a halogen lamp (power of ∼30 mW measured at the fiber input) as broadband source (350-1700nm) and an optical spectrum analyzer (Ando AQ-6315A OSA) for detection in the Vis-NIR range (400-1600 nm) and (ii) using a fiber supercontinuum source (Le Verre Fluoré, Targazh IRguide, emission range from 0.8 to 4.2 µm), a monochromator (Brucker) and a PbSe detector sensitive in the infrared range (1500-4800 nm). Quality of cleaving was systematically inspected with microscope objective and special attention was paid to limit the fiber curvature during measurement.

3. Results and discussion

First, we investigate the ability of germano-gallate oxide glasses to be drawn from the preform. The investigated glass materials (acronyms, nominal compositions in molar and cationic percent)

are summarized in Table1. Their physicochemical properties are displayed in Table2. The

as a reliable criterion for evaluating the glass thermal stability vs crystallization, in particular for its shaping by thermal process like glass fiber drawing, with targeted values of ∆T ≥ 100-120 °C. In

our previous work [9], we identified the glass of composition 28Ga2O3–37GeO2–23BaO–12K2O

(mol.%) (hereafter Ga28Ge37Ba23K) as prone to support preform-to-fiber drawing conditions on

the basis of its thermal properties (Tg= 671 °C, ∆T = 191 °C). The Ga28Ge37Ba23K glass was

further modified by replacing potassium oxide with the rare earth elements lanthanium and yttrium

oxides in equimolar ratios, following a work by Wen et al. on the fabrication of fiber optics [7]. It

is worth noting that in the latter work, GeO2content was 65 mol.%, i.e. the main glass constituent.

Here the three selected glasses were 28Ga2O3–37GeO2–23BaO–6La2O3–6Y2O3 (hereafter

Ga28Ge37Ba23LaY), 21Ga2O3–43GeO2–24BaO–6La2O3–6Y2O3(hereafter Ga21Ge43Ba24LaY)

and 15Ga2O3–60GeO2–13BaO–6La2O3–6Y2O3(hereafter Ga15Ge60Ba13LaY).

Table 1. Investigated glasses (acronyms, nominal compositions in molar and cationic percent)

the time regarded as a reliable criterion for evaluating the glass thermal stability vs crystallization, in particular for its shaping by thermal process like glass fiber drawing, with targeted values of ΔT ≥ 100-120 °C. In our previous work [9], we identified the glass of composition 28Ga2O3–37GeO2–23BaO–12K2O (mol.%) (hereafter Ga28Ge37Ba23K) as prone

to support preform-to-fiber drawing conditions on the basis of its thermal properties (Tg = 671 °C, ΔT = 191 °C). The Ga28Ge37Ba23K glass was further modified by replacing potassium

oxide with the rare earth elements lanthanium and yttrium oxides in equimolar ratios, following a work by Wen et al. on the fabrication of fiber optics [7]. It is worth noting that in the latter work, GeO2 content was 65 mol.%, i.e. the main glass constituent. Here the three

selected glasses were 28Ga2O3–37GeO2–23BaO–6La2O3–6Y2O3 (hereafter

Ga28Ge37Ba23LaY), 21Ga2O3–43GeO2–24BaO–6La2O3–6Y2O3 (hereafter Ga21Ge43Ba24LaY)

and 15Ga2O3–60GeO2–13BaO–6La2O3–6Y2O3 (hereafter Ga15Ge60Ba13LaY).

Table 1. Investigated glasses (acronyms, nominal compositions in molar and cationic percent)

The measured characteristic temperatures of the glasses are reported in Table 2. The Tg of the La/Y-containing glasses are ~60 °C higher than the potassium-containing glass, suggesting a significant change of the glass network with the introduction of the rare earth. As compared with the Ga28Ge37Ba23K glass, the stability criterion ΔT of the Ga28Ge37Ba23LaY

glass decreases while it remains stable for the Ga21Ge43Ba24LaY glass and even cancels for

the germanate Ga15Ge60Ba13LaY glass (no crystallization phenomena detected). The density

of the rare earth containing glass ranges from ρ = 4.97 g.cm-3_{up to 5.13 g.cm}-3_{, while for the}

potassium-containing composition the value is 4.34 g.cm-3_{. One can also note a larger}

refractive index for the rare earth containing glass as compared to the glass Ga28Ge37Ba23K

(Δn ~ 0.08).

Table 2. Physicochemical properties of the investigated glasses

Nominal compositions (mol.%) Nominal compositions (cation mol.%) Sample Ga2O3 GeO2 BaO K2O La2O3 Y2O3 GaO3/2 GeO2 BaO KO1/2 LaO3/2 YO3/2 Ga28Ge37Ba23LaY 28.0 37.0 23.0 - 6.0 6.0 40.0 26.4 16.4 - 8.6 8.6 Ga21Ge43Ba24LaY 21.0 43.0 24.0 - 6.0 6.0 31.7 32.3 18.0 - 9 9 Ga15Ge60Ba13LaY 15.0 60.0 13.0 - 6.0 6.0 23.7 47.3 10.2 - 9.4 9.4 Ga28Ge37Ba23K 28.0 37.0 23.0 12.0 - - 40.0 26.5 16.5 17.0 - -Tg Tx ΔT ρ n532nm Sample (± 2°C) (± 2°C) (± 4°C) (±0.005 g/cm3₎ (±0.005) Ga28Ge37Ba23LaY 735 910 175 5.13 1.780 Ga21Ge43Ba24LaY 728 925 197 5.06 1.805 Ga15Ge60Ba13LaY 736 - - 4.97 1.796 Ga28Ge37Ba23K 671 832 191 4.34 1.701

Table 2. Physicochemical properties of the investigated glasses

The measured characteristic temperatures of the glasses are reported in Table2. The Tgof the

La/Y-containing glasses are ∼60 °C higher than the potassium-containing glass, suggesting a significant change of the glass network with the introduction of the rare earth. As compared with

the Ga28Ge37Ba23K glass, the stability criterion ∆T of the Ga28Ge37Ba23LaY glass decreases

while it remains stable for the Ga21Ge43Ba24LaY glass and even cancels for the germanate

Ga15Ge60Ba13LaY glass (no crystallization phenomena detected). The density of the rare

earth containing glass ranges from ρ = 4.97 g.cm−3_{up to 5.13 g.cm}−3_{, while for the}

potassium-containing composition the value is 4.34 g.cm−3_{. One can also note a larger refractive index for}

Polarized Raman spectroscopy was carried out on the investigated germano-gallate oxide glasses in bulk samples. The polarized vibrational spectrum for each studied glass is shown in

Fig.1.

Polarized Raman spectroscopy was carried out on the investigated germano-gallate oxide glasses in bulk samples. The polarized vibrational spectrum for each studied glass is shown in Fig. 1.

Figure 1. Raman spectrum of the investigated glasses (a)Ga28Ge37Ba23LaY, (b) Ga21Ge43Ba24LaY, (c) Ga28Ge37Ba23K and (d) Ga15Ge60Ba13LaY (d) (λExc = 532 nm)

For all glass compositions, two main contributions can be observed with maximum at around 525 cm-1 _{and 800 cm}-1_{. For the germanate glass (Fig. 1(d)), addition vibrations are}

observed above 900 cm-1_{. The band located between 400 cm}-1 _{to 600 cm}-1 _{is attributed to}

bending modes involving mainly T−O−T bridges (T used for Ge or Ga in tetrahedral coordination), while the bands between 700 cm-1 _{to 900 cm}-1 _{are assigned to localized}

symmetric and antisymmetric stretching modes of tetrahedral units Ge−O or Ga−O bonds [5]. One can notice for all samples a shoulder at around 760 cm-1_.

Previous investigation on sodium germano-gallate glasses [5] has ascribed the bands at 760 cm-1 _{and 860 cm}-1 _{to ring structure of germanium and gallium tetrahedral sites in which}

the sodium ions act as charge compensators. An increase of the sodium concentration, giving rise to a more intense band in relative intensity at around 830 cm-1, was assigned to non-bridging oxygens (NBOs) on the germanium tetrahedral units. Here, for the GeO2-rich

composition Ga15Ge60Ba13LaY, the band located at 830 cm-1 could be attributed to such

NBOs with barium or rare earth ions. The spectrum corresponding to the Ga28Ge37Ba23K (Fig.

1(c)) glass shows also a maximum around 830 cm-1 _{while this vibration is shifted to 805 cm}-1

for the Ga28Ge37Ba23LaY and Ga21Ge43Ba24LaY glasses. Furthermore, the Raman signal of

the Ga28Ge37Ba23LaY and Ga21Ge43Ba24LaY glasses increases significantly between 620 cm-1

and 730 cm-1_{. Such occurrence appearing for larger amount of gallium oxide could be related}

to the vibration in this wavenumber range of [GaO4]- and GaO6 polyhedral units, as suggested

in glasses in the Ga2O3-La2O3 system or in gallium oxide based garnets [12, 13]. At this stage

of investigation, one cannot exclude the presence of gallium in octahedral sites as also suggested in the Ga2O3-La2O3 glasses.

Then, we conducted drawing experiments on the rare earth containing glass preforms Ga28Ge37Ba23LaY, Ga21Ge43Ba24LaY and Ga15Ge60Ba13LaY (Fig. 2). The preforms were

drawn using dedicated optical fiber drawing towers, both at the COPL/University Laval and at the ICMCB/University of Bordeaux. The towers are equipped with furnaces having a sharp temperature profile, a diameter and tension controllers, a UV coating unit (optional) and a collecting drum (Fig. 2). The heating chamber is kept under continuous dry nitrogen gas flow (0.5 L.min-1). Preforms are fed into the furnace and the temperature is gradually increased up

200 300 400 500 600 700 800 900 1000 1100 1200 0,0 0,5 1,0 Ga15Ge60Ba13LaY 200 300 400 500 600 700 800 900 1000 1100 1200 0,0 0,5 1,0 Ga28Ge37Ba23K 200 300 400 500 600 700 800 900 1000 1100 1200 0,0 0,5 1,0 Ga28Ge37Ba23LaY 200 300 400 500 600 700 800 900 1000 1100 1200 0,0 0,5 1,0 Ga28Ge37Ba23LaY Raman shift (cm-1₎ N or m al ized i nt ens ity ( a. u. ) Raman shift (cm-1₎ N or m aliz ed in te ns ity ( a. u. ) Raman shift (cm-1₎ N or m al iz ed i nt en sit y ( a. u. ) Raman shift (cm-1₎ N or m al iz ed i nt en sit y ( a. u. ) (a) (c) (b) (d) Ga21Ge43Ba24LaY Ga15Ge60Ba13LaY Ga28Ge37Ba23K Ga28Ge37Ba23LaY

Fig. 1. Raman spectrum of the investigated glasses (a) Ga₂₈Ge₃₇Ba₂₃LaY, (b) Ga21Ge43Ba24LaY, (c) Ga28Ge37Ba23K and (d) Ga15Ge60Ba13LaY (d) (λExc= 532 nm)

For all glass compositions, two main contributions can be observed with maximum at around

525 cm−1_{and 800 cm}−1_{. For the germanate glass (Fig.1(d)), addition vibrations are observed}

above 900 cm−1_{. The band located between 400 cm}−1_{to 600 cm}−1_{is attributed to bending modes}

involving mainly T−O−T bridges (T used for Ge or Ga in tetrahedral coordination), while the

bands between 700 cm−1_{to 900 cm}−1_{are assigned to localized symmetric and antisymmetric}

stretching modes of tetrahedral units Ge−O or Ga−O bonds [5]. One can notice for all samples a

shoulder at around 760 cm−1_.

Previous investigation on sodium germano-gallate glasses [5] has ascribed the bands at 760

cm−1_{and 860 cm}−1_{to ring structure of germanium and gallium tetrahedral sites in which the}

sodium ions act as charge compensators. An increase of the sodium concentration, giving rise

to a more intense band in relative intensity at around 830 cm−1_{, was assigned to non-bridging}

oxygens (NBOs) on the germanium tetrahedral units. Here, for the GeO2-rich composition

Ga15Ge60Ba13LaY, the band located at 830 cm−1could be attributed to such NBOs with barium or

rare earth ions. The spectrum corresponding to the Ga28Ge37Ba23K (Fig.1(c)) glass shows also a

maximum around 830 cm−_{1while this vibration is shifted to 805 cm}−_{1for the Ga}

28Ge37Ba23LaY

and Ga21Ge43Ba24LaY glasses. Furthermore, the Raman signal of the Ga28Ge37Ba23LaY and

Ga21Ge43Ba24LaY glasses increases significantly between 620 cm−1 and 730 cm−1. Such

occurrence appearing for larger amount of gallium oxide could be related to the vibration in

this wavenumber range of [GaO4]−and GaO6 polyhedral units, as suggested in glasses in the

Ga2O3-La2O3system or in gallium oxide based garnets [12,13]. At this stage of investigation, one

cannot exclude the presence of gallium in octahedral sites as also suggested in the Ga2O3-La2O3

glasses.

Then, we conducted drawing experiments on the rare earth containing glass preforms

Ga28Ge37Ba23LaY, Ga21Ge43Ba24LaY and Ga15Ge60Ba13LaY (Fig.2). The preforms were

drawn using dedicated optical fiber drawing towers, both at the COPL/University Laval and at the ICMCB/University of Bordeaux. The towers are equipped with furnaces having a sharp temperature profile, a diameter and tension controllers, a UV coating unit (optional) and a

collecting drum (Fig.2). The heating chamber is kept under continuous dry nitrogen gas flow (0.5 L.min-1). Preforms are fed into the furnace and the temperature is gradually increased up to

∼940 °C. The bottom-section of the preform is locally softened until forming a drop, which falls

down by gravity until being hooked to the collecting drum system (drum velocity: ∼4 m.mn−1_,

preform feed rate: ∼1 mm.mn−1_{). When required, a UV-cureable polymer coating (Desolite) was}

applied on-line during fiber drawing.

(Desolite) wa Preforms w = 200 µm, d preform prior Ga28Ge37Ba23L selected mate centers during Figur Schem and c In order to performed by meters. (Fig. 2

1 _{for the glass}

Experimental overcome ove namely when preform prior preform neck of fibers of parameters, is s applied on-lin were successfu down to 100 r to drawing, a LaY fiber (Fi erials effective g the drawing p

re 2. Preform-bas me of the fiber dra

cane (b) XRD pat drum, an o confirm the o y the cut-back m 2(c)). We mea ses Ga28Ge37Ba results on the er the process n the glass visc r to drawing s -down and can optical quality s confirmed by

ne during fiber

ully drawn into µm. Fig. 2(a) glass drop and g. 2(b)) show ely prevents t process. ed optical fibers m awing process an ttern of Ga28Ge37B d (c) Loss measur optical transpa method at the sured optical a a23LaY, Ga21Ge Ga28Ge37Ba23K is avoiding c cosity is close shows no sign ne show detrim y (Fig. 3(b)). y XRD analysi r drawing. o tens of meters ) depicts a ph d a few centim ws that the res the nucleation

manufacturing fo nd photograph of

Ba23LaY fiber alo rements (inset: fi

arency of the fi wavelengths o attenuations of e43Ba24LaY an

K glass are dep crystallization to 107 _{Pa.s. W} ns of heteroge mental surface c This result, w is of the Ga28G s long fibers w hotograph of G meters long can

sistance toward n and growth

or GaGeBaLaY g Ga28Ge37Ba23LaY ong with a photog iber cross-section fibers, attenuati of 1310 nm on f 6.2 dB.m-1_{, 5.} nd Ga15Ge60Ba1 picted in Fig. 3 in the softenin While the macro eneities or incl crystallization which is irresp Ge37Ba23K cane with diameters r Ga28Ge37Ba23L ne. XRD analys d devitrificatio of parasitic c glass composition Y glass preform, d graph of the fiber nal view)

ion measureme n initial section

9 dB.m-1 _{and 3} 13LaY respectiv

3. The main dif ng temperature oscopic Ga28G lusions (Fig. 3 preventing the pective of the e (Fig. 3(c)). T ranging ϕ LaY glass ses of the on of the crystallite s (a) drop r on ents were ns of ~3.0 3.7 dB.m -vely. fficulty to e regime, Ge37Ba23K 3(a)), the e drawing drawing The XRD

Fig. 2.Preform-based optical fibers manufacturing for GaGeBaLaY glass compositions (a) Scheme of the fiber drawing process and photograph of Ga28Ge37Ba23LaY glass preform,

drop and cane (b) XRD pattern of Ga28Ge37Ba23LaY fiber along with a photograph of the

fiber on drum, and (c) Loss measurements (inset: fiber cross-sectional view)

Preforms were successfully drawn into tens of meters long fibers with diameters ranging

φ = 200 µm, down to 100 µm. Figure2(a) depicts a photograph of Ga28Ge37Ba23LaY glass

preform prior to drawing, a glass drop and a few centimeters long cane. XRD analyses of the

Ga28Ge37Ba23LaY fiber (Fig.2(b)) shows that the resistance toward devitrification of the selected

materials effectively prevents the nucleation and growth of parasitic crystallite centers during the drawing process.

In order to confirm the optical transparency of the fibers, attenuation measurements were performed by the cut-back method at the wavelengths of 1310 nm on initial sections of ∼3.0 meters.

(Figure2(c)). We measured optical attenuations of 6.2 dB.m−1_{, 5.9 dB.m}−1_{and 3.7 dB.m}−1_for

the glasses Ga28Ge37Ba23LaY, Ga21Ge43Ba24LaY and Ga15Ge60Ba13LaY respectively.

Experimental results on the Ga28Ge37Ba23K glass are depicted in Fig.3. The main difficulty

to overcome over the process is avoiding crystallization in the softening temperature regime,

namely when the glass viscosity is close to 107_{Pa.s. While the macroscopic Ga}

28Ge37Ba23K

neck-down and cane show detrimental surface crystallization preventing the drawing of fibers

of optical quality (Fig.3(b)). This result, which is irrespective of the drawing parameters, is

confirmed by XRD analysis of the Ga28Ge37Ba23K cane (Fig.3(c)). The XRD pattern presents

mixed crystalline peaks indexed by the isostructural phases KGaGeO4(JCPDS 052-1595) or

BaGa2O4(JCPDS 046-0415). pattern prese (JCPDS 052-Figure macros Surface ro potential issu improvement samples draw the microcry parasitic optic the nucleation crystallization as compared t The altern the open-cruc preform draw glass flowing is detailed in Ga28Ge37Ba23K melting and adjust the gla

tens of meters range 180-20 process. The slice (t = 5 m drawn glass extends from stretching vib produced fibe nm. No purif observed. nts mixed cry 1595) or BaGa 3. Preform-based scopic Ga28Ge37Ba oughness’ at t ue. Yet, despite

could be achi

wn from the Ga

stal formation cal losses. Thi n and growth p n of zeolite-typ to its volume. native approach cible technique wing, to reach a through a pin n Fig. 4(a). A K glass chunk the absence o ass viscosity un s of crystal-fre 0 µm (Fig. 4(b absorption co mm) is depicted fiber (black li ~280 nm to ~ bration of free O ers extends up t fication protoc ystalline peak a2O4 (JCPDS 04 d optical fibers m a23K glass preform

pattern of

the surface of e polishing tre ieved. Finally, a28Ge37Ba23K p n from the fib is is in good a

phenomena in pe crystalline p h explored to p e. Here the gla a sufficiently l nhole located a After loading

ks are first hea of any crystalli ntil allowing it ee Ga28Ge37Ba2 b)). An acrylic oefficient spect d in Fig. 4(c) ( ine). The optic

~6.0 µm. The OH− _{groups. L}

to 2.8 µm in th col was introd

s indexed by 46-0415). manufacturing for m prior to drawin f the Ga28Ge37Ba2 the Ga28Ge37B eatments to el no loss measu preforms as ou ber surface pr agreement with Ga28Ge37Ba23K phases mainly produce Ga28G ass is heated at low viscosity t the bottom o in a specifica ated up to 120 ites. Then the ts drawing thro

23K glass fiber

c coating was trum recorded (blue line) alon cal transmissio broad absorp Loss measurem he mid-IR, with duced at this the isostruct r Ga28Ge37Ba23K g ng (b) Ga28Ge37Ba 23K cane Ba23K preform liminate surfac urement could utput signal wa roduces scatte h our previous K glass strong occurs on the Ge37Ba23K optic t a much high (typically 103 -of the crucible. ally designed 00-1300 °C to e temperature oughout the ho r was obtained also applied o d on a Ga28Ge ng with the op on of the bulk ption near 3.0 ments show that h minimum los stage, which

tural phases K

glass (a) Photogra a23K cane and (c)

ms was conside ce flaws, no n d be performed as undetectable ering centers work [9] show gly overlapped surface of the cal glass fibers her temperature -104 _{Pa.s) to e} Scheme of th platinum cruc o ensure their is slowly decr ole. Using this d with a diame on the fibers d e37Ba23K glass ptical attenuati k Ga28Ge37Ba2 µm correspon t the transmissi sses of 3.1dB/m explains the h KGaGeO4 aph of ) XRD ered as a noticeable d on fiber e. In fact, and then wing that d and that e samples s relies on e than for enable the he method cible, the complete reased to s method, eter in the during the polished ion of the 23K glass nds to the ion of the m at 1310 high loss

Fig. 3.Preform-based optical fibers manufacturing for Ga₂₈Ge₃₇Ba₂₃K glass (a) Photograph of macroscopic Ga28Ge37Ba23K glass preform prior to drawing (b) Ga28Ge37Ba23K cane

and (c) XRD pattern of the Ga28Ge37Ba23K cane

Surface roughness’ at the surface of the Ga28Ge37Ba23K preforms was considered as a potential

issue. Yet, despite polishing treatments to eliminate surface flaws, no noticeable improvement could be achieved. Finally, no loss measurement could be performed on fiber samples drawn

from the Ga28Ge37Ba23K preforms as output signal was undetectable. In fact, the microcrystal

formation from the fiber surface produces scattering centers and then parasitic optical losses.

This is in good agreement with our previous work [9] showing that the nucleation and growth

phenomena in Ga28Ge37Ba23K glass strongly overlapped and that crystallization of zeolite-type

crystalline phases mainly occurs on the surface of the samples as compared to its volume.

The alternative approach explored to produce Ga28Ge37Ba23K optical glass fibers relies on the

open-crucible technique. Here the glass is heated at a much higher temperature than for preform

drawing, to reach a sufficiently low viscosity (typically 103_-104_{Pa.s) to enable the glass flowing}

through a pinhole located at the bottom of the crucible. Scheme of the method is detailed in

Fig.4(a). After loading in a specifically designed platinum crucible, the Ga28Ge37Ba23K glass

chunks are first heated up to 1200-1300 °C to ensure their complete melting and the absence of any crystallites. Then the temperature is slowly decreased to adjust the glass viscosity until allowing

its drawing throughout the hole. Using this method, tens of meters of crystal-free Ga28Ge37Ba23K

glass fiber was obtained with a diameter in the range 180-200 µm (Fig.4(b)). An acrylic coating

was also applied on the fibers during the process. The absorption coefficient spectrum recorded

on a Ga28Ge37Ba23K glass polished slice (t = 5 mm) is depicted in Fig.4(c) (blue line) along

with the optical attenuation of the drawn glass fiber (black line). The optical transmission of the

bulk Ga28Ge37Ba23K glass extends from ∼280 nm to ∼6.0 µm. The broad absorption near 3.0 µm

corresponds to the stretching vibration of free OH−_{groups. Loss measurements show that the}

transmission of the produced fibers extends up to 2.8 µm in the mid-IR, with minimum losses of 3.1 dB/m at 1310 nm. No purification protocol was introduced at this stage, which explains the high loss observed.

In summary, we proposed here two methods for the fabrication of germano-gallate optical fibers. This is to the best of our knowledge the first report on the production of crystal-free, light guiding fibers from a glass composition having gallium oxide as main constituent. While for the

Figure 4 (a) Sch on a 25 thick b In summa fibers. This is light guiding for the Ga28G introduction technology f spectroscopy network form expected. The Ga21Ge43Ba24L of the [GaO4] the origin of structures. Th Ga28Ge37Ba23K by the introdu al. have obse presence of N previous inve and vibration alkali ions oc oxide tetrahed The prefo softening at a the complete by a controlle then the fibe represents an glass drawing For all the practical appl implemented glass chemic core/cladding

4. Open-crucible heme of the fiber mm diameter op bulk glass (blue li techniq ry, we propose s to the best of fibers from a g Ge37Ba23K glass of lanthanum for various co investigation, med of ring st e introduction LaY glasses te - _{and GeO} 4 pol f the disappea he relatively hi K glass compo uction of the ra

erved that the NBOs on gall estigation did n nal spectroscop ccupy a compe dra [9]. orm drawing t a viscosity clos melting of the ed decrease of er drawing thr alternative ap g, such as for th e investigated g lications. It is i to purify starti cal purity, esp

preforms requ

technique-based draw process (b) tical post (c) Abs ne) and optical at ue. Acrylic coatin

ed here two me f our knowledg glass compositi s composition, m and yttrium ontent of gall the Ga28Ge37 tructure of [G of lanthanum ends to promot lyhedral based arance of the igh Tg value o osition suggest are earth ions. e gallium can lium ions. In not evidenced pies [5]. One h ensator role in technique relie se to 107 _Pa.s. e glass (corresp temperature an roughout the p pproach for pr he glass Ga28G glass composit important rem ing materials a pecially to re uired for the fab

optical fiber man ) Photograph of th orption coefficien ttenuation of the ng was removed p

ethods for the f ge the first rep ion having gall no fiber could oxides enab lium to germ 7Ba23K glass s GaO4]- and Ge m and yttrium o e the formation glass network surface cryst of the La/Y-co ts that the glas In the Ga2O3-L occupy tetrah GeO2-Ga2O3-N any NBOs on has suggested the structure es on the hea On the other h ponding to a v nd thus of visc pinhole of the reventing unde e37Ba23K.

tions, the mini

inding that at and glasses. Wo duce hydroxy brication of sin nufacturing for th he glass fiber on i nt spectrum of th

glass fiber drawn prior to measurem

fabrication of g port on the pro

lium oxide as m d be obtained b les to implem manium. Accor structure prese eO4, with lim

oxides for the n of NBOs wit k. This structura tallization of ontaining glass ss structure m La2O3 glass sys

hedral and oct Na2O for Ga/G

n gallium ions that ring struc to balance the ating of the g hand, the cruci viscosity of ab

cosity to attain e crucible. Th esired crystalli

imum loss rem this stage, no orks are curren yl group’s con

ngle-mode opti

he Ga28Ge37Ba23K its drum and enr he Ga28Ge37Ba23K n by the open-cru ments. germano-galla oduction of cry main constitue by preform tec ment preform rdingly to the ents a german mited amount o e Ga28Ge37Ba23 th the depolym al change is pr zeolite-type c ses, as compar may be strongly stem [12],Yosh tahedral sites Ge ratio aroun using combin cture occurs w e charge of the glass until atta ible technique bout 102 _Pa.s), n 103_-104 _Pa.s, his technique ization to occu mains relatively specific protoc ntly ongoing to ntent, and to ical fiber. K glass rolled 5-mm ucible ate optical ystal-free, ent. While chnology, drawing e Raman no-gallate of NBOs 3LaY and merization robably at crystalline red to the y affected himoto et with the nd 1, our ned NMR where the e gallium aining its relies on followed enabling therefore ur during y high for cols were o improve produce

Fig. 4.Open-crucible technique-based optical fiber manufacturing for the Ga₂₈Ge₃₇Ba₂₃K glass (a) Scheme of the fiber draw process (b) Photograph of the glass fiber on its drum and enrolled on a 25 mm diameter optical post (c) Absorption coefficient spectrum of the Ga28Ge37Ba23K 5-mm thick bulk glass (blue line) and optical attenuation of the glass fiber

drawn by the open-crucible technique. Acrylic coating was removed prior to measurements.

Ga28Ge37Ba23K glass composition, no fiber could be obtained by preform technology, introduction

of lanthanum and yttrium oxides enables to implement preform drawing technology for various content of gallium to germanium. Accordingly to the Raman spectroscopy investigation, the

Ga28Ge37Ba23K glass structure presents a germano-gallate network formed of ring structure of

[GaO4]−and GeO4, with limited amount of NBOs expected. The introduction of lanthanum and

yttrium oxides for the Ga28Ge37Ba23LaY and Ga21Ge43Ba24LaY glasses tends to promote the

formation of NBOs with the depolymerization of the [GaO4]−and GeO4polyhedral based glass

network. This structural change is probably at the origin of the disappearance of the surface

crystallization of zeolite-type crystalline structures. The relatively high Tgvalue of the

La/Y-containing glasses, as compared to the Ga28Ge37Ba23K glass composition suggests that the glass

structure may be strongly affected by the introduction of the rare earth ions. In the Ga2O3-La2O3

glass system [12], Yoshimoto et al. have observed that the gallium can occupy tetrahedral and

octahedral sites with the presence of NBOs on gallium ions. In GeO2-Ga2O3-Na2O for Ga/Ge

ratio around 1, our previous investigation did not evidenced any NBOs on gallium ions using

combined NMR and vibrational spectroscopies [5]. One has suggested that ring structure occurs

where the alkali ions occupy a compensator role in the structure to balance the charge of the

gallium oxide tetrahedra [9].

The preform drawing technique relies on the heating of the glass until attaining its softening at

a viscosity close to 107_{Pa.s. On the other hand, the crucible technique relies on the complete}

melting of the glass (corresponding to a viscosity of about 102_{Pa.s), followed by a controlled}

decrease of temperature and thus of viscosity to attain 103_-104 _{Pa.s, enabling then the fiber}

drawing throughout the pinhole of the crucible. This technique therefore represents an alternative approach for preventing undesired crystallization to occur during glass drawing, such as for the

glass Ga28Ge37Ba23K.

For all the investigated glass compositions, the minimum loss remains relatively high for practical applications. It is important reminding that at this stage, no specific protocols were implemented to purify starting materials and glasses. Works are currently ongoing to improve glass chemical purity, especially to reduce hydroxyl group’s content, and to produce core/cladding preforms required for the fabrication of single-mode optical fiber.

4. Conclusion

In this work, we report on the drawing into optical fibers of germano-gallate glasses having various contents of gallium to germanium. We established that lanthanum/yttrium oxides-containing glasses allow the production of crystal-free, light guiding fibers by conventional preform pulling, with optical losses of about 4-6 dB/m at 1310 nm on bare, unpurified fibers. Alternatively, we demonstrated that the crucible technique is a successful approach to suppress the surface

crystallization issues faced during the drawing of Ga28Ge37Ba23K glass preforms, leading to the

fabrication of tens of meters-long fiber with minimum losses of 3.1 dB/m at 1310 nm. These results provide insights and practical steps in the shaping into optical fibers of mid-IR vitreous materials having a strong technological potential in optics and photonics.

Funding

Canada Excellence Research Chairs, Government of Canada (CERC); Natural Sciences and Engineering Research Council of Canada (NSERC); Agence Nationale de la Recherche (ANR) (ANR-10-IDEX-03-02, ANR-17-CE08-0042-01).

Acknowledgments

This research was supported by the Canadian Excellence Research Chair program (CERC) in Photonics Innovations and the Aquitaine Region. The authors are also grateful to the Natural Sciences and Engineering Research Council of Canada (NSERC), the Fonds de Recherche Québecois sur la Nature et les Technologies (FRQNT) and the Canadian Foundation for Innovation (CFI) for the financial support. Mobilities were supported by grants of the French Consulate in Québec (by the Frontenac Program), the association of Campus France and Mitacs Globalink as well as grants of the Agence Nationale de la Recherche (ANR) with the program “Investissement d’avenir” number ANR-10-IDEX-03-02 and by the ANR PROTEUS (PRCI ANR-17-CE08-0042-01).

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